2005 chenault and griggs - quantifying long-term beach width change in the oceanside littoral cell,...

8/3/2019 2005 Chenault and Griggs - Quantifying Long-term Beach Width Change in the Oceanside Littoral Cell, California

http://slidepdf.com/reader/full/2005-chenault-and-griggs-quantifying-long-term-beach-width-change-in-the 1/5

Proceedings of the 14th

Biennial Coastal Zone Conference

New Orleans, Louisiana

July 17 to 21, 2005

1

QUANTIFYING LONG-TERM BEACH WIDTH CHANGE IN THE

OCEANSIDE LITTORAL CELL, CALIFORNIA

Carla Chenault, University of California at Santa Cruz, Earth Science Department

Gary B. Griggs, University of California at Santa Cruz, Earth Science Department and Institute of Marine Sciences

Keywords: aerial photography, Oceanside, San Diego, shoreline change, beach mapping,beach change, littoral cells

INTRODUCTION

The Oceanside Littoral Cell contains some of the most widely used beaches in southernOrange and northern San Diego Counties. From 1940 to 1980 California’s population

exploded from 6.9 to 23.7 million. During this population boom California was in a

climatically calm La Niña dominated period characterized by moderate storm events.

Consequently, coastal development flourished as houses and roads were built perilouslyclose to the shore (Figure 1). The succeeding 20 years, from 1978 to 1998, saw a climate

shift to El Niño conditions dominated by more frequent winter storms, heavier rainfall,elevated sea levels, and larger waves. With this shift, coastal erosion and protection for

property owners and public infrastructure in the coastal zone became an issue at the

forefront of public agendas.

Figure 1. Houses perched on cliff edge, Solana Beach (californiacoastline.org)

It is commonly asserted that Southern California beaches are eroding and that funding isneeded for beach nourishment, but as yet, no study has quantified a significant reduction

in beach width for any Oceanside Cell beaches. Our goal is to determine the extent of beach width change over the last seventy years and attempt to correlate these changes to

fluctuations in climate or reductions in beach sand delivery to the littoral system.

Quantifying beach width change will allow us to better understand the causes of shorelineerosion and enable coastal managers to make informed decisions locating any future

beach nourishment projects.






July 17 to 21, 2005

2

BACKGROUND

The Oceanside Cell stretches 48 miles from Dana Point to the Scripps and La JollaSubmarine Canyons (Figure 2). The primary sources of littoral sediment to the Oceanside

Cell are: fluvial contributions from coastal rivers and streams, sea cliff and gully andupland terrace erosion, and artificial beach nourishment. Net sediment transport is to thesouth where sediment enters the head of submarine canyons and is lost from the system.

Figure 2. Oceanside Littoral Cell location map

San Juan Creek, San Mateo Creek, Santa Margarita River, and San Luis Rey River

supply fluvial sediment to the Oceanside Cell. Sediment supply from coastal rivers and

streams is extremely episodic and directly related to climatic events (Inman and Jenkins,

1999). Under natural conditions sediment delivery from rivers averages approximately288,000 yds3 /yr. As a result of the construction of dams, the actual sediment delivery by

rivers is approximately 113,000 yds3 /yr, a reduction of 55% annually. Dams currently

control 44% of the drainage area of the Oceanside Cell, and 28 miles of the 48 miles of shoreline are affected by the sediment reduction (Willis and Griggs, 2003).

Seventy-three percent of Oceanside Littoral Cell beaches are backed by eroding sea cliffsranging in height from 25 to 100 feet except in the Torrey Pines area where sea cliffs

reach heights of 300 ft (Runyan and Griggs, 2002). The sea cliffs consist mostly of two

units: a more resistant Eocene bedrock, composed of sedimentary rocks ranging from

mudstone to sandstone to conglomerate, and a capping unit of unconsolidated Pleistocenemarine terrace sands.

Benumof and Griggs (1999) found annual cliff erosion rates in this littoral cell to varyfrom four to eight inches depending on bedrock type, rock strength, wave climate, and






July 17 to 21, 2005

3

terrestrial processes. Depending on local uplift patterns, sediments of various

composition and hardness are subject to erosion. Runyan and Griggs (2003) found

armoring of sea cliffs in the Oceanside Cell has reduced sediment supply to beaches by18% from a natural contribution of approximately 67,000 yds3 /year to approximately

55,000 yds

3

/year with the current level of coastal armor. Kuhn and Sheppard (1984) alsofound sand from gully and upland terrace erosion is significant to the sediment budget inthe Oceanside Cell, which Runyan and Griggs (2002) reported as contributing

approximately 240,000 yds3 /year.

A significant amount of additional sediment has been added to Oceanside beachesthrough various beach nourishment projects with sediment dredged from offshore and

from harbor construction and maintenance. Between 1940 and 1990, an average of

248,500 yds3 /year of sand was artificially deposited on Oceanside Cell beaches, a total of

almost 12.5 million yds3. The rate of artificial sediment supply has decreased over the last

30 years since construction was completed on the harbors (Wiegel, 1994; Flick, 1993). A

large effort was made in 2001 with the addition of 1 million yds3of sediment by a local

government beach restoration project (Coastal Frontiers, 2002).

METHODS

We selected 15 beaches throughout the littoral cell to represent the range of beach

settings. The beaches selected encompass the possible variations in back beach type,

coastal armoring, position within sub-cells, and lithology of sea cliffs. These beaches

span approximately 20 miles of the 48-mile long cell.

We are using aerial photographs taken between 1938 and 2001, obtained from a varietyof sources including University, public agency, and private collections. The photographs

being used are at 5 to 10 year intervals and taken during the months of June through

October to minimize a signal from seasonal beach width fluctuations.

Erdas Imagine software is being used to georectify the photographs with USGS Digital

Ortho Quarter Quadrangles (DOQQ) for ground control. We will use the DigitalShoreline Analysis System (DSAS) extension in ArcView to measure beach width and

calculate beach width change. In this program a back beach feature is delineated on each

set of photos as well as the wet/dry line on the beach and then transects are cast tomeasure the distance between the two. This process is repeated for each set of photos and

then beach widths can be compared for each of the years.

PRELIMINARY RESULTS

Anecdotal accounts suggest that the beaches of the Oceanside Cell have narrowed

significantly over the last several decades and especially since the storms associated with

the 1982-1983 El Niño event. We would expect to see this response based on thedocumented sediment reductions to the littoral system. In the large portions of the cell

where the back beach location is fixed by seawalls or revetments we would also expect

passive erosion to have resulted in beach narrowing. However, results from severalearlier studies conducted in this area have found beach width change to be minimal

(Welin and Flick, 2004 and U.S. Army Corps of Engineers, 1991).






July 17 to 21, 2005

4

The U.S. Army Corps of Engineers Storm and Tidal Wave Study (1991) surveyed the

Oceanside Cell beaches from Dana Point to Point La Jolla using historical aerial photosand historical beach profiles taken between 1940 and 1989. Their comparison showed

that most areas have maintained a stable beach width. The most significant change wasnarrowing of the beaches south of Oceanside Harbor by as much as 35 meters between1980 and 1989. They also found the beaches between Oceanside and Cardiff and those at

Dana Point narrowed during the same time period. The beaches of Camp Pendleton and

those between Cardiff and La Jolla widened during the 1980 to 1989 period. It should be

noted that not all photographs and profiles used in this study were from the same time of year and thus results may reflect seasonal oscillations in beach width.

Welin and Flick (2004) compared 855 beach profiles from 26 ranges taken in northernSan Diego County. The profiles were taken between 1934 and 2002 but for many

transects, profiling began following the 1982-1983 El Niño event. Overall, 6 transects

showed an average negative trend while the remaining 20 transects showed a positivetrend, with annual change ranging from –1.27 m/yr to 2.97 m/yr. All but one of the

profiles showing positive trends include only the years since 1983 and thus may reflect

recovery from the large storms of the 1982-1983.

CONCLUSIONS

The assertion is commonly made that the beaches of Oceanside Littoral Cell are eroding.

On many beaches observable changes have been noted over the past few decades (Kuhnand Shepard, 1984). However, preliminary studies by the U.S. Army Corps of Engineers

(1991) and by Welin and Flick (2004) showed little change in beach width throughout theperiod of their studies. Quantifying the extent of change since the 1930s will help us gain

a better understanding of the impacts of human alterations to the sediment budget through

harbor construction, damming, and beach nourishment and also of climatic events such asEl Niño and La Niña. Additionally, understanding which beaches have narrowed will

enable coastal managers decide where tax dollars will be best spent for future

nourishment projects.

ACKNOWLEDGEMENTS

We would like to thank the UC Marine Council for funding this work through theCalifornia Coastal Environmental Quality Initiative. Thank you also to the CSBPA

Wiegel Award and the Friends of Long Marine Lab.

LITERATURE CITED

Benumof, B.T., G.B. Griggs (1999). The Dependence of Seacliff Erosion Rates on Cliff

Material Properties and Physical Processes: San Diego County, California. Shore

& Beach. 67:29-41.Coastal Frontiers (2002). SANDAG Regional Beach Monitoring Report, Spring 2002

State of the Coast Report. San Diego Association of Governments. 37 p.

Flick, R.E. (1993). The myth and reality of Southern California beaches. Shore & Beach61:3-13.

2005 chenault and griggs - quantifying long-term beach width change in the oceanside littoral cell,...

Documents