Narrow Results

The development and evolution of crescentic patterns in double-barred systems is explored using a morphodynamic stability model. The description of the surf zone hydrodynamics is based on depth and wave averaged conditions while sediment transport is calculated using a total load formula. The linear stability analysis predicts that an initially rectilinear coast, subject to infinitesimal perturbations and under normal wave incidence, is unstable and can result in the development of crescentic shapes that can be coupled either in-phase (highs and lows of both sandbars are at the same alongshore position) or out-of-phase (highs and lows of one sandbar correspond to lows and highs of the other sandbar). Results of numerical simulations are sensitive to some of the parameterizations used in the description of hydrodynamics. Changes in the breaker index can have an effect not only on the spacing and growth rate of the pattern but also on the type of pattern that develops. An increase in the breaker index leads to a faster growth of the pattern but also to a smaller alongshore spacing. The role of parameterizations in lateral mixing and roughness length appear to be limited.

Long-term migrations of sandbars on Chirihama Beach, Japan, have been investigated using a set of field surveys collected over 9 years. The typical profile configurations are characterized by the presence of multiple bars. The height of the outer bar exceeds 4 m, when the bar evolves most significantly. The movements of the bars are significant. For the time variation, it is shown that the net offshore periodic migrations of bars have been repeated. The length of one cycle is approximately 8 years, and the interval between each cycle is approximately 3 to 4 years.