Changing Size Distribution of Suspended Sediment in Arid-Zone Flash Floods

  1. J. D. Collinson and
  2. J. Lewin
  1. L. E. Frostick1,
  2. I. Reid1 and
  3. J. T. Layman2

Published Online: 29 APR 2009

DOI: 10.1002/9781444303773.ch7

Modern and Ancient Fluvial Systems

Modern and Ancient Fluvial Systems

How to Cite

Frostick, L. E., Reid, I. and Layman, J. T. (1983) Changing Size Distribution of Suspended Sediment in Arid-Zone Flash Floods, in Modern and Ancient Fluvial Systems (eds J. D. Collinson and J. Lewin), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303773.ch7

Author Information

  1. 1

    Birkbeck College, University of London, UK

  2. 2

    La Sainte Union College, Southampton, UK

Publication History

  1. Published Online: 29 APR 2009
  2. Published Print: 7 FEB 1983

ISBN Information

Print ISBN: 9780632009978

Online ISBN: 9781444303773



  • changing size distribution of suspended sediment in arid-zone flash floods;
  • drainage basin character and flood hydrology;
  • total concentration of suspended sediment;
  • size of suspended sediment;
  • II Kimmer –suspended sediment load and storm-flow characteristics


Water and sediment discharge are analysed in detail over several floods in a moderate-sized (7 km2) ephemeral stream network in Kenya's northern arid zone. Negligible interception by sparse desert scrub and measured low surface infiltration capacities dictate overland flow within minutes of rainfall onset and result in rapid stream response to multi-celled rainstorms (lag response averages 34 min). Surface wash samples, collected in troughs sunk into a variety of slopes, indicate slope sediment yields up to 0·219 kg m−2 for single flood events. The contributions of six major tributaries are marked by clearly distinguished discharge pulses in the main channel. These have significant consequences for the size of suspended sediment (a parameter hitherto largely ignored) which responds sympathetically to water discharge pulses. Suspended sediment concentration rapidly achieves a peak with the passage of the flood bore over the dry channel bed and declines from values as high as 15,800 mg 1−1 in direct relationship with declining water discharge. The average particle-size of suspended sediment declines from 46 to 2 µm, though it is shown that clay-size particles are less responsive to changes in flow. Comparison between expected concentrations in size classes above 63 µm using Laursen's semi-empirical equation, and observed concentrations yields a high correlation coefficient, r = 0.88, but the relationship is poor for silt and clay fractions. These are derived almost entirely from surface wash and constitute 56% of annual suspended sediment discharge. Size analyses of flash-flood suspended sediment provide an important and unique insight into the hydrodynamics of arid-zone ephemeral streams, giving a running commentary on the flexible boundary separating bedload and suspended load.