Distorted Froude-scaled flume analysis of large woody debris

This paper presents the results of a movable-boundary, distorted, Froude-sc aled hydraulic model based on Abiaca Creek, a sand-bedded channel in northe rn Mississippi. The model was used to examine the geomorphic and hydraulic impact of simplified large woody debris (LWD) elements. The theory of physi cal scale models is discussed and the method used to construct the LWD test channel is developed. The channel model had bed and banks moulded from 0.8 mm sand, and flow conditions were just below the threshold of motion so th at any sediment transport and channel adjustment were the result of the deb ris element. Dimensions and positions of LWD elements were determined using a debris jam classification model. Elements were attached to a dynamometer to measure element drag forces, and channel adjustment was determined thro ugh detailed topographic surveys. The fluid drag force on the elements decreased asymptotically over time as the channel boundary eroded around the elements due to locally increased bo undary shear stress. Total time for geomorphic adjustment computed for the prototype channel at the Q(2) discharge (discharge occurring once every two years on average) was as short as 45 hours. The size, depth and position o f scour holes, bank erosion and bars created by flow acceleration past the elements were found to be related to element length and position within the channel cross-section. Morphologies created by each debris element in the model channel were comparable with similar jams observed in the prototype c hannel. Published in 2001 by John Wiley & Sons, Ltd.