Modelling shoreline sand waves: application to the coast of Namibia

Abstract

The SW coast of Africa (Namibia and part of Angola) features very long sandy beaches and a wave climate dominated by energetic swells from the SSW, therefore approaching the coast with a very high obliquity. Satellite images reveal that along that coast there are many shoreline sand waves with wavelengths ranging from 2 to 8 km. A more detailed study, including a Fourier analysis of the shoreline position, confirms a high spectral density concentration at these lengths scales. Also, it becomes apparent that at least some of the sand waves are dynamically active rather than being controlled by the geological setting. A morphodynamic model is used to test the hypothesis that these sand waves could emerge as free morphodynamic instabilities of the coastline due to the obliquity in wave incidence. It is found that the wave period, Tp, is crucial to establish the tendency to stability or instability, instability increasing for decreasing period, whilst there is some discrepancy in the observed periods. Model results for Tp = 7 s clearly show the tendency for the coast to develop free sand waves at 2 km wavelength within a few years, which migrate to the north at rates of 0.6-0.7 km/yr. For Tp = 8 s, instability is weaker and rather sensitive to other factors as the underlying bathymetry. In this case, the coast seems to be nearly at neutral stability so that sand waves originated from other mechanisms can propagate downdrift with little decay.