The Influence of Wave Spectral Shape on Dune Erosion.

Dune erosion is strongly influenced by the maximum landward extent of wave runup during storms, which in turn is a function of the tide, storm surge, wave setup and incident waves. However, complete spectral wave information is often not available and coastal engineers must rely on empirical relationships such as JONSWAP, that define the distribution of energy with frequency within the ocean. Importantly, the JONSWAP peak enhancement factor (termed 'gamma') is a crucial factor in shaping the synthetic wave spectra, with many engineering applications generally relying on the default value of 3.3. To evaluate this assumption, this paper presents results from an extensive physical modelling laboratory experimental program that provided the opportunity to observe and evaluate how the shape of the JONSWAP wave spectra can potentially impact the erosion of sandy dunes. Observations confirm that the highest value of the JONSWAP peak enhancement factor tested (=7) produced waves with a greater proportion of the spectral energy concentrated at the spectral peak for both the offshore incident waves and measured swash timeseries. This led to lower maximum wave runup and slight reduction in the dune erosion modelled. In contrast, the lowest value of the JONSWAP peak enhancement factor tested (=1) resulted in more frequency spread within the wave spectra, particularly at frequencies lower than the peak. This resulted in slightly increased extreme values of individual waves, higher individual wave run up events, and greater resulting dune erosion. These results confirm that the assumed wave spectral shape can influence the magnitude of dune erosion that is observed, and that the influence of swash spectra should be considered further when using models (physical or numerical) to predict dune erosion. [ABSTRACT FROM AUTHOR]