Coastal Model Applications and Field Measurements- Field Measurements and Model Applications

Science Center Objects

Several components of this project are applications to evaluate the model against critical field measurements or to test new model components. Data from field measurements is described in our publications and available in our databases.

Bathymetry of the New England inner shelf and south shore of Martha’s Vineyard with model grids

Bathymetry of the New England inner shelf and south shore of Martha’s Vineyard with model grids

Comparison of observed near-bed velocities and modeled near-bed velocities using several bottom-roughness formulations.

Comparison of observed near-bed velocities and modeled near-bed velocities using several bottom-roughness formulations. Velocity vectors are overlaid on map of backscatter from the sea floor showing regions with coarse sand (light color) and fine sand (dark colors). White lines are bathymetry contours. Mean velocities were averaged over entire period (top panel), flood and ebb velocities were determined from dominant alongshore flow direction, and separately averaged for flood- and ebb-oriented flows with speeds >5 cm/s after removing mean velocities (bottom panel). Note change in arrow scale between two panels

Barnegat Bay residence times in days

Barnegat Bay residence times in days for (a) scenario T, only tidal forcing; (b) scenario TB, combined offshore hydrodynamic forcing; (c) scenario TBR, streamflow with combined hydrodynamic forcing; and (d) scenario TBRM, full suite with meteorological forcing

Impact of Model Resolution

Numerical models sometimes require high resolution to get accurate results. Comparison of model results at various resolutions with measurements made by collaborators at WHOI revealed an average circulation pattern on the inner shelf south of Martha’s Vineyard that was generated by tidal forcing, and only resolved in the high-resolution model. The loss of model skill with decreasing resolution was attributed to insufficient representation of the bathymetric gradients.

Evaluation of Modeled Bottom Roughness near Martha’s Vineyard Coastal Observatory

Bottom roughness is an important parameter that modifies the current velocity in coastal ocean models. Methods for specifying bottom roughness in models range from very simple specifications of constant, uniform values to complicated nested that base roughness on ripple geometry which, in turn, is governed by sediment type and wave-current combined bottom stresses. We tested several roughness formulations in the COAWST modeling system simulation of flow near MVCO, and compared the results with measurements made by the USGS and collaborators during the ONR Ripples Directed Research Initiative. We found that the more complicated formulations, which incorporated empirical ripple roughness estimates, do not improve our modeling skill.

Floc Model Dynamics near MVCO

We are using data from our OASIS profiling arm measurements to evaluate the recent implementation of floc dynamics in our sediment-transport model (CSTMS). The OASIS data includes measurements by both optical and acoustic instruments, which respond very differently to different types of suspended sediment. Acoustic backscatter sensors respond well to solid, high-density particles like sand, whereas optical sensors are sensitive to larger, density particles like flocs. Idealized model runs for waves, currents, and bottom sediments that resemble measurements at MVCO produce flocs and resuspend bottom sands in a realistic manner.

Using particle tracking to assess estuarine residence time

Estuarine water quality is strongly related to circulation and residence time. Particle tracking is one method to evaluate how long a parcel of water may remain within an estuary. We used the COAWST model and the LTRANS particle-tracking model to assess the influence of tides, remote forcing, and winds on residence time in Barnegat Bay, New Jersey. We found that tides are relatively inefficient at flushing the northern bay, where water quality is poorest.

Scenarios of seagrass change with coupled hydrodynamic-optical models

Estuarine eutrophication due to nitrogen loading has led to numerous ecological changes, including loss of seagrass beds. One potential cause of these losses is a reduction in light availability due to increased attenuation by phytoplankton. Future sea level rise will also tend to reduce light penetration and modify seagrass habitat. We integrated a spectral irradiance model into a biogeochemical model coupled to COAWST, with an offline link to a bio-optical seagrass model. We assessed potential seagrass habitat in a eutrophic estuary under future nitrogen loading and sea-level rise scenarios, focusing on West Falmouth Harbor, a shallow estuary located on Cape Cod. Scenarios of future nitrate reduction and sea-level rise suggest an improvement in light climate in the landward basin with a 75% reduction in nitrate loading. This coupled model may be useful to assess habitat availability changes due to eutrophication and sediment resuspension and fully considers spatial variability on the tidal timescale.