The Effect of Large Woody Debris Dams Upon Flow Resistance

The flow resistance offered by vegetation is a critical issue in fluvial studies. One type of vegetation that has a significant impact upon flow resistance is Large Woody Debris (LWD). LWD accumulations, such as debris dams, can account for significant proportions of flow resistance despite only covering a small proportion of the channel bed. Current approaches to predicting the resistance rely upon the drag force approach that is often used for rigid standing vegetation (Petryk and Bosmajian, 1975). However, this does not take into account the different types of LWD accumulations that are present within river systems. For example, accumulations with high blockage ratios or those in high-energy, high-gradient channels have a significant amount of spill resistance that is not accounted for in the drag-force approach. Data collected from the Highland Water, a 3rd order stream flowing through the New Forest in southern England, together with Froude-scaled flume experiments are herein combined with data from previous studies (Shields and Gippel, 1995; Curran and Wohl, 2003) to identify some general trends associated with the flow resistance offered by LWD accumulations in a range of fluvial environments. The results show that there is a trend of increasing resistance with slope and blockage ratio. When using the drag force approach, large residual errors between measured and predicted resistance have been identified for LWD dams subject to spill resistance. A method to account for the spill resistance associated with different types of LWD dams is required in order to allow accurate prediction of the flow resistance contributed by LWD accumulations. Spill resistance is known to be related to Froude number which can affect the friction factor when variations in free-surface occur. Therefore, Froude number was used to account for some (~70%) of the residual error between measured and predicted flow resistance especially where channel gradient is high (S>0.01), and where blockage ratios tend to one.