Transportation 



241 



shore. Waves at the outer plunge zone were 

 75 cm high and at the inner plunge zone 25 

 cm during the sampUng period. The results 

 (Fig. 200) show an increase from about 75 

 grams of sediment per liter of water at the 

 shore to about 0.004 grams/liter at the water 

 surface 55 meters seaward. Median diame- 

 ters also decreased outward from 185 to 160 

 microns. Sediment was more concentrated 

 and coarser near the bottom than higher 

 above it. A similar vertical variation was 

 found by Inman (1949^?) who placed multi- 

 sock sediment traps in 1 to 4 meters of water 

 near the surf zone off Scripps Institution of 

 Oceanography. The socks 10 cm above the 

 bottom caught 2 to 20 times as much as those 

 at 30 cm, and 5 to 50 times as much as those 

 at 78 cm, with the amount and vertical dis- 

 tribution of sand dependent on the size and 

 period of the waves. Another multisock 

 sediment trap placed in 1.4 meters of water 

 off Huntington Beach by Terry (1951) col- 

 lected 5 times as much sediment in a sock 

 near the bottom as at 1.2 meters above it, 

 and the median diameters decreased regu- 

 larly from 180 microns in the bottom sock 

 to 130 in the top one. Measurements of 

 suspended sediment along a pier at Mission 

 Beach near San Diego were made by Watts 

 (1953), who used a submersible electric pump 

 that forced measured volumes of water 

 through a sediment filter. The results again 

 showed a dependence of the amount and 



DISTANCE FROM SHORE -METERS 



Figure 200. Concentration and grain size of suspended 

 sediment in surf zone off Redondo Beach. Adapted from 

 Marlette (1954). 



grain size of suspended sediment on the wave 

 height and water depth. According to 

 Watt's computations the wave-induced long- 

 shore current is capable of transporting sev- 

 eral hundred thousand cubic meters of sand 

 past the pier annually. Stirring of sediments 

 in the surf zone is of course the result of high 

 turbulence there. According to orbital cur- 

 rent meter measurements by Inman and 

 Nasu (1956), these velocities are as much as 

 100 times the settling velocity of sand grains 

 present on the floor of that zone. 



Work on suspended sediment was carried 

 farther seaward by Gorsline (1954) by sam- 

 phng from several depths using a pump and 

 hose lowered from a boat to bring water to 

 the surface where the sediment was allowed 

 to settle out. At a station in 7.5 meters of 

 water 1700 meters off Seal Beach, he found 

 at depths of 7, 5, and 2 meters concentra- 

 tions of 0.24, 0.12, and 0.05 grams/liter and 

 median diameters of 70, 32, and 15 microns. 

 The sediment near the surface was finer than 

 that flooring the shelf between the samphng 

 site and the shelf break, so we might con- 

 clude that suspended sediment in apprecia- 

 ble quantities is able to make its way across 

 the shelf to the basin beyond. Supporting 

 evidence is provided by reports by divers of 

 a kind of haze near the bottom in shelf areas. 

 In addition to traveling across the shelf-break 

 to deep water, some of the suspended sedi- 

 ment slowly makes its way down submarine 

 canyons. A sediment trap left in La Jolla 

 Canyons for 2 months (Revelle and Shepard, 

 1939) was covered with about 2 mm of sedi- 

 ment that graded upward from 75 microns 

 for median diameter in a bottom tray to 41 

 microns in a top one, in contrast to 110 

 microns for the underlying sediment surface. 



The fact that diffused sediments reach 

 deep water by crossing the shelves or mov- 

 ing down canyons is shown by the discovery 

 of fine silt particles in water samples taken 

 in Nansen bottles between the water surface 

 and a depth of 300 meters in surveys made 

 in October and December 1957 (Gunnerson, 

 1957 b). Concentrations at the north side of 

 San Pedro Basin were found to be as great 

 as 200 grains/ml decreasing to 20 grains/ml 

 farther offshore. Commonest among the 



