Asymmetric Estuarine Responses to Changes in River Forcing: A Consequence of Nonlinear Salt Flux

Abstract

AbstractA linear theory for estuarine adjustment to river forcing as put forth by MacCready is extended to allow for quantification of nonlinear salt flux induced by gravitational exchange flow. It has been shown that, under a steplike change of river forcing, the estuarine responses are asymmetric, with the salinity field adjusting faster during the rising discharge. The asymmetry arises because the up-estuary salt flux due to exchange flow is a nonlinear function of estuarine length ∝ L−3. During the rising discharge, the estuary is longer, and the salt flux is comparatively less sensitive to the length variations. As a result, the up-estuary salt transport cannot keep pace with the rate of discharge changes, leading to a larger net salt flux and thus a shorter response time. A simple theory accounting for the nonlinear effect is then applied to Hudson-like systems and shown to capture the asymmetric response. The asymmetry is generalizable to other estuarine regimes where up-estuary salt fluxes are expressed as nonlinear power laws.