A note on salt intrusion in funnel-shaped estuaries: Application to the Incomati estuary, Mozambique

Salt intrusion in estuaries is important for ecological reasons as well as water extraction purposes. The distance salt intrudes upstream depends on a number of factors, including river discharge, tidal and wind mixing and gravitational circulation. In this paper, an analytical solution is presented for the salt intrusion in a well mixed, funnel-shaped estuary whose cross sectional area decreases exponentially (with decay coefficient beta) with distance, x, inland, and in which longitudinal mixing is constant along the length of the estuary. The solution predicts that a graph of the logarithm of salinity against exp (beta x) should be a straight line, with slope proportional to the mixing coefficient K-x. The solution is tested against observations from 15 surveys over a four-year period in the Incomati estuary. Good straight line fits, as predicted, are observed on all surveys, with a mean R-2 = 0.97. The average value of K-x for all surveys is 38 m(2) S-1. The solution is used to make predictions about the minimum river flow required to prevent salt intruding to an extent where it causes a detrimental effect on water extraction. The minimum recommended river flow required to prevent this is 35 in 3 S-1. In recent years, flow has fallen below this level for several months each year. (c) 2005 Elsevier Ltd. All rights reserved.