Research On Hydrodynamic Characteristics Of Slope Surface Flow And Sediment Transport Mechanisms

Research on hydrodynamic characteristics of slope surface flow and mechanisms of sediment transport by the surface flow is not only a hot spot, but also a weak aspect in the area of soil erosion. There are some important problems which should be addressed in the resistance law, flow regime discrimination, and the dynamics of sediment transport because of the particularity of slope surface flow, though a great many research achievements have been reached. By the flume experiment of fixed bed resistance with different sizes of roughness under simulated rainfall, the mechanisms of increased resistance of surface flow are discussed and a new method for flow regime discrimination is found. Meanwhile, the indexes of surface flow strength are summarized and a general expression of sediment transport capacity is given. Finally, a formula for calculating sediment transport capacity by surface flow is presented based on particle swarm optimization. The work can provide a reference to the construction of soil erosion prediction model. The main results are as follows:(1) Hydrodynamic characteristics of surface flow are revealed through theoretical derivation combined with the experiment of fixed bed resistance. All of the average velocity, average depth, Froude number, and resistance coefficient of surface flow have a power function relation with unit width discharge, slope, and roughness. Accordingly, an empirical formula for calculating hydraulic parameters is given. The result indicates that the relationship among the hydraulic parameters can be analyzed based on flow regime index, m, which varies from 0.317 to 0.418 and increases with increased bed roughness under the experimental condition. The flow regime index may increase with increased bed roughness. Correction coefficient of flow velocity ranges from 0.2 to 0.75 and it increases with increased bed roughness and increased Reynolds number, but decrease with increased slope. This indicates that the vertical velocity distribution of surface flow is different from that of open channel flow.The phenomena of increased resistance is proved and discussed and the essence of increased resistance is elucidated. The resistance of surface flow has a reverse relation with Reynolds number. With the same Reynolds number, the resistance coefficient of surface flow is greater than that of laminar flow in open channel and increases with increased bed roughness. One reason for the increased resistance is the pressure difference resistance caused by trailing vortex of obstacles under the detouring action of surface shallow flow. Another is the increased resistance caused by the instability of free surface.(2) The evolvement of rolling wave in surface flow is revealed and the critical condition for the instability of free surface is found. The velocity of rolling wave has a power function relation with unit width discharge and Weber number. With the Weber number less than 1.0, the velocity of rolling wave increases greatly. When Weber number is greater than 1.0, its influence on the velocity decays gradually and there is a linear relationship between the velocity and Onsager number. The relationship between wave length and flow path length is approximately linear. The wave length in special section is changed with unit width discharge and Weber number in a form of single peak, but has a good linear relation with slope. In the unstable region of laminar flow, the critical Froude number ranges from 0.5 to 0.7 and the critical Onsager number ranges from 3.0×10-3 to 3.9×10-3, while in the turbulence flow, the critical Froude number ranges from 1.59 to 2.2.(3) A new method for the discrimination of surface flow regime is put forward. The new discrimination is given as the ratio of viscous sublayer depth to its thickness according to the critical condition of rolling wave recession. The ratio indicates not only the contrastive relation of inertial force, fluid viscosity force, and gravitational force, but also the contrastive relation of Froude number and Reynolds number. Surface flow is in the unstable region of laminar flow (waving flow) when the ratio is greater than 0.12 and is in turbulence flow region when the ratio is less than 0.12. The essence of sediment transport by surface flow is the"lifting by the rolling wave and sediment transporting by the flow".(4) Hydrodynamic characteristics of slope shallow flow on loess sandy slope under simulated rainfall are clarified. When simulated rainfall intensity is between 1.0 and 2.33 mm/min and slope is between 6°and 21°, the thickness of viscous sublayer ranges from 0.25 to 0.45 mm, the ratio of viscous sublayer depth and its thickness ranges from 0.95 to 0.30, and surface flow is in the unstable region of laminar flow. Slop shape is in lower energy status and transition status. With flow strength increasing, slope shape is changed from sand waves to calm bed. Surface flow on steep slope is the subcritical flow as the bed appears as sand waves. On steep slope, contrariwise, it is basically the torrent flow as the bed is shifted from sand waves and sand dunes to calm bed. A formula for calculating resistance coefficient incorporating the influences of circum flow, namely transition from sand waves to calm bed, is given.(5) The relationships between flow strength indexes and sediment transport strength on loess sandy slope and incohesive sandy bed under simulated rainfall are clarified and a formula for calculating sediment transport capacity on slope under the experimental condition is given. There is a good relationship between sediment transport strength and flow power under the two experimental conditions. The averaged velocity index is relatively better for cohesive sand and however, it may not be used for incohesive sand. Based on the understanding, a combined parameter of flow strength incorporating flow velocity, slope, and flow depth is determined and a general expression of sediment transport capacity is presented, after analyzing the flow strength indexes. Finally, a formula for calculating sediment transport capacity under the specific experimental condition is found by using the improved particle swarm optimization algorithm. Error analysis shows that the predicted values are close to the measured data.