Modelling prospective future processes, sedimentation patterns and ecosystem service delivery at Stillerust Vlei, KwaZulu-Natal Drakensberg Foothills

Abstract

South Africa’s climate is projected to become warmer and drier, except for the Eastern Sub-escarpment region. The climate change projections for the Thukela catchment are for warmer, wetter conditions with increased inter-annual variability, and an increase in the frequency and intensity of convective storm events. These projected climatic changes have great implications on hydrogeomorphic processes, ecosystems and ecosystem services of floodplain environments as they lead to changes in river flow and flood regimes. Floodplains host the largest of South Africa’s wetland systems, supply the largest store of ecosystem services, and are strongly coupled with social and economic development. However, the potential responses of floodplain hydrogeomorphic processes and ecosystem service delivery to climate change are not currently understood. This research aimed to improve understanding on how the Stillerust Vlei floodplain will respond to the projected climatic changes of the region, which is essential for better assessment and management of the ecosystem services provided by floodplain wetland. The research addressed questions related to whether the wetland will be able to attenuate floods in the future, trap sediments to the same degree in the future, and the potential impact on nutrients and contaminants for better management of downstream water quality. This study investigates how sediment trapping dynamics, hydrogeomorphic processes and ecosystem service delivery might change in future as a form of floodplain response to climate change. This was achieved by applying a 2D hydraulic modelling approach, where the CAESAR-Lisflood model was used in flow only mode to simulate inundation depth and flow velocities. The model was used to run a series of numerical experiments that investigated sediment dispersal processes outcomes associated with three hydrological scenarios representing drought, intermediate and wet conditions derived from historical flow records. Spatial outputs of inundation depth and flow velocity were analysed to assess potential floodplain sediment dispersal patterns. Modelled flow velocities were analysed using the Hjulström curve to determine threshold velocities for sediment entrainment and settling within the floodplain as a function of particle size. The Wet-Ecoservices Version 2 was used to evaluate the potential ecosystem service provision associated with different flow and sediment dispersal patterns. The results show that flow magnitude and floodplain connectivity have a strong influence on sediment dispersal patterns. The results show that reduced floodplain inundation during the drought conditions resulted in limited sediment distribution which reduced the wetlands capacity to trap sediments and attenuate floods.