Resistance to overland flow on desert hillslopes

doi:10.1016/0022-1694(86)90099-5

Journal of Hydrology

Volume 88, Issues 3–4, 30 November 1986, Pages 343-363

https://doi.org/10.1016/0022-1694(86)90099-5 Get rights and content

Abstract

This study examines the relation between the Darcy-Weisbach friction factor, f, and the Reynolds number, Re, for overland flow on six runoff plots in semiarid southern Arizona. The surfaces of these plots are irregular and covered with stones. As overland flow increases, the stones and microtopographic protuberances, which constitute the major roughness elements, are progressively inundated, thereby altering the flow resistance. Analyses of 14 cross-sections reveal that the resulting f-Re relations have two basic shapes: convex-upward and negatively sloping. These shapes bear little resemblance to the conventional f-Re relation for shallow flow over a plane bed, whose shape is a function of the state of flow. Rather, they are explained in terms of the simultaneous operation of two processes. The first is the progressive inundation of roughness elements and increase in their wetted upstream-projected area as discharge increases. This process causes f to increase. The second is the progressive increase in the depth of flow over already inundated parts of the bed as discharge increases. This process causes f to decline. Whether the f-Re relation has a positive or a negative slope depends on whether the first or second process dominates, and this depends on the configuration of the bed and level of discharge.

These findings have profound implications for the mathematical modeling of overland flow on desert hillslopes. Whether a model is based on the Saint-Venant equations or makes use of the kinematic-wave approximation, it requires the specification of a relation between f and Re (or surrogates thereof), and the computed hydrograph is very sensitive to the form of this relation. Hitherto, the most widely used relation has been the conventional one for shallow flow over a plane bed. However, the present study suggests that this relation does not apply to desert hillslopes. Additional field and laboratory studies are needed to learn more of the behavior and controls of f-Re relations on such hillslopes so that models of overland flow can employ more realistic relations and better simulate runoff hydrographs.

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Citation Excerpt :
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Resistance to overland flow on desert hillslopes

Abstract

J. Hydrol.

J. Hydrol.

Flow resistance estimation in mountain rivers

J. Hydraul. Eng.

Open-channel Hydraulics

Fluvial Hydrology

Experimental study of Horton overland flow on tropical hillslopes. 2. Hydraulic characteristics and hillslope hydrographs

Z. Geomorphol., Suppl. Band

The hydraulics of overland flow on hillslopes

U.S. Geol. Surv., Prof. Pap.

Simulation of overland flow on short field plots

Water Resour. Res.

Overland flow and groundwater flow from a steady rainfall of finite duration

J. Geophys. Res.