The influence of planform on flow resistance in mobile bed compound channels J. F. Lyness, W. R. C. Myers, J. B. C. Cassells and J. J. O'Sullivan A large-scale test facility, the United Kingdom Flood Channel Facility, has been used to compare observations of a range of overbank flows in straight compound chan- nels and meandering compound channels with similar geometries, floodplain roughnesses and mobile main channel bed material. For straight and meandering channels with smooth floodplains, the single channel Manning's n values were similar, but single channel Manning's n values for the meandering channel with roughened floodplains are approximately 50% greater than those for the straight channel with similarly rough- ened floodplains. At higher relative depths, Y r 4 0Í25, the floodplain Manning's n values become similar for both the straight and meandering channels indicating that most of the overbank flow for the meandering channel is in the valley direction at these depths. The meandering main channel with roughened floodplains was found to have the largest mobile bed features. NOTATION A area B top width of main channel d stage L m path length of full meander l path length to any point on meander n Manning's roughness coef®cient P perimeter Q discharge Q r ratio of main channel discharge to ¯oodplain dis- charge R hydraulic radius (= A/P) S o bed slope Y r relative depth (= depth of ¯ow on ¯oodplain/total depth of ¯ow) y meander cross-over angle y 0 angle to main axis at any point on the sine wave c main channel f ¯oodplain 1. INTRODUCTION There is an increasing recognition that the con¯icting pressures of engineering and environmental objectives must be rationa- lized and harmonized to ensure optimum use of rivers, which constitute one of our most valuable natural resources. River engineering design is becoming more environmentally sensitive, seeking solutions that are sustainable, enhance the environment and ensure ¯ood protection. Compound or two-stage river channels, consisting of a deep central main channel ¯anked by one or two ¯oodplains offer environmental advantages by ensuring depths at low ¯ows that provide a suitable habitat for ®sh and other wildlife, while the ¯oodplains provide extra conveyance for ¯oods. The ¯oodplain areas are, however, often used for agricultural, commercial or housing purposes and this raises the issues of ¯ood risk and ¯ood protection. This is further complicated by the dif®culty of estimating the discharge capacity of rivers of a compound cross-sectional shape comprising a main channel inset in a ¯oodplain ¯ow zone. Methods appropriate to simple cross-sectional shapes are not accurate when applied to overbank ¯ows and may lead to over- estimation of discharge capacity, which is dangerous, or under- estimation causing uneconomic over, design. 2. DESCRIPTION OF OBJECTIVES The objectives of this paper are as follows. (a) To outline the use of the United Kingdom Flood Channel Facility for comparisons of ¯ow behaviour in straight and meandering compound channels with mobile main channel beds and smooth and roughened ¯ood plains. (b) To show the effects of main channel and ¯oodplain boundary roughness and channel sinuosity on the dis- charge capacity of mobile bed compound channels. (c) To show the effects of main channel and ¯oodplain boundary roughness and channel sinuosity on the single channel Manning's n value of ¯ow resistance for mobile bed compound channels. (d ) Using vertical divisions between the main channel and ¯oodplains to compare the behaviour of zonal discharges and zonal Manning's n values for straight and meandering compound channels with mobile main channel beds and roughened ¯oodplains. 3. FLOW RESISTANCE IN STRAIGHT AND MEANDERING TWO-STAGE CHANNELS Only overbank ¯ows in straight and meandering two-stage channels with mobile beds in the main channel and smooth and roughened ¯oodplains are considered in the present study. The variety of perimeter surfaces, the compound section geometry and the planforms contribute many factors to the ¯ow resistance of the channels. For the mobile bed Einstein and Proceedings of the Institution of Civil Engineers Water & Maritime Engineering 148 March 2001 Issue 1 Pages 5^14 Paper 12119 Received 22/07/1999 Accepted 18/08/2000 Keywords: river engineering/floods and floodworks/hydraulics and hydro- dynamics J. F. Lyness Reader in Civil Engineering, School of the Built Environment, University of Ulster W. R. C. Myers Senior Lecturer in Civil Engineering, School of the Built Environment, University of Ulster J. B. C. Cassells Research Assistant, School of the Built Environment, University of Ulster J. J. O'Sullivan Research Assistant, School of the Built Environment, University of Ulster Water & Maritime Engineering 148 Issue 1 The influence of planform on flow resistance Lyness et al. 5