Overland flow resistances on varying slope gradients and partitioning on grassed slopes under simulated rainfall

Abstract

It is still unclear how slope steepness (S) and revegetation affect resistance (f) to overland flow. A series of experiments on runoff hydraulics was conducted on granular surfaces (bare soil and sandpaper) and grassed surfaces, including grass plots (GP), GP with litter (GL), and GP without leaves (GS) under simulated rainfall and inflow (30<Re<1400) with varying slopes ranging from 2.6% to 50%. The results show that the observed f based on a small‐size runoff plot under rainfall conditions tends to be overestimated due to the increase in flow rate, or Re (Reynolds number), with downward cross sections and a good f‐Re relation (f = KRe−1). There exists a good f‐Re relation for granular surfaces and a good f‐Fr relation (Fr, Froude number) for grass plots. A greater f occurred at the gentle and steep slopes for the granular surfaces, while f decreased with increasing slopes for the grass treatments. The different f‐S relations suggest that f is not a simple function of S. When Re≈1000, the sowing rye grass with level lines increased f by approximately 100 times and decreased bed shear stress to approximately 5%. The contribution of grass leaves, stems, litter, and grain surface to total resistance in the grass plots were averagely 52%, 32%, 16%, and 1%. The greater resistance from leaves may result from the leaves lying at the plot surface impacted by raindrop impact. These results are beneficial to understand the dynamics of runoff and erosion on hillslopes impacted by vegetation restoration.Key Points:Different relations between resistance and slope gradient exist on granular and grassed surfacesThe resistance contribution in grass plots follows leaves>stems>litter>soil grainGrass plantation greatly increases overland flow resistance and decreases erosion dynamics