length of the trap (a very extreme case). As illustrated in the figure, the
maximum capacity of each trap is limited by the maximum submerged weight of
sand that the trap can hold, based on the volume of the trap, rather than the rated
capacities of the load cells. Selection of the size of load cells used on each trap
was made to maximize the accuracy of the measurements, while at the same time
protecting the load cells from the possibility of over loading.
2200
Total capacity of three
2000
load cells
1800
Maximum submerged
1600
weight of sand based
1400
on volume of trap
1200
1000
800
600
400
200
0
-2 -1 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
Sediment Trap No.
Figure 23. Design capacity of the sediment traps in terms of the total load cell
capacity and the maximum submerged weight of sand that each trap
can hold based on the volumetric capacity of each trap
Sealing the perimeter of the traps
To prevent sand from filling in the gap beneath a trap, each trap is sealed on
the upstream edge and on the two side walls using neoprene rubber sheeting, as
shown previously in Figure 22. The seal between a flow channel side wall and
the inside wall of the trap is made by compressing the rubber sheet between the
wall and an aluminum clamping bar and then bolting the clamping bar to the
wall. The sheet is clamped on both the trap and flow channel walls. At the
upstream end of the traps, a single rubber sheet was laid along the entire width of
the concrete beach to form the upstream seal for all the traps. The sheet was slit
at each flow channel wall and slid into place. Within each trap, this rubber sheet
was clamped to the upstream edge of the trap in the same manner that the side-
walls were sealed. The other end of the sheet was compressed between the
concrete beach and the stack of bricks used to build the adjustable boundary at
the downstream end of the beach. The sand beach was back-filled against the
bricks to cover any exposed rubber sheeting which extended beyond the
upstream end of the brick boundary. A sand-tight seal was created between the
40
Chapter 4
Sediment Trapping and Dredging Systems
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