DEVELOPMENT OF A PORTABLE SAND TRAP FOR USE IN THE NEARSHORE 



PART I: INTRODUCTION 



Background 



Characteristics of sediment transport 



1. Sediment transport by water is a complex phenomenon that is little 

 understood despite more than 200 years of intense study by engineers and 

 scientists. Riverine sediment transport occurs in predominantly unidirec- 

 tional and slowly varying flows, with the transport rate dependent primarily 

 on flow speed, water depth, turbulence, sediment grain size, and channel mor- 

 phology. Although riverine sediment transport may appear to be a relatively 

 simple problem because the flow is unidirectional, a universally valid 

 predictive formula for sediment transport in rivers does not exist. Instead, 

 specific formulae have been developed for particular rivers. Sediment trans- 

 port in the nearshore -- the narrow zone of the coast in which waves shoal, 

 break, and run up on the beach --is even more complex because of the presence 

 of wave-induced oscillatory flow that acts simultaneously with the unidirec- 

 tional and quasi -steady current moving along the shore (the longshore 

 current) . 



2. In general terms, sediment transport occurs as bed load and suspen- 

 ded load. Bed load is defined as those particles which are supported by the 

 bed and move in continuous or near -continuous contact with the bed by rolling, 

 skipping, or sliding along the bottom. Suspended sediment is that part of the 

 transported material which is supported by the surrounding fluid during its 

 motion (Shen 1971). Many measurements of bed-load and suspended load trans- 

 port rates have been made in the unidirectional flow regime of rivers and 

 laboratory tank facilities. In contrast, few corresponding measurements have 

 been made in the nearshore . 



3. This report is concerned with sediment transport along the coast. 

 Of particular interest is longshore transport in the nearshore zone, since 

 permanent beach change on natural and engineered beaches is primarily con- 

 trolled by the movement of sediment alongshore under prebreaking and breaking 



