Modelling tidal energetics of the Columbia River Estuary
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The Columbia River Estuary is shallow and has a mixed diurnal and semidiurnal tide, strong riverflow, broad tidal flats and variable channel cross-section. It is weakly non-linear with respect to tidal forcing, as measured by a ratio of tidal amplitude to depth. These features, common to many shallow estuaries, were incorporated into a one-dimensional harmonic transport model. The harmonic method utilizes the fact that nearly all of the energy in the system is in a few fundamental tidal frequencies, the first overtides thereof and the mean flow. This allows the representation of the flow as a series of harmonic components whose spatial variability is determined by the model. The model provides a qualitative explantation for and accurate quantitative predictions of along-channel variations in tidal properties in terms of the momentum balance. Near the mouth of the estuary, the boundary shear stress, the pressure gradient and the acceleration terms are all important in the force balance, and the tide behaves like a damped oscillator. Far upriver the pressure gradient is primarily balanced by friction at the bed and tidal wave propagation can be described as a diffusion process. Changes in the channel width in mid-estuary cause partial reflection of the tidal wave and a maximum in tidal range. This partial reflection decreases the tidal energy flux in the upstream direction. The distribution of the time averaged energy fluxes was also determined from the model. The analysis shows that there are two regions of high energy dissipation. Energy is supplied near the mouth of the estuary by the divergence of the landward flux of tidal potential energy and far upriver by the divergence of the seaward flux of fluvial potential energy. These two regions of high dissipation are separated by a region of low dissipation, the energy flux divergence minimum. This spatial division of the estuary is mirrored in the system biology and geology. It is likely that the energy flux divergence minimum is a common feature of shallow estuaries with strong riverflow. 1989.
