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Estuarine, Coastal and Shelf Science 92 (2011) 195e204

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Estuarine, Coastal and Shelf Science

journal homepage: www.elsevier .com/locate/ecss

Anthropogenic influences on shoreline and nearshore evolution in the SanFrancisco Bay coastal system

Kate L. Dallas a,*, Patrick L. Barnard b

aDept. of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USAbU.S. Geological Survey, Pacific Coastal and Marine Science Center, Santa Cruz, CA 95060, USA

a r t i c l e i n f o

Article history:Received 30 April 2010Accepted 27 December 2010Available online 14 January 2011

Keywords:ebb-tidal deltatidal inletcoastal morphologylong-term changesman-induced effectsUSACaliforniaSan Francisco BarSan Francisco Bay

* Corresponding author.E-mail address: katedallas@gmail.com (K.L. Dallas

0272-7714/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.ecss.2010.12.031

a b s t r a c t

Analysis of four historical bathymetric surveys over a 132-year period has revealed significant changes tothe morphology of the San Francisco Bar, an ebb-tidal delta at the mouth of San Francisco Bay estuary.From 1873 to 2005 the San Francisco Bar vertically-eroded an average of 80 cm over a 125 km2 area,which equates to a total volume loss of 100 � 52 million m3 of fine- to coarse-grained sand. Comparisonof the surveys indicates the entire ebb-tidal delta contracted radially, with the crest moving landward anaverage of 1 km. Long-term erosion of the ebb-tidal delta is hypothesized to be due to a reduction in thetidal prism of San Francisco Bay and a decrease in coastal sediment supply, both as a result of anthro-pogenic activities. Prior research indicates that the tidal prism of the estuary was reduced by 9% fromfilling, diking, and sedimentation. Compilation of historical records dating back to 1900 reveals thata minimum of 200 million m3 of sediment has been permanently removed from the San Francisco Baycoastal system through dredging, aggregate mining, and borrow pit mining. Of this total, w54 million m3

of sand-sized or coarser sediment was removed from central San Francisco Bay. With grain sizescomparable to the ebb-tidal delta, and its direct connection to the bay mouth, removal of sediments fromcentral San Francisco Bay may limit the sand supply to the delta and open coast beaches.

SWAN wave modeling illustrates that changes to the morphology of the San Francisco Bar have alteredthe alongshore wave energy distribution at adjacent Ocean Beach, and thus may be a significant factor ina persistent beach erosion ‘hot spot’ occurring in the area. Shoreline change analyses show that the sandyshoreline in the shadow of the ebb-tidal delta experienced long-term (1850s/1890s to 2002) and short-term (1960s/1980s to 2002) accretion while the adjacent sandy shoreline exposed to open-ocean wavesexperienced long-term and short-term erosion. Therefore, the recently observed accelerating rates of baysediment removal, ebb-tidal delta erosion, and open coast beach erosion are all correlated temporally.

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1. Introduction

Anthropogenic activities within coastal estuaries may reducesediment delivery to the coastal ocean, decreasing sand supply toopen coast beaches and ebb-tidal deltas (e.g., Davis and Barnard,2000, 2003; Elias and van der Spek, 2006; Smith et al., 2008).Ebb-tidal deltas are bodies of sediment deposited by ebb-tidalcurrents on the seaward side of tidal inlets and in some locations,are huge sand reservoirs that have increasingly been examined asprospective sources of sand for beach nourishment (Cialone andStauble, 1998). Ebb-tidal deltas can also strongly influence coastalprocesses in the vicinity of the tidal inlet (e.g., Davis and Fox, 1981)

).

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through partial wave sheltering of the adjacent shoreline, therebyreducing shoreline erosion (Marino and Mehta, 1987).

Studies of ebb-tidal deltas have shown that a dynamic balancebetween tidal energy and waves determines delta morphology,where ebb-tidal currents induce a net-offshore directed sedimentfluxandoffshorewaves induce anet-onshoredirected sedimentflux(Hayes, 1975; Walton and Adams, 1976). Previous studies haveshown that anthropogenic activities in estuaries canhaveadramaticinfluence on delta size and shoreline position (Barnard and Davis,1999; Davis and Barnard, 2000; Elias and van der Spek, 2006).Analysis of a series of bathymetric surveys of an inlet and ebb-tidaldelta system in the Netherlands by Elias and van der Spek (2006)demonstrated that changes to a back basin’s tidal prism causedmorphologic changes to the delta and erosion of the adjacentshoreline. Carter et al. (1982) and Cooper and Navas (2004) alsoshowed that changes in ebb-tidal deltamorphology altered incident

K.L. Dallas, P.L. Barnard / Estuarine, Coastal and Shelf Science 92 (2011) 195e204196

wave energy and caused long-term shoreline change. Knowledge ofthe processes that impact ebb-tidal delta evolution is thereforefundamental for successful coastal management in these regions.

The morphology of San Francisco Bay’s ebb-tidal delta haschanged significantly over time (Gilbert, 1917; Battalio and Trivedi,1996; Hanes and Barnard, 2007; Barnard et al., 2007), but little isknown about why the delta has changed. The objectives of thisstudy were to quantify long-term bathymetric change of the SanFrancisco ebb-tidal delta, assess shoreline change along the adja-cent sandy shoreline, and investigate the processes drivinggeomorphic change in the coastal system.

2. Study area

San Francisco Bay, located at the mouth of the Sacramento-SanJoaquin River system, is the largest estuary on theWest Coast of theUnited States, draining over 40% of the state of California. Theestuary consists of three sub-embayments e North Bay (San Pabloand Suisun Bays), Central Bay, and South Bay (Fig. 1). San FranciscoBay is an urbanized estuary, and with the surrounding area home toover 7 million people (United States Census Bureau, 2009), iswidely considered to be the estuary most impacted by humanactivities in the United States (Nichols et al., 1986).

San Francisco Bay is connected to the Pacific Ocean by thenarrow Golden Gate inlet (Fig. 1). The large surface area of the bayand diurnal tidal range of 1.78 m creates an annual maximum tidalprism of w2 billion m3 (w2 trillion liters), which causes peak ebbflow depth-averaged currents in the inlet to exceed 2.5 m/s(Barnard et al., 2007). After the ebb jet emerges from the inletthroat, the velocity decreases and coarse sediment is deposited. Thetidally transported sediment, and sediment supplied by littoraldrift, accumulates to form the w150 km2 ebb-tidal delta, the SanFrancisco Bar (Fig. 1).

The San Francisco Bar is shaped by tidal currents and waves,which regularly exceed 6m in height on the continental shelf duringmajor winter storms (Coastal Data Information Program [CDIP],2010). The ebb-tidal delta has a horse-shoe shape with a dredgedshipping channel across its central crest and two peripheral floodchannels adjacent to the coast. Grain size varies across the delta,ranging from coarse sand and gravel near the inlet entrance to fine

Fig. 1. Location of the San Francisco ebb-tidal delta at the mouth of the San Francisco Estuain meters.

sand on the outer reaches. Numerical modeling results indicate thatthe ebb-tidal delta plays an important role in dissipating waveenergy throughwave refraction, and in some cases can reducewaveheights by 50% (Barnard et al., 2007). Wave refraction and focusingdirectly impact beach morphology at adjacent Ocean Beach.

Ocean Beach is a 6.5 km long sandy beach located just to thesouth of the entrance to San Francisco Bay (Fig. 1). The proximity tothe inlet creates strong alongshore tidal currents that can exceed1 m/s (Barnard et al., 2007). This area is also exposed to high waveenergy with a mean annual offshore significant wave height of2.4 m, but winter offshore storm heights can exceed 9 m (CDIP,2010). Since 1997 there has been a trend of shoreline accretion inthe northern and central portions of Ocean Beach and shorelineerosion in the southern reach (Barnard et al., 2007). The southernreach of the beach has been eroding for decades (Domurat et al.,1979; Hapke et al., 2006; Hansen and Barnard, 2010), witha recent mean shoreline retreat of 15.1 m since 1997 (Barnard et al.,2007). Chronic erosion in this location has claimed portions of twoparking lots and badly damaged a major roadway.

3. Methods

3.1. San Francisco ebb-tidal delta bathymetric change

Sounding data from four hydrographic surveys of the SanFrancisco ebb-tidal delta were used to analyze long-term bathy-metric change: 1873, 1900, 1956 and 2005. For the 1873 and 1900surveys, soundings were digitized from hydrographic sheetsobtained from the National Ocean Service (NOS) and registered toa common horizontal datum using graticules and triangulationstations. For the 1956 survey, registered soundings were obtaineddirectly from NOS. The 2005 multibeam bathymetry data set wasreceived in registered, grid format from the surveyors at the SeaFloor Mapping Lab at California State University, Monterey Bay.

Continuous bathymetric surface representations of the 1873,1900, and 1956 surveys were created using triangulated irregularnetwork surfaces that were converted to raster grids with a hori-zontal resolution of 25 m. For all years, grids were compared tooriginal sounding data to check for problem areas. All historic gridswere adjusted from mean lower low water (MLLW) to a common

ry, California. Dashed box indicates study focus area. Bathymetric contours from 2005

Table 1Bathymetric grid uncertainties (m) for historic surveys.

Uncertainties (m) 1873 1900 1956

Gridding interpolation 0.03 0.01 0.01Sounding uncertainty 0.40 0.21 0.21MLLW uncertainty 0.09 �0.09 e

Total Grid Uncertainty (m) 0.41 0.23 0.21

Fig. 2. Index map showing the three shoreline change analysis regions and variouslocations mentioned in the text.

K.L. Dallas, P.L. Barnard / Estuarine, Coastal and Shelf Science 92 (2011) 195e204 197

vertical datum (NAVD88) to account for sea-level rise and tidal staffvariations through time and differenced to create bathymetricchange grids. Bathymetric change grids were used to calculatevolume change for each survey period. Ebb-tidal delta crest loca-tion was determined by extracting the shallowest depth along 40transects cast roughly perpendicular to the crest.

3.2. Bathymetric change error and uncertainty analysis

The total bathymetric grid uncertainty is a combination ofpotential errors and uncertainties, including gridding interpolationerror, sounding measurement uncertainty, and MLLW datumuncertainty (Table 1). The total grid error (i.e., systematic error) isbelieved to be far less than the grid uncertainty of an individual cell.Gridding interpolation error (error that results from interpolatingbetween point sounding data) was calculated by removing 10% ofthe sounding data and gridding the remaining data. Every soundingremoved was compared to its associated grid cell value and theaverage difference for all of the removed data represents thegridding interpolation error. Gridding interpolation error was smalland was removed from the grids, but to be conservative wasretained in the uncertainty calculation.

Measurement uncertainty for the historic surveys can only beestimated based upon the error criteria employed during surveying.During the historic surveys sounding error was determined in thefield by comparing separatemeasurements at trackline crossings andwas not to exceed 3% of the water depth (Shalowitz, 1964). Compar-ison of soundings at trackline crossings and observations of similardepths offshore of the delta from different surveys indicate thatsystematic sounding error was not significant (i.e.,<<individual celluncertainty). Sounding uncertainty was conservatively estimated foreach survey bymultiplying themaximumerror permitted at the timeof surveying by the mean survey depth.

The 1873 and 1900 surveys also have MLLW datum uncer-tainties due to the MLLW datum chosen during surveying. Unlikethe 1956 and 2005 surveys, where the MLLW datumwas calculatedover a 19-year tidal epoch, the earlier surveys used a 3-monthaverage of MLLW. To assess the uncertainty that arises from usinga shorter time period, a 19-year average MLLW was calculated foreach survey using historical tide gauge records and compared to thesurvey MLLW.

A total uncertainty for each historical survey was estimated bytaking the square root of the sum of the squares of the griddinginterpolation error, sounding uncertainty, and MLLW uncertainty(Table 1). An uncertainty of �0.12 mwas assigned to the 2005 dataset, which was provided by the surveyors based on robust harbortesting and evaluation of trackline overlaps. Volume changeuncertainties were calculated by multiplying the square root of thesum of the squares of total survey uncertainties by the surface areaof the grid. These estimates are very conservative and assumea systematic error throughout the surveys, even though there wasno evidence that the error was this large for any of the volumecalculations. However, because it was not possible to providea more quantitative assessment of systemic survey error, theconservative approach of simply applying the grid cell uncertaintyto the entire survey was chosen.

3.3. SWAN wave modeling

The SWAN wave model (Holthuijsen et al., 1993; Delft3D, 2007)was run in a stand-alone, stationary mode with grids and modelparameters from Eshleman et al. (2007). Three nested grids withconsecutively finer resolutions (500 m, 200 m, and 100 m) wereused. The bathymetry files were interpolated onto thewave grids inthe following order of importance: ebb-tidal delta bathymetry fromthe 2005 multibeam survey (Kvitek, 2010), modern nearshorebathymetry collected using Personal Watercraft from 2004 to 2006along Ocean Beach (Barnard et al., 2007) used to fill in gaps inshallow water areas (for methods see Ruggiero et al., 2005), andbathymetry data compiled by the National Marine SanctuaryProgram in 2003 for areas offshore of the ebb-tidal delta. Themodelwas runwith parameterizedwave forcing and did not includewind,tidal currents, or waveewave interactions. Model runs wererepeated using the ebb-tidal delta bathymetric data sets and outputmaps of significant wave heights were differenced to analyzehistorical changes in wave height.

3.4. Sediment removal

The volume of sediment permanently removed from the SanFrancisco Bay and the San Francisco Bar (i.e., the San Francisco Baycoastal system) through dredging and borrow pit mining wasquantified by a literature search (Markwart, 1915; Scheffauer, 1954;United States Army Corps of Engineers, 1975; Ogden Beeman andAssociates and Ray Krone and Associates, 1992). Hardcopydredging files were not reviewed for this research, so dredgingresults are minimum values. Dredging volumes reported hererepresent only those sediments that were disposed of outside thebay. These volumes include dredge spoils placed on land, in the deepocean, or those used in beneficial reuse projects. Records of borrowpit mining events are scarce and the volume of material removedwas often not reported, so borrow pit mining results are alsominimum values. Aggregate mining volumes were collected fromreports submitted by aggregate mining companies to the San Fran-cisco Bay Conservation and Development Commission (BCDC) andCalifornia State Lands Commission. Aggregate mining has occurredin the San Francisco BayeDelta estuary since the 1930s, however

Fig. 3. Volume of documented sediment removed from the San Francisco Bay coastalsystem through dredging, borrow pit mining, and aggregate mining from 1900 to2008.

K.L. Dallas, P.L. Barnard / Estuarine, Coastal and Shelf Science 92 (2011) 195e204198

records prior to the establishment of BCDC in 1974 are incompleteand unreliable and were not incorporated into this research.

3.5. Shoreline change

Long-term (1850s/1890s to 2002) and short-term (1960s/1980s to2002) shoreline changes were evaluated from Crissy Field Beach toPoint San Pedro (w30 km) (Fig. 2). To assess shoreline change alongsections of coast that experience similar wave and tidal energy, thecoastline was broken into 3 separate regions e San Francisco, OceanBeach, and San Mateo (Fig. 2). Existing shorelines were acquireddigitally from the United States Geological Survey National Assess-ment of Shoreline Change (Hapke et al., 2006) and the NationalOceanic and Atmospheric Administration Shoreline Data Explorer(NOAA, 2009) and are originally from topographic sheets (T-sheet),digital raster graphics (DRG), and light detection and ranging (lidar)data sets. In addition, aerial imagery (1983) and lidardata (1997,1998,and 2002) were used to supplement the existing shorelines.

To compare historic highwater line (HWL) shorelines andmodernmean high water (MHW) shorelines it was necessary to apply a biascorrection (Ruggiero et al., 2003; Hapke et al., 2006; Moore et al.,2006). Previous studies have found large horizontal offsets of up to50 m (Ruggiero et al., 2003) between these shoreline indicators. Thebias correctionwas adapted fromHapke et al. (2006) andwas appliedto all historic HWL shorelines. Shoreline change rateswere calculatedfor the sandy shoreline at shore perpendicular transects spaced 50mapart and averaged for each region using the Digital ShorelineAssessment System(Thieler et al., 2005). Short-termshoreline changerates were calculated at each transect using the endpoint methodcomparing the 1960s/1980s and 2002 shoreline positions. Long-termrates of shoreline change were calculated using linear regressionapplied to all shorelines from the earliest (1850s/1890s) to 2002.

3.6. Shoreline change errors and uncertainty analysis

The total error for the short-term shoreline change rate wascalculated by taking the square root of the sum of the squares of T-sheet/DRG error, georeferencing error, digitizing error, and shore-line position error (Hapke et al., 2006) (Table 2). T-sheet/DRG errorreflects errors present in the original surveying methods. Geore-ferencing error applies to shorelines derived from T-sheets, DRGs,and aerial photographs and reflects the maximum root meansquare error found during the georeferencing procedure. Digitizingerror reflects the ability of the digitizer to accurately digitize theshoreline and was taken as the maximum error specified inprevious studies (Anders and Byrnes, 1991; Crowell et al., 1991;Moore, 2000). Shoreline position error is the average bias uncer-tainty for historical shorelines (Hapke et al., 2006) and themaximum error associated with the derivation of a lidar shorelinefor lidar data (Stockdon et al., 2002).

Separate total position errors were calculated for each shoreline(Table 2). Short-term, annualized uncertainty was calculated foreach region by taking the square root of the sum of the squares ofthe older shoreline error and recent shoreline error and dividing by

Table 2Maximum estimated errors (m) for individual shorelines.

Errors (m) 1857 1877 1866_1899 1929 19

Shoreline extenta SF SF OB, SM OB, SM SFT-sheet/DRG position 10 10 10 10 10Georeferencing 15 8.5 4 4 4Digitizing 1 1 1 1 5Shoreline position uncertainty 8.7 8.7 8.7 8.7 8.7Total position error (m) 20 16 14 14 15

a SF ¼ San Francisco, OB ¼ Ocean Beach, SM ¼ San Mateo.

the total time between shoreline dates (Hapke et al., 2006). Long-term shoreline change rate uncertainty was based on the 90%confidence interval of the linear regression for each transect andthe uncertainty of the proxy-datum offset (see Hapke et al., 2006for a more thorough discussion).

4. Results

4.1. Sediment removal

Between 1900 and 2008 a minimum of 200million m3 (Mcm) ofsediment were removed from the San Francisco Bay coastal systemthrough dredging (120 Mcm), borrow pit mining (54 Mcm), andaggregate mining (26 Mcm) (Fig. 3). As stated prior, the dredgingtotal represents events where sediment was removed from the SanFrancisco Bay coastal system, and does not include projects wheresediment was simply relocated within the system. The total of200 Mcm is a minimum estimate because not all records have beencompiled (missing 1990e1996 dredge records and likely manyadditional borrow pit, aggregate mining, and dredging records).A majority of the sediment from 1900 to 1990 (missing spatial datafrom 1997 to 2008) was removed from Central Bay (113Mcm), withlesser amounts removed from the North Bay (41 Mcm), San Fran-cisco ebb-tidal delta (21 Mcm), and South Bay (3 Mcm). Grain sizesare unknown for much of the sediment, but where data wererecorded 75 Mcm were sediment that was fine sand or coarser.

4.2. Historical bathymetric change to the San Francisco ebb-tidaldelta

4.2.1. Change from 1873 to 1900In 1873 the San Francisco ebb-tidal delta had a continuous crest,

with a broad outer region where depths ranged from 10 to 11 m

41 1950s 1963 1977 1983 1997 2002

OB, SM SM SF SF, OB, SM SF, OB, SM SF, OB, SM15 3 3 e e e

4 6.7 3.5 6 e e

1 5 5 5 e e

8.7 8.7 8.7 8.7 1.5 1.518 12 11 12 1.5 1.5

K.L. Dallas, P.L. Barnard / Estuarine, Coastal and Shelf Science 92 (2011) 195e204 199

(Fig. 4a). By 1900 most of the delta was erosional and the crest hadretreated landward along nearly its entire length (Figs. 4 and 5). From1873 to 1900 a volume of 75� 58Mcm eroded across the delta, withanaveragedepth changeof�0.61m(Fig. 4b).During this periodmorethan85%of the deltawas erosionalwithwidespread erosion along itssouthern crest where it approaches the shoreline at Ocean Beach.

Fig. 4. (a) Bathymetry maps of the San Francisco ebb-tidal delta from 1873 to 2005

4.2.2. Change from 1900 to 1956From 1900 to 1956 a volume of 51 � 38 Mcm of sediment

accreted on the ebb-tidal delta, with an average depth change ofþ0.42 m (Fig. 4b) (this volume change calculation includes the shipchannel, which was dredged during this time period. When thechannel was excluded the volume changewas 18% greater). Overall,

and (b) maps of bathymetric change of the ebb-tidal delta from 1873 to 2005.

Fig. 5. (a) Location of the crest of the San Francisco ebb-tidal delta from 1873 to 2005 and (b) a cross-section profile through the crest.

K.L. Dallas, P.L. Barnard / Estuarine, Coastal and Shelf Science 92 (2011) 195e204200

about three-quarters of the delta was accretional. Accretion wasmost pronounced in the central part of the delta landward of thecrest and within the flood channel adjacent to Ocean Beach. Theprimary erosional areas were along the length of the crest andwithin the ship channel where dredging began in 1931. Althoughthe delta experienced net accretion from 1900 to 1956, the crestcontinued to contract radially with a maximum crest retreat of1.6 km (Fig. 5).

4.2.3. Change from 1956 to 2005Comparison of the 1956 and 2005 surveys reveals net erosion of

76 � 30 Mcm of sediment, with an average depth change of�0.45 m (Fig. 4b) (volume change is 20% less if the dredged shipchannel is excluded). More than 75% of the deltawas erosional withmajor sediment loss around the crest and within the ship channeldue to modified dredging practices to deepen and widen thechannel (Fig. 4b). A distinct accretionary mound can be seen southof the ship channel as a result of dredge disposal occurring in this

Fig. 6. Model predicted changes in significant wave height (m) (a) from 1873 to 2005 andDp ¼ 300�). SF-8 is a federally designated dredge disposal site. Box in Fig. 6b shows locatio

location since 1971. Accretion is also evident along the floodchannel offshore Ocean Beach andmay represent a decrease in flowthrough this channel as a result of increased hydraulic efficiency ofthe main channel due to dredging (Hanes and Barnard, 2007).Similar to other time periods, the crest of the delta retreatedlandward from 1956 to 2005 (Fig. 5).

4.3. SWAN numerical wave modeling

Results from SWAN wave modeling simulations illustrate thatobserved long-term changes to the morphology of the San Fran-cisco ebb-tidal delta have altered wave heights and wave focusing.Changes in modeled significant wave heights from 1873 to 2005(with parameterized forcing of significant wave height (Hs) ¼ 3 m,peak period (Tp) ¼ 12 s, and mean direction (Dp) ¼ 300�, which ischaracteristic of a majority of waves seen in this region) are shownin Fig. 6a. Modeled results show an increase inwave heights of up to0.6 m in the northern part of the delta landward of the crest and in

(b) from 1956 to 2005 (model run with parameterized forcing of Hs ¼ 3 m, Tp ¼ 12 s,n of erosion hot spot. Depth contours from 2005 in meters.

Table 3Average shoreline change rates (m/yr).

Region No. oftransects

Average rate (m/yr) Erosion rate (m/yr) %Erosion(m/yr)

Accretion rate(m/yr)

%Accretion

LT ST LT ST LT ST LT ST LT ST LT ST

San Francisco 64 60 0.2 � 0.2 0.6 � 0.4 �0.1 � 0.2 �0.2 � 0.4 14% 13% 0.3 � 0.2 0.7 � 0.4 86% 87%Ocean Beach 138 141 �0.1 � 0.1 1.5 � 0.6 �0.6 � 0.1 �0.9 � 0.6 48% 32% 0.3 � 0.1 2.6 � 0.6 52% 68%San Mateo 204 184 �0.4 � 0.1 �0.6 � 0.3 �0.5 � 0.1 �0.6 � 0.3 93% 98% 0.2 � 0.1 0.1 � 0.3 7% 2%

Note: LT ¼ long-term (1850e1890s to 2002), ST ¼ short-term (1960se1980s to 2002).

K.L. Dallas, P.L. Barnard / Estuarine, Coastal and Shelf Science 92 (2011) 195e204 201

the vicinity of SF-8, a disposal area for sediments dredged from theship channel since 1971. Wave heights have decreased up to 0.5 min the shipping channel and along the outer crest of the delta. Thesame general pattern of changes in modeled significant waveheights are also seen when forced with waves originating froma more westerly approach (not shown).

Changes in significant wave heights from 1956 to 2005 along thesouthern portion of the ebb-tidal delta adjacent to Ocean Beach areshown in Fig. 6b. Wave heights along the shoreline generally showa decrease in the northern section of the beach and an increase inthe south. This same trend is also seen when forced with typicalwinter storm conditions (Hs ¼ 7 m, Tp ¼ 15 s, Dp ¼ 270�) and showsa 10% increase in wave power from 1956 to 2005 in the section ofbeach experiencing chronic erosion.

4.4. Shoreline change

Shoreline change rates were calculated for long-term (1850s/1890s to 2002) and short-term (1960s/1980s to 2002) time periods.The long-term shoreline change rate in the San Francisco region,averaged along 3.4 km of coastline (where sandy shoreline existed),was found to beþ0.2� 0.2 m/yr, at the limit of error analysis, whilethe short-term average shoreline change rate was þ0.6 � 0.4 m/yr(Table 3). Accretion was observed along a majority of the sandyshoreline during both time periods (Fig. 7).

The long-term average shoreline change rate for the OceanBeach region showed no significant change at �0.1 � 0.1 m/yr,while the short-term average shoreline change rate wasþ1.5 � 0.6 m/yr (Table 3). In both the long-term and short-termtime periods there was a pronounced trend of accretion at thenorth end of the beach and erosion in the south (Fig. 7). In thecentral and southern sections of Ocean Beach shoreline change andsignificant wave height change at the 10 m contour are correlatedfor winter storm conditions from the 1950s to 2000s, with anincrease in wave height linked to shoreline erosion (Fig. 8). Thestatistical correlation for the entire beach is poor, but afterexcluding part of the northern beach sheltered by the ebb-tidaldelta, roughly 50% of the shoreline change can be explained bywave height change.

In the SanMateo region the average long-term shoreline changerate, measured along 10.2 km of sandy coastline, was �0.4 � 0.1 m/yr, while the short-term average shoreline change rate was�0.6 � 0.3 m/yr (Table 3). Erosion was observed at virtually all thetransects along the San Mateo region during both time periods(Fig. 7).

Fig. 7. Shoreline change rates from San Francisco to Pacifica for long-term (1850s/1890s to 2002) and short-term (1960s/1980s to 2002) time periods.

5. Discussion

5.1. San Francisco ebb-tidal delta evolution

The substantial change in morphology of the San Francisco ebb-tidal delta over the past 130 years provides an example of theresponses of an ebb-tidal delta to changes in boundary forcing

conditions. Results show the San Francisco ebb-tidal delta experi-enced periods of both erosion and deposition over time, with a totalnet loss of 100� 52 million m3 of fine- to coarse-grained sand from1873 to 2005. Consistent radial contraction of the crest of the delta,despite overall volume gain between 1900 and 1956, demonstratesa change to one or more of the factors influencing its morphologicevolution, including an increase in wave height, reduction in tidalcurrent strength due to a decrease in San Francisco Bay’s tidalprism, and/or a decrease in sediment supply.

Ebb-tidal delta size decrease associated with increasing waveenergy has been well documented at deltas exposed to differentdegrees of wave action around theworld (Walton and Adams, 1976;Hicks and Hume, 1996). Analysis of the long-term variability ofstorminess (a proxy for coastalwaveactivity) in Central California byBromirski et al. (2003) showed no substantial change since 1858.Similarly, Allan and Komar (2006) showonly a negligible increase inaverage winter coastal wave heights (1.2 � 1.8 cm/yr) in the studyarea since 1980. Recent observations of increased coastal waveheights, though not statistically significant trends, could havecontributed to ebb-tidal delta contraction over the past fewdecades,but cannot account for contraction documented over the entire spanof this study. The negligible short-term trend and absence of anylong-term trend suggest waves are not the main driver of the long-term contraction of the San Francisco ebb-tidal delta.

Fig. 8. Shoreline change rate and modeled significant wave height change at the 10 mcontour (with parameterized forcing of Hs ¼ 7 m, Tp ¼ 15 s, Dp ¼ 270�) along OceanBeach from the 1950s to 2000s.

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A second hypothesis to explain erosion and contraction of theebb-tidal delta is a change to the tidal prism of San Francisco Bay,and thus a relative increase in the balance of wave forcing versustidal currents. A reduction in the tidal prism of San Francisco Bayhas been previously documented due to anthropogenic filling ofshoals, sedimentation within the bay, and diking of tidal marshes(Gilbert, 1917; Keller, 2009). Gilbert (1917) recorded an averagetidal prism of 1.63 billion m3, Conomos (1979) cited a 1931 value of1.59 billionm3, and Keller (2009) cited a current tidal prismvalue of1.48 billion m3. Together, these results indicate a 9% reduction inthe estuary’s tidal prism. Based on published empirical relation-ships that relate the volume of a back basin’s tidal prism to thevolume of sediment in the associated ebb-tidal delta, (Gilbert, 1917;Walton and Adams, 1976; Marino and Mehta, 1987; Hicks andHume, 1996; Fontolan et al., 2007) even a modest decrease in thetidal prism of San Francisco Bay could lead to profound impacts onthe size of the ebb-tidal delta.

A third hypothesis to explain long-term erosion of the ebb-tidaldelta is a decrease in sediment supply. Hydraulic gold miningdebris and damming of rivers that flow into San Francisco Bay incombination with dredging, borrow pit mining, and aggregatemining within the estuary have altered sediment dynamics. Gilbert(1917) estimated that hydraulic gold mining dislodged roughly1.3 billion m3 of material into watersheds that drain into SanFrancisco Bay from 1849 to 1914, and was the primary reason for0.9 billion m3 deposited in the Bay during this time period(Porterfield, 1980). However, Gilbert (1917) estimated that only38 million m3 of sediment made it through the Bay to the ocean,including the San Francisco Bar, and that the signal would wane bythe middle of the 20th century.

High rates of accretion in Suisun Bay (Cappiella et al., 1999) andSan Pablo Bay (Jaffe et al., 1998) from the mid 1800s to 1887 reflecttransport of this material from the mines to San Francisco Bay.Dominant accretion in Central Bay (Fregoso et al., 2008) and on theebb-tidal delta from 1900 to 1950s may also reflect a lag in trans-port of this material farther down the estuary. Erosion of the deltafrom 1956 to 2005 may reflect the decrease in hydraulic miningdebris supplied to the system and a return to an equilibrium state.

Damming of rivers that flow into San Francisco Bay has alsodecreased sediment supply to San Francisco Bay. Wright andSchoellhamer (2004) calculated that the three largest dams inSan Francisco Bay’s watershed have impounded over 80 Mcm ofsediment and, along with other anthropogenic impacts, havecaused a w50% reduction in suspended sediment flux from theSacramento River to San Francisco Bay from 1957 to 2001. Whileit is unknown if the coarser sediment discharged from the

Sacramento River as bedload ultimately settles on the ebb-tidaldelta, the dramatic decline in sediment yield indicates the systemas a whole is receivingmuch less sediment as compared to 50 yearsago. Since the mid-1950s, Suisun Bay, San Pablo Bay, Central Bay,and the San Francisco ebb-tidal delta have all experienced neterosion (Jaffe et al., 1998; Cappiella et al., 1999; Hanes and Barnard,2007; Fregoso et al., 2008; Dallas and Barnard, 2009; Barnard andKvitek, 2010), suggesting a connection between ebb-tidal deltachange and change to sediment influx.

In addition to declining sediment input from the SacramentoRiver, results from this study indicate that a minimum of 200 Mcmof sediment has been removed from the estuary and ebb-tidal deltathrough dredging, aggregate mining, and borrow pit mining. Of thetotal, at least 75 Mcm was fine-grained sand to gravel and iscomparable with grain sizes on the delta. A majority of the coarsesediment (63%) removed was from Central Bay. Barnard and Kvitek(2010) demonstrated that the rate of sediment loss for Central Bayaccelerated three-fold from 1997 to 2008 relative to the rate from1947 to 1979 (Fregoso et al., 2008), with coarse sediment extractionby aggregate mining playing a dominant role. Analysis by Barnardet al. (in press) of over 3000 bedforms in the area coupled witha validated numerical model strongly suggests net seaward-directed bedload sediment transport. Removal of coarse sedimentfrom Central Bay could therefore potentially reduce sedimentsupply to the ebb-tidal delta as well as open coast beaches.

5.2. Nearshore wave height change and shoreline change

Changes in morphology of the ebb-tidal delta have altered waverefraction in the region and impacted wave heights along adjacentOcean Beach. We demonstrate that spatial variation in wave heightchange along Ocean Beach is a result of morphological changes toshoals offshore of these regions. In the north, deposition offshorehas served to protect this section of the beach, dissipating waveenergy as waves break farther offshore. In the south, contraction ofthe delta has resulted in widespread erosion of offshore shoals, andhas consequently left this section of the beach more exposed towave energy. Focusing of waves along southern Ocean Beach due tothe shape of the delta (Barnard et al., 2007; Eshleman et al., 2007)and potential increases in nearshore wave height over the past 50years along the same stretch of beach are likely significant driversof persistent, ongoing erosion in this region.

Shoreline change rates for the area show that a majority of theopen coast shoreline from southern Ocean Beach to Pt. San Pedrohas experienced net erosion since the late 1800s. Shoreline changeresults for the state of California by Hapke et al. (2006) demon-strated that the coastline from Pt. Lobos to Davenport (w80 kmsouth of Pacifica), which includes the Ocean Beach and San Mateoregions covered in this study, has the highest regionally averagedlong-term erosion rate in the state. As San Francisco Bay is a majorcontributor of sediment to the San Mateo coast down to the end ofthe littoral cell at Pt. Pedro, this pervasive erosional trend indicatesthat sediment supply from the Bay to the adjacent coastal regionhas been sharply reduced.

In summary, net long-term erosion of the San Francisco ebb-tidal delta and continued radial contraction of its crest suggestsa change to the tidal currents, waves, and/or sediment supply.A slight increase in wave activity and height may have contributedto erosion of the ebb-tidal delta within the past few decades, butthe absence of any long-term trend suggests changes in waveenergy are not the main driver of long-term delta erosion. Instead,a reduction in the tidal prism of San Francisco Bay has contributedto persistent contraction of the ebb-tidal delta. A decrease insediment supplied to San Francisco Bay and historic and ongoingremoval of coarse sediment from Central Bay may also limit

K.L. Dallas, P.L. Barnard / Estuarine, Coastal and Shelf Science 92 (2011) 195e204 203

sediment supply to the ebb-tidal delta and open coast beaches.Bathymetric change of the ebb-tidal delta has caused an increase inwave height along southern Ocean Beach and may be a significantfactor in the location of an erosional hot spot. Furthermore, accel-erating rates of sea-level rise will require an increased supply ofsediment to maintain the present-day volume and morphology ofthe ebb-tidal delta and the wave sheltering benefits it provides.

This research is the most comprehensive study on long-termbathymetric change of the San Francisco ebb-tidal delta to date, andthe only study relating anthropogenic activities, ebb-tidal evolu-tion, and shoreline change. A long-term, system-wide perspective,as presented here, is an effectiveway to study the connectivity of anestuarineecoastal system. The results of this study can be used asan analog for similar systems world-wide, especially in developingcountries, many of which have been or will be strongly modifiedby anthropogenic influences, including damming of drainages,changes in upland land-use (e.g., urban development, agriculture,over-grazing), and elimination of tidal wetlands by development.Future research and management of these systems needs toconsider sediment transport pathways from the drainages feedingthe estuary out to the open coast, and recognize the cumulativeimpacts of modifications to the sediment supply. With rising sea-level increasing the accommodation space and therefore thedemand for sediment to maintain estuarineecoastal systems intheir current form, the efficient management of sediment resourceswill be essential for preventing additional stresses on thesesystems, many of which are already experiencing loss of tidalmarshes and beaches due to recent limits of the sediment supply.

6. Conclusions

Quantitative analysis of a series of historical and recent bathy-metric surveys of the San Francisco ebb-tidal delta provides infor-mation on its long-term morphologic evolution and the processesdriving the observed change. It is concluded that:

(1) From 1873 to 2005 the San Francisco ebb-tidal delta experi-enced periods of both erosion and deposition, with total netloss of 100 � 52 million m3 of fine- to coarse-grained sand;

(2) A minimum of 200 million m3 of sediment has been perma-nently removed from the system by dredging, aggregatemining, and borrow pit mining. At least 50 million m3 of thistotal was sand or coarser grained material removed fromCentral San Francisco Bay and is comparable with grain sizes onthe ebb-tidal delta;

(3) Changes to the morphology of the San Francisco ebb-tidal deltahave altered alongshore wave energy distribution along adja-cent Ocean Beach. Over the past 50 years wave heights havedecreased along northern Ocean Beach and increased alongsouthern Ocean Beach, and this increase is coincident with thelocation of a beach erosion ‘hot spot’ that has persisted fordecades;

(4) Shoreline change results indicate a majority of the sheltered,sandy shoreline from Crissy Field Beach to northern OceanBeach has been stable or experienced net accretion since thelate 1800s, with an increase in accretion rates since the 1980s.In contrast, a majority of the exposed, open coast beaches fromsouthern Ocean Beach to Pt. San Pedro have experienced neterosion since the late 1800s, with an increase in erosion ratessince the 1960s;

(5) Long-term erosion of the San Francisco ebb-tidal delta andaccelerating rates of shoreline erosion along open coast bea-ches correlate temporally with a reduction in the tidal prism ofSan Francisco Bay and a decrease in coastal sediment supply,both as a result of anthropogenic activities.

Acknowledgments

This research was supported by the U.S. Geological SurveyCoastal and Marine Geology Program, United States Army Corps ofEngineers, San Francisco District, Friends of Long Marine LabScholarship, and the Robert L. Weigel Scholarship for CoastalStudies. We thank Cheryl Hapke, Amy Foxgrover, and 3 anonymousreviewers for their comments andhelp in improving themanuscript.We also thank Theresa Fregoso and the San Francisco Bay Conser-vation and Development Commission for help with this research.

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