几个有关问题的试验研究

（一）引河淤泥的絮凝沉降特性

1.静水中的絮凝沉速

渤海湾北部河口挡潮闸引河淤泥颗粒细，中值粒径d50一般为0.005～0.009mm，其级配曲线见图9。取子牙新河、陡河等处挡潮闸引河纯淤积区淤泥（d50=0.005～0.009mm）去盐分散后，用沉降天平测出其按沉速重量百分数的加权平均沉速为0.0016～0.0035cm/s。这种细泥在含盐水体中的絮凝作用很明显，在含沙量相近的条件下，絮凝沉速随着含盐量的增高而增大，当含盐量在15‰以内时，絮凝沉速增加较快，其絮凝沉速可增加10倍以上，含盐量超过15‰絮凝沉速不再有继续明显增大的变化。图10为陡河淤泥沉速随含盐量的变化情况。同一泥样，含盐量增加后，相应代表粒径变粗，沉速加大的情况见图11。

2.动水中的泥沙絮凝沉速

絮凝沉速在静水中与颗粒大小、含沙浓度、颗粒及液体的物理化学性质有关；在动水中还与水流紊动强度有关。

M.W.Owen〔1〕在英国泰晤士河口进行潮汐悬移质的絮凝沉速现场观测表明：含沙量大沉速大，含沙量小沉速小；大潮紊动强度大沉速小，小潮紊动强度小沉速大。Owen的研究仍然是采用静水沉速测验方法，尚不能定量确定在实际水流中的絮凝沉速值。为研究动水对絮凝沉速的影响，根据Dobbins的不平衡输沙公式级数解，我们建立了不平衡输沙的近似公式

图9　淤泥颗粒级配曲线

图10　淤泥静水沉速和含盐量的关系

图11　含盐量对土样沉速影响

式中：；h为水深；X为运行距离；Sm（0）为起始断面平均含沙量；为X断面的平均含沙量；S为水流的饱和挟沙力。

图12　关系图

简化公式和级数解比较见图12。根据范家骅等人水槽淤积试验资料，试验采用天津北大港附近海滩淤泥，水流含沙量0.5～2.5kg/m3，水流含盐量22.5‰～24‰，水深0.22～0.35m，平均流速0.027～0.122m/s，水温17～23℃。考虑床沙不能起动Sc=0，考虑泥沙絮凝沉降特性用平均沉速ω表示，由式（12）推求每组试验值，再根据已知的水力条件解ω值，这个ω值即为动水中的细颗粒絮凝沉速。图13系以计算所得ω值与实测V*及Sm（0）值点绘而成。图13表明：在试验资料范围内，动水絮凝沉速随含沙量的增加而增加，随水流摩阻流速增加而减少。

图13　絮凝沉速ω和摩阻流速V*起始含沙量S（0）关系

（二）淤泥的起动特性

（1）引河淤泥容重随着沉积历时增大而增大，1975年在子牙新河海口闸下实测泥面下不同深度的淤泥湿容重如表2：

表2　不同深度的淤泥湿容重值

但引河中经常裸露出水面部位的淤泥经实测其湿容重可在1.60t/m3以上。随着淤泥容重的增大，其起动和扬动流速也相应加大。但在一定水深条件下取得的室内试验资料，因实际引河中水深大，水质不同，判别引河冲刷条件时，在很多情况下不能直接引用。

（2）临界起动摩阻流速的确定。在图7的实测资料中看到，从涨潮到平潮，流速及摩阻流速减小，含沙量减小，水流处于淤积。转入落潮，流速及摩阻流速从零开始增加，但含沙量继续减小，说明水流的摩阻流速小于淤积物的V*c，当水流摩阻流速增大到V*c附近，水流含沙量降到最低值，当摩阻流速超过V*c值时，水流含沙量增加。因此，我们可取落潮过程第一次出现的含沙量最小值所对应的水流摩阻流速，作为潮汐淤积物的临界起动摩阻流速V*c。分析子牙新河及青静黄引河的自然潮汐实测资料，引河新淤床面淤泥的V*c值约为0.6cm/s。

（三）拖淤机具的效率试验(www.xing528.com)

拖具作功等于拖力F乘以拖淤距离L。这个功作用于河床及水流，使得泥沙离开床面并悬浮到水中，由水流带出拖淤河段。床沙悬扬到水流中的悬浮功为

则拖具的效率为

式中：γs为床沙比重；γ为水的比重；U为流速；S（y）为由拖淤纯增加的测点含沙量；y为测点水深；B为宽度；t1为机船通过拖淤段的时间；t2为水流通过整个拖淤段所需的时间；F为平均拖力；L为拖淤长度。

目前一般使用的拖具是一个平卧于河床的方框，底部装尖齿，对于湿容重较小的淤泥河床，φ值约为0.02。经初步试验，对拖具的结构形式作些改进，特别是增加一些促使水流扰动掺混的部件，效率可以提高2～3倍。

目前采用的机船是适用于浅海捕鱼的120马力木壳机船，根据现场试验，这种拖船仅有1/6～1/10的动力可以转化为带动拖耙作的功。其余动力消耗在克服船体阻力及其他。虽然采用目前的机船及拖具，在解决闸下引河淤积已经取得了一定成效。如果能对目前采用的机船及拖具作些改进，拖淤的效率还可以进一步提高。

最后指出，现场观测试验表明了机船拖淤的措施可以维持挡潮闸下游短引河在潮汐涨落过程中达到冲淤平衡。在一定条件下，如在陡河、子牙新河海口枢纽还可以成功地作为清除引河淤积或恢复河道设计断面的工程措施。在挡潮闸工程管理运用中，采用这种措施能使引河保持一定河槽断面行水，既可确保闸门安全开启不致淤死，又可为上游来水时充分利用有限的水源增大泄流冲刷的效果，为有利于及时扩大引河过流断面创造条件。

参考文献

1　Owon M.W.The effect of turbulence on the settling velocities of silt flocs.14th congress of the International Association for Hydraulic Research Vol.4 pp.27-32

2　水利水电科学研究院.渤海湾淤泥质河口挡潮闸泥沙淤积问题，1979

Sedimentation problems caused by tidal barriers in muddy estuaries along Bohai Bay shore

Du Guohan Peng Runze Wu Deyi Xu Mingquan Zhu Zongfa Chang Deli

（Water Conservancy and Hydroelectric Power Research Institute）

Abstract：Since the founding of New China，a large number of estuary barriers have been completed.These water projects have played important role in the protection against storm-tide，flood and waterlogging and in irrigation.However，sedimentation in estuaries is widespread，and reduces the benefit of these water conservancy works.In the muddy estuaries on the shore of Bohai Bay，the tidal barriers have to be closed for a long time in a year，because the water in the rivers is deficient.Consequently，among the completed 20 tidal barriers，18 are silted extensivly，for example，the Douhe Tidal Barrier had been silted up twice during a period of 6 years.

In this paper，based on the data observed from the estuaries of Ziyahe floodway and others on the shore of Bohai Bay，the statistical analysis shows that during a natural tidal cycle over 70%of the sediment transported by flood tide deposits on the channel bed downstream of the tidal barrier；but after adoption of the measures of agitation dredging the silt concentration of ebb tide becomes greater than that of flood tide.This has rendered the survival of the silted tidal barrier on Douhe River，and the reduction of the cost of agitation dredging by 90%or more as compared with dredger dredging.

This paper analyses the behaviour of tidal flow in a short channel downstream of the tidal barrier and presents the approximate equations of tide level，tide velocity and discharge.By these equations the variation of tide velocity and shear velocity along the channel downstream of a tidal barrier is calculated for judging the deposition and erosion characteristics of sediment-laden tidal flow.

On the basis of analysing the observed data and the behaviour of sediment-laden tidal flow，this paper shows that with a tidal barrier in closed condition，the existence of a deposition reach in the downstream channel，the deformation of tidal wave due to the construction of a tidal barrier，and the nonequilibrium of sediment transportation during flood and ebb tides are the factors contributing to the deposition of the channel.However，the main effect is the nonequilibrium sediment transport character in a tidal cycle，since the primary period of flood tide is characterized by super-saturated deposition and that of ebb tide period by sub-saturated erosion.Agitation dredging not only can remedy the defect of motionless bed materials in the deposition reach，but also can supply with sufficient sediment from channel bed during ebb tide，thereby changing a sub-saturated ebb tide flow into a super-saturated sediment-laden flow（even with silt concentration greater than that of flood tide），and achieving the aim of removal of the silt deposited downstream of the tidal barrier.

Based on the results of experimental study，this paper describes the settling characteristics of silt flocs，and gives the definition of the falling velocity of silt flocs in moving water and its simplified computation method by using the theory of the nonequilibrium of sediment transportation.Moreover，by utilizing the field data this paper also develops a method for determining the critical shear velocity of the newly deposited silt.On the basis of field and laboratory data，the effect and efficiency of the current equipments of agitation dredging are discussed.