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Flow resistance and hydraulic geometry in contrasting reaches of a bedrock channel.

Ferguson, R.I. and Sharma, B.P. and Hardy, R.J. and Hodge, R.A. and Warburton, J. (2017) 'Flow resistance and hydraulic geometry in contrasting reaches of a bedrock channel.', Water resources research., 53 (3). pp. 2278-2293.


Assumptions about flow resistance in bedrock channels have to be made for mechanistic modeling of river incision, paleoflood estimation, flood routing, and river engineering. Field data on bedrock flow resistance are very limited and calculations generally use standard alluvial-river assumptions such as a fixed value of Manning's n. To help inform future work we measured how depth, velocity and flow resistance vary with discharge in four short reaches of a small bedrock channel, one with an entirely rock bed and the others with 20%-70% sediment cover, and in the alluvial channel immediately upstream. As discharge and submergence increase in each of the partly or fully alluvial reaches there is a rapid increase in velocity and a strong decline in both n and the Darcy-Weisbach friction factor f. The bare-rock reach follows a similar trend from low to medium discharge but has increasing resistance at higher discharges because of the macro-roughness of its rock walls. Flow resistance at a given discharge differs considerably between reaches and is highest where the partial sediment cover is coarsest and most extensive. Apart from the effect of rough rock walls, the flow resistance trends are qualitatively consistent with logarithmic and variable-power equations and with non-dimensional hydraulic geometry, but quantitative agreement using sediment D84 as the roughness height is imperfect.

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Publisher statement:R. I. Ferguson, B. P. Sharma, R. J. Hardy, R. A. Hodge, J. Warburton, (2017) Flow resistance and hydraulic geometry in contrasting reaches of a bedrock channel, Water resources research, 53(3): 2278-2293, 10.1002/2016WR020233 (DOI). To view the published open abstract, go to
Date accepted:22 February 2017
Date deposited:09 March 2017
Date of first online publication:22 March 2017
Date first made open access:22 September 2017

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