coarse and fine, angular and rounded, and light and heavy grains leads to 

 grading. Field visits to a locality are often repeated to assess temporal 

 cvariability of these phenomena. Simultaneous measurement of energy 

 processes such as current and waves is often required for understanding the 

 rates and mechanisms of movement. 



Measurement of sediment movement 



The measurement of suspended and bed-load sediment movement in the 

 surf zone is an exceedingly difficult process. There are a variety of sampling 

 devices available for measuring suspended and bed-load transport in the field 

 (Dugdale 1981; Seymour 1989), but these devices have not performed 

 properly under some conditions or have been expensive and difficult to use. 

 For these reasons, new sampling procedures are being developed and tested at 

 CERC and other laboratories. Point measurements of sediment movement can 

 be performed by two general procedures: 



* Direct sampling and weighing of a quantity of material. 



• Detection of the fluid flow by electro-optical or acoustic instruments 

 deployed in the water. 



Two general methods are available to directly sample the sediment in 

 suspension and in bed load. First, water can be collected in hand-held bottles 

 or can be remotely sucked into containers with siphons or pump apparatus. 

 The samples are then dried and weighed. The second method is to trap a 

 representative quantity of the sediment with a mesh or screen trap through 

 which the water is allowed to flow for a fixed time. A fundamental problem 

 shared by both methods is the question of whether the samples are truly 

 representative of the sediment in transport. For example, how close to the 

 seabed must the orifice be to sample bed load? If it is high enough to avoid 

 moving bed forms, will it miss some of the bed load? Streamer traps made 

 from mesh are inexpensive to build but difficult to use. The mesh must be 

 small enough to trap most of the sediment but must allow water to flow 

 freely. Kraus (1987) deployed streamers at Duck, NC, from stainless steel 

 wire frames (Figure 21). Kraus and Dean (1987) obtained the distribution of 

 longshore sand transport using sediment traps. A fundamental limitation of 

 traps is that they can usually only be used in mild conditions. In winter and 

 during storms it is too hazardous for the field technicians to maintain the 

 equipment. Perversely, it is under these harsher conditions when the greatest 

 sediment movement occurs. Another fundamental problem is relating the 

 instantaneous measured suspended and bed-load transport to long-term sedi- 

 ment movement. Because of the extreme difficulty of conducting research in 

 the surf zone, answers to these questions remain elusive. 



Chapter 3 Field Data Collection and Observation 



47 



