沙坑演变对桥下河床冲刷影响研究
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摘要
近年来,大量河沙被开采用于建筑工业等领域,在河床上造成了大量的沙坑。沙坑引起水流条件的改变进而导致的河床演变致使许多桥梁毁坏,预测评估沙坑演变对桥下冲刷的影响对于桥梁的安全使用有着重要的意义。本文根据床沙卷吸率的概念及理论,结合一维水流方程和Exner河床变形方程,建立了模拟沙坑演变的一维泥沙数学模型,并通过实测资料对其进行了验证,结果符合良好。利用模型研究了采沙河流中水流的变化及河床演变的规律,并修改模型相关条件模拟计算了桥渡压缩冲刷,然后结合已得到的沙坑演变和压缩冲刷的特点模拟研究了近距离沙坑演变对于桥下冲刷的不利影响。结果表明：
(1)沙坑导致了水流、泥沙以及河床相互之间的关系失去平衡。具体表现为由于水力梯度的增大,导致沙坑上游河段的溯源冲刷；沙坑起到蓄沙池的作用而发生淤积；沙坑下游河段由于含沙量的减少而发生冲刷。
(2)沙坑上游段溯源冲刷的强度随着离沙坑上缘口距离的增大而减小；沙坑随着前端的逐渐淤积会向下游迁移；当沙坑迁移到沙坑下缘口时,沙坑下游河床变形趋势由冲刷转为淤积。
(3)沙坑河床演变的强度会不断降低,随着沙坑的淤平,河床最终会重新达到平衡状态,但此时已造成了河床的下切及水位的下降。
(4)不同水流泥沙条件、不同沙坑几何边界条件下的沙坑演变之间有着明显的差异。大体表现为上下游河段的河床冲刷以及沙坑内的河床淤积的强度随着水流流量、沙坑尺度的增大而增大。
(5)多个沙坑同时存在时,河床变形比单个沙坑的情形下更加剧烈。位于沙坑之间的河床变形最为剧烈,会逐渐形成上下沙坑群之间的串连。
(6)桥渡压缩冲刷跟压缩影响范围边界、压缩比等条件相关。不同影响范围边界对桥下河床变形影响较大,另外桥下河床冲刷程度跟压缩比密切相关,压缩比越小,压缩的程度就越大,冲刷越强烈。
(7)桥梁下游的近距离沙坑对于桥下冲刷的影响主要体现为沙坑引起的上游河段溯源冲刷导致的河床下切,桥梁上游的近距离沙坑对于桥下冲刷的影响主要体现为沙坑向下游的迁移。两种工况下的影响程度都跟沙坑与桥之间的距离、沙坑尺度等相关。距离越近、尺度越大,不利影响的程度就越大。
In recent years, a large amount of sand in rivers is excavated for the industrial purpose, therefore a dredged hole or holes are formed on the river bed. The hole usually evolves due to the flow dynamic role, which may do great damage to the bridges near the dredged site. As a result, it is significant to predict and assess the effect of the dredged hole on the scour under bridge to bridge safety. In presented paper, A one-dimensional flow dynamic model, coupled with the Exner equation, is adopted to describe the variation of the flow characteristics and the dredged hole evolution in riverbed. The model is verified by comparing with measured data and it is shown that the results are in good agreement. Modify the relevant conditions so as that the model is capable of simulating compression scour caused by the bridge piers. Combine with the characteristics of the above two aspects to research the adverse effects on the scour under bridge of evolution of the dredge hole. The following conclusions can be obtained.
(1)The hole results in the temporary loss of the equilibrium among the flow, sediment and riverbed. And the erosion occurs in the upper reaches of the hole due to the increase of hydraulic gradient, deposition occurs in the hole and scour occurs due to the decrease in sediment concentration in the lower reaches of the hole. (2)Gradual decrease in the depth of upstream erosion is along with the increase in the distance from upstream of the hole until the depth of upstream scour becomes negligibly small at a certain location upstream from the hole. With the gradual silting of the hole, the hole is tend to migrate downstream. When the hole migrated to the mouth of the lower edge of the hole, the trend of downstream bed is converted into deposit.
(3)The dredge hole makes a very substantial influence on the hydraulics and morphodynamics, but the influence is bound to decay and the ultimate state is naturally equilibrium. However, the bed elevation and the water surface elevation inevitably lower when the hole reaches the state of equilibrium.
(4)The processes with different conditions of water discharge, size of the hole, sediment concentration and time effect are analyzed. It is shown that the variations of scour depth and deposition depth increase with the water discharge, size of the hole and the time.
(5)When there are many holes in the riverbed, the bed deformation tends to be more intense than that under the condition of single hole. Deformation in bed between holes is the most intense so that the holes connect each other to form a bigger hole.
(6)The compression scour of the bridge piers is related to the boundary setting of compression and compression ratio. Different boundaries produce different calculated results. As the compression ratio becomes smaller, which means the degree of compression is greater, the erosion is tend to be more intense.
(7) If the dredge hole is located at downstream of bridge, the influence of a dredged hole evolution in riverbed on the scour under bridge is mainly caused by river incision caused by upstream erosion; If the dredge hole is located at upstream of bridge, the influence of a dredged hole evolution in riverbed on the scour under bridge is mainly caused by the migration downstream of the hole. The influences of the two conditions are both related to the distance between the hole and the bridge and scale of the hole. The degree of adverse effects becomes greater as the distance becomes smaller or the scale becomes greater.
(1)沙坑导致了水流、泥沙以及河床相互之间的关系失去平衡。具体表现为由于水力梯度的增大,导致沙坑上游河段的溯源冲刷；沙坑起到蓄沙池的作用而发生淤积；沙坑下游河段由于含沙量的减少而发生冲刷。
(2)沙坑上游段溯源冲刷的强度随着离沙坑上缘口距离的增大而减小；沙坑随着前端的逐渐淤积会向下游迁移；当沙坑迁移到沙坑下缘口时,沙坑下游河床变形趋势由冲刷转为淤积。
(3)沙坑河床演变的强度会不断降低,随着沙坑的淤平,河床最终会重新达到平衡状态,但此时已造成了河床的下切及水位的下降。
(4)不同水流泥沙条件、不同沙坑几何边界条件下的沙坑演变之间有着明显的差异。大体表现为上下游河段的河床冲刷以及沙坑内的河床淤积的强度随着水流流量、沙坑尺度的增大而增大。
(5)多个沙坑同时存在时,河床变形比单个沙坑的情形下更加剧烈。位于沙坑之间的河床变形最为剧烈,会逐渐形成上下沙坑群之间的串连。
(6)桥渡压缩冲刷跟压缩影响范围边界、压缩比等条件相关。不同影响范围边界对桥下河床变形影响较大,另外桥下河床冲刷程度跟压缩比密切相关,压缩比越小,压缩的程度就越大,冲刷越强烈。
(7)桥梁下游的近距离沙坑对于桥下冲刷的影响主要体现为沙坑引起的上游河段溯源冲刷导致的河床下切,桥梁上游的近距离沙坑对于桥下冲刷的影响主要体现为沙坑向下游的迁移。两种工况下的影响程度都跟沙坑与桥之间的距离、沙坑尺度等相关。距离越近、尺度越大,不利影响的程度就越大。
In recent years, a large amount of sand in rivers is excavated for the industrial purpose, therefore a dredged hole or holes are formed on the river bed. The hole usually evolves due to the flow dynamic role, which may do great damage to the bridges near the dredged site. As a result, it is significant to predict and assess the effect of the dredged hole on the scour under bridge to bridge safety. In presented paper, A one-dimensional flow dynamic model, coupled with the Exner equation, is adopted to describe the variation of the flow characteristics and the dredged hole evolution in riverbed. The model is verified by comparing with measured data and it is shown that the results are in good agreement. Modify the relevant conditions so as that the model is capable of simulating compression scour caused by the bridge piers. Combine with the characteristics of the above two aspects to research the adverse effects on the scour under bridge of evolution of the dredge hole. The following conclusions can be obtained.
(1)The hole results in the temporary loss of the equilibrium among the flow, sediment and riverbed. And the erosion occurs in the upper reaches of the hole due to the increase of hydraulic gradient, deposition occurs in the hole and scour occurs due to the decrease in sediment concentration in the lower reaches of the hole. (2)Gradual decrease in the depth of upstream erosion is along with the increase in the distance from upstream of the hole until the depth of upstream scour becomes negligibly small at a certain location upstream from the hole. With the gradual silting of the hole, the hole is tend to migrate downstream. When the hole migrated to the mouth of the lower edge of the hole, the trend of downstream bed is converted into deposit.
(3)The dredge hole makes a very substantial influence on the hydraulics and morphodynamics, but the influence is bound to decay and the ultimate state is naturally equilibrium. However, the bed elevation and the water surface elevation inevitably lower when the hole reaches the state of equilibrium.
(4)The processes with different conditions of water discharge, size of the hole, sediment concentration and time effect are analyzed. It is shown that the variations of scour depth and deposition depth increase with the water discharge, size of the hole and the time.
(5)When there are many holes in the riverbed, the bed deformation tends to be more intense than that under the condition of single hole. Deformation in bed between holes is the most intense so that the holes connect each other to form a bigger hole.
(6)The compression scour of the bridge piers is related to the boundary setting of compression and compression ratio. Different boundaries produce different calculated results. As the compression ratio becomes smaller, which means the degree of compression is greater, the erosion is tend to be more intense.
(7) If the dredge hole is located at downstream of bridge, the influence of a dredged hole evolution in riverbed on the scour under bridge is mainly caused by river incision caused by upstream erosion; If the dredge hole is located at upstream of bridge, the influence of a dredged hole evolution in riverbed on the scour under bridge is mainly caused by the migration downstream of the hole. The influences of the two conditions are both related to the distance between the hole and the bridge and scale of the hole. The degree of adverse effects becomes greater as the distance becomes smaller or the scale becomes greater.
引文
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[2]毛野,张志军.初析长江河道采沙的利弊得失[J].河海大学学报,2001,29(6)：68-72.
[3]赖永辉,谈广鸣,曹志先.河道采沙对河流河道演变及人类生产活动影响研究述评[J].泥沙研究,2008,(6)：74-80.
[4]赖永辉.河道采沙影响评价指标体系的构建[J].水力水电科技进展,2009,29(3)：29-32.
[5]李健,杨文俊.对国内外采沙研究的思考与总结[J].人民长江,2007,38(11)：196-198
[6]袁林,季成康.长江下游沙料开采对河道演变的影响[J].人民长江,1997,28(7)：33-35.
[7]张玲丽,谭政江,李昆.赣江中-下游河段采沙对河床的影响[J].水电与新能源,2010,91(5)：63-65.
[8]周作付,罗宪林,罗章仁,杨清书.近年珠江三角洲网河区局部河段洪水位异常壅高主因分析[J].热带地理,2001,21(4)：319-322.
[9]J.V. Bonta. Runoff-energy factors for MUSLE sediment-yield model for surface mines[J].International Journal of Sediment Research,2000,15(2):162-181.
[10]任杰,曾学智,贾良文.东江下游河段溯源侵蚀特征与机理[J].水科学进展,2010,21(1)：84-88.
[11]季荣耀,陆永军,左利钦.东江下游博罗河段人类活动影响下的河床演变[J].泥沙研究,2010,28(5)：48-54.
[12]毛野.初论采沙对河床的影响及控制[J].河海大学学报,2000,28(4)：92-96.
[13]王刚,茜平一,邹勇.靠近堤脚采沙对堤防稳定影响分析[J].中国农村水力水电,2004,(7)：68-72.
[14]王刚,茜平一,邹勇.河道采沙对堤防安全影响分析[J].人民长江,2004,35(2)：27-28.
[15]Monammad Akram Gill. Hydrodynamics of mining pits in erodible bed under steady flow [J].Journal of Hydraulic Engineering,1994,120(11):1337-1348.
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