Experiment on hydraulic characteristics of shallow open channel flow on slope

Abstract: Hydraulic characteristics of overland flow are of importance in elucidating the mechanisms of soil erosion and sediment yield on slope. By a fixed-bed resistance experiment, the relationship of hydraulic parameters and the resistance characteristics for the shallow flow are quantitatively studied under conditions of six roughness beds and five gradients. Results showed that the index of the shallow flow regime exhibited a saddle-shape changing trend with increased gradient. The index of the shallow flow regime would gradually decrease with the increase of gradient when the gradient was less than 0.15 rad, but the trend was opposite when the gradient was higher than 0.15 rad. The index of the shallow flow regime presented a parabolic changing trend with increasing roughness, and the average index was 0.367. For the bed roughness less than 0.38, the flow range per unit width in unstable flow region became narrower with the increase of roughness, and the flow extended to the transitional zone; For the bed roughness greater than 0.38, the flow range per unit width in the unstable flow region gradually became wider with the increase of roughness. Generally, the shallow flow may be summarized by the category of rolling slope flow zone and transition flow zone. The Froude numbers of the shallow flow were both a power function with the flow of per unit width and the gradient, and their exponents are 0.436 and 0.278, respectively. The flow per unit width of the critical flow increased with the increase of roughness but decreased with the increase of gradient. It had a positive relationship with power 0.573 of the roughness and power 0.636 of slope, and the patterns of the shallow flow were in the critical flow and the supercritical flow. The resistance coefficient was in an opposite relationship with power 0.872 of per unit width flow, a positive relationship with power 0.445 of slope, and also an inverse relationship with the Reynolds number. However, the resistance coefficient had large difference with open channel flow, and increased with increasing roughness and slope. The increased resistance was likely to be mainly associated with the pressure drag by flow around a body and the local resistance by rolling slope flow. A formula of resistance in rolling slope flow zone was proposed based on the resistance characteristics of the shallow flow. Results from the study may provide information on the prediction of soil erosion on slope, and promote the application of open channel flow theory in overland flow.