| Sponge city is the trend of urban ecological construction in the future.As a typical low impact development measure,grass swale is widely used in the construction of sponge city.But the runoff regulation mechanism and sediment transport characteristics of grass swale are not clear,which leads to the design and application of grass swale in practical engineering is not standardized.This study takes the grass swale as the object.Through the water discharge simulation test,the hydraulic parameters of grassed swale runoff under the conditions of different flow,different slope and different sediment concentration were systematically measured under the conditions of clear water and sediment-laden flow.The variation law of runoff hydraulic characteristics and section velocity distribution in grass swale was analyzed,and the hydraulic characteristics of sediment-laden flow through grass swale were studied.On this basis,the sediment transport characteristics of grass swale were clarified.The main conclusions are as follows:（1）With the increase of flow rate and slope,the flow velocity,Reynolds number and Froude number gradually increase,and the Manning roughness coefficient and Darcy-Weisbach friction coefficient gradually decrease.The velocity（V）could be expressed as a power function V=0.3387Q0.5550J0.6601 of flow rate（Q）and slope（J）.The variation ranges of Reynolds number and Froude number were 1160.95～6596.82 and 0.17～1.21,respectively.The runoff flow patterns were all turbulent,and the flow pattern was greatly affected by the slope.When flow rate and slope were small,they have great influence on friction coefficient.Under the experimental conditions,the Darcy-Weisbach friction coefficient was negatively correlated with Reynolds number.The velocity distribution of cross section showed symmetrical distribution on both sides of the center,and the maximum velocity point was located at the center of water surface.With the increase of the flow rate and slope,the velocity contours of cross section gradually became dense and the gradient of the velocity change increased.The maximum flow velocity of the section increased significantly and the area of the minimum flow velocity area decreased.（2）The flow velocity of sediment-laden flow in grass swales increased gradually with the increase of slope,showed a power function relationship.It gradually decreased with the increase of sediment concentration,showed a linear relationship.There was no regular change with the increase of slope length.The relationship between flow velocity and erosion duration was mainly affected by slope.There was a positive correlation under 1%and 2%slopes,and a negative correlation under 3%to 10%slopes.The Reynolds number increased first and then decreased with the increase of slope.The Froude number increased with the increase of slope.Both Reynolds number and Froude number decreased with the increase of sediment concentration.With the increase of slope and sediment concentration,the Darcy-Weisbach friction coefficient of sediment-laden flow increased gradually.The Darcy-Weisbach friction coefficient was significantly negatively correlated with the Reynolds number（P<0.01）,and the correlation with the Froude number was poor.The shear force and flow power of sediment-laden flow all increased with the increase of slope.With the increase of sediment concentration,the flow shear force gradually increased,the flow power gradually decreased,and the larger the slope,the greater the decrease.（3）The characteristics of sediment transport in grass swale were as follows:When the slope was small,the sediment-laden flow through the grass swale showed siltation,and gradually became scouring with the increase of slope.Under the experimental conditions,the deposition rate ranged from-18.87%to 22.02%.The deposition rate decreased with the increase of slope,increased with the increase of sediment concentration,and does not show obvious regularity with the increase of slope length.It showed a trend of random alternation of erosion and deposition,and erosion or deposition may occurred in any slope section.Deposition rate was negatively correlated with flow velocity,flow shear force and flow power,and showed a good correlation.The deposition rate could be well fitted by linear equations with flow velocity.The deposition rate could be well fitted by logarithmic function with flow shear force and flow power.The sediment transport capacity of the grass swale ranged from30.50 to 243.65 kg·m-3,and increased with the increase of slope.The deposition amount under different slopes showed a decreasing trend with the increase of the potential sediment transport capacity of the water flow,and there was a significant correlation between the two under each slope.Grass swale communicates the effect of increasing resistance and deceleration to regulate runoff.The runoff reduction effect was stronger under the condition of small slope,and the transport efficiency of runoff and sediment was higher under the condition of large slope.The research results can provide reference for the design and application of grass swale. |
