山区变比降河道卵砾石溯源淤积的数值模拟
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摘要
山区河床组成多为宽级配卵砾石颗粒、沿程河床比降陡缓相接,受泥沙补给及水流过程影响,不同河段冲淤变化复杂多变,溯源淤积引发山洪现象频繁发生。为探究泥沙补给条件的变化对于山区河流河床冲淤变形以及水位变化的影响,作者以变坡陡比降河道为研究对象,通过计算流体力学(computational fluid dynamics,CFD)与离散元(discrete element method,DEM)耦合的方法对不同来水来沙条件下典型山区河道中卵砾石的溯源淤积过程进行了数值模拟,并将计算结果与室内水槽试验结果进行了比对。计算结果表明:强输沙水流由陡比降河道进入缓比降河道后,泥沙一旦在下游发生淤积,便会迅速向上游传播,进而导致河床大范围淤积抬高。溯源淤积的起始位置,发展速度以及淤积床面的厚度与上游泥沙补给强度、水流速度、颗粒粒径等因素有关,来流流量越小、泥沙补给强度越大,溯源淤积的起始位置越靠近河道上游,淤积发展速度越快,淤积床面厚度越小。泥沙淤积会导致淤积段沿程水位显著升高,在淤积锋面处,水位变动最为剧烈,水位增加明显,随着溯源淤积的继续发展,该位置处的水位会稍加回落并且趋于稳定。由此可见,山区河流泥沙补给条件的改变对于河床变形以及水沙灾害具有重大的影响,为揭示山洪泥沙灾害的致灾机理提供了理论基础。
Most of the riverbeds in the mountainous area are composed of pebble sediment with a wide size distribution, and the riverbed slope changes obviously. Affected by sediment supply and water flow, the erosion and deposition processes in different river reach are complex and changeable, and the retrogressive deposition and flash flood occurs frequently. In order to analyze the influence of sediment supply on the riverbed deformation and water level change in mountainous rivers, the process of retrogressive deposition of gravels in a typical mountainous river is simulated based on the CFD-DEM coupling method, and the calculated results are compared with the flume test data. The calculation results show that once sediment is deposited in the downstream due to the slope of river bed becoming gentle, it will rapidly spread to the upstream,which will lead to a wide range of river bed rise. Besides, the initial position of deposition, the rate of deposition and the thickness of deposition are related to the concentration of sediment, the velocity of water flow and the particle size. The decrease of the flow rate and the increase of the intensity of sediment supply will result in a faster rate of the retrogressive deposition and the thinner thickness of river bed. Furthermore, the sediment deposition in the mountainous river may lead to a significant rise in the water level along the channel. In the front of the deposition, the water level increases most sharply. As the development of the retrogressive deposition, the water level at this position will fall slightly and tend to be stable. The results demonstrate that sediment supply conditions play important roles in the deformation of rivereds and the occurance of flow-sediment disasters in mountainous rivers. This provides a theoretical basis for revealing the mechanism of the disasters caused by flood and sediment.
Most of the riverbeds in the mountainous area are composed of pebble sediment with a wide size distribution, and the riverbed slope changes obviously. Affected by sediment supply and water flow, the erosion and deposition processes in different river reach are complex and changeable, and the retrogressive deposition and flash flood occurs frequently. In order to analyze the influence of sediment supply on the riverbed deformation and water level change in mountainous rivers, the process of retrogressive deposition of gravels in a typical mountainous river is simulated based on the CFD-DEM coupling method, and the calculated results are compared with the flume test data. The calculation results show that once sediment is deposited in the downstream due to the slope of river bed becoming gentle, it will rapidly spread to the upstream,which will lead to a wide range of river bed rise. Besides, the initial position of deposition, the rate of deposition and the thickness of deposition are related to the concentration of sediment, the velocity of water flow and the particle size. The decrease of the flow rate and the increase of the intensity of sediment supply will result in a faster rate of the retrogressive deposition and the thinner thickness of river bed. Furthermore, the sediment deposition in the mountainous river may lead to a significant rise in the water level along the channel. In the front of the deposition, the water level increases most sharply. As the development of the retrogressive deposition, the water level at this position will fall slightly and tend to be stable. The results demonstrate that sediment supply conditions play important roles in the deformation of rivereds and the occurance of flow-sediment disasters in mountainous rivers. This provides a theoretical basis for revealing the mechanism of the disasters caused by flood and sediment.
引文
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[3]Zheng Yuanyu,Wang Huifeng,Wang Haizhou,et al.Experimental study on channel response with different flow and sediment conditions in mountain river[J].Advanced Engineering Sciences,2017,49(Supp1):8-13.[郑媛予,王慧锋,王海周,等.不同来水来沙条件下山区河流的河床响应试验研究[J].工程科学与技术,2017,49(增刊1):8-13.]
[4]Hassan M A,Egozi R,Parker G.Experiments on the effect of hydrograph characteristics on vertical grain sorting in gravel bed rivers[J].Water Resources Research,2006,42(9):2286-2292.
[5]Wang Qiang,Nie Ruihua,Fan Niannian,et al.Study on effects of incoming sediment change on river bed-form and bed load transport[J].Journal of Sichuan University(Engineering Science Edition),2016,48(Supp1):20-24.[王强,聂锐华,范念念,等.来沙条件变化对河床形态及推移质运动的影响研究[J].四川大学学报(工程科学版),2016,48(增刊1):20-24.]
[6]Liu Fuqiang,Zhang Xiaolei,Mao Yu,et al.Study on scouring and deposition characteristics of typical flood processes in a reservoir based on numerical simulation of flow and sediment[J].Journal of North China University of Water Resources and Electric Power(Natural Science Edition),2016,37(5):46-50.[刘富强,张晓雷,毛羽,等.基于水沙数值模拟的某水库典型洪水过程冲淤特性研究[J].华北水利水电大学学报(自然科学版),2016,37(5):46-50.]
[7]Wang Xiaoming,Lv Suiju,Li Chunguang,et al.Three-dimensional numerical simulation of fluvial processes in Daliushu reach of Yellow River[J].Hydro-science and Engineering,2016(6):52-60.[王晓明,吕岁菊,李春光,等.黄河大柳树河段河床演变的三维数值模拟[J].水利水运工程学报,2016(6):52-60.]
[8]Xiong W,Tang P,Kong B,et al.Computational simulation of live-bed bridge scour considering suspended sediment loads[J].Journal of Computing in Civil Engineering,2017,31(5):04017040.
[9]Cundall P A,Stack O D L.Discrete numerical model for granular assemblies[J].Geptechnique,1979,29(1):47-65.
[10]Han Y,Cundall P A.LBM-DEM modelling of fluid-solid interaction in porous media[J].International Journal for Numerical&Analytical Methods in Geomechanics,2013,37(10):1391-1407.
[11]Wu K,Yang D,Wright N.A coupled SPH-DEM model for fluid-structure interaction problems with free-surface flow and structural failure[J].Computer&Structures,2016,177:141-161.
[12]Gupta P,Sun J,Ooi J Y.DEM-CFD simulation of a dense fluidized bed:Wall boundary and particle size effects[J].Powder Technology,2016,293:37-47.
[13]Chen X L,Zhong W Q,Zhou X G.CFD-DEM simulation of particle transport and deposition in pulmonary airway[J].Powder Technology,2012,228:309-318.
[14]Jing L,Kwok C Y,Leung Y F,et al.Extended CFD-DEMfor free-surface flow with multi-size granules[J].International Journal for Numerical&Analytical Methods in Geomechanics,2016,40(1):62-79.
[15]Sun Rui,Xiao Heng.SediFoam:A general-purpose,opensource CFD-DEM solver for particle-laden flow with emphasis on sediment transport[J].Computers&Geosciences,2016,89:207-219.
[16]Li Bin,Guo Zhixue,Chen Ridong,et al.Experimental study on sediment deposition and water level surge under high sediment concentration in variable steep slope river[J].Journal of Sediment Research,2015(3):63-68.[李彬,郭志学,陈日东,等.变坡陡比降河道强输沙下泥沙淤积与水位激增的试验研究[J].泥沙研究,2015(3):63-68.]
[17]Guo Qingchao.Sediment-carrying capacity in natural rivers[J].Journal of Sediment Research,2006(5):45-51.[郭庆超.天然河道水流挟沙能力研究[J].泥沙研究,2006(5):45-51.]