maximum submerged weight of sand that each trap can hold. In this report, the
term "submerged weight" will be used rather than "mass," because this is the
quantity the traps measure. These calculations assume that the specific gravity of
quartz sand is 2.65, and that the in situ bulk porosity of the sand in the trap is 0.4.
Therefore, the buoyant unit weight of the sand in the traps is 1,122-kg/m3. As
shown in Figure 20, the height of the traps decreases in the offshore direction.
This is dictated by the fact that the elevation of the beach decreases in the
offshore direction. One of the design criteria was to select the height of each trap
so that the brim elevation of the trap is slightly lower than the elevation of the
concrete beach. Therefore, in an extreme event, if a portion of the moveable bed
eroded close to the elevation of the concrete, sediment could be transported into
the traps and not be obstructed. This decision also makes it possible to remove
the entire sand beach in the future to conduct additional fixed-bed experiments
without the traps obstructing the longshore current. In addition, to simplify
construction and assembly, the trap height decreases in sets of two or three traps.
Therefore, there are only 10 different sizes. This also allows mechanical
components on adjacent traps to be temporarily interchanged, if in need of repair.
Table 2
Dimensions and Capacities of the 20 Sediment Traps
Maximum
Submerged
Inside
Inside
Inside
Volumetric
Weight of
X-Loc.
Height
Length
Width
Capacity
Sand
m3
Trap No.
m
m
m
m
kg
-2
1.875
0.55
2.44
0.50
0.67
749
-1
2.625
0.55
2.44
0.50
0.67
749
1
3.375
0.50
6.10
0.56
1.71
1917
2
4.125
0.50
6.10
0.56
1.71
1917
3
4.875
0.46
6.10
0.56
1.58
1772
4
5.625
0.46
6.10
0.56
1.58
1772
5
6.375
0.37
6.10
0.56
1.28
1432
6
7.125
0.37
6.10
0.56
1.28
1432
7
7.875
0.37
6.10
0.56
1.28
1432
8
8.625
0.32
6.10
0.56
1.10
1238
9
9.375
0.32
6.10
0.56
1.10
1238
10
10.125
0.32
6.10
0.56
1.10
1238
11
10.875
0.27
6.10
0.56
0.93
1044
12
11.625
0.27
6.10
0.56
0.93
1044
13
12.375
0.25
6.10
0.56
0.84
947
14
13.125
0.20
6.10
0.56
0.67
752
15
13.875
0.20
6.10
0.56
0.67
752
16
14.625
0.15
6.10
0.56
0.52
582
17
15.375
0.15
6.10
0.56
0.52
582
18
16.125
No Trap
19
16.875
0.11
6.10
0.56
0.39
437
20
17.625
No Trap
Trap support frames and degrees of freedom
A pair of traps is supported by two mounting frames, which are made of
rectangular galvanized steel tubing. Each frame spans two flow channels and
supports two traps, one frame at the upstream end of the trap and one at the
downstream end, as shown in Figure 21. The support frames are bolted to the
concrete floor of the wave basin. Each trap is suspended at two points near the
upstream corners of the trap, where most of the sand accumulates, and one
37
Chapter 4
Sediment Trapping and Dredging Systems
Previous Page
