Measuring And Estimating Shallow Water Flow Velocity Under Complex Underlying Soil Surface Conditions

The overland flow of hillslope runoff or melt water flow is an important parameter to characterize the hydrodynamics characteristics,which is a key paramater of soil erosion and sediment transport.The measurement of overland flow velocity is important to the studies on mechanism and prediction of soil erosion.This research used electrolyte tracer method,dye tracer and the flow front as tracer to measure the flow velocity to establish the relationship between the measured velocities and the accurate velocity.A method and procedures were proposed to velocity of water flow in sand layer.Electrolyte tracer and dye tracer method were used to measure flow velocity in sand layer,under three flow rates of 0.25,0.5 and 1 L/min and three slope gradients of 4°,8°and 12°,as pulse boundary velocity and peak velocities.The results show that under different conditions,the ratio between pulse boundary velocity and leading edge velocity was 0.26;that between pulse boundary velocity and leading edge velocity was 0.48.The overall trend of leading edge velocity and pulse boundary flow velocity increased with slope gradient and flow rate.The study verified the feasibility of measuring velocity of water flowing in a sand layer.A device was made to create a saturated soil slope to measure water flow velocity over saturated slopes.Pulse boundary velocity and leading edge velocities under slope gradients of 5°,10°,15°,and 20°and flow rates of 2,4,and 8 L/min,for slopes with sand and soil mixing ratios of 0%,20%,50%,and 80%.Both velocities increased with slope gradient,flow rate and sand and soil mixing ratio,the slope gradient had the most significant influence.The pulse boundary velocity over slopes of 20%and 50%sand and soil mixing ratio was lower than that of pure soil slope.The pulse boundary velocity over slopes of 80%sand and soil mixing ratio was greater that of pure soil slope.The pulse boundary flow velocity over saturated slopes with sand and soil mixing ratio from 20%to 80%overall increased with increase in sand and soil mixing ratio.The pulse boundary flow velocity was slightly lower than that leading edge velocity,with proportional coefficient of 0.93.The leading edge velocities and peak velocities increased exponentially with slope gradient and flow rate over frozen and non-frozen soil slopes.Under experimental conditions,the leading edge velocity and peak velocity over frozen slopes were 43%and 40%higher than those over non-frozen slopes.Peak velocities were approximately 0.63 and 0.61 times of leading edge velocities over frozen and non-frozen soil slopes.The slope gradient and the flow rate had greater effect on velocity over frozen soil slope than that of non-frozen soil.Under different slope gradients of 4°,8° and 12° and flow rates of 12,24 and 48 L/min,conceptual peak velocity of shallow water flow over loess slope were between 0.151?0.571 m/s,which increased with slope length with decreased magnitude.Fitting peak velocities were between 0.24?0.57 m/s,more impacted by flow rate than by slope gradient.Mean peak velocity were between 0.291?0.773 m/s,which did not vary much with distance.All three peak velocities above increased with slope gradient and flow rate.Fitting peak velocity and mean velocity were approximately equal.Mean peak velocity was about 1.3 times of fitting peak velocity and mean velocity.Under different slope gradients of 4°,8° and 12° and flow rates of 3,6 and 12 L/min The velocities within sand layer,including conceptual peak velocity,segmental peak velocity and mean peak velocity,calculated with different computational methods remained stable at different distance,the effect of flow rate on velocity was not significant,but the slope gradient had great influence on velocity.All the peak velocities were about equal to the velocity measured by electrolyte centroid method and electrolyte pulse boundary method.All these measured velocity can be used as water flow velocity.Mean velocity can be got by the leading edge velocity multiplied with a correct factor 0.8.The results can help to understand hillslope hydrodynamics.It is also helpful for the study on soil erosion and sediment transport on various soil surfaces and for allocation of different soil and water conservation practices.