| It has steep slope and deep gully,complex terrain and serious soil and water loss in the loess plateau area of China.With the high quality development of the Yellow River Basin,a large number of high and steep slopes above 45° had appeared in the engineering construction,which often caused serious soil erosion under the action of rainstorm runoff.To find out high and steep slope runoff water dynamic characteristics of erosion and sediment transport mechanism under the condition of rainfall in this research,the methods of indoor and outdoor simulated rainfall test were used to study the process of runoff and confluence,the characteristics of hydrodynamic parameters and the law of erosion and sediment yield and transport of the high and steep loess slope under different slope conditions and different rainfall intensity combinations.The following research results were obtained:(1)Characteristics of rainfall runoff production and confluence in high and steep slopes.In view of the weak research on the law of runoff production and confluence in high and steep slopes under different working conditions,the rainfall experiments were carried out on high and steep slopes under different slopes,rainfall intensities and working conditions.The results showed that,the time of runoff generation and confluence decreased with the increase of slope,and the time of runoff generation at a slope of 70° was about 16% of slope at 5° under the same working condition.The time of runoff generation and confluence decreased with the increase of rainfall intensity,and the runoff generation time under 135 mm·h-1 rainfall intensity was about 7% of that under 30 mm·h-1 intensity.The overland flow after runoff production was an unsteady flow,and the runoff rapidly increased with time and then gradually reached a stable equilibrium state.In the later stage of rainfall,the runoff can be regarded as a constant flow.The equilibrium time under 120 mm·h-1 rainfall intensity was about 42% of that under 60 mm·h-1.The relationship between the time of runoff generation and confluence of high and steep slope and rainfall intensity was figured out,which indicated that the runoff generation process of high and steep slope was similar to that of gentle slope,but more affected by slope and rainfall intensity.(2)Influence of rainfall intensity on flow pattern and flow state of thin layer runoff in high and steep slope.The Reynolds number of slope runoff is positively correlated with the rainfall intensity under the experimental condition of 60~120 mm·h-1.Compared with the previous experimental research with a slope less than 45°,the Reynolds number of slope runoff increased rapidly with the enhance of rainfall intensity after runoff production on a steep slope greater than 45°.The Reynolds test indicated that the flow was in a turbulent state.There was a "slope characteristic value" of Reynolds number with the change of slope under the same rainfall intensity.The relationship between Reynolds number and slope change was not obvious when the slope less than that of 25°,and it increased rapidly with the slope enhancing when the value was exceeded.It followed a similar rule with above results that the change of Froude number with rain intensity.The characteristic value of the slope was about 55°,and the Froude number of the slope above this value was all greater than 1 after the stability of the slope flow,at which time,the flow pattern was a jet stream.The above research indicated that,the main reason of the significant effects on the flow pattern and flow state of high and steep slope through rainfall intensity was that the impacts of rainfall on the slope runoff caused turbulence on the thin runoff surface.The splashing effect of raindrops was the main factor that made the thin layer runoff different from the conventional open channel flow.(3)The "double turbulent source hypothesis" for rain runoff on slope was proposed.According to the above test,the runoff of the high and steep slope is affected by the double turbulence of the rough bed surface and raindrop impact.The turbulence caused by raindrop blow and water movement spreads toward the bed surface,and the turbulence of the bed surface spreads toward the water surface.Both of these turbulences obey Prandtl’s blending length theory.Two-way diffusion,or " double turbulence sources ",causes rain runoff movement on high and steep slopes to be different from open channel flow movement.(4)Based on the " double turbulence sources hypothesis ",the influence of rainfall on the overland flow was considered,and the original overland flow calculation method was revised.Based on the decentralized motion wave model proposed by Ligget and Woolhiser,which increases the "dual turbulence source" influencing factors of rainfall intensity and bed surface roughness,the calculation method of the water surface line of high and steep slope and other hydraulic dynamic parameters based on the dual turbulence source model were proposed,and the indoor and outdoor verification was carried out to solve the problem of the difficulty in measuring the depth of thin-layer runoff.Compared with the traditional method,the accuracy of this method in calculating the equilibrium time,equilibrium flow at slope toe and water level is improved by more than 80%.(5)The resistance characteristics of rainfall runoff on high and steep slopes were studied.The results showed that the flow pattern calculation of single turbulent source in open channel could not accurately describe the flow pattern of rainfall runoff in high and steep slope under the influence of "double turbulent sources".The test results showed that under the conditions of classical Reynolds experiment,the rainfall-runoff of high and steep slope was mixed by the strong influence of "double turbulent sources",and the flow pattern is turbulent;high and steep slope rainfall runoff negative quadratic power function relationship between drag coefficient and Reynolds number through the calculation of open channel flow,which indicated flow is in a state of transition layer flow to turbulent flow and when the Reynolds number of high steep slope rainfall runoff flow pattern is larger than 500,Water flow pattern was inconsistent with the law of turbulent flow,and there was a different influence mechanism with pipeline flow.The conclusion was of great significance to the understanding of the conflict of flow state,flow pattern and resistance law on slope.(The larger the Reynolds number,the smaller the drag coefficient).(6)Based on the "double turbulent source hypothesis",the erosion and sediment transport law of rainfall on high and steep slope runoff was studied.Through the analysis of the effects of rainfall intensity,bed roughness,slope and flow velocity on erosion and sediment transport,the influence of rainfall on erosion and sediment transport of high and steep slope were revealed.Improving the existing bed load sediment transport rate formula based on energy dissipation,an erosion transport formula of high and steep slope based on the hypothesis of double turbulent sources was proposed.This formula can not only be used to calculate bed load sediment transport of river runoff,but also to calculate rainfall erosion runoff of high and steep slope,which provided a new method for the process calculation of rainfall runoff erosion of slope. |
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