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Proceeding Paper

The Hydraulic Performance of Green Roofs in Urban Environments: A Brief State-of-the-Art Analysis of Select Literature †

by
Erica Orsi
*,
Gaetano Crispino
and
Corrado Gisonni
Department of Engineering, Università degli Studi della Campania Luigi Vanvitelli, 81031 Aversa, Italy
*
Author to whom correspondence should be addressed.
†
Presented at the International Conference EWaS5, Naples, Italy, 12–15 July 2022.
Environ. Sci. Proc. 2022, 21(1), 1; https://doi.org/10.3390/environsciproc2022021001
Published: 11 October 2022
(This article belongs to the Proceedings of EWaS5 International Conference: “Water Security and Safety Management: Emerging Threats or New Challenges? Moving from Therapy and Restoration to Prognosis and Prevention”)
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Abstract

:
In recent years, green roofs have been used as a control measure for urban stormwater management, as they retain, detain and slowly release rainwater. Green roofs present many economic, social and environmental benefits. After a general overview of the main features of green roofs, the paper focuses on the main outcomes highlighted by selected literature studies on the hydraulic performance of green roofs. The hydraulic efficiency has been specifically assessed through the definition of two parameters of great interest to design the tailwater drainage system: the peak reduction and the volume reduction indexes.

1. Introduction

The world has observed a rapid urbanization over the past decades, with the increase in construction to accommodate the shift in population from rural to urban areas [1]. According to a recent report from the United Nations, 60% of people will live in urban settlements by 2030 [2]. This results in several environmental issues on a global scale, such as urban floods and Urban Heat Island (UHI) effects [3]. Moreover, the growing urbanization influences the natural water cycle: the impermeable surfaces augment and, consequently, there is an increase in surface runoff and a reduction in infiltration [4]. Climate change further aggravates these problems by increasing both the frequency and intensity of climatic extremes [5]. These critical issues can be partially solved by installing Green Roofs. A green roof (GR) installation consists of introducing plants and soil on a building rooftop and representing a strategy to make buildings more suitable under environmental, economic and social points of view [6].
GRs increase surface permeability by imitating the hydrologic functions of cities before the intense construction. Rainwater landing on GRs enters a complex hydrological system [7]. The system retains water in vegetation, substrate and layered materials, thus providing runoff retention capacity for stormwater management [8]. In addition, water leaving the GR systems through evapotranspiration contributes to a cooling effect, the mitigation of UHI effect [9], and the indoor thermal environment isolation [10]. Other significant GRs benefits include the improvement of the water runoff quality by the filtration of pollutants and heavy metals out of rainwater and the reduction in air pollution and greenhouse gas emissions thanks to the ability of the vegetation to filter the air [11,12,13,14]. Nowadays, the scientific and technical literature studies on the hydraulic behavior of GRs can be considered as quite exhaustive. However, some gaps have to be still filled to define the main hydraulic performance parameters of GRs and to characterize them by adopting standard commercial numerical models such as the Storm Water Management Model (SWMM) [15], Hydrus-1D [16] and Soil Water Atmosphere and Plant (SWAP) [17]. In this regard, this paper aims to resume the main outcomes highlighted by selected literature studies in the range of the assessment of the hydraulic performance of extensive green roofs. Firstly, both history and modern numerical applications are discussed in order to present a state of the art of the GR technology. Later, a classification into extensive and intensive GR installations is presented. Finally, the benefits of GRs with a focus on their hydraulic performance are described in detail. In particular, two indexes are analyzed: the peak reduction and the volume reduction rates. Among various performance parameters, they were chosen because they directly provide the essential variables (incoming discharges and volumes) to design the part of the drainage system downstream from the GR system. The literature studies considered herein were selected accordingly.

2. History and Modern Applications of Green Roofs

The existing literature shows that planting vegetation on building rooftops has been used for centuries as passive cooling practices in different countries [18]. The most famous ancient GRs were the Hanging Gardens of Babylon constructed in the fifth century [19], the Mysteries Villa in Pompei (Italy) [20] and the applications in the ziggurats of ancient Mesopotamia [21]. Modern GRs have been implemented in Germany, France and Switzerland in the early 1960s. However, Germany is regarded as the world leader in the employment of this strategy, because GRs on the large scale were being developed, designed and implemented [22].
Currently, several countries are making a strong initiative to install GRs in the construction of new buildings and to retrofit old ones in existing structures. As a result of the regulations for new and renovated flat roofs, 15% of flat roofs in Basel (Switzerland) have been greened [23]. In Toronto (Canada), the green roof by-law mandates all newly established development with a floor area of 2000 m2 to include GR on 20–60% of the roof area [24].

3. Classification and Components of Green Roofs

GRs are generally classified as extensive or intensive, depending on the depth of soil, vegetation type, construction material, management and allocated usage. Intensive GRs have a substrate thickness above 200 mm, while extensive vegetated roofs present a substrate thickness below 200 mm. Extensive GRs are less heavy, and they can be installed more easily, so that they are often added to existing roofs. Moreover, extensive GR requires less depth of soil and assumes self-maintenance of the roof, and less water needs. Intensive roofs encompass comparatively better potential for improved insulation, enhanced stormwater management and energy performance. However, their heavy weight may require a reinforced structure, and furthermore, a specific drainage and irrigation system must generally be utilized with a consequent increase in the technical complexity and associated costs [22].
A typical GR structure consists of multiple layers, each one playing an important role in the entire roof system. The structure mainly includes the vegetation layer, the growing medium (or soil layer), the filter layer, the drainage layer and the protection layer. Many other layers can be required, such as the root barrier and the waterproofing membrane.

4. General Benefits of Green Roofs

GRs mitigate energy demand by enhancing thermal performances of buildings [25]. The vegetative layer stabilizes the indoor building temperature and reduces the daily temperature fluctuations [26]. The increase in shading, a better insulation and a larger thermal mass contribute to improve the thermal building behavior. An experimental analysis of an extensive GR installed on a building of the University of Calabria (Italy) [27] showed that, if compared with a standard black bituminous roof, the GR is able to reduce the temperature at the interface with the structural roof by 12 °C, on average, in summer and to maintain a value that is 4 °C higher in winter. Moreover, Tsang and Jim [28] showed that GRs could reduce solar energy absorption by 50% compared with conventional roofs.
The UHI can be reduced by increasing albedo or vegetation to support evapotranspiration [29,30]. The albedo of green roofs ranges from 0.70 to 0.85, which is larger than the albedo of bitumen, tar and gravel roofs (typically from 0.10 to 0.20) [6]. Santamouris [31] compared several mitigation technologies to fight UHI effect and observed that the large-scale application GRs could reduce the ambient temperature from 0.3 °C to 3 °C. Gill et al. [32] showed that an increase of 10% of the urban green in Manchester (UK) could avoid the predicted increase of 4 °C of the ambient temperature over the next 80 years.
Plants can mitigate air pollution through direct and indirect processes. In the direct process, plants consume gaseous pollutants through their stomata. In the indirect process, plants mitigate air pollution by modifying microclimates [33]. Yang et al. [33] reported that a GR with an area of 19.8 ha could remove 1675 kg of air pollutants in only one year. Other authors [34] estimated that 2000 m2 of grass on the GR can remove up to 4000 kg of air particles.

5. Hydraulic Benefits of Green Roofs

5.1. Stormwater Attenuation

GRs can be used as a control measure for urban stormwater management as they retain rainwater, promote evapotranspiration (ET) and delay peak flows [35]. When precipitation enters a GR, a portion of the water runs downwards and it is absorbed by the growing substrate or retained in pore spaces. The remaining water passes through the filter fabric and goes into the drainage element, where it is detained. If the storage capacity of the drainage space is exceeded, then the overflow drains and the retained water inside the GR evaporates or it is used by plants being partially transpired. Several studies [19,35,36,37] evidenced that GRs can significantly reduce the amount of stormwater runoff compared to that of conventional roof designs. Moreover, many literature investigations [38] also showed that GRs modify the runoff hydrograph, with a delayed runoff starting time, a reduced runoff peak rate (decrease of 60–80%) and a slow released runoff at the end of the precipitation event. This paper is just focused on the description of the efficiency of extensive GRs to reduce volume and peak outflows, as follows.

5.1.1. Peak Flow Reduction

The reduction of the peak outflows from the catchment equipped with one or more GRs is typically compared with the conventional roof scenario. The Peak flow Reduction (PR) is usually defined as:
PR [%] = 100 × (Q0max − QGRmax)/Q0max
where Q0max and QGRmax are the outflow peaks of the reference conventional roof and GR implementation, respectively.
Several studies have investigated PR affected by GRs (Figure 1 and Table 1). Stovin et al. [39] observed the hydraulic performance of a standard commercial extensive GR system (with a plan surface of 3 m2) in the UK, composed of 80 mm substrate layer. They identified 22 significant events over 29 months, and they pointed out that PR was 60% for these significant storms. In New Zeland, Fassman-Beck et al. [40] monitored four extensive GRs with depths from 50 to 150 mm: a 217 m2 GR situated on the Faculty of Engineering roof at the University of Auckland, a mini GR (4 m2) installed at the Research office in East Tamaki (Auckland) covered with either 10 mm or 150 mm depth of substrate and a 500 m2 GR in the Waitakere Civic Center. They observed PR ranging between 73% and 89% compared to the control roofs. Palla et al. [41] performed continuous simulations (26 years) of the small urban catchment of Colle Ometti (Genoa, Italy) retrofitted with GRs. The 60% of the catchment is covered with an impervious surface and rooftops account for 31% of total area. The GR system consists of a 120 mm growing medium produced by Harpo Seic Verde Pensile. They observed that PR was equal to 60% on average, while a maximum value of 96%. Palla and Gnecco [42] examined the hydraulic performance of three case studies of GRs characterized by different climate conditions across the Italian territory. The model setting concerned a 0.6 ha hypothetic urban block, where the impervious area occupies 57% of the urban block and rooftops account for 33% of the total area. The selected GR system consists of a 120 mm growing medium produced by Harpo Seic Verde Pensile. The continuous simulations reported that the median value of the PR was moderately constant for all climate conditions and equal to 60%. In Poland, Mroweic et al. [43] studied the peak flow reduction by varying the retention capacity (hR), which is the capacity of a GR system to hold water. The test sites of GR were located on the roof at the Institute of Environmental Engineering (Czestochowa, Poland). The total catchment area was equal to 2.05 ha, almost the whole area was covered with an impervious surface and roofs occupied 50% of the total area. The results of the simulations showed that the GR with hR ≥ 35 mm reduced peak flows in 80–90% of the rainfall events, while for hR = 15 mm the peak flow reduction was equal to 25%.
In general, the simulation’s results reveal that climatic conditions do not influence peak reductions, but it is mainly affected by rainfall-event characteristics and retention capacity.

5.1.2. Volume Reduction

Volume Reduction (VR) is influenced by many factors as the initial water content, the slope of the roof, the vegetation, the growth media, the meteorological conditions and the precipitation. VR can be computed as:
VR [%] = 100 × (V0 − VGR)/V0
where V0 and VGR are the outflow volumes of the reference conventional roof and GR implementation, respectively.
According to the main literature studies, VR varies from 30% to 52% (Figure 2 and Table 1). Stovin et al. [39] observed a VR of 30%. Masseroni and Cislaghi [44] the hydraulic effect of three GRs at the catchment scale with a conversion of 5%, 30% and 100% of impervious areas to green roofs. The studied subcatchment was the Seveso Basin (Lombardy, Italy), with a total area of about 95 km2 (9500 ha), characterized by approximately 54% impermeable surfaces. The corresponding simulations demonstrated that green roof implementation with 100% conversion scenario reduced the runoff volume by up 35%. In their continuous simulations, Palla et al. [41] reported a VR equal to 30% on average, with the maximum value equal to 86%. In the investigation of Palla and Gnecco [42] it emerges that VR depends on climate characteristics in terms of rainfall depth and actual evapotranspiration. VR increases when the site is characterized by the minimum rainfall depth together with the highest actual ET rates. In fact, this parameter varied from 30% (Genoa) to 52% (Castelbuono). As for PR, Mroweic et al. [43] evaluated VR by varying the retention capacity. The results of simulations showed a reduction of volume by 11.3% for hR = 15 mm to 35.9% for hR = 50 mm.

6. Numerical Models

Besides empirical and conceptual hydrological models, there is some commercial software to model the rainfall–runoff process numerically. According to the selected literature, most of the applications described in the case-study papers [41,45,46] are HYDRUS and SWMM. For this reason, an overview of these models is provided as follows.

6.1. Hydrus

Hydrus is a commercial software simulating water, heat and multiple solute movement in variably saturated media. It is a physically based model, using the Richards equation, and it can operate in one- (i.e., the vertical), two- or three-dimensional domains.
Hilten et al. [45] evaluated the hydraulic performance, in terms of peak flow and volume reduction of runoff, of a modular block green roof using Hydrus-1D. The study confirmed the influence of the precipitation depth on the performance of green roofs. PR and VR were 86.1 and 65.6% for a 2.54 cm precipitation, respectively, and they were 0.4 and 21.5% for a 7.93 cm precipitation. Palla et al. [46] investigated the hydraulic response of a GR system site at the University of Genoa (Italy) through both a conceptual linear reservoir model and Hydrus-1D. Simulations demonstrated that Hydrus-1D was more accurate. However, since the prediction errors of the conceptual model were limited when compared with the observed hydraulic performance, conceptual models were suitably used to simulate the hydraulic behavior of a GR. Raha et al. [47] pointed out that Hydrus-1D generally performed better in the estimation of the hydraulic performance of GRs as the drainage area increases. This study also revealed that a 1D infiltration model needs field-specific data, such as substrate saturation conditions.
In the same way, Soulis et al. [48] calibrated model parameters with Hydrus-1D, showing an acceptable relationship between observations and corresponding model results. Zhang et al. [49] studied the stormwater retention and detention performance of 6 GRs with different types and the depth of substrates under extreme storms. The Hydrus-1D simulations revealed that the GR stormwater retention performance decreased exponentially with the increase of the storm return period and it increased as the substrate depth augments.

6.2. SWMM

SWMM [15] is the most common commercial model for the evaluation of the performance of GRs [50]. It simulates the rainfall–runoff process in the drainage system and models the quality and quantity of runoff from sub-catchments. Early versions of SWMM did not include a specific green roof module, whereas Low Impact Development (LID) control modules have been recently implemented in SWMM (Environmental Protection Agency, Washington, D.C., U.S.A.). LIDs can be modeled by defining properties of different layers such as thickness, conductivity and porosity. Moreover, these modules can be used for both long-term and single-event simulations [51]. Palla and Gnecco [52] demonstrated that SWMM can be successfully used to assess the GRs hydraulic performance at the catchment scale. However, this research did not consider ET, and simulations for single events were only performed. Conversely, several studies [50,53] focused on the role of the ET on the recovery of retention capacity. The latter is, indeed, essential to account for ET across long-term simulations. Cipolla et al. [50] conducted long-term simulations of a full-scale GR, by using the bioretention module of SWMM. The numerical results confirmed a good relationship between the SWMM simulation results and the corresponding monitored runoffs. Palla et al. [41] analyzed the hydraulic performance of a GR at the catchment scale and they implemented continuous simulations by also developing an approach to evaluate the actual ET. The results confirmed that actual ET plays a significant role in the long-term retention process.
The suitability of the SWMM is limited because this model does not consider soil water potential and flow under unsaturated conditions. The software adopts the field capacity concept so that the soil water flow can only be simulated if the soil moisture content exceeds field capacity. Given that Hydrus does not take into consideration the conceptual and computational frameworks to simulate the hydrological and hydraulic processes of stormwater in urban areas, Baek et al. [54] developed a coupled SWMM–Hydrus-1D (SWMM-H) model to improve the simulation of GR hydrologic processes. This study demonstrated that SWMM-H simulates soil hydrology and hydraulics in a GR system with excellent results.

7. Conclusions

This paper resumed the role, history, benefits and performance of GR systems. Factors that typically influence the hydraulic performance of GRs can be grouped into two categories: weather conditions (length of the antecedent dry weather period, characteristics of rainfall event, season/climate) and GR physical characteristics (layers and materials, substrate depth, its hydraulic characteristics, percentage of drainage area, roof geometry, type of vegetation).
The design of the drainage systems serving the urban area retrofitted with GRs implementation should account for the necessity to safeguard the hydrologic and hydraulic invariance criterion. In this regard, the design phase of the hydraulic system placed downstream from GRs is based on the flow discharge and volumes, which are retained, detained and then released by GRs. The description of the volume and peak reduction indexes is, therefore, of primary importance for engineers involved in the design of urban sewers downstream from GRs. At this aim, the present paper provided some practical indications to estimate these performance indicators according to the selected literature studies. In particular, the peak and volume reduction indexes can be safely made equal to 0.60 and 0.30, respectively. However, each country has different climatic conditions and urbanization levels and, for this reason, local research aiming to derive the expected field-based GRs performance is preferred. For this purpose, further investigations are needed to outline specific performance protocols as a function of the system and climate characteristics.

Author Contributions

Conceptualization, E.O.; methodology, E.O. and C.G.; validation, E.O. and G.C.; formal analysis, E.O.; investigation, E.O.; writing—original draft preparation, E.O.; visualization, E.O. and G.C.; supervision, G.C. and C.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Environsciproc 21 00001 g001 550
Figure 1. PR reported in GR studies [39,40,41,42,43,44].
Figure 1. PR reported in GR studies [39,40,41,42,43,44].
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Environsciproc 21 00001 g002 550
Figure 2. VR reported in GR studies [39,41,42,43,44].
Figure 2. VR reported in GR studies [39,41,42,43,44].
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Table
Table 1. Details of selected literature studies on the peak and volume reduction efficiency of GRs.
Table 1. Details of selected literature studies on the peak and volume reduction efficiency of GRs.
AuthorsSiteRainfall Max. Depth [mm]Substrate Depth [mm]PR [%]VR [%]Numerical Model
[39]Sheffield99.680.060.030.0-
[44]Seveso Basin-192.058.035.0SWMM
Auckland-50.0–150.073.0–89.0--
[41]Genoa462.8120.060.030.0SWMM
[42]Bergamo96.4120.060.040.0
Genoa462.8120.060.030.0SWMM
Castelbuono91.8120.060.052.0
[43]Częstochowa70.750.0–70.080.0–90.035.9SWMM
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Orsi, E.; Crispino, G.; Gisonni, C. The Hydraulic Performance of Green Roofs in Urban Environments: A Brief State-of-the-Art Analysis of Select Literature. Environ. Sci. Proc. 2022, 21, 1. https://doi.org/10.3390/environsciproc2022021001

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Orsi E, Crispino G, Gisonni C. The Hydraulic Performance of Green Roofs in Urban Environments: A Brief State-of-the-Art Analysis of Select Literature. Environmental Sciences Proceedings. 2022; 21(1):1. https://doi.org/10.3390/environsciproc2022021001

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Orsi, Erica, Gaetano Crispino, and Corrado Gisonni. 2022. "The Hydraulic Performance of Green Roofs in Urban Environments: A Brief State-of-the-Art Analysis of Select Literature" Environmental Sciences Proceedings 21, no. 1: 1. https://doi.org/10.3390/environsciproc2022021001

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