Modeling Green Roof Potential to Mitigate Urban Flooding in a Chinese City

Li Liu,Jie Niu,William J. Riley,Liwei Sun

doi:10.3390/w12082082

Abstract

The Middle and Lower Reaches of the Yangtze River (MLRYR) region, which has humid subtropical climate conditions and unique plum rain season, is characterized by a simultaneous high-frequency urban flooding and reduction in groundwater levels. Retrofitting the existing buildings into green roofs is a promising approach to combat urban flooding, especially for a densely developed city. Here, the application potential of the Green Roof System (GRS) and the Improved Green Roof System (IGRS) designed to divert overflowing water from green roofs to recharge groundwater were analyzed in a densely developed city, Nanchang, China. For the first time, the influence of GRS on the hydraulic condition of Combined Sewage System/Storm Water System (CSS/SWS) is analyzed, which is a direct reflection of the effect of GRS on alleviating urban flooding. The simulation results show that GRS can retain about 41–75% of precipitation in a 2-hour timescale and the flooding volumes in the GRS/IGRS region are 82% and 28% less than those of the Traditional Roof System (TRS) in 10- and 100-yr precipitation events, respectively. In the continuous simulations, GRS also enhances Evapotranspiration (ET), which accounts for 39% of annual precipitation, so that reduces the cumulative surface runoff. Considering the IGRS can provide more hydrological benefits than the GRS under the same climate conditions, we may conclude that the widespread implementation of both the GRS and the IGRS in Nanchang and other densely developed cities in the MLRYR region could significantly reduce surface and peak runoff rates.

Highlights

The permeability of urban land has been changed substantially with urban sprawl [1,2,3] since the original permeable soil is replaced by relatively impervious surfaces
Considering that most surface runoff is discharged via the drainage layer of GRS into the Combined Sewage System/Storm Water System (CSS/SWS) in heavy precipitation because the retention volume of GRS decreases as precipitation intensity increases [16], we propose an Improved Green Roof System (IGRS) that combines green roof and rooftop disconnection to decrease drainage system loads and better recharge groundwater
We find that runoff coefficient (Q/P) of the GRS increases from 57.41% to 72.19% when the precipitation intensity changes from 2-yr storm to 100-yr storm (Table 6), which indicated that the retention capacity of the GRS decreases with the increase of the precipitation intensity

Summary

Introduction

The permeability of urban land has been changed substantially with urban sprawl [1,2,3] since the original permeable soil is replaced by relatively impervious surfaces. Lower Reaches of the Yangtze River region (MLRYR), one of the most densely populated areas in. Total annual precipitation in this region has increased significantly since the end of the 1970s. There is a decrease in the number of precipitation days and a significant increasing precipitation intensity as proved by previous studies [4,5,6,7,8,9]. Drought in the MLRYR has significant sustainability in the past 50 years with increasing intensity over the past two decades [10]. Severe droughts have been detected in the MLRYR in 2000, 2001, 2004, 2007, 2011, and 2013 [11]

Objectives

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Results

Conclusion