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不同规格鱼鳞坑坡面侵蚀过程及特征研究

2020-06-04谢欣利吴发启

农业工程学报 2020年8期
关键词:产流坡面径流

侯 雷,谢欣利,姚 冲,吴发启

·农业水土工程·

不同规格鱼鳞坑坡面侵蚀过程及特征研究

侯 雷1,谢欣利1,姚 冲1,吴发启2※

(1. 西北农林科技大学水土保持研究所,杨凌 712100;2. 西北农林科技大学资源环境学院,杨凌 712100)

探究不同雨强下鱼鳞坑措施的坡面侵蚀规律及特征,对进一步明晰该措施的径流泥沙调控机制具有重要意义。该研究采用小区模拟降雨试验,研究3种雨强(40,70和100 mm/h)条件下2种规格鱼鳞坑坡面的蓄水保土效益及侵蚀过程,并探讨鱼鳞坑坡面水沙特征及其与细沟形态之间的关系。结果表明,鱼鳞坑的径流泥沙调控作用明显但也存在阈值,在措施被冲垮前,各处理减流效益和减沙效益分别为66.02%~82.20%和85.77%~91.91%;平均径流强度和输沙速率分别为平整坡面的18.87%~33.21%和10.04%~15.38%;当鱼鳞坑被冲垮后,坡面产流量和产沙量分别为平整坡面的1.00~1.22倍和1.39~3.53倍,其平均径流强度和输沙速率分别为鱼鳞坑冲垮前的4.60~7.46倍和13.70~16.75倍。鱼鳞坑坡面径流含沙率在措施冲垮前后的差异较大,水沙关系线性拟合的斜率在措施被冲垮前为0.020和0.035,而冲垮后斜率为0.205和0.231,同时坡面总产沙量和峰值产流产沙量与细沟形态显著相关。因此,鱼鳞坑措施对坡面产流产沙特征及形态的影响在其调控阈值前后有较大差异。研究可为鱼鳞坑坡面水土流失的研究可为区域水土保持措施配置及管理提供相应的依据。

侵蚀;径流;泥沙;鱼鳞坑;水沙关系;调控作用;细沟

0 引 言

鱼鳞坑是黄土高原地区坡面水土流失治理的典型工程措施,是破碎地形条件下重要的造林整地方式[1-4]。鱼鳞坑措施通过改变地表微地形使坡面形成洼地,降雨初期洼地以蓄水为主,同时增加降雨入渗率[5-9],而在产流过程中洼地起到了拦截径流和沉积泥沙的作用[10-13]。坡面鱼鳞坑大小和空间分布差异,会影响坡面洼地蓄满时间及汇流路径,当鱼鳞坑被蓄满后,径流冲刷作用加大,会对鱼鳞坑的形态造成破坏[14]。唐小娟[15]研究表明,当鱼鳞坑能正常发挥拦蓄作用时的蓄水保土效益显著,然而一旦被冲垮失去拦沙功能,反而对坡面造成更为严重的破坏。吴淑芳等[16]的研究表明,在坡度为20°且上方汇水流量为3.0 m3/h时,鱼鳞坑内壁会被径流冲击和剥蚀,最终导致鱼鳞坑毁坏失去拦蓄作用。Guo等[17]的试验同样表明在90 mm/h的间歇性降雨条件下,鱼鳞坑措施会出现拦蓄作用失效的现象。由此表明,鱼鳞坑径流泥沙调控能力存在阈值,该阈值的大小与坡度、雨强等因素密切相关。黄土高原地区地形破碎、沟壑纵横,且降雨主要集中在夏季,多为历时短、强度大的暴雨[18-19],因此有必要对不同降雨强度条件下的鱼鳞坑坡面径流泥沙调控作用进行分析和研究。

目前多数研究只是对次降雨条件下鱼鳞坑措施的侵蚀过程及特征进行探究,多集中于坡面侵蚀对坡度、雨强的响应研究,而对于不同规格及空间分布的鱼鳞坑坡面侵蚀过程及特征的研究则鲜有报道,且缺乏以径流调控阈值为界进行的分析和研究。因此,本文以鱼鳞坑冲垮时间为阈值,定量分析该措施相对平整坡面的蓄水保土效益,探究2种规格鱼鳞坑坡面的产流产沙过程及水沙关系,并进一步分析鱼鳞坑坡面水土流失特征与细沟形态的关系,以期为该区水土流失治理及措施配置提供依据。

1 材料与方法

1.1 供试材料

本试验在西北农林科技大学资源环境学院水土保持与荒漠化防治工程实验室进行,试验土壤为塿土,取自杨凌农田表层耕作土(0~20 cm),土壤填至径流小区后静置沉降,使其接近自然状态。试验小区规格为长5.0 m,宽1.5 m。降雨设备为中科院水土保持研究所研制的侧喷式降雨机降雨高度为7.5 m,降雨均匀度大于80%。

1.2 试验设计

根据杨凌地区降雨强度特点[20],设计降雨强度范围为20~120 mm/h,因此试验雨强分别设定为40、70和100 mm/h。试验对黄土高原地区的鱼鳞坑措施进行模拟,坡度设置为25°,并在坡面挖长径×短径×深分别为0.6 m×0.4 m×0.1 m(大鱼鳞坑)和0.4 m×0.3 m×0.1 m(小鱼鳞坑)的2种不同规格鱼鳞坑,鉴于试验主要为探究不同规格及空间分布的鱼鳞坑对坡面蓄水保土效益的影响,因此在布设鱼鳞坑时控制坡面的初始填洼量(开挖方量)相同,即设置大鱼鳞坑4个,小鱼鳞坑8个,其排列方式如图1。同时以平整坡面(无措施坡面)为对照,因此共设计9个试验处理(3个降雨强度、3种坡面条件)。

图1 坡面鱼鳞坑布设方式

1.3 试验过程

在降雨开始后记录产流时间,从坡面产流开始,0~3 min每1 min接取径流泥沙样,之后每间隔2 min接取径流泥沙样并称质量,降雨持续时间为1 h,同时在降雨过程中观察并记录鱼鳞坑最先被冲垮的时间。降雨结束后,利用精度为1 mm的直尺测量和记录坡面细沟的沟宽和沟深,同时对所收集的径流泥沙样静置处理并去除上层清液,采用105 ℃烘箱对径流泥沙样进行烘干处理,并对烘干后的泥沙进行称质量。

1.4 数据处理

输沙速率(T)为单位时间单位面积侵蚀的土壤输送到坡面以外的质量,计算公式如下:

T=/(·) (1)

式中T为输沙速率,kg/(m2·h);为(h)时段内的坡面侵蚀量,kg;为坡面面积,m2。

径流强度(I)为单位时间单位面积表土的水流深,计算公式如下:

I=1 000/(2)

式中为径流强度,mm/h;为径流率,m³/h。

2 结果与分析

2.1 鱼鳞坑措施的减水减沙效应

坡面鱼鳞坑的存在增加了地表填洼量,增加径流在坡面的停滞时间,提高入渗的可能性,进而影响坡面产汇流过程。图2表示了不同降雨强度下鱼鳞坑坡面和平整坡面初始产流时间的对比关系,可知鱼鳞坑措施的存在延迟了径流产生的时间,图中1∶1线的距离反映了初始产流的延迟效果,因此鱼鳞坑在该空间分布条件下,小鱼鳞坑相较于大鱼鳞坑具有更强的径流延滞作用。由表1可知,小鱼鳞坑相对于大鱼鳞坑坡面,其初始产流时间分别增加了1.81、2.12和1.07 min。同时鱼鳞坑的径流延迟作用还与降雨强度相关,降雨强度越小,径流延迟作用越显著。大鱼鳞坑和小鱼鳞坑坡面与平整坡面相比,其初始产流时间分别增加27.79%~59.12%和72.42%~116.85%。在试验的时间为60 min的条件下,40 mm/h的降雨强度未对鱼鳞坑的形态造成破坏,当降雨强度为70和100 mm/h时,大鱼鳞坑和小鱼鳞坑形态均存在不同程度的毁坏,且小鱼鳞坑被冲垮的时间更短,因此在坡度和雨强一定时,小鱼鳞坑形态的稳定性弱于大鱼鳞坑。因此本研究以鱼鳞坑措施被冲垮的时间点为界,将坡面水土流失过程划分为2个子过程,分别探究鱼鳞坑冲垮前后坡面的蓄水保土效益。

表1 鱼鳞坑坡面与平整坡面的初始产流时间和冲垮时间

不同雨强条件下的鱼鳞坑对坡面的减流减沙效益存在差异,本研究以相同时间段内的平整坡面产流产沙量为对照,并以鱼鳞坑措施最先出现冲垮现象的时间点为界,分析和对比鱼鳞坑坡面的减流减沙效益(表2)。当降雨强度为40 mm/h时,与平整坡面相比,大鱼鳞坑和小鱼鳞坑坡面的总径流量分别减少69.78%和76.04%,总侵蚀量分别减少85.77%和88.65%。当降雨强度为70 mm/h时,2种规格的鱼鳞坑均出现不同程度的毁坏,在鱼鳞坑措施未被冲垮前,大鱼鳞坑坡面的总径流量和总侵蚀量相对平整坡面分别减少67.88%和86.61%,小鱼鳞坑坡面的总径流量和侵蚀量相对平整坡面分别减少82.20%和91.91%;而冲垮后大鱼鳞坑的总径流量和总侵蚀量分别为平整坡面的1.21倍和3.53倍,小鱼鳞坑的总径流量和总侵蚀量分别为平整坡面的1.00倍和1.89倍。当降雨强度增加为100 mm/h时,大鱼鳞坑和小鱼鳞坑坡面同样出现被冲垮的现象,鱼鳞坑在被冲垮前,大鱼鳞坑和小鱼鳞坑坡面的总径流量相对平整坡面分别减少66.02%和76.06%,总侵蚀量分别减少86.49%和89.08%;当鱼鳞坑被冲垮后,大鱼鳞坑和小鱼鳞坑坡面的总径流量分别为平整坡面的1.22倍和1.09倍,总侵蚀量分别为平整坡面的1.91倍和1.39倍。

表2 不同降雨条件下鱼鳞坑坡面和平整坡面的径流量和泥沙量

2.2 鱼鳞坑措施对坡面水土流失过程的影响

2.2.1 径流强度

鱼鳞坑措施坡面和平整坡坡面的径流强度随着坡度和降雨强度增加而逐渐增大,平整坡面的径流强度呈现先增大后稳定的趋势,而鱼鳞坑坡面的径流强度因措施被冲垮前后而存在较大差异(图3)。

图3 不同降雨强度下鱼鳞坑坡面和平整坡面的径流强度变化过程

当降雨强度为40 mm/h时,大鱼鳞坑和小鱼鳞坑坡面的平均径流强度分别为7.92和6.77 mm/h,相对应的平整坡面的径流强度为23.85 mm/h。当降雨强度为70 mm/h时,大鱼鳞坑坡面的径流强度在被措施冲垮后出现陡增现象,其径流强度峰值达到97.62 mm/h,随后上下波动并逐渐降低,该雨强下平整坡面的平均径流强度为51.50 mm/h,小鱼鳞坑在被冲垮后,其径流强度峰值达到87.82 mm/h。当降雨强度增加到100 mm/h时,大鱼鳞和小鱼鳞坑在被冲垮前平均径流强度分别为20.87和15.70 mm/h,而坡面措施被冲垮后其平均径流强度则分别为100.37和94.34 mm/h;相对应的平整坡面的平均径流强度为78.61 mm/h。在鱼鳞坑措施完整的条件下,有措施坡面的平均径流强度为平整坡面的18.87%~33.21%;当鱼鳞坑措施被冲垮,径流强度均值分别为鱼鳞坑冲垮前的4.60~7.46倍。

2.2.2 输沙速率

鱼鳞坑措施坡面和平整坡面的侵蚀输沙速率差异明显(图4),在不同坡度及降雨强条件下,平整坡面的输沙速率呈现先增大后减小,并逐渐达到稳定的趋势。当降雨强度为40 mm/h时,大鱼鳞坑和小鱼鳞坑坡面的输沙速率分别在0.13和0.10 kg/(m2·h)上下范围内波动且变化稳定,相对应的平整坡面的平均输沙速率为0.78 kg/(m2·h)。当降雨强度为70 mm/h时,大鱼鳞坑和小鱼鳞坑在未被冲垮前的坡面平均输沙速率分别为0.42和0.33 kg/(m2·h),而在措施被冲垮后,大鱼鳞坑和小鱼鳞坑坡面的输沙速率陡增且其峰值分别达到18.75和12.73 kg/(m2·h),而平整坡面在整个降雨过程中的平均输沙速率为3.04 kg/(m2·h)。当降雨强度增加到100 mm/h时,大鱼鳞坑和小鱼鳞坑坡面未被冲垮前的输沙速率的范围分别介于0.24~0.79和0.14~0.60 kg/(m2·h)之间,而当坡面措施被冲垮后,输沙速率的峰值则分别达到22.34和16.77 kg/(m2·h),峰值之后存在上下波动和减小的趋势,随后逐渐趋于稳定,平整坡面的平均输沙速率为5.20 kg/(m2·h)。鱼鳞坑措施在完整条件下坡面的平均输沙速率为平整坡面的10.04%~15.38%;鱼鳞坑措施被冲垮条件下输沙速率均值分别为鱼鳞坑冲垮前的13.70~16.75倍。

图4 不同降雨强度下鱼鳞坑坡面和平整坡面的输沙速率变化过程

2.2.3 水沙关系

坡面径流和泥沙之间存在一定的关系[21-23],因此对不同雨强条件下的鱼鳞坑坡面径流强度和和输沙速率之间的关系进行分析。以100 mm/h降雨强度下的坡面为例,平整坡面拟合斜率为0.038(²=0.454),由图5可知,在鱼鳞坑措施被冲垮前,拟合方程的斜率小于平整坡面的斜率,而鱼鳞坑措施一旦被冲垮,其斜率显著增大。说明坡面鱼鳞坑措施具有较好地蓄水拦沙功能,能显著降低坡面径流含沙量;但措施若被冲垮,则会失去拦沙功能,甚至产生负的减沙效益。

注:*表示显著性水平在0.05,**表示显著性水平在 0.01。下同。降雨强度为100 mm·h-1。

2.3 鱼鳞坑坡面细沟特征及对侵蚀的影响

细沟形态与坡面径流泥沙之间具有相互反馈的作用,径流侵蚀力和土壤抗侵蚀力是影响细沟的最直接因素[24-27],鱼鳞坑可以有效抑制坡面细沟的产生,然而鱼鳞坑形态一旦遭到破坏,则会加剧细沟的形成并造成更为严重的水土流失。由于40 mm/h的降雨强度下坡面鱼鳞坑形态完整且并未产生明显的细沟,因此分别对70和100 mm/h雨强条件下鱼鳞坑坡面的细沟特征进行分析(图6),在相同降雨条件下,大鱼鳞坑坡面的细沟宽度的平均值和最大值都大于小鱼鳞坑坡面,主要是由于大鱼鳞坑蓄积径流和泥沙的能力较强,一旦被冲垮对鱼鳞坑坑壁的毁坏更加明显。

图6 不同雨强下鱼鳞坑坡面细沟形态特征

表3揭示了细沟形态特征与产流产沙量之间的相关关系,可以看出细沟的宽度与总产沙量和峰值产流产沙量呈现出显著的相关性(<0.05),其中细沟的宽度与峰值产沙量呈极显著相关(<0.01);细沟深度与总产沙量和峰值产流量呈极显著相关(<0.01),因此表明细沟的形态与坡面侵蚀的关系密切,也与措施是否被冲垮具有一定关系。一方面,鱼鳞坑被冲垮促进了细沟的形成和发育,并且增加细沟的宽度和深度;另一方面,细沟的出现为坡面径流和泥沙提供了输送通道,从而显著提高了坡面土壤流失量。

表3 细沟形态参数与产流产沙量的相关性分析

3 讨 论

鱼鳞坑措施防治坡面水土流失主要是由于该措施的存在增加了地表填洼量,能够延缓坡面径流、增加入渗,起到了提高坡面径流拦蓄和减弱泥沙剥离和搬运的作用[17]。同时坡面鱼鳞坑能够切断汇流路径并减弱径流速率,削减径流从上坡到下坡的能量,从而抑制坡面细沟的形成,有效调控坡面径流和泥沙。由试验结果可知,鱼鳞坑规格和空间分布都会对鱼鳞坑的蓄水保土效果产生不同的影响,其影响不仅表现在减流减沙效益,而且也会影响鱼鳞坑措施被蓄满及冲垮所需的时间。在初始填洼量(开挖方量)一致且鱼鳞坑形态都未被破坏时,小鱼鳞坑坡面的减流减沙效益强于大鱼鳞坑,分析认为主要与鱼鳞坑空间分布有关,相同面积的坡面条件下小鱼鳞坑会将坡面划分为更多的单元,缩短了上下2个坑间的径流长度,削弱了径流能量,从而增强了坡面的蓄渗能力。而当鱼鳞坑的形态一旦被破坏,坑内的水沙将快速泄出,增加产流产沙量的同时较大程度地促进细沟的产生和发育[28],并且随着降雨过程的持续进行,会加剧细沟沟头溯源侵蚀、沟壁扩张以及沟底的不断下切,所以在次降雨过程中鱼鳞坑措施若出现被冲垮的现象,对鱼鳞坑径流泥沙调控作用的研究则应建立在其功能是否得到有效发挥(措施是否会被冲垮)的基础上。

因此探究坡面径流泥沙调控临界点对农业耕作措施及工程措施配置下坡面侵蚀过程及蓄水保土效益的研究具有重要作用。试验中可知,当坡面鱼鳞坑被蓄满后就会发生漫流现象,措施会在较短时间内被冲垮,主要是由于坑内溢流的水对坑边缘侵蚀力大于坑边缘临界剪切力[29-30],因此明确鱼鳞坑出现溢流现象的时间节点,对鱼鳞坑规格的选择与措施配置具有一定的借鉴意义。假定单个鱼鳞坑正上方汇水面积内的径流在均匀降雨条件下(除入渗以外)全部流入坑内,则鱼鳞坑出现溢流的时间节点可由式(3)表示为

式中为蓄满时间,min;为鱼鳞坑体积,m3;1为鱼鳞坑底面积,2为鱼鳞坑正上方汇水面积,m2;为降雨强度,mm/min,为入渗速率,mm/min。

此外,由于鱼鳞坑对坡面入渗、产汇流特征及减沙机制的影响较为复杂,因此以鱼鳞坑调控阈值为界的泥沙调控机理有待进一步的研究。

4 结 论

1)鱼鳞坑坡面在较大雨强且持续时间较长的条件下其蓄水保土能力存在一定阈值,即措施被冲垮前后的产流产沙量存在明显的正负效益,该阈值大小与鱼鳞坑蓄积泥沙的能力密切相关。

2)鱼鳞坑措施能够有效延滞径流,增加坡面初始产流时间,且在鱼鳞坑功能正常发挥的条件下,其减流效益和减沙效益分别在66.02%~82.20%和85.77%~91.91%的范围之内,但在较大雨强且历时较长的条件下坡面鱼鳞坑会出现被冲垮的现象,与平整坡面相比,反而会增加坡面水土流失量,其产流和产沙量分别为平整坡面的1.00~1.22倍和1.39~3.53倍。

3)在鱼鳞坑措施完整的条件下,有措施坡面的平均径流强度和输沙速率均低于平整坡面,分别为平整坡面的18.87%~33.21%和10.04%~15.38%。但在较大雨强(70和100 mm/h)条件下时,径流强度和输沙速率均存在以鱼鳞坑被冲垮为时间节点的突变现象,其均值分别为鱼鳞坑冲垮前的4.60~7.46倍和13.70~16.75倍。根据对不同规格鱼鳞坑坡面水沙关系的线性拟合可知,措施冲垮前斜率分别为0.020和0.035,冲垮后斜率为0.205和0.231,即鱼鳞坑措施被冲垮前后径流含沙率差异明显。

4)细沟的宽度和深度与鱼鳞坑坡面的总产沙量和峰值产流产沙量均具有显著相关性。

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Erosion process and characteristics of different specifications of fish-scale pit slope

Hou Lei1, Xie Xinli1, Yao Chong1, Wu Faqi2※

(1.,,712100,;2.,,712100,)

This study explored the characteristics of fish-scale pit slope erosion under different rainfall intensities. Two specifications of fish-scale pits (0.6 m×0.4 m×0.1 m and 0.4 m×0.3 m×0.1 m) were adopted in the simulated rainfall experiment including three rainfall intensities (40, 70 and 100 mm/h). Runoff and sediment samples were collected to calculate the benefits of runoff and sediment reduction, and the widths and depths of rills were measured to describe the rill morphology. Runoff generation time and fish-scale pit damage time were also observed and recorded during the rainfall process. Moreover, runoff rate and sediment transport rate were calculated and the relationships between runoff and sediment yield and rill morphology were analyzed. The results showed that when the rainfall lasted for one hour, the morphology of fish-scale pits were intact under the 40 mm/h rainfall intensity, but the fish-scale pits were damaged under the 70 and 100 mm/h rainfall intensities. The fish-scale pit had an obvious effect on runoff and sediment yields and the threshold existed. Based on the fish-scale pit damage time, the fish-scale pit slope erosion was divided into two processes. The runoff generation time increased with decrease of the rainfall intensity. The two specifications of fish-scale pits delayed the runoff generation time, which was 27.79%-59.12% and 72.42%-116.85% higher than smooth slope. The small fish-scale pits were easier to be damaged than the big fish-scale pits under the same rainfall intensity. The benefits of runoff and sediment reduction were 66.02%-82.20% and 85.77%-91.91%, and the mean value of runoff rate and sediment transport rate were 18.87%-33.21% and 10.04%-15.38% of the smooth slope before the fish-scale pits damage. After fish-scale pits damage, slope measures would lose the regulatory function and would have the negative effects on soil and water reduction. The runoff rate and sediment transport rate were suddenly increased and the mean value of runoff rate and sediment transport rate were 4.60-7.46 and 13.70-16.75 times that before fish-scale pits damage. The mean runoff rate and sediment transport rate of the smooth slope were 51.50 mm/h and 3.04 kg/(m2·h) under the 70 mm/h rainfall intensity, and the mean runoff rate and sediment transport rate were 78.61 mm/h and 5.20 kg/(m2·h) under the 100 mm/h rainfall intensity. The flow-sediment relationship under 100 mm/h rainfall intensity showed that the sediment was greatly different before and after fish-scale pits damage. The sediment yield could be decreased by fish-scale pit measures application. The slopes of linear regression equations for fitting the relationship between runoff and sediment were 0.020-0.035 before fish-scale pit damage and they were increased to 0.205-0.231 after fish-scale pit damage. The slopes of linear regression equations for fitting the relationship between runoff and sediment on smooth slope was 0.038 under 100 mm/h rainfall intensity. The width of rill had significant correlation with peak sediment yield (<0.01) and the depth of rill had significant relationship with total sediment yield and peak runoff yield (<0.01). The overflow time of fish-scale pit was an important indicator of the regulation threshold of runoff and sediment, and slope erosion characteristic and rill morphology showed obvious difference before and after fish-scale pits damage. This study will provide valuable information for allocation and management of regional soil and water conservation measures.

erosion; runoff; sediments; fish-scale pit; flow-sediment relationship; regulation function; rills

侯雷,谢欣利,姚冲,等. 不同规格鱼鳞坑坡面侵蚀过程及特征研究[J]. 农业工程学报,2020,36(8):62-68.doi:10.11975/j.issn.1002-6819.2020.08.008 http://www.tcsae.org

Hou Lei, Xie Xinli, Yao Chong, et al. Erosion process and characteristics of different specifications of fish-scale pit slope[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2020, 36(8): 62-68. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2020.08.008 http://www.tcsae.org

2020-01-17

2020-04-01

国家自然科学基金“黄土坡耕地作物覆盖与管理的防蚀作用研究”(41977065)

侯雷,博士生,主要从事土壤侵蚀过程研究。Email:houleizd@163.com

吴发启,教授,博士生导师,主要从事土壤侵蚀与水土保持研究。Email:wufaqi@263.net

10.11975/j.issn.1002-6819.2020.08.008

S157.1

A

1002-6819(2020)-08-0062-07

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