University of Washington | College of the Environment | School of Oceanography | Physical Oceanography

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Estuarine Dynamics: Adjustment Time & Sensitivity

Estuaries form the transition between river and ocean waters, and their distinct combination of circulation and salinity gradients makes them sites of high biological productivity. I have developed a theory for the rate at which estuaries adjust the changes in forcing due to river flow or tidal mixing.

aestus figure

This figure shows a model simulation of an idealized version of the Husdon River Estuary over several months. The fields are all tidally-averaged. The forcing is varied (a) as the river flow goes from low to high and back to low (gray line) while the amplitude of the tidal velocity goes through a Spring-Neap cycle. The three panels on the right show salinity sections down the channel axis at different times, incidated by the symbols. The estuary goes through a significant portion of its parameter space during this simulation, being partially-mixed (day 10), well-mixed (day 17), and approaching a salt wedge (day 57). The response of the tidally-averaged stratification (the bottom to top salinity difference, averaged over the channel length, normalized by oceanic salinity) is shown in (b). There are two curves here, the solid one showing the actual model response, and the dashed one showing a "quasi-steady" response in which the model is allowed to fully adjust to the instantaeous forcing at each time step. The stratification increases during high river flow, and during neap tide. In (c) the length of the tidally-averaged salt intrusion vs. time is plotted, again with full-model and quasi-steady curves. The salt intrusion decreases markedly when river flow is high.

The effects of the adjustment time in this system can be seen by comparing the full-model and quasi-steady model solutions in (b) and (c). The adjustment time of this system varies between 3 days (during high river flow) and 31 days (during low river flow). During high river flow the adjustment time is short compared with the spring-neam cycle of tidal mixing, and the length of the salt intrusion varies approximately as predicted by quasi-steady theory. During times of low river flow, on the other hand, the length of the salt intrusion cannot adjust to changes in tidal mixing, although the stratification varies by a factor of three.