Advancing Numerical Calculation of Mixed Sized Sediment Transport and Morphology Change
Watts, Irene Margaret
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Numerical modeling of sediment transport and morphologic change has become a powerful tool for studying and working in the coastal zone. Numerical calculation can assist in a broad range of projects from regional sediment management to beach nourishment as well as evaluating environmental aspects of dredging and placement and implications to water quality and turbidity. Success in application of these tools is partially dependent on the quality of characterization of the sediments in the field. These parameters could include grain size, mineralogical composition, specific gravity and fall velocity. Sediment transport formulations are sensitive to the spatial representation of the sediments in the model (Watts and Zarillo, 2015). The focus of this work is to improve computation of sediment movement of the coarser end of the grain size distribution. In the region of focus for this study, Sebastian Inlet, Florida the coarser fractions beyond 0.35 mm are composed of nearly 100% carbonate material. The methodology developed can be applied to any region with a broad span of grain sizes with different specific gravities, mineralogical composition and shapes. Sediments with concentrations of carbonate sediments exist throughout the Atlantic coast of Florida, the Caribbean (as described in Hine, Wilber Hine et al. (1981), Hawaii in Eversole et al. (2002), Australia (Nielsen, 1992) and the Mediterranean covered in Goldsmith et al. (1980)). As in the case of Florida, beach nourishment projects exist and models that effectively calculate the movement of these types of sediments can assist in the management of these projects. The purpose of this work is to advance the state of knowledge by improving the performance of sediment transport models in carbonate rich environments. The goal will be accomplished by a combination of a comprehensive field campaign to include both hydrodynamics and bottom topography and calculation of the resulting morphology change, with validation through numerical modeling of sediment dynamics. Sebastian Inlet, Florida, the focus region for this study, is characterized with a significant multimodal sediment distribution including carbonates (Watts et al., 2015). To measure hydrodynamics and related data, six (6) bottom mounted Acoustic Doppler Current Profilers (ADCPs) are deployed in the nearshore areas spanning alongshore of the Sebastian Inlet on the Atlantic coast of Florida in water depths ranging from 7.5 m to 9 m relative to Mean High Water (MHW). These gauges are recording wave height and direction, water elevation, water temperature, as well as current magnitude and direction. To document bathymetry and resulting morphology change, an instrumented personal watercraft was developed to record bottom topography at frequent temporal resolution, supplementing other less frequent existing and ongoing surveys. These bathymetric measurements extend from the nearshore to the 13-meter (45 ft) contour. Data from a meteorological station that has been in operation since 1997 and located at the distal end of the north jetty of Sebastian Inlet provides wind speed and direction, water temperature and elevation, barometric pressure and air temperature. The unique distribution of nearshore gauges allows for an analysis of hydrodynamic variations alongshore of a tidal inlet not previously observed in other large field campaigns. This hydrodynamic characterization will provide an enhanced understanding on the change in hydrodynamics in vicinity of a tidal inlet which directly contributes to multimodal sediment transport magnitudes and directions, and the subsequent morphology change. Repeated bottom topography surveys will provide net sediment transport rates and morphological changes with varying forces (quiescent and energetic periods). Previous modeling studies applied the Coastal Modeling System (CMS) at Sebastian Inlet indicated the present configuration of the numeric model is insufficient to calculate sediment transport in this carbonate rich environment. The hypothesis to be tested is that predictions of carbonate rich sediment movement can be improved by altering the fall velocities input to model sediment transport algorithms to match measured fall velocity of the carbonate fraction.