c . Acoustic . 



Downing (1981) describes an acoustic bed-load device that 

 detects the impact of bed-load particles. Bubbles and fine 

 materials such as clay and silt do not have sufficient inertia 

 to be counted as they hit the device; therefore only sand-sized 

 material is detected. The device consists of a "needle" par- 

 tially embedded in the bed. The Helley- Smith bed- load sampler 

 described in the riverine pressure-difference section was used 

 to calibrate the acoustic bed - load device in a riverine environ- 

 ment; the correlation coefficient between measured and Helley - 

 Smith sampler predicted rates was 0.898. Use of the device is 

 limited to regions with well-sorted material, grain speeds 

 exceeding . 6 m/sec , and high sediment transport rates . The 

 present study indicates that the Helley- Smith sampler may give 

 unrepresentative high transport rates of sand. 



Total load apparatus 



15. Instruments developed to measure the total load, both the suspended 

 and bed-load transport in the coastal nearshore or surf zone include a pit 

 sampler developed for use in the nearshore zone and the streamer trap. 



a. Pit sampler. 



Anderson (1987) placed a marine sampler approximately 6 ft 2 

 (.56 m 2 ) in the nearshore zone (50-ft depth) off Carmel Point, 

 California. The trap was made of 1-in. (2. 5 -cm) marine grade 

 plywood and divided into sections so that transport from each 

 direction could be distinguished (Figure A17). Suspended load 

 was not expected to travel more than 3 ft, and could, in prin- 

 ciple, be differentiated from bed-load material by the section 

 in which it settled. Divers measured the quantities of col- 

 lected sediment. In use of the trap, eddies on the leading edge 

 of the interior of the box tended to resuspend sediment that had 

 already settled. Significant flow disturbance was not observed 

 except around the corners of the trap, where scour occurred. 

 The pit trap became the abode for many animals and began growing 

 seaweed by the third week of installation. In practice," deposi- 

 tion of suspended material could not be distinguished from 

 deposition of bed load. 



b. Streamer trap. 



The streamer trap consists of vertically mounted mesh collection 

 bags (streamers) attached to nozzles mounted in a metal frame 

 (see Figure 2 in Part I) and is the subject of the main portion 

 of this report. The bottommost streamer rests on the bed and 

 collects both bed and suspended load, whereas, the higher-eleva- 

 tion streamers collect suspended material. Integration of the 

 vertical distribution of sediment transport yields the total 

 longshore sediment transport in the water column. The streamer 

 trap is discussed in detail in Part II. 



A3 2 



