Hydrodynamic Model of Coos Bay

What is a hydrodynamic model?

A hydrodynamic or “circulation” model is a three-dimensional computer simulation used to describe the motion of water in a defined body of water (e.g. lake or estuary). The model informs us about water circulation and related water quality attributes such as temperature or salinity. For example, a hydrodynamic model simulating water movement in an estuary will be able to inform us how far up the estuary salt water can be found during different tides and at different times of the year, or what the water current speed and direction is at a given location in the estuary during different tides and at different times of the year.

Hydrodynamic models for estuaries rely on computer simulation techniques to describe water circulation, temperature and salinity in the estuary. A computer model of the estuary consists of a three-dimensional grid made of thousands of individual three-dimensional cells. Each cell is programmed using data collected from instruments deployed in the estuary, including information about water temperature, salinity, current speed and velocity, river discharge, and wind direction and speed (among other data). Computers determine how all these cells act together to create a full three-dimensional simulation of water flow and changes in water temperature and salinity in any given part of the estuary- all based on the values assigned to every grid cell in the model!

Model Development

The Coos estuary circulation model was developed by Dave Sutherland, Associate Professor, and Ted Conroy, prior master’s student, from the Department of Earth Sciences at the University of Oregon.

Model Validation

The success of any hydrodynamic model depends on how well the modelled values compare with real, measured observations. Validation of the Coos estuary model was a collaborative effort with information provided by multiple sources: South Slough National Estuarine Research Reserve (SSNERR) contributed data on water level, salinity, temperature, and current velocity and direction. Bathymetric data were obtained from a variety of sources: NOAA charts and Digital Elevation Maps (DEM), United States Army Corps of Engineers Light Detection and Ranging (LiDAR) and channel surveys, and Oregon State University’s Coastal Profiling System, a high-speed maneuverable personal watercraft-based system. The ability to merge these bathymetric datasets allowed for model resolution of 15 meters within the estuary! Freshwater river discharge was provided by stream gaging efforts of the Coos Watershed Association.

Meteorological data was supplemented by location weather stations to account for the influences of wind. The integration of these various data sets allowed the Coos estuary model to be validated in its ability to accurately simulate water level, salinity, and current velocity and direction. Please see figures 2-4 in for further information regarding the validation of the model.

How is the model used?

The Coos estuary hydrodynamic model is a useful tool to examine numerous questions.

The model is utilized to study estuarine dynamics such as how salinity changes on a seasonal basis due to differing freshwater inflows. Water quality concerns are also being addressed such as the fate and transport of secondary wastewater effluent released into the bay. The model can help address ecological questions and concerns, for example, the hydrodynamic connectivity of larvae between oyster beds, the likely settlement sites of invasive green crabs, and the potential factors causing the eelgrass decline in the South Slough reserve. It is important to understand how the estuary historically behaved and may change into the future due to further dredging of the present estuary. Therefore, the hydrodynamic model has been modified with historic (1865) and proposed dredging bathymetries. Furthermore, sediment dynamics are examined using the historic, present day, and potential future dredging models.

Re-routing of the major river that feeds the estuary has caused sediment to be more efficiently routed into the primary channel; this channel has at the same time been deepened, meaning sediment now finds an excellent "sink" in the navigation channel, and is less likely to traverse the intertidal flats. This creates a feedback loop demanding ongoing dredging, as seen in other modified systems. Coos Bay has a diversity of sedimentary environments, including broad intertidal flats and deep navigation channels. Shallow tidal-flat environments are typically characterized by mud and sand, and in some places provide important habitat for eelgrass and oysters. Channels are scoured in some places by fast currents, leaving a gravel and shell layer; elsewhere channels accumulate mud delivered by rivers, especially during rainy periods. Sediment dynamics have been incorporated into the hydrodynamic model of Coos Bay, in order to explore where sediment travels during periods of high and low river discharge, and during different tidal phases.

Validation of the sediment model was done using samples from the seabed of the estuary (which were analyzed in laboratories for sediment sizes) and measurements of water clarity (or amount of sediment mixed throughout the water column) from instrumentation stations throughout the system.