11th ICRS Abstract book - Nova Southeastern University

Oral Mini-Symposium 15: Progress in Understanding the Hydrodynamics of Coral Reef Systems
15-9
Nonlinear Wave Transformation Over Shallow Fringing Reefs
Okey NWOGU 1 , Zeki DEMIRBILEK* 2 , Mark MERRIFIELD 3
1 University of Michigan, Ann Arbor, MI, 2 U.S. Army Engineer R & D Center,
Vicksburg, MS, 3 Univeristy of Hawaii, Honolulu, HI
A set of field experiments were conducted to investigate spectral wave transformation
over shallow fringing reefs due to nonlinear wave-wave interaction and wave energy
dissipation. The experiments were conducted at a site located along the southeast coast of
the Island of Guam. A cross-shore transect of four Aquadopp current profilers and three
pressure transducers were used to measure the waves and currents over the reef. The
offshore wave conditions were recorded with a directional wave buoy.
Numerical simulations were conducted with a nonlinear Boussinesq wave model. The
effect of wave breaking is parameterized in the Boussinesq equations with an eddy
viscosity based turbulent shear stress while the effect of bottom friction is parameterized
with a quadratic drag law shear stress term. The Boussinesq model was initialized at the
offshore boundary with buoy data. Comparisons of the measured and predicted spectral
densities over the reef flat showed that the quadratic drag law parameterization for
bottom friction could not adequately reproduce wave energy dissipation over rough coral
surfaces. Although the bottom friction factor could be tuned to match the overall wave
height, it dissipates wave energy across a broad range of frequencies including the
infragravity band, in contrast to the field data that showed a preferential dissipation of the
wave energy in the incident wind-wave frequency band.
One hypothesis for wave energy dissipation over shallow rough surfaces with the
characteristic roughness scales of the order of 10 percent of the water depth is that the
turbulent boundary layer is no longer restricted to a thin layer near the bottom but rather
permeates the entire water column. This would lead to an eddy-viscosity type formulation
for bottom friction as opposed to the quadratic shear stress formulation. Preliminary tests
conducted with an eddy-viscosity based formulation of frictional dissipation matched the
measured data much better.
15-11
Hydrodynamics And Circulation On Monsoonally Influenced Reef Platforms:
Interactions Between Processes And Morphology
Paul KENCH* 1
1 Geography, Geology and Environmental Science, The University of Auckland,
Auckland, New Zealand
Improved understanding of hydrodynamics and controls on circulation processes in coral
reefs systems has been highlighted in numerous studies due to the importance of such
processes on ecological and biological functions. In contrast, few studies have considered
the importance of reef platform hydrodynamics on geomorphic processes (e.g. sediment
transport and island building) or interactions between reef morphology (other than the
reef rim) and hydrodynamic processes. The hydrodynamics of reef platforms are
controlled not only by the deep water to reef flat transition but subsequent
transformations of energy on reef surfaces. This study examines the wave and current
processes on reef platforms in the Maldives. Specific aims are to examine the influence
of predictable shifts in monsoons on reef platform hydrodynamics and to evaluate
morphological feedbacks on reef circulation. Detailed wave and current records were
obtained from three small reef platforms in the central Maldives. The reef platforms vary
in shape, have similar size, and all have vegetated reef islands that occupy 21 to 45% of
reef area. Instruments were deployed at six locations on each reef to capture spatial
variations in wave and current processes and experiments were repeated under differing
monsoon conditions. Results show that the magnitude of wave energy and wave direction
is modulated by variations in monsoon conditions (from the west and northeast). The
presence of reef islands and moats exerts a major control on circulation patterns.
However, circulation changed markedly between different monsoon phases. The degree
of alteration in circulation is related to reef shape, reflecting the sensitivity of different
shaped reefs to changes in wave approach and wave transformation. Consequently,
circulation is shown to reverse between monsoon phases on circular reef platforms but
exhibited more subdued changes on elongate reefs.
15-12
Aster Bathymetry in Computational Fluid Dynamic Simulation Of Rongelap Atoll
Hydrodynamics, Marshall Islands
Eric PETERSON* 1
1 Institute for Sustainability and Innovation, Victoria University, Melbourne MC VIC, Australia
ASTER green, red and near infra-red (NIR) imagery with a resultion on the order of handheld
GPS echo soundings was calibrated to model bathymetry of Rongelap Atoll to a depth of 10
meters. Beyond that depth nautical charts and echo soundings were used to model bathymetry.
The combined result is illustrated in Figure 1.
Figure 1 Bathymetric model of Rongelap
Shape files of depth contours were converted to ordered ASCII X,Y,Z vertex files input into the
finite element meshing program GAMBIT (FDI and ANSYS). These data were then ordered in
volumetric and substrate surface elements of coral reefs and input to the FLUENT
computational fluid dynamics (CFD) package. Bluelink (Australian Bureau of Meteorology
and CSIRO) was used to apply boundary conditions, and results are hydrodynamic charts of the
atoll. The resulting map of substrate shear stress at Rongelap is displayed in Figure 2.
Figure 2 Benthic shear stress of Rongelap Atoll
15-13
Finite-element model of the Great Barrier Reef circulation
Jonathan LAMBRECHTS* 1 , Eric DELEERSNIJDER 1 , Vincent LEGAT 1 , Emmanuel
HANERT 2 , Eric WOLANSKI 3
1 Institute of Mechanics Materials and Civil Engineering, Université catholique de Louvain,
Louvain-la-Neuve, Belgium, 2 Department of Meteorology, University of Reading, Reading,
United Kingdom, 3 James Cook University, Townsville, Australia
An unstructured-mesh, finite element, depth-integrated model of the hydrodynamics of the
whole Great Barrier Barrier Reef (GBR), Australia, has been developed and implemented on a
parallel computer. Far away from reefs, islands and important bathymetric features, the mesh
size may be as large as a few kilometres,whereas, in the vicinity of reefs and islands, the grid is
drastically refined, leading to meshes that can be 100 metres in size. This enables our model to
simulate motions characterized by a wide range of space and time scales. Large scale currents,
i.e. the tides, the wind-induced circulation and the bifurcation of the East Australian Current,
are reproduced with an accuracy that is comparable to that achieved by today's large-scale
models of the GBR. The model is also successful at representing small-scale processes, such as
tidal jets, their instabilities, as well as the eddies developing in the wake of islands and
headlands. Both large and small scales have been validated.
A study of multi-scale reef connectivity has been undertaken.
Biological applications based on this hydrodynamical model have been undertaken such as a
model of the threat from lethal jellyfish at the GBR coast and a study of multi-scale reefconnectivity.
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