A holistic environmental flows (EFlows) assessment, undertaken as part of Ecological Reserve determination studies for selected surface water, groundwater, estuaries and wetlands in the Usuthu/Mhlatuze Water Management Area, South Africa, led to recommendations for modified releases from the Jozini Dam to support the socially, economically and ecologically important Pongola Floodplain situated downstream of the dam. The EFlows study analysed various permutations of flow releases from the dam based on the recommendations of pre-dam studies, and augmented by more recent observations, inputs from farmers and fishermen who live adjacent to the floodplain and discussion with the operators of Jozini Dam. The EFlows method used, DRIFT, allowed for the incorporation of detailed information, data and recommendations from a decades-old research project on the Pongola Floodplain that was undertaken prior to the construction of the Jozini Dam into a modern-day decision-making framework. This was used to assess the impact of a series of different flow releases on nature and society downstream of the dam. It was concluded that, within historic volumetric allocations to the floodplain, a release regime could be designed that considerably aided traditional fishing and grazing without necessarily prejudicing other uses, such as agriculture.

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The Pongola Floodplain in the Makhathini Flats is an area of low topographic relief between the 1973-commissioned Jozini Dam, and the Usuthu River which borders Mozambique. The floodplain system is characterised by a complex mosaic of meandering river channels, levees, and floodplains interspersed with pans (or depressions) and wetlands. The landmark 1982 study of the floodplain, Man and the Pongolo Floodplain, suggested a pattern of flows to ‘maintain the floodplain’ based on socio-ecological criteria. Since 1998, however, annual releases have been primarily targeted at the needs of recession agriculture and inundation of the floodplain in the Ndumu Reserve. No releases have been specifcally aimed at maintaining the floodplain ecosystem and the services it delivers to support the livelihoods of local communities. In 2013, the Department of Water and Sanitation commissioned an Ecological Reserve study of the Usuthu/Mhlatuze Water Management Area, which incorporates the Pongola Floodplain. This paper describes two-dimensional hydrodynamic modelling using RMA2 to inform this flow assessment. Four computational Pongola Floodplain models have been developed since 1979, including cell-based, one- and two-dimensional approaches. The RMA2 model is based on existing topographical, hydrological and hydraulic information, and was calibrated and verifed for the period 2008 to 2010 using water-level data from the local hydrometric monitoring network. Generally, good replications have been achieved in terms of peaks, rising and recession limbs, recession of ponded pan water-levels, and low-flow river stages. The RMA2 modelling represents an advancement of previous hydrodynamic studies of the floodplain and contributes to an improved understanding of its hydraulic behaviour. Model application was for the 15-year period 1990 to 2004, and simulations included naturalised, present management (2014), and 7 potential dam operational scenarios. The results were post-processed for analyses in the DRIFT DSS, described in the companion paper.