Numerical Investigation of Tidal and River Flow Interaction in the Willametteand Columbia River Tri-Channel System

摘要

The Willamette River is a major tributary to the Columbia River and joins the Columbia River at Columbia river mile (CRM) 100. There is a bypass channel, called the Multnomah Channel, which connects the Willamette River at Willamette river mile (WRM) 3.5 with the Columbia River at CRM 86.5. This tri-channel system has several unique hydrodynamic features. The hydrodynamics are heavily influenced by strong interactions between fresh water and tides, timing difference in hydrographs between the Willamette River and Columbia River, which are both regulated, tidal lag through the Multnomah Channel and the Columbia River, and fluctuations of dry and wet seasonal river flows. The hydrodynamic conditions in the Willamette River and Multnomah Channel confluence area are strongly dependent on tidal and river flow interactions. Understanding such complex and dynamic interactions is important for some ongoing studies, such as a remedial investigation/feasibility study (RI/FS) in a lower part of the lower Willamette River, called the Portland Harbor Superfund site, and for long-term maintenance of the Willamette River Federal navigation channel.This paper presents an investigation of how a combination of river flows and tides influences the flow regime both in the Willamette River and in the Multnomah Channel using an existing unsteady HEC-RAS model. The model was run for a range of flow conditions, and the results are evaluated to describe the flow conditions in the Willamette and Multnomah Channel confluence area. In particular, the paper describes how the split of the Willamette flows between the Multnomah Channel and the Willamette River is related to the tidal and river flow conditions, and when flow reversal in the Willamette River occurs.