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 depthaveraged 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.

Abstract:Output from a three-dimensional numerical flow model (SSIIM) is used in conjunction with high-resolution topographic and velocity data to assess such models for eco-hydraulic applications in river channel design and habitat appraisal. A new methodology for the comparison between field measurement and model output is detailed. This involves a comparison between conventional goodness-of-fit approaches applied to a spatially structured (riffle and pool) sample of model and field data, and a 'relaxation' method based upon the spatial semivariance of model/field departures. Conventional assessment indicates that the model predicts point-by-point velocity characteristics on a 0Ð45 m mesh to within š0Ð1 m s 1 over 80% of the channel area at low flow, and 50% of the area at high in-bank flow. When a relative criterion of model fit is used, however, the model appears to perform less well: 60-70% of channel area has predicted velocities that depart from observed velocities by more than 10%. Regression analysis of observed and predicted velocities gives more cause for optimism, but all of these conventional indicators of goodness of fit neglect important spatial characteristics of model performance. Spatial semivariance is a means of supplementing model appraisal in this respect. In particular, using the relaxation approach, results are greatly improved: at a high in-bank flow, the model results match field measurements to within 0Ð1 m s 1 for more than 95% of the total channel area, provided that model and field comparisons are allowed within a radius of approximately 1 m from the original point of measurement. It is suggested that this revised form of model assessment is of particular relevance to eco-hydraulic applications, where some degree of spatial and temporal dynamism (or uncertainty) is a characteristic. The approach may also be generalized to other environmental science modelling applications where the spatial attributes of model fits are of interest.

This paper presents an analysis of data from the UK Environment Agency's River Habitat Survey database relating to bedform type and frequency. Emphasis is given to the gravel-bed riffle which (a) traditionally has been considered to be a fundamental morphological unit diagnostic of river stability, and (b) more recently has become an important design component in channel restoration and rehabilitation schemes. Data were sufficient to support the identification of distinct, catchment-scale controls determining the circumstances under which step-pools and riffles occur, and key local-scale influences upon riffle frequency and spacing. This demonstrates that the database can be used to support a broad geomorphological analysis. Suggestions are made for the interpretation of results obtained from the existing database, and for possible future modifications to enhance its geomorphological value.