The Elephant Marsh, Malawi – Part 2: two-dimensional hydrodynamic modelling in support of an eco-social assessment

Abstract

The Elephant Marsh lies on the floodplain of the lower Shire River, in southern Malawi. It is both a cultivated, seasonal floodplain and a wetland, characterised by a complex mosaic of meandering channels, marshland and shallow lakes. In 2016, the Marsh was granted RAMSAR status based on a series of supporting studies, including the modelling of eco-social options for managing its ecological condition. This paper describes two-dimensional (2-D) hydrodynamic modelling using RMA2, in support of this eco-social assessment. The hydraulic model was parameterised using mostly existing data but augmented with bathymetric channel and lake surveys. Calibration and verification used water level data from the hydrometric network for the period 1999 to 2009. Generally, observed water level time-series are well replicated in the model, but there are large discrepancies prior to this period. These are due to temporal changes in hydraulic controls, mainly sedimentation, but also breaching of an embankment at the downstream end of the Marsh. Selected hydraulic-habitat variables (based on daily hydrological time-series 1976 to 2009) provide the indictors of hydrological and hydraulic change used to inform a DRIFT eco-social assessment. The Marsh displays moderate flood attenuation characteristics, but the importance of this will likely increase under continued sedimentation and predicted climate futures, which include more severe storms. The 2-D modelling contributes to an improved understanding of the Marsh’s hydraulic behaviour, particularly regarding anthropogenic influences since the early 1900s.