Changing characteristics and influencing factors of Manning's roughness coefficient along grass swales
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摘要:
随着海绵城市的快速发展，植草沟得到了广泛应用。目前关于植草沟沿程流量的计算一般采用谢才公式，但其适用条件与植草沟存在显著差异，导致计算结果存在一定误差，其中曼宁粗糙系数变化是引起误差的原因之一。在假定谢才公式适用的前提下，通过足尺试验，系统研究了恒定流量、变流量进水条件下植草沟曼宁粗糙系数的变化特征，并进一步分析了植被高度对曼宁粗糙系数的影响。结果表明：植草沟沿程流量以及曼宁粗糙系数均随流动距离增加而降低；不同进水流量条件下，曼宁粗糙系数与进水流量密切相关，随进水流量增加而增加，试验条件下其最大值为0.22；另外，在植草沟不同植被高度以及不同变流量进水条件下，曼宁粗糙系数在峰值流量时刻的变化范围为0.19~0.22，试验条件下植被高度对曼宁粗糙系数影响较小。因此，植草沟的曼宁粗糙系数受沿程流量的影响较大，研究成果可以为植草沟沿程流量计算时曼宁粗糙系数的精确取值提供参考。
Abstract:With the rapid development of sponge cities, grass swales have been widely used. Recently, the Chezy formula is generally used to calculate the flow rate along the grass swales. There are differences between the application conditions of the Chezy formula and grass swales, which often cause remarkable calculation errors. One of the reasons for the errors is the Manning roughness coefficient variation. Assuming that the Chezy formula was appropriate, the variation characteristics of the Manning roughness coefficient of grass swales under different constant and variable inflow conditions were systematically researched through full-scale experiments, and the effect of vegetation height on the Manning roughness coefficient was further analyzed. The results showed that the flow rate and Manning roughness coefficient decreased with the increase of flow distance along the grass swale. Under different inflow conditions, the Manning roughness coefficient was closely related to the inflow rate, and the higher the flow rate, the greater the Manning roughness coefficient, with the maximum value being 0.22. In addition, the maximum Manning roughness coefficient of grass swale with different vegetation heights was 0.19-0.22 under different variable flow inflow conditions, and the vegetation height had little influence on the Manning roughness coefficient under the experimental conditions. Therefore, the Manning roughness coefficient of grass swale was markedly affected by runoff volume. The research results can provide reference for selecting the Manning roughness coefficient in the calculation of grass swale drainage capacity.
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图 1 植草沟试验装置
1—进水箱；2—进水管；3—电磁流量计；4—配水槽；5—溢流挡板；6—堰；7—植草沟；8—出水槽；9—出水箱；10—水泵；11—橡胶管；12—回流管。
Figure 1. Experiment device of grass swale
图 2 不同降雨重现期条件下植草沟进水流量过程线
Figure 2. Inflow hydrograph of grass swale under different rainfall return periods
图 6 植草沟沿程实测流量和理论流量
Figure 6. Monitored and theoretical calculated flow rates along grass swale
图 7 植草沟内沿程流量和n变化
Figure 7. Changes of flow rate and Manning roughness coefficient along grass swale
图 8 植草沟不同沿程距离n变化特征
Figure 8. Variation characteristics of Manning roughness coefficient at different monitoring locations along grass swale
图 9 植草沟植被高度在不同沿程距离处对峰值流量时刻n的影响
Figure 9. Effect of vegetation height on Manning roughness coeffiencent at peak flow time of different monitoring locations
表 1 不同恒定Q进下植草沟沿程n衰减率
Table 1. Manning roughness coefficient attenuation rate along grass swale under different constant inflow rates
% 植草沟沿
程距离/mQ进/(m3/h) 1 1.5 2 3 4 5 19.7 8.3 6.6 2.9 3.7 10 40.5 20.0 12.9 5.6 4.8 15 51.2 35.5 20.1 8.8 6.5 20 71.5 45.2 26.4 12.2 9.5 
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