Effects of surface roughness and vegetation coverage on Manning'sresistance coefficient to overland flow

Abstract：Resistance to flow of surface water determinesthe study hydrodynamics in the slope. The common resistance typesin the wild are grain resistance that mainly exerted by soil particles,and thevegetation resistance that exerted by the vegetationbelonging to form resistance. However, there are no consensus conclusions about the laws of overland flow that influenced bythe grain and vegetation resistance, especially for the resistance to overland flow that calculated by manning’s resistancecoefficient. Simultaneously, the superposition principle that used to calculate composite resistance needs to be verified in thesesituations for the overland flow. Therefore, the artificial scouring experiments in the fixed bed have beenconducted at the slopegradient of 5° in Jinyun Forest Ecosystem Research station, Chongqing Province, China. The waterproofs with differentroughness were selectedto simulate the surface roughness, whilethe circular cylinders with different diameterswere used tosimulate the vegetation coverage. In this study, 9 different unit discharges varyingfrom 0.2´10-3 to 0.5´10-3 m3/(m ·s) were setas the water inflow; 3 different grain sizes ks of surface roughness of 0.12, 0.18, 0.38 mm were selected to simulate the grainresistance, while the ks of smooth flume bed equaled to 0.009; and 4 different vegetation coverage Cr of 0, 4.0%, 6.6%, 12.2%were selected to simulate the vegetation resistance. To calculate more accurately, the resistance was calculated by theequivalent manning’s resistance coefficient ne which usedthe equivalent hydraulic radius, instead of hydraulic radius. Theequivalent hydraulic radius were considered the contact area of vegetation and flow, while the hydraulic radius did not. Thevelocity of overland flow were measured using a trace method, and repeated for 10 times. Afterwards, the flow depth h andnewere calculated. Results showed that 1) the ne negatively relates with discharge for non-vegetated slope, while positivelyrelates with discharge for vegetated slope. The ne increases as the increasing ks and Cr. 2) Assuming the composite resistanceequals to the sum of grain resistance and vegetation resistance, the equivalent manning’s resistance caused by grain resistanceneb and caused by vegetation resistance nevcan be deduced. The neb negatively relates with h, while positively relates with ks.The nev linearly positive relates with h for larger h, while the nev is larger for the lower h. This phenomenon indicates that thelinear superposition principle wasnot suitable for calculating the overland flow resistance, because the vegetation resistanceshould be linearly positively related with h for the fully flow depth if the linear superposition principle was suitable based onthe results of previous works. The larger nev for the lower hcan beattribute to the effect of additional resistance. Because of theshallow flow depth of the overland flow, the region impact of surface roughness was overlapped with the region impact ofvegetation. Therefore, twotypes of the resistance interfered each other, resulting the additional resistance. Afterwards, theequivalent manning’s resistance that caused by additional resistance nawas used to verify nev, resulting in nev linearly increasedas the increasing h. The na negatively related with h, while positively related with ks and Cr. 3) The multiple regressionanalysis was used to simulate ne, and the results was well accordance with the observed ne (correlation coefficient 0.98).Finally, by comparing correlation coefficient R after rejecting corresponding resistance components, the vegetation resistanceis the major factor of ne and the grain resistance is the second major factor, while additional resistance has the smaller impacton ne. This finding provides sound supports for building the model of soil erosion, and for the conservation of soil and wateron the slope.