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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #100904
Title: AN EXPERIMENTAL STUDY OF ALLUVIAL CHANNEL RESPONSE TO LARGE WOODY DEBRIS

Author
item WALLERSTEIN, N. - UNIVERSITY OF NOTTINGHAM
item Alonso, Carlos
item Bennett, Sean
item THORNE, C. - UNIVERSITY OF NOTTINGHAM

Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 4/1/1999
Publication Date: N/A
Citation: 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: Field investigation in northern Mississippi has shown that streams are actively eroding their channel banks, introducing into the channel large numbers of fallen trees. Accumulations of large woody debris (LWD) can affect the direction and magnitude of the stream flow, and it can significantly increase river stage and channel roughness. Yet in an evolving stream system, it is difficult to separate the alluvial response to LWD from other stimuli such as channel straightening, bank stabilization, and anthropogenic effects. Thus a flume model study was constructed to examine the effects LWD can have on alluvial flow processes. Using Abiaca Creek as the field prototype, a distorted Froude-scaled model was constructed, and four differently sized elements were used to simulate the LWD. As the debris elements neared the water surface, the drag coefficient increased dramatically due to the effects of wave drag. It was shown that the velocity around an element decreased exponentially with time as the channel widened and deepened due to flow constriction and diversion, and also caused the formation of bars downstream. The largest elements produced the greatest amount of channel adjustment, but channel response within each experiment was fast. Experimental results agreed well with observations on Abiaca Creek and other studies.