Reach-scale controls of a pool-rapid channel during an extreme flood event : Sabie River, South Africa
The Sabie River exhibits a unique macrochannel incised 10 – 20 m into the Cainozoic planar surface, producing a mixed-bedrock-alluvial system draining a semi-arid catchment of 7096 km2. The channel sequences between bedrock outcrops alternating with alluvial features and are influenced by rare, high magnitude flood events, which act as a major control to riparian landscape maintenance. Pool-rapid channels found in the Sabie River are particularly influenced by large floods because of their vulnerability to both sedimentation and scour brought on by non-uniform hydraulics. Through an aerial LiDAR survey within Kruger National Park, Mpumalanga, 50km of 1 m spatial data were investigated to isolate a single reach consisting of a pool-rapid channel and an anastomosing channel immediately upstream. These data were used to create a 2D flow model (JFlow) simulating high magnitude discharge ￼￼scenarios up to ~5000 m3s-1, incorporated with post-processed spatial data including channel\ud velocity, depth, water surface elevations. Further analysis occurred through the development of spatial models comprised of Froude numbers and bed shear stress. In addition, observations of time sequenced aerial photographs involving multiple channels similar to and including the study area were scrutinised for obvious alterations in morphology following both Cyclone’s Eline and Dando. Model outputs indicate comparably reduced longitudinal flow velocity, Froude and bed shear stress as flow passes through an area of increased gradient of the anastomosing channel prior to the pool-rapid during increased discharge. Substantial rises in cross-sectional area following the upstream pool-rapid boundary (control point one) as a result of island networks becoming submerged, greatly influenced Froude numbers and bed shear stress as macro-channel settings within this area allowed floodwater to spread much wider than elsewhere in the reach. Further downstream at subsequent control points representative of individual rapid units, channel hydraulics displayed an irregular template of relative highs and lows associated with differential slope and geometric characteristics at each control point. Geometric settings situated both upstream and downstream of each control point also influenced the hydraulic nature of the channel during extreme discharge, in addition to other features associated with the channel (e.g. tributary confluences and alluvial bedforms). Geomorphic response is mixed because of such irregular hydraulics, however deposition is much more prevalent, with progressive aggradation occurring years following 2012’s Cyclone Dando as seen in photography leading up to 2015. Such conditions allow for areas of refugia for riparian species such as Combretum erythrophyllum (River Bushwillow) to develop, enabling pool-rapids in the Sabie River to possess a unique platform for riparian habitat recovery.
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