Flow resistance and sediment transport by concentrated overland flow in a grassland valley

Flow resistance and sediment transport data are needed from well vegetated humid environments to evaluate surface wash erosion and channel incision by overland flow. In humid environments, runoff in valley floors can reach depths of several centimetres but erosion is often limited by dense grass cover. Intense grazing reduces grass cover but the impacts of this on sediment transport processes are poorly understood. We conducted flume experiments in a grassed valley of coastal California to investigate flow resistance and sediment yield under natural conditions and with progressive clipping of grass cover. Flow resistance has a laminar-like relationship with Reynolds number but we attribute this to very low velocity beneath submerged stems, and not to the state of flow. The sediment transport relations provide support for the concept of a threshold shear stress below which erosion is effectively prevented by surface resistance. Shear stress partitioning suggests that on a densely grassed surface over 90% of flow resistance is exerted on plant stems. This effectively prevents sediment transport at boundary shear stresses as high as 1000-1800 dyn/cm². Complete clipping of the grass cover reduces the critical shear stress for sediment transport to 11-38% of that under natural conditions. Continued surface wash erosion and channel initiation are prevented, however, by strong soil cohesion provided by a dense root mat. Even with reduction of root density, boundary shear stresses of at least 250-430 dyn/cm2 are required for channel incision.