Field measurements of drag coefficients for model large woody debris

Woody debris in rivers can be a significant source of roughness and consequently influences flow at both the local and reach scale. In order to develop a better quantitative understanding of the interaction between wood in rivers and stream flow, we thus performed a set of field measurements of the drag on model woody debris for conditions that prevail in typical natural streams. Our model debris consisted of PVC “logs” with diameters between 4 and 30 cm. The field setting allowed us to consider the hydrodynamic influence of a rough stream bottom, and our measurements thus complement previously published flume-based measurements. We found that, owing to the variation of velocity with water depth, some of our results differed appreciably from measurements made in smooth flumes. We determined the effects of (i) the orientation of the log, (ii) the size of the log relative to the water depth, (iii) the depth of the log in the water column, and (iv) leafless branches on the log. We found that the orientation of the log had no significant effect on the apparent drag coefficient. By contrast, because the water velocity varies with depth, the position of the log in the water column influenced the apparent drag for small logs. For large logs (diameter >30% of the water depth), however, the position of the log had little effect on drag. The ratio of the diameter of the log to the water depth, a quantity called “blockage,” also affected drag. As blockage increased, drag increases. For blockages greater than about 0.3, however, the drag becomes independent of blockage. Finally, we found that the presence of leafless branches does not increase the drag (within measurement sensitivity).