Effects of flood dynamics on island geomorphology in a large mixed bedrock-alluvial anabranching river: a case study of the Vaal River near Parys.

Abstract

Rare-infrequent magnitude floods of shorter duration events are significant natural hazards and play a key role in shaping river channel and island geomorphology. For a given river or channel, there is a relationship between the magnitude of a flood, frequency of a flood and duration of a flood, once a flood exceeds a reach-averaged erosional threshold geomorphic change can begin to occur. Expansion of conceptual models for the response of mixed bedrock alluvial influenced dryland rivers to such floods is of increasing scientific importance. The Vaal River near Parys in the Free State Province is characterised by a variable degree of mixed bedrock-alluvial anabranching channels which divide and re-join around the islands. In this study the historical aerial images and flow data from 1938 to 2016 were used to determine the effects of flood dynamics on island geomorphology in a large mixed bedrock-alluvial anabranching river: Vaal River near Parys. The historical aerial images and flow data reveals some minor island geomorphological changes during flood of rare magnitude, infrequent and shorter duration. The highest flood in the record was the one which was found having a recurrence interval of 20-50 years. The changes observed in the mixed bedrock-alluvial anabranching river in the Vaal River near Parys, indicate some minor decrease in the island bar area during flood of rare magnitudes, infrequent and shorter duration with recurrence interval of 20-50 years. The findings in this study area also reveals that the island bars in the area of study shows some degree of stability, however for the past 78 years the islands bar have not change the position. The impact of flood dynamics on island geomorphology in a large, mixed bedrock-alluvial anabranching river is not yet researched in the area of study (Vaal River), these findings will contribute to enhanced analysis of the Vaal River, relatively the impacts of extreme floods in island and channel geomorphology.