Advancing Numerical Calculation of Mixed Sized Sediment Transport and Morphology Change
Abstract
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.