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

N Entwistle,S Tooth,D Milan,G Heritage

doi:10.5194/piahs-367-215-2015

N Entwistle, S Tooth + Show 2 more

Open Access

https://doi.org/10.5194/piahs-367-215-2015

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Abstract

Abstract. Rivers in the Kruger National Park, South Africa, have variable degrees of bedrock and alluvial influence. Pre-2000 aerial imagery for the Sabie River (catchment area 6320 km2) reveals downstream alternations from alluvial single thread or braided, to bedrock anastomosed or mixed anastomosed channel types, with pool-rapids also present locally. In 2000 and 2012, extreme floods resulted in significant alluvial erosion, widely exposing the underlying bedrock. Since the 2012 flood, aerial LiDAR surveys reveal the strong gradient control exerted by the bedrock and mixed anastomosed channel types, which influences hydraulic conditions and sediment dynamics. Two dimensional hydraulic modelling of moderate floods (<1500 m3 s−1) reveals reduced velocities upstream of bedrock or mixed anastomosed channel types, which promotes deposition. During more extreme floods (>3500 m3 s−1), the bedrock or mixed anastomosed channel types are drowned out, resulting in dramatically increased velocities along the entire river and widespread alluvial stripping regardless of initial channel type or location.

Highlights

Many of the large rivers draining southern Africa are characterised by incised bedrock ‘macrochannels’ cut 10–20 m into ancient planation surfaces, but with variable amounts of unconsolidated alluvial sediment infill
This paper presents additional evidence from two significant cyclone-driven flood events (2000 and 2012) that have impacted on the Sabie River, South Africa, and which illustrate the controlling role of bedrock anastomosing and mixed anastomosing channel reaches in influencing the pattern of alluvial sediment loss and gain
During periods characterised by a flow regime dominated by low and moderate floods, local low velocity areas in the anastomosed reach would allow cohesive sediments to accumulate over bedrock to form bars and islands, and these tend to remain unimpacted by high flood velocities

Summary

Introduction

Many of the large rivers draining southern Africa are characterised by incised bedrock ‘macrochannels’ cut 10–20 m into ancient planation surfaces, but with variable amounts of unconsolidated alluvial sediment infill. This paper presents additional evidence from two significant cyclone-driven flood events (2000 and 2012) that have impacted on the Sabie River, South Africa, and which illustrate the controlling role of bedrock anastomosing and mixed anastomosing channel reaches in influencing the pattern of alluvial sediment loss and gain.

Results

Conclusion