A note on salt intrusion in funnel-shaped estuaries: Application to the Incomati estuary, Mozambique Rachel Brockway a , David Bowers a, * , Antonio Hoguane b , Veronica Dove b , Valentina Vassele b a University of Wales, Bangor, School of Ocean Sciences, Menai Bridge, Anglesey, LL59 5AB, UK b Eduardo Mondlane University, Department of Physics, Maputo, Mozambique Received 19 May 2005; accepted 21 July 2005 Available online 19 September 2005 Abstract Salt intrusion in estuaries is important for ecological reasons as well as water extraction purposes. The distance salt intrudes up- stream 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 b) 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 (bx) should be a straight line, with slope pro- portional 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 Z 0.97. The average value of K x for all surveys is 38 m 2 s ÿ1 . The solution is used to make predictions about the minimum river flow required to prevent salt in- truding to an extent where it causes a detrimental effect on water extraction. The minimum recommended river flow required to prevent this is 35 m 3 s ÿ1 . In recent years, flow has fallen below this level for several months each year. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: salt intrusion; Incomati estuary; Mozambique 1. Introduction Salt intrusion in estuaries presents a management as well as a scientific problem. In many rivers, water is ex- tracted for irrigation and for drinking, and if it is contam- inated by salt from the sea, it is no longer useable (Aerts et al., 2000). There can be conflict between the needs of those who wish to extract water from the higher reaches of the river, and those who are affected by the resulting increased salt intrusion in the lower reaches of the estu- ary. From the scientific point of view, we wish to under- stand better the mechanisms by which salt mixes upstream, and to make predictions of the relationship be- tween water extraction, on the one hand, and salt intru- sion, on the other, to aid the management of such issues. In south-east Africa, the Incomati River rises in South Africa, and then flows through South Africa, Swaziland and Mozambique to reach the Indian Ocean at Maputo Bay, Mozambique (see Fig. 1). In recent years, tributary rivers feeding the Incomati have been extensively dammed, and this has had an impact on the estuary, where salt water is intruding further inland (Hoguane, 2000). This has had an immediate social and economic impact: agricultural land adjacent to the estu- ary which relied on freshwater for irrigation has had to be abandoned as the estuary has become more saline (Hoguane, 2000). * Corresponding author. E-mail address: oss063@bangor.ac.uk (D. Bowers). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.07.014 Estuarine, Coastal and Shelf Science 66 (2006) 1e5 www.elsevier.com/locate/ecss