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Little Corona Beach
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Sediment Budget Analysis, Dana Point to Newp01t
Bay by Craig Everts
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REMARKS: Hi Bob -Enclosed is a report that may help you with Little Corona.
Please call if you need any help with it. Thanks.
BY: C.z=-~~---
Chris Webb
M&N File: 5815
FORMS/fRANSMIT0397
COAST OF CALIFORNIA STORM AND TIDAL WAVES STUDY
ORANGE COUNTY COAST
APPENDIX A
SEDIMENT BUDGET ANALYSIS
DANA POINT TO NEWPORT BAY, CALIFORNIA
Final Report prepared for:
U.S. Army Corps of Engineers
Los Angeles District
P.O. Box 2711
Los Angeles, CA 90053-2325
Subcontract through:
Coastal Frontiers Corporation
9420 Topanga Canyon Blvd., Suite 101
Chatsworth, CA 91311
Report prepared by:
Everts Coastal
1810 Emery Street
Eau Claire, WI 54701
June 1997
EXECUTIVE SUMMARY
The 13-mile long, high-relief coast between Dana Point and the entrance to Newport
Harbor is irregular in plan, consi1ting of small sftndy pockets backed by seaclijfs and
separated from one another by headlands with reef extensions that function like
underwater groins, retarding the alongshore movement of sand. This report documents
the sediment budgets of 23 short beaches in this soutliern Orange County, California
region (Fig. A). Each beach is separated from neighbor by a projecting rocky point.
j,,,,,,,.;;;;a;'·ooo~...,1!'"0,oooiaaa;a;;;;';;i5'000 Feot
! ,,,• -120· ·1--_.w' ••-• •••• Pe,:;lfic ()(;tum ··, ..
I -300' IMLLW) ~i11i-Ce/ls -'
I 7 9 I 13
1 2 3 5 6 8 JO 11 12 1415 I6 17 18 1920 21 22 23
Figure A. Lo-cation map, coast of southern Orange County, California.
SEDIMENT BUDGET APPROACH Analyses were performed to hindcast sediment
budgets for most of the twentieth century and to estllblish a means to predict list
century behavior of the pocket beaches if the pastc1mses of that behavior change. To
accomplish this a relationship was established between changes in the volume of
sediment in the littoral zone (littoral sediment lens) and changes in mean beach width
or beach volume. Mean beach width, the mean annual tl'latance between the shoreline
and the back boundary ef a specified length of beach, is probably the best single
measure of theprotective capacity of a beach. is also a useful and direct indicator
its recreational potential Mean beach parameters were obtained data spanning
decade:,. The sediment budget approach cannot be used to forecast storfflr<caused or
seasonal changes.
\•
Together, changes in mean hMch width and beach pemion define much of the essence of beach behavior needed in coastal zone management, coastal engineering, and the evaluation of proposed human interventions in the littoral zone. Mean beach width is an important control on the rate at which a beach changes position. Before the backbeacl1 line can be attacked by breaking wavM the fronting beach must be temporarily removed. The frequency at which this occurs is directly related to the size of the fronting beach. Forecasts of beach position change on the recessionai southern Orange County ceast are of special concern. When a beach retreats, valuable property behind it is eroded.
Reversible movements of sediment on a seasonal ba,'fjs define the surface of tlte littoral sediment lens and its landward and seaward limits. Alongshore boundaries of the lens are ideally specified at major headlands.
A sediment budget analysis is typically done in two parts. In the calibration stage, an historical sediment budget for a specified coastal reach is balanced. The change in the volume of the lens is obtained using survey data, or more commonly shoreline change data, which are often all that are available. Shoreline changes are converted to volume changes and the sediment budget is balanced by adjusting one or more of the
measured or inferred fluxes that caused the change ill volume. The volume change, US'ually assumed to be more accurate than the fluxes, is usually not adjusted. the predictive stage of a 11(tdiment budget, the calibrated mndel is used to forecast future changes in the mean width or volume of the beach.
In southern Orange County, sediment passes across the backbench line as stream-
carried inputs from watersheds and seacliff contributions; (ICNJ/SS the shorebase due to downwelling, shore-normal deflections at headlands, and a one-time onshore transport to Big Corona Beach from the pre-;jetty delta at the entrance of Newport Bay; in an alongshore direction around headlands, at the east jetty at Newport Harbor, and al'Ound Dana Point; and across the base of the littoral sediment lens. Also included in tlte sediment budget at Big Corona Beach is beachfill placed when the entrance to Newport Harbor WIIS dredged.
SEDIMENT BUDGET RESULTS. The IMSt complete set of reduced shoreline position data a11ailablefor all the beaches was tlte period 1927-1984. Backbeach line retreat estimates are also available for tltis 57-year interval Changes in sediment
volume were obtained using thMe shoreline and backbeacl1 line data sets.
Mean beaeh widths have not cltangedgreatly since 1917. Shoreline positions are generally near where they were 70-years ago and in this period backbeach line retreat rates averaged only an estimated 0. 07 to 0.2 ft/yr. Crystal Cove gained sand at a small annual rate while Victoria Coast (Lagunita 1Jeach, Victoria Beach, and Blue Lagoon Be,u:h) lost a small amount of sand on an annual basis. Losses and gains in the other mini-cell, weYe within the limits of uncertainty. In total, the southern Orange County coast experienced an average 8200 cubic yards sediment gain per year (cyy)
11
plus or minus 50%. Limited evidence suggests some of the beaches may have been
narrower in the last century than t11ey were in the early part of this one, suggesting the
slight accretional trend identified in this investigation has in progress for at least
JOO years.
Table A is a smrun,;uy of the sediment budgets of the 23 mini-cells. Coastal streams
carrying sediment from small watersheds in San Joaquin Hills and Santa Ana
Mountains were the largest contributors to the littoral zone. The over 18, fJOO cyy (plus
or minus 50%) was unevenly distributed. Well over half entered throagh Aliso Creek.
Some mini-cells received none. Et·odml! seaclijfs were the next largest source of
sediment, contributing abo,it 8000 cyy (plus or minus 50%).
The largest loss of sediment, an estimated 15,000 cyy (plus or minus 50%), was and
will continae to occur at Dana Point. Net longshore sediment transport rates vary with
location as shown in Figure B. The largest rates were found ut and downcoast of the
large Aliso Creek point source of sediment.
1&.Qtl0
i 14,Q00
1:.a-
12,000 ~ 1 10,000
'ii ., a ~ ,S 8,001)
"ii . ' . ~ ~ •. non : ~
1 lli,000
]
~ 2,000
]
; 0 ~
-2,000
-0 ,e m II: ,:::
t z
0 ~
~ u I = "' ~ I"----..
0 10,000
.....
:i' t -'d.
1l a
0 =-~ a m ·c fl l> r
,. J
2:11,000. 40,000 50,000
Distance.Southeast of Newport ff arbor, ft
~ -~-...
m = =--A
'
60,000 30,000
Figure B. Distribution lo11gshore sediment transport rates between Newport
Harbor and l)a11a Point (1927-1984).
Lesser losses were found at the shorebase (about 4600 cyy, plus or minus 100%) and
beneath the littoral sediment lenses (about 20()0 cyy, plus or minus 50%). The latter
flux was partially controlled by the 0. 008ft/yr rise in s.ea level relative to the coast in
this century. The magnitude of tl1e slwrebase and !en, base losses varied from mini-
cell to mini-cell At Dana Strand Beach, tlte flux across the of the le11s was
positive, meaning sediment was added from beneath lens. E;fforts to significamly
reduce these losses to improve the mean width of tlte beach Rre unlikely to be cost-
effective,
iii
Beach Name of Beach or
Number Coas(ai Reach
1 8k1 Corona Beach
2 Uttte Corona Beach
3 East Corona Coast
4 C,vstal Cove
5 Irvine Cove
6 Emerald Ba\/
7 Crescent Bay
<' 6 Shaws Cove
9 Dtvers Cove coast
10 Rockpile
11 M~ln Beach
12 Blu~bi~ Car:iyqn
·13 Woods Cove Coast
14 Rockledge Coast
15 Victoria Coast
16 Treasure-Island
17 Aliso Beach
18
Thousand Sleos Coasl
19 northwest Side of Three
Arch Ba.v
20 Three Arch Bav
21 southeast side of Three
Arch Bav
22 Salt Creek Beach
23 Dana Strand Beach
TOTAL
Table A. Selfime11t butlget summaries: mi11i-cells of south em Ora11ge Co1111ty {192 7-
1984).
Flux. Across Back 8eacb line,
C "'
length of Volume Change Seadiff Watershed Modified Flux Across Flux Across Base Artificial
Beach, ft in Lens, cyy contribuU011s Cohtribtitidns Shorebase, cyy af Letts, cyy flux
1800 3,100 0 -0 2890 0 1000
400 51 <200 370 -20 -30 0
1300 36 300 60 -110 -120 0
1660,0 3210 1400 1,600 -1060 -o 0
1700 -97 <100 60 -210 -130 0
2300 -579 <200 410 .JOO -260 0
1100 126 40 10 -140 -125 0
600 0 60 130 -70 -60 0
1500 100 <100 -o -190 -o 0
700 -50 <70 -0 -110 -40 0
6100 742 200 1900 '640 -350 0
2100 231 300 160 ·180 -110 0
1000 163 >100 ,o -!10 -50 0 -o 0 >10b -o 0 0 0
2300 -2582 <300 140 -190 -100 0
700 54 50 -o -40 -30 0
5900 1,033 <1 000 12,000 -370 -250 0
3000 430 400 60 -190 -140 0
•O 0 <300 -0 11 0 0
14()0 238 <200 60 -!10 -70 0
600 58 <400 -.0 .Jo 0 0
4300 973 <1 000 1200 -280 -170 0
4300 1 003 <1100 -o -280 25 0
59;700 8,241 7,920 18,180 -1,640 -2,010 1000
Alongshore Rux, cyy
NW SE
801mdary Boundary
0 790
790 1,259
1,259 1,383
1,383 113
113 30
30 659
659 318
3.18 378
378 188
188 188
188 556
556 515
516 311
311 411
411 3;143
3,143 3,069
3069 14,417
14,417 14,117
14,117 14,417
14417 14,279
14,279 14,591
14,591 15,368
15;368 15,209
SPECIAL CONCERNS IN SOUTHERN ORANGE COUNTY. In the initial stages of
the Cmist of California Study, two concerns were identified with respect to human
impacts on the southern Orange County coast. The first was the impact of seawalls on
the sediment budget. second concern focused on the rate of fill loss that could be
expected if beaches were artificially widened to improve protection for property behind
them. Other concerns were the impact rising sea level on the mean width of the
beaches and the impact of changing land use in watersheds on sediment
budget.
Impacts of Seawalls on the Sediment Budget. Seaclijf armoring will have two
important negative impacts on the sediment budget. By protecting the base of a cliff, a
seawall will reduce or eliminate the contribution of sediment entering tl1e littoral zone
from that source, In addition, by stabilizing the landward end of the littoral sediment
lens, armoring will impact the amount of sediment required to support the len!i llS sea
level rises and/or the retreats.
The decline in the baekbeach line sediment contribution, and the impact on the flux of
sediment at tlte base of the littoral sediment lens, will vary from one mini-cell to the
next. A loss of till seaclijf sediment contributions due to armoring would deprive the
southern Orange County coast of an estimated 7900 cyy, or almost 40% oftlte
estimated total contribution it receives each year. With the present low seacliff retreat
rates and a relatively high rate of sea level rise, tlte sediment flux raquired to .mpport
the lens will increase as the le:iigth of armored eeast e.>cpands. As limiting example, if
tlte entire backbeach tine in Strlilthern Orange County WllS armored and the rise in sea
level remained at its 20th century rate, the total loss flux at the base of the mini-cell
littoral sediment lenses would be approximately 10,000 cyy, or jive times tlte present
loss at this boundary.
The complete armoring of the southern Orange County coast would shift the present
positive sediment budget (an estimated 8200 eyy) to a negative budget of about-10,000
cyy. The result would be a cltange from nn advance to a continuing loss of 0.1 ft of
beach per year. Some of the beaches of1,wuthern Orange County would disappear in
less titan 100 years.
Expected Beachfill Losses. In soutltern Orange County there seems to be little current
intere/lt in enhancing beaches for recreational pu1poses. However, since almost all of
the beaches are narrow and slowly rBtreating into tlte a<?joining seacliffs, there is a
definite intere:rt in artificially widening them to increase their protective capacity. The
most uncertainty in a beach enhancement project for this purpose is the cost of
maintaining an artificially-widened beach over its project life. Often the largest cost of
a beach enhancement project is the expense to maintain design widths. cost is
directly proportionel to the rate at which the beachfill is lost following the initial
placement. The best local ex.ample of a liigh-cm;t tnaintenance effort is the Surfside
project in the Huntington Beach Littoral Cell which requires nourishment at an
annual rate of about 350,000 cy.
V
Sediment budget results provide guidance to determine whether the loss of beachfill
will be low because of the containment effect of the headlands, or whether it is likely to
be high, like it is at Surfside, for some less obvious reason. Most beaches in southern
Orange County are apparently at or very near their natural "holding cH.pacity". The
imposition of large quantities of artificial beachfill to widen them would probablJJ be
lost quite rapidry. The actual loss rates would depend on storminess and bathymetric
factvrs, among many. The rate would vary frvm mini-cell to mini-cell.
Headlands bordering most vf the cells extend out into relativery deep water and clearly
fimction as effective barriers to the mvvement of sediment along the coast. Without
them, the beaches would be much smaller, and in some places absent. However, there
is also a net shore-normal movement of sediment ant of these cells across the
shorebase (Table A). One reason for this movement has been documented at Little
Corona Beach. New contributions of sediment from Buck Gully are typically deposited
on the inner shore/ace during high discharge events in the stream Later, during
periods of high wave energy, the material is carried further seaward past the
shorebase. The result is a relatively na"ow beach that is in dynamic equilibrium with
a critical shore/ace slope. Sediment inputs lll:fflSS the backbeach line are balanced by
losses acr(lff the shorebase. Without a continuous artificial nourishment program at
high cost, Little Corona Beach will ntJt hold much more sand it carries today.
Longshore sediment transport southeast of Aliso Beach is clearzy to the southeast at
I fJ,fJOO to 2/J,000 cyy as shown in Figure B. This material does not collect in deepry-
indented Three Arch Bay which experienced a 1927-1984 annual accretion of less
than 3 00 cy (from 1. 5 to 3 % of the net longshore transport .rate past it). The material
either transited seaward of the headlands and did not enter the embayment, or if it did
enter, it did not remain there. The most likely reason for the lack of accumulation is
that the shore/ace is near or at its critical slope. As at Little Corona Beach, a shore/ace
at critical slope acts as a long-tt!rm transport surface, not a depositional or erosional
suiface.
Impact o(Sea Level Rise on the Sediment Budget. Together, the sea level rise rate and
the backbeach line retreat rate affect the magnitude of the flux across the base of the
littoral sediment lens. Assuming the backbeach line retreat rate remains the same as it
was the 20th century, but the rfJte of sea level rise doubles to 0. 01 ti ft/yr, the total
negative flux at the base of the littoral sediment lenses would quadruple to about -8000
cyy the coming century. A daubling of the sea level change rate is within most
scenarios of sea surface rise /87 the 21st century.
Impact of Changing Land Us.e on Sediment Contributions from the Watersheds.
Watersheds contribute the most sand to the beaches of southern Orange County.
Changing conditio,,i in the watersheds could therefore play an important ro[,.; in
upsetting the sediment budgets ofthe mini-cells. The apparentry narrower beaches of
vi
the nineteenth century imply that watershed contributions before the advent of intensive ranching «nd development were less th«n they were between 1927 and 1984.
Future land usage could reverse the positive impact of increased watershed contributions in this centKry. As more ef tlte natural watersheds and agricultural lands are built on, paved, and landscaped, a lesser yield of sediment from tlteir surfaces is to be expected. Until development completed, however, sediment contributions from construction sites will likely increase the yields over past rates. Every effort should be to retain the presrmt average annual discharge of sediment to the littoral zone from the watersheds.
TABLE OF CONTENTS
1. I Sediment Budget Method .................................................................................... I
1.2 Objective ............................................................................................................. 1
I. 3 Approach ............................................................................................................. 3
2.1 Littoral Sediment Lens ......................................................................................... 5
2.2 Sediment Budget Procedure ................................................................................. 8
3.0 BOUNDARIES OF THE LITTORAL SEDIMENT LENS .......... m,, .................. 11
3.1 Alongshore Boundaries of the Littoral Sediment Lens ........................................ 11
3 .2 Landward Boundary of the Littoral Sediment Lens (Backbeach LlllllJ.. .. ........... 13
3 .3 Seaward Boundary of the Littoral Sediment (Shorebase) ........................... 13
3.4 Base of the Littoral Sediment Lens ..................................................................... 16
3.5 Sediment Sizes ................................................................................................... 17
4.0 HISTORIC VOLUME CHANGES IN THE LITTORAL SEDIMENT LENS .. 18
4.1 Historic Shoreline Changes ......................................................... ,. .................... 18
4.2 Historic Backbeach Line Changes ...................................................................... 20
4.3 Relationship Between Changes in Mean Beach Width ood Lens Volume ............ 21
4.4 Mean Beach Width ............................................................................................ 25
5.-0 HISTORIC SEDIJ\1ENT FLUXES U•u11~••uoo ♦oHn.••un♦••••••unono•oo1u,.,.,uuo••U•.wMn•••n•• 27
5.1 Alongshore Sediment Fluxes .............................................................................. 27
:5.1.1 Net Flux Across the Entrance to Newport Harbor ..................................... 27
5.1.2 Net flux at Dana Point.. ............................................................................ 28
5 .1.3 Net Longshore Sediment Transport Past Headlands ................ ,. ................ 30
5 .2 Sediment Flux Across Backbeach Line ............................................................... 30
Stream Discharge ...................................................................................... 32
Seadiff Sediment Contributions ............................................................... .
5 .3 Se1:litt1ent Transport Across the Shorebase ......................................................... .
1 Shoalings and Deepenings ......................................................................... 36
5.3.2 Tracer Study ............................................................................................. 38
5.3 .3 Shoreface Slopes ....................................................................................... 39
5 .4 Sediment Transport Across the Base of the Littoral Sediment Lens .................... 39
Sediment Gains Losses Within the Littoral Sediment Lens ........................... 43
5 .6 Artificial Beach Enhancement and Sand :ti4'lning. . . . .... .... . .................. 44
6.0 CALIBRATION OF IDSTORIC SEDIMENT BUDGETS ................................. 46
6.1 Total Shorebase Sediment Flux .......................................................................... 46
6.2 Alongshore Distribution of the Shorebase Sediment Flux ................................... 48
6.3 Individual Longshore Sediment Fluxes and Mni-Cell Sedimimt Budgt'lts ............ 50
TABLE OF CONTENTS (CONT'D)
7.0 CONCLUSIONS: APPLICATION OF THE CALIBRATED SEDIMENT
7.1 Future Mean Beach Widths ................................................................................ 59 7.2 Impact of Seacliff Armoring ............................................................................... 60 7.3 Impact of Sea Level Rise ................................................................................... 6.2 7.4 Impact of Changing Land Use in the Watersheds ................................................ 62 7,5 Estimated Beachfill Losses if Beaches are Artificially Widened ........................... 62
TABLE OF FIGURES
Figure 1. Location map, southern Orange County, California ........................................ 2
Figure 2. Definition sketch: a change in the mean width of the beach is proportional to a
change in the volume of the littoral sediment lens ........................................... 6
Figure J. Definition ski:ltcl!i: three types of shore platform in southern Orange County .. 8
Figure 4. Three possibilities of sediment flux across the base of the littoral sediment lens
demonstrating the relationship between sea level rise and littoral sediment lens
retreat under conditions of a balanced sediment budget. ............................... 44
Figure 5. Measure of potential for seaward-directed transport across the shorebase ..... 52
Figure 6. Measure of potential for transport pallt headlands from one mini-cell to the
next mini-cell. .............................................................................................. 52
Figure 7. Estimated net longshore sediment transport rate and potential for longshore
sediment transport past headlands ................................................................ 53
Figure 8. Estimated net Jongshore sediment transport flux between mini-cells between
Ne,wt,ort Harbor and Dana Point. ................................................................. 53
Figure 9. Alongshore gradient in the net longshore sediment transport flux ................. 55
Figure 10. Seacliff sediment contributions between Newport Harbor and Dana Point (note
the lack of an alongshore gradient) ............................................................... 56
Figure 11. Watershed sediment contributions between Newport Harbor and Dana Point
(note the lack of an alongshore gradient) ...................................................... 56
Figure 12. Sediment flux across the shorebase between Newport Hilrbor and Dana Point
(note the lack of an alongshore gradient) ...................................................... 57
Figure B. Sediment flux ru::ro!ll! the base of the littoral sediment lens between Newport
Harbor Point (note the lack of an alongshore gradient) ................. 57
Figure 14. Sediment volume change within the littoral sediment lenses of mini-cells
between Newport Harbor lll!d Dana Point. ................................................... 58
Figure 15. Net imbalance in the mini-cell sediment budgets without longshore sediment
transport ...................................................................................................... 64
TABLE OF TABLES
Table 1. Tasks to complete the mini-cell sediment budget analyses ............................... 3
Table 2. Boundaries of the littoral sediment lens ......................................................... 11
Table 3. General planfonn characteristics of mini-cells in southern Orange County ..... 12
Table 4. Shorebase locations from the geometric method: southern Orange County .... 14
Table 5. Volume changes that exceed the 75 cubic yards per foot of beach assumed to
be outside the normal bounds of uncertainty in acoustic surveys ................... 15
Table 6. Characteristics of Shore Platfonns in Orange County• .................................. 16
Tabl.e 7. Historic shoreline chmges ............................................................................ 19
Table 8. Comparison of historic long-tenn changes in shoreline position ..................... 20
Table 9. Summary: Mean seacliffretreat rates in southern Orange County north of Dana
Point.* ......................................................................................................... 21
Table 10. Values used to estimate the historical change in the volume of the littoral
sediment lens. .............................................................................................. 23
Table 11. Mean all!1ual volume change in tbe littoral sediment lens between Newport Bay
and Dua Point (1927-1984) ........................................................................ 24
Table 12, Mean beach widths in mini-cell!l between Newport Harbor and Dana Point ... 26
Table 13. Sediment fluxes in southern Orange County (1927-1984)* ............................ 27
Table 14. Net longshore sedimenttrnnsport estimates at Dana Point.. ........................... 30
Table 15. Characteristics of headlands and the submeiged extensions ofheadlands in
southern Orange County (from City of Laguna Beach, 1988) ....................... 31
Table 16. Previous estimates of mean ammal coarse sediment discharge between
Newport Harbor and Dana ru.m,....... ............. .. 32
Table 17. Sand yield estimates for southern Orange C01Jnt11• ........................................ 33
Table 18 Variables used in coarse sediment yield estimates for coastal !01.lthern Orange
Cmmty ........................................................................................................ 37
Table 19. Summary: Estimated sfll!ellft' sediment fluxes between Newport Harbor and
Dana Point. .................................................................................................. 38
Tnble 20. Potential for offshore sediment transport the shorebase southern Orange
County ........................................................................................................ 40
Table 21. Conditions for sediment release, capture, or zero change as shore platform
retreats landward, and rises with a rising sea level. ........................................ 43
Table 22. Estimated sediment flux across the base of the littoral sediment lens (from
Equation 12) ................................................................................................ 44
Table 23. Su!llmary of fluxes m:roos boundaries of the littoral sediment lens (control
volume) between Newport Harbor and Dana Point for the period 1927-
1984 ............................................................................................................ 47
Table 24. Parameters used to evaluate the probability of alongshore sediment transport
past headlands and cross-shorebase sediment transport ................................ 50
Table 25. Groupings of alongshore anC: cross-shorebase sediment transport parameters 51
Table 26. Sediment budget summary for mini-cells between Newport Harbor and Dana
Point. ........................................................................................................... 53
SEDIMENT BUDGET ANALYSIS:
DANA POINT TO NEWPORT BAY, CALIFORNIA
1.0 INTRODUCTION
1.1 Sediment Budget Method.
Long-term trends in beach width and net rate at which a beach retreats or advances
can be forecast using the sediment budget approach. Together, these dynamics define
much of the essence of beach behavior needed in coastal zone management, coastal
engineering, and the evaluation of proposed huruan interventions in the littoral zone.
width is the difference between the positions of the shoreline and the landward
boundary of the beach. Mean beach width is a measure of the recreational potential and
protective capacity of a beach. Changes in mean beach width are directly proportional to
changes in the littoral sediment budget. Beach position change is partly controlled by the
sediment budget because beach retreat into previously stable land is inversely proportional
to the fronting beach. Until the beach is stripped away, usually during a storm,
waves ci!lnOt attack and erode the backbeach line. Wide beaches provide protection
longer periods and against more severe storms than narrow beaches.
In addition to its primary use in predicting future changes in mean beach width and
position, a sediment budget a:mdvscis may be employed to identify and quantify sediment
sources and sinks within the littoral zone. It can also be useful in establishing paths
sediment transport there. Another use is to prioritize the sediment fluxes that affect the
budget order to manage them to improve beach conditions. Artificial beach
enhancement is an excellent example where a sediment budget analysis may be used to
predict post-placement beachfill loss rates (Marine Board, 1996). The sediment budget
method can be used to evaluate the quality and quantity of available coastal knowledge in
order to rank future needs to collect, analyze, and interpret coastal data. Sediment budget
results are sometime used as checks on the results of more detailed analytical methods.
Where accurate sediment flux data are not available and cannot be estimated by the
process of elimination, it is sometimes possible to obtain relative impacts of the fluxes
involved using the sediment budget approach.
1.2 Objective.
This report details the sediment budget of the coastal reach between Dana Point
Newport Harbor, California as shown Figure 1. The report was prepared under
subcontract to Coastal Frontiers Corporation who, in tum, were under contract to the
Corps ufEngineers (Contract DACW09-96-D-0001). The stated requirements of the
Corps of Engineers contract were to describe the purpose and uses of the sediment
budget, identify the littoral suhcells, dll!icril1e the present state of knowledge of the main
I
sediment sources and sinks in the study area, and develop and present the sediment
budgets of the littoral subcells (mini-cells).
S.000 10,000 15,000 Feet
Paclllc Coast I-ii way
I 2 J' 5 6 8 JO 11 12 1415/6 17 18 19Z011 22 13
Figure 1. Location map, southern Orange County, California.
Coastal soothern Onm,!l:eCounty is one of projecting headlands, deep and not,so-deep
bays with sandy beaches, and seacliffs. Littoral terrain such as this is
sometimes referred to as a "pocket beach" coast. The sediment budget results provide
information on the five documented needs, articulated in question form, below. Topics 4
and 5, potential beneficial and adverse impacts of human interventions, have been
identified as the most imt10rt:ant concerns in southern County (USACE-LAD,
1993a).
J. Changes in Mean B11ach Width and Beach (Segclifj) Retreat Rates. What are
the present mean wid~bs of the beaches in southern Orange County, and what were the
recent pas.t mean widths? Are they presently changing? What were the past rates of seacliff
retreat and what are likely to be in future?
2. Causes ofBe:at:h Width Change. What presently are the causes of change in
mean beach width? Are these mechanisms of change likely to differ in future?
2
3. Impact q.f Sea Level Rise. Will estimated accelerm:ions in the rate of sea level
rise have a measurable effect on the beaches and seacliffil of southern Orange County in
the next 50 or 100 years? How should sea level rise be considered in managing the coastal
sand resource?
4. Human Interventions to Arrest Beach (Seacli(f) Retreat, What is the impact of
seawalls on the mean width of1;he h .. ,,,1,?
5. Human Interventions to Enhance Beach Widths. At present, there seems to be
llttle interest in widening beaches for recreational purposes. However, since almost of
the beaches in southern Orange County are narrow they are slowly retreating landward as
the base of the adjoining seacliffs are eroded. Is beach enhancement the best way to
protect those seacliffs, or will the beachfill be so rapidly that this method of protection
will prove too costly? the beaches now near their "holding capacity" or could they
accept significantly more material without high losses?
1.3 Approach.
This report is a compilation of documented littoral processes between Dana Point and
Newport Harbor, the reach re.furred to as the Laguna Beach Mini-Littoral Cells. Tw,Mtv-
three discrete coastal segments within this reach are treated as littoral entities. Within each
of these pockets, the historical sediment budget is calibrated. Table 1 is a list of the steps
used to perform sediment budget analyses and predict future beach behavior.
Table 1. Tasks to complete the mini-cell sediment budget analyses.
Task
Establish spatial bou:mlllries of littmal
sediment lens; establish temporal limits
for the analvsis
Determine the historic net rate Qf volume
change in the linoral sediment lens or the
historic shoreline chan e rate
'·'·"' Estimate historic sediment fluxes across
the boundaries of the control volume
Address the need!/ of the investigation by
predicting futnre beach behavior using the
cllllbrated historical sediment budget
model
This roport follows the routine of Table L Following a methodology and definitions
section, control volume boundaries are designated, historic sediment volumes are
determined, historic sediment fluxes are estimated, the historical sediment budget is
calibrated, and the calibrated model is used to ansW'l:lr the questions posed in the preceding
3
section. Because this is a summary report, the fluxes, shoreline change rates, md
backbeach line change rates are as presented in the original reports. The reader is
encouraged to peruse those referenced sources for detailed discussions of the variables.
Only in the few situations where the fluxes have not been defined are they developed in
detail in this report.
4
2.0 METHODOLOGY
The key rellrtionship in a sediment budget analysis is between a predicted change in the volume of sediment in the littoral zone and the desired change in the mean width or volume of the beach. The volume change is equal to the sum of all sediment fluxes into and out of a designated coastal reach during a specified time interval. Results are annual or longer mean rates for the specified coastal reaeh. Beach position changes, which occur when the backbeach line is eroded or advll.llces, are significantly controlled by the amount of sediment contained in beach. Sediment movements within the littoral zone have no effect on the mean width or mean volume of the beach. A sediment budlget analysis, such as the one described in this report that considers the. entire littoral zone, cannot be used to forecast the amount ofbeach that will be eroded during a storm, nor can it efficiently provide information needed to forecast seasonal changes in the position of the shoreline.
l,l Littoral Sediment Lens.
A change in the mean width of the beach could theoretically be made proportional to a change in sediment volume between any set ofboundllries in the littoral zone. The seaward margin could, for example, be located at the msl-shoreline, but it would be virtually impossible to obtain the mean flux across that boundary over a long time period. In practice, the most realistic boundaries are the maximum landward and seaward limits of seasonal reversible movements of sediment normal to shore. Fluctuations ta these limits occur with some statistical predictability OIi a seasonal basis.
The body of sediment within these limits is herein defined as the littoral sediment lens (Fig. 2). The littoral sediment is the control volume used in the sediment budget analyses. immobile surface that supports the is the littoral substrate. The . landward subaerial part of the littoral sediment is the beach. The subaqueous part is the shoreface. The beach and shorefuce are three-dimensional features. The hackbeach line, or landward boundary of the the shorebase at the seaward edge of the lens, and the shoreline that separates the and the shoreface at a designated sea-surface elevation, are two-dimensional features. Beach width is the horizontal distance between the shoreline and the backbeach line. Mean beach width is the horizontal distance between mean position of the shoreline and the mean position of the backbeach line during an averaging period, typically a year in length.
Reversible movements of sediment within the littoral sediment lens define the shape of the lens surface, landward and seaward limits, and to some extent the position of its base. They have no effect on the sediment budget. Sediment movements and changing bottom elevations outside the lens similarly have no on the sediment budget unless sediment crosses the boundaries of the lens. Mixing is intense near the top of the lens around its sea (shoreline) intercept. Transport and sediment level changes decline toward the landward, seaward and underlying boundaries of the lens. At the backbeach line and 1he shorebase, reversible movements of sediment are zero. Sediment level changes are consequently zero, or near zill:o there also.
5
HINTERLAND
:r u < "" "" ""z ~-u ..J < =
sbiu·ebue b 11ea1r■rd bou■dary orllttonil ZDH
bi.tk.bea:t:1111 Hae il Ja■dward boM•dary ol llttorai ut'lt
11JJORE-NORMAL BOUNDARIES
OF Ll'TTORALZONE
BEACH
LJITORALZONE
SHOREFACE
w "' < = lol = 0 ili
h,
""""'.-,---,-------y, _______ ,
• SHORE ANGLE
littoral substrate surface z,
Figure 2. Definition sketch: a change in the mean width of the beach is proportional
to a change in the volume of the littoral sediment lens.
6
Beach retreat occurs when the littoral sediment lens retreats as illustrated in Figure 2 (top). Changes in mean beach width occur when there is a loss or gain in the volume of sediment in the littoral sediment lens. Beach retreat should not be confused with a reduction in mean width. Beaches can, and often do, retreat without a diminution in the volume of contained littoral sediment. The slope of a newly-created inner continental shelf profile created as the shoreface retreats on a recessional coast reflects the ratio of the rate of beach retreat to sea level rise.
I
Littmal sediment is any material small enough to be moved by waves and/or currents, but large enough that it remains a part of the littoral sediment under most conditions.
Littoral sediment is usually in the size range from sand through shingle. Boulders are littoral sediment if they are moved during severe storms. The fine-sediment cutoff along the open coast of southern California is near the silt-sand boundary (0.062 mm; USACE,
1991 ). Silt finer fractions ten.d to be carried through the littoral sediment lens to less energetic regions further seaward, or landward into estuaries, inlets and harbors.
As illustrated in Figure 2, the littoral sediment lens rests on and against an inactive littoral substrate. On a high-relief, recessional coast such as southern Orange County, the littoral
substrate may be a two-part erosional surface composed of an underlying gently-sloping
shore platform, and a steep seacliff that fixes the backbeach line of the littoral sediment lens. The shore platform is usually composed of the same material as the seacliff. The
shore angle is the junction between the mildly-sloping platform and the steeper seacliff.
Three types of rocky shore platform are common to the high-relief, recessional coast of
southern Orange County (Fig. Low-type, covered shore platforms are typically cut into weak rock or unconsolidated sediments. This type of platform is protected its entire length by the overlying littoral sediment lens or by an unconsolidated sediment layer
beneath the lens. Low, covered-type platforms are typical oflow•relief coasts. Low-type,
exposed shore platforms are uncovered along part of their length, usually the seaward part. In sorne places a beach against a seacliffis the only component part of the littoral
sediment lens. High-type shore platforms are almost always cut into resistant rock. They
rarely support a littoral sediment lens. ?,fost high-type platforms are found at headlands.
A littoral cell is a finite segment with arbitrarily-defined alongshore boundaries, usually at a impediment to alongshore sediment transport. The alongshore
boundaries used in a sediment budget analysis may conform to littoral cell boundaries,
often differ to best meet project needs. Like littoral cell boundaries, the alongshore limits of the littoral sediment lens control volume) are specified wham net alongshore sediment transport are known or can be reasonably estimated. Examples are major headlands; nearshore submarine canyons; large inlets, river mouths, or baymouths; major changes in the orientation of the coast; and at long jetties and harbor breakwaters.
7
LOW-li'l"E. COVERED SHORE PU TFORM
EL Cracai.t 8ay
littoral :sedime.at lens
,-stnte
LOW-1YP&. EXPOSED SHORE PUTFORM
tz.Cryu!C.tt
p■ tdi .. of littoral sediment
HIGII-1YPE SHORE PlA TFORM
SHORE ANGLE
Figure 3. Definition sketch: three types of shore platform in southern Orange
County.
2,2 Sediment Budget Procedure.
A sediment budget is usually thought of as a conservation of mass approach, but in reality
it is almost always dependent upon an assumed conservation of sediment volume.
Sediment mass fluxes are rarely measured in the field; surveys establish sediment volume
changes. Implicit is the assumption that the '.mil, specific gravity of the material in the lens
remains constant.
8
Two elements of a sediment budget analysis are often not addressed. Backbeach line
retreat must be quantified on a high relief coast because it is required to estimate the
seacliff sediment flux 11Cross that boundary, The impact of a change in sea level must also
be quantified because of the effect it has on the amount of s.edimentthat passes across the
basal boundary of the littoral sediment lens. A vertical shift in the surface of the littoral
sediment lens follows a long-term rise or fall in the sea surface, which will affect the
sediment flux across the base of the lens.
A sediment budget is the sum of all of the fluxes that pass across the boundaries of the
littoral sediment lens as well as the littoral-type sediment that originates or is destroyed
within the lens. A net change in the littoral sediment lens, or control volume, dV, / dt, is
in which t!cQ, = mean difference in longshore sediment transport fluxes across the two
shore-normal boundaries of the cotttrol volume, L V, = sum of all shore-normal fluxes at
the backbeach boundary (seacliff sediment contributions, overwash losses, aeolean losses
or gains, gains or losses a harbor entrance, bay or inlet, and river, stream, and ravine
contributions through backbeach line), L v; = sum of all more-normal fluxes at the
shorehas!l boundary ( downwelling, onshore transport, losses to submarine oanyons, and
losses due to deflections of sediment across the shorebase at headlands other long,
shore-normal barriers), ;E Vw = sum of all volume changes that occur internally ( sediment
volume by mechanical abrasion or chemical solution and volume gain u shells are
mechanically broken up and new littoral-type sediment such as calcium carbonate Sll.lld is
created by chemical precipitation), I; v;, = sum of all artificial fluxes (sand mining ls a
negative flux;
flux), and v.
placi:,ment of artificial beachfill for enhancement purposes is a positive
sediment addition to or loss across the base of the lens.
The
shape
vo11un:te can be approximated by assigning beach a trapezoidal cross-sectional
the shoreface a triangular shape. Thus,
(2)
in which x1 = alongshore length of littoral sediment lens, w = mean beach width, zb =
elevation of beach berm, z, = depth of shorebase, and fJ = slope of forel!hore.
Time is an importa.,t consideration in a sediment budget analysis. The balan~ins; interval
determines what Gan and cannot be forecast. To some extent, uncertainties in the results
are inversely proportional to the time interval of the budget. Data mll!lt um.ially be
9
collected over a decade or more, and frequently, for calibration purposes. If the three-
dimensional surface of the littoral sediment lens could be accurately mea!lured only twice,
it would be possible to establish changes in the sedimelll budget on the interval of the
surveys. Survey inaccuracies and unknown changes between survey points, especially over
the underwater portion of the lens, introduces enough variability in the volume change
results that many profiles over a Jong time period are required to establish a useful net
volume change. An advancing or recessional lens provides the best circumstances to
optimize the net change.
Surveyed shoreline positions or shoreline positions from aerial photographs are often the
only data available. Many years coverage is required in order to separate the large bias
introduced by seasonal fluctuations in beach width or shoreline position from the usually
smaller net changes that occur. Large year-to-year variations in many of the
sediment fluxes also bias the budget results unless a long-time series ofinput data is used.
River and stream fluxes, lasses to submarine caeyons, and the net gain or loss of sediment
moving along the coast, are examples ofunateady year-ta-year :fluii:es.
A sediment budget analysis is done in two The sediment budget is balanced in the
calibration phase by adjusting the historical fluxes based on measurements, judgment, and
sometimes consensus, until their sum is equal to the net volume change for the period of
record. Changes in lens volume are eo11sidered to be more accurate than the sediment flux
data, some of which may unknown. Next, in the forecast stage, one or more of the
historic fluxes, or relative sea level are modified and the calibrated model is used to
estimate the change in mean beach width or volume likely to result.
10
3.0 BOUNDARIES OF THE LITTORAL SEDIMENT LENS
Lens boundaries in the pocket segments of the southern Orange County coast are
discussed in the order given in Table 2.
Table 2, Boundaries of the littoral sediment lens,
longshore sediment transport in
and out pockets (mini•
cells)
Slre!!m discharge, seacliff
contributions
net onshore or offshore transport
gain if substrate below lens is
eroded; loss if new substrate is
created
3,1 Alongshore Boundaries of the Littoral Sediment Lens.
Pocket beaches between Dana Point and Newport Harbor are separated from one another
by headlands of varying size. Reef extensions of the headlands function like underwater
groins, retarding or eliminating the alongshore movement of sand between pockets. The
!JO(~ beaches behave in a different way than the Jong, relatively uninterrupted beaches in
the Oceanside Littoral Cell to the south and the Huntington Beach Littoral Cell to the
The unlike behavior is mostlv due to the presence, or lack thereof, of headlands.
Short beaches separated by sediment-blocking structures in southern Orange County are
near-dynamic equilibrium. In contrast, most of the long beaches in the adjacent littoral
cells experience a large net annual loss of sediment.
Sediment-blocking structures are e&sentially two-dimensional, shore-normal features that
act as physical barriers to the alongshore transport of sediment. They affect the amount of
sediment moving past the structure, the volume that is retained upcoast of it, and the
amount that reaches downcoast beaches. The ubiquitous headlands in southern Orange
County are natural sediment-blocking structures; the harbor entrance jetty at the west
of Big Corona Beach is an artificial sediment..lJlocking structure.
The extent to which headlands act to reduce of eliminate the alongshore movement of
littoral sediment in southern Orange County is one of the topics addressed in this sediment
11
budget investigation, To assist in this effort it is realistic to select alongshore mini-cell
boundaries at the larger of these headlands, Selections, shown in Table 3, were made in
consideration of three types of short beach, "Pocket beaches:' are defined as being
confined between high headlands that extend well seaward of the beach (example is
Crescent Bay), "Contained beaches" are also confined between headlands, but internal
structures permit the passage of sediment along the coast above ms! ( example is Crystal
Cove beaches), "Sand patches" in rocky shore platforms are series of very small beaches
held within indentations of extensive shore platforms ( example is the coast of Corona de!
Mar southeast of Little Corona Beach),
Table 3. General planform characteristics of mini-cells in southern Orange County.
Name of Beach
(mini-cell)
Bi Corona Beach
Little Corona Beach
East Coron.a Coast
Crystal Cove
Irvine Cove
Emerald Ba
Crescent Ba
Shaws Cove
northwest side of Three
Arch Ba
Dana Strand Beach
12
Plan Shape of Beach:
1 = crescent
2 =hook
modest hook at north end
4: three straight beaches with hook at
north end of north beach
4: crescent with sli ht hook at north end
1
I
4: two relatively long straight beaches with
hook at north end of north beach
ht hook at north end
ht hook at north end
4: crescent with a modest hook at the
north end
convex.-ward planfmm
4: straight beach with slight hook at north
end
3.2 Landward Boundary of the Littoral Sediment Lens (Backbeach Line).
The backbeach line is the base of the seacliffin most locations. All of the beaches except
for Big Corona are relatively narrow. Within an interval of perhaps 25 to 50 years the
seadiffs behind these beaches will probably be exposed to breaking wave forces when the
fronting beach is temporarily stripped by a severe storm. As a consequence, all seacliffs
between Dana Point and Big Corona Beach are erosional, but their rates of retreat are
variable.
Big Corona Beach is mostly an artificial feature created with beachfill the Newport
Harbor navigational channel. It is held in place by the east jetty at the entrance to the
harbor. Seacliffs behind the western two-thirds of Big Corona Beach are protected by the
artificially-wide oeiicn.
3.3 Seaward Boundary of the Littoral Sediment Lens (Shorebase ).
The most difficult boundary to locate is the shorebase. As defined in Section 2, the
shorebase is the seaward limit of reversible sediment movements that occur over the
period of the historical sediment budget. A number of clues are available to locate the
limits of reversible sediment transport. Whenever possible it is desirable to employ more
than o.ne to establish redundancy and increase confidence in the results.
If available, it is usually most effective to employ a time-series of precision sediment-level
survey data, such as might be obtained using stakes implanted in the seafloor and
measured by divers, and overlay the resulting repetitive profiles to define a "pinch-out"
depth beyond which bottom level cll.anges are undetectable. Another method is to analyze
a time-series of non-precision surveys, such as might be obtained using sonic devices, to
locate the seaward limit of declining standard deviations in the bottom level. Because of
the elevation variability inherent in such surveys, the standard deviation ofbottom level
change is large landward of the shorebase, declines toward it, and is nearly constant
seaward of it. A third method that sometimes helps is to analyze sediment samples taken
normal to the shoreline to determine if there is a distinct boundary beyond which the
sediment size no longer declines with depth, and/or material in deeper water is either relic
of past conditions, and/or mostly silts and muds that are never found in equilibrium in the
littoral sediment lens. The geometric method, which requires only a single shore-normal
deep water profile, is to plot the shape of the profile to establish the location where it
changes in a landward to seawMd direction from concave-up and steep to planer with a
milder slope. This method works best when profiles are plo1tted with an exaggerated
vertical scale, but it is often difficult to establish the slope junction with a precision
exceeding plus or minus 2 ft. The method is based on the assumption that the littoral
sediment lens is steep and usually concave-up in shape and that when it retreats a linear
profile is left behind with a slope equal to the ratio of the sea level rise rate to the beach
retreat rate.
13
The shorebase has not been established using a long time-series of repetitive surveys of any type in southern Orange County. A series of five recent surveys at selected locations
were used to define the shape of the shoreface and the slope of the inner-continental shelf
The surveyed profiles are presented in USACE-LAD (1995, Appendix F) along with calculated volume changes based on the surveys. Table 4 is the estimated shorebase
location based on the geometric criterion obtained from the surveyed profiles. In most
cases, the slope of the southern Orange County shoreface is concave-up in shape and steeper than the adjacent, usually planer continental shelf
Table 4. Shorebase locations from the geometric method: sonthern Orange County.
Name of Beach or
Coastal Reach
Crystal Cove
Crescent Bay
Aliso Beach (includes WtRSt
Street Beach)
northwest side of Three
ArchBav
Tbree Arch Ba
southeast side of Three Arch 1 ,,, ,,,
Ba
Salt Creek Beach
Dana Strand Beach
Shorebase
Distance from
Shoreline, ft
650
650
540
540/430
NA
560
500/440
NA
330
550 ?)
700
near Abalone Point,
apparent pinchout
and slo break
slo e break
straight profiles to a
depth of 40 ft; slope
is 0.0047 rise/run
slo e break
Myrtle St. and
Ocean Ave.
shotebase obliterated
b reef
West Street Beach,
slope break and
• chout
indistinct shorebase
slo e break
Volume changes from existing repetitive surveys indicate the profile envelope for recent
years cannot be used to effectively establish the pinch-out depth. If one lll1J1Umes the acoustic surveys used to calculate the volume changes were accurate to plus or minus 0.5 ft (probably the best possible uncertainty range), the total volume contained within the
14
bounds of this accuracy from one survey to the next is about 75 cubic yards (cy). Not
considered in this volume is the uncertainty introduced when the profile line strays from
the previously surveyed line. Changes of75 cy or more only occurred at the few places
and dates shown in Table 5.
Table 5. Volume changes that exceed the 75 cubic yards per foot of beach assumed
to be outside the normal bounds of uncertainty in ac11ustic surveys.
AB43
'.l\.1344
Survey Dates
May-Nov92
May-Oct 93
May-Oct93
Remarks
Buck Gully di!!Charge'I
o evidence of !his
ranges in
stream discharge and/or
seasonal alongshore
shift is sediment
Stakes were implanted along ranges on the east and west sides of Crescent Bay in water
depths of about 15 to 36 ft (mllw) and distances to the sediment surface measured
between June 1994 May 1995 USACE-LAD (1996a). Changes in bottom elevation
to October 1994 lndicate little change (0.1 to 0.2 ft, perhaps caused by ripple migration
and not cross,.shore transport) although a number of stakes were lost to unknown causes.
Changes in bottom elevation of0.2 to 0.8 ft occurred during the storm season of October
1994-May 1995. Accretion was found in water depths of 19 to 35 ft (mllw) near the
center of the bay, and erosion off the east and west headlands in water depths ofabout 35
ft (mllw). The mllw-depths at the ends ofthe submerged extensions of the headlands are
45 ft (west headland) and 49 ft ( east headland) acaording to City of Laguna Beach (1988).
Stakes were not driven off the ends of the headlands (USACE-LAD, 1996a).
Lll'\/!lli11e profiles were also acquired during the June 1994-0ctober 1995 period at
Crescent Bay. Accretion, as evidenced by a maximum 3•ft rise in bot:toirn elevation in
ft (mllw), and extending as a progressively declining rise out to a water depth of about
ft (mllw) was measured in the central part of the bay during the winter of 1994-95.
Sediment was lost from the profile above a depth of about 9 ft. Between June and July
1994, sediment was moved along the trend of the coast from the southeast to the
northwest in water depths out to about 25 ft (mllw) although the presence of a rock
outcrop on the east range complicates the picture. The bottom elevation was lowest on
both the east and west ranges after the winter of 1994-95.
15
3.4 Base of the Littoral Sediment Lens.
The backbeaoh line retreat rate and the net rate of sea level change control the flux of coarse material that passes across the base of the littoral sediment lens. In some places in southern Orange County, the base of the littoral sediment lens is rocky substrate, in others, it is unconsolidated sand. Where rocky substrate is cut by wave action, it is known as a shore platform. platform characteristics are listed in Table 6. Where the lens not rest on a shore platform, the envelop and base of the lens can be estimated using Equation 2.
Table 6. Characteri1tics of Shore Platforms in Orange County*
TYPE OF SHORE
PLATFORM
H = high-type pla!fonn,
M = low•!ype tl<J)osed platform,
C = low-.e latfonn
H
M
C
M
MIC
C
M
C
MIC
HIM
H/C
M
H
• observations from aerial photographs taken on 3 Dece:nber 1991
16
3.5 Sediment Sizes.
City of Laguna Beach (1988) and USACE-LAD (1993b) describe the sediment size
distribution in the littoral zone of southern Orange County. The results, based on about
100 samples collected from the beach and shoreface, are summarized in City of Laguna
Beach (1988) as composite size distributions by pocket. Most of the samples were in the
very fine to medium sand range. They contained little mud or silt. USACE-LAD collected
four samples each at 14 ranges. Elevations were +12 ft, mllw, -12 ft and-30 ft (mllw)
between Big Corona Beach and Dana Point. Beach and •hr"'"f"""' samples at and above -
12 ft (mllw) all contained less than 8% silt and mud. The (mllw) samples contained
up to 49% silt and mud (Aliso Creek location). These samples were collected seaward of
the shorebase (Table 4). In general the City of Laguna Beach composite samples are finer
than the composite of the +12 to -12-ft (mllw) samples collected by USACE-LAD.
17
4.0 IDSTORIC VOLUME CHANGES IN THE LITTORAL SEDIMENT LENS
A sediment budget model is balanced by adjusting one or more uncertain sediment fluxes until the net of all the fluxes on the right side of Equation 1 equals the measured change in
the volume of the littoral sediment lens on the left side of the equation. Historic sediment
volume changes have not been measured in the littoral sediment lenses of the study
Nevertheless, volume changes based on measured shoreline and backbeach line changes
can be calculated. These changes are, as they are in many other places, probably more
accurate than the existing sediment flux data.
4.1 Historic Shoreline Changes.
Beaches in southern Orange County have been remarkably stable the past 50 years or
so. Sparse evidence suggests these beaches were narrower in nineteenth century. In
the past decade there were four investigations that to various extents measured the movement of the shoreline, but not all beaches were investigated the results were not
all quantitative. One investigation, Everts Coastal (1996) only defined changes that
occurred on Little Corona Beach. The others focused on beaches further to the southeast.
In some instances, the studies provide information on the same beaches, so there is a
means to compare results.
County of Orange (1985) measured shoreline positions at selected county-controlled
beaches. Six sets of aerial photographs taken over a 14-year period were used. Recent
shoreline positions were compared, and those shorelines were compared to the 1885
shoreline on US Coast and Geodetic Survey T-sheets. Results of the 1967-1981
comparison are shown in Table 7. Results of the 1885 shoreline comparison with the
recent results is at Table 8. County of Orange found the shoreline fluctuated seasonally in
a longshore direction, typically with an advance along the southeast part of a pocket and a retreat along northwest part between November and April. FrotnMay through
October the shift was typically in the opposite direction. Shoreline :fluctuations in the
centers of the pockets were less than along the ends.
City of Laguna Beach (1988) conducted the most detailed analysis of aerial photographs
in the southern Orange County region. other investigations, however, only
selected reaches of coast were analyzed. This study focused on the position of the wetted bound. Nineteen sets of photographs for the period 1927-1984 were used to establish
historical shoreline behavior. Two sets were the 1920's, three sets from the 1930's,
six sets from the l960's, four sets from the 1970's, and four sets from the 1980's. The
mean shoreline change rate for the 57-year period was compared with 1934-1982 NOS
shoreline data. The later data set suggests a mean shoreline advance of O .5 ft/yr more than the aerial photographs for the entire study area. City of Laguna Beach concluded the
shoreline was stable in all but one of the shoreline compartments studied. Victoria Coast, the exception, exhibited a net shoreline retreat ofa~oo, 60 ft between 1927 and 1984.
18
Table 7. Historic shoreline changes.
Name of.Beach or
Coastal Reach
East Corona Coast
Crystal Cove
Can on
Treasure Island
Aliso Beach (includes
West Street Beach)
mlrthwest side of Three
ArehBav
Three Arch Bay
southeast side of Three
Arch Ba
Salt Creek ml!ICh
Dana Strand Beach
*see text (USACE-LAD, 1996a)
+o.6 ft/yr (southeast
end of Crystal
Cove **
.0.2** +LO***
-0.3** (+o.3)
o•• -0.4
-0.1 ••
ahout 0 with r!IOO!lt
-1.1 (-1.2)
**within range of uncertainty of method used to estimate shureline change rates
' .. 1934-1982 d!ange on NO8 maps in parenth<lses
***"llllt of photographs ru1 lyzed by USACE-LAD ( 1996); photographs were taken every six months bev •en 1991
through 1995 in mipport of the Coast of California Study-Orange County
19
Table 8. Comparison of historic Iong-tenn changes in shoreline position.
Name of Beach or
Coastal Reach
Main Beach
Bluebird Canvon
Victoria Coast
'Thousand Ste s Beach
Three Arch Bay
Salt Creek Beach
Dana Strand Beach
• estimate probably accurate to plus or i:nirrmi 40 ft
Historic Shoreline
Changes (City of
Laguna Beach, 1988)
shorelines from 1875, 1934, lmiO.
1972, and 191\2
shoreline advance
light shoreline advance, l~ss
recessional
stable (West Street Beach was
recessional
stable
USAGE-LAD (1996a; section entitled "Historical Air Photograph Analysis of Laguna
Beach Mini Littoral Cells, Laguna Beach, California") plotted shoreline positions
displayed on aerial photographs taken on 21 June 1938, 2 September 1960, January
1988, 3 December 1991 and November 1993. The authors of the report state the plots
provide " ... general qualitative guidance ... " and conclude there are significant seasonal
alongshore shifts in the shoreline. They also remark on the absence of a delta off the
mouth of Aliso Creek, and suggest this deficiency following high flow events is probably
due steep shoreface in that area. A quantitative listing of shoreline change rates was
not provided for the 55-year period covered by the photo set
USACE-LAD (1996a) also describes msl-shoreline changes at Crescent Bay during the
period January 1992-May 1995. Shoreline fluctuations were in a range of 45 ft at the west
side pocket, only 15 ft in the center of the pocket, and ft at the east end of the
pocket.
4.2 Historic Backbeach Line Changes,
Backbeaeh line change , ates are no less important than shoreline change rates in :letl . ..ing
historic variations in the volume of the littoral s:ediment lens and mean beach width. A
geomorphic method was employed by Everts Coastal (1995a) to estimate the historical
20
rates ofseacliffretreat summarized in Table 9. These rates are appropriate for sediment
budget purposes, but are not of sufficient accuracy to be to define building setbacks,
or to estimate the time-dependent potential for damage to structures in the path of a
retreating seacliff. Rates are averages for the coastal segments listed. At any location the
rate will likely differ from the average. Seacliff retreat is an episodic phenomenon. Years
or even decades of negligible retreat are typically followed by a short-lived, but severe
episode of scour.
Table Summary: Mean seacliff retreat rates in southern Orange County north of
Dana Point.*
Seacliff Retreat lute, feet/ ear
0.19
0.14
0.17
0,13
0.12
0.075
0.107
0.H?
0.12
0.13
• uncertainty in mean seacliff retreat rates is estimated to be plus or minus 50%
The current mean rate of seacliff retreat in southern Orange County is O. 07 to 0.20 ft/yr,
depending on location. Cliff retreat in recent years was least per unit sea level at
Crescent Bay. It was comparatively large in the Dana Point to Main Beach reach that
constitutes over half the high-relief portion of the Orange Coun1tv coast. Seacliffretreat
per unit sea level rise was midway between the two extremes in the reach between Crystal
Cove Corona de! Mar.
Headland seacliffs are composed of more resistant material than the seacliffs In the
pockets. Headland cliffs retreat at a lower rate than shown in Table 9 even though they are
not protected a fronting beach. Everts Coastal (1995a) conduded the outer shore
platforms headlands have been quite stable in the past 5000 years.
4.3 Relationship Between Changes in Mean Beach Width and Lens Volume.
The beach and shoreface profile fluctuates about a dynamic equilibrium shape. Although
the littoral sediment lens may shift position and its volume may vary, the dynamic
equilibrium shape of the upper surface ofthe lens is maintained through time as long as the
distribution of sediment sizes within it remains nearly constant, the wave climate does not
significantly change, and the local topography and bathymetry are not modified in such a
way as to affect the movement of sediment.
21
Under the assumption that an equilibrium upper lens profile will be maintained if the position of the beach shifts and/or littoral sediment is lost from or gained by the littoral sediment lens, the relationship b:etween a change in the volume of the lens, L\ Vi/ L\t , and a
change in the mean width of the beach, tw/ L\t, can be approximated by
(3)
in which x 1 = length of beach, = elevation of berm or backbeach line, z, = depth of
shorebase, and Pb, p, = portions, respectively by elevation or depth, of the beach and
shoreface that are not covered by the littoral sediment lens. The shorebase, which is fixed by the wave climate, may be on the top of an exposed shore platform or at the seaward
boundary littoral sediment lens. The rate of change in .the mean width of the beach is
--L\W L\y s /1y b -=-----L1t L\t L1t (4)
in which A.y, / 1,t = mean rate of shoreline change, and Liyb J !t = mean rate of change in
the position of the backbeach line.
Table 10 is a list of the data in Equations 3 and 4 to estimate the historical (1927-1984)
sediment volume changes in the littoral sediment lenses between Newport Bay and Dana Point. Shoreline change data are from Table 7; hackbeach line change data were
interpolated from Table 9, The Crystal Cove shoreline change rate, based on
measurements at the Laguna Beach end of the reach, was reduced to a mean of O .2 ft/yr for the entire reach, The littoral sediment lens is absent on parts of the beach and
shoreface that !Ire exposed rock. This reduces volume of the from that which would be obtained using Equation 3.
Results of the sediment volume c01np1L1tatim:m are at Table 11. Respectively, :from column 3 through column 5, the results ue mean annual change in the volume of the littoral sedimem lens, the mean volume change per foot of sandy beach, and the mean of change in beach width for the 57-year period. The total calculated mean change in the volume of the littoral sediment Jens from Newport Bay to Dana Point was +8240 cyy.
22
Table 10. Values used to estimate the historical change in the volume of the littoral
sediment lens.
Beach
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
17
18
19
20
21
22
23
1.5
0
0)
0.6
-0.2
-0.3
0
-0.1
!)
-0.3
0
0
0.2
NA
-1.1
0
NA
0
0
0
0
t;y,
At
z,
ft mlfw
0 10
-0.15 10
-0.15 10
-0.1 12
-0.15 (12)
-0.1 13
..0.1 12
-0.1 12
--0.1 12
-0.1 12
-0.15 12
-0.15 12
-0.15 (12)
NA NA
-0.15 13
-0.15 13)
-0.15 15
00.15 14
NA NA
-0.15 (13
-0.2 13)
-0.2 13
-0.2 12
Z, p, p,
fl (mllw)
21 0 0
25 0.2 0.4
(22) 0.5 1
22 0.05 0.5
(21 0.1 0.05
21 0 0
21) 0 0.1
21) 0.1 0
(21) 0.2 0.6
21) 0.2 1
21 0.05 0.5
0.05 0.6
21 0.3 0.8
NA NA NA
21 0 0.1
(21) 0.1 0.9
20 0.1 0.1
0.1 0.4
NA NA NA
(22) 0 0.2
24) 0 1
27 0 0.35
(26) 0 0.25
• parentheses indicate extrapolated or interpolated value; Xe Is length of actual sandy beach; three
columns on right from large scale aerial photographs dated 5 November 1992; Rockledge Coast
and northwest side of Three Arch Bay contain so little sand that a sediment budget is futile so
these reach~ are con11lderiild conservative in littoral sediment.
23
x,
fl
1800
400
1300
16600
1700
2300
1100
600
1500
700
6100
2100
1000
NA
2300
700
5900
3000
NA
1400
600
4300
4300
Table 11. Mean annual volume change in the littoral sediment lens between
Newport Bay and Dana Point (1927-1984).
Name of AV, AV, Aw ----
Beach Beach of At x,At At
Number Coastal Reach cv/yr cy/ft-yr ft/yr
1 Big Corona
Beach 3,100 1.72 1.5
2 Little Corona
Beach 51 0.13 0.15
3 East Corona
Coast 36 0.03 0.15
4 Crystal Cove 9,640 0.58 0.7
5 Irvine Cove -97 -0.06 -0.05
6 Emerald Bav -579 -0.25 -0.2
7 Crescent Bay 126 0.11 0.1
8 Shaws Cove 0 0.00 0
9 Divers Cove
Coast 100 0.07 0.1
10 Rockpile -50 -0.07 -0.2
11 Main Beach 742 0.12 0.15
12 Bluebird
Canvon 231 0.11 0.15
13 Woods Cove
Coast 163 0.16 0.35
14 Rockledge
Coast NA NA NA
15 Victoria Coast -2,582 -1.12 -0.95
16
Treasure Island 54 0.08 0.15
17 Aliso Beac.h 1,033 0.18 0.15
18 Thousand
Steps Coast 430 0.14 0.15
19 northwest side
of Three Arch
Bay NA NA NA
20 Three Arch
Bay 238 0.17 0.15
21 southeast side
of Three Arch
Bay 58 0.10 0.2
22 Salt Creek
Beach 973: 0.23 0.2
23 Dana Strand
Beach 1,003 0.23 0.2
24
4.4 Mean Beach Width.
The recent mean width of the beach in each of the mini-cells wu established using two sets of mapping-quality aerial photographs, one from the early summer (26 June 1992) and one from the winter (11 January 1994). Beach widths were measured at five locations at each pocket beach, one-sixth of the total length of the bead! in from its outer edges and at one-sixth distances for the remaining three &tations. Results are at Table 12.
25
N
"'
Number
1
2
3 ·4
5
6
7
8
' 10
11
12
13
14
15
15
17
18
19
20
21
22
23
Table 12. Mean beach widths in mini-cells between Newport Harbor and Dana Point.
NameofBeach 8126/92 6/26/92 6/26/92 6/26/92 6/26/92 AVERAGE 1111/94 11/1/94 11/1/94 1111/94 or Coastal Reach W1 W2 W3 W4 ws WIDTH W1 W2 W3 W4 It ft fl ft It 6/26/jl2 ft I! ft ft Bia Corona Beach 360 216 160 144 120 200 383 233 200 150 little Corona Beath BB 40 88 72 48 67 67 33 117 100 East corona Coast 24 40 80 24 64 48 33 17 33 67 crvstal cove 86 86 60 69 86. n 33 67 50 67 69 86 103 34 112 Bf 50 67 67 67 86 129 95 103 86 100 83 117 83 67 Irvine.Cove 69 69 95 86 103 84 50 83 100 117 Emerald Bav 112 129 120 86 95 108 too 117 117 83 Crescent Bav 60 60 86 69 69 69 50 67 100 83 Shaws Cove 43 60 52 34 86 65 67 67 83 67 Dive.rs Cove 34 52 43 69 78 65 83 0 67 so Ro·cknile 26 43 52 43 34 40 17 17 33 50 Main Beach, includes 155 !59 52 95 86 91 183 100 50 17 C.:ntal Coast from
Cheneys Potnt to
"'-
erueblrd· Canvon 60 17 17 0 60 31 49 33 16 0 Woods Cove 9 9 ij 43 26 19 0 0 33 0 Rockle.dae 16 8 8. 8 33 15 16 0 0 16 Victoria Coast 130 57 106 0 65 72 114 114 130 65 Treasure Island 49 49 57 49 57 52 65 65 49 49 Aliso Beach, including • 122 245 204 188 122 176 196 212 244 163 West Street Beach
. Thousand Sten.,i; Beach 24 122 147 122 163 118 16 114 81 147 northwest side of Three 24 Q 8 0 33 13 16 0 0 0 Archl!av
Three .Arch Bav 86 95 120 69 69 BB 81 114 130 81 southeast side of Three 43 34 17 26 17 28 16 16 16 16 Arch Bav
Salt Creek Beach 258 215 163 163 189 198 293 212 261 147 Dana Str.ind Beach 95 n 86 172 172 120 81 65 49 65
11/1/94 AVERAGE AVERAGE
ws WIDTH WIDTH
It 11/1/94 92&94
133 220 210
67 11 72
33 37 42
100 83 70
67 83 72
67 83 92 117 93 89
117 101 108
50 10 69
50 87 61
67 63 5,1
50 33 37
33 77 ll4
86 33 32
86 20 19
33 13 14
130 111 91
65 59 55
196 202 189
98 91 11U 33 10 11
98 101 94
16 1B 22
196 222 210
65 65 93
5.0 msTORIC SEDJMENT FLUXES
The sediment fluxes that affected the mini-cells between 1927 and 1984 in southern Orange County are discussed in this section in the order listed in Table 13.
Table 13, Sediment fluxes in southern Orange County (1927-1984)*,
Sediment Flux
5 .1.1 net flux at Ne ort Harbor
5.1.2 net flux at Dana Point
5.1.3 net transport prurt headlands
5 ,2.1 stream sediment contributions
5 .2.2 sediment contributions from
seacliffs
• 1927-1984 is the period eruoompassing most of the shoreline chllZJ:!lC data
5.1 Alongshore Sediment Fluxes.
Littoral sediment lenses are rather small in southern Orange County alongshore sediment transport may be II significant part of both the credit and debit side of their budgets. There have been no longshore sediment transport measurements at the shore-normal boundaries of these mini-cells. Some may be completely elosed from material entering or leaving in a shore-parallel direction; some may allow large quantities and out Only at the entrance to Newport Harbor and Dana Point have alongshore fluxes been defined.
5.1.1 Net Flux Across the Entrance to Newport Harbor.
Two investigations have been completed on Balboa Peninsula th11t indicate the quantity of sediment moving past the west jetty at the entrance to Newport Harbor is low or negligible. Everts Coastal (1995b) describes the results ofa Longshore current study, including a station at "The Wedge" adjacent to the jetty, and concluded the net longshore
sediment transport rate there was zero. Longshore currents, measured daily over a 14-month period were slightly to the west at The Wedge, or away from the entrance. Waves were higher (mean height= 2.5 ft) at that location of any of the other 9 stations monitor ,.,d. Waves on Balboa Peninsula were highest during the rnnL1er months of more frequent south swell. Longshore currents, when averaged by month, tended to be directed to the east in the winter and slightly to the west in the summer. Everts Coastal concluded
27
net longshore sediment transport was neither onto the Peninsula nor away from it during
the study period. Rip currents were unevenly distributed~ong the Peninsula, with their
most frequent occurrence at The Wedge (about 35% of the time).
Everts Coastal (1996) also reported on an eight-year long study offshore the west jetty.
The purpose of the investigation was to quantify the rate of movement, if any, of sediment
outbound along the jetty and perhaps alongshore at its seaward end. Given the frequency
of rip currents along the jetty the possibility exists for a significant offshore loss of Balboa
Peninsula sediment at that location. Results of the Everts Coastal study, however, show
only 400 cyy of sand-sized material were transported to the end of the jetty, thence slightly
to the west in the late l980's and early 1990's. None of the deflected material was found
to move east across the harbor entrance to Corona Beach. Since the end of the west
jetty terminates in a depth of about 50 ft and the entrance channel is deep has only
required maintenance dredging once since the jetties were extended in the 193 O's, it is
quite likely that the material was removed from the channel entered through the west
jetty, not around it.
USACE-LAD (1985-Inman) also concluded the net longshore sediment transport rate is
small along Balboa Peninsula and it is unlikely that sand is transported in quantity past the
harbor entrance. The lack of a substantial fillet against west jetty also suggests net
transport the east is low or non-existent. The small fillet that does exist is probably a
result of jetty sheltering rather than net longshore sediment transport the east.
5.1.2 Net flux at Dana Point.
Surveys made since Dana Point Harbor was completed in 1970 indicate huge quantities of
littoral material have not passed from the north to the south around Dana Point A sandy
beach has not developed against the breakwater connected to Dana Point, nor has a
subaerial fillet developed along it. A submerged fillet, however, formed inside the
breakwater. This deposit is composed of sand that passed through the breakwater.
Additionally, sediment has accumulated in the indentation between the rocky headland of
Dana Point and the south breakwater to about the location of the dogleg in the structure.
The entrance channel at the distal end of the breakwater has required no maintenance.
USACE-LAD (1990) noted that between October 1968, when the breakwater was
completed, and 20-years in October 1988, approximately 19,000 cy of sand-sized
material passed through and was deposited adjacent to the permeable breakwater in the
west harbor basin. USACE-LAD (1990) also stated a comparison of a 1965 pre-
construction survey and a 1988 condition survey indicated 130,000 cy (plus or minus
40,000 cy) of sediment had accumulated below sea level in the triangular ofrnhore region
between the base of the breakwater and Dana Point From these data USACE-LAD
(1990) concluded the average bypass rate from north to south was 7500 cyy (plus or
minus 2000 cyy) between 1963 a1,J 1988.
28
USACE-LAD (1996a) also estimated the Dana Point b~assrate and noted 27,500 cy was dredged above a depth of 10 ft (mllw) within the west basin in 1990. Doouments indicate the design depth was deeper, between 12 and 25 ft, so USACE-LAD (1996a) concluded volume that passed into the basin, but was not removed, was 65,000 cy (I 968-1989 period), for a total 90,000 cy that passed through the breakwater (a mean 4300 cyy which varied greatly from year to year). Based on an accumulation of sediment detected at the harbor entrance in a l990's bathymetric survey, they also concluded the 4300 cyy measured within the harbor represents only 25% of the quantity that passed Dana Point. Since the depositional volume at the entrance was not quantified, they based the total amount that passed Dana Point on this percentage, or 17,200 cyy. This was done by assuming an offshore diversion of sand at Dana Point. Although not stated, the justification for the offshore deflection hypothesis is probably Fisher et al' s (1983) conclusion that a large amount of material had been deposited to a depth of500-ft off the point in the past thousands of years. No estimated """'"m,zv limits were given for the USACE-LAD {1996a) bypass rate around Dana Point.
Tom Rossmiller of Orange County compared bathymetry from September 1965, obtained before the harbor was constructed, and a post-construction October 1988 survey. Both surveys covered the region inside and outside the present harbor structures. Profiles were scaled from the two mrveys 200-fl: normal to a common base line, and the area between the two bottom surfaces determined. Pinch-out to the plus or minus 1.5-ft range of uncertainty of the acoustic llUrveys was typically at a depth of about 30 ft (mllw) and the enclosed envelopes declined to this vmtical distance on all profiles. Most of the deposition was in a triangular area between the "dogleg" in the breakwater and Dana Point outside the breakwater. Significant deposition was not found within or in the vicinity of the entrance channel near the breakwater. With a correction for the quantity of sand that had been deposited within the breakwater, Rossmiller found about 15,000 cyy had been deposited in the region of the harbor.
The USACE-LAD (1990 and 1996a) and Rossmiller' s estimates are listed in Table 14. Both are for essentially the same period. It is not possible determine which is more accurate, the range of estimated bypass is 5 500 cyy to 17,200 cyy, or a difference of over 300%. It appears the majority of the discrepancy between USACE-LAD (1990) and USACE-LAD (1996a) is not in the amount of material that was deposited in the west basin of the harbor, in the amount that was assumed to have been deflected offshore and deposited near the entrance to Dana Point Harbor. The larger amount, though, is questionable. If about 360,000 had moved along the breakwater much ofit would have deposited around the distal end of the structure and accumulated in the harbor, which did not happen. The most detailed estimate is probably Rossmiller' s, which was based on a comparison of detailed, large-scale bathymetric survey sheets.
For purposes of this sediment budget, the nrt longshore sediment transport rate around Dana Point in the past 25 years is assumed to be 15,000 cyy plwl or minus 2500 cyy.
29
5.1.3 Net Longshore Sediment Transport Past Headlands.
Headlands are formidable barriers to the alongshore movement of sand The flux around
these features in unknown, but the submerged character of some of the headlands has been
determined. The City ofLaguna Beach (1988) sponsored an extensive underwater
investigation of each headland and reef separating the pocket beaches along that city's
coast. Later, the character of the headlands in Corona Del Mar were studied as well
(Everts Coastal, 1996). The geometry of the Laguna Beach and Corona de! Mar headlands
is described in Table 15. Dimensions of the remaining submerged headland extensions in
the study region have not been quantified.
Table 14. Net longshore sediment transport estimates at Dana Point.
Reference
USACE-LAD (1990-Everts)
USACE-LAD (1996a-
Coastal Frontiers Corp)
T. Rossmiller (1997, pa.
comm.
Reef extensio!lll of the headlands in Laguna Beach are typically without breaks to their
seaward tips. These extensions are from 10 to 20-ft higher than the adjacent sandy
shoreface or continental shelf indicating sand does not pass over them. The submerged
headlands are not covered with littoral-type sand even in depressions. Vegetation is thick,
and the adjoining bottom is horizontal at contact with the rock rather than sloping.
Ripples lllong and off the ends of the reefs were found City ofLaguna Beach (1988) to
be nearly aligned with the shoreline at times when underwater measurements were made.
This !IUggests that if sediment passes it does so during high energy events. Sediment at the
ends of the reefs is only slightly smaller than the composite size of the sediment within the
pockets (Table 15).
5.2 Sediment Flux Across Backbeach Line.
Sediment fluxes across the backbeach line are
contnbutors. Therll are no backbeach line
30
upland sources and seaclif.ls. Both are
fur littoral sediment in the study area.
Table 15, Characteristics of headlands and the submerged extensions of headlands
in southern Orange County (from City of Laguna Beach, 1988).
; nortllEtriefala!J Bk il'ofut 'i'''
Pocket on
southeast
side
Big Corona
Beach
Little Corona
Beach
small Corona
Del Mar
pockets
Irvine Cove
Emerald Bay
Crescent Bav
Shaws Cove
Coast
Victoria
Coast
Treat.\' ·e
lslandBeach
Distance
from
Headland
Seacliff to
Seaward
End of
Headland,
ft
700
230
150
800
600
1450
600
1400
1800
450
700
250
900
31
5.2.1 Stream Discharge.
Watersheds are the largest natural sources of coarse sediment reaching the southern Orange County coast. The amount varies greatly depending on the size of the contributing watershed. Delivery streams are typically short, but steep. Field measurements have not been made of the quantity of coarse, littoral-type sediment discharged in any of the southern Orange County streams. Where estimates have been made different methods were applied .. Previous discharge estimates are listed in Table 16. Most were based on sediment yield detenninations. Measured water discharges were used to estimate sediment discharge in Lagulll! Canyon and Aliso Creek. Sediment discharge estimates have not been made for Thousands Steps Beach, Three Arch Bay, individual pocket beaches between Newport Bay and Irvine Cove, and Emerald Bay.
Table 16. Previous estimates of mean annual coarse sediment discharge between Newport Harbor and Dana Point.
Beach
Number
2
3
4
5
6
7
8
9
11
12
13
14
15
16
]7
18
19
20
21
22
23
Laguna USACE-LAD (1996a)
Beach 1988
50
10
100
10
1900
150
10
10
JOO
2000 tolal for beaches 2-10* • • • •
• • •
1800
18,600 (USACE-LAD, 1996a)
reference; adjusted value is
lower as desotibed in the
calibration section oftlris r art
900 to 1200 (p47)
• seaward-draining San Joaquin Hills watersheds betwoen Newport Harbor and Lllj!Ullli Canyon are considered as one 15-sq mi unit with a comb.inedm-annual disc!rarge of 2000 cyy (p54)
32
City of Laguna Beach (1988) estimated the coarse sediment discharge indirectly by assuming it was, over a long time interval, equal to the coarse sediment yield in the watersheds within and upstream of the city's borders. Sediment yield is the rate at which sediment is mobilized from the watershed surface and delivered to charmels in the watershed. City of Laguna Beach (1988) assumed sediment storage or depletion in channels or on floodplains in the small, steep watersheds is negligible in periods of decades or more. Sand yield values used by the City ofLaguna Beach were obtained from nearby and similar small stream basins, mostly where entrapment rates had been measured in debris basins and reservoirs. The yield estimates are summarized in Table 17. Data are from Mustafa (1978), Boyle Engineering Corp. (1982), Moffatt & Nicho~ Engineers (1986), Stow and Chang (1987), Simons and Li (1985), Taylor (1983) Inman (1976).
Table 17. Sand yield estimates for southern Orange County*.
a:Vera 'eJ6t:aifWat&Shdf
'City of Laguna Beach (1988)
Sand Yield Estimate in c
200
100
100
60
2500
USACE-LAD (1996a; section ootitled "Pluvial Sediment Investigation, Orange County Coast") :fi:le:ll.!le1d attention on the three largest watersheds in the southern coastal reach, Laguna Canyon, Aliso Creek, Salt Creek. They used the City ofLaguna Beach (1988) mean estimate of 1800 cyy for Laguna Canyon, with an estimated 1000 cy discharge during a 10-year rainfall event, and 20,000 cy for a 100-year event. For Aliso Creek, the recurrence estimates were apparently made m;ing a sediment discharge rating curve prepared by Camp, Dresser & McKee (1982). The total-load estimate they used for existing conditions Aliso Creek (1982) was 87,000 tons. USACE-LAD (1996a) assumed the coarse fraction was 30% of the total sediment load and the relationship between volume and mass was 0. 7 cy of per ton of sediment. Accordingly, they estimated the recent mean annual discharge of coarse material at 18,600 cyy. Camp, Dresser & McKee (1982) measured the bed sediment and found the mean size to be between 0.3 and 0.6 mm. USACE-LAD (! 996a) estimated the coarse sediment discharge in Salt Creek at 900-1200 cyy for existing partly developed conditions.
For uniformity in the mini-cell sediment budget analyses, it is desirable to apply a single method to obtain relative, if not also absolute, stream discharge estimates. Without conducting a new study, probably most efficient method is to rely on the sediment yield of the individual watersheds as was done by City of Laguna Beach (1988). To do this a number of assumptions are involved. Material mobilized on the surface of the watersheds is assumed moved to the stream channel and thence transported to the coast in a short time. Aggradation md scour in the channel and on the floodplain are assumed to be in balance. Given the steep nature of the watersheds and the absence oflarge
33
floodplains this is probably a reasonable assumption. The portion of coarse sediment with
respect to the total sediment population is considered constant. Since sediment size
distributions for the watersheds are not available, this is the only realistic assumption that
can be made. In this analysis, the yield rates are assumed to be those defined by City of Laguna Beach (1988), and illustrated in Table 17. Last, it is assumed coarse sediment
discharges from the undeveloped foothill parts of the watersheds can be weighted in the analysis with respect to relief.
The estimated annual sediment yield from a watershed, q,, excluding Laguna Canyon,
Aliso Creek, and Salt Creek, is
(5)
in which c,, coarse sediment yield in an undeveloped terrace region (Table 17), cat =
coarse sediment yield from a developed terrace terrain, c df = coarse sediment yield from a
developed foothill region, cef coarse sediment yield from undeveloped foothills, A,. =
area of undeveloped terrace in the watershed, m = coefficient used to retain variance in sediment yield within plus or minus about I 00% of the mean in undeveloped foothill
regions, and y is
Yi,+ (6)
in which y, = measure of the varian<}e in the sediment yield in the undeveloped foothill
part of the ith watershed, and where
e, /!,
Yli =-,-r, (7)
in which y 11 = me111ure of the variance of the undeveloped foothill part of the ith
watershed related to its steepness, e, = maximum elevation the foothill part of the
watershed, I, = length of watershed, with r; mean value for all watersheds, so
(8)
in which n number of streams considered, and r 21 = measure of the variance of the
undeveloped foothill part of the watershed based on its narrowness or broadness, thus
(9)
34
and y; , the mean value for all watersheds, so
1 " I y; =-1:-1-.
n 1 Al/ff
Over time, undeveloped foothills are the most prolific producers of sediment. The topography is steepest, and the probability that the chaparral cover will be removed by fire thereby exposing surface to time-limited, but very high denudation rates, is greatest in the upper parts of these watersheds. Undeveloped foothill regions also constitute largest parts of the coastal watersheds of southern Orange County. These regions thus deserve special recognition in the development of the total sediment yield. Because resources did not allow a Modified Universal Soil Loss analysis, the approach was to recognize the special contributions of the undeveloped foothills by considering their longitudinal and lateral steepness. The assumption is that sediment yield is proportional to slope along the axis and in cross-section of the main sediment-carrying channels. The recognition of this control is included in the y coefficient. The power m (0.5 is used) was arbitrarily included to restrain the variance in sediment yields per unit area to about plus or minus 50% of the mean for all of the smllll southern Orange County watersheds.
Data used in the analysis are listed in Table 18; results are presented in the last column of the table. Laguna Canyon and Salt Creek estimates are from previous, more detailed analyses, but the coarse sediment yield estimates are similar to those made using the method discussed above. The large Aliso Creek: estimate by USACE-LAD (1996a) from Camp, Dresser & McKee (1982) is much Jargerthan one would get using the sediment yield method. This method is provisionally accepted, but later evaluated in the context of the balanced sediment budget for the entire region.
5.2.2 Sea cliff Sediment Contributions.
On a high-relief, recessional coast, seacliffs typically contribute significant quantities of littoral-type sediment to the fronting beaches. Where the backbeach line is armored and scour by sub aerial processes above the structure is negligible, or the fronting beach is at its critical protective width,. the contribution from beyond the backbeach line is zero. Neither of these conditions pertains. in the area except at Big Corona Beach and isolated locations where seawalls have been in place for long periods. The flux of littoral-type sediment from the remaining seaclifl1i, v v , can be approximated as
(11)
in which p v = portion of the seaclif':'that is oflittoral sediment size, ha = mean vertical
distance between the shore angle and the crest of the seacliff, and Ayb / At mean
horizontal rate ofbackbeach line retreat.
35
Seacliff fluxes, estimated by Everts Coastal (1996a) for 10 contiguous reaches that encompass the entire study reach, were adjusted using Equation 11 for each of the mini-cells. Results are summa.rized in Table 19. The portion of littoral-type sediment in the seac!iffs w!I!! assumed to be 0.4; no samples were collected to quantify this parameter. Seacliffretreat rates are from Table 9. Seacliffheights were taken from 7.5 min. topographic maps. The entire length of each mini-cell was used to calculate its seacliff sediment contribution.
5,3 Sediment Transport Across the Shorebase.
The flux across the shorebase are treated as an unknown in the historical sediment budget analysis. This flux is almost always difficult to quantify. Often it is not even possible to determine direction prior to completing a budget analysis. This is certainly the case in southern Orange County. Surveys have not been made with the precision and frequency, and over a long enough time interval, to quantify shorebase flux.
5.3.1 Shoalings and Deepenings.
An investigation completed for the Coast of California Study, though, provides qualitative data suggesting large movements of sediment onshore across the shorebase from the continental shelf, or onto the shelf from the littoral sediment lens did not occur in recent years. USACE-LAD (1996a-Coastal Frontiers Corp; section entitled" Historical Bathymetric Changes, Orange County Coast") compared bathymetry from 1934 with that 1975 out to a depth of about 180 ft (mllw). Only relatively small regions of seabed with changes in elevation over 2 ft were identified. While localized areas of significant seabed drop and rise were noted, USACE-LAD stated "Such discrepancies may be due to the existence of rock reefs ... which may yield dramatically different depth saundings dependent the exact survey vessel trackline." Given the general lack of a trend in the measured shoalings and deepenings it is reasonable to assume many of them may be· survey artifacts and do not represent actual bottom level changes. None occurred within the 30-ft isobath, nor seemed to be associated with a headland. The largest changes occurred beyond the 120-ft isobath.
The only exception is a deepening of over 10 ft off the east jetty at the entrance Newport Harbor. USACE-LAD (1993, p5-61) suggests between 1000 and 5000 cyy sand was carried into the Big Corona Beach pocket between about 1930 and 1990 from this source. According Everts Coastal (1996), material came ashore in the years following the completion of the Newport Harbor jetties II!! the offshore region adjusted to the new hydraulics of the stabilized inlet. At present, it is unlikely that any appreciable quantity is entering the littoral system from offshore sources at Big Corona Beach.
36
w _,
Beach
Numbe
1
:!
3
4
5
6
1
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
TOTAL
max
min
mean
Name of Beach
or Coastal Reach
Bio: Corona Beach
Little Corona Beach
East Corona Coast
Crvstal Coye
Irvine Cove:
Emerald Bav
Crescent Bav
Shaws Cove
Divers Cove
Rockoile
Main Beach, lf!ciudes
cental Coast from
Sheneys Pt to
Ha"'•=• Rock
Bluebird Canvon
Woods Cove
Rockledoe
Victoria Coast
Treasure Island
Aliso Beach, including
West S! 8eaeh
Thousand steps
Beach
northwest side of
Three All!h Bav
Three Arch Bav
s·outheast side of
Three Arch Bav
Saft Creek Beach
Dana Strand Beach
namma1
2.005
0.476
1.000
Table 111. Variables used iu coarse sediment yield estimates for coastal southern Orange County (ut = undeveloped terrace terrain, dt = developed terraces, uf= undeveloped foothills, and df= developed footllills).
Areas sn.miJ Maximim Wat•rshed Length of gamma Estimated Annual Course ut di uf di Elevation ff Watershed mi Sediment Oise.hat•• re•-·•
nen 0.09 1.75 1164 3.13 1.071 370 0.53 0.16 716 1.58 0.720 60 1.39 7.27 1164 3.45· 1.1147 1,630 0.13 0.14 700 0.92 2.941 60 .0.25 2.07 1013 3.38 0.916 410 0.09 400 0.46 1.382 10 0.03 0.48 912 1.92 1.827 130
nen
n= 0.94 1063 5.60 1,900
0.08 0.45 0.53 1000 1.77 1.947 180 n~
n= 0.20 0.48 824 1.08 . 1.819 140
n= 0.35 0.22 2400 17.00 18,600
0.11 0.16 820 0.65 3.098 60
neg
0.20 0.13 760 0.92 3.232 60
neg
700 5.00 1,200 ·-nea
24 810 cw
narnma2 Qamma
1.920 3.232
0.439 0.720
1.000 1.818
Table 19. Summary: Estimated seacliffsediment flnxes between Newport Harbor and Dana Point.
Name of Beach or Coastal
Reach
Big Corona Beach
northwest side of Three Aroh Ba
Three Arch Bav
southeast side of Three Arch Ba
Salt Creek Beach
Dana 81rand Beach
"' long .. tenn annual t:tintributions of coarse sediment Mt1Ul'ate to an estimated plus or minus 50%
5.3.2 Tracer Study.
USACE-LAD (1996a) reports the results of an 11 month tracer study at Crescent Bay. In June 1994, 625 pounds of dyed native beach was released onan 87-ft long, shore-no111llll line between elevations of+ 1.5 and +8. 7 ft (mllw) on the beach in the center of the pocket. Samples were collected in small, 6-in deep cores July 1994, October 1994, and May 1995.
One month after release, the dyed material had moved alongshore, but remained on the beach. By October about 25% of the samples collected offshore the beach contained dyed sand. The quantities were small, but the deepest tracer was found at about 40 ft (mllw). In May 1995, after an abnormally stormywinter, 19 of35 samples collected offshore eontained traces of dyed sand out to a depth of about 35 ft. Most cf the dyed material was recovered on the beach. Beach sample11 contained larger concentrations of dyed s.ediment than were found offshore. One sample that contained between I and 5 dyed grains was recovered in Emerald Bay, apparently after passing around a 49-:ir deep extension of the west headland at Crescent Bay.
38
5.3.3 Shoreface Slopes.
An indirect indication of the relative probability that littoral-type sediment will be moved
either onshore or offshore is the slope of the shoreface and the frequency aud intensity of
storm wave activity. All else being equ!!l, sediment is more likely to move seaward past
the shorebase on a steep than a mildly-sloping shoreface, and on one eKposed to the most
severe storm waves. In some places the shoreface may be at a "critical'' slope where any
material that is deposited near the shoreline is likely to move offshore, and any depo.sited
near the shorebase is likely to move onshore. This is the case at Little Corona Beach
where sediment that enters the system from Buck Gully is passed seaward through the
littoral sediment lens md out at the shorebase (Everts Coastal, 1995a); Steepenings occur
when the upper part of the shoreface is loaded with Buck Gully sediments; the shoreface is
adjusted back toward its dynamic equilibrium slope during storms when the sediment is
transported downslope and offshore.
The slope of the shoreface and the severity of storm waves are listed in Table 20 for
selected reaches. Slopes were taken from Corps of Engineers survey locations that were
profiled for the Coast of California Study (Transects 30-49). In about half the locations
where the slope was measured, the lens covered the entire rocky shore platform out to the
shorebase. In the remaining half, the rocky platform was partially exposed along the
outer portion of the shoreface. Wave data shown in Table 20 were assembled for the
Coast of California Study (USACE-LAD, I 995, draft). These data allow a detailed look at
storm waves of the past 60 years as projected into water depths of30 to 40 ft. The
spectral back-refraction model of O'Reilly (1991) was used to adjust deep water wave
heights and approach directions as affected by island sheltering, shoaling, refraction and
diffraction. Measured wave data and hindcasts were made using a nUl!)erical model
developed by Pacific Weather Analysis (1994). Note the large spatial variation in hindcast
storm wave heights.
5.4 Sediment Transport Across the Base of the Littoral Sediment Lens.
A coastal sediment budget is based on the movement oflittoral-type material across all of
the boundaries of the littoral sediment lens. Two circumstances set the stage for a
movement of sediment across the base of the lens. First, if the volume of the lens is less
than its critical volume and the Ions remains in place, or if the Jens retreats toward the
mainland, the underlying substrate may be susceptible to scour. A lens at critical volume
provides complete protection for the underlying substrate. If the lens volume is less th!lll
critical, however, the substrate beneath it will occasionally be exposed to the ercishre
action of waves and currents as sediment moves reversibly onshore and offshore. Released
sediment will pass into the lens across its base resulting in a positive sediment flux.
Seoond, when a stable or slowly-retreating lens moves upward in response to a rise in sea
level, or a lens advances seaward, support would be lost beneath the lens without sediment
released to uphold it. The result in this situation is a dow,. ward movement of material
from the lens across its base resulting in a negative sediment flux. New substrate,
composed of lens material, is created in this way.
39
Table 20. Potential for offshore sediment trallllport past the shorebase in southern Orange County.
Reach
CorOl!a de! Mar
to Abalone Pt.
Abalone Pt. to
Laguna Cyn.
Laguna Cyn. to
Gotflfi.
Gcffis. to
Mussel Cove
M'ussel Cove to
Dana Pt.
Type of Shore
Platform at
mostly exposed
part covered, part
exposed (slope is 0,05
at Divers Cove; Table
4
indistinct slope breaks
at Bluebird C:,n, Aliso
Beach, and Thousand
Ste s Beach (Table 4
mo.Uy exposed
*data are storm return wave heights in feet
25-yr 100-yr
return return
interval, interval,
ft• ft*
11.3/8.1 14.8/9,6
17.3/11.6 23.1/13.9
15.5/8.4 19,9110. I
14,6/7.5 19.2/9,Q
16.1/8,7 21.1/10.7
On a scale ofyean; or decades, flux across the base of the lens is usually small in to transport across other boundaries of the Exceptions include some conservative, headland-bounded pockets where: (1) the backbeach line is resistant to erosion and sea level is rising so thereby the flux is negative, and (2) the backbeach line is retreating rapidly into highly erodable material and sea level rise, if any, is small so the flux is positive.
Sea level rise affects magnitude and even the direction of this flux, but there will be a even in the absence of a rising or falling sea surface. Atake-up or release of !llldiment across the base of the littoral sediment lens is the only direct effect a relative rise in sea level has on the sediment budget. The base of the littoral sediment lens responds in one of three ways on the recessioual ooast of southern Orange Coumy.
As shown in the upperidealized diagram ofFigure 4, if the ratio of sea rise to backbeach line retreat is relatively small, the underlying substrate will be scoured. Littoral-type sediment produced by this erosion will be incorporated into the If the sea level to backbeach line retreat ratio is equal to the horizontal distance the backbeach line to the shorebase divided by the vertical distance from ms! to the shorebase as shown in the middle diagram, the littoral sediment lens will move landward Md upward with little soour or accretion at base. The net flux across this surface will be nearly zero. If, however, the ratio of sea level rise to backbeach line retreat is relatively large as illustrated in lower diagrru:n, the base of the lens will become a constructional feature. New substrate will be created to support the rising This material will be provided at the expense of the lens.
40
Case I.
Sediment flux into the
lens across its base
Case II.
Negligible sediment
flux across the base
of the lens
Case III.
Sediment flux out of
the lens across its base
lens base c11ts into substrate beneath
and landward of earlier lens base
-.z:P'.,,..-
-o.--
lens base moves landward alo11g a
projection of the earlier lens base
base rises above earlier /e11s base
position of lens base at later time
position of lens base at earlier time
sea surface reference datu~,m~~=.:_:.:=-:.:_:.::==:_:.:_:.:~~3Z _ shore angle
(hackbeach line)
Y,
Definition Sketch
Figure 4. Three possibilities of sediment flux across the base of the littoral sediment
lens demonstrating the relationship between sea level rise and littoral sediment lens
retreat under conditions of a balanced sediment budget; solid and dashed bold lines
represent, respectively, the initial position and a later position of the base of the
littoral sediment lens as it retreats a constant distance, but rises various distances in
response to a rise in the sea surface.
41
The long-term flux across the base of a littoral sediment lens where the shore platform is low and covered, V, , can be approximated as
(12)
in which z, = vertical distance between the elevation of the shore angle and the shorebase,
a length controlled by the wave climate, y1 = horizontal distance from the backbeach line
to the shorebase, a length also controlled by the wave climate, Liy b / !:i.t = mean rate of
change in the position of the backbeach line, Ila/ fit= mean rate of sea level rise, and
p; = 1 when !:ia/ l:iyb > z, /y,, i.e., when the platform flux is negative and littoral
sediment from the becomes a part of the substrate, while p; = p, when Aa/ Ay b < z ,/ y, , i.e., when the platform flux is positive and the shore platform is an
erosional feature. Sediment from the shore platform is released to the lens in this situation. Table is a list of the three categories of basal sediment flux under conditions of sediment balance.
Low platforms that are only partially covered by a littoral sediment lens may not retreat at the same rate as the backbeach line, or may not maintain a constant shape as they retreat. The flux between these platforms and the littoral sediment lens will likely be less than if they were fully covered by lens sediment. This type of platform is common in southern Orange County as shown in Table 6. Shore platforms at Crystal Cove, for example, are mostly of this low, partially covoced type. High platforms (Fig. 3) are near-static features that change form only very slowly with time as Sun.amura (1992) suggested. The release of sediment over the-interval of a sediment budget eotlducted for engineering and most planning purposes is almost always negligible on these features. Seacliffs alone provide the substrate contribution where there is a high-platform without a littoral sediment lens.
The flux associated with low, covered platforms was approximated using Equation 12. Low, but exposed shore platforms were assumed to release sediment at a rate between the flux calculated for a low, covered platform !llld zero a high-type platform. This ratio was chnsen compensate for the reduced release from what appears to be at least moderately resistant rock. The results are shown in Table 22 (last column).
In the twentieth century, the sediment flux across base of the lens in most of the southern Orange County .mini-cells has been negative. Movement has been out of the littoral sediment lens to create new underlying substrate (Case III in Table 21 and Fig. 4). This situation apparently also existed in the past few thousands of years because the sand lens above bedrock genorall:r thickens in a seaward direction along all of the southern Orange County coast (Fischer et al, 1983). USACE-LAD (1993c) report vibrocore findings on the continental shelf off Big Corona, Little Corona, and Main Beaches that
42
indicate a seaward-thickening sand deposit above bedrock in water depths of 50-60 ft. The
dominant sediment found was sand, with a coarsening trend that, in a downward direction,
contained progressively less silt and mud-sized material. USACE-LAD (1996a) also
found the sandy shoreface of Crescent Bay underlain by rock and cobbles, which in tum
were overlain by a thickening sand cover in a seaward direction.
Table 21. Conditions for sediment release, capture, or zero change as the shore
platform retreats landward, and rises with a rising sea level.
Flux to or from the littoral
sediment lens*
• 11 y b is negative since it is assumed the backbeach line is retreating
5.5 Sediment Gains and Losses Within the Littoral Sediment Lens.
Sediment may be produced within or on the surface of the littoral sediment lens. The most
common type of in-situ production occurs when shells are worn down or fractured and the
fragments become a part of the littoral sediment deposit. A small amount of material may
enter southern Orange County littoral sediment lenses in this way. Another type
aa:mlion occurs when calcium carbonate ls precipitated out of seawater. This has not
been reported in southern California.
Sediment losses due to mechanical wearing or chemical solution are also possible.
Wearing is caused by the movement of soft minerals and rocks on abrasive surface$. The
Amount of shell-source littoral sediment produced is herein assumed countered by the
amount lost due to abrasion. Chemu:al solution to any significant extent has not been
documented southern California.
43
Table 22. Estimated sediment flux across the base of the littoral sediment lens (from Equation 12).
Beach Name of Li y. V,, Number Beach of Xt Z., y, Lit Case
Coastal Reach ft ft ft ft/yr cyv 1 Big Corona
Beach 1800 21 860 0.00 1 0 2 Little Corona
Beach 400 25 720 -0.15 2 -30 3 East Corona
Coast 1300 22 (720) -0.15 2 -120 4 Crystal Cove 16600 22 620 -0.10 ~3 ~D 5 Irvine Cove 1700 21 (650) -0.15 2 -130 6 Emerald Bay 2300 21 650 -0.10 2 -260 7 Crescent Bav 1100 21 (650) •0 .. 10 2 -125 8 Shaws Cove 600 21 (640) -0.10 2 -60 9 Divers Cove
Coast 1500 2i (620) -0.10 ~3 ~0 10 Rockpile 700 21 (600) -0.10 2 -40 11 Main Beach 6100 21 580 -0.15 2 -350 12 Bluebird
Canyon 2100 21 (570) -0.15 2 -110 13 Woods Cove
Coast 1000 21 (560) -0.15 2 -50 14 Rockledge
Coast NA NA NA NA NA NA 15 Victoria Coast 2300 21 (540) -0.15 2 -100 16
Treasure Island 700 21 (530) -0.15 2 -30 17 Aliso Beach 5900 20 520 -0.15 2 -250 18 Thousand
Steps Coast 3000 22 (570) ·0.15 2 -140 19 northwest side
of Three Arch
Bay NA NA NA NA NA NA 20 Three Arch
Bav 1400 22 (580\ ·0.15 2 -70 21 southeast side
of Three Aroh
Bay 600 24 (600) .Q.20 2 0 22 Salt Creek
Beach 4300 27 810 -0.20 2 -170 23 Dami Strand
Beach 4300 26 (630) -0.20 2 25
44
5.6 Artificial Beach Enhancement and Sand Mining.
There have been no documented beach enhancements except at Big Corona Beach where some of the sediment dredged from the Newport Harbor entrance channel was placed in the early part of this century, at Little Corona Beach in 1988, and at Salt Creek Beach when the Ritz Carlton Hotel was constructed. B_ig Corona Beach was enhanced a second time in the late 1980's with material dredged from the navigation channel. Between 1927 and 1984, an estimated annual average 1000 cyy was artificially placed on Big Corona Beach. The 1988 Little Corona Beach enhancement was about 2000 cy with the source of material being a small accretional beach in Newport Harbor. Over the 57-year interval of the sediment budget this quantity is negligible. The amount of sediment placed on Salt Creek Beach is unknown. No mining of beach sediment has been documented.
45
6.0 CALIBRATION OF IDSTORIC SEDIMENT BUDGETS
The goal of the calibration phase of this investigation is to use Equation 1 to balance the
historic budgets of the mini-cells using measured changes in the volume of the littoral
sediment lenses and the most realistic fluxes. In southern Or1111ge County, the net annual longshore sediment transport rates between the mini-cells, and the sediment fluxes across the shorebase boundaries of the cells, have not been established as yet. Only one of these
unknowns at a time can be quantified.
The calibration of a sediment budget is an imprecise and subjective enterprise. In each step judgment must be exercised to adjust questionable fluxes. This is done to obtain the
summary group of fluxes that produce the most reasonable balanced budget. All fluxes are hind cast mean values. None have been measured or established with certainty. Those
considered to be more clearly defined than the others are taken to be accurate. As the calibration process proceeds, the less refined fluxes are open to adjustment. Before the unknown shorebase flux is addressed, which sets the stage quantifying the net longshore sediment transport rates between mini-cells, it seems prudent to revisit the
coarse sediment discharge estimate in Aliso Creek
6.1 Aliso Creek Coarse Sediment Flux.
The largest single contrihution to the sediment budget of southern Orange County is the provisional 18,600 cyy discharge estimate from Aliso shown Table 18. The coarse yield, about 520 cyy/sq. mi. for the 36-sq.mi. watershed as defined by USACE-LAD
(1996a) using Camp, Dresser & McKee (1982) finid discharge estimates, is two to almost
five times the quantity one would get using sediment yield methodology that produced
the estimates for the other watersheds (Table 18).
USA.CE-LAD (1997) notes sediment yield estimate of 62,000 tons yield of all
sizes and equal to about 45,000 cyy) in the Camp, Dresser & McKee report are cursory
and may be in error by as much as 200 percent (presumably 22,000 to 90,000 of all
sizes). USA CE-LAD (1997) also states long-term trends in sediment discharge ln Aliso Creek cannot be quantified due to a lack of ediment transport and irediment yield
information. They :!late, however, that sediment yields in the watershed will decline irl future as a result of development and increased channel armoring. Assuming the portion of coarse material in the diacharged sediment varies between 0.1 and 0.3, coarse sediment
discharge range is between 2200 and 27,000 cyy. This res1Jlts in an average coarse yield in
the watershed of between 60 and 730 cyy/sq.mi.
Comparing the Aliso Creek yield r!lllge with the infilling rate data from local debris basins and reservoirs (Table I 7), and using the method presented in Section 5.2.1, suggests the average yield should be between 130 and 260 cyy/sq.mi. Channel downcutting, high yields
in the Santa Ana Mountains lll!ld its foothills (the othe1 watersheds are wholly in the less
rugged San Joaquin Hills), and abnormal yields due to development, probably increased
the average Aliso Creek watershed yield over the high end of this range the 1927-1984
46
sediment budget period. Assuming an increase of25 percent over the 260 cyy/sq.mi. high end of the range results in an average estimated yield from the Aliso Creek watershed of 325 cyy/sq.mi., or a mean annual coarse sediment discharge of 11,700 cyy. Accordingly, the coarse sediment flux in Aliso Creek was adjusted to 12,000 cyy, or about two-thirds of the USACE-LAD (1996a) estimate.
6.2 Total Shorebase Sediment Flux.
Historic fluxes across each of the boundaries of the littoral sediment lens and the historic change in the volume of the littoral sediment lens between Dana Point and Newport Harbor are listed in Table 23. The sediment budget, balanced with the adjusted Aliso Creek discharge estimate, indicates the previously unknown sediment flux across the shorebase (second to the last row), is -1630 cyy.
Table 23. Summary of fluxes across boundaries of the littoral sediment lens ( control volume) between Newport Harbor and Dana Point for the period 1927-1984.
Sediment Flux
measured average annual gain
based on sw:veys and aerial
hoto a h. analvses
difference between the amount
entering and leaving parallel to
shore
stream ll;!IQ seacliff contributions
diment to snpport tbe littoral
• . . . level mse fill
beach enhancement at Big
Corona Beach
sediment created Qr destroyed
within the control volmne
--::::::=..r.;,;::;.
-s,;r:;!;%'.'os
*By applying the sediment yield approach Ulled in calcnlating the coarse sediment dil!charge in tbe otber watersheds, the a!ijllsted Aliso Creek contriblltio~ is 12,000 cyy ratber than the lll,600 cyy estimate of USACE-LAD (l'i96a)
47
6,3 Alongshore Distribution of the Shorebase Sediment Flu.x.
In order to estimate the alongshore flux between mini-cells, it is first necessary to establish an alongshore distnbution of the 4850 cyy that is "naturally" lost across the shorebase (Table 23, last row). The method chosen was to evaluate the impact of two different shorebase flux distributions on the longshore sediment transport flux, a uniform alongshore distribution bruied on beach length, and a variable distribution based on the slope of the shorebase and the alongshore distribution of storm wave energy. The first assumption is that the movement of sediment from one mini-cell to the next is controlled in part by the magnitude of the headland barriers separating them, the sheltering effect of the headlands, and the alongshore distribution of storm wave energy as described by USACE-LAD (1996c). The cumulative impact of these controls was correl!rted with cak;ulated longshore sediment transport fluxes found using the uniform the non. uniform distributions of shorebase fluxes. It was assumed that the "best" distribution would be associated with the best correlation coefficient. In practice, little difference was found between the correlation coefficients.
Two sets of qualitative measures were developed. One addresses the potential for longshore transport around the headlands. This measure is used to establish the correlation coefficients with respect to the longshore sediment transport rate between cells. The second measure addresses the potential for seaward-directed transport across the shorebase. This becomes the basis for assigning a non-uniform shorebase flux distribution. The alongshore transport measure considered, qualitatively, the sediment-blocking
potential ofa headland, ,;1, the sheltering effect ofthe surrounding headlands on the
enclosed beach, ,; 2 , and wave energy av:ailable to deflect sediment seaward along a
headland during storms, ,; 3 • Sediment transport across &horebase considered the slope of the shareface, and the relative amount of en erg}' expended during severe wave
storms., ,; 3 •
Headlands function as sediment-blocking structures such tllat the difference between the depth at the submerged end of the headland, z h, and the depth of the shorebase is a measure of the limiting seaward extent of the sediment transport conduit around the headland, so
As the value
(mini-cell) will
(13)
,; 1 increases past LO, the probability that sediment from the upcoast bay
the headland to the downcoast headland declines.
Headlands also intercept incoming waves sudi that some of their energy is lost before it reaches the beach and shoreface 0<f the enclosed bay. This loss of energy may act to reduce the quantity of sediment moved seaward during a storm. A simple measure of the sheltering of the headlands is
48
(14)
in which y h = shore-normal distance between the most indented part of the bay and a line
between a line connecting the shorelines of the bounding heedlands, and xb = straightline
distance between the headlands. As the magnitude of this measure increases, the assumption is that the potential for transport around the bordering headland is reduced.
Most sediment that moves around the headlands is probably transported during extra-tropical storms in the northern hemisphere. According to USACE-LAD (1996c), storm wave energy is not evenly distributed along the coast of southern Orange County. A measure of the relative difference in wave energy reaching the coast during extra-tropical storms, q 3 , is
E, = E,m (15)
in which = wave height squared for the largest storm waves in a given reach of coast,
and E,,. = wave squared of the largest storm waves in reach of the southern Orange
County coast. vlllue of 1.0 indicates storm wave energy is highest in that reach. ,{;3 becomes less the amount of wave energy reching the coast is assumed to decline.
Cross-shorebase transport is assumed to vary with and the Ill.ope of the shoreface. As
the shoreface slope increases the probability of downslope transport also increases.
The aforemei:ttioned measures of alongshore transport past the headlands, and net cross-shorebase transport, are listed in Table 24. Shorebase depths are :from Table 4. Depths at the end of the submerged headlands, where they are available, are from City of Laguna Beach (1988). Wave data and shoreface slopes are :from Table 20. The absolute values shown in the table were subjectively separated into the high, medium, and low groupings shown in Table 25 according to whether they are likely to assist or hinder shore-parallel and shore-normal movements of sediment.
Shorebase fluxes were adjusted in an alongshore dirention to reflect the cumulative ranking:; in Figure 5 (lower curve is slope; upper cumulative curve includes the alongshore distribution of storm wave energy. These weightings account for the apparent greater probability of offshore transport between Irvine Cove and Main Beach. The longshore sediment flux between mini-cells was ealculated using both distributions of the shorebase flux. These longshore fluxes were then correlated with the cumulative measure shown as the upper curve in Figure 6 (lower curve is ,; 1, middle cumulative curve includes ,; 2 , and
the upper cumulative curve includes ,;,). A test of the adjustment, however, indicated it
did not significantly affect the relatiomihip be.tween the alongshore movement of sediment around the headlands and the asllll.med cumulative potential for that traniport. The
49
Table 24. Parameters used to evaluate the probability of alongshore sediment transport past headlands and cross-shorebase sediment transport.
Beach Sediment Headland Wave Beach Name of Beach of Orhintation1 Blocking Sheltering Transport Number Coastal Reach degrees from Parameter *1 Parameter, Parameter,
true north ,;, c, l, 1 Bin Corona Beach 90 1.3 0.40 0.4 2 Little Corona Beach 110 1.0 0.30 0.4 3 East Corona Coast 138 1.5 0.10 0.4 4 Crystal Cove 128 2.0 0.10 0.4 5 Irvine Cove 113 1.8 0.60 1.0 6 Emerald Bay 126 2.1 0.40 1.0 7 Crescent Bav 113 2.3 0.40 1.0 8 Shaws Cove 90 1.3 0.70 1.0 9 Divers Cove Coast 118 2.6 0.60 1.0 10 ROCkoile 98 1.9 0.50 1.0 11 Main Beach 148 0.0 0.25 1.0 12 Bluebird carivon 155 1.2 0.50 0.8 13 Woods Cove Coast 132 1.6 0.50 0.8 14 Rockledae Coast 150 1.1 0.40 0.8 15 Victorta coast 147 1.9 0.20 0.8 16 Treasure Island 97 0.0 0.40 0.7 17 AIISO Beach 143 <0.7 0.10 0.7 18
Thousand Steps Coast 137 0.0 0.05 0.7 19 northwest side of
Three Arch Bay 172 <1.7 0.00 0.7 20 Three Arch Bay 145 <1.7 0.40 0.8 21 southeast side of
Three Arch Bav 100 0.0 0.15 0.8 22 Salt Creek Beach 160 0.0 0.25 0.8 23 Dana Strand Beach 155 1.9 0.10 0.8
• from Tables 3 & 13
-values in parentheses obtained interpolation where measurements are unaval!llble
Shoref,11:e
Slape *1\
y.
0.049
0.049
0,049
0.049
0.06
0.06
0.06
0.06
0.06
0.06
(0.052)
(0.052)
0.052)
Q.052)
0.052)
0.045
0.045
0.045
0.045
0.045
0.033
0.033
0.033
correlation coefficient remained about 0.4 as shown in
(for the non-uniform shorebas.e flux).
adjusted relationship in Figure 7
6.3 Individual Longshore Sediment Fluxes and Mini-Cell Sediment Budgets.
The goal of the mini-cell budget calibrations is to develop the most reali!,tic grouping of sediment fluxes that produce a balanced budget. Table 26 is a sutlllnll!y of these component fluxes for the cells. Hi11torical sediment volume changes and fluxes listed in Table 26 are probably accurate to no.better than plus or minus 50%.
50
Table 25. Groupings of alongshore and cross-shorebase sediment transport
parameters.
Sediment Headland Wave Shoreface
Beach Name of Beach of Blocking Sheltering Transport Slope,
Number Coastal Reach Parameter•, Parameter •, Parameter•, ,;, c, c, y.
1 Bia Corona Beach 2 2 1 2
2 Little Corona Beach 2 2 1 2
3 East Corona Coast 2 3 1 2
4 Crystal Cove 1 3 1 2
5 Irvine Cove 2 1 3 3
6 Emerald Bay 1 2 3 3
7 Crescent Bay 1 1 3 3
8 Shaws Cove 2 1 3 3
9 Divers Cove Coast 1 1 3 3
10 Rockpile 2 2 3 3
11 Main Beach 3 2 3 2
12 Bluebird Canvon 2 2 2 2
13 Woods Cove Coast 2 2 2 2
14 Rockledqe Coast 2 2 2 2
15 Victoria Coast 2 3 2 2
16 Treasure Island 3 2 2 1
17 Aliso Beach 3 3 2 1
18
Thousand Steos Coast 3 3 2 1
19 northwest side of
Three Arch Bav 2 3 2 1
20 Three Arch Bay 2 2 2 1
21 southeast side of
Three Arch Bav 3 3 2 1
22 Sall Creek Beach 3 2 2 1
23 Dana Strand Beach 2 3 2 1
• H=3, M=2, L=1: respectively, ll!gh, medium, and low relative probability of alongshore
l!Wlsport aroound headlands.
An estimated 7900 cyy was contributed as seacliffs eroded and 18,000 cyy entered from
the watersheds across the backbeach boundary. An estimated net 1800 cyy was lost across
the shorebase, with 3000 cyy moving landward at Big Corona Beach and 4850 cyy
probably lost unevenly across the shorebase between Newport Harbor and Dana Point. An
estimated additional 2000 cyy was lost across the base of the littoral sediment lenses as sea
lev~l rose. About 1000 cyy was artificially added at Big Corona Beach as beachfill. An
estimated 15,000 cyy was lost around Dana Point. The net longshore sediment transport
rate was everywhere to the southeast. It remained near zero from Newport Harbor to
Victoria Beach and it increased to about 14,000 cyy at Aliso Beach (Fig. 8).
51
7
e
I
I
/ -I/
/
10,000 20,000 30,000
\
\
~
\
40,000
Otstam:a, ft
'
1
\
~
'
50,000 60,000 70,000 80,000
Figure 5. Measure of potential for seaward-directed transport across the shorebase.
'
' /'\ I \ '\
I I
\
", V ' '\ \ \ ' j
~ I \ ~ _) I
' !'-------·~-\ /\ I \ \
" ,I/\ ""
0
0 10,00G 20,000 30,000 50,000 SQ,000 70,000 80JJOO
Olmim±aSouthe'a.lrl ofNewp:ort Harbor, ft
Figure 6. Measure of potential for transport past headlands from one mini-cell to
the next mini-cell.
52
,.o
s.o
.-1000
~
0
v ,.
~ ---
~ V _,.
_,.,,,._.,-y"' 0.0001553x + 5,8558934
,..,,.,,..,. R2 " 0.3852664 .
1000 2000 4000 5000 6000 7000 JOOO OOtlO 10000 11000 Oa.lculatad Net t.ongshOm Sediment Transport Rate, eyy q BOU'lheast
Figure 7. Estimated net longshore sediment transport rate and potential for longshore sediment transport past headlands.
16,000
-!!-~ 14,000 0 -
'5 e-12,000 C e ... c • 10,000 . §
'l!-"' ; {1,000 •• -"' . -I~ j ' 6,000 s: " z
'O 4,0llll ; • .!< 2,000 ~
1 = 0 'o
r
~ "-... ,,... ...... J
<:
·2,000
" 20,000 30,000 40.000 60,000 60,00fl 70,000 eo,ooo
Ois,ance ..1outheut m'Newport Barbor, ft;
Figure 8. Estimated net longshore sediment transport flux between mini-cells between Newport Harbor and Dana Point.
53
V, .i,.
Beach
Number ~1
2
3
4
5
il
7
8
ll
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Table 26. Sediment budget summary for mini-cells between Newport Harbor and Dana Point.
Flux Across Back Beach Lme, C" Alongshore Hux, cyy
Name· of Beach or Length of Volume Change Seacliff Watershed Modified Flux Across Flux Across Base Artificial NW SE Coastal Reach Beach, ft in Lens, r:yy Contributions Contributions Shorebase, cyy of Lens, cyy Flux Boundary Boundary
Big Corona Beach 1800 3.J.00 0 --0 2890 0 1000 0 790 Little Corona !each 400 51 <200 370 -20 -&l 0 790 1259 East Corona Coast 1300 36 300 60 -80 -120 0 1,259 1,383 Crvstal Cove 16600 3,210 1400 1600 -1060 -o 0 1,383 113 Irvine Cove 1700 -97 <100 60 -210 -130 0 113 30 .Emerald Bav 2300 -579 <200 410 -300 -260 0 30 659 Crescent Bav 1100 126 40 10 -140 -125 0 659 318 Shaws.Cove 600 0 60 130 -70 -60 0 ~18 378 Divers Cove Coast 1500 100 <100 -o -190 -0 0 378 188 RockPile 700 -50 <70 -o -80 -40 0 188 188 Main Beach 6100 742 200 1,900 -640 -350 0 188 656 Bluebird Cal')YOn 2100 231 300 180 -180 -110 0 556 615 Woods Cove Coast 1000 163 >100 -o -80 -50 0 515 311 Rockfe'dqe-Coast -o 0 >1'00 -o 0 0 0 311 411 Victoria Coast 2300 -2582 <300 140 -190 -100 0 411 3,143 Treasure· Island 700 54 50 -0 -40 -&l 0 3,143 3,069 AUso Beach 5900 1,033 <1 ODO 12.noo -370 -:!SO 0 3,069 14417 3000 430 400 60 -190 -140 0 14,417 14,117 Thousand Steps Coast
northwest sicl,e of Three -o 0 <300 "0 0 0 0 14,117 14,417 Arch Bay
ihree Arch Bav 1400 238 <200 60 -90 -w 0 14,417 14,279 southeast side of'Three 600 58 <400 -o -JO 0 0 14,279 14,591 Arch Sav
$alt Creek-.Beach 4300 973 <1 000 1,200 -280 -170 0 14,591 15,3GB Dana Strand Beach 4300 1,003 <1100 -o -280 25 0 15,368 15,209 TOTAL 59,700 8,241 7,920 18,180 -1,640 -:!,010 1,000
Only the net !ongshore sediment transport rate showed any semblance of a spatial gradient (Fig. 9). The other fluxes exhibit no alongshore gradient as illustrated in Figures 10-13. Similarly, the predominant gain and the rare loss of sediment in the mini-cells between 1927 and 1984 does not vary in an alongshore direction as shown in Figure 14.
'16,000
e: .
i ::::::
~ 0 ! 10,000
~ _ 8,000
:0: ,,,. 6,000
M E 4,ooo
li ! 2,000
"' f 0 0 ~ ;;i, = j "2,DO:O
~
-4,000
• • •
/~
0 Hl',000
... . I .• • • • .,,.,,.,
/ ,
y = 0.2594619x -5075.2037981 / R2 = 0.5982311
/
/ /
/ •• /
1/. • ••• . .... . .... /
20,0QtJ 30,0QO 40,000 80,000 71).®0
Distance Southea5t ofNtwport Harbor~ ft
Figure 9. Alongshore gradient in the net longshore sediment transport flux.
55
80,000
11400
1,200
•
~ 1,000
" ~ ' D
'~ 800
0 ~
'
I
~ ! • ~ 800 • 0
' 400 0
'°'
0
ly = 0,0061342x + 105.4230467
L---'" R2 = 0.0944555 . -L--+---~ • • • • ----i . ♦ ••
~· ♦ ♦
• 10,000 30,000 40,000 50,000 eo,ouo 70,000 80,1'00
Dlstam:e Soutllaai.t of NAAwplffl Humor, ft
Figure 10. Seacliff sediment contributions between Newport Harbor and Dana Point
(note the Jack of an alongshore gradient).
7,000 •
!
I I I
I ly = 0.0071120x + 287.34049101
R2 = 0,0090268
• •
• 1,000
I. • i I . I . I . • ♦ 0
0 ro,ooo 20,000 30,000 40,000 S0,000 00,000 70,000 80,000
O!stanoo Southeast n:f N·ewport Harbor, H
Figure 11. Watershed !lediment contributions between Newport Harbor and Dana
Poiut (note the lack of an alongshore gradient).
56
• • •
·200
!::: " i -400
• 0 ~ m • • ,600 2 ;i
~ "' ~ ~ .. ,. ;a
0 ~
·1,000
10,000
• • • • ' • •
• !
: •
· l'Y = 0.0023001X. 299.361~06811
R2 = 0.0299870
•
•
50,000
mmna Southeast of Newport HBor, ft
•
•
• •
60,000 70.000 so.oon
Figure 12. Sediment flux ae:ross the i,horebase between Newport Harbor and Dana
Point (note the lack of an alongshore gradient).
so
I • I •
• I • • I
• ' I ! •
' . • • i • i •
• y = -o.00004a1x -85,51772541
R2 = 0.0000925 I
•
~300
.350 • 10,000 3.0,000 50,000 70,000
01$1.l:llM SUuU\eaJtotNewpott Hiirbor, ft
Figure 13. Sediment flux aeross the base of the littoral sediment lens between
Newport Harbor and Dana Point(note the lack of an alongshore gradient),
57
• •
1; ,,, 2,000
! y = -0.0140359x + 905.0129117 ..
& 1,000
R2 = 0.0591240
5 • • . . • • • •
♦ . •
•
-2,000
•
-3,000
0 10,000 20,000 30,000 40,UOO 60,000 70,000 60,fJOO
Dlstanctt Southeast of Nt\0/POrt Hatoor, ft
Figure 14. Sediment volume change within the littoral sediment lenses of mini-cells
between Newport Harbor and Dana Point.
58
7.0 CONCLUSIONS: APPUCATION OF THE CALIBRATED SED1MENT BUDGET MODEL
Analyses were made to forecast future beach behavior in response to anticipated changes in one or more of the sediment fluxes that enter or leave the littoral sediment lenses, or to a change in the long-term rate of sea level rise. Qualitative forecasts were made using the calibrated sediment buc;!get modeL
It was not the intention to estimate future changes in the forcing functions that affect the sediment budget. To be meaningful, the effort would require sophisticated forecasting. For example, a forecast of future land use practice in each of the coastal watersheds would be needed along with a sediment yield analysis to predict the mean width of any of the beaches in future. Instead, this section focus' on the causes of the past change in mean beach width, how those causes are likely to change in future, and on the two most important concerns, the impact of seawalls on the mean width of the beaches and the most likely beachfill loss rate if any of the beaches are artificially enhanced. All of the questions posed in the introductory section were either answered in the previous section or are addressed here.
The use of these sediment budget results should reflect the fact that any methodology to predict the future is provisional .. While a predictive sediment budget analysis is based on a reasoned surmise of how future fluxes will differ from measured or estimated historic fluxes, unforeseen flux variations will likely occur. In addition, historical flux and beach behavior estimates are often inexact because sediment budget analyses are typically made with existing data and/or data that can be relatively easily and quickly gotten. It is difficult to obtain a long time-series specifically for a project sediment budget. In all sediment budgets the anticipated range of variation in the predictive results should be stated. possible, all assumptions should be tested.
7.1 Future Mean Beach Widths.
Mean beach widths did not change greatly over the past 70 years (Table 10). Backbeach line retreat rates averaged for this century were 0.07 to 0.2 ft/yr (Table 9). Crystal Cove gained sand at a small annual rate while the Victoria Coast (Lagunita, Victoria Beach, and Blue Lagoon Beach) lost a small amount on an annual basis. Lo!;ses gains in the other mini-cells were within the limits ofunc.ertainty of the shoreline change data used to obtain the volume changes. One NOAA shoreline position and a few old ground photographs suggest some of the beaches may have been nmower the I 9th century than they are today.
A variation in the volume ofa littoral sediment lens is translated to a change in the mean width ofits beach. A sediment budget analysis is used to estimate this change as a function o:thv cumulative quantities of sediment entering and leaving 4 sp1::cified segment of coast in a specified time interval. The primary contributing sources in southern Orange County between 1927 and 1984 were: (I) the upland watersheds, (2) seacliffs, (3) transport
59
alongshore and around headlands from the northwest to the southeast into the mini-ee!ls, (4) transport across the base of and into the littoral sediment lens at Dana Strand Beach, and (5) l1111dward tr11J1sport across the shorebase from the natural delta ofNewport Bay to Big Corona Beach. Losses were due to: (1) longshore transport from the northwest to the southeast out of the mini-cells, (2) sea level rise, and (3) offshore transport across the shorebase from almost all mini-cells to depositional sites in deep water. Big Corona Beach was received a significant volume of artificial beachfiil; Little Corona Beach and Salt Creek Beach were artificially filled, but the amounts were small.
Future beach behavior will differ :from past behavior if the flux contributions change as seacliffs erode, watersheds downwear, backbeach lines become armored, and sea level rises. The 1927-1984 contribution of sediment from the pre-development delta at the outlet ofNewport has ended. An approxil!llltion of the impact of a change the
sediment budget on the mean width of a beach, !iw/ M, is
!iw 11V, /M
At = x1(zb +z,) (16)
in which AV,/ M rate of <.:hange in the volume of the littoral sediment lens, X;
alongshore dimension of the mini-cell, z. ~ benn elevation, and z, = depth ofshorebase.
Values of the dimensional parameters are listed in Table 10.
In future, no great variation in the amount of sediment passing the east jetty at Newport Harbor nor aro1Jlll1Dana Point is anticipated. Similarly, there is no reason to believe great changes will occur in the amount of sediment that is lost across the shorebase. Major alterations to the wave <.l!imate would have take place for that to occur. Humans, at least locally, wlll have no imluence over the frequency, duration, and severity of wave stonns. Infrequent artificial oontributions to Big Corona Beach wlll likely continue as they have in the past as entrance channel is dredged to maintain navigable depths.
With the exception of Big Corona Beach, beaches throughout southern Orll!lge County are too narrow to provide complete protection for the backbeach line. Seacliffs are vulnerable because the sacrificial volumes of the fronting beaches are not large enough to prevent wave atta<.lk severe stonns. One obvious solution to seacliff erosion is to construct some form of armor. The other is to artificially widen and maintain the fronting beach to increase its protective capacity. The most important consideration in the latter is the rate ofbeachfill loss after the initial fill has been placed.
7 .2 Impact of Seacliff Armoring.
Annoring the base of a seacnff on a recessional coast may result in the eventual disappearance of the fronting beach. Under natural cundmons, beaches on high-relief coasts migrate landward at the rate of retreat of the backing seacliffs, and at the expense of whatever is immediately behind them. Because the backbeach line is free to erode, the
60
beach retains its width. This is the present state of most slowly-retreating beaches in front
of non-annored ll!l&:llfll! in the study area. The Marine Board (1995) recognized this
property of natural beaches by noting "Unless there is a natural obstruction or an obstruction of human origin, beaches experiencing erosion simply move landward in
response to the forces of waves and currents, while retaining their general shape and . " szze.
When a seacliff is annored this natural progression ends. If the shoreline continues its
retreat because of a negative imbalance in the sediment budget, the beach will eventually
disappear. The time over which this occurs is equal to mean width beach when it
is armored divided by the post-annoring shoreline retreat rate.
Seacliff armoring will have two important negative impacts on the sediment budget in
southern Orange County. By protecting the of a cliff, a seawall will reduce or
eliminate the contribution of sediment entering littoral zone that source. By stabilizing the landward end of the littoral sediment lens, annoring will impact the amount
of sediment required to support the lens as sea level rises (Equation 12 and Fig. 4).
A decline in sediment contributions armored seacliifs will affect each of the mini-cells to a different degree. The impact, which depends on the makeup of the sediment budget as quantified in Table 26, can be estimated using Equations l, 11 and 15. In future, a loss of
all seacliff sediment contn1mtions due to armoring would deprive the southern Orange County coast of an estimated 7900 cyy, or almost 40% of the estimated total it will
receive each year.
Because oflow (0.07 to 0.2 ft/yr) seacliffretreatrates and a relatively high (0.008 ft/yr) rate of sea level rise, there is presently a negative sediment flux of about 2000 cyy at the base of the lens. This is the quantity required to support the lens as sea surface rises.
This lll'lgative component of the budget will increase as the length of armored coast expands and the rate of sea level rise accelerates in future, if that occurs. If the entire
backbeach line in southern Orange Coi.mtywas armored and the rise in !l!lll. remained
constant at its 20th century rate, the total loss flux at the base ofthe mini-cell littoral
sediment lenses would be approximately 10,000 cyy, or five times. the present loss at this
bo,.mdary.
To demonstrate the worst case scenario, with the impacts of the loss of the seacliff
sediment source and the increased loss of sediment at the base of the littoral sediment
lenses, the complete armoring of the southern Orange County coast would shift the
present positive sediment budget ( an estimated 8200 cyy) to a negative budget of about •
8000 cyy, From Equation 16, the result would be a continuing loss of0. l ft of beach per year. Many of the beaches of southern Orange County would disappear in around 100
years.
61
7 .3 Impact of Sea Level Rise.
Sea level rise, as it affects the mean width of a beach, impacts the flux across the base of the littoral sediment lens as shown in Figure 4. The means of calculating this flux is detailed in Table 21 and Equation 16. Because the magnitude oftbe impact is related to the rate ofbackbeach line retreat, the two cannot be separated. Assuming the backbeach line retreat rate remains the same as if was in the 20th century (Table 10, column 4), but the rate of sea level rise doublesto 0.016 ft/yr, the total negative flux at the base of the littoral sediment lenses would quadruple to about 8000 cyy. A doubling of the sea level rise rate is within most scenarios of sea level rise for the 21st century.
7.4 Impact of Changing Land Use in the Watersheds.
Watersheds contribute more sand to the beaches of southern Orange County than any other single source. Changing conditions in the watersheds could therefore play a large role in upsetting the sediment budgets of the mini-cells. The !!,pparently narrower beaches of the nineteenth century imply that watershed contributions before the advent of intensive ranching and development were less than they were between 1927 and 1984.
Future usage could reverse this positive impact Developments in the natural watersheds of the San Joaquin Hills, and to a lesser extent in the valleys, will dictate how much the yield ohediment from these drainages will be modified in coming years. Tho impact of any changes can be forecast by incorporating the changed sediment aontribution from a specific stream into Equation 1. Impediments to the movement of sand in the streams are presently small and ineffectual. Watershed .contributions of sediment to the mini-cells are not significantly altered and this situation is not likely to change greatly in the future. The southern Orange County situation is. in contrast to the nearby Huntington Beach Littoral Cell where Prado Datn on the Santa Ana River is a complete barrier to the downstream movement of sediment. The only significant natural source of sediment left in that cell is now the river channel downstream of the dam.
7.5 Estimated Beachfill Losses if Beaches are Artificially Widened.
In southern Orange County, there seems to be little current interest in enhancing beaches for recreational purposes. However, since almost all of the beaches are narrow and slowly retreating into the adjoining seacliffs there is a definite interest in artificially widening them to increase their protective capacity. This situation was recently noted by USACE-LAD (1993a). For artificial beach enhancement to be successful, post-emplacement fill losses must be low enough that the project will be cost-effective. This sediment budget investigation provides useful qualitative infurmation to speculate on the expected level of beachfill retention if southern Orange County beaches are enhanced.
Of paramount importance L the question of whether the mini-cells are at or near thf<ir natural "holding capacities'' or whether they could accept additional quantities of material without rapidly losing it. The headlands bordering most of the cells ext,eod out into
62
relatively deep water. They clearly function as effective barriers to the movement of sediment along the coast. Without them, the beaches would be much smaller, and in some places absent.
It also appears there is a net movement of sediment out of these cells across the shorebase (Table 26). The reason for this movement has been documented at Little Corona Beach.
Everts Coastal (1996) has shown new contributions from Buck Gully are typically
deposited on the inner shoreface during high discharge events in the stream. Later, during periods of high wave energy, the material is carried further offshore past the shorebase.
The result is a relatively narrow beach that is in dynamic equilibrium with a critical
shoreface slope. When this slope is exceeded by sediment loading on the upper part of the shoreface, the next wave storm carries some or all of that material seaward, reducing the
slope. Sediment inputs across the backbeach line are balanced by losses across the
shorebase. Without a continuous artificial nourishment program at high cost, Little Corona Beach will not hold much more sand than is presently there. The beach is at its dynamic
equilibrium width.
A number of lines of evidence suggest most of the other beaches in southern Orange
County are also at or very near their natural "holding capacity". The imposition oflarge
quantities of artificial beachfill on these beaches would probably be lost quite rapidly. The actual loss rates would depend on storminess and bathymetric factors, among many. The rate would vary from mini-cell to mini-cell.
As shown in Figure 15, the sediment budgets of most mini-cells were nearly in balance
without considering longshore sediment transport between cells. Longshore sediment
transport rates between mini-cells (Fig. 8) are within the plus or minus 50% uncertainty of the net sediment :fluxes except at Aliso Beach. This suggests that alongshore transport
from one cell to the next northwest of Aliso Beach, or perhaps northwest of Victoria
Coast, might be nearly zero. Further, based on the measure of the physical impediment to alongshore transport, the potential for shore parallel transport between cells was found to be less in the northwestern part of the study area than southeast of Aliso Creek as shown in Figure 6.
Southeast of Aliso Beach longshore sediment transport is clearly to the southeast at
10,000 to 20,000 cyy. This material does not collect in Three Arch Bay which experienced
a 1927-1984 annual accretion of less than 300 cy (from 1.5 to 3 % of the net rate past the
mini-cell). The most reasonable explanations are that the material passed seaward of the
headlands and did not enter Three Arch Bay, or if it did, it did not remain. The most likely
reason for the lack of accumulation there is that the shoreface is near or at its critical
slope. As at Little Corona Beach, a shoreface at critical slope acts as a transport surface,
not a depostional or erosional surface.
63
12000 I I
I I A I I
I
'
-.. ·-·
~ /\ ........_ . ~ ~
"' . -vi ,.
I I ' -2000
0 10,000 20,000 40,000 50,ot.ll'J 70,000 80,000
Diittance s11m.1:u:nml or Newport Harbor, ft
Figure 15. Net imbalance in the mini-cell sediment budgets without Iongshore
sediment transport.
64
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the city hy Moffatt & Nichol, Engineers, 62p plu!I appendix.
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Everts, Craig H., 1991, "SeacliffRetreat and Coarse Si:ldiment Yields in Southern
California", Proceedings, Coastal Sediments '91, ASCE Specialty Conference,
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San Gabriel River", unpublished report prepared for the U.S. Corps of
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65
Inman, D.L., 1976, Summruy Report on Man's Impact on the California Coastal Zone", report published by the California Department of Navigation and Ocean
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Moffatt & Nichol, Engineers, 1994b, "Geotechnical Appendix to Reconnaissance
Investigation ofHuntington Cliffs," unpublished report by Coastal Frontiers Corporation, prepared for US Anny Corps of Engineers, Los Angeles District.
Munro, Rosalind, 1992, "Marine Terraces Along the Frontal Slopes of the Newport Coast, Orange County, California", in The Regressive Pleistocene Shoreline,
Southern California, E. Heath and W. Lewis, eds., p I 05-113.
Mustafa, M.G., 1978, "Sedimentation in Upper Newport Bay, California", report prepared for the Irvine Company by Williamson and Schmidt, Civil Engineers, October.
O'Reilly, William C. and Guza, Robert T., 1991, "Modeling Surface Gravity Waves in the Southern California Bight", SIO Reference Series No. 19-25, September
Pacific Weather Analysis, 1994, in USACE-LAD, 1995
Simons, Li and Associates, 1985, "Analysis of the Impacts of Dams on Delivery of Sediment from the Santa Margarita River, California", US Bureau of Reclamation,
final report.
Stow, D.A., and Chang, H.H., 1987, "Coarse Sediment Delivery by Coastal Streams to the Oceanside Littoral Cell, California", Shore and Beach, Januruy, p30-40.
Sunamura, Tsuguo, 1992, Geomorphology of Rocky Coasts, John Wiley & Sons, New York, 302p.
Trenhaile, Alan S., 1987, The Geomorphology of Rock Coasts, Clarendon Press, Oxford, 384p.
USA CE-LAD, 1990, "Sediment Budget Report: Oceanside Littoral Cell", Los Angeles District, US Army Corps ofEngineers, C.'...STWS 90-2, l l0p plus 12 appendices.
66
USACE-LAD, J99Ja, "Existing State of the Orange County Coast", Final Report No. 93-1, Los Angeles District, Corps of Engineers.
USACE-LAD, 1993b, "Sama Ana River Delta Beach and Offshore Profile Survey and Sediment Sampling", Final Report No. 94-1, Los Angeles District, Corps of Engineers, December, 16p plus appendix.
USACE-LAD, 1993c, "Beach Nourishment Sediment Sources: Previous Studies and Results ofVibracoring Field Program", Los Angeles District, Corps of Engineers, Coast of California Storm and Tidal Waves Study Report 94-2, December.
USACE-LAD, 1994, "Preliminary Report, Comprehensive Coastal Study of Crescent
Bay", draft report prepared by Coastal Frontiers Corp. for the US Anny Corps of Engineers, Los Angeles District.
USACE-LAD, 1995, "Shore History, Surveys and Changes", Preliminary Report No. 95-2, Los Angeles District, Corps of Engineers, 22p plus appendices.
USACE-LAD, J996a, "Comprehensive Coastal Study of Crescent Bay, Laguna Beach, California", Final Report No. 96-1, Los Angeles District, Corps of Engineers,
January, 25p.
USACE-LAD, 1996b, "Coastal Sediment Budget Summary, Orange County, California", Final Report No. 96-2, Los Angeles District, Corps of Engineers, prepared by Coastal Frontiers Corp, 2 lp plus additional sections.
USACE-LAD, 1996c, ''Nearshore Hydrodynamic Factors and Wave Study of the Orange County Coast", Final Report 96-3, Los Angeles District, Corps of Engineers, prepared by Noble Consultants, January.
USACE-LAD, 1997, "San Juan and Aliso Creeks; Watershed Management Study, Orange County, California", Reconnaissance Report, Los Angeles District, Corps of Engineers, February, 275 p plus appendices.
67
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Ml...~~•MOFFATT & NICHOL ~--~E N G I N E E R S
, 3780 Kilroy Airport Way, Long Beach, CA 90806 Telephone: {5f52} 4~6-9551 Suite 600 Fax: (582) 424-7489
TEL.ECOPIE MESSAGE AND TRANSMISSION HEADER PAGE
PAGE 01
COMPAN ·"'-+-~----'--+ Beach I-en f.A.,,/~J ATIENTIO -------------------'-'-~ ---·--~·-FROM: _C~~~~~~-f~-DATE'. Fe 06 FAX NUMBERS: 9 NO. OF PAOf S (In~ r, H&ader): ._!L JOB NUMBER: _s_a_,....c...5 __ _.,__~·~---
MESSAGE:
Hi Bob -· Attached ate pages from a report by a former M&N employee (Crai Everts) that addresstlr'Llttte!Corona. Mike Sinacora has this entire report In his offi at this time if you need to s,e the original. It refers to an M&N 1986 study and I'll t to track lhat down when I haye a chan<:e, but this is an update-to-date version of the earlier work. I also have a ~ore extensive report from 1997 on this area that 1111 ma I to you. Please call if you ne♦d more stuH or assistance translating this stuff.
Thanks.
Chris
03/97
02/23/2005 19:23 94%733055 NBLG HQ
' 1hat LCB was a roclcy beach practically free of sand in 1875, In l 875 (USC&OS mooth
Bheet), kelp w~, grtjwing in the depression, inuica1ins the depression wa& not nU d with
sand 1u it is today, ij111 th~t rock was ll!X:posed and had be~n e,cpo,ed Jang enough for the
kelp to fonn.
9.:Z Sediment Budaet. LCB is a semi-clo~e,;i syslt!!m that is maintained in nature' delicate
balance. Sand entel'? at it.s lindward and leave5 at its seawanl end. The mean wld of the
beach is dependent ~pon the volume of sand on the beach and in th chute (shore ace),
When the lens vob.1'1r is large the shoret'ace is ~teep. The potential for tnmsporl ut of the
lens is enhanced in ~his situation, Altemataly, if there is a paucity of sa!ld in the I , th11
shoreface slope !1 ltjw and the pot~ntial for seaward-directed flow is less. The m slcpe
of the shoreta~e $U,fac~ is about 0.06 (Fig.44) . .
When sand dischar9ed from Buck Gully under low to moderate Hows is deposite on the
beach and •t the upper end of the depression, 1he slope out to a depth of about 2 f\
(mllw) is increased. :A return lo a dy11amic equilibrium ,iope is effected by a do slope
movement of some pf the newly.formed deposit At higher B,tck Gully flows, th
discharged ,edimenj, p41rhaps elong wlth ;ome of the existing beach m11t~rial. is c rri~d
further out inti; the ~hute, or even out of the chute.
The a~tion of large ~vnv~s cause dow11welling and rip currents which tr,rnsport sa d
seaward in the dept~Mion, An intense combimi.tion of downwelling. tip current£, nd
stre&m discharg~ mh transport ~and thmuah the chute and out of the littoral std ment
lens at its deepwater end. OHkhore-dir~cted transport from the beach onto the ~h reface
typkllllj· occurs duilni. stonny peri.ods of largo, long-period swell from the south The
anount rn,wed incr~a~es with the tidal elevation, and large quantities are s□meti es
transported offshor~ during storms th6t occur coir.cidert with perigean spring tid &. Sand
returns to the beac~ during periods of swell from the south and iOuthW!!&t.. w~st ly and
northwesti!rly swell!hns little dfe~t beCll\lse of the orientation of the beach, the o 'entatillll
Clf the Poppy Aven~e headland, and the sheltering effect of the east jelly al thee ranee lo
Newport Bay. !lip .torrent, in the chute are common during peri::ids of moderate o heavy
~outh 1well.
Sand carried offshore from the beach will usually return to the beach ifit does no pass
below the prnnoundecl break in 11lope. shown in figure 44, at a depth ofal;mut 27 ft
(mllw). Once sand i~ carried a~r0£S this boundary (the shorebase M defined in Fi . J), it is
lost to ·th~ littoral ddimeot lens. The beach will subsequently remain narrow un,i the ne~1
discharge of sand i~ Bu.ck Gully replace~ it
This Buck Gully tr~nsport conduit has been in operation during a significant ped d oft1mt
given th!! quanrity qf ~edim~nt that hat. ace1imuh1.ted on the Continental Shelf. Fi~ her, et
al ( 1983 ), for example, show the sedimen1 deposit seaward of the chute i, about a.ft
thick. In June !91/Jj C oast!I Frontiers (USACE•LAD, 19941.,J samrled ! 9.4 feet fthe
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' !
Jj
;.l r. " =
shelf bottom in a. water depth uf 57 f~et off Buck Gutty. An analysis of four sampl s taken
at illt!IV!lls indicati, a lonsis1~nt reduction in the media.n grain size from 0.11 mm t the
top to D 23 mm at the lbase of the: core. Sedirnen1 finer than sand in the core deer sed
from 18 percent at thdtop to 9 percent at the bottom. This upward•finill8 indicat : (1) a
progressive reduction In energy at the site with time. possibly caused by deepenin
conditions, and1or (2)ln progre$sivc reduction in the size distribution of~~dirnent eaching
the site from the 11platjds. Both ate likely, and are quite possibly rela1cd to a Ions i tcrval
of sea.ward-directed sipdirnent transpor! associated with sea level iise and a retreat of the
littoral [ediment lens. i
:11.llll)lh"' l)ltl~tt !1111.11)/
• ",Ull ,,~ill t r.o
'"
:11 [--~,u.j,, i~ ,.,...,_
I 11~~, r ,11,1
'" • 1Jll f,11
I 111 ! ,,
J11 j HIJ'lll.rmim11,,i i ---~~..,.....
"'
Figure 44, Profiles tiormal to the shoreline at Little Corona Be,11:h; note th position
of the hfdrnck surrlice and the distribution or Uttornl udiment Rbove bed ck; !he
thorebase I~ nt 11bo/1t •27 ft (mllw) lit an outcropping.
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I '
Sa.nd apparently does ndt entrr LCB from either Big Corona Beach, or frtim the
,O\Jtheast, nor is lt lost i~ these alongshore direction& The hcadiands at Poppy Avenµe and
A.r,.h Rr>~k appear to fu*ction zs near-complete barriers tQ the long!hore movement of
sediment. The expo5ed ~hore platform on either side of!he depression at Little Coro,ia
Beach has, reHefof,w1jr 10 ft (Fia. 44) Seac,lifferosion wu $imilai:ty round to pla)! a
mhtor role in the ~up ply !of sand to LCB
I
!1,2.l Surveys, in 12 April 1994 1111d 1 February l 995, Moffatt & Nichol,
En11lneers ( 1995) surve ed Litt!~ Corona neach and its offhllore i!!11virons. Tile 1995! field
effort came after a cl11st r of high dischai:ge events in Buck Gi.llly and cofocident wave
storms in January The ~bjective of the survey and measurement progrnm was to estku,lish
the v1,lurne of sand and~otll'ser sediment in the littoral sediment lens at different tim~s. and
tc determine whether t lens volume chanaed as a result ofhiah stream flows !Uld •ev11rc
wave events. The beac depression and c!lntinental shelf to II depth of about 35 ft Were
surveyed. The undulyitjg rocky substrate was located by water-jet probing through ~he
sflnd cover 011 a grid sp,clng of 20 ft Offshore surveys were accompli,hed using 11110n-
stret~h tag litie for hori~ontal position a~d e precision gage for depth detenninations.
Standard leveling technjques were used to survey (he beach and locate and orient tiw tag
tin~. The origin of all ,4rve-1s was the County o{ Onm11e bench mark al Little Co1oria
Bench ('Ne. 3 l).
The two rurveyed pro flies an: shown with 1he underlying substrate suriace in Figur¢ 44.
figure 4~ shows tbe th~\:ness of the sa11d lens at the upper end nfthe littoral zone ~n 28
April, and Figure 46 sll.pws the elevation of the subsua1e ~urface (bedrock, and cobbles
and ~oulders on the ~rook) on that date. The sand level chan!je between 12 .April! 1994
and l Febn,ary 1995 is/shown in Figure 47.
Fifty.five hundred cu~if yards of sand were lost from the littoral sediment lens between
the twc s1,1rveys. Aoovt a dupth of -4 ft (rnllw) the sand loss was 9700 cubic yards;.
between -4 and -27 ft ¢ere was a ne! gain of about 4200 r.ubic yards. Sand that enjenid
t.h~ litloral system. fl-01~ Buck: Gully end sand from the beach wu transported onto ll,nd
partially through the slforeface con1~onent of the littoral sediment lens.
Quite likely, all oflhe ~and was lollt in January 1995, On two oCCBSion~. storm wa\Jes,
abnormally large flow~ in Buck GuUy, s.nd high-velocity onshore wind~, created llOllditlons
rhat ,roded the be:,.ch *oo resulted in downwel\ing that 1r1;nsported the mobilized ~a1erial
offshore. Twc"hour arf! three-hour dutation discharges in Bu~k Gully wen, respetjtively,
5-yr return frequency. ~nd IO to 25-yr return frequency events F'lows irt Buck Gully were
near bankfull. This typ~ of su8tsined !low has not occurred since 1983 and µnwiou~ly,
since 1969 (A. Nestliri~~,. Oru,ge County Flood Control, pers comm.). Wa11es d~ring the
storms epp1oach~d fr,tn all directions. Significant heights reached 8 to 10 ft severtl tim~~
as reported on south· f~cing Balboa P~ninsuh1. Wind speeds reached a maximum 62 knots
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on 6 January at tHe Newport tle~c;h Mwne Department Headquaners. The fre¢Juency of occurrence of this combination of erosive phenomena is unknown, bu: Moffatt & Nichol, Engineers ( 1995) lspe.culated tkat the rec1.1m:nce in1cr1·al was between once in jO and unce in JOO years.
.
' 1 ...... ,---
(_~ ·': _,
J .
11 ti: "., 1i,I IHi
h It► L .. b..........:.1 1!1(\'\fOfi"lll''!'j ~1 i~lt,,fll!
l1f~"•~;l, t'lrHU,m • lil"..lJ ;l I..,,) l""'f,1
Figure 45. S1uid "1ickness on 28 April l 994 at Little Coron11 Beach.
This duster of uru1suai events ts part of the natural cycle of beach advances and retreats ar Little Corona Bea~h. The sand loss from the littoral sediment lens re.1ulted in h 15 to 20 ft retreat of the rneuf1 pn~ition of the shoreline. The mean position of the shoreline will remain recessed u~til the next Buck Gully discharge of sand, or an ar1iJicial replenishment
proj~ct nourishes the heach.
9.2.2 Bottdm Control Structure. The bottom control structurt at the outlet of Buck Gully was dttigned to reduce headcut,ing upstream of Little Corona Beach and eliminate meam-c~used tot erosion of the cliffs near the outlet. The siructurt was coMtruct~d ~bout J 970 to address these noeo, articulated by local residents. Eslimates of the volume ofsedilhel\t impounded upstream of the stmcture are shown in Tabll.! 23. TI1e impounded v0Ju,.n11 is about one-third of the equilibrium volume of sand in the littoral sediment Je,ns,
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j ilfl•":) 1ff\ \\', ~lt/H\1111 $ I,..~,., lll, 1!\'<li!~t, ~ t ~.-l,I fr, 1 I •H1", l~"~J
Figure 46. E:lev~tfon of sulutrate surface on 23 April 1995 at Little Corona Beach.
l'able 23. Sedl~e~t lmpoundment Upstrei:im of the Bottom Control Stru~ture at the qut1M of Buck Gully.
TJJ,lfEPERlOD ESTIMATE: ltEFB~~CE IMPOUNDED .,, .. , !': '
SEDIMENT
VOLUME, cubic
i vard,
1970 1, 19115 4000 r~o to 90 ~eimt .,,,._, • Mo.f!&\11 & Nl,Ml,
~
)5 .,,,,,)
,,_ foijn.J lo b1 SU!d) En~jn"""' ( rnio) n"i, 1995 6000 phu: or minus 40 •lllil!IOIC 1-.1 ll!l field·-1(25 \OU~) peroen1 obsrlfii'atjo.ns) this i 1n•elliaatlon
02/23/2005 19:23 94%733055 NBLG HQ
l'ig11l'e ~-7. San~ ltvel (hange H Little Corona Beach bern·on 28 April lll94 and 1 F~bru~.ey l99Sj
PAGE 07
The catchment ~f Buck Gully is about 1200 icres of gentle to rugged foothill terrain, The use ofup to sod 11Ctes of the watershed is rapidly changi~g es develqpment expands on the Irvine ((last ~ other 400 acre, in Corona del Mar and Newport Beach has already been developed hS m~dlum,denslty residential prop(n"l)'. Prior to development, this walershed ,.vas u~ed for gr~i:ing, which is typioa.l of undeveloped coast.al footlillls in southern California. M1,fflm & Niehol, Enginee.rs ( 1986) estimated the annual discharge of coarse sediment in Buclc Gully at 30D cy fo1 the period p1ior to the construeti(ln of the bottom cont ml struetur~ Tati!~ 24 is a compilation of sediment yield meuurements from nearby watersheds that \vas the basis fur the Moffatt & Nichol, Engineers (1966) estimate.
9.3 Pres,~nt SP.a~liff Ilehnvior. Seacliffs surroundinl,'l Li1tle Corona Beach ar~ composed ofMontMey Sh~la Moffart & Nichol, Engineers (1986) est\mMed the conc~nlration of rnnd in thes~ erOJsinnal features and concluded the cliff retreat rate probably averaged O l fli)'T west of the putie! of Buck Gully and 0,2 fl/yr eas1 tifit They also condullect that !\ano contributio*s fh1m the cliffE probably average between 25 and 35 c11bic yards per yr-ar, respect'.vc!y, v.e,t and east of the outlet. The approxlmately 60 cubic yards per year
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total contributi4n is about 20 percent of the estimated coarse sediment contribution from. Bu~k Gully
Table 24. Sedll11ent Yield E!!imate1 from seled~d Watersheds in Sol.lthern Calfomin.
WATERSHEn: SEDl'.MF.:NT YltLD ESTlMA TI;, cubic REFER.ENCE ·;•,, i, vards/nar ·. :·,,,,, '.. . .. . ,· .,.: . .-' . .-; . S1111 DJ,..n Cll!ek 1. 115 =••·mile <all oed1"'-I i,n lll1dcvei"""" ateu\ Mllflafa 119781.·. • ,.'.,,•· San Ditgo Creek (I) l~□o toLBI f,"' sq, 111il• (/or op•n foothill U<IIS: •bout !lay!, l.'llp...-ins t: JOO p<r ''l mi e, for S6M), (l) !llllld" HDO p,:r •~-mile <°'!IJlOll!!UJU (1982) whor• constru,Oon i, in pio,re!O, !nd (3) 11ad ~,so p<r :.:,·' '''"' •• "' '. "'· m~ for dcwl£l)Cd ~!M!l}ill 111m .. •'.
---• -Sillld Cwwott 11.,..,.,,,olr 4DO (>11!1~) p,:r sq. mile
Boyl•~in"""ina • .·:·',/,, mo to 1QO (slllld) per "I· mile fITTntite waler$hld
('.""""raU<!llll?Ul • . Ban Di•so Cr~k
}.19(1,tt &. Nitt,at: ... :. ,\f .. : Wimers!><rg,Oml;,n Qiovo I < I I){) [linu) per ,q. mil~ for thi1 Urh1>11 \\·•tmhed
Fri,.;••er,'llh~i ''
Moll',m & Nii:hcl, • ~.· Channel , Eilsine~,,(i9li5)
Buck 01111)· '"'....,_-+-,~
2.10 {or ahout t IQ per $G, rnil, bmd on th, sand volum• Moffiill ll. Nichol, im,pQnndeo 11p~ll'l8:ilttl of tl1c bottom cootroi Git11icturt in EngJM,,"' (l~86) ' 1986 i,.-,-, ...... --
ni ('4!nd) i11 \llld;,·,lop•d fllothlll ot,as
___ ,.. ___ f)t,.-1, OulJc•; osiinrnt• I••~-
Moifi1t & Nichol, term \.'ield 40 ( "nd) iu ikwel<lpod foorhill •ree, E,ngin,m (l986) I i,.re-ra!?I! '111.nd rieltl estim~~ •n J 9116 ;::_ 300
98