摘要
河流边滩和近海水域浅滩的水深相对于其平面尺度很小,属于浅水流动的范畴。河流边滩和近海浅滩为水陆交界带,受人类活动的影响很大。由于河流海岸的岸线复杂,在岸线突出点的下游将形成回流区,在此区域内多会发生泥沙淤积和污染物的聚集,其水流泥沙运动具有特定的规律。泥沙的淤积过程、淤积形态对该区域的水流特性、生态环境、航道管理等均具有重要的影响,如漂浮物的聚集和清除、泥沙吸附的污染物质的解吸扩散、航道清淤等。
我国漫长的海岸线和广阔的浅水带具有重要的经济开发价值,如港口建设、水产养殖、矿产资源开发、湿地保护、航运交通及旅游景区等。为此,需要研究浅水水流的运动特性、泥沙输移规律和污染物的传播机制及防治措施,为浅水区开发提供科学依据。
在对浅水流动回流区中水流运动及泥沙淤积规律的研究中,较多采用数学模型为工具。但由于水流泥沙和污染物在此区域内的运动规律非常复杂,采用实体模型进行研究将能获得更系统的资料。
本文采用实体模型试验的方法对明渠浅水水流进行系统深入的研究,专门设计了明渠水槽,配置了先进的供水供沙系统和测量仪器,重点探讨了明渠加糙的简便方法,为开展边墩尾流的试验研究提供了基础。
1试验条件
1.1模型设计
模型长18.9m,宽2.90m,固定坡降0.005,模型表面为光滑的混凝土,模型布置见图1.1。
模型进口安装4台轴流水泵,供水流量范围为2～100 l/s,采用50cm等宽矩形堰测进口流量。为研究浅水半岛的流动特性,从模型进口到纵向坐标4.5m处,分别在模型左测加0.25m、0.5m、0.75m和1.0m的直角矩形半岛。
1.2测量仪器
表面流场测量系统:采用流场实时测量系统VDMS测量水流的表面流速场。在模型上方5.0m处垂直安装4部摄像机,示意如图1.1,在尾流区上方3m处增设2部摄像机,同步测量表面流速场并进行实时数据处理。悬移质加沙机:采用专门设计的悬移质加沙机在模型进口加沙,根据设计的含沙量调节向模型施放的浑水流量,以基本保持模型中的含沙量稳定。浓度计:采用前向接收式红外线光电浓度计测量悬移质泥沙的浓度。试验沙:采用比重1.056的塑料沙,级配基本均匀。中值粒径D50=0.14 mm,几何标准偏差:
1.3参数计算
试验研究中采用的各种参数定义如下。
1. 4加糙方法在矩形明渠的两侧底部加5mm宽的木条,木条的上平面与渠底齐平。在木条上间隔10cm钉上铁钉。试验前预先采用1.0mm和1.5mm的尼龙绳对称挂在铁钉上,保持尼龙绳垂直于流向且贴紧床面,尼龙绳的间距分别为80、40、20、10cm。
2矩形明渠浅水流动的试验研究
首先进行了5种糙率、6级流量共30组矩形明渠浅水流动的基础性试验。经初步分析,加糙试验的粗糙雷诺数Re k = KνSu*与绳距Re d = DνS U的关系为:式中Ds为绳距。设:阻力系数C f~ Rη的关系为:
3 50cm边墩浅水流动的试验研究
在模型左上侧加50cm边墩,共进行了30组试验。试验时调节尾部竖板,使上下游的水深相等,水流参数均采用下游断面的计算结果。
3.1阻力系数
类似于(2-9)式,50cm边墩加糙试验的粗糙雷诺数与绳距关系的回归方程为:按(2-11)式定义的参数回归计算得出方程为:实测结果与(3-2)式符合良好,说明阻力系数的变化规律与宽浅明渠的试验结果相似。
3.2 S数
S数随流量加大而迅速减小;在小流量时,加糙间距的影响较大。当流量大于20 l/s后,S数的变化已不明显。
S数与水流条件和加糙间距有关,定义参数:
则根据实测资料回归得出:
上式与实测资料符合良好。从(3-3)式可知,
由(3.4)式反映出浅水半岛流动的稳定系数与水流强度和加糙绳距成反比,水流强度越弱,紊动强度越小;加糙绳距越密,对水流的阻力越大,其综合结果就是S数越大,水流越稳定。
3.3回流区长度
从流场分布图可以确定右边壁处的流速滞点,滞点以上即为回流区,进而可以测量回流区的长度Lc。典型的回流区测量结果见图3.16.
分析表明,回流区长度与床面阻力系数有关,根据实测资料回归得出:式中Ds为边墩的宽度,由(3-2)式有:
上式与实测资料的相关关系良好。
4 25cm、75cm和100cm边墩的试验结果
三种边墩的试验结果与50cm边墩的结果相似,下面只列出主要的回归计算成果。
4.1 25cm边墩的试验结果
4.2 75cm边墩的试验结果
4.3 100cm边墩的试验结果
5综合参数的变化规律
在已完成的4种边墩宽度(加明渠)共5种工况的试验中,分别研究了加糙绳距的当量粗糙度、阻力系数、S数和回流长度的变化规律,将前述各章得出的回归公式写成统一的表达式。
各参数的数值统计如表5-3。
式中:Bt为加边墩以后的有效宽度,B0为明渠宽度。实测资料表明,式(5.9)～(5.12)中的参数与ε有关,建立其相关关系,最后得出:阻力系数:回流区长度:
上述公式与实测阻力的相关性良好,完善地描述了浅水明渠流流动的规律性,为相关课题的深入研究提供了基础。
6泥沙淤积试验研究
通过试验,系统研究了边墩下游各种工况下的泥沙淤积特性,分析了泥沙淤积的发展过程和变化规律。在泥沙淤积大致平衡的条件下,建立了泥沙淤积厚度、淤积面积和淤积体积与水流阻力系数、S数和Re数的回归关系。
6.1试验条件
在边墩为50cm宽的模型上进行了5级流量、5种加糙条件的加沙试验。试验开始时,首先调节一定流量下的均匀流流态,达到恒定均匀流后采用PTV测量该组试验的初始流场,记录各种试验参数。在模型的进口加沙,采用浓度计实时监测水流的含沙量。每组试验的持续时间为2～5小时,当边墩下游的淤积体顶部达到水面,淤积体的形态基本不变化后停止试验。
6.2泥沙淤积的统一规律
各种工况的试验结果表明,当试验运行3h左右,回流区的泥沙淤积已基本平衡,各淤积参数保持稳定,故以各组试验运行3h的测量结果为基础,研究泥沙淤积的统一规律性。
分别定义无量纲的淤积厚度Hd、淤积面积Ad和淤积体积Vd为:
式中:H、A、V分别为淤积体的平均厚度、面积和体积,h为水深,D为边墩宽度,Lc为回流区长度。
6.2.1淤积厚度的变化规律
当试验运行3h时,泥沙淤积的沙脊已露出水面,按回流区中有泥沙淤积的面积上的淤积量求出平均淤积厚度。从试验结果可以看出,泥沙的淤积厚度约为水深的0.3倍,与阻力系数、S数和Re数无关,即:
6.2.2淤积面积的变化规律
泥沙淤积面积随水流阻力的增加而加大:
由于S数越大,水流越稳定,将有利于泥沙的淤积,所以淤积面积随S数的加大而增加:
随着Re数的加大,水流强度增加,紊动增强,水流的挟沙力加大,淤积的
泥沙将相对减少,即泥沙的淤积面积将随Re数的加大而减小:
6.2.3淤积体积的变化规律
泥沙淤积体积随水流阻力的增加而加大:
淤积体积随S数的加大而增加:
随着Re数的加大,泥沙淤积的体积将因淤积面积的减小而减少:
7结论与展望
7.1结论
本文以试验为主,研究了浅水区边墩下游的水流运动特性和泥沙淤积规律,获得了下述主要成果:
7.1.1试验方法研究
A、专门为研究浅水流动的特性而设计、建造了大型的宽浅水槽,供水流量自动控制,大范围的VDMS表面流场测量系统快速准确。
B、已有的研究结果表明,床面阻力是控制水流运动特性的重要参数。本文提出了一种采用绳索加糙的试验方法,可使阻力系数的变化范围达到5倍以上,该方法操作简单、调整灵活。
C、设计了完善的泥沙淤积试验方案,包括浑水水流循环系统,悬移质自动加沙和监测系统及床面淤积跟踪测量系统。试验过程表明,水流控制系统调整灵活,流场测量快速准确,悬移质加沙过程稳定,床面淤积测量满足精度要求,本系统为进行大量的回流区流动特性的试验研究提供了基本保证。
7.1.2清水试验成果
A、明渠阻力与水流强度和加糙绳距有关,绳距越密,阻力越大,在小流量时绳距的影响更加明显。(3-2)式能很好地描述各种水流强度和加糙绳距的阻力变化规律。
B、S数与阻力系数成正比而与水深成反比,其变化规律可用(3-4)式表达。
C、回流区长度与边墩的宽度成正比,并与阻力系数有关,阻力越大,回流区长度越短。回流区长度的回归公式见式(3.6)。
D、根据试验成果,得出了浅水明渠流动的当量粗糙度、阻力系数、S数和回流区长度等参数变化的规律性,各参数的回归计算公式分别为(5.15)、(5.16)、(5.18)和(5.19)式,这些公式较完善地描述了浅水明渠流流动的规律性,为相关课题的深入研究提供了基础。
7.1.3泥沙淤积试验成果
A、在水流强度较弱时(流量Q<44.0 /s),泥沙主要在主流与回流区的交界带淤积,当水流强度中等( Q≈70 l /s)时,主流因接触边壁反射而在其下游又形成第二个回流区,试验初始阶段重要淤积在两个回流区,其后逐渐发展成较连续的淤积体。在最大水流强度的试验条件下( Q≈90 l /s),第二个漩涡更大,泥沙主要在两个回流区内淤积,形成两个相互分离的淤积体,下回流区内的淤积有随试验时间的推移而增加并逐渐下延。
B、在中等水流强度以下( Q < 70 l /s),泥沙大致在流速矢量小于0.1m/s、涡量强度大于1 (1/s)的范围内淤积。当Q≈90 l /s时,泥沙仍然是集中淤积在流速矢量小于0.1m/s的区域,但泥沙在涡量大于1.0 (1/s)的区域难以落淤。
C、泥沙基本淤积平衡时,泥沙淤积的统一规律性为:
1)、泥沙的淤积厚度约为水深的0.3倍,不随阻力系数、S数或Re数而变化。
2)、泥沙淤积面积随水流阻力和S数的增加而加大,随Re数的加大而减小。
3)、泥沙淤积体积的变化规律与淤积面积的变化相似,亦是随阻力系数和S数的增加而加大,随Re数的增加而减小,其相关关系见(6.9)～(6.11)式。
7.2展望
为完善试验成果,需要进一步研究如弧度形、半圆形、折线形等各种边墩形状的试验;进行多种泥沙比重和粒径的试验研究,深入探讨泥沙研究过程、淤积形态的规律性。
深入分析水流运动特性与泥沙运动、沉积过程的相关关系,浅水区植物对流动和泥沙淤积的影响亦是值得系统研究的课题。
The countries with long coastline, broad shallow rivers, shallow seas, and tidelands shallow water belt have the important comprehensive development value. The silt deposition process in these regions i.e., deposition shape and flow characteristic, the ecological environment, the land utilization, the route management and so on has the important influence. This laboratory study is primary pillars in studies for nearby the shallow water region and downriver water flow state of motion. The study of sediment deposition provides the scientific approaches and basic rules for the shallow water area’s development. The main features of the study are given one by one below:
1st, testing method research
Construction of special model width ripple flume well equipped, with advanced and automatic metering equipments. Use of thread lining to produce different kinds of bed resistance testing method enabled the coefficient of friction range variation 5 times the actual bed resistance. The experiments conducted by this system provide the basic guarantee for the problems in the massive shallow recalculating flow, mentioned above.
2nd, clear shallow water experiment
Relationships between main parameters like bed resistance, the fluent intensity and placing thread lining at different distances are derived by regression method. Placing thread linings closely produces the bed resistance higher. The S number is directly proportional to the coefficient of friction, Also the S number as well as coefficient of friction is inversely proportion to the water depth. The recirculation length is directly proportional to the abutment width, and inversely proportional to the coefficient of friction. If bed resistance is higher recirculation area falls shorter. Among the variables like equivalent roughness, friction coefficient, S number and recirculation flow length variation rules has been discovered and regressive formula are established. These formulas perfectly described the shallow water flow rules and provide the fundamental knowledge.
3rd, silt deposition experimental achievement.
A. When the fluent intensity is weaker, the silt mainly deposit in the mainstream of recirculation flow region. Medium flow has capacity to create two recirculation regions deposition and in case of higher flows silt mainly deposit in two recirculation flow regions and form two separate deposit bodies.
B. In the regions where flow velocity vector is less than 0.1m/s and vortices intensities less than 1(1/s) silt deposition take place.
C. The silt deposition depth is approximately 0.3 times the water depth. The sediment depositions not only depend on coefficient of bed resistance friction but also on variation in S number and Re number. The silt deposit area and deposit volume increase with increase in bed resistance and the S number. The silt deposition area and deposition volume increase while Re number decrease.
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