COASTAL ENGINEERING 2004
2631
15
R
Phase
0
0
10
0.5
π/2
π
5
3π/2
0
-5
-10
-15
0
20
40
60
80
Alongshore Distance (m)
Figure 8. Comparison of shoreline evolution in enclosed groin compartment at steady-state
conditions with no offshore losses (R = 0) and a similar situation with offshore losses corresponding
to R = 0.5 when breaking-wave angle varies sinusoidally with time for ζ = 1.
expressed in terms of a relaxation time. The value of this is site-specific but
may be easily quantified from local conditions. By assuming that the offshore
loss of sediment in these structure-induced rip currents is proportional to the
transport gradient near the structure, the examples showed that the offshore
losses of sediment are proportional to the phase shift between forcing and
response. Thus, if the time between large changes in forcing (wave) direction is
considerably shorter than the relaxation time, offshore sediment losses due to
structure-induced rip currents may be significant.
In a groin compartment the relation between the two time scales is
expressed by the morphodynamic response factor. As the value of this factor
approaches 1, the variation in the forcing is slow enough for the response to
keep even pace. As a result, offshore losses are kept at a minimum. When the
ζ-value increases, the phase shift between forcing and response also increases as
do the offshore losses of sediment.
In summary, groins and jetties might best be considered for coastal
protection in situations where offshore losses of sediment may be kept at a
minimum. For this reason, the temporal behavior of wave directionality must be
known. Based on this, a relaxation time for an open-coast single-groin system
and a morphodynamic response factor for a groin compartment must be
determined as they control the offshore losses from the system. If changes in
wave direction are infrequent enough, offshore losses from structure-induced rip
currents will be small, increasing the possibility that a groin (system) could
function successfully.
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