relate these to sediment properties and bed form types; and (c) to deter- 

 mine what the longshore current distributions are on both sides of the 

 breaker zone. All of these relate to the need for understanding coastal 

 sediment transport. 



Several models have been proposed in the past for the distribution of 

 longshore currents, the causes of current generation, and the resulting 

 littoral drift. However, systematic studies for testing these models in 

 the field have not been carried out, even for simple topographies, mono- 

 chromatic waves, and unobstructed coastlines. Verification of the shore- 

 normal distribution of longshore currents is important, the studies to 

 begin under the simplest environmental conditions, progressing to include 

 complex bathymetry, tides, and low-frequency components of general 

 circulation, to application of the modified models to the design, evalua- 

 tion, and maintenance of coastal structures. 



Since numerical models of coastal currents are currently based on 

 mean velocities, the vertical distribution of currents combined with their 

 lateral variability is unknown. First-order importance in sediment trans- 

 port studies is knowing the structure of the velocity field and being 

 able to extrapolate profiles to the bottom, especially where entrainment 

 is concerned as boundary layer development and the boundary where dis- 

 tribution governs the motion of sediments. Therefore, experiments must 

 be designed to obtain current data near the bottom. 



In surface current measurements it is necessary to keep a constant 

 distance between the reference water level (mean lower low water (MLLW) 

 where tides are present) and the point at which currents and waves are 

 measured (Fig. 28a). If the nearshore slope is uniform, all sensors can 

 be arranged to move downward only and parallel to the mast at constant 

 increments. Although this operation is not difficult, it may require 

 extended periods of underwater work to adjust spars and realine meters 

 into the three coordinates during which the main features of the nearshore 

 circulation system (wave, wind, and current directions) can change. 



In the study of sediment entrainment, transport, bed form generation 

 and maintenance, and bottom friction, the flow of water near the bottom 

 must be measured (Fig. 28b). For this measurement (but not exclusively) 

 at least three points of flow readings in the vertical are needed to 

 define the velocity gradient and in turn imply the form of the boundary 

 layer, and the distribution of boundary shear. This is a relatively 

 straightforward procedure when operating on gentle, uniform slopes; how- 

 ever, both steep or barred offshore profiles can complicate the design of 

 an experimental grid. The problem in data analysis of such a scheme is 

 the evaluation of the variable pressure-response factor resulting from the 

 continuously changing depth at the wave gage. Since equidistant or "equi- 

 depth" spacing of the sled positions usually generates data gaps (Fig. 28c), 

 it is more advantageous to survey the profiles beforehand and use the fa- 

 thometer profile to determine positions for the sled; i.e., on bar crests, 

 bar troughs, or midpoints on bar slopes where data will contain similarities 



50 



