and the overbar and prime denote time average and instantaneous fluctuation 

 from the average, respectively. 



By assuming that suspended sand moves at approximately the same 

 horizontal velocity as the water, temporal characteristics of the fluctuating 

 portion of the transport can be examined through the application of the co- 

 spectrum (Huntley and Hanes 1987), as shown schematically in Figure 10-1. 



1 





Fluctuating 

 Component of 

 Cross-shore 

 Suspended 

 Transport 



at co = co 1 



Qt (col) 



Integration 



over 



Frequency 







Fluctuating 



Portion of 



Net 



Cross-shore 



Suspended 



Transport 



Qf=CV r 



(qs=Uc1 Steady Portion of Net Cross-shore Suspended Transport 



Net 



Cross-shore 

 Suspended 

 Transport 



Q=Qs+Qf 



Figure 10-1 . Summary of cross-shore suspended transport 



Objectives 



The purpose of the intermittent near-bed sediment suspension measurement 

 at the SUPERTANK Laboratory Data Collection Project was to study small- 

 scale sediment dynamics under controlled laboratory conditions at prototype 

 scale. The fluid-sediment interaction which occurs in large-scale laboratory 

 tests probably embodies much of the same physics as the small-scale fluid- 

 sediment interaction in the field. Laboratory conditions provide a degree of 

 control and reproducibility which is not possible in the field. 



Emphasis is on measurement of the small-scale, short-time response of 

 suspended sand to waves. Specific phenomena to be studied are the vertical 

 and temporal structures of suspension events, the relation of suspension events 

 to cross-shore sediment transport, and the reference concentration near the 

 bed. The acquired data will be used to evaluate models of reference 

 concentration as well as to quantify the significance of intermittent suspension 

 to cross-shore transport of sand. 



186 



Chapter 10 Intermittent Near-Bed Sediment Suspension 



