山区河道粗颗粒床面垂线流速分布规律试验研究

Experimental Investigation on Flow Velocity Distribution in Mountain Rivers with Coarse Sediment Bed

粗颗粒床面普遍存在于山区河道中,在浅水条件下,水深与床面颗粒粒径的比值h/d较小,此时河道中水流垂线流速分布难以用传统的对数公式或对数补偿公式进行准确描述。在室内概化水槽中,利用玻璃珠及乒乓球模拟粗颗粒床沙,对不同h/d下的垂线流速分布规律进行了系统的试验研究。前人研究发现,不同淹没度(h/d)下的理论床面取值并不统一。考虑到工程应用方便,作者采用普通粗糙床面河道下的理论床面值,即y0为0.2d。将试验测得数据进行无量纲化处理后与对数补偿公式计算得到的理论值对比,并率定公式中的系数B与尾流系数Π到最佳。结果表明:当h/d较小时,系数B随着h/d的增大减小,尾流系数Π随着h/d的增大而增大;而当h/d较大时,B与Π都趋于一个常数。最后,建立了适用于山区粗颗粒卵砾石泥沙河道的垂线流速分布公式,并将其与众多前人试验数据进行拟合检验。检验结果表明,作者提出的垂线流速分布公式具有较高的计算精度,同时对于普通粗糙床面河道,该公式也具有很好的适应性。

In mountain rivers with coarse sediments,the ratio of water depth to sediment diameter(h/d)is relatively small especially under the condition of shallow water depth.Therefore,the vertical velocity distribution along the river cannot be described accurately with traditional logarithmic or logarithmic-wake law.In this paper,flume experiments were carried out with two kinds of sediments(glass beads d=1.4 cm and pinpong d=4 cm)to test the influence of coarse sediment on vertical velocity distribution under different h/d.In the presence of coarse sediment bed,one of the problems encountered is how to determine the reference level y0.For practical purpose,the present study assumes that y0 does not varies with h/d,namely,y0=0.2d.The measured data were first converted to dimensionless and then compared to theoretical results calculated by logarithmic-wake law.In addition,the coefficient B and another wake coefficient Ⅱin in logarithmic compensation formula were calibrated to fit for the measured data.The results indicate that when the value of h/d is small,the coefficient B decreases with the increase of h/d while the wake coefficient Ⅱ presents an opposite trend;when the value of h/d becomes larger,both B and Ⅱ tend to a constant.Finally,a modified logarithmicwake law is derived to describe the velocity distribution in mountain river flows with small h/d.The modified model agrees well with the experimental data conducted in the present laboratory flume.Furthermore,it also has a good adaptability to riverbed with coarse sediments.