COASTAL ENGINEERING 2004, pp.2620-2632.
IMPLICATIONS OF MORPHODYNAMIC TIME SCALE
FOR COASTAL PROTECTION
HANS HANSON and MAGNUS LARSON
Department of Water Resources Engineering, Lund University, Box 118,
Lund, Sweden S-22100, Hans.Hanson@tvrl.lth.se, Magnus.Larson@tvrl.lth.se
As reported in several studies, even though properly designed, it is possible that groin
systems not only cause down-drift beaches to erode, but also contribute to the generation
of rip currents. These rip currents run along the updrift side of the structure, moving
sediment offshore where it may be, at least in part, lost from the system. It is well known
that the directionality of the incident waves is a central factor for the shoreline response
to groins. Until now, however, this directionality has been characterized only by the ratio
of net transport rate Qn to gross transport rate Qg. In this study it is concluded that the
phase lag between the forcing and the morphodynamic response is another key factor
responsible for these offshore losses. Based upon this, a relaxation time for open-coast
systems and a non-dimensional morphodynamic response factor for groin compartments
are introduced as new design parameters for groin systems. These parameters provide an
indication about the time it takes for the morphodynamic system to adjust to a change in
the hydrodynamic forcing conditions.
1.
Introduction
The use of groins for coastal protection has become somewhat tarnished over the
years. Even though groins undoubtedly may maintain beach width, reduce
losses from beach fills, prevent sediment transport into inlets and channels, etc.,
the world has seen many cases where groins have contributed to down-drift
erosion. Clearly, our understanding of relevant design parameters needs to be
enhanced to improve the functional design of these structures. The wave-
induced longshore sediment transport rate depends on the angle between the
breaking waves and the shoreline. By splitting the shoreline into shorter
stretches by a groin or groin field, each compartment may be more easily
reoriented by the incident breaking waves. Ideally, the shoreline will be
reshaped to become locally parallel with the breaking wave crests, at which
point the wave-induced longshore sediment transport rate approaches zero. At
that time, an equilibrium plan shape will be reached (Figure 1a).
The situation remains stable until the incident waves arrive and break at a
different angle (Figure 1b). If the change is significant, a considerable
longshore current may be generated. At the location of the down-drift groin, the
current is re-directed offshore in the form of a rip current. This is a well-known
phenomena reported in several text books (see, e.g., Silvester and Hsu 1993). In
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