HR: 08:30h
AN: H51L-03    [Abstracts]
TI: A critical evaluation of grid-by-number sediment sampling using laser scanner derived clast population statistics across a gravel bar
AU: * Milan, D J
EM: dmilan@glos.ac.uk
AF: University of Gloucestershire, Department of Natural & Social Sciences, FCH, Swindon Road, Cheltenham, GL50 4AZ, United Kingdom
AU: Heritage, G L
EM: g.l.heritage@salford.ac.uk
AF: University of Salford, Built and Human Environment Research Institute, School of Environment and Life Sciences, Peel Building, Manchester, M5 4WT, United Kingdom
AB: Water flow level in river channels is moderated by the interaction with the roughness of the surface over which it flows. The interaction is highly complex and remains poorly understood despite its economic and social importance in flood level forecasting. The empirical and semi-rational nature of approaches used to estimate hydraulic roughness makes them very difficult to apply and much of the hydraulic resistance has been attributed to grain roughness using various forms of the Colebrook-White equation where the grain diameter is modified by a multiplier to account for the non-uniform nature of gravel-bed surfaces. Fundamental to the accuracy of the particle size approaches is the sampling of river-bed gravels where sample size, operator bias, particle shape and surface heterogeneity can greatly affect the result. Despite these problems a standard surface sample of the intermediate axis of 100 clasts remains the accepted method for grain-size characterisation amongst scientists and engineers concerned with channel hydraulics. Surface roughness has also been measured using a random field of spatial elevation data. The success of this approach has been tempered by the lack of high-resolution topographic data covering all roughness scales, however, improved data-point resolution is now achievable using terrestrial laser scanning technology. The aim here is to reliably quantify the population grain-size distribution of a natural gravel surface using random field terrestrial laser scanner x,y,z data and by direct comparison to demonstrate the errors inherent in the conventional particle-size approach. Application of the random field approach, using a terrestrial laser scanner, across a gravel bar surface on the River South Tyne at Lambley, UK, generated an effective sample of 120,000 clasts yielding a D84 for use in the Colebrook White equation of 0.110m. Monte Carlo sampling within the 12000 measured clasts from the bar surface generated 560 simulated grid-by-number D84 estimates. Grain-size D84 values ranged from 0.100m to 0.195m with a median value of 0.130m. This represents an average 18% and a maximum 77% over-estimation of the grain-size value in the flow resistance equation. Such potential errors, inherent with the conventional grid-by-number sampling technique, impact significantly on flood level estimation options.
DE: 1821 Floods
DE: 1825 Geomorphology: fluvial (1625)
DE: 1847 Modeling
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting