Effects Of Canopies An Roots Of Grass On Runoff And Sediment Transport Over Slope Lands

Vegetation restoration is one of the important measures for construction of ecological environment on Loess Plateau. However, there is a predicament in the ecological environment construction on Loess Plateau which is called"it looks green from afar and the muddy water was seen from nearby". In production, the effect of vegetation structure on soil erosion is in urgent need. In fact, it is very meaningful to enhance the quality of ecological restoration by optimizing the vegetation structure. A sandy soil from water-wind erosion crisscross region was used in this study and a grass which was usually used in vegetation restoration on Loess Plateau was planted in laboratory. The hydrodynamic characteristics and erosion and sediment rule of runoff on the bare slope, grassroots slope and grassland under different slopes and rainfall intensities were analyzed by rainfall simulation. The conclusion was as follows:(1) The effect of canopies and roots of grass on hydraulics of overland flowIn the experiment conditions, the Reynolds number of the runoff on grass underlying surface, root underlying surface and bare underlying surface varied from 60 to 240, 71 to 210 and 54 to 221 respectively when the rainfall intensity changed in the range of 30~90mm/h and slope in the range of 1~12°, which indicated that the runoff was laminar flow on all three conditions and the energy dissipation and flow velocity distribution obey that of laminar flow; the Froude number of runoff under the three underlying surfaces varied from 0.2 to 0.9, 0.3 to 2.6 and 1.5 to 7.3 respectively, showing that the runoff on grassland was tranquil flow and supercritical flow on bare slope, while on root underlying surfaces, the runoff was tranquil flow on the slope of 1°and supercritical flow on the slope of 3~12°. The froude of the overland flow could be decreased from 79.8% to 85.8% by canopies and roots of grass together, that's why the grass played an important role on the runoff stability and sediment reduction. The average flow depth could be increased by 2.13~2.9 times on grassland compared that on bare slope, which reflects the effect of the canopies and roots of grass on the increase of resistance to some extent; the resistance of overland flow could be increased by 27.9 to 52.3 times on grassland compared that on bare slope, the average flow velocity could be decreased by 65.3~75.7%, which indicate that he canopies and roots of grass reduce the soil erosion by decreasing flow velocity and increasing resistance and flow depth; the rainfall disturbance has much little effect on overland flow velocity compared to the flow and slope, when the Chezy formula which used in uniform flow of open channel applied in nonuniform overland flow, it needs to be corrected.(2)The effect of canopies and roots of grass on soil erosion and sediment transportIn the experiment conditions, the sediment transport rate of unit width could be reduced by 58.5%~79.0% under the interaction of canopies and roots of grass. The sediment transport rate was reduced by 30% and 36% respectively by canopies and roots of grass under small rainfall intensity and 7% and 73% respectively under large rainfall intensity. Since both tranquil flow and supercritical flow existed in the root underlying surface, the flow was often unstable in the critical conditions, resulting in the instability of the sediment transport. The sediment yield was reduced through increasing the flow depth and decreasing flow velocity under the interaction of canopies and roots of grass. The canopies had an indirect effect on reducing erosion. The roots had significant influence on erosion reduction, which could reduce sediment yield by 77%.