FLOW RESISTANCE AND SEDIMENT TRANSPORT BY CONCENTRATED OVERLAND-FLOW IN A GRASSLAND VALLEY

Flow resistance and sediment transport data are needed from well veget ated humid environments to evaluate surface wash erosion and channel i ncision by overland flow. In humid environments, runoff in valley floo rs can reach depths of several centimetres but erosion is often limite d by dense grass cover. Intense grazing reduces grass cover but the im pacts of this on sediment transport processes are poorly understood, W e conducted flume experiments in a grassed valley of coastal Californi a to investigate flow resistance and sediment yield under natural cond itions and with progressive clipping of grass cover. Flow resistance h as a laminar-like relationship with Reynolds number but we attribute t his to very low velocity beneath submerged stems, and not to the state of flow. The sediment transport relations provide support for the con cept of a threshold shear stress below which erosion is effectively pr evented by surface resistance. Shear stress partitioning suggests that on a densely grassed surface over 90% of flow resistance is exerted o n plant stems. This effectively prevents sediment transport at boundar y shear stresses as high as 1000-1800 dyn/cm(2). Complete clipping of the grass cover reduces the critical shear stress for sediment transpo rt 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 den sity, boundary shear stresses of at least 250-430 dyn/cm(2) are requir ed for channel incision.