Modeling of water quality in estuaries and estuarine networks (Virginia, Maryland)

A one dimensional finite element model for the prediction of transient water quality in an estuary is developed. Ten water quality constituents, salinity, coliform bacteria, organic nitrogen, ammonia nitrogen, nitrite-nitrate nitrogen, organic phosphorous, inorganic phosphorous, phytoplankton as measured by chlorophyll \u27a\u27, DO and CBOD are included in the model. The effects of temperature, solar radiation and zooplankton predation are incorporated into the model as external forcing functions. The concentration distributions are obtained by solving the conservation of mass equations successively. A compatible hydrodynamic model is also developed to provide the driving transport processes to the water quality model. The finite element method is chosen in the numerical formulation because of its flexibility in grid layout. Linear interpolation functions are used for the spatial discretization. The time scale of the model is tidal time. The downstream boundary condition is related to the direction of tide. On flood tide the concentration of the inflowing water is prescribed. On ebb tide the dispersive flux is determined by the concentration distribution within the estuary. Two time integration methods, trapezoidal and explicit method with split time scheme, are developed. The trapezoidal scheme is found to be very stable and the accuracy is adequate for most applications. The explicit version of the computer program is shorter and the execution time is faster, but the size of the integration time step is more restricted. Both versions are validated against known analytical solutions for a rectangular channel with constant upstream boundary concentrations and uniform velocities. The implicit model was used to simulate the water quality in the James River estuary from March to October 1971. The model has also been applied to the Potomac River in Virginia, and is being applied to the Chester River in Maryland. A network version of the water quality model is also developed. The model is capable of simulating water quality in estuaries that have interconnecting channels or estuarine networks such as the lower portion of the Appomatox River. This model was used to simulate average water quality conditions in the Appomatox River