Fluvial erosion, transport and deposition
Detachment- versus transport-limited
Assume steady, uniform flow, consistent downstream hydraulic geometry, as shown on succeeding slides
Notes:
A fundamental distinction occurs in nature between streams which have a mobile alluvial bed, such that the flow is generally loaded to capacity with those grain sizes present on the bed, and bedrock-floored channels within which the flow has excess transport capacity. As with the similar case for mass wasting, alluvial channels are transport-limited, whereas the bedrock channels are called detachment-limited. Transport-limited channels will be discussed later.
In bedrock channels a variety of processes may determine the rate at which the bed is eroded, including plucking from the bed, the weathering rate of the bedrock exposed in the stream, solution, and abrasion by sediment in transport. As a result, there is no universal rate law for erosion of detachment-limited channels. However, Howard and Kerby (Howard, A.D. and Kerby, G., 1983, Channel changes in badlands, Geol. Soc. Am. Bull., 94, 739- 752) proposed that in badland channels the hydraulic plucking and abrasion may be proportional to the shear stress (?=? in the equation above) exerted on the bed by the flow. Others have suggested stream power or stream power per unit channel bed (?=?=?V in the above equation). By assuming steady, uniform flow and a consistent downstream hydraulic geometry, the rate of bed erosion can be re-expressed as a power function of contributing drainage area, A, and channel gradient, S). Howard and Kerby measured long term erosion rates in artificial badlands and found that the data supported a power function of gradient and drainage area with exponents consistent with erosion proportional to shear stress.