戴矜君,程金花,张洪江,相莹敏,周柱栋,杨帆(北京林业大学水土保持学院,100083,北京)
野外放水条件下坡面流水动力学特征
戴矜君,程金花†,张洪江,相莹敏,周柱栋,杨帆
(北京林业大学水土保持学院,100083,北京)
摘要:坡面流是土壤侵蚀的主要动力因素之一,也是侵蚀泥沙搬运、农业面源污染的重要载体,为探讨坡面流水力学特征变化规律,本文通过径流小区放水冲刷试验,研究不同坡面覆盖和放水条件下坡面流水动力学特征。结果表明:坡面流雷诺数和弗劳德数受植被覆盖情况和坡面坡度的影响较小,受放水流量影响较显著。其中,坡面流入流断面雷诺数和弗劳德数随时间基本保持不变;出流断面雷诺数动态变化呈增加趋势,弗劳德数呈缓慢下降趋势,其动态变化幅度随植株密度增加而趋于平缓,植株布设方式对雷诺数和弗劳德数的动态变化影响较小。流量和植株密度增加会引起阻力系数增长,阻力系数随冲刷历时呈增长趋势,坡度和植株密度可以控制这种增长趋势。
关键词:坡面流;径流小区;放水冲刷试验;流态;雷诺数;弗劳德数;阻力系数;动态变化
项目名称:北京高等学校青年英才计划“北方土石山区坡面侵蚀水动力学机理研究”(YETP0750)
坡面流是土壤侵蚀的主要动力因素之一,也是侵蚀泥沙搬运、农业面源污染的重要载体[1]。研究野外实际冲刷条件下,坡面流水动力学特征对定量预测土壤流失,建立土壤流失方程具有重要意义[2]。目前,坡面流相关研究以室内人工降雨结合模拟变坡土槽为主[3 4],结合经验分析和水文模型计算对坡面流水力学特征值的平均状态进行半定量性统计分析[5]。如李勉等[6]通过室内放水冲刷试验,研究表明,坡面流同时存在层流、紊流和过渡流3种流态。Hu Shixiong等[7]采用阻力分割方法,发现阻力系数与含沙量呈正相关关系,并得出坡面流阻力的计算模型。王玲玲等[8]通过室内放水试验,发现有植被坡面的阻力系数随放水流量的增加而减少,裸坡的阻力系数随放水流量的增加而增加。李毅等[9]通过室内模拟降雨试验,研究表明植被增加能改善坡面流水力性质,增加坡面阻力和粗糙度,降低坡面流紊动性。
限于野外原型坡面水流流动十分复杂,获取的相关参数准确度较低,因此,坡面流研究多采用变坡土槽进行室内模拟试验,忽略下渗和侵蚀动态变化对坡面流水动力学特征参数的影响[10 11],但野外实际坡面流条件下,坡面侵蚀、下渗都会引起坡面流水动力学参数变化。由于水力侵蚀中,坡面流的侵蚀力和携沙力主要是受坡面流水动力学性质影响[12 15],要准确描述实际情况下坡面流的侵蚀输沙规律,建立侵蚀模型,就必须研究实际坡面下,坡面流的水动力学平均状态和动态变化[16 19]。本研究设定不同放水流量、坡度、植株密度和植物布设方式,研究野外条件下,坡面流水动力学特征和动态变化,对揭示坡面侵蚀机理、建立侵蚀模型具有重要意义。
野外模拟冲刷试验在北京市高庙屯小流域上辛庄水土保持教育基地进行(E116°3'11″~116°4'19″,N40°26'19″~40°27'26″),研究区属典型华北土石山区,地貌类型主要为山地;大陆性季风气候,年均降水量467 mm,时空分布不均;土壤类型为褐土,土壤机械组成见表1。研究区内植被覆盖良好,以灌草、农作物和林地为主;区域内以水力侵蚀为主,土壤侵蚀模数为530 t/(km2·a)。
2.1试验设计
试验坡面覆盖植物采用紫色苜蓿(Medicago sativa L.),研究方法采用野外径流小区放水冲刷试验。试验冲刷坡面设计如表2;试验冲刷过程中流量设定3种(1.0,2.0,3.0 m3/h)。于2014年春季按照实验设计种植紫花苜蓿。为避免坡面砾石覆盖对坡面流的影响,试验前对坡面砾石进行清理。
表1 土壤机械组成Tab.1 Soil mechanical composition %
表2 试验组设计Tab.2 Design of experimental treatments
2.2试验方法
为保证低流量下冲刷坡面水流连续,经过预试验设定冲刷区宽度为0.7 m、冲刷区长度为4 m。利用钢板垂直砸入土壤20 cm,将试验径流小区(5 m× 2 m)分割为2个宽度为0.7 m的条状地块。条状地块上部设置溢流槽(0.3 m×0.7 m×0.3 m),下部10 cm处设置入流观测断面;地块下部设置梯形集水槽(0.7 m×0.5 m),上部10 cm处设置出流观测断面;每一观测断面水平布置5个水深和流速观测点。坡上部配置潜水泵(WQD25102.2QG)对坡面供流,连接DN40流量计保证水流恒定。坡面布置如图1。
图1 试验坡面布置图Fig.1 Layout for the arrangement of experimental slope
放水冲刷过程中,每30 s测定并记录各观测断面基础数据,并记录水温,计算相关水动力学特征参数。
2.3数据获取及计算
水深:假定水深均匀连续状态下,坡面流水深测定采用游标卡尺(精度0.02 mm),每30 s测定一次各观测点水深,取5个观测点的平均值为观测时段内该观测断面的平均水深。
流速:采用高锰酸钾染色法进行测定,3次重复,根据流态乘以修正系数(层流取0.67,过渡流取0.70,紊流取0.80[20]),由于实测流速误差较大,采用公式(1)对流速进行修正:
式中:V为观测时段内平均流速,m/s;Q为断面瞬时流量,m3/s,入流断面取放水流量,出流断面取集水区收集到的径流量;B为过水断面宽度,取0.7 m;h为观测断面5个观测点平均水深,m。
雷诺数(Reynolds Number):Re为流体惯性力与黏滞力的比值,用以判断坡面流流态[21]。
弗劳德数:Fr为流体惯性力和重力的比值,用以判断惯性力和重力对流体的主导作用[22 23]。式中:Fr为坡面流弗劳德数(Froude Number);g为重力加速度,取9.8 m/s2。
阻力系数:根据试验实测雷诺数,判定冲刷过程中流态为层流与过渡流;因此,选用Darcy-Weisbach阻力系数对阻力进行研究[25 27]
式中:f为阻力系数;J为水力坡度。
3.1雷诺数变化特征
根据前人研究[22 24],坡面流上临界雷诺数取6 500;下临界雷诺数取575。对24组冲刷试验数据进行分析,得到雷诺数总体特征和动态变化(表3和图2)。
表3所示,不同坡面下,各坡面入流断面雷诺数随时间变化不明显,基本稳定在 300~330(1 m3/ h)、620~740(2 m3/h)、940~990(3 m3/h)之间,表明坡面流入流断面流态与放水流量关系密切,与坡面覆盖情况和坡度关系不明显。不同地表覆盖情况下,各坡面出流断面雷诺数基本稳定在240~300(1 m3/h)、550~650(2 m3/h)、800~1 000(3 m3/h)之间;流量为1 m3/h时,出流断面坡面流流态为层流,流量为2 m3/h时,流态介于层流和过渡流之间,流量为3 m3/h时,坡面流流态为过渡流,出流断面流态受放水流量影响较明显;相较于入流断面,在相同植被布设和放水流量下,出流断面雷诺数较入流断面小,表明坡面流经过冲刷区后,流体的紊动性降低,这是由于坡面流流动过程中,阻力降低了流体的动能。
表3 各坡面雷诺数统计特征Tab.3 Statistical characteristics of Reynolds number in each slope
不同坡面覆盖情况下,坡面流出流断面雷诺数动态变化过程如图2。横向对比4个坡面出流断面雷诺数增加趋势,4号坡面(30株/m2,行排列)<2号坡面(20株/m2,行排列)≈3号坡面(20株/m2,随机排列)<1号坡面(裸坡),表明植被密度越小,坡面流雷诺数的增加趋势越明显,其中,植株行排列和随机排列对雷诺数动态变化过程的影响并不明显,说明植被布设方式对雷诺数动态变化的影响较小。放水流量为1 m3/h时,5°裸坡(1号坡面)雷诺数变化于23~375、均值283,低于10°裸坡(5号坡面)变化于178~427、均值337;有植被5°坡面雷诺数均值分别为268(20株/m2、行排列)、241(20株/ m2、随机排列)和232(30株/m2、行排列),低于同放水流量下10°坡面的308(20株/m2、行排列)、311 (20株/m2、随机排列)和 266(30株/m2、行排列),表明放水流量一定时,坡度对坡面流流态影响较大,坡度增加,坡面流的惯性力相对较大,流动趋于紊乱;同时,随着植株密度的增加,雷诺数下降,当变化相同植株密度时,5°坡面较10°坡面雷诺数变化小,这表明植被增加,能有效降低坡面流的紊动性,其中,坡度越大,植株减低流体紊动性的作用越明显。3.2弗劳德数变化特征
不同坡面覆盖情况下,2个观测断面的弗劳德数均<1,属缓流。各坡面入流断面弗劳德数基本稳定在0.3(1 m3/h)、0.4(2 m3/h)和0.5(3 m3/h)左右,随放水历时增加无明显的定向趋势,表明入流断面坡面流弗劳德数,只随放水流量的增加而增加,与坡面地表覆盖情况、地面坡度关系不明显。
图2 雷诺数动态变化Fig.2 Dynamic changes of Reynolds number at different section
不同坡面覆盖情况下,坡面流出流断面弗劳德数动态变化过程如图3。出流断面弗劳德数除前期(0~1 min)波动不稳定外,冲刷过程弗劳德数基本呈缓慢下降趋势。对比4个坡面的变化趋势,4号坡面(30株/m2,行排列)<2号坡面(20株/m2,行排列)≈3号坡面(20株/m2,随机排列)<1号坡面(裸坡),表明随植株密度增加,弗劳德数的动态变化趋于平缓,其中,植株布设方式对弗劳德数动态变化过程影响不显著。
3.3阻力系数变化特征
为探究坡面流阻力动态变化影响因素,选择坡度、放水流量、植株密度、植株布设方式和冲刷时间为因变量,对坡面流阻力系数进行逐步回归分析。
f=2.350t0.012q-3.178c0.084n=480R2=0.538式中:t为放水冲刷时间,s;q为放水冲刷流量,m3/ h;c为植株密度,株/m2。
逐步回归结果表明:坡面流阻力系数与放水流量、植株密度关系密切,受坡度、植株布设方式的影响较小。阻力系数公式中,时间因子 t的指数为0.012,表明阻力系数动态变化呈增加趋势,对各坡面冲刷流量为1 m3/h时的阻力系数动态变化数据做回归分析,结果如表4。坡度为5°时,坡面流阻力系数动态变化的时间指数分别为 0.971(裸坡)、0.633(20株/m2,行排列)、0.884(20株/m2,随机排列)、0.433(30株/m2,行排列)略大于同坡面覆盖情况下10°坡面的时间指数0.869(裸坡)、0.584(20 株/m2,行排列)、0.658(20株/m2,随机排列)、0.418(30株/m2,行排列),表明随坡度增加,坡面流阻力系数动态变化趋于平缓;时间 t的指数随植被覆盖增加而降低,表明植被能有效控制阻力系数的增加趋势。对比相同植株密度、不同植被布设方式下阻力系数的动态变化过程,3号坡面(5°,20株/ m2,行排列)的时间因子指数>2号坡面(5°,20株/ m2,随机排列),7号(10°,20株/m2,行排列)的时间因子指数>6号坡面(10°,20株/m2,随机排列),表明坡面流阻力系数与植株布设方式关系密切,其中,植株随机布设坡面的阻力系数变化较行排列布设坡面更平稳。
图3 弗劳德数动态变化Fig.3 Dynamic changes of Froude number
表4 坡面流阻力系数动态变化关系Tab.4 Dynamic changes of resistance coefficient in the slopes
1)坡面流入流断面流态与地表覆盖情况、坡度关系不显著,只与放水流量关系密切,雷诺数随时间基本保持平稳。出流断面雷诺数与冲刷流量、坡度和植株密度关系密切,雷诺数动态变化呈明显增加趋势,增加幅度随植株增加而趋缓,其中,植株布设方式对其动态变化影响较小。
2)入流断面弗劳德数只随放水流量的增加而增加,与地表覆盖情况、坡度关系不显著,弗劳德数动态变化无明显定向趋势。出流断面弗劳德数呈现缓慢下降趋势,随植株密度增加,动态变化趋于平缓,其中,植株布设方式对弗劳德数动态变化过程影响不明显。
3)坡面流阻力系数与放水流量、植株密度关系密切;坡度、植株布设方式对阻力系数的影响较小。阻力系数动态变化呈增加趋势,坡度和植株密度的增加能控制阻力系数增加趋势,其中,植株随机排列坡面的阻力系数动态变化相对行排列坡面更平稳。这与李勉等[28]通过室内放水试验的研究结论不一致:有草被覆盖坡面阻力系数随时间呈增加趋势。这是由于本实验为野外试验,根据华北地区的水文条件,选定冲刷单宽流量为1.43、2.86、4.29 m2/h,文献[28]的试验区选定为黄土高原,采取室内模拟试验,设定冲刷单宽流量为0.384、0.624 m2/h。
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Hydrodynamic characteristics of surface runoff on field scour
Dai Jinjun,Cheng Jinhua,Zhang Hongjiang,Xiang Yingmin,Zhou Zhudong,Yang Fan
(College of Soil and Water Conservation,Beijing Forestry University,100083,Beijing,China)
Abstract:[Background]Soil erosion harms the development of national economy.Water erosion is the most important form of soil erosion in China.Overland flow is the main driving force of water erosion and main carrier of sediment transport which is a key factor of water erosion.Reynolds number,Froude number and resistance coefficient of overland flow are important parameters reflecting overland flow characteristics.[Methods]To investigate mechanical properties of overland flow,the study on the dynamic changes of Reynolds number,Froude number and resistance coefficient with 3 plant density(0,20,and 30 plants/m2),2 vegetation distributions(row arrangement and random arrangement),two slopes(5°and 10°)and 3 scouring flow(1,2,3 m3/h)were studied by field scour simulation experiment.[Results]The results showed that effect on Reynolds number of inflow section by plant coverage density and slope was not significant,and only effected by flow rate.Dynamic changes on Reynolds number of inflow section remained stable over time,while that in outflow section showed an increasing trend which was affected by plant density.And the increasing rate of dynamic changes in outflow section slowed down with the increase of the plant density,however,influence by plant arrangement was not significant.Compared to the inflow section,Reynolds number of outflow section was smaller,indicating that the kinetic energy of the overland flow reduced during scouring process.Froude number was both less than 1 in two observation section,which were slow flow.Froude number of inflow section only increased with flow rate increasing,and the influences on Froude number of inflow section byplant coverage density and slope were not significant.The same as Reynolds number of inflow section,Froude number of inflow section also remained stable over time.The dynamic change of Froude number in outflow section decreased with time slowly,which was effected by plant densiy significantly,and the influence on it by plant arrangement was not significant.The dynamic changes of Froude number leveled off even plant density increased.Effect on resistance coefficient by slope was not significant,which was effected by plant density and flow rate,and the influence of plant arrangement was not significant.Stepwise regression analysis on resistance coefficient showed that f=2.350t0.012q-3.178c0.084,indicating that dynamic changes of resistance coefficient showed an increase trend that could be controlled by increase of slope and plant density.[Conclusions]Dynamic changes of resistance coefficient on bare slope had greater increase rate than slopes covered by vegetation,and resistance coefficient changing with time tend to be gentle with the plant density increased.Meanwhile,change range of resistance coefficient in plant random arrangement slope was smaller than slope in plant row arrangement.
Keywords:overland flow;runoff plots;scouring experiment;flow pattern;Reynolds number;Froude number;resistance coefficient;dynamic changes
中图分类号:S157.1
文献标志码:A
文章编号:1672-3007(2016)03-0052-08
DOI:10.16843/j.sswc.2016.03.007
收稿日期:2015 05 29修回日期:2016 03 03
第一作者简介:戴矜君(1992—),女,硕士研究生。主要研究方向:土壤侵蚀。E-mail:645059050@qq.com
通信作者†简介:程金花(1979—),女,副教授,硕士生导师。主要研究方向:土壤侵蚀与植被恢复。E-mail:jinhua_cheng@ 126.com