The Influence of Flooding on the Erodibility of Cohesive Sediments along the Sabie River, South Africa

  1. N. D. Smith4 and
  2. J. Rogers5
  1. G. L. Heritage1,
  2. A. L. Birkhead2,
  3. L. J. Broadhurst2 and
  4. B. R. Harnett3

Published Online: 17 MAR 2009

DOI: 10.1002/9781444304213.ch10

Fluvial Sedimentology VI

Fluvial Sedimentology VI

How to Cite

Heritage, G. L., Birkhead, A. L., Broadhurst, L. J. and Harnett, B. R. (1999) The Influence of Flooding on the Erodibility of Cohesive Sediments along the Sabie River, South Africa, in Fluvial Sedimentology VI (eds N. D. Smith and J. Rogers), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304213.ch10

Editor Information

  1. 4

    Department of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, NE 68588-0340, USA

  2. 5

    Cape Town, South Africa

Author Information

  1. 1

    Department of Geography, Peel Building, Salford University, Manchester M5 4WT, England

  2. 2

    Centre for Water in the Environment, Department of Civil Engineering, Hillman Building, University of Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa

  3. 3

    Kings College, London University, London, England

Publication History

  1. Published Online: 17 MAR 2009
  2. Published Print: 7 OCT 1999

ISBN Information

Print ISBN: 9780632053544

Online ISBN: 9781444304213

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Keywords:

  • influence of flooding on erodibility of cohesive sediments along Sabie River South Africa;
  • fluvial geomorphological work in terms of volume of sediment transported through a reach;
  • characteristics of Sabie River and study sites location;
  • hydraulic shear resistance of cohesive sediments;
  • American society of civil engineers (asce) task committee on erosion of cohesive sediments;
  • applied shear stresses during extreme flood events;
  • flooding and erosion of cohesive sediments

Summary

The dominance of extreme flood events in controlling geomorphological change in incised semi-arid river systems has been demonstrated by many authors. Catastrophic stripping of cohesive sediment may occur, particularly where vegetation cover is sparse. This paper investigates the existing potential for large-scale stripping of the cohesive fluvial sediment deposited in the incised macrochannel of the Sabie River, South Africa. The frequency distribution of critical resistance of the cohesive bed material was estimated using in situ shear-vane strength measurements, combined with laboratory testing of plasticity index and grain-size distributions. Topographical and hydraulic data were used to compute the distribution of applied hydraulic shear stress, corresponding to a large flood event of between 1705 and 2259 m3 s−1 (1-in-60 yr return period on the annual maximum flow series at the downstream reaches), for cross-sections along representative reaches of bedrock-anastomosing, mixed-anastomosing, pool–rapid, braided and alluvial single-thread channel types. The probability of eroding the cohesive sediments was determined by statistical analysis of all possible combinations, where the applied hydraulic shear stress was greater than the critical resisting shear strength of the bed material. The results show that 68% of the area of cohesive bed material along the Sabie River potentially may be eroded by such an extreme flood. Field observations indicated that this did not occur during a recent flood of this magnitude. The important role of in-channel vegetation in dissipating energy by increased resistance, thereby reducing boundary shear, was identified as a major factor preventing widespread cohesive sediment stripping. This was clearly evident in the densely vegetated bedrock-anastomosing reaches, where flow resistance was found to be highest and geomorphological change as a result of erosion of the cohesive sediment was minimal. The inability of the flood to erode sediment also may be attributed partly to the river transporting material at capacity. This is likely to be a function of antecedent conditions, where a recent prolonged drought resulted in sediment accumulating over the catchment. Furthermore, the short duration of the flood may have contributed, by inundating the large-scale consolidated deposits for a relatively short period, insufficient to instigate widespread erosion. This study demonstrates that there is considerable potential for large-scale stripping of the macrochannel deposits along the Sabie River and emphasizes the need to manage the riparian vegetation, which acts to protect the cohesive bed from entrainment, if the physical and ecological diversity of the river is to be maintained.