The material used for beach replenishment was dredged from the San Diego 

 Bay. The median grain size was 120 micrometers (Muslin, 1978) . The 

 average composition of the dredged material consisted of 70 to 85- 

 percent sand, 5- to 15-percent silts and clays, and 5- to 15-percent 

 shell material (App. A). The composition of Imperial Beach before the 

 dredge disposal was 95- to 99-percent sand with only small amounts of 

 shell and silt. 



Beach replenishment started at rock groin No. 1 (Fig. 1) and 

 proceeded slowly south at an average rate of 15 meters per day, and 

 eventually terminated 1.36 kilometers south near station B. Conse- 

 quently, the full impact of beach dredge disposal occurred at different 

 times at the sampling stations. For example, during survey II (6 April 

 1977) the 3.7-meter offshore stations were sampled 15 days (137,700 

 cubic meters deposited) after the start of beach replenishment. Thus, 

 the 3. 7- cubic meter depth at station A had been affected by dredge 

 disposal (Fig. 1) while there was no discernible dredging impact at the 

 3.7-meter depth at station B, 1.19 kilometers south of station A. 



The ^material deposited on Imperial Beach averaged approximately 10- 

 percent silts and clays. Most of this fraction was rapidly washed 

 offshore either during the sediment deposition or on exposure to wave 

 regimes. Therefore, it is estimated that approximately 76,500 cubic 

 meters of the material applied to the beach was rapidly transported into 

 the sublittoral zone. An estimate of the area impacted by these fine 

 sediments (length of disposal area = 1.34 kilometers times distance 

 offshore to the 9.2-meter isobath) is 1.25 x 10^ square meters. This 

 area, if covered evenly by 76,500 cubic meters of sediment, would be 

 buried under 6.1 centimeters of silts and clays. This is consistent 

 with observations made during the beach disposal period (survey III, 2 

 June 1977) which found 2 to 6 centimeters of silt in the upper part of 

 the 10-centimeter-deep cores from 6.1 meters of water. This layer of 

 silt appeared responsible for the burial not only of infauna but also 

 larger macrofauna such as the sand dollar, Dendraster exoentricus . 



IV. PHYSICAL AND CHEMICAL RESULTS 



1 . Beach Topography . 



Profiles of the beach are biologically important because from them 

 estimates of sediment gain or loss (sediment stability) can be ascer- 

 tained. Additionally, significant features (e.g., surge channels and 

 sandbars), which partition the intertidal area into different habitats, 

 can be quantified. Sandbars create low-energy zones while surge chan- 

 nels generally represent areas of rapid water and sediment movement. 



The approximate beach profiles of the three intertidal stations are 

 shown in Figure 4. These profiles show the extent of beach 



28 



