Oil Spill Prevention & Response
Texas Water Development Board (TWDB), in partnership with the Texas General Land Office (TGLO) Oil Spill Prevention and Response effort, maintains near real-time and forecast simulations of water circulation in the major bays for use in the event that an oil spill occurs within Texas bays. For bays with heavy ship traffic, these models quickly provide information to TGLO responders on the potential path of an oil spill by predicting water current direction and speed throughout the bay. TWDB has models for Sabine Lake, Galveston Bay, Matagorda Bay, and Corpus Christi Bay and is in the process of developing a model for San Antonio Bay.
TxBLEND Modeled Currents with Associated Winds and Tides
TxBLEND Hydrodynamic Model for Oil Spill
Currents are computed with a hydrodynamic and salinity transport model called TxBLEND. The model was developed at Notre Dame University and was subsequently adopted, modified, and applied by TWDB to Texas' bays and estuaries. TxBLEND is a two-dimensional (vertically-averaged) finite-element model which solves the continuity equation, momentum equations, and the advection-diffusion equation for conservation of salt. The model uses unstructured, numerical grids based on the bathymetry of each bay, thus each one has different sizes and shapes so that the grid point, or nodes, vary. For example, Sabine Lake has 2,341 nodes while Corpus Christi Bay has 4,218. The computational time step for the estuaries also varies. Matagorda Bay model has a 150 second time-step, and the other estuaries have 300 second time-steps. Water velocities and depths are calculated by TxBLEND at each node for each time step, but results are displayed only every three hours within the three-day window described above, and only for the few nodal locations shown.
Currents predicted by this model are input into another type of computer model known as a trajectory model. This model, called GNOME, uses currents, winds, and other information to predict where oil will move in the near future. Oil-spill emergency response teams coordinated by the Texas General Land Office (TGLO) use information provided by these models to deploy clean-up teams and other resources to minimize a spill's impact.
TxBLEND Model Input and Related Data
Tides are the slowly changing rise and fall of the ocean and bay levels in response to interactions between the sun, moon, and earth. Tides are normally the dominant force in generating currents in Texas' bays. Rising tide levels in the Gulf of Mexico push water into the bays through the passes. For example, tidally driven currents enter Galveston Bay through San Luis Pass, the pass at Bolivar Roads, and Rollover Pass. Likewise, falling tides in the Gulf drain the bays through the passes.
Gulf tides are measured along the Texas coast by the Texas Coastal Ocean Observation Network (TCOON) and the National Water Level Observation Network (NWLON) under the management of the National Oceanic and Atmospheric Administration (NOAA). TWDB computes forecasts for over 20 of these stations based on tidal harmonic constituents from several years of measured data. Comparisons between the measured tides and the hindcasts/forecasts are shown on the TWDB Tide Forecasts for Texas. The differences in the measured data and the hindcasts/forecasts are largely due to meteorological events.
For information on offshore and nearshore currents and winds, go to the: Texas Automated Buoy System (TABS).
Wind values used in the bay models are derived from the NAM Model of the National Centers for Environmental Prediction (NCEP). Staff in the Texas A&M University Oceanography Department process the NAM output and provide TWDB with wind data files particular to the bay or estuary. Winds are from the model. None are direct observations, although observations closely agree with the model values at the time they were created, since observations are interpolated to the NAM model grid.
Wind plots on the respective model pages show the hindcast/forecast wind speed and direction that is input into TxBLEND for the respective bay, including a "stick plot".
Rivers provide the dominant source of freshwater inflow to the estuaries. The model input for the principal stream flows are mean daily flows based on the recorded measurements from the United States Geological Survey (USGS) stream gages, which are updated daily. The last daily mean available is the day previous to when the model is executed, so that day's value is used subsequently. For example, a model that is executed today will use daily mean flows as measured through yesterday. Yesterday's inflow is extended to the value for today's and the following days' inflows. Other minor sources of inflows which may be included are: constant value inflows derived from historical flows or from estimates of ungaged surface run-off averaged over a normal year; and monthly estimates of flows from power plants based on their historical water intake and returns.