Instrumentation to measure drag on idealized vegetal elements in overland flow

Information on shear stress acting on soil particles is necessary to quantify potential soil erosion. Movement of water across a rough surface generates a resistive force, part of which acts on the large-scale roughness elements while the remainder acts on the intervening soil surface. An important large-scale roughness element is vegetation, which acts to reduce shear stresses on the soil and thereby reduces potential erosion. Insight into the drag force acting on individual vegetative elements is necessary for understanding this dynamic. An instrumentation system was developed to measure the drag force on individual elements representative of vegetation. Vegetative elements were modeled in a hydraulic flume using rigid circular cylinders and idealized shapes to account for differences in the rate of change in upstream frontal area with flow depth. Data were gathered for a horizontal flume and for a 1% flume slope. Flow rates ranged from 0.004 to 0.028 m³s^-1 with average flow depths ranging from 1.9 to 9.8 cm and average flow velocities ranging from 0.41 to 1.02 m s^-1. Sixteen element shapes were considered, resulting in a total of 80 test scenarios. The test conditions resulted in both partial and complete submergence of the elements. Results are presented as actual drag force and dimensionless drag coefficient. The relationship between drag force divided by velocity squared and upstream projected area is well represented by a straight line for cylinders and all other elements. Drag coefficient is a function of element shape and is represented by an average value over the range of flow depths investigated. The overall error in the estimated drag coefficient is 7.3%.