Experimental Study on Characteristics of Bouder Bars Evolution in Mountainous Sediment-laden River
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摘要: 山区多沙河流漂石河段受水沙条件影响,洲滩发育丰富,改变了漂石河段的水沙运动规律及洲滩演变特征。基于野外调查与室内物理模型试验,探讨了漂石对多沙河段水动力特征及洲滩发育过程的影响。通过都江堰市岷江支流白沙河与龙溪河的野外调查,分析了典型漂石河段床沙组成及漂石河床形态特征。由于山区河流暴雨洪水及挟带的泥沙及河道地形多变等因素,大量上游来沙在漂石河段落淤,河床内洲滩大量发育。通过系列室内物理模型试验,分析不同泥沙补给条件下漂石河床的洲滩形成过程及河床形态变化与水位响应过程。研究结果表明:暴雨山洪产生丰富的泥沙补给,导致白沙河与龙溪河的粗颗粒平均粒径增大,由于大量漂石的存在,形成漂石河段。漂石河段水力要素多变,河床形态剧烈调整。当上游泥沙补给时,受漂石周围水流变化影响,漂石河床局部以横向带状溯源淤积为主,下游段两侧淤积突出,极易形成漂石洲滩,从而增大局部河床比降,减缓河道下切。上游来沙不均匀过程显著改变了漂石河床局部水位及泥沙淤积规模。因此,通过野外调查与室内物理模型试验,揭示了水沙条件下的漂石河床形态变化特征,山区河流上游来沙与漂石共同影响洲滩发育过程,表明漂石对山区多沙河流的水沙运动产生了较大影响。Abstract: The water and sediment conditions influence the channel evolution around the river boulder regions in mountainous sediment-laden rivers, which significantly changes the characteristics of water and sediment movement and riverbed deformation. Through field investigation of Baisha River and Longxi River, the tributaries of Minjiang River in Dujiangyan City, the bed sand composition, and morphological characteristics of the typical boulder reaches were analyzed. Field investigations found that due to the torrential rains and floods of mountain rivers and the sediment carried by them, as well as the changing terrain of the river, a large amount of upstream sand was silted in the section of the boulder river, resulting in the development of many beaches in the riverbed. Based on the field investigation of typical riverbed and physical model experiments, the effects of the boulder and sediment supply on hydrodynamic characteristics and beach development in river reach were analyzed. According to the experimental results, it can be concluded that the heavy rain and flash floods produced abundant sediment supply including boulders, resulting in the large average particle size of the Baisha River and the Longxi River, and forming a large number of boulder reaches. The hydraulic factors around the boulder abruptly, lead to considerable changes in the water level and riverbed forms. Under the sediment supply, the upstream sediment is mainly lateral banded source deposition, while the downstream sediment deposition is prominent on both sides, which is easy to form a drift stone beach. The development of boulder beaches can increase the local riverbed gradient and slow down the river channel cutting. While the non-uniformity of the upstream sediment supply changes the local water level and sediment deposition scale around the boulder. Therefore, the morphological characteristics of the boulder riverbed under the condition of water and sediment are revealed. It shows that the sediment and boulders from the upper reaches of the mountainous rivers jointly affect the development process of the beach, and the boulders have a prominent impact on the movement of water and sediment in the mountainous sediment-laden river.
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暴雨洪水、滑坡、泥石流或冰川将大量漂石搬运输至山区河流上游段[1-2]。山区河流坡陡流急,水沙条件多变,山区漂石河段常形成孤立、阶梯、交错的漂石洲滩形态[3-4]。巨大漂石打破原有河床平整性,漂石极大影响着其局部近底水流结构,从而制约着推移质输沙快慢与尺度[5]。漂石分布影响漂石河床水流结构[6-7]。叶晨[8]及Cao[9]等指出阵列漂石能显著影响水流流速及其紊动强度。漂石增加床面阻力,降低输沙率[10],随着漂石等粗颗粒聚集,进一步导致输沙率下降[11-12]。此外,洪水涨落及丰富来沙也改变床面切应力分布、水沙交换速率及输沙率大小,影响着河道水沙运动和水位升降[13],并制约局部河段河床形态变化[14-15]、洲滩发育及河势迁移[16-17]。当河床切应力超过泥沙起动临界值时,造成漂石河床局部区域冲刷[18],随着起动条件的增加,漂石局部的冲刷范围及最大冲深显著加大[19]。上游推移质运动可改变漂石周围水流能量分配,促使漂石河床局部水流紊动增强,加之推移质不断在深潭中淤积,使深潭内部水流紊动大为减弱,消能率降低[20-22]。受山区河流卵石推移质运动加剧影响,卵砾石沙洲广泛发育[23]。由于漂石阻水作用,漂石下游较易发育形成洲滩,促进了河道中浅滩的发育,加速了河流地貌演变[24-25],大量泥沙落淤,造成河床形态急剧调整,引发河床变形致灾[26-27]。漂石与洲滩发育相互关联、相互影响,本文采用野外调查与模型试验相结合的方法,测量水沙变化条件下的水流运动特征,探讨漂石对多沙河流冲淤变形及洲滩发育的影响,深入研究漂石与洲滩发育之间的相互作用机制,对于河道整治及水沙灾害防治具有一定的理论意义与应用价值。
1. 白沙河和龙溪河漂石河段调查分析
白沙河、龙溪河均为岷江支流,位于四川省都江堰市境内,属山区河流,汛期暴雨影响易发生崩塌、滑坡和泥石流,大量松散泥沙汇入河道。白沙河与龙溪河床沙级配宽,卵石混合分布,大量漂石零散分布于河道,采用漂石取样框分析了白沙河和龙溪河的典型河段漂石特征,分别为96处和46处。选取观凤沟与白沙河交汇口上游0.10 km处为白沙河研究段起点,起点上游2.98 km河段取样47个,起点距下游1.13 km取样49个。龙溪河研究段选取南岳庙到八一沟与龙溪河的汇口之间河段,总长3.12 km,沿程共取样46个。采样点的位置以GPS技术测量。图1为白沙河与龙溪河取样方法。泥沙取样框为1.0 m×1.0 m木质方框(图1(a)),以网格计数法研究了卵石河床粗颗粒粒径[28],采用钢尺和皮尺测量研究段范围内超过1 m的漂石粒径及河床几何形态(图1(b))。利用无人机拍摄白沙河与龙溪河研究河段地形,结合Photoscan软件分析提取白沙河和龙溪河的地形数据。
表1为白沙河与龙溪河沙样的粗颗粒粒径分布参数。由表1可知,白沙河与龙溪河粗颗粒粒径较大,且均值接近,分别为17.7和17.4 cm。与白沙河相比,龙溪河粗颗粒的中值粒径与暴露度略大,龙溪河的年平均流量3.34 m3/s也大于白沙河的16.1 m3/s,表明龙溪河水力分选作用较强,粗颗粒暴露更充分。在1~100 cm的床面粗颗粒中,白沙河与龙溪河的漂石占比分别达13.7%与12.6%,通过对大漂石的粒径调查,发现大漂石的粒径极值可达7.3与5.8 m。
表 1 白沙河与龙溪河沙样的粗颗粒粒径参数Table 1 Summary of coarse particle parameters河流 均值/
cm中值/
cm暴露度/
cm中小型漂石
20 cm≤D≤100 cm极大值/cm 均值/cm 占比/% 白沙河 17.7 14.9 10.8 28.1 13.7 7.3 龙溪河 17.4 16.1 12.1 25.3 12.6 5.8 图2为龙溪河与白沙河漂石洲滩照片。龙溪河与白沙河研究河段内漂石河床形态各异。漂石与上游来沙可能影响漂石洲滩的分布形态。
利用无人机拍摄地形照片与Photoscan软件分析提取白沙河与龙溪河地形数据,得出研究河段的漂石洲滩比降变化情况,图3为漂石洲滩比降。
由图3可知,凸起部分是漂石,在图3(a)中比降J1最小,在图3(b)中,J1与J2明显小于J3,邻近漂石的河段比降明显变小,这是由于因大量泥沙在漂石之间落淤,在漂石前后的坡度明显变缓,表明漂石洲滩的形成能降低河道比降。
2. 漂石河段概化试验
2.1 室内模型试验设计
基于对龙溪河龙池镇河段的野外调查,该河段存在大量的漂石洲滩,为典型的漂石洲滩河段(图2(a)~(d))。在四川大学水力学及山区河流开发保护国家重点实验室修建比尺为1∶30的相似物理模型进行系列试验,如图4所示,模型总长度为30 m,该试验段长度21 m,河床高程总体呈沿程减小,平均比降为2.38%;横断面呈矩形,纵断面宽窄相间不规则变化。
试验工况见表2,分为无漂石、漂石等5种上游来沙试验,共24个工况,研究漂石对局部河床水流及河床形态影响,以及来沙过程对漂石河床形态的影响。间隔1 m布置测量断面,共计22个。流量包括40、60 L/s,工况R1~R6为无漂石河床加沙,R7~R24为漂石河床加沙。其次,在上游采用泥沙颗粒粒径1~2 cm(d50=1.5 cm)的卵石作为上游补给用沙在上游河段加沙。试验包括5种加沙方案如图5所示,来沙方式分别为等量间歇加沙(2 kg与4 kg两种等量方式)、单峰间歇加沙(包含正态单峰与单峰前置两种)和双峰间歇加沙以模拟天然来沙的间歇性与不均匀性,加沙间隔为30 s。每组试验加沙后,利用测量各断面两岸水位和床面高度,泥沙主要淤积于宽河段(9#~13#断面),测量该河段河道淤积形态。
表 2 试验工况设计Table 2 Summary of experimental cases工况 Q/(L·s–1) D/cm N/个 漂石位置 加沙类型 R1 40 — — — — R2 60 — — — — R3 40 22.8 1 12#断面中点 — R4 60 22.8 1 12#断面中点 — R5~R9 40 — — — C1~C5 R10~R14 60 — — — C1~C5 R15~R19 40 22.8 1 12#断面中点 C1~C5 R20~R24 60 22.8 1 12#断面中点 C1~C5 注:Q为流量,D为漂石粒径,N为漂石个数。 2.2 试验结果分析
2.2.1 清水条件水动力参数变化
图6为漂石河床清水条件下的水位变化。
如图6可知,在11#~22#断面之间,此河段发生了水跃现象。这是由于河道由展宽到缩窄,河道坡降S从6.70%减小到0.25%,导致水位陡升,该河段弗洛德数从Fr>1降到Fr<1,水流流态由急流变为缓流。当漂石置于12#断面中间位置时,漂石上游水位变化不明显,其下游断面(13#断面)出现水位下降。从流态来看,漂石邻近位置的Fr均增大明显,且Fr>1,此河段水跃现象消失,漂石显著影响局部河床水位及水流流态,漂石局部河段的水流结构发生改变。
图7为Q=40 L/s工况断面平均流速与泥沙起动流速对比,泥沙起动采用沙莫夫公式计算:
$$ {u_{{\rm{sc}}}} = 1.47\sqrt {gd} {\left( {\frac{h}{d}} \right)^{\frac{1}{6}}} $$ (1) 式中:usc为起动流速,m/s;g为重力加速度,m/s2;h为水深,m;d为泥沙粒径,m,取中值粒径d50=15 mm。
由图7可知,由于模型河道展宽,河床比降由陡变缓,水流流速减小,导致在11#和14#断面平均流速与泥沙起动流速几乎相等,泥沙先后开始在11#和14#断面落淤。当漂石放置于11#断面中点,过流断面束窄,漂石邻近河段,即在11#~13#断面水位急剧变化,断面平均流速大于泥沙起动流速,与试验现象一致,泥沙在漂石上游的邻近位置开始落淤。表明漂石的存在改变其局部水流条件,导致水流的挟沙能力变化,改变泥沙落淤的位置与形态,从而影响河床的输沙过程。
2.2.2 来沙条件漂石河床发育过程
图8为R6工况(Q=40 L/s, 无漂石河床4 kg等量来沙)与R16工况(Q=40 L/s,单漂石河床4 kg等量来沙)的泥沙淤积形态变化。泥沙从12#断面开始淤积,淤积形态从单一横条溯源淤积发展成多翅拱形的横向淤积条带,随着来沙增加,淤积条带联结成片状。相同水沙条件及相同时刻下,泥沙覆盖面积相近,但漂石减小了泥沙向下游输移速率,促使淤积带的曲率更大,且淤积带大致向上游移动15 cm,而漂石邻近位置是极少泥沙淤积。表明漂石改变局部河床泥沙淤积位置,形成明显不同的淤积地形,对局部河床泥沙输移过程产生较大影响。
2.2.3 漂石河床淤积断面特征
图9为非均匀来沙下漂石河床的典型断面高程,其中,B为河宽,Z为高程。由图9可知,无漂石河床,在不同类型的来沙过程中,其淤积形态不同,来沙过程为双峰及单峰型时,淤积带溯源淤积至上游11#断面位置,当来沙过程为等量及单峰前置型时,泥沙淤积则集中于12#断面。在12#断面处,单峰前置型来沙时,其泥沙淤积高度最大,在11#断面,双峰型来沙时,淤积高度最大。在漂石河床,不同的来沙方式下,漂石局部淤积形态差异明显。双峰型来沙,河床淤积形态左右不对称显著,左岸的淤积高度大于右岸。 并且相较于不均匀来沙,等量来沙多的沉积于漂石上游,导致4 kg等量来沙在11#断面的淤积高度最大;而12#断面中正态单峰型最大,4 kg等量来沙最小。
2.2.4 来沙条件漂石洲滩河段水位变化
图10为等量来沙4 kg条件下淤积河段河床高程及水位变化。由图10可知,当上游来沙时,在11#~13#断面河段水位明显增加,此河段为淤积段,而漂石淤积河段的水位波动显著。相同来沙量下,随着流量的增大,泥沙的淤积位置由漂石上游迁移至漂石下游,这是由于小流量水流挟沙动力不足,泥沙的铺床作用显著,降低河床比降,水位变幅减小,大流量则抬高下游河床,水位上升。
图11为不同来沙类型下漂石河床水位变化。由图11可知,在3个断面中,断面12#水位最大,12#和13#两断面水位无明显差别。但在当Q=40 L/s时,上游的11#断面水位受影响较大,这主要是由于漂石上游壅水,导致水流挟沙小,大部分泥沙沉积于漂石上游11#断面附近。表明漂石对局部水位的变化有显著影响,上游来沙易在漂石局部河段落淤,促使河床的剧烈调整和抬高,促使在来沙条件下,漂石河床局部的水位陡涨,偏离正常水位,造成巨大的安全隐患。
3. 结 论
基于野外漂石河流的调查与水槽试验,探讨不同来水来沙条件下漂石河段的泥沙淤积发展的规律、河床变形规律、水位发展规律及漂石洲滩形成机理,取得主要结果如下:
1)漂石的壅水作用及强尾流作用,促使水深、流速及水流流态急剧变化,漂石周围河床的剧烈调整,易造成局部河段水位陡增。
2)上游来沙时,漂石的存在明显影响着河床的泥沙输移过程,在漂石上游以横向带状溯源淤积为主,下游则两侧淤积突出,易形成漂石洲滩,漂石洲滩发育能增大局部河床比降,减缓河道下切。
3)上游来沙不均匀性影响着漂石河床的水位与床面形态变化过程,上游来沙不均匀性导致淤积过程淤积形态规模差别较大,并对漂石上游水位影响较大。
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表 1 白沙河与龙溪河沙样的粗颗粒粒径参数
Table 1 Summary of coarse particle parameters
河流 均值/
cm中值/
cm暴露度/
cm中小型漂石
20 cm≤D≤100 cm极大值/cm 均值/cm 占比/% 白沙河 17.7 14.9 10.8 28.1 13.7 7.3 龙溪河 17.4 16.1 12.1 25.3 12.6 5.8 表 2 试验工况设计
Table 2 Summary of experimental cases
工况 Q/(L·s–1) D/cm N/个 漂石位置 加沙类型 R1 40 — — — — R2 60 — — — — R3 40 22.8 1 12#断面中点 — R4 60 22.8 1 12#断面中点 — R5~R9 40 — — — C1~C5 R10~R14 60 — — — C1~C5 R15~R19 40 22.8 1 12#断面中点 C1~C5 R20~R24 60 22.8 1 12#断面中点 C1~C5 注:Q为流量,D为漂石粒径,N为漂石个数。 -
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