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Lookup NU author(s): Professor Andy Large,
Dr Graham Jewitt
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The effects of the February 2000 extreme flood (6000-7000 m3 s-1, 200 year return period flow) on channel morphology are examined over 108 km of the semiarid bedrock-influenced Sabie River, South Africa. The river is incised into bedrock forming a macrochannel with a planform that is fixed over the timescale of change being investigated. Within the confines of the macrochannel, the river may be zoned into distinct channel types (pool rapid, bedrock anastomosing, etc.) between 1 and 20 km long each characterised by a distinctive assemblage of smaller morphologic units ranging from large lateral bars filling most of the macrochannel to metre-scale lee bars and sand sheets. Changes in morphologic unit composition and spatial coverage are presented for each channel type, along with reconstructed flood hydraulics. This appraisal is based on 1:10,000 scale aerial photographs taken 5 months before and 7 months after the event, the latter verified by post-flood field observations. Some channel types (e.g., cohesive mixed anastomosing) displayed widespread sedimentary stripping, while others (e.g., bedrock anastomosing) remained essentially unchanged. At the morphologic unit scale, large cohesive lateral deposits were largely unaffected by the flood, whereas smaller active channel unconsolidated and consolidated features associated with perennially flowing sub-channels were stripped. At the scale of channel type, change appears to be controlled by a complex combination of factors including (i) spatial and temporal flow energy level change affecting sediment transport and (ii) tributary location. Susceptibility to erosion at the scale of the morphologic unit appears to be linked to elevation above the low flow active channel with features at lower elevations in the channel being stripped. Field observations suggest that unconsolidated, active channel units are re-forming at stripped locations. © 2004 Elsevier B.V. All rights reserved.
Author(s): Heritage G, Large ARG, Moon B, Jewitt GPW
Publication type: Article
Publication status: Published
Print publication date: 28/10/2004
ISSN (print): 0341-8162
ISSN (electronic): 1872-6887
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