Measurement of Estuarine Physical Processes and Dredged Channel Response, Willapa Bay, WA

This paper describes a monitoring program to obtain high resolution and continuous measurements of estuarine physical processes and bathymetry in and near the Bay Center Entrance Channel in Willapa Bay, Washington. The field data provide valuable insights for identification of relevant processes and mechanisms controlling sediment transport in the channel and for establishing their relative importance in causing channel shoaling of a dredged navigation channel. The goal is to develop and improve numerical models used to predict estuarine sediment transport and morphological change at a range of scales. Planned maintenance dredging of the Bay Center Entrance Channel provided the opportunity to monitor bathymetry, hydrodynamics and suspended sediments in the channel to document pre- and post-dredging morphology and sediment transport processes. Dredging removed 178,00 cu yd from the channel in a 40-day period. Channel depth was excavated from 0 ft to 15 ft at its most constricted location. Process measurements were made at three stations, two in the Bay Center Entrance Channel (East and Middle Stations) and one in the Nahcotta Channel near the confluence with Bay Center Entrance Channel (West Station). The measurements during the period preceding and immediately following dredging span the season in which the channel has been observed to change location seasonally over a period of two decades. Measurements are interpreted with morphology changes to specify processes that are responsible for the observed pattern of channel changes. Varying asymmetry in the tidal flow induces a net transport toward the channel mouth that combined with local deceleration of the current results in deposition in the channel reach that has historically shoaled most quickly.