Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2001
Publication Date: N/A
Interpretive Summary: Field investigation in northern Mississippi has shown that streams are actively eroding their channel banks, introducing into the channel large numbers of fallen trees. These trees have been observed to accumulate at specific locations within the streams, thus causing an accumulation of large woody debris (LWD). LWD can affect the direction and magnitude of the stream flow, and it can significantly increase river stage and channel geometry. Yet in an evolving stream system, it is difficult to separate a river's response to LWD from other possible causes such as channel straightening, bank stabilization, and anthropogenic effects. Thus a flume model study was constructed to examine the effects LWD can have on river flow processes. Using Abiaca Creek, MS as the field case, an experimental model was constructed, and simple cylinders were used to simulate trees that have fallen into the creek. As the debris elements neared the water surface, the drag forces acting on the elements increased dramatically due to the effects of wave formation. The velocity around an element decreased with time as the channel widened and deepened due to flow constriction and diversion, and channel erosion also caused the formation of new sediment accumulations downstream. Alluvial channel response within each experiment was relatively short, and these experimental results agreed well with observations within Abiaca creek. This new knowledge will enable action agencies to develop better tools to assess the impact of LWD accumulations on river flow conveyance and flood risk potential.
Technical Abstract: This paper presents the results of a movable-boundary Froude-scaled hydraulic model based on Abiaca Creek, a sand-bedded channel in northern Mississippi. The model was used to examine the geomorphic and hydraulic impact of simplified Large Woody Debris (LWD) elements. The theory of physical scale models is discussed and the method used to construct the LWD test channel is developed. The channel model has bed and banks molded from 0.8 mm sand, and flow conditions were just below the threshold of motion so that any sediment transport and channel adjustment were the result of the debris element. Elements were attached to a dynamometer to measure element drag forces, and channel adjustment was determined through detailed topographic surveys. The element's drag force decreased asymptotically over time as the channel boundary eroded around the elements due to increased shear stress. Total time for geomorphic adjustment computed for the prototype channel at the Q2 discharge was as short as 6 hours. The size, depth and position of scour holes, bank erosion and bars created by flow acceleration past the elements was 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 channel.