The Hydrodynamics of the Incomati Estuary – An Alternative Approach to Estimate the Minimum Environmental Flow

Abstract

The Incomati Estuary, with a length of about 30 km, a width of about 1.5 km, and an average depth of 10 m, is located in the , with peak and lower flows during the wet season (November to March) and dry season or winter (June-August), respectively. The morphology of the estuary is characterized by meanders, islands and sand banks, fringed by mangroves. The tides are semidiurnal, with amplitudes varying from 0.5 to 3.5 m during the spring and neap tides, respectively. The predominant winds are trade winds, with average speed of 4 m s À1 . The estuary plays an important ecological role and sustains important socioeconomic activities. It is a nursery ground for important fisheries, a tourism attraction, and hosts significant agriculture and livestock activities. The sustainability of the natural resources and the activities in the estuary is threatened by the effects of the dams. The three basin countries agreed in 1991, during the "Piggs Peak Agreement," to the maintenance of a minimum environmental flow of 2 m 3 s À1 , which has been violated several times by South Africa. The present work investigates how the hydrodynamics of the estuary, which controls most of the ecological processes, is influenced by the river discharges. Tides, currents and water masses in the estuary are described, based on historical hydrodynamic data. A simple 1-D, depth and width integrated, tidal hydrodynamic model forced by the observed tides at the mouth, was used to reproduce the hydrodynamics of the estuary. In addition, an analytical solution of the advection-diffusion of salt is applied to reproduce the salt intrusion. The model was then used to determine the extent of salt intrusion as a function of the river discharge, and so used to estimate the minimum environmental flow required for a healthy estuarine ecosystem in Incomati. The results indicated the estuary shifts, from a stratified condition in the wet season to vertically-mixed in the dry season. The tidal wave takes about 30 minutes to propagate from the mouth to the head of the estuary. The current varied between 0 and 1.2 m s À1 , mostly driven by tides. The density driven circulation is weak, with a density gradient of about 0.10 kg m À3 per km, and a density-driven velocity of about 0.04 m s À1 .The hydrodynamic model applied explains 87% of the current observed in the estuary,

Birds and birding 2013-2020 at Macaneta, Southern Mozambique

“…However, Hoguane and Antonio (2016) measured salinity at 24 locations from the tip of Praia da Macaneta up to 30 km upstream in March and June 2002, and found that salinity decreased to less than 1 ppm at 19 km upstream in both dry and wet seasons, which is downstream of the main study site described herein (for reference the bridge at Marracuene is 22 km upstream). Despite these measurements, their modelling suggested salt water intrudes 22-25 km upstream in periods of low freshwater flow.…”

Improved Hydrological Understanding of a Semi-Arid Subtropical Transboundary Basin Using Multiple Techniques – The Incomati River Basin

“…and Macamo et al(2015) described the reduction of mangrove area and the negative impact on Incomati estuary ecosystems, attributed to human activities (deforestation, altered flow regime). According toHoguane and Antonio (2016), the minimum cross border flow established in the Piggs Peak agreement (2 m 3 /s) is not sufficient to meet the minimum environmental flow requirements of the estuary. They estimated using hydrodynamic and water quality models that at least 20 m 3 /s would be required to prevent salinity intrusion.…”