Trap Operation 



Longshore sand transport sampling 



18. The streamer trap has been used in the surf zone under significant 

 breaking wave heights exceeding 1 m and average longshore currents up to 



0.60 m/sec . The trap can be used in these conditions because scour around the 

 rack legs and bottom nozzle is usually relatively minor for short sampling 

 intervals, and the trap and its operator can remain stable and in the same 

 location during sampling. 



19. Prior to deployment of the streamer trap, streamers are secured on 

 the rack at selected elevations . For operation in average water depths on the 

 order of 1 m, typically five streamers have been used per trap. The nylon 

 string fastening the end of each streamer is checked to ensure that it is 

 tight and secure. Each trap is carried by one or two operators to a specified 

 position in the surf zone. Vertical poles previously placed across the surf 

 zone assist trap operators in accurate positioning (Figure 5). The traps are 

 held out of the water until sampling begins. 



20. At the start of the collection period, the traps are simultaneously 

 thrust into the sea bottom, and the operators secure the traps by rocking the 

 rack and pushing the rack legs into the bed by standing on the horizontal bars 

 located at the bottom. When the bars reach the seabed, preventing further 

 penetration, the bottom edge of the lowest nozzle should lie on the bed. Trap 

 operators sometimes use dive masks to ensure that the bottom edge of the 

 lowest nozzle is flush with the bed and to monitor development of scour. 

 During the collection period of typically 5 to 10 min, the trap operator 

 stands near the trap on the down-current side, thereby avoiding any collection 

 of operator- induced suspended sand. A 5- to 10-min collection interval is 

 recommended to avoid excessive filling of the streamer and scour at the bed. 

 Trap operators are instructed to note excessive scour and report this condi- 

 tion after the collection period as an indication of data quality. High waves 

 passing through the surf zone may tip the trap. If tipping occurs, the trap 

 operator immediately stands on the rack bars, righting the rack and again 

 aligning the bottommost nozzle with the bed. The streamer trap is usually 

 more stable in the surf zone environment than a human. In weak longshore 



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