柔性植被和刚性植被水流水动力学特性研究

为了深入揭示植被水流的内在规律,系统研究了刚性和柔性植被覆盖下坡面流水动力学特性。结果表明:刚性或柔性植被覆盖下,水流平均流速均随流量增大而增大,刚性植被对流速的减缓效果优于柔性植被;刚性植被覆盖下,弗劳德数小于1,处于缓流流态,阻力系数随雷诺数和流量增大呈单峰形式变化;柔性植被覆盖下,随流量增大,弗劳德数逐渐增大,由缓流过渡到急流,阻力系数随雷诺数和流量增大均呈减小趋势。在生态工程建设过程中,同种植被应该选取刚度较大的植被类型。     

底坡变化对坡面流水动力学特性影响研究

坡度是影响坡面流水动力学特性的重要因素之一,采用变坡试验水槽在变化水深条件下模拟坡面流,用于研究7种不同坡度对水动力学特性的影响。结果表明:水力坡度、弗劳德数、雷诺数随坡度增大而增大,阻力系数随坡度增大先增大后减小;在相同坡度条件下,弗劳德数随水深增大先减小后趋于稳定,雷诺数随水深增大而增大;当0≤J≤1.5%时,水力坡度随水深增大先减小后趋于稳定;当2%≤J≤3%时,水力坡度随水深增大先基本不变后减小。     

植物茎秆影响坡面径流水动力学特性研究

为深入揭示植被覆盖下坡面流阻力的内在规律,在考虑植被段前壅高和段后水深的基础上,系统研究了植被茎秆覆盖度为32.32%时6个坡度和12个流量组合下坡面流的流动特性。结果表明:在植被茎秆覆盖度为32.32%条件下,当坡度小于6°时,坡度对平均流速影响与流量相当,而坡度大于6°时,坡度对流速的影响甚微;试验过程中,壅水区水深与平均水深均随着流量增加而增大,而尾水深则略有波动;植被阻力系数随雷诺数的增加而增大,而随弗汝徳数的增加而减小,这是植被的粘滞阻力和压差阻力相互作用的结果。该成果为进一步研究植被覆盖下坡面流水动力学特性提供理论依据,进而对水土流失治理及生态修复均具有一定的指导意义。     

不同边壁条件下矩形明渠水力特性实验研究

为了研究明渠水流水力特性变化规律,在室内实验的基础上,采用矩形明渠变坡水槽,通过改变边壁条件、坡度和流量进行实验,得到了较好的实验数据。实验结果表明:边壁条件和坡度相同的情况下,随着流量的增加,水深呈线性增加,平均流速和雷诺数呈现幂函数增大;当Fr1时,弗劳德数随着流量的增加而增加;当1Fr1.526时,弗劳德数随着流量的增加先增加而后减小;当Fr1.526时,弗劳德数随着流量的增加而减小;曼宁糙率系数随着流量的增加呈对数减小趋势。边壁条件和流量相同的情况下,随着坡度的增大,水深呈减小趋势,平均流速、雷诺数、弗劳德数和曼宁糙率系数均呈现增加的趋势。并由实验数据得出了雷诺数与坡度和流量关系的经验公式。     

植被缓流效果试验研究

为深入研究植被减流减沙特征及对流域水土保持效益的影响,本文通过5个覆盖度,6个坡度和7个流量的组合条件下的室内放水试验,系统研究了植被对坡面水流的缓流特征。结果表明:随着植被覆盖度增加,植被对坡面水流的阻滞作用逐渐凸显,即缓流系数减小,缓流效果增强,当植被覆盖度大于4.650%时,缓流效果趋于稳定。在试验流量范围内,缓流系数随流量的增加呈现逐渐减少的趋势,而随坡度的增加略有增加。缓流系数随水深和雷诺数增加而减小,而与弗劳德数呈反C型分布。最后,建立了基于试验控制变量(植被覆盖度,流量和坡度)的缓流系数预测公式。     

地表粗糙度对坡面流水动力学参数的影响

坡面流阻力的研究对于深入认识坡面流水力特性及土壤侵蚀产沙机理具有重要意义。采用变坡试验水槽,利用定床阻力冲刷试验系统研究了坡面流Darcy-Weisbach阻力系数与粗糙度、流量、坡度和水流型态之间的关系。结果表明:阻力系数随着床面粗糙度的增加呈指数函数增大;阻力系数随着雷诺数的增加呈幂函数减小,且随粗糙度的增加;阻力系数与弗劳德数呈良好幂函数关系,且粗糙度的影响不显著,流量为主要影响因素;人工模拟粗糙度床面下,阻力系数可由粗糙度、流量和坡度的简单幂函数良好表示,并且粗糙度为主要影响因素,流量和坡度的影响相对较小。由因子分析可知,阻力系数可由粗糙度和弗劳德数良好表示,经检验该模型可用来计算不同粗糙度坡面薄层水流的阻力系数。     

黄土坡面细沟水动力学特性试验研究

在室内放水冲刷试验的基础上,开展了不同流量和不同坡度条件下的均匀细沟和侵蚀细沟的水动力学特性研究,结果表明:在均匀细沟和侵蚀细沟下,平均流速均受坡度和流量的影响,且可用坡度和流量的幂函数来表示;在均匀细沟和侵蚀细沟中,水流雷诺数变化范围分别在252~1 965、248~1 903之间,均主要受流量的影响,且均随流量的增大呈线性关系;弗劳德数在均匀细沟和侵蚀细沟中随坡度的变化趋势基本一致,但随流量的变化趋势有较大差异;阻力系数在均匀细沟中随坡度、流量的增加而增大,且可以用坡度和流量的幂函数来表示,侵蚀细沟的阻力系数随坡度和流量的增大呈先增大后减小的趋势;阻力系数在均匀细沟和侵蚀细沟中随弗劳德数的变化趋势基本一致,但随雷诺数变化趋势有较大差异。     

不同布置形态下刚性淹没植被对水流特性的影响

自然河道中植被呈现多种多样的分布状态,为探究不同布置形态下刚性淹没植被对水流特性的影响,利用室内水槽模拟含刚性淹没植被的明渠进行试验研究。结果表明:刚性淹没植被的布置形态对明渠流水流特性影响显著。在本文研究的植被布置形态下的水深、植被阻力系数与植被粗糙系数均增大,表现为交错型布置斑块型布置线性布置;流速与雷诺数均减小,表现为线性布置斑块型布置交错型布置;糙率n随着平均流速与水力半径之积VR的增大而减小。     

工程堆积体坡面细沟流水力学参数特性研究

为采用水力学参数特性揭示黄土高原沟壑区工程堆积体坡面土壤侵蚀过程,开展了野外放水冲刷试验。通过组合不同流量(35、45、55L/min)、不同坡度(24°、28°、32°)的试验,对坡面沟蚀发生发展过程中的水力学参数特性进行了研究。结果表明,坡面细沟水流的平均流速与水深主要受流量控制,二者均与单宽流量呈幂函数关系,且流速随冲刷时间的持续和冲刷形态的变化呈现上下起伏—瞬时增大—稳定的波动或减小—趋于稳定的变化历程。在所有坡度和流量范围内,雷诺数Re在42.9~1 082.8之间变化,雷诺数Re随坡度的增大呈先减小后增大的抛物线趋势。由坡面流弗劳德数Fr随冲刷历时变化情况表明,坡面径流形态是由急流向缓流发展。阻力系数f在0.019~0.94之间变化,f随坡度的增加先减小后增加,且随雷诺数Re的增大而增大,二者之间呈显著的指数函数关系。粗糙系数n随雷诺数增大而增大,且其与雷诺数存在显著性的指数函数关系。研究结果可为工程堆积体土壤侵蚀预报模型的建立提供一定的参数,对解决生产建设项目弃土弃渣场、取土场等水土流失问题具有重要的作用。     

冲积床面形态及阻力的水槽试验研究

本文通过水槽试验,对中径为0.22mm的床沙,比较系统地研究了冲积床面形态及阻力规律.试验中观察到沙垄消亡时的床面形状比较规则,沿程分布比较均匀.试验结果表明,同一沙粒阻力条件下,沙波相对高度及陡度随水深增大而减小,床面形态过渡区,水流阻力不仅随沙粒阻力增大而减小,而且与相对水深有关.同一沙粒阻力的水流,其阻力系数随水深增大而减小,在床面形态过渡区,阻力系数相差不大,沙粒阻力是影响床沙质含沙量的主要因素.     

黄土区陡坡径流水动力学特性试验研究

通过黄土水槽冲刷试验，初步研究了黄土区坡面径流的水动力学特性。陡坡径流的流态属于急紊流，且具有时空分异特征。随着坡度的增加，径流雷诺数Re和Darcy-Weisbach阻力系数f均呈先增后减的抛物线趋势。f与Re之间存在幂函数正相关关系，而f与弗劳德数(Fr)之间存在幂函数负相关关系。     

A STUDY OF DRAG COEFFICIENT RELATED WITH VEGETATION BASED ON THE FLUME EXPERIMENT

This study is focused on the effects of ecological factors (diameter and flexibility) and vegetation community composition on the drag coefficient related with vegetation. The single leafy shrub and three mixed communities (including shrub-grass, shrub-reed and reed-grass community) were studied. The flow velocity and water level were measured and used to calculate the drag coefficient based on the Bernoulli's equation, Darcy drag formula and the expression for the drag coefficient related with Darcy drag factor. The trend of the drag coefficient in the vertical direction was analyzed against flow depth, diameter, diameter Reynolds number, flow depth Reynolds number and relative roughness height in different discharges. The results show that beside the dense leafy shrubs community, the vertical trend of the drag coefficient among other cases against flow depth, diameter, diameter Reynolds number, flow depth Reynolds number and relative roughness height can be approximately expressed by power law functions under different flow discharges. Moreover, in a mixed community with two plants with distinctly different ecological factors, the one with the most distinct variations of ecological factors determines the vertical trend of the drag coefficient; the other one only affects the magnitude of the drag coefficient. Furthermore, if the ecological factors of the vegetation in the vertical direction are kept almost not changed, the drag coefficient can be approximately regarded as a constant.     

INCIPIENT MOTION OF NON-COHESIVE SEDIMENT UNDER ICE COVER - AN EXPERIMENTAL STUDY

Based on a series of experiments under both ice-covered and free surface conditions, the present article discusses the role of flow velocity and critical shear Reynolds number for incipient motion of bed material. The influence of the resistance coefficients of both the underside of the ice cover and the channel bed on the location of the maximum velocity has been discussed. In addition, the impacts of ice and composite resistance coefficients on flow velocity for incipient motion of bed material have been assessed. The diagram describing the critical shear Reynolds number and the dimensionless shear stress for the incipient motion of sediment under ice covered conditions with different under cover resistance coefficient has been established. The effects of grain size on densimetric Froude number for incipient motion of bed material have been investigated. A relationship between the densimetric Froude number for incipient motion of bed material and the median grain size of bed material as well as the roughness coefficient of channel bed and roughness coefficient of ice cover has been established.     

浮帘群水流阻力特性试验研究

为探明含浮帘群水流的阻力特性和各阻力参数计算方法,对不同间距、不同高度浮帘群在不同恒定均匀明渠流中的阻力特性开展了92组水槽试验。通过拟合水面线获得水力坡降,探讨浮帘拖曳力系数、曼宁糙率和达西韦斯巴赫阻力系数的变化规律。研究结果表明:浮帘群布置存在临界间距,此时浮帘间的相互干扰作用几乎消失;浮帘群消能增阻作用明显,其拖曳力系数在不同水流条件下变化较小,取值介于1.60~2.05之间;曼宁糙率主要与浮帘相对高度及浮帘分布密度有关,并获得了曼宁糙率的修正计算方法;达西韦斯巴赫阻力系数可由浮帘相对高度、浮帘分布密度及浮帘雷诺数组成的拟合公式获得。     

模拟降雨下枯落物调控坡面产流产沙过程及特征研究

本研究采用模拟降雨实验的方法,研究了30、60和90 mm/h降雨强度下,5种枯落物覆盖坡面径流的水动力学特征,以期揭示枯落物覆盖条件对坡面产流产沙过程及其径流水动力学特征影响。研究结果表明:坡面平均产流率和产沙率随降雨强度增大而增大,随枯落物生物量增加而减小,并且与降雨强度和枯落物生物量之间均为指数函数关系;坡面流量-历时曲线稳定流量值主要受降雨强度的影响;产沙率-历时曲线稳定产沙率受降雨强度和枯落物生物量等多种因素的影响;枯落物覆盖对坡面径流水动力学特性有显著影响,相比于裸坡覆盖有50、100、150、200 g/m~2枯落物坡面径流平均流速相比裸坡减少率分别在17.7%、26.8%、37.9%、53.7%;雷诺数减少率分别在11.2%、18.7%、26.3%、33.4%左右;弗劳德数减少率在21.3%、30.3%、42.9%、61.2%左右,Darcy-Weisbach阻力系数增大了0.5~5.6倍;产沙率与Darcy-Weisbach阻力系数呈对数关系,与剪切力和径流功率呈线性关系,枯落物能显著增大径流临界剪切力和功率;产沙率(Dr)和各水动力学因子间有良好的相关关系,相关程度顺序为:RewVPFrft。     

An Approximately Semi-Analytical Model for Describing Surface Runoff of Rainwater Over Sloped Land

Accurate prediction of surface runoff is critical to watershed management. In this research a semi-analytical model was adopted to solve the kinematic wave equation based on the assumption that the rate of overland-flow depth change is proportional to the rainfall excess. Simulations were compared with the results from laboratory experiments at various rain intensities. Parameters of infiltration rate and Manning’s roughness coefficient were determined. The accuracy of the semi-analytical model was evaluated by numerical simulations. The predicted outflow rates from the numerical simulations agreed well with the observed data. Further, our study indicated that the ratio (c) of the overland-flow depth change to the rainfall excess was a power function of the rain intensity. The depth and velocity of water flow at any time and distance could be calculated with the semi-analytical model. Hydraulic parameters including Reynolds number, Froude number, hydraulic shear stress, stream power and Darcy-Weisbach friction factor characterizing the dynamic features of overland flow of rainwater were calculated based on calculated overland-flow depth and velocity. The proposed analytical method can provide a new way to predict infiltration and runoff over sloped land.     

EXPERIMENT OF IMPACT OF RIVER HYDRAULIC CHARACTERISTICS ON NUTRIENTS PURIFICATION COEFFICIENT

Hydraulic characteristics of a river are the key factors for river water quality improvement and river restoration. A simulated river was exploited to study the interactions between water contaminant purification coefficients and the characteristics of the river, such as the cross-section velocity, water depth, Reynolds number, and Froude number. To enhance the purification capacity of the river, detritus with 5 mm to 10 mm in diameter and contact surface area per volume of 87.6 m2/m3 were constructed naturally at the bed and along one side of the bank to form half shape of the river cross-section. During the one-month experiment (including 3 periods) from the third of November to the fourth of December, 2005, three categories of hydraulic conditions were investigated. Results show that the purification coefficient (K) of Total Nitrogen (TN) is closely coherent with the hydraulic retention time (T), river length (L), Reynolds number (Re) and Froude number (Fr). The relationship of K and T generally agrees with the power law under the three experimental water hydraulic conditions. Based on these results, the optimal Re and Fr ranges can be obtain to serve as a guideline for ecological re-engineering design to improve river water quality and restore river ecosystem.