Abstract

1. The River Idle is a low gradient river flowing through north Nottinghamshire. The river was comprehensively channelized (early 1980s) for flood defence and land drainage. The scheme has produced a trapezoidal channel with a lack of ecological diversity, a poor fishery and low aesthetic value. A rehabilitation design was required to tackle these deficiencies through improvements which did not compromise the other obligations of the managing authority. 2. Rehabilitation proposals were based on a semi-quantitative assessment of the sources and pathways of sediment transport through the system. Geomorphological reconnaissance was used to define conservation value, to prioritize the most suitable reaches for restoration and to identify a reference reach. On this basis, structures were designed to achieve short-term objectives without compromising long-term geomorphological sustainability. 3. After design, the siting and impacts on flow conveyance were tested using appropriate hydraulic models. BENDFLOW was used to refine the positioning of deflectors while the estimated reduction in flow conveyance (HMODEL2 and FCFA) did not exceed 10% of capacity or 0.05 m in water surface elevation (HECRAS) under high flow conditions. 4. As river restoration and rehabilitation schemes become more widely employed in rivers with multi-functional management requirements, so detailed design principles set into their catchment context and hydraulic testing will become increasingly critical to protect designs from the criticism that flood defence and land drainage interests are adversely affected. © 1998 John Wiley & Sons, Ltd.

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