Delivered by ICEVirtualLibrary.com to: IP: 202.170.51.231 On: Fri, 10 May 2013 08:05:40 Hydraulic characteristics of meandering mobile bed compound channels J. F. Lyness, MSc , PhD, MICE, W. R. C. Myers, BA, BSc , PhD, MICE and J. J. O'Sullivan, BA, BAI , MSc & The experimental arrangement for exam- ining the hydraulic characteristics of overbank ¯ows in a meandering mobile bed compound channel at the UK Flood Channel Facility (FCF) is described. The FCF was designed to bridge the gap between small and prototype scales and has been used in a series of experiments on straight compound, skewed and ®xed- bed meandering channels. For the present study the sinusoidal meandering main channel had a mobile sand bed. Several overbank ¯ows were measured with smooth or roughened ¯oodplains. Flood- plain roughness was provided by surface- penetrating rod elements. It was found that, for ¯oodplain ¯ows with a relative depth 40´2, the average ¯ow velocities over the ¯oodplain, in the valley direction, were greater than in the meandering main channel. The principal main channel bed- forms were repeating long dunes. There was also evidence of plunging ¯ow over the main channel at the apex of bends. Keywords: ¯oods & ¯oodworks; hydraulics & hydrodynamics; river engineering Notation B top width of main channel d stage L m path length of a full meander l path length to any point on meander n Manning's roughness coecient Q discharge V depth-average ¯oodplain velocity com- ponent Y r relative depth (=depth of ¯ow on ¯ood- plain/total depth of ¯ow) y meander cross-over angle y 0 angle to main axis at any point on the sine wave l Darcy±Weisbach friction factor Subscripts x longitudinal (valley) direction y transverse direction FP ¯oodplain MC main channel Introduction Rivers represent one of mankind's most impor- tant environmental assets. They are home to a splendid diversity of wildlife, provide water and wastewater facilities to urban populations, they give pleasure to many people and they enhance the landscape. River ¯ows are often aected by man's activities and therefore require careful management for water supply, waste disposal, ¯ood alleviation and power generation, as well as amenity uses. 2. While, previously, river design and man- agement tended to be dominated by engineering concerns, there is now an increasing recognition that the con¯icting pressures of engineering and ecological objectives must be rationalized and harmonized to ensure optimum use of a valuable resource. Thus river design is becom- ing more environmentally sensitive, seeking solutions which are sustainable and which enhance the ecological environment as well as meeting ¯ood protection standards. 3. A ¯ood alleviation solution which also oers environmental advantages is that of a two-stage or compound river geometry consist- ing of a deep main channel ¯anked by one or two ¯oodplains. Such cross-sections ensure reasonable depths at low ¯ows, which provide a suitable habitat for ®sh and other wildlife, while the ¯oodplain areas provide both storage and additional conveyance during high ¯ows. 4. The ¯oodplain areas are, however, often developed for agricultural, commercial or housing purposes and this raises the issue of ¯ood risk and ¯ood protection. The assessment of ¯ood discharge capacity and associated ¯ood levels in compound river channels is uncertain. The ¯ow patterns in such situations are com- plicated by interaction between ¯ow in the central main channel and that over the ¯ood- plains. Added to this may be increased com- plexity of ¯ow patterns due to variations in planform, such as meandering geometry which is common. Furthermore, bed and bank material may be ®xed or erodible. Not surprisingly therefore, there is as yet no commonly accepted standard method for predicting discharge capa- city in non-straight two-stage river channels. Description of objectives 5. The objectives of this study are to 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. J. O'Sullivan, Research Assistant, School of the Built Environment, University of Ulster 179 Proc. Instn Civ. Engrs Wat., Marit. & Energy, 1998, 130, Dec., 179±188 Paper 11545 Written discussion closes 15 April 1999