Mechanisms that influence the performance of beach nourishment: A case study in Delray Beach, Florida, USA

Since 1973, Delray Beach, located on the southeast coast of Florida, U.S.A, has been periodically nourished. The morphological evolution of these nourishments has been influenced by design aspects (e.g., changes in shoreline orientation), extreme events (hurricanes, northeastern storms), and dredging of offshore borrow areas created to provide the nourishment sediments. This paper describes a thorough analysis of the physical processes influencing nourishment performance using the process-based model Delft3D. The analysis distinguishes three steps: (1) a wave analysis, (2) a hydrodynamic analysis, and (3) a morphological analysis. The wave analysis shows that the oldest, deeper, borrow pits (dredged in 1973 and 1978) have the largest effect on the nearshore wave climate, while the effects of the reefs can be considered negligible. Results from the hydrodynamic analysis indicate that differences in wave height cause variations in alongshore current velocities related to differences in wave setup and local wave angle. The morphologic analysis also shows a large influence of the older borrows on fill performance. Bar formation behind the borrows is predicted by the model; however, this feature couldn't be confirmed with 300-m spaced beach profile data. Higher current velocities were predicted in the shadow zone behind borrow areas because of gradients in wave-induced water levels and changes in wave direction induced by the borrows. The higher than average erosion in the southern end of the fill appears to be caused by changes in shoreline orientation and to a lesser degree, borrow area effects. Modeling played a key role in understanding the observed morphological development of the fill. The use of different bathymetric scenarios proved to be a useful method in isolating the physical processes relevant to nourishment Performance and assessing their relative effects.