DEVELOPMENT OP A SUSPENDED SEDIMENT SAMPLER FOR 



LABORATORY USE UNDER WAVE ACTION 



by 



John C. Fairchild, Hydraulic Engineer 



Research Division, Beach Erosion Board, Corps of Engineers 



INTRODUCTION 



General . Studies of sand (littoral) movement in the recently com- 

 pleted Shore Processes Test Basin of the Beach Erosion Board will 

 eventually require the quantitative measurement of the amount of sand 

 thrown in suspension by wave action. On an open coast, this sand would 

 be available as suspended load for transport by the longshore currents 

 developed by the wave action. Pump-type samplers have heretofore been used 

 successfully in the field under conditions of oscillatory flow and to 

 some extent in the laboratory. It was felt that this type of sampler 

 probably offered the best chance of successful operation, and certain 

 exploratory tests were carried out to determine the best working pro- 

 cedures, and to establish some idea of the degree of error and re- 

 producibility to be expected. 



The accuracy and efficiency of numerous types of samplers for use 

 in unidirectional stream flow have received extensive study and 

 investigation^' and, in general, their use in river flow is considered 

 quite satisfactory. For efficient operation however the intake nozzle 

 should be oriented directly into the stream flow. Such orientation in 

 oscillatory flow is, of course, impossible due to the reversible flow 

 accompanying the wave action which causes the sand to be intermittently 

 in motion first in one direction and then in the other. 



Review of the literature treating sediment suspension under wave 

 action revealed no direct method whereby the tr,ue suspension at a point 

 can be determined; however, a report by Watts presents the results 

 of the laboratory development and field testing of a pump-type suspended 

 sediment sampler for use in (full scale, ocean) wave action. The report 

 gives a sampling procedure designed to collect a suspended sample re- 

 presenting the average concentration over a number of wave cycles. 



Due to the absence of any reliable method applicable to wave action 

 for determining the instantaneous concentrations at a point and the ex- 

 pected complexity plus added cost of considering an instantaneous 

 sampler, it was decided to collect the fluid-sediment mixture over some 

 measured period of time as had been done in the field sampling. This 

 would give a concentration value averaged over a number of wave cycles. 

 It was felt that such long-time samples would minimize the variance 

 between successive values and therefore more accurately reflect the 

 average concentration. The greater variance for a short interval sample 

 results from the erratic nature of the turbulent forces influencing the 

 concentration; by sampling over several wave cycles, however, the effects 



41 

 ♦Numbers in parentheses refer to references at end of report. 



