Citation:

Frick, W E. AND A C. Sigleo. ESTIMATING NITROGEN AND TIDAL EXCHANGE IN A NORTH PACIFIC ESTUARY WITH EPA'S VISUAL PLUMES. Presented at 3rd International Conference on Marine Waste Water Discharges, Catania, Italy, September 27-October 2, 2004.

Impact/Purpose:

A main objective of this task is to combine empirical and physical mechanisms in a model, known as Visual Beach, that

● is user-friendly

● includes point and non-point sources of contamination

● includes the latest bacterial decay mechanisms

● incorporates real-time and web-based ambient and atmospheric and aquatic conditions

● and has a predictive capability of up to three days to help avert potential beach closures.

The suite of predictive capabilities for this software application can enhance the utility of new methodology for analysis of indicator pathogens by identifying times that represent the highest probability of bacterial contamination. Successful use of this model will provide a means to direct timely collection of monitoring samples, strengthening the value of the short turnaround time for sampling. Additionally, in some cases of known point sources of bacteria, such as waste water treatment plant discharges, the model can be applied to help guide operational controls to help prevent resulting beach closures.

Description:

Relatively large fluctuations in temperature, nitrate, and other water properties were observed in August 2000 three kilometers from the entrance jetties to Yaquina Bay, Oregon, USA. Exhibiting periods comparable to the tides, several hypotheses were proposed to explain the variations, including internal tides, ocean patchiness, and Yaquina Bay tidal outflow. The Prych-Davis-Shirazi (Windows) model, PDSW, as found in the US EPA Visual Plumes model, was used to test the viability of the outflow hypothesis. PDSW is a steady state model in which plume development time is short relative to times during which ambient conditions, particularly currents, change significantly. The model was originally designed for relatively small industrial discharges. However, it has been used successfully to assess large power plant cooling flows, with 50m wide discharge channels. The Yaquina Bay jetty is half an order of magnitude wider than that and, at the time of first testing the model on this case, represented the largest known channel to be simulated with the model. Subsequent application to the much larger Columbia River tidal plume shows that it might be useful for understanding even larger outflows. The results indicate that, within the limitations of the model, it is appropriately applied to the Yaquina plume and represents verification evidence for the use of the model when ambient ocean currents are steady. However, where ocean currents change considerably in a few hours, the steady state assumption requires further consideration as plume trajectories will meander horizontally and often fractionate, or become patchy. Data from two depths below the nitrate sensor indicate no vertical temperature gradient to support the internal tide hypothesis. While fluctuating by up to three degrees over tide cycles, temperature changes are relatively abrupt at both levels, without discernible lag. Oceanic patchiness cannot be discounted out of hand but may often be related to the movement of previously discharged estuarine water into the site region as alongshore currents reverse directions. The PDSW analysis supports the tidal Yaquina Bay outflow hypothesis as a cause for nitrate and water property variations near the Yaquina Bay jetties. However, further verification of PDSW focuses on analyzing another data source in the effort to establish that the relatively high bay temperatures, needed to support the plume hypothesis, were recorded. If that effort supports the plume hypothesis, it will suggest that PDSW predictions, while useful for predicting the horizontal penetration of large plumes onto the coastal shelf, underestimate the depth of plume penetration into the water column.

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