Design with respect to net long-shore sediment transport

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with a confining wall may relate to potential scour if a wall terminates adjacent to

the navigation channel, and it could also be a hazard to navigation.

Deposition basin size is usually based on how often dredging can be scheduled

and the estimated longshore sediment transport rate. The optimal design of a basin

and associated structures is intended to capture the net longshore transport volume,

as described below. (Such capture of sediment assumes the basin is placed on the

side of the inlet where the greater amount of longshore sediment volume originates.)

Capturing only the net longshore transport volume will facilitate reduced sediment

handling. The recommended procedure is to design for some amount greater than

the yearly net, but less than the yearly transport directed toward the basin. This

value may then be doubled to provide reserve if, for some reason, a dredging cycle

is missed. Ways to achieve capture of only the net transport are described in the

next section.

3.2.2. Design with respect to net long-shore sediment transport

Ideally, a weir jetty system should provide a deposition basin that can contain one to

two years of the net longshore transport. In order to capture only the net longshore

transport rather than the total right-to-left transport (use Fig. 11 as a reference), the

Deposition

Basin

Sediment pathways

Low wave energy

deposition

High wave energy

deposition

Channel

QL= Longshore transport to left

QR= Longshore transport to right

QN= Net longshore transport

= QL- QR (for this example, QL>QR)

Fig. 11. Sediment pathways over weir for low and high wave conditions.

Figure 11. Sediment pathways over weir for low and high wave conditions.