Methodology of Sandy Beach Stabilization by Nourishment: A Long-Term Morphodynamic Modelling Approach

Abstract

Many attempts have been made to control beach erosion, but in the long term none have succeeded in stabilizing sandy beaches that are being eroded. Coastal structures are of little use for stabilization as they cause beach profiles to gradually become steep. The equation for a stable sandy beach is derived from the set of the equations of continuity for beach change and longshore sediment transport under unsteady and non-uniform conditions, so that theoretical shoreline configurations can be obtained for both static and dynamic stable sandy beaches.

One methodology for beach erosion control is beach nourishment; in which a strip of borrowed offshore sand is reconstructed along the eroded coastline and subject to wave climate to establish new dynamically stable sandy beaches. Most bed evolution models neglect slope-related sediment transport, which though generally weak compared to other mechanisms, is extremely important to the stability of the system. In fact, it is the downslope gravitational transport that enables the beach profile to reach a state of equilibrium. A Lagrangean particle displacement approach has been adopted for bed morphology calculation in the present model since it include gravitational and sheltering effects.

Simplification of equilibrium monotonic profiles (One-line N-line, Genesis) has been shown to be able to match for long-term shoreline change well. In a long-term simulation, the inclusion of downslope gravitational transport by itself may not ensure sensible equilibrium topography, especially when cross-shore profile evolution is important. This is because cross-shore profile evolution depends on the vertical velocity structure comprising of undertow, mass flux and bottom boundary drift. In long-term simulation of a situation where the cross-shore mechanisms are important, a 3D description of the flow structure is required. To improve this aspect the present model includes the role played by undertow, wave asymmetry and bottom boundary drift in sediment transport. Laboratory measurement comparison will also be discussed in this study.