Process controlling beach nourishment performance at Delray Beach, Florida, USA

Beach erosion is a chronic problem in the southeast coast of the State of Florida, where Delray Beach is located. To mitigate increasing erosion problems (subsequent) beach nourishment projects were constructed along the entire sandy coast of the state of Florida. At Delray Beach, the beach was nourished 7 times since 1973 using approximately 5.7 million m3 of sediments along a stretch of beach of about 4km. Sand for these beach nourishment projects was extracted from an area immediately offshore of the project site, from blanket deposits that overlay bedrock and are located just landward of offshore coral reefs. Critical evaluation of sediment data for several years was initially conducted to investigate the hypothesis raised by previous authors that higher erosion in the south end of the project is due to the occurrence of finer beach sand at this segment. Data from multiple annual monitoring reports demonstrated that alongshore grain size distribution on Delray Beach varies significantly, temporally and spatially, and that there is no persistent trend of finer grain sizes being located in the erosional hot spot segment compared to the rest of the beach; in fact, grain size differences within the project area can be considered to be minor. This data provided enough evidence to conclude that grain size differences alongshore were not the cause of increased hot sport erosion. Wave modelling and morphological modelling also suggested that the gaps in the offshore shore parallel reef system were not responsible for hot spot erosion. The effects of the barrier reefs on the nearshore waves and currents were relatively minor as evidenced by numerical model results. Strong alongshore variability in wave height and associated pronounced areas wave shadowing and focusing zones were observed along the project area in initial numerical modelling efforts, but these were attributed to wave transformation over the dredge pits located offshore of the nourished beach. Near the erosion hot spot segment on the south end of the project, however, these variations in nearshore wave heights were relatively small compared to other segments of the project. Five methods of wave climate schematization designed to reduce a full wave time-series into a representative set of conditions for coastal morphological modeling were evaluated to achieve the first task. Of all the methods of wave climate schematization tested the ´Energy Flux Method´ and ´Opti Method´ showed best results in terms of representing accurately the sediment transport of the benchmark wave climate with a reduced set of wave conditions. The Energy Flux Method was identified as the preferred technique because it was relatively easy to apply, it is not subjective since waves are selected as a function of wave energy distribution and shows satisfactory performance even when compared to more complex and time intensive methodologies such as the Opti method. The research conducted in this thesis demonstrates that some engineering solutions such as the backfilling of all the dredge pits or introduction of permeable structures at the downdrift (south) end of the Delray Beach nourishment project can improve the performance of the nourishment reducing volumetric losses with mild downdrift effects. Since the downdrift beaches are receiving sediment lost from the nourishment project “free of costs”, a 50 per cent reduction of these losses implies that the downdrift beaches will still be receiving sediments, but half the amount. Each intervention will, however, impact updrift and downdrift beaches in a different way; have its costs and its environmental impacts, therefore, before further consideration for implementation of these significant project modifications a complete engineering, economic and environmental feasibility analysis is recommended.