11th ICRS Abstract book - Nova Southeastern University

Oral Mini-Symposium 15: Progress in Understanding the Hydrodynamics of Coral Reef Systems
15-1
Hydrodynamic Modeling Of A Fringing Reef Embayment: Hanalei Bay, Hawaii
Ronald HOEKE* 1 , Curt STORLAZZI 2
1 Joint Institute of Marine and Atmospheric Research(JIMAR), University of
Hawaii/NOAA, Honolulu, HI, 2 Coastal and Marine Geology Program (CMG), USGS
Pacific Science Cente, Santa Cruz, CA
Concerns about watershed management and its connection to nearshore ecological health
at Hanalei Bay, Kauai, Hawaii, USA, have prompted a concerted interagency effort to
gain better understanding of circulation, sedimentation, and water quality within the bay.
Recent advances in computational hydrodynamics have resulted in the development of
suites of software to model such coastal processes. These models have proven successful
at a number of sandy, continental coastal sites, but few have been applied to insular coral
reefs. Here we present the application of the Delft3D computational hydrodynamic
software package to Hanalei Bay, a wave-dominated, microtidal coral reef embayment.
Numerical output using model parameters traditionally used for sandy continental
margins is contrasted with adaptations for microtidal, oceanic fringing reef environments
and compared to in situ observations. Analyses reveal the importance of: (1) wave
refraction/diffraction effects, (2) spatially varying bottom friction values that range over
an order of magnitude, and (3) shortcomings of numerical solutions of these two
processes when applied to fringing reef environments. The relative contribution of tides,
winds, and waves to the bay’s hydrodynamics are calculated for a range of conditions.
The contribution of waves is several orders of magnitude greater than other physical
processes in determining overall circulation and flushing of the bay, and dominates the
near-bed shear stresses at most locations in the bay.
These findings show the precision of current computational hydrodynamics in highenergy,
morphologically complex environments, such as Hanalei Bay, to be relatively
poor at this time. Despite this, these techniques do provide valuable order-of-magnitude
estimates of hydrodynamic processes and create a qualitative synoptic picture. These
spatially explicit, fine scale estimates would be difficult with other methods.
15-2
Modelling Larval Retention Around Reefs By Local Scale Circulation Features
Paulina CETINA-HEREDIA* 1 , Sean CONNOLLY 2,3 , Michael HERZFELD 4
1 School of Marine & Tropical Biology, James Cook University, Townsville, Australia,
2 School of Marine and Tropical Biology, James Cook University, Townsville, Australia,
3 ARC centre of Excellence, Coral Reef Studies, Townsville, Australia, 4 Division of
Marine and Atmospheric Research, CSIRO, Hobart, Australia
Larval transport is mediated by circulation patterns. Low frequency, large scale currents
can advect larvae for long distances, connecting populations over 100 km. However,
local scale circulation features, such as lee reef eddies, can retain larvae near reefs and
enhance self-recruitment. To accurately estimate larval dispersal, it is necessary to
consider these local scale circulation processes. This study aims to approximate larval
retention around reefs as a consequence of recirculation and stagnant flows provoked by
the interaction of currents with the complex reef bathymetry that reefs normally display.
To characterize eddies formed in the lee of reefs (different shapes) under different
circulation regimes (low and/or high frequency dominant) and quantify the retention of
larvae we simulate larval transport with a 3D finite difference hydrodynamic model
(Sparse Ocean Hydrodynamic Code) in the Northern Section of the Great Barrier Reef.
The life span, strength and size of eddies is quantified, using vorticity and the Okubo
Weiss invariant as diagnostic variables. Finally, an approximation of larval retention as a
function of reef geometry and prevalent circulation regime is attempted to provide a
novel tractable approach accounting for local scale circulation features on larval dispersal
in regional scale metapopulation models.
15-3
Characterization Of Hydrodynamic And Biophysical Anomalies On The Florida Reef
Tract
Lewis GRAMER* 1 , Elizabeth JOHNS 2 , James HENDEE 2
1 Cooperative Institute of Marine and Atmospheric Studies, University of Miami, Miami, FL,
2 Atlantic Oceanographic and Meteorological Laboratory, National Oceanographic and
Atmospheric Administration, Miami, FL
NOAA's Integrated Coral Observing Network (ICON) project uses an artificial intelligence
software system to implement heuristic models of coral reef ecosystem response to physical,
chemical and biological conditions. These heuristic models use categorical, “if-then” rules to
recognize and report patterns in environmental data integrated in near real-time from multiple
external sources. One such model is described that detects and distinguishes episodic,
biologically significant hydrodynamic processes acting upon coral reefs in the Florida Keys
National Marine Sanctuary. Data are gathered from in situ sensors, satellites, and highfrequency
radar at three shallow reef locations 100-200m inshore of the outer edge of the reef
crest: Sombrero Reef in the Middle Keys, Molasses Reef in the Upper Keys, and Fowey Rocks
off the Miami coast. The model recognizes apparent changes in biological production and
circulation that may impact the reef ecosystem. Primary model criteria are in situ sea
temperature variability occurring at near-tidal periodicities, wind velocity variability, and sea
color-derived satellite chlorophyll a concentrations. Model forecasts are then verified using
secondary data not utilized by the model as input, including satellite-derived regional sea
surface temperature and ocean color imagery, radar-derived ocean surface currents, in situ
salinity, and divers’ visual reports. Three classes of nutrient delivery events are characterized
by the model products: those forced respectively by wind-driven upwelling and downwelling;
by net tidal transport of eutrophic water from Florida Bay; and by interaction of Florida Current
frontal features with topography, which may be modulated by internal waves breaking on the
reef slope. Multiple events of each type are characterized within a 23-month period in 2006-
2007.
15-4
Spatial And Temporal Variability in Velocity Shear Over Fringing Coral Reefs And Its
Implications On Water Column Structure And Particulate Flux
Curt STORLAZZI* 1
1 Coastal and Marine Geology Team, US Geological Survey, Santa Cruz, CA
Long-term hydrodynamic data from a number of bottom-mounted instruments and spatiallyextensive,
but temporally-limited, hydrographic surveys have been collected to better
understand coastal dynamics on and among fringing coral reefs in Hawaii, USA. These highresolution
data (waves, currents, temperature, salinity, and turbidity) suggest that wave- and
wind-driven flows appear to be the primary control on flow over shallow portions of the reefs
while tidal and subtidal currents dominate flow over the deeper, outer portions of the reefs and
insular shelf. Near-surface current directions over the fore reef vary on average by more than
40º from those near the seafloor, and the orientation of the currents over the reef flat differed on
average by more than 65º from those observed over the fore reef. This shear occurred over
relatively short vertical (O~meters) and cross-shore (O~100’s of meters) scales, causing
material distributed throughout the water column, including the suspended particles causing
turbidity (e.g. sediment or larvae) and/or dissolved nutrients and contaminants, to be transported
in different directions under constant oceanographic and meteorologic forcing, depending on its
initial location. When the direction of the flows over the fore reef and the reef flat are counter
one another, which is quite common, they cause a zone of cross-shore horizontal shear and
often form a front, dividing turbid, lower-salinity water inshore from clear, higher-salinity water
offshore. It is not clear whether these zones of high shear and fronts are the cause or the result
of the location of the fore reef, but these features appear to be correlated alongshore over
relatively large horizontal distances (O~kilometers). When two flows converge or when a
single flow is bathymetrically steered, eddies can be generated that, in the absence of large
ocean surface waves, tend to accumulate suspended particulate material.
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