Estuarine–ocean exchange in a North Pacific estuary: Comparison of steady state and dynamic models

Nutrient levels in coastal waters must be accurately assessed to determine the nutrient effects of increasing populations on coastal ecosystems. To accomplish this goal, in-field data with sufficient temporal resolution are required to define nutrient sources and sinks, and to ultimately calculate nutrient budgets. Models then are required for the interpretation and analysis of data sets. To quantify the coastal ocean nitrogen input to Yaquina Bay, Oregon, nitrate concentrations were measured by a moored sensor hourly for one month during summer upwelling some distance outside the estuary entrance jetties. The time series results then were interpreted using a steady state model (Visual Plumes’ PDSW) and a hydrodynamic model, the Finite Volume Coastal Ocean Model (FVCOM). The physical scales of many stream and river plumes often lie between the scales for outfall mixing zone plume models, such as those found in EPA’s Visual Plumes, and larger-sized grid scales for regional circulation models like FVCOM. A potential advantage of relatively simple, steady state plume models is that they use entrainment terms to close the plume equations, theory that has proven useful in simulating turbulent plume discharges from various sources, some approaching the dimensions of rivers. Important advantages of models like FVCOM are that they are dynamic and include the effects of the Earth’s rotation. The results showed that the steady-state plume model simulates observed velocity and concentration data fairly well during periods of strong discharge velocity and weak ambient coastal currents. FVCOM was judged to give better estimates under all other ambient current conditions, although the data from the mooring cannot be used to prove this assertion as stronger currents would deflect the plume away from the mooring. Nevertheless, plume models may be useful in establishing boundary and initial conditions for hydrodynamic models.