Modelling prospective future processes, sedimentation patterns and ecosystem service delivery of a wandering reach of the Olifants River floodplain, Western Cape

Abstract

Changes in river flow regimes driven by climate change or anthropogenic developments have the potential to alter flood and sediment dispersal patterns, and the delivery of ecosystem services. Climate change projections for the winter rainfall region of the Western Cape suggest warmer, drier conditions with more prolonged and intense drought periods, shorter duration but high intensity rainfall events, and overall increased inter-annual rainfall variability. Considering this, it can be expected that current processes of river flow, sediment dispersal patterns and the delivery of ecosystem services may be affected by these estimated climatic changes. This research therefore aimed to assess how sediment dispersal patterns and ecosystem service provision vary during periods of low, intermediate and high river flow scenarios, within a multi-thread wandering river reach of the Olifants River floodplain, Western Cape. The rationale for this approach is that, although it is difficult to predict the exact nature of future flow regimes, it is possible to evaluate how a river floodplain system may respond to different river flow patterns that have characterised a system in the recent past, and to use such insight to inform how the system may respond in the future if one type of flow regime becomes more dominant. The first objective was to develop representative flow time series for different flow scenarios representing low, intermediate, and high flow periods. This was achieved through collection and analysis of existing daily average river flow data and collected river flow data. The second objective was to evaluate the sediment dispersal patterns under the different flow scenarios. This was achieved by using the CAESAR-Lisflood landscape evolution model in reach mode. To address this objective the developed flow time series scenarios, field collected sediment data, a LiDAR-derived digital elevation model, and field estimates of the Manning’s roughness coefficient were used as the fundamental input parameters to run simulations for the above-mentioned flow scenarios. The last objective was to assess the implications of model output flow and sediment dispersal patterns on the potential delivery of flow and sediment-associated ecosystem services which are the flood attenuation, sediment trapping and phosphate assimilation services. The WET-EcoServices tool and output results from the CAESAR-Lisflood simulations were used to address this objective.