Allowance for variable water level

An initial 1:30 plane-sloping beach was constructed using sand having D50 =

0.11 mm, which was the same initial beach slope and sediment size to be used for

the fine-grained moveable-bed test series in the LSTF. In each experiment, the

maximum depth of erosion was located just offshore of the still-water shoreline.

For the design wave condition, the maximum depth of erosion did not exceed

0.1 m; however, this conclusion is based on 2-D flume tests, and not 3-D tests in

the LSTF.

For design purposes, it was assumed that the entire 18-m width of the

moveable-bed beach would erode by 0.1 m. Although this was only a rough

assumption, it was conservative in that it allowed for the maximum depth of

erosion to occur at any location across the beach profile. In particular, this was a

very conservative assumption near the offshore end of the beach where accretion

would occur (as opposed to erosion) as the offshore bar developed. Nonetheless,

over designing the LSC recirculation system near the offshore region of the

beach would give the system more flexibility in the future when conducting

moveable-bed experiments with coastal structures.

Note that this analysis assumed that as the offshore bar and trough feature

developed in the moveable-bed experiments, the depth-averaged LSC at any

cross-shore location would not change substantially from the case with a plane

sloping fixed-bed beach. Although this may not be the case, it was assumed that

as the beach profile developed changes to the depth-averaged LSC would be

relatively small, since the initial beach slope was relatively gentle (1:30) and the

total profile adjustment would be relatively small. Therefore, this assumption

would be adequate for design purposes.

Allowance for variable water level

The design of the LSC recirculation system also accounted for the

requirement to conduct experiments with variable water levels in the facility. As

mentioned previously, the plan was to conduct the majority of the moveable-bed

LST experiments with an offshore water depth of 0.9 m. However, if the water

level is increased in the future, the LSC distribution would need to be translated

shoreward.

A number of physical constraints in the vertical dimension of the facility

dictated that the maximum operating water level could not exceed 1.0 m (at least

for energetic wave conditions). Lower water levels could be used, however, the

wave generation capability of the wave makers would decrease with decreasing

water level. Therefore, for design purposes, the critical design parameter was the

maximum operating water level. The first curve in Figure 14 shows the

longshore flux distribution for the design wave condition, with an offshore water

depth of 0.9 m. This curve includes the increased capacity required by using a

factor of safety of 1.1 and the assumption that the moveable bed eroded 0.1 m

across the entire width of the beach. The abrupt change in cross-shore gradient at

X = 18 and 21 m was caused by the sudden change in beach slope at these

locations. The second curve was obtained by increasing the offshore water depth

to 1.0 m for the same design wave condition. Essentially, this 0.1-m increase in

operating water level translates the LSC distribution 3.0 m shoreward, because

Chapter 3

Longshore Current Recirculation System