be considered in deriving estimates of the LST rate. The maximum error
associated with sand that bypasses the trap is about 2.5 percent, but this
magnitude of error occurs only in flow channels with the highest longshore
transport rates (high longshore currents and high levels of sand entrainment).
Measurements to quantify this error were made under some of the most energetic
wave and current conditions that are to be generated in the facility. In most
channels, especially those in the vicinity of the shoreline, the error is much less
than this value. Therefore, sand bypassing should only have minimal influence
on the estimates of LST. The last two issues are discussed in more detail here,
because they are potentially the largest sources of error in the measurements of
trap weight, and consequently, estimates of LST that are made based on the
weight of sand that accumulates in the traps.
Sand accumulates on the neoprene rubber seals around the periphery of the
sand traps. Figure 91 provides a representative example of the magnitude of
these quantities, as a percentage of the total weight that accumulated in the traps,
for the plunging-breaker case. Two quantities are shown: one represents sand
that accumulated on the side seals, and one reflects sand that accumulated on the
front, or updrift, seal. Generally, the quantity of sand that deposits on the updrift
seal greatly exceeds that which accumulates on the side seals. However, for a
few of the traps shown in the figure, the two quantities are comparable. The
updrift seals do not have uniform surface areas. Generally, the total quantity that
accumulates on all rubber seals is about 3 to 12 percent of the total that actually
settles into the trap. As seen in the figure, the percentage of sand accumulating on
the seals can approach 15 to 20 percent in individual traps. Results shown in
Figure 91 are similar to those observed for the spilling breaker case (see
Figure 52).
Earlier in this chapter, the anomalous 3-D beach evolution that occurred at
the downdrift boundary was discussed. These anomalies must be accounted for,
as corrections to the measured trap weights, to maximize the accuracy of
computed LST rates. The extent and magnitude of the anomalies change with
cross-shore position; therefore, the magnitude of the corrections varies with
cross-shore position. In general, anomalies were restricted to the region of beach
within 1 to 3 m of the downdrift boundary. The method used to correct trap
weights is illustrated using Figure 92, and described below.
Figure 92 shows the total volume change on each of the beach profiles
measured between the alongshore coordinates of 11 and 44 m (representing the
entire length of beach). Profiles were measured at 0.5-m intervals. A persistent
pattern of volume loss is evident at the updrift end of the beach, between
alongshore coordinates of 32 and 44 m, which arises because no sand is
introduced at the updrift boundary during the experiments. Between alongshore
coordinates of 16 and 32 m, small changes are evident but no persistent volume
gains or losses are obvious. This is the region where the highest degree of
longshore uniformity in process measurements is observed, and the region where
the bathymetric contours remain nearly straight and parallel. At the downdrift
end of the beach, between alongshore coordinates of 11 and 16 m, larger volume
changes are evident. Volume changes in this zone are assumed to be anomalous
and caused by lateral boundary imperfections. These volume changes are used to
develop corrections to the quantities of sand that accumulate in the traps.
147
Chapter 10
Longshore Sediment Transport Experiments
Previous Page
