绿色屋顶降雨径流削减效果监测与过程模拟

选择平面、斜面、绿色、滞蓄4种不同形式屋顶,采用"液位计+三角堰"的径流测量方法,开展降雨径流对比监测。结果表明,相比平面与斜面屋顶平均径流系数(分别为0.68和0.84),绿色与滞蓄屋顶平均径流系数分别为0.41和0.43,均明显有所减小,并且数值相差不大,反映出相近的径流总量削减效应。比较分析绿色与滞蓄屋顶降雨径流过程,2类屋顶径流过程接近,产流后水流运动机制相似,均受上部积水水头作用影响,并以孔流(孔隙、孔口)形式出流,可以采用相似的水库概念模型进行简化模拟。根据土壤水饱和下渗理论,考虑到绿色屋顶基质层厚度有限,推导建立了描述绿色屋顶饱和产流后降雨径流过程的基本方程,并应用于模拟绿色屋顶降雨径流过程。结果表明,径流过程计算值与实测值间决定系数值为0.85、Nash-Sutcliffe效率系数值为0.84,初步验证了模型的合理性。研究成果有助于深入认识绿色屋顶对降雨径流的削减效应与出流机制。

Monitoring and simulation of rainfall-runoff reduction effects and process on green roofs

Green roof is an important facility to mitigate urban flood resulted by increasing urbanization and impermeable surface. A thick substrate material layer with plants, grass, and/or trees, and extra structural support is one of main components of green roof. Due to the complexity of rainwater storage mechanism in the substrate material layer, the simulations results on rainfall-runoff on green roof are mostly unsatisfactory. In this study, the runoff reduction effect of green roof was simulated by considering the effect of substrate material layer on runoff. The green roof was compared with slope roof, plane roof and retention roof in order to understand the effect of effect of substrate material layer on runoff. The experiment was carried out in Beijing Water Science and Technology Institute, China. The precipitation was measured by using a self-recording tipping bucket rain gauge. Runoff was monitored by using an equipment including water level gauges and triangular weir. The runoff coefficient and rainwater retention rate were calculated. A total of 11 rainfall-runoff events were monitored in 2015. The results showed that the change of the runoff coefficient was highly complex and it was influenced by many factors, e.g., precipitation, average/maximum rainfall intensity, and interval time between rainfall events. For all the monitored rainfall events, the average runoff coefficient values for plane roof, slope roof, green roof and retention roof were 0.68, 0.84, 0.41 and 0.43, respectively. The average rainwater retention rate values of green roof and retention roof were 58.9% and 57.2%, respectively. It indicated that the runoff reduction effects of green roof and retention roof are similar but both were better than the other two types of roofs. The runoff process of green roof was also similar with that of retention roof under the same rainfall conditions. Both the outflow processes from green roof and retention roof could be regarded as an unsteady orifice flow under the effect of accumulated water head on the roof-top. Thus, a unit hydrograph model to simulate runoff from green roof was proposed based on the saturated infiltration theory that included the thickness limitation of the substrate material layer and the influence of water accumulated on the surface. By using the proposed unit hydrograph model, the rainfall-runoff processes on green roof were simulated. The simulation results showed that the trends of the simulated and measured runoff were consistent well. The coefficient of determination and Nash-Sutcliffe efficiency were 0.85 and 0.84, respectively. This study will be helpful for further understanding the runoff reduction effect and the outflow mechanism of green roofs.