Estuarine Overmixing

Abstract

Abstract H. Stommel and H. G. Farmer derived a theory for the maximal exchange between an estuary and the adjacent shelf sea based on a combination of mass and salt conservation (also known as Knudsen’s relation) and hydraulic control theory at the estuary mouth. This upper limit of exchange flow is termed overmixing, because there is more than enough mixing in the estuary to maintain the density difference and exchange flow at the mouth. This seminal work inspired a number of papers on hydraulic control of baroclinic flow through constrictions. However, none of these papers returned to the idea that the limitations on the exchange imposed by hydraulic control could in turn modify the outflow density at the mouth: that is, that the mass and salt flux at the mouth must obey Knudsen’s relation in a steady state. Numerical simulations of an idealized estuary with artificially enhanced mixing show that the estuary never quite reaches the theoretical overmixing limit, although some of the qualitative features of the overmixing solution are reproduced. In particular, it is possible to locate the point in parameter space where constriction begins to limit the exchange flow. However, the exchange is always submaximal with regard to inviscid, two-layer hydraulic control.