Surface Wave Forces Acting on Submerged Logs

Drag forces exerted on cylindrical simulated logs by flowing water are measured in a laboratory flume. Drag coefficient values calculated from these data are presented for a range of log 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 drag created by stationary surface waves which causes the drag coefficient to increase considerably when a log is positioned near the free surface. This phenomenon, which has not been accounted for in previous studies on large woody debris, is shown to depend on log slenderness, submergence, and Froude number.