constructing the boundaries similar to the average beach profile. Additionally,
because of their density, mortar bricks are less likely to be displaced under waves
and currents than other materials.
At the onset of an experiment, the bricks are arranged so that the boundary
elevation closely matches the initial beach profile (Figure 34). It is important
that the brick elevations at the downdrift boundary closely match the average
profile of the beach. A lower brick elevation in relation to the beach may induce
sand to enter the traps and cause an overestimate of sediment transport.
Additionally, sand near the downdrift boundary scours to the upper elevation of
the brick, which alters the local profile and uniformity of longshore currents.
Conversely, if the brick elevation is higher than the beach elevation, the bricks
act as a barrier and transport into the traps is restricted and results in
underestimation of sediment transport. For this reason, the downdrift boundary is
observed carefully during each experiment and, if necessary, bricks are removed
or added so that the boundary profile is representative of the average beach
profile.
Figure 34. The elevation of the lateral boundaries is adjusted using bricks.
Initially, the bricks are set as close as possible to match the initial
beach profile. During the experiment, the downdrift boundary is
observed carefully, and if necessary, bricks are removed or installed
so that the downdrift brick boundary has almost the same profile
shape as the average profile along the beach
Despite careful monitoring of the downdrift boundary to maintain the
boundary elevation at the average profile elevation, not all transported sand
reaches the traps. For mild to moderate longshore currents a shoal may form at
the downdrift boundary, which must be included in calculations of longshore
sediment transport. This correction is based on postexperiment surveys in which
the region near the downdrift boundary is compared with the main portion of the
beach. If a shoal is present, excess sand at the downdrift boundary is included
52
Chapter 5
Beach Morphology Control
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