COASTAL ENGINEERING 2004
2629
100
300
Left Groin
Right Groin
75
250
50
200
25
150
0
100
-25
50
-50
0
-75
-50
-100
-100
0
2
4
6
8
10
12
Time (months)
Figure 6. Simulated offshore losses and associated shoreline locations adjacent to lateral groins in a
groin compartment with periodic, instantaneous change in wave direction.
H = 0.87 m, depth of closure D = 10 m, and distance between groins B = 80 m,
the different -values corresponds to morphodynamic response factor ζ-values
of 6, 4, 3, 2, and 1, respectively. The higher values correspond to a situation
where the variation in the hydrodynamic forcing is significantly faster than that
of the morphodynamic response whereas the smaller values correspond to
situations where the forcing and the response are more in phase. Thus, with
smaller values, we would expect smaller offshore losses due to a structure-
induced rip current. All simulations were run for a (prototype) duration of 252
days with R set to 0.5. The value of R is arbitrary as the analysis focuses on
relative losses as a function of the morphodynamic response factor ζ.
Figure 7 shows that for the slower change in wave direction (ζ = 1), the
offshore loss of sediment was determined to about 10 per cent of the gross
longshore transport rate Qg half-way between the groins. With a more rapid
change in wave directionality the relative losses are fairly stable up to about ζ =
4 from were the relative losses increase more quickly. However, in absolute
terms (m3 103) there is an almost linear increase of offshore losses with ζ.
Although this is a specific example setting it highlights the importance of
keeping the value on the morphodynamic response factor ζ down to reduce
offshore losses.
Figure 8 illustrates the shoreline response in an enclosed groin compartment
at steady-state conditions with no offshore losses (c.f. Fig. 2) and a similar
situation with offshore losses corresponding to R = 0.5 when breaking-wave
angle varies sinusoidally with time for ζ = 1. The shape of the shoreline seems
to be independent of R, whereas its location is gradually receding as a result of
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