11th ICRS Abstract book - Nova Southeastern University

Oral Mini-Symposium 15: Progress in Understanding the Hydrodynamics of Coral Reef Systems
15-14
Momentum Balances Across A Wave-Dominated Coral Reef
James HENCH* 1 , Hunter LENIHAN 2 , Stephen MONISMITH 1
1 Environmental Fluid Mechanics Laboratory, Stanford University, Stanford, CA,
2 University of California, Santa Barbara, Santa Barbara, CA
For many coral reefs, circulation is dominated by waves breaking over the reef crest that
drive a strong circulation over back reef habitats. Unlike classical work on beaches,
where the primary momentum balance is between radiation stress gradients and set-up of
the free surface, flows over coral reefs typically have mean flows into a lagoon, and thus
friction can also be important in the momentum balance. Unfortunately friction on coral
reefs is difficult to estimate a priori due to the remarkably high rugosity. Here we studied
a nearly tideless system in Moorea, French Polynesia to isolate the effect of wave forcing
on coral reef circulation. An array of 7 stations spanning the forereef, reefcrest and
backreef areas, were each instrumented with pressure sensors and velocity profilers. The
resulting data were used to estimate the contribution of each term in the cross-reef
momentum balance, as well as corresponding bottom friction coefficients. Initial results
show that the momentum balance varies markedly with distance from reefcrest and
transitions from a radiation stress and set-up balance, to a friction and set-up balance.
15-15
Defining Drag Coefficients For Coral Reefs
Johanna ROSMAN* 1 , James HENCH 1
1 Environmental Fluid Mechanics Laboratory, Stanford University, Stanford, CA
Coral reefs form a very rough bottom boundary that is porous to water flow. Drag due to
a coral reef is typically modeled using a simple quadratic drag law with a bulk drag
coefficient; however, several different definitions have been used in the literature and a
wide range of values have been reported. We have developed a formal framework for
estimating drag coefficients for flow over coral reefs in the absence of surface waves. We
consider three different definitions, in which the above-canopy, within canopy and depthaverage
velocity are used for the drag parameterization. Using a simple two layer model
we derive expressions for the drag coefficient in terms of morphological and flow
parameters. We show that drag coefficients can vary by more than an order of magnitude
for the same geometry and flow, depending on the definition used.
15-16
Roughness and the Effects of its Spatial Variability on Coral Reefs
Clifford HEARN* 1
1 ETI Professionals at USGS, St Petersburg, FL
Topographic and bathymetric complexity are key controls on the hydrodynamics of coral reefs.
They exist on all spatial scales from microns to kilometers on reefs and within hydrodynamic
studies are all treated as part of the spatial spectrum of the generalized concept of ‘roughness’.
Coral reef hydrodynamics treats roughness in a variety of ways at different spatial scales.
Roughness has a major influence on the way that currents flow over reefs and the processes by
which energy is transported and dissipated. A fundamental property of roughness is that the
average roughness below any specified spatial scale is itself spatially variable on that scale.
This means that a hydrodynamic model of a reef will necessarily include spatially varying
smaller-scale roughness. This paper considers one feature of this phenomenon which is the
change in roughness length, or frictional drag coefficient, encountered between distinct
topographic zones of a reef such as the fore-reef, reef-flat and back-reef. The analysis is based
on frictional Reynolds numbers for the individual zones of the reef which are combined into a
‘reef Reynolds number’ which is a measure the relative magnitudes of large scale and small
scale roughness. The paper examines the implications of the larger scale variability of
roughness on the connectivity between the zones of a reef as provided by the transport of
materials and turbulence.
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