175
Pre-Test Weight
Post-Test Weight
150
Side Rubber Cleaned
125
All Rubber Cleaned
100
75
50
25
0
-2
1
3
5
7
9
11 13
15
17
19
Sediment Trap No.
Figure 52. Average sediment trap weights for Test1H_Case2, showing the
amount of sand that had accumulated on the neoprene rubber seals
The third issue related to measuring high quality LST data is the observation
that, in some cases, a very small amount of sand tends to by-pass some of the
sediment traps, even though Sediment Trap No. 1 to 17 and 19 are 6.0 m in
length. This was a concern during the design phase, and every feasible step was
taken to maximize the length of the traps. Most importantly, the beach was
constructed as far upstream as possible to maximize the length of the downstream
flow channels and sediment traps in an effort to provide sufficient time for the
suspended sediment to fall out of suspension. Nonetheless, for wave conditions
that generate a strong longshore current magnitude near the peak of the
distribution, small amounts of sand tend to bypass the traps where the longshore
current magnitude is strongest. It should be mentioned that sand bypassing is not
a problem where the longshore current magnitude is weaker, for example, farther
offshore near the tail of the longshore current distribution. To quantify the
magnitude of this problem, several streamer traps (similar to those used by Rosati
and Kraus 1989) were installed above the downstream end of the traps during
several test segments. The objectives were to quantify the vertical distribution of
sediment flux and to integrate this distribution across the 0.75-m width of the
flow channel to estimate the total sediment flux past the trap. After collecting
data during several test segments, results showed that the maximum quantity of
sand bypassing the traps was 1.5 percent of the quantity collected in the sediment
trap, which only occurred in the three flow channels with the strongest longshore
current magnitude. Because it would be very time consuming to perform these
measurements during every experiment, the decision was made to accept this as a
potential error in the sediment trap data sets. It should be mentioned that this
only occurs near the peak of the longshore current distribution and for
96
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
Previous Page
