High‐Magnitude Floods and Stream Channel Response

Abstract

This paper discusses two premises: (a) large floods may play a bigger role than normally recognized in determining channel morphology, and (b) such effects tend to follow a geographical distribution of drainage basins. Post-flood observations of stream channels are common but inductive conclusions are not. Field observations from Jamaica, Maryland, U.S.A. and South-east Asia, and a review of case-studies indicate that high-magnitude floods leave a stable imprint on channel form, if certain conditions are fulfilled. Such conditions involve the presence of a considerable supply of coarse detritus, length of the recurrence interval of large floods, the ratio of peak flood discharge to bankfull flow, etc. Stability of flood-affected forms does not only depend on the erosive, or transporting or depositional ability of the flood discharge, but also on the inability of smaller flows to modify such forms in the inter-flood period. Attempts are made to construct and locate environments where the effect of large floods will be too pronounced to explain the channel entirely in terms of the most commonly used parameter: the bankfull discharge. Under such circumstances channel forms may be visualized as lag effects of a high-magnitude flood being slowly altered in most cases by flows of smaller magnitude. Erosional and depositional effects of such channels may also be preserved in the floodplain alluvium or in terrace deposits, thereby acting as a palaeohydrological indicator.