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There are several methods that can be used to determine the extent to which foredunes are undercut by wave runup during extreme storm events (Sdune). Changes in the position of the vegetation line or the primary dune crest, for example, are often readily discernible in aerial photographs. Such information can be used to obtain actual values for Sdune. Field surveys can also be used to establish the possible extent of foredune retreat. The difficulty with such measurements, however, is that it is often not possible to associate them with event magnitude. As a result, such empirical observations are probably best used as a reality check on the results of the other types of analyses considered below. |
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Figure 4. Geometric model used to evaluate the maximum potential erosion during an extreme storm. |
Komar et al. (1999) describe a simple geometric model that can be used to estimate the maximum extent of foredune retreat. The model translates the existing beach/dune form landward in response to elevated storm water levels according to the following relationship:
Sdune = (WL - Hj) +  BL
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tan
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where Sdune is the maximum extent of foredune
retreat;WL is the total water level; Hj is the elevation of the
beach/dune junction; BL is vertical shift in the beach profile that can
result from the presence of a rip current, in effect a safety factor;
and tan
the beach slope. Values for the total water level are a combination
of mean water level and wave runup. Representative values for 50 and
100 year events, for example, can be found in published reports or
obtained through analysis of tide gauge and buoy records. Typically,
values for the elevation of the beach/dune junction and the beach
slope will need to be obtained from field measurements.
In using the calculation described above to calculate Sdune, it is important to note that the simple geometric model is not time dependent: The model assumes that the extreme storm event is of unlimited duration, i.e., the eroded beach/dune system reaches a steady state. Numerical simulations using the U.S. Army Corps of Engineers SBEACH model suggest that much if not most of the profile change induced by storms occurs rapidly. However, time periods as long as one month may be needed to develop the fully eroded profile predicted by the geometric model. This suggests that estimates of the extent of foredune retreat obtained from the simple geometric model are best viewed as possible maxima.
Numerical models have also been used to assess the response of the beach profile to storms (Komar et al., 1997). These models include the U.S. Army Corps of Engineers SBEACH model noted above, Kriebel and Dean's (1985) EBEACH model, and Southgate and Nairn's (1993) COSMOS model. Such models provide a potentially robust methodology for determining Sdune. However, in their current form they are are limited by their inability to fully simulate the process of foredune retreat. Still, they do provide useful information regarding beach profile response to storms. In the context of calculating Sdune, results obtained from these models can be viewed as possible minima when compared to the results obtained from the geometric model.
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