Modelling the morphology of sandy spits

Abstract

The shape, dimensions and growth rate of an accumulating sandy spit is investigated by a theoretical and experimental study. The idealised case of a spit growing without change of form under a constant wave forcing is considered. The longshore wave-driven sediment transport is taken to be dominant in the formation of the spit. The shape of the spit is first described by an analytical one-line model for the coastline development and by a simple littoral drift model. The analytical model is valid for a spit with so large dimensions, that quasi-uniform conditions can be applied. It is found that with this assumption the dimensions of the spit cannot be determined. The width and shape of a finite spit is therefore determined from simulations with an area model for the wave-driven current and sediment transport along the spit. In this case the curvature effects from the spit on the longshore sediment transport (inertia of the longshore current, wave focussing) are includes. Several solutions for a uniformly growing spit may exist, and it is assumed that the fastest growing spit is the one to emerge. The simulations indicate the width of the spit to be proportional to the width of the surf zone. In experiments conducted in a wave tank an accumulating spit was formed at the down-drift end of a uniform stretch of coast exposed to waves approaching at an angle. The spit approached equilibrium dimensions when a constant wave climate was applied. The radius of curvature of the spit varied according to the height of the incoming waves.