Effect of flow discharge and median grain size on mean flow velocity under overland flow

Abstract

Precise estimation of mean flow velocity (Umean) is imperative for accurate prediction of hydrographs and sediment yield. For overland flow, Umean is normally estimated by multiplying the dye or salt based velocity measurement with a correction factor (a). A wide range of correction factors is available in the literature, all of which were derived under different experimental conditions. The selection of a suitable a has become a main challenge for accurate mean flow calculations. This study aimed to assess the variability of a with grain size (D50) and slope (S), and to evaluate the dependency of Umean on flow rate (Q), D50 and S by regression analysis. Flume experiments were performed at Q varying from 33 to 1033 10 6 m3 s 1, S ranging from 3 to 10 , and D50 ranging from 0.233 to 1.022 mm. Flow velocities were measured directly with the dye tracing technique (Udye), and derived indirectly from flow depth measurements (Udepth). The Udepth measurements were considered as Umean. The derived a (Udepth/Udye) values did not remain constant with sediment size and increase significantly with the increase of D50. The mean a values for 0.230, 0.536, 0.719 and 1.022 mm sands were 0.44, 0.77, 0.82 and 0.82, respectively. Hence, due to the substantial variation of a with D50, no absolute a value is applicable to all hydraulic and sedimentary conditions. However, mean a values for 0.230, 0.536 and 0.719 mmsands were found comparable with a values available in the literature for similar grain sizes. The influence of Q, S, and D50 on Umean was studied by regression analysis. Regression analysis depicted the significant influence of Q and D50 on Umean, while the effect of slope was found to be non-significant. Comparison of the derived regression equation with five literature datasets showed that the model can predict mean flow velocities in overland flow at a reasonable accuracy as long as the mean velocity is below 0.4 m s 1. At higher velocities the error becomes unacceptably large.