Introduction
The permeability of urban land has been changed substantially with urban
sprawl (Czemiel Berndtsson, 2010; Chen, 2013; Berndtsson, 2010) since
the original permeable soil is replaced by relatively impervious
surfaces. As a result, the Middle and Lower Reaches of the Yangtze River
region (MLRYR), one of the most densely populated areas in China, is
suffering from serious urban flooding and groundwater depletion. On the
one side, total annual precipitation in this region has increased
significantly since the end of the 1970s. In addition, there is a
decrease in the number of precipitation days and a significant
increasing precipitation intensity as proved by previous studies (Li et
al. 2015; Ye and Huang, 1991; Wang and Zhou, 2005; Zhang et al., 2008;
Feng, 2012; Wang et al., 2016). On the other side, drought in the MLRYR
has significant sustainability in the past 50 years with increasing
intensity over the past two decades (Wang, el. at, 2014). For example,
severe droughts have been detected in the MLRYR in 2000, 2001, 2004,
2007, 2011, and 2013 (Liu, 2017). Therefore, the
MLRYR climatic characteristics
result in relatively high frequency of urban flooding and groundwater
depletion.
Green Roof Systems (GRS), which include a substrate for vegetation, have
considerable potential to alleviate urban flooding caused by excessive
surface runoff (Carson et al., 2015; Cipolla et al., 2016; Sims et al.,
2016; Claudia et al., 2019). In comparison to a Traditional Roof System
(TRS), the GRS can intercept, detain, and delay surface runoff
(Berndtsson, 2010; Li and Babcock, 2014). For example, the evaluated
results in Li and Babcock (2014) showed that the GRS could reduce total
runoff volume by 30 - 86%, peak flow rates by 22 - 93%, and delay peak
surface runoff flows by up to 30 minutes. Furthermore, GRS is feasible
to apply since it can be retrofitted to existing buildings and does not
require as much additional space as other approaches. Considering that
existing buildings in many cities account for a large fraction (often
40-50%) of impermeable area (Dunnet and Kingsbury 2004; Sims et al.,
2016), GRS is a potential Low Impact Development (LID) method to address
urban flooding issue in this region because it can be implemented
widely.
Recognizing the advantages of GRS, more and more research has been
devoted to this field such as measuring the rainwater retention of green
roofs over a certain period of time (Mentens et al., 2006; Simmons et
al., 2008; Liu et al., 2019), comparing the rainwater retention amount
of green roofs under different rainfall intensities (Hilten et al.,
2008; Talebi et al., 2019), detecting the impact of roof slope and
thickness of the substrate on the retention effect (Voyde, et al.,
2010;Yio et al., 2013; Carter and Jackson, 2007; Feitosa and Wilkinson,
2016; Bollman et al., 2019; Peng et al., 2019), comparing the rainwater
retention ability of common roofs, green roofs and white roofs (VanWoert
et al., 2005; Yang and Bou-Zeid, 2019), and the selection of green roof
vegetation (Schroder et al., 2019; Du et al., 2019; Tran et al., 2019).
Previous research has shown that GRS performance depends strongly on
climate condition (Sims et al., 2016; Chen, 2013; Wong and Jim, 2014;),
rooftop configuration, and plant species (Li and Babcock, 2014; VanWoert
et al., 2005; Sendo et al., 2010; Metselaar, 2012; Liu et al., 2019).
However, the existent literature is scarce in the following aspects: (1)
Few studies focus on the application potential of GRS in the MLRYR
region with its unique climate condition that is the main factor
determining the performance of GRS. In the MLRYR region, the studies on
GRS mainly focus on the landscape design, vegetation selection and
energy saving (Xiao et al., 2014; Jiang, 2011; Li et al., 2019), and
only a few studies on the stormwater retention capacity which includes
the calculation of the runoff mitigation of green roofs under different
rainfall intensity, the interception of rainwater by statistical
analysis of the measured rainfall data, and the feasibility study of
using waste sludge from sewage treatment plant as green roof soil layer
(Shen et al., 2017; Liu and Chen, 2018; Li et al., 2019). These studies,
however, did not aim at the special climate characteristics of MLRYR,
nor did they run the simulation for a long period with the long-term
rainfall data. They only used the experimental method to analyze the
short-term rainfalls. Therefore, it is necessary to study long-term
rainfall for many years and short-term rainfall with different
intensities according to the rainfall characteristics of MLRYR,
especially the unique plum rain season every year. (2) Previous research
did not give a comprehensive analysis of the important effect of
evapotranspiration in the hydrodynamic process of GRS (Ebrahimian et
al., 2019; Zhang et al., 2019; Li et al., 2019; Cascone et al., 2019).
We therefore need to better understand the evapotranspiration of GRS by
analyzing the PET (Potential ET), AET (Actual ET) and RET (Reference
ET). (3) Although there are many researches on the retention and
detention of GRS, these studies did not analyze the mitigation potential
of GRS for urban flooding by calculating the overload of CSS / SWS which
is the most direct part to determine whether urban flooding will occur.
(4) Many studies have recognized the effect of soil layer on the
retention efficiency of GRS, but the sensitivity of soil parameters was
not comprehensively analyzed which is important because each soil
parameter has a different effect on the retention results. The
sensitivity analysis of soil parameters will be helpful to the
structural design of GRS in future studies, so as to obtain better
retention efficiency. Moreover, considering that most surface runoff is
discharged via the drainage layer of GRS into the CSS / SWS in heavy
precipitation because the retention volume of GRS decreases as
precipitation intensity increases (Li and Babcock, 2014), we propose an
Improved Green Roof System (IGRS) that combines green roof and rooftop
disconnection to decrease drainage system loads and better recharge
groundwater.
Therefore, the objectives of this work are to address the following
questions: (1) What are the impacts of the GRS and IGRS on hydrology
characters (e.g., surface runoff, flood, evaporation, and infiltration)
of an urban catchment in Nanchang that has typical rainfall
characteristics of MLRYR? (2) Based on the comprehensive analysis of
PET, AET, and RET, what role does evapotranspiration play in the
hydrological cycle of GRS? And (3) Does the GRS or IGRS have the
potential to be applied in cities like Nanchang? To answer these
questions, this study simulated the hydrological process of runoff,
flooding flow, evaporation, and infiltration of GRS under different
rainfall intensities and durations, and tested the sensitivity of soil
parameters of GRS, so as to explore the potential of GRS to mitigate the
urban flooding problems. The novelty of the urban flooding mitigation
study stems from the fact that the hydraulic condition of CSS / SWS is
the most direct factor to decide whether urban flooding will occur. We
analyzed flooding nodes and overloaded conduits of CSS / SWS as well as
runoff retention of GRS and IGRS under the unique climatic and
high-density developed conditions in MLRYR. On the whole, we first
analyzed the potential application of GRS to reduce surface runoff and
peak flow rates and recharge groundwater in a densely developed city in
the MLRYR region. We performed the analyses using the United States
Environmental Protection Agency (USEPA) Storm Water Management Model
(SWMM). Then we explored the
potential and impacts of green roof application by analyzing
hydrological characteristics of TRS, GRS, and IGRS in terms of surface
runoff, flood, evaporation, and infiltration, to see if the GRS or IGRS
is superior to the TRS in the studying city. Finally, we synthesized and
discussed results in Section 3.