【作者】 张颖；

【导师】 谢宝元； 余新晓；

【作者基本信息】 北京林业大学 ， 工程绿化， 2007， 博士

【摘要】 由于人类不合理地利用自然资源,导致森林植被受到极大的破坏,引发了严重的土壤侵蚀。黄土高原就是是严重的水力侵蚀地区。森林作为陆地上最重要的生态系统,以其林冠层、林下茂密的灌草层和林地上富集的枯枝落叶层以及发育疏松而深厚的土壤层截持和蓄储大气降水,发挥着其特有的水文生态功能,在防治土壤侵蚀方面有其不可缺少的意义。本文在查阅了大量国内外文献资料,全面了解了坡面流水动力学特性及坡面侵蚀动力过程等方面的研究现状及最新进展的基础上,以揭示森林植被加入后对坡面薄层水流水动力学特性及坡面侵蚀的影响为目标,采用室内人工降雨模拟实验的方法,变换雨强和坡度,运用水动力学与侵蚀理论以及统计理论,对试验资料进行分析,研究了森林植被影响下,林冠降雨的雨滴动能、坡面薄层水流流态、坡面流阻力系数变化规律、坡面流泥沙运动规律;建立了坡面流阻力系数公式、泥沙起动临界剪切力公式,坡面流侵蚀数学模型,并采用实测资料进行了验证,取得了具有重要理论意义和应用价值的研究成果。主要研究内容与成果有以下几个方面:(1)实验得出油松、侧柏、元宝枫林冠丛冠滴雨有相当数量的大雨滴形成,其单位水体雨滴潜在动能分别为28.44 J·m-2mm-1,29.19 J·m-2mm-1和30.20 J·m-2mm-1,不同树种之间的差别很小,而林外降雨的雨滴动能为17.39 J·m-2mm-1,因此当所有林冠雨滴都达终速时,林冠对雨滴的动能增大63.5%、67.91%、74.4%,如果林冠足够高,那么对林地的击溅是非常严重的,将远远高于无林地的击溅侵蚀量。由于本实验用的是小树苗,因此计算得出林冠雨滴动能油松、侧柏、元宝枫冠层分别为e1=1.862 J·m-2mm-1,e2=1.345 J·m-2mm-1,e3=4.379 J·m-2mm-1。有效的降低了雨滴动能。(2)在同一坡度下各处理小区的雷诺数差别不大,说明雷诺数受坡度影响不大,而不同雨强雷诺数变化大,雨强从60mm/h——100mm/h变化时,雷诺数变化范围为从5—30。由于裸地和各树种的雷诺数坡面水深很小,一般在0.2mm左右,表征惯性力与粘性力之比的雷诺数Re也很小。在试验条件下,都在层流范畴。说明雨滴的扰动并没改变坡面流的流动型态,未改变水流质点沿着水流流线垂直方向即流线法线方向上的位置进而产生涡体引起水流质点混掺而形成紊流。(3)由水力学及流体力学理论结合试验结果,得出,降雨条件下,有森林植被覆盖的坡面流在地面没有枯落物等覆盖时,仍属层流范畴,因此阻力系数变化符合层流阻力规律,通过理论直接推求,可得:f=24/Re;有枯落物覆盖时,坡面流符合水流在多孔介质中的流动规律,需用用渗流理论研究,通过简化为多组规则圆管的层流运动,得出坡面流在枯落物中的流动规律。坡面流在枯落物中流动的数学模型,可用下述模型描述hf/L=f’(a(1-ε)_ε3)u2,其中包括一个未知的待定系数f’,在流速较低时,可用欧根(Ergun)方程描述:f’=4.17/Re+0.29。(4)分析侵蚀泥沙的颗粒,可得,树种覆盖后,击溅作用变小,侵蚀量变少,侵蚀的泥沙中,粗颗粒变少,平均粒径变小,覆盖的坡面,加剧粘粒和量少的粉沙侵蚀量,坡面粗化不明显,裸地坡面溅蚀作用加剧的是粉沙颗粒和少部分较大颗粒的侵蚀量,而土壤中大颗粒还是留了下来,裸地坡面出现明显的粗化层。虽然林冠层能有效防止溅蚀的发生,但坡面流还是能带走粘粒部分,粘粒还是容易流失。去除林冠和表层土后,侵蚀量变得更少,但表层还是会被溅蚀,有水流后,溅蚀作用减弱,颗粒起动的比裸地的少,所以,去除表层扰动土后,侵蚀量更少,粉沙多,粘粒比裸地更少一些,但不稳定,雨滴击溅和水流作用用于克服泥沙分离起动损耗的能量较大,很难被起动输移,这说明森林植被根系的作用增加了林地植被的抗蚀力,使降雨一水流系统仅能溅起输出比裸地坡面更细小的泥沙颗粒。(5)借鉴前人的研究成果,从单颗粒泥沙起动受力分析入手,研究泥沙颗粒起动的临界状态,根据泥沙颗粒滚动受力力矩平衡,粗略推导出有降雨击溅情况下裸地坡面单颗粒泥沙起动临界剪切力公式:有植被覆盖的小区和有枯落物覆盖的小区,可视为无雨滴击溅作用,泥沙起动临界剪切力表达式可直接采用杨具瑞等建立的公式(杨具瑞,曹叔尤,2004):(6)通过坡面流侵蚀动力学模型数值拟合结果,得出,森林植被覆盖的坡面,减少了雨滴击溅侵蚀,抗侵蚀力增加。更多还原

【Abstract】 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;更多还原