Anastomosing reach control on hydraulics and sediment distribution on the Sabie River, South Africa

Entwistle, NS; Heritage, G; Tooth, S; Milan, D

Abstract

Many of the large rivers in southern Africa exhibit a strong bedrock influence, being characterised by a channel incised 10–20 m into ancient planation surfaces. Under alluviated conditions, these channels display downstream sequences of channel types, including alluvial single thread, braided, bedrock anastomosed, mixed anastomosed, and pool-rapid. This sequence of channel types has been reviewed using aerial imagery for the Sabie River, which drains a 6320km2 catchment and flows across the Lowveld of South Africa. Prior to 2000, the river exhibited a downstream sequence of channel types that broadly alternated from alluvial single thread or braided to bedrock anstomosed or mixed anastomosed, with pool-rapid types also present locally. Unconsolidated (predominantly sandy) alluvial sediments were significantly eroded by cyclone-driven extreme floods both in 2000 and 2012, exposing the underlying bedrock template along considerable lengths of the river. This bedrock-dominated state was surveyed using aerial LIDAR following the 2012 flood. Long profile data revealed the strong gradient control exerted by the bedrock anastomosed and mixed anastomosed channel types, which creates hydraulic conditions suitable for deposition in the upstream alluvial reaches. The role of these bedrock-influenced channel types on hydraulic character is also revealed in the results of 2D hydraulic modelling of moderate flood (<1500 m3s-1) conditions for the channels, where velocities are much reduced upstream of bedrock or mixed anastomosed channel types, thereby promoting deposition. This hydraulic effect is lost under more extreme flood conditions (>3500 m3s-1) as the bedrock or mixed anastomosed channel type is drowned out, resulting in dramatically increased velocities along the entire river and a general stripping of unconsolidated and consolidated sediments regardless of initial channel type or location.

Citation

Entwistle, N., Heritage, G., Tooth, S., & Milan, D. (2015). Anastomosing reach control on hydraulics and sediment distribution on the Sabie River, South Africa. Proceedings of the International Association of Hydrological Sciences, 367, 215-219. https://doi.org/10.5194/piahs-367-215-2015

• Journal Article Type: Article
• Online Publication Date: Dec 14, 2014
• Publication Date: Jan 1, 2015
• Deposit Date: Jan 16, 2015
• Publicly Available Date: Apr 5, 2016
• Journal: Proceedings of the International Association of Hydrological Sciences
• Print ISSN: 2199-8981
• Publisher: Copernicus Publications
• Peer Reviewed: Peer Reviewed
• Volume: 367
• Pages: 215-219
• DOI: https://doi.org/10.5194/piahs-367-215-2015
• Publisher URL: http://dx.doi.org/10.5194/piahs-367-215-2015
• Related Public URLs: http://iahs.info/uploads/dms/16687.367Abstracts-for-website-28.pdf