Classifying the hydraulic performance of riffle-pool bedforms for habitat assessment and river rehabilitation design

Abstract

Riffle-pool sequences are the dominant bedforms in gravel and mixed bedded channels of intermediate slope. Their fundamental importance in determining the mesoscale habitat environment is demonstrated in their widespread recreation in channel restoration and rehabilitation schemes. This paper explores the hydraulic functioning of riffle-pool bedforms, particularly the variations in the hydraulic performance of different bed oscillation morphologies. It addresses the need for a quantitative means of classifying flow behaviour that can be applied in functional ecohydraulic river rehabilitation designs. Information from reaches on two physically contrasting UK rivers with well marked riffle-pool topography are used to illustrate the approach. The reaches are mapped to obtain a detailed channel morphology. Surveys describing the streamwise depth-averaged velocities at three flow stages are interpolated to a common regular grid, grouped using cluster analysis, and then the validity of each cluster as a distinct hydraulic patch class is assessed statistically using analysis of variance. The spatial pattern of the hydraulic patch classes is then overlain on the bed topography to link the patches to the bed morphology. The procedure groups locations along the channel which display similar suites of velocity values at different flow stages and thus differentiates between areas in the channel within which the hydraulic habitat is spatially relatively invariant from those where abrupt changes occur. It also allows the quantitative description of different hydraulic patch classes. Overlay of the hydraulic patch class boundaries on channel reach topography provides a simple but innovative method of exploring and defining the spatial hydraulic habitat implications of riffle-pools of different topographic forms. © 2003 John Wiley & Sons, Ltd.