滩地植被对弯曲漫滩河道主槽二次流发展的影响研究
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摘要
滩地植被对河道水沙运动及河床演变具有重要的影响,尤其当洪水期水流漫滩时。为了弄清高密度植被对弯曲主河道水流特性的影响,作者采用模型试验,在弯曲复式河道滩地铺设模型草,模拟天然植被,通过改变上游来流量,研究了高密度植被对主槽二次流涡团高度及涡团中心位置的影响。结果表明,滩地植被对弯中断面附近的二次流涡团高度影响最大,二次流涡团高度在滩地无植被时等于主槽高度,而在滩地有植被后等于主槽高度加上滩地植被冠层高度,此时,滩地植被等效增大了主槽高度。进入弯段后,滩地植被对二次流涡团高度的影响逐渐减小,在弯顶断面可以忽略滩地植被的影响。根据试验结果,提出了不同断面二次流涡团中心位置的预测方法并用不同来源的数据进行检验,发现该方法可以准确预测弯曲主槽中不同断面的二次流涡团中心位置。最后,讨论了二次流相对强度与主槽过流率之间的关系,发现无论滩地是否有植被,最大主槽过流率总是出现在二次流相对强度较小的弯顶断面。在相同水深条件下,主槽过流率在有植被漫滩河道中比无植被河道大。随着主槽水深(相对水深)增大,有植被和无植被工况各断面的过流率差值减小。当主槽水深接近或小于滩地植被冠层高度和主槽高度之和时,弯曲主槽中的二次流涡团沿程变化特性将与非漫滩弯曲河道的二次流涡团沿程变化特性十分相似。
Floodplain vegetation has an important influence on the flow and sediment transport and river bend migration, especially when flood overflows the main channel. The influence of dense floodplain vegetation on the height of secondary current cell and the position of the center of the cell was investigated through physical experiments in which the artificial grass was deployed on the floodplain to simulate floodplain vegetation. The experimental results indicated that the floodplain vegetation affected the height of secondary current cell most significantly at cross-over sections. The height of the cell was equal to the bankfull level in the non-vegetated channel, but was equal to the sum of the bankfull level and the canopy height. The floodplain vegetation increased the bankfull level equivalently. In a meandering bend, the influence of vegetation on the height of secondary current cell reduced gradually, and then was negligible at the apex section. Based on experimental observations, a method for predicting the position of secondary current cell was proposed and verified using the data from a different source, indicating that this method was capable of accurately predicting the position. Finally, the relation between the relative intensity of secondary current cell and the ratio of main channel discharge and total discharge was discussed. Regardless of whether vegetation was fixed on the floodplain or not, the maximal discharge ratio occurred at apex section in all scenarios. As the main channel depth increased, the difference between the ratios in non-vegetated channel and vegetated channel was reduced. When the main channel depth was close to or smaller than the sum of canopy height and bankfull level, the characteristics of secondary current cell along a meandering reach in the channel with overbank flows would be similar to those in a channel with inbank flows.
Floodplain vegetation has an important influence on the flow and sediment transport and river bend migration, especially when flood overflows the main channel. The influence of dense floodplain vegetation on the height of secondary current cell and the position of the center of the cell was investigated through physical experiments in which the artificial grass was deployed on the floodplain to simulate floodplain vegetation. The experimental results indicated that the floodplain vegetation affected the height of secondary current cell most significantly at cross-over sections. The height of the cell was equal to the bankfull level in the non-vegetated channel, but was equal to the sum of the bankfull level and the canopy height. The floodplain vegetation increased the bankfull level equivalently. In a meandering bend, the influence of vegetation on the height of secondary current cell reduced gradually, and then was negligible at the apex section. Based on experimental observations, a method for predicting the position of secondary current cell was proposed and verified using the data from a different source, indicating that this method was capable of accurately predicting the position. Finally, the relation between the relative intensity of secondary current cell and the ratio of main channel discharge and total discharge was discussed. Regardless of whether vegetation was fixed on the floodplain or not, the maximal discharge ratio occurred at apex section in all scenarios. As the main channel depth increased, the difference between the ratios in non-vegetated channel and vegetated channel was reduced. When the main channel depth was close to or smaller than the sum of canopy height and bankfull level, the characteristics of secondary current cell along a meandering reach in the channel with overbank flows would be similar to those in a channel with inbank flows.
引文
[1]Yang K,Cao S,Knight D W.Flow patterns in compound channels with vegetated floodplains[J].Journal of Hydraulic Engineering,2007,133(2):148-159.
[2]Shan Y,Liu X,Yang K,et al.Analytical model for stage-discharge estimation in meandering compound channels with submerged flexible vegetation[J].Advances in Water Resources,2017,108:170-183.
[3]Ervine D A,Willetts B B,Sellin R H J,et al.Factors affecting conveyance in meandering compound flows[J].Journal of Hydraulic Engineering,1993,119(12):1383-1399.
[4]Shiono K,Muto Y.Complex flow mechanisms in compound meandering channels with overbank flow[J].Journal of Fluid Mechanics,1998,376:221-261.
[5]Ishigaki T,Shiono K,Rameshwaran P,et al.Impact of secondary flow on bed form and sediment transport in a meandering channel for overbank flow[J].Proceedings of Hydraulic Engineering,2000,44:849-854.
[6]Shiono K,Spooner J,Chan T,et al.Flow characteristics in meandering channels with non-mobile[J].Journal of Hydraulic Research,2008,46(1):113-132.
[7]Shiono K,Chan T L,Spooner J,et al.The effect of floodplain roughness on flow structures,bedforms and sediment transport rates in meandering channels with overbank flows:Part I[J].Journal of Hydraulic Research,2009,47(1):5-19.
[8]Liu C,Wright N,Liu X,et al.An analytical model for lateral depth-averaged velocity distributions along a meander in curved compound channels[J].Advances in Water Resources,2014,74:26-43.
[9]Liu C,Shan Y,Liu X,et al.The effect of floodplain grass on the flow characteristics of meandering compound channels[J].Journal of Hydrology,2016,542:1-17.
[10]Liu C,Luo X,Liu X,et al.Modeling depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation[J].Advances in Water Resources,2013,60(8):148-159.
[11]Liu Chao,Yang Kejun,Liu Xingnian,et al.Analytical models for overbank flows in meandering channels with vegetated floodplains[J].Journal of Sichuan University(Engineering Science Edition),2012,44(6):7-12.[刘超,杨克君,刘兴年,等.植被作用下的弯曲复式河槽漫滩水流2维解析解[J].四川大学学报(工程科学版),2012,44(6):7-12.]
[12]Huai W,Gao M,Zeng Y,et al.Two-dimensional analytical solution for compound channel flows with vegetated floodplains[J].Applied Mathematics and Mechanics,2009,30(9):1121-1130.
[13]Ismail Z.A study of overbank flows in non-vegetated and vegetated floodplains in compound meandering channels[D].Leicestershire:University of Loughborough,2007.
[14]Jing H,Guo Y,Li C,et al.Three-dimensional numerical simulation of compound meandering open channel flow by the Reynolds stress model[J].International Journal for Numerical Methods in Fluids,2010,59(8):927-943.
[15]Rameshwaran P,Shiono K.Quasi two-dimensional model for straight overbank flows through emergent[J].Journal of Hydraulic Research,2007,45(3):302-315.
[16]Sun X,Shiono K.Flow resistance of one-line emergent vegetation along the floodplain edge of a compound open channel[J].Advances in Water Resources,2009,32(3):430-438.
[17]Neumeier U.Velocity and turbulence variations at the edge of saltmarshes[J].Continental Shelf Research,2007,27(8):1046-1059.
[18]Kadlec R H.Overland flow in wetlands:Vegetation resistance[J].Journal of Hydraulic Engineering,1990,116(5):691-706.
[19]Nepf H M.Flow and transport in regions with aquatic vegetation[J].Annual Review of Fluid Mechanics,2012,44(1):123-142.
[20]Yang Kejun,Liu Xingnian,Cao Shuyou,et al.Turbulence characteristics of overbank flow in compound river channel with vegetated floodplain[J].Journal of Hydraulic Engineering,2005,36(10):1263-1268.[杨克君,刘兴年,曹叔尤,等.植被作用下的复式河槽漫滩水流紊动特性[J].水利学报,2005,36(10):1263-1268.]
[21]Knight D W,Omran M,Tang X.Modeling depth-averaged velocity and boundary shear in trapezoidal channels with secondary flows[J].Journal of Hydraulic Engineering,2007,133(1):39-47.
[22]Liu C,Shan Y,Liu X,et al.Method for assessing discharge in meandering compound channels[J].Water Management,2016,169(1):17-29.
[2]Shan Y,Liu X,Yang K,et al.Analytical model for stage-discharge estimation in meandering compound channels with submerged flexible vegetation[J].Advances in Water Resources,2017,108:170-183.
[3]Ervine D A,Willetts B B,Sellin R H J,et al.Factors affecting conveyance in meandering compound flows[J].Journal of Hydraulic Engineering,1993,119(12):1383-1399.
[4]Shiono K,Muto Y.Complex flow mechanisms in compound meandering channels with overbank flow[J].Journal of Fluid Mechanics,1998,376:221-261.
[5]Ishigaki T,Shiono K,Rameshwaran P,et al.Impact of secondary flow on bed form and sediment transport in a meandering channel for overbank flow[J].Proceedings of Hydraulic Engineering,2000,44:849-854.
[6]Shiono K,Spooner J,Chan T,et al.Flow characteristics in meandering channels with non-mobile[J].Journal of Hydraulic Research,2008,46(1):113-132.
[7]Shiono K,Chan T L,Spooner J,et al.The effect of floodplain roughness on flow structures,bedforms and sediment transport rates in meandering channels with overbank flows:Part I[J].Journal of Hydraulic Research,2009,47(1):5-19.
[8]Liu C,Wright N,Liu X,et al.An analytical model for lateral depth-averaged velocity distributions along a meander in curved compound channels[J].Advances in Water Resources,2014,74:26-43.
[9]Liu C,Shan Y,Liu X,et al.The effect of floodplain grass on the flow characteristics of meandering compound channels[J].Journal of Hydrology,2016,542:1-17.
[10]Liu C,Luo X,Liu X,et al.Modeling depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation[J].Advances in Water Resources,2013,60(8):148-159.
[11]Liu Chao,Yang Kejun,Liu Xingnian,et al.Analytical models for overbank flows in meandering channels with vegetated floodplains[J].Journal of Sichuan University(Engineering Science Edition),2012,44(6):7-12.[刘超,杨克君,刘兴年,等.植被作用下的弯曲复式河槽漫滩水流2维解析解[J].四川大学学报(工程科学版),2012,44(6):7-12.]
[12]Huai W,Gao M,Zeng Y,et al.Two-dimensional analytical solution for compound channel flows with vegetated floodplains[J].Applied Mathematics and Mechanics,2009,30(9):1121-1130.
[13]Ismail Z.A study of overbank flows in non-vegetated and vegetated floodplains in compound meandering channels[D].Leicestershire:University of Loughborough,2007.
[14]Jing H,Guo Y,Li C,et al.Three-dimensional numerical simulation of compound meandering open channel flow by the Reynolds stress model[J].International Journal for Numerical Methods in Fluids,2010,59(8):927-943.
[15]Rameshwaran P,Shiono K.Quasi two-dimensional model for straight overbank flows through emergent[J].Journal of Hydraulic Research,2007,45(3):302-315.
[16]Sun X,Shiono K.Flow resistance of one-line emergent vegetation along the floodplain edge of a compound open channel[J].Advances in Water Resources,2009,32(3):430-438.
[17]Neumeier U.Velocity and turbulence variations at the edge of saltmarshes[J].Continental Shelf Research,2007,27(8):1046-1059.
[18]Kadlec R H.Overland flow in wetlands:Vegetation resistance[J].Journal of Hydraulic Engineering,1990,116(5):691-706.
[19]Nepf H M.Flow and transport in regions with aquatic vegetation[J].Annual Review of Fluid Mechanics,2012,44(1):123-142.
[20]Yang Kejun,Liu Xingnian,Cao Shuyou,et al.Turbulence characteristics of overbank flow in compound river channel with vegetated floodplain[J].Journal of Hydraulic Engineering,2005,36(10):1263-1268.[杨克君,刘兴年,曹叔尤,等.植被作用下的复式河槽漫滩水流紊动特性[J].水利学报,2005,36(10):1263-1268.]
[21]Knight D W,Omran M,Tang X.Modeling depth-averaged velocity and boundary shear in trapezoidal channels with secondary flows[J].Journal of Hydraulic Engineering,2007,133(1):39-47.
[22]Liu C,Shan Y,Liu X,et al.Method for assessing discharge in meandering compound channels[J].Water Management,2016,169(1):17-29.