Submitted to: American Society of Civil Engineers Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2001
Publication Date: N/A
Interpretive Summary: PROBLEM: Large woody debris (LWD) refers to accumulations of trees and large branches in stream channels. Ecologists and engineers are considering the use of LWD as integral parts of stream stabilization techniques and river restoration programs. However, little is known about the potential channel adjustment to LWD accumulations. SOLUTION: Experiments were carried out in a laboratory channel model to study the hydraulic and geomorphic impact of large woody debris (LWD). In the course of this work it became necessary to determine the drag forces experienced by isolated debris elements in an open channel flow. For the purpose of simplification cylinders with varying length to diameter ratios were chosen to represent debris pieces. The element dimensions tested were determined from scaling criteria developed for the prototype. This a reach on Abiaca Creek, northern Mississippi, where extensive field surveys of LWD jams had been conducted by the Corps of Engineers. BENEFITS: The data collected in the model provided new information concerning drag on LWD elements. The measurements also demonstrated that drag forces may be significantly underestimated if the resistance of LWD elements is characterized without considering the effect of the element's slenderness and proximity to the free surface. This new knowledge is important in developing engineering guidelines for the management of debris formations to reduce adverse effects on stream stability and habitats.
Technical Abstract: Drag force on cylindrical large woody debris (LWD) elements is measured in a laboratory flume. Drag coefficient values calculated from these data are presented for a range of element submergence and slenderness values. Drag coefficients for large submergence values are consistent with those previously published. However at submergence values less than eight element diameters, the observed drag coefficients are consistently higher than those previously published. This discrepancy is due to the additional form drag created by stationary surface waves that causes the drag coefficient to increase significantly when an element comes close to the free surface. This phenomenon, which has not been accounted for in previous studies on LWD drag, is shown to depend on element Froude number and submergence, in addition to element Reynolds number and slenderness, rather than stream flow blockage.