Dynamic Effects Of Forest Vegetation On Hill-slop Water Erosion In Loess Area

Because of irrational utilization of natural resources human is destroying the forest vegetation and thus leading to serious erosion. the loess plateau is one of serious water erosion areas. As the most important ecological system on the earth terrain, forest plays its specific function in hydrological ecology via forest crown, dense shrubs and grasses under trees, litters and soils. Moreover forest has roles in protecting head waters, purifying water quality, conserving soils and water and reducing natural disasters through its interaction with soils, air and water.On the base of consulting a large number of literatures domestic and aboard, the present situation and latest development of hydrodynamic characteristics of overland flow and slope erosion dynamic process were reviewed. Aiming at revealing the effect of rainfall splash intensity on hydrodynamic features of sheet flow and slope erosion, with a simulated rainfall experiment in condition of changeable intensities and changeable gradients.applying hydrodynamic,erosive theory and statistics theory this text studied the influence of forest vegetation on raindrop kinetic energy of leaf canopy precipitation, flow regime and resistance coefficient of overland flow and sediment motion law on hill-slop, established resistance coefficient formula of overland flow, critical shear formula of sediment incipient motion and mathematical model of overland flow erosion, and validated all of them with field data, obtained important achievements of having theortical significance and appliedcost.By means of the study the main results are listed as follows: (1) Experiments showed that the forest canopy drip rain of Pinus tabulaeformis Carr, Acertruncatum Bunge and Platycladus orientalis were made up of a large number of big rain drops, their rain drop potential kinetic energies of unit water bodies were 28.44 J·m^-2mm^-1, 29.19 J·m^-2mm^-1 and 0.20 J·m^-2mm^-1, differ trees had fine distinctions, while the rain drop kinetic energies outside of forests was 17.39 J·m^-2mm^-1, so when all of forest canopy rain drips arrived at final speed, the forest canopy kinetic energies would increase 63.5%,7.91% and 4.4% respectively, if forest canopy was enough high, the splash of rain drop to woodland would be very serious, and its splash erosion quantities were well above non-forested. The forest canopy kinetic energies of l,2,and 3 usingsaplings were e₁ = 1.862 J·m^-2mm^-1,= 1.345 J·m^-2mm^-1, = 4.379 J·m^-2mm^-1, they reduced kineticenergies effectively.(2)The Reynolds numbers of each disposal area in the same slope had fine distinction, it explainedthat the influence of slope on Reynolds number was little, while rainfall intensity on the contrary,Reynolds number changed from 5 to 30 with rainfall intensity varying from 60mm/h to 100mm/h,because the slope water depth of bare ground and forests were very small, commonly in the 0.2mm,so Reynolds number in token of the ratio of inertia force to viscous force was also very small, thetest that belonging to laminar flow category explained that the disturbance of raindrop did notchange overland flow form.(3)Combined hydraulics and hydromechanics theory with test results, we could get that in thecondition of rainfall, the overland flow that forest land without litters belonged to flow category, sothe changes of resistance coefficient followed the rules of flow resistance, through theory we could24 directly get that f =((24)/(Re)); the overland flow with litters followed the flow rules of porous medium,using filtration theory, overland flow was simplified into laminar flow with several regular tubules,and the flow laws of overland flow in litters could get out. The model (h_f)/L=f((a(1-ε))/(ε³))u² coulddepict the overland flow in the litters, there f' was a undetermined coefficient, when the flowvelocity was very low, the Ergun equation f'=(4.17)/(Re')+0.29 could depict the overland flow.(4)The particles in the erosion sediments showed that the splash function of forests lessened, the erosion amount, coarse granules in erosion sediments and average particle tailed off, the covered slope aggravated the erosion amount of clay and a little fine sediment, and the slope coarsening was not obvious; the slope splash function of bare land aggravated the erosion amount of fine sediment and a little part of coarse granules, while the coarse granule retained in the soil, the obvious rough bedding texture layer appeared in bare slope.Although forest canopy layer could prevent splash erosion effectively, clay was taken away by overland flow and run off easily, the erosion amount became less after removing the forest canopy and surface soil, while the splash erosion still produced in the surface layer; the splash function weakened after having water current, the starting particles were less than bareland, so removing the surface disturbed soil made the erosion amount and clay less, silty sand more than bareland, but unsteady, the wear and tear energy used by raindrop splash and current function for overcoming sediments separating and starting were comparatively large, sediments were very difficult to be started and transported, it explained that forest root systems could increase anti-erosion capability of forest vegetation, make rainfall-current system splash and export finer particles than bareland.(5) Using the achievements of predecessors, from the force that single particle sediment endured analyzed the critical state of sediment starting, according to the endure force moment balance of sediment rolling, the cursory single particle sediment starting critical shear formula of bareland slope in the condition of rainfall and splash as follows: The area with vegetation or litters could be regarded as having no raindrop splash function, the critical shear expression of sediment starting could adopt the formula;...