Hydrological Performance Of Green Roofs Research Articles

Effects of Design Factors and Multi-Stage Environmental Factors on Hydrological Performance of Subtropical Green Roofs

Environmental and design factors determine the stormwater management capacity of green roofs; however, the design and environmental factors that impact their hydrological performance in subtropical humid regions are poorly understood. In particular, meteorological factors have received little attention. Meteorological factors vary greatly at different stages of a rainfall event (e.g., during the rainfall and outflow). Therefore, the impact of meteorological factors at different stages on hydrological performance should be considered separately to obtain a more accurate picture of their effects on hydrological performance. In this study, experimental green roofs were established based on four substrate types and two depths. For the first time, this study systematically explored the effects of design factors for the substrate (type and depth) and multi-stage environmental factors on the hydrological performance of green roofs. Environmental factors, including meteorological factors, from three critical stages (before and during a rainfall event and during the outflow), and rainfall characteristics (e.g., rainfall depth and rainfall duration) were incorporated to determine the variation in hydrological performance. The effects of multi-stage environmental factors on retention and peak reduction were analyzed, with a ranking of each factor’s relative importance. Environmental factors played a leading role in determining hydrological performance. However, the impact of multi-stage environmental factors was not as important as that of rainfall depth and antecedent volumetric water content. Differences in hydrological performance were compared across combinations of design factors. No significant differences were observed across substrate types and depths. However, potential interactive effects might exist, though these were not significant compared to environmental factors (e.g., rainfall depth and rainfall duration). These results confirmed that the meteorological factors in the different event-related stages significantly impacted the hydrological performance. Quantifying the effects of design and environmental factors is critical for hydrological performance evaluation. The results provided a broader perspective on understanding influence mechanisms of hydrological performance and highlighted the impact of microclimates on hydrological performance.

A Critical Review of Existing Methods to Evaluate the Performance of Nature-Based Solutions (NBS) on Commercial Roofs (CR) to Mitigate Urban Flooding

The intensity and frequency of extreme rainfall events have increased in North America and the world due to climate change. Extreme rainfall events, characterized by a heavy volume of rainfall in a short duration, have triggered the onset of urban flash floods. Over the years, flash flooding has been reported in different cities in Canada, which resulted in many losses. Subsequently, different green roofing systems have been adopted to control urban stormwater runoff as part of Nature-Based Solutions (NBS) to mitigate urban flood and build a flood-resilient city. Currently, no specific widely recognized standard or code is dedicated to determining the hydrological performance of green roofs as a whole system. Moreover, there are no test protocols to regulate the design of green roof systems in the market. A comprehensive literature review examines existing research methods adopted to evaluate influencing parameters affecting the hydrological performance of NBS-CR. The results indicate several limitations in experimental and field investigations. Consequently, to address these limitations, it is essential to formulate a multi-functional work plan to develop a standardized test method that can become a common platform for the roofing industry to test and quantify the hydrological performance of their systems.

Hydrological Analysis of Green Roofs Performance under a Mediterranean Climate: A Case Study in Lisbon, Portugal

In this paper, the hydrological performance of eight pilot green roofs (GR) installed in Lisbon, Portugal, under a Mediterranean climate is analyzed. The pilot units were installed at Instituto Superior Técnico campus of Lisbon University. The pilots present different plant species and different substrate types, with some of the units incorporating recycled construction and demolition waste (RCW). The green roofs pilots’ hydrologic performance was evaluated through the simulation of artificial precipitation events between March 2021 and July 2021. Considering the results obtained, it can be concluded that the inclusion of RCW in the substrate composition did not hinder the development of vegetation or the hydrological performance of GR. The results showed a rainfall water retention per event ranging from 37% to 100%, with an average rainfall retention of about 81%. The runoff delay ranged from 2 to 18 min, and the peak attenuation ranged from 30 to 100%. The results indicated that previous substrate moisture strongly influences the hydrological performance of GR. As the inclusion of RCW in the substrate composition promotes a more effective drainage of the substrate during dry conditions, it is considered that RCW may have positive impacts on GR’s hydrological performance.

A Simple Modelling of Green Roof Hydrological Performance Using Response Surface Methodology

Response Surface Methodology is an optimization tool used for modelling works and has been widely used for modelling, optimizing, and identifying the interrelationship between input parameters and output variables. However, it has yet to be discovered as a tool for modelling green roof performance. This paper aims to explore the feasibility of Response Surface Methodology in Minitab to investigate the green roof hydrological performance in terms of peak runoff, peak attenuation, and water retention. In this study, Response Surface Methodology is used to identify model equations to predict hydrological performance of green roof, as well as to investigate the relationship between green roof slope, water absorption of waste material and water absorption of natural fibre on the hydrological performance of green roof system. The findings showed that the generated mathematical model equations can forecast the peak runoff, peak attenuation, and water retention of a green roof system. The results of the relationship between the input parameters and the output variables are shown using 2D contour plot and surface plot. The generated 2D contour plot and surface plot revealed that the input parameters have significant impact on the peak runoff, peak attenuation, and water retention of a green roof system.

Green roofs in the humid subtropics: The role of environmental and design factors on stormwater retention and peak reduction

Numerous factors affecting the hydrological performance of green roofs vary by climate and region. But meteorological factors have received relatively little attention, while no empirical research has focused on a quantified assessment of design and environmental factors on retention and peak reduction in humid subtropical climates. In this study, the hydrological performance of green roofs with designed substrates and vegetation was monitored in a humid subtropical climate. The influence of design and environmental factors was quantitatively assessed by ANOVA and regression. Results showed that substrates based on shale and perlite performed best in combination with grasses or forbs. The order of factors influencing retention was rainfall depth > relative humidity > substrate > antecedent volumetric water content > mean intensity. As for peak reduction, the order was rainfall depth > substrate > mean intensity. These findings suggest that the role of design factors is limited compared to environmental factors. However, the peak reduction or retention of runoff on a green roof can still be improved by adjusting the materials and relative proportions of the substrates. The key factors and optimal design features identified in this study may help promote the application of green roofs in humid subtropics.

Hydrological Performance of Green Roofs in Mediterranean Climates: A Review and Evaluation of Patterns

The capacity of green roofs to intercept rainfall, and consequently store and slow runoff resulting in a reduction in flood risk, is one of their main advantages. In this review, previous research related to the influence of green roofs on the hydrological cycle is examined with a special focus on studies for Mediterranean climate conditions (Csa and Csb according to the Köppen–Geiger climate classification). This climate is characterized by short and intense rainfall occurrences which, along with the increased area of impervious surface on Mediterranean regions, intensify the risk of flooding, particularly in the cities. The analysis covers the variables rainfall retention (R, %), runoff delay (RD, min or h), peak delay (PD, min or h), peak attenuation (PA, %), and runoff coefficient (RC, −), in relation to physical features of the green roof such as layers, substrate depth, slope, and vegetation, as well as, weather conditions, such as monthly temperature and monthly precipitation. Following a statistical analysis, some patterns for the average rainfall retention (%) were found in the published literature for green roofs under Mediterranean climate conditions—namely, that the most significant variables are related to the substrate depth, the existence of certain layers (root barrier, drainage layer), the origin of the vegetation, the types of green roofs (extensive, semi-intensive, intensive), and the precipitation and temperature of the location. Moreover, a multivariate analysis was conducted using multiple linear regression to identify the set of green roof features and weather conditions that best explain the rainfall retention (%), taking into consideration not only the studies under Mediterranean conditions but all climates, and a similar pattern emerged. Recommendations for future research include addressing the effect of physical features and weather conditions on the other variables (RD, PD, PA, RC) since, although present in some studies, they still do not provide enough information to reach clear conclusions.

Experimental study on the hydrological performance of green roofs in the application of novel biochar

AbstractBiochar has the potential to be a soil amendment in green roofs owing to its water retention, nutrient supply, and carbon sequestration application. The combined effects of biochar and vegetated soil on hydraulic performance (e.g., saturated hydraulic conductivity, retention and detention, and runoff delay) are the crucial factor for the application of the novel biochar in green roofs. Recent studies investigated soil water potential (i.e., suction) either on vegetated soil or on biochar‐amended soil but rarely focused on their integrated application. With the purpose of investigating the hydraulic performance of green roofs in the application of biochar, the combined effect of biochar and vegetated soil on hydrological processes was explored. Artificial rainfall experiments were conducted on the four types of experimental soil columns, including natural soil, biochar‐amended soil, vegetated natural soil, and vegetated biochar‐amended soil. The surface ponding, bottom drainage and the volumetric water content were measured during the rainfall test. Simulation method by using HYDRUS‐1D was adopted for estimating hydraulic parameters and developing modelling analysis. The results indicated that the saturated hydraulic conductivity of vegetated soil columns were higher than bare soil columns. The addition of biochar decreased the saturated hydraulic conductivity, and the magnitude of decrease was much significant in the case of vegetated soil. The influence of vegetation on permeability is more prominent than biochar. The vegetated biochar‐amended soil has the highest retention and detention capacity, and shows a preferable runoff delay effect under heavy rain among the four soil columns. The results from the present study help to understand the hydrological processes in the green roof in the application of biochar, and imply that biochar can be an alternative soil amendment to improve the hydraulic performance.

Investigating the hydrological performance of green roofs using storm water management model

The green roof, also known as vegetated roof covers, or eco-roofs or natural roofs, is one of the recently developed low impact development (LID) techniques, which significantly increases water retention and thus helps to mitigate the urban flooding. In the present study, the installation of green roofs is assumed as a hypothetic retrofitting scenario according to a sustainable storm water mitigation strategy for a selected urban catchment and performance has been evaluated using Storm Water Management Model (SWMM). The model has been calibrated for the measured flow data of eight events using Nash-Sutcliffe Efficiency (NSE) Coefficient. The lowest and highest value of NSE was found 0.67 and 0.93 respectively. The hydrologic performance of the green roof was conducted under event-based as well as continuous simulation. Based on simulation results, peak flow has been reduced from 7.13 ltr/sec to 6.36 ltr/sec for Green Roof – 1 (GR-1) and 1.45 ltr/sec to 1.30 ltr/sec for Green Roof – 2 (GR-2). The simulation results point out that the integration of green roof LID contributes to the reduction of the quantity of storm water runoff by increasing the retention period. However, the hydrological behavior of a green roof is site-specific, and therefore the site-specific conditions and parameters like characteristics of the vegetation, physical properties of its layers, etc. have to be considered in the evaluation of its performance. Further, the research at the micro-level is necessary to evaluate the role of the green roof slope, the effect of vegetation cover, and the antecedent period on the performance of the green roofs for more reliable results.

Detention-based green roofs for stormwater management under extreme precipitation due to climate change

Abstract Rooftops cover a large percentage of land area in urban areas, which can potentially be used for stormwater purposes. Seeking adaptation strategies, there is an increasing interest in utilising green roofs for stormwater management. However, the impact of extreme rainfall on the hydrological performance of green roofs and their design implications remain challenging to quantify. In this study, a method was developed to assess the detention performance of a detention-based green roof (underlaid with 100 mm of expanded clay) for current and future climate conditions under extreme precipitation using an artificial rainfall generator. The green roof runoff was found to be more sensitive to the initial water content than the hyetograph shape. The green roof outperformed the black roof for performance indicators (time of concentration, centroid delay, T50 or peak attenuation). While the time of concentration for the reference black roof was within 5 minutes independently of rainfall intensity, for the green roof was extrapolated between 30 and 90 minutes with intensity from 0.8 to 2.5 mm/min. Adding a layer of expanded clay under the green roof substrate provided a significant improvement to the detention performance under extreme precipitation in current and future climate conditions.

Evaluation of Green Roof Performance in Mitigating the Impact of Extreme Storms

Green roofs are used in urban areas to mitigate the adverse effects of stormwater. Through numerical modeling, this study evaluates the impacts of design parameters on green roof hydrological performance under different rainfall characteristics. A calibrated model is run with long-term precipitation data series for three locations (Hong Kong, China; Beltsville, MD, USA; and Sidney, NY, USA). The results show that the amount of peak runoff reduction increases with the duration of the storm return period in Beltsville and Sidney; while the trend is opposite in Hong Kong. Percentage peak reduction generally shows a decreasing trend with the storm return period in three locations. For average runoff reduction, the amount of reduction increases with the storm return period, whereas the percentage reduction presents an opposite trend in all three locations. The actual values vary between the three locations due to differences in rainfall characteristics. Both peak and average runoff reduction increase with green roof thickness, but in practice, it is not cost effective or feasible to increase the thickness beyond a certain threshold. The hydraulic conductivity can then be optimized for peak runoff reduction and it is found to increase with the return period. However, hydraulic conductivity has a minimal effect on average runoff reduction. Overall, this paper studies green roof hydrological performance in response to different rainfall characteristics and provides recommendations on green roof designs related to soil thickness and hydraulic conductivity.

Hydrological performance of extensive green roofs in response to different rain events in a subtropical monsoon climate

Rapid urbanization transforms permeable land into developed areas with predominantly impervious surfaces, significantly increasing stormwater runoff and exacerbating the risk of pluvial flooding. Green roofs provide an attractive strategy for increasing surface permeability by mimicking pre-development hydrologic functions and mitigating flood risks in compact cities. However, the potential of this strategy has not been rigorously assessed, despite advances in global stormwater management. This is mainly due to insufficient scientific knowledge of hydrologic performance and a lack of experimental studies of rainwater-harvesting capacity under specific climatic conditions. This study evaluated the hydrologic performance of a real-scale extensive green roof (EGR) constructed in a subtropical monsoon climate in Nanjing, China. Overall, the EGR showed considerable ability to retain rainfall (mean retention ~ 60%, accumulated retention ~ 30%), although retention performance varied from 11% to 100% depending on the rainfall event considered, and decreased with increasing rainfall. Event-based rainfall–runoff comparisons demonstrated that the EGR retained rainwater efficiently during the early stages of a rainfall event and significantly attenuated peak runoff flows compared to bare roofs. Statistical analysis showed that total rainfall depth, rainfall duration, and substrate layer moisture influenced the overall retention most strongly, but also the percentage retention and runoff depth, highlighting the impact of substrate properties in addition to rainfall characteristics on EGR hydrologic performance. These findings provide new knowledge of and important insights into the hydrological performance of green roofs in subtropical monsoon climates, which could be used to guide EGR construction to increase landscape permeability, mitigate the risk of pluvial flooding, and enhance the climatic resilience of urban regions.

Impacts of Vegetation on Hydrological Performances of Green Roofs Under Different Rainfall Conditions

Vegetation is an important component of green roofs and may affect their hydrological performance through the processes of rainwater interception and evapotranspiration. Based on the rainfall-runoff observations of green roofs with four types of vegetation covers (Portulaca grandiflora, Sedum lineare, Festuca elata, and bare substrate) located in Beijing during 26 rainfall events from April to October 2017, the impacts of vegetation cover on the hydrological performance of green roofs were investigated using runoff and peak discharge reduction rates and time-delay of runoff generation and peak discharge as indices. For the 12 green roofs, there was a significantly negative correlation (P<0.01) between runoff reduction rate and rainfall event volume. For low rainfall (<10 mm), the runoff reduction rates of all the green roofs were equal or close to 100%. When the rainfall volume increased to about 30 mm, the runoff reduction rates dropped to below 70%. For the heaviest rainfall event during the observation period (81.4 mm), the runoff reduction rates of all the green roofs were less than 55%. The impacts of vegetation on the hydrological performance of green roofs changed with rainfall conditions. The differences between runoff reduction rates of green roofs with different types of vegetation cover were largest for the heavy rainfall events. For the moderate rainstorm events, the differences were slightly lower. For light rainfall events, however, no significant differences were observed among the runoff reduction rates of green roofs with different types of vegetation cover, as little runoff was generated. Vegetation cover could enhance the hydrological performance of green roofs, as the runoff and peak discharge reduction rates and time-delay of runoff generation and peak discharge of green roofs covered with vegetation were all better than those of the bare substrate for all the groups of rainfall events except the light rainfall. Vegetation-covered green roofs with P. grandiflora performed the best, as the average height and aboveground biomass per unit area of P. grandiflora were the largest.

Hydrological and plant performance of green roofs in the climate context of the Veneto Plain (north-eastern Italy): preliminary results

A research was conducted at the Agripolis Campus of the University of Padova (Italy) to evaluate in the climate context of the Veneto plain the hydrological and plant performance of twelve different extensive green roof configurations in plastic trays (green roof microcosms), obtained from a factorial combination of two drainage/ storage layers, two substrates, and three vegetation treatments. The drainage/storage layers tested were a plastic preformed layer and a mineral layer (expanded perlite). The substrates tested were a volcanic medium and a recycled crushed bricks medium. Three types of vegetation were tested: a sedum mix (S. acre, S. album, S. reflexum, S. sexangolare), an herbaceous perennial mix (Bromus erectus, Melica ciliata, Campanula spicata, Potentilla pusilla, Euphorbia cyparissias), and a suffruticose mix (Dianthus spp., Thymus serpillum, Clinopodium nepeta, Artemisia alba, Anthemis tinctoria). In addition, as a control, gravel was considered to mimic a conventional roof with storage layer. The experiment was set in May 2014. The runoff volumes were evaluated by weighing, after each rainstorm, the water collected in tanks placed under the trays. Plant closure was evaluated elaborating digital image analysis taken on monthly basis. The results related to the preliminary phase from July 2014 to June 2015 indicate seasonal effects on the hydrological performance of green roof components; in fact, probably because of the low losses by evapotranspiration, the reduction of rainfall runoff in fall and winter was quite low (on average, about 31.5%), with little differences among the different microcosm configurations. Considering the entire period, the best performance in terms of water retention in our climate context was shown by the green roof configuration composed by suffruticose mix, recycled crushed bricks substrate and expanded perlite drainage/storage layer, which allowed a reduction of 62.7% of the total rainfall runoff compared to 14.1% of the conventional roof with gravel. Canopy closure of suffruticose and herbaceous mixes were higher than that of sedum mix throughout the experiment, and values were increased by crushed bricks medium and perlite drainage/storage layer.

Hydrological performance of green roofs in the context of the meteorological factors during the 5‐year monitoring period

AbstractGreen roofs as solutions that can offer varying levels of stormwater management are the topic of current interest. In order to use this form of reconstructing retention capacity, it is important to understand the influence of meteorological conditions on the functioning of living roofs. The study presents the results of research, using of the ANOVA variance analysis method, on retention performance (i.e. volumetric control) and detention (temporal delay run‐off) carried out in the years 2009–2014. The results indicate that the green roof can effectively retain rainfall and delay the initiation of run‐off from the rainfall events included in the analysis. Understanding the hydrological performance of green roofs in different local meteorological conditions is key to the successful implementation and development of sustainable practices to control run‐off in urban areas.

Hydrological Performance of Green Roofs at Building and City Scales under Mediterranean Conditions

Green roofs are one specific type of sustainable urban drainage system (SUDS); they aim to manage runoff at the source by storing water in its different layers, delaying the hydrological response, and restoring evapotranspiration. Evidence of their performance in the Mediterranean is still scarce. The main objective of this paper is to analyse the hydrological performance of green roofs at building and city scales under Mediterranean conditions. A green roof and a conventional roof were monitored over one year in Benaguasil (Valencia, Spain). Rainfall and flow data were recorded and analysed. Hydrological models were calibrated and validated at the building scale to analyse the hydrological long-term efficiency of the green roof and compare it against that obtained for the conventional roof. Results show that green roofs can provide good hydrological performances, even in dry climates such as the Mediterranean. In addition, their influence at the city scale is also significant, given the average runoff coefficient reduction obtained.

Green roofs: Effects of plant species used on runoff

AbstractGreen roofs are becoming a major nature‐based solution worldwide to reduce urban stormwater runoff. Runoff reduction and retention mainly depend on the hydrological characteristics of substrates and the water use strategies of plants. However, little is known about how plant species affect the hydrological performance of green roofs. Two commonly used succulent plants and four turfgrass species were investigated for their impacts on the hydrological performance and runoff reduction of green roof lysimeters under controlled conditions using a rainfall simulator. The results showed that two succulent plants (Sedum lineare and Callisia repens) did not make significant contributions to canopy interception in any of the four seasons and made only minor contributions to evapotranspiration (ET) water loss in autumn and winter, resulting in minor effects on runoff reduction by the green roofs. Festuca arundinacea contributed 47.4–116.7% of water loss via transpiration and 36.5% of runoff reduction under a 25 mm day−1 rainfall intensity in spring and 48.0% of runoff reduction under a 10 mm day−1 rainfall intensity in winter. Poa pratensis, Lolium perenne, and Agrostis stolonifera also significantly reduced runoff. Runoff reductions by extensive green roofs were mainly caused by ET rather than by their canopy interceptions. Plant shoot biomass and ET were the primary factors controlling the runoff reduction by extensive green roofs. Our results strongly suggest that the runoff reduction capacity of extensive green roofs can be greatly improved by selecting plant species with a higher shoot biomass and ET rate.

A Longitudinal Microcosm Study on the Effects of Ageing on Potential Green Roof Hydrological Performance

Green roofs contribute to stormwater management through the retention of rainfall and the detention of runoff. These processes are reasonably well understood, and runoff responses can be accurately modelled given known system properties. The physical properties of the substrate are particularly relevant to the hydrological response. The substrate is a living biological system, whose properties may change over time. Two sizes of green roof microcosms (50 mm and 150 mm diameter) were observed over a 12-month period. Six system configurations were considered, with two contrasting substrates and three vegetation treatments. Multiple approaches were used to characterize the microcosms’ physical and hydrological properties: standard physical tests, bespoke laboratory detention tests, and visualization of the substrate and the root systems using X-ray microtomography. Results suggests that both the substrates’ maximum water holding capacity and its capacity to detain runoff tend to increase with age. However, there were inconsistencies in the data and these are discussed within the paper. The noted increases were generally not statistically significant as a result of substrate heterogeneity. Notably, the observed differences after one year were relatively small when compared with differences resulting from original substrate compositions and seasonal changes reported elsewhere.

A Review on the Controlling Factors that Affecting the Stormwater Retention Performance of Green Roof

Green roofs have become a popular strategy in urban stormwater management in the recent years. However, the hydrological performance of green roofs can vary substantially due to design characteristics and environmental conditions. This study is aimed to review all the controlling factors that affecting the stormwater retention performance of green roofs. Current hydrological research on green roofs involving different plant species, substrate types and environmental conditions are reviewed and evaluated in detail. Better understandings on the controlling factors that affecting the hydrological performance of extensive green roofs will be benefiting to the successful implementation of green roof system in the future.

Retention performances of green roofs worldwide at different time scales

AbstractUrbanization process is rapidly converting forested areas and grasslands to residential, commercial, or industrial spaces, triggering soil degradation and significantly increasing impervious surfaces. Consequently, man‐made environments experience significant decrease of infiltration and groundwater recharge, enhancing the problems related to the management and protection from increased stormwater runoff. Green roofs are structural measures able to mitigate these negative drawbacks due to anthropic transformation, retaining and detaining stormwater runoff, with the main advantage of reducing hydraulic loads on combined storm sewer systems. Hydrological performance of green roofs have been studied in different parts of the world under different climatic conditions, but a comprehensive study that provides global responses, even in an approximate way, is still missing. The aim of this work is to assess the convenience and the feasibility of installing green roofs in any target area of the world, providing a rough indication of geographic regions where green roof installation is possible with good hydrological performance, and feasible in rainfed conditions with low vegetation water stress. To do that, we explored in silico green roofs' hydrological performance worldwide, using climatological forcings from ERA‐Interim database as input for a simple conceptual hydrological model to estimate green roof outputs, in terms of evapotranspiration and runoff.

Using a Hydrological Model to Simulate the Performance and Estimate the Runoff Coefficient of Green Roofs in Semiarid Climates

Green roofs offer a series of benefits to buildings and to the urban environment. Their use in dry climates requires optimizing the choice of their components (i.e., vegetation, substrate and drainage layer) for the specific local climatic conditions, in order to minimize irrigations needs while preserving the attributes of the roof. In this study, we calibrated and validated an existing hydrological model—IHMORS—for the simulation of the hydrological performance of green roofs. Simulated results were compared to experimental data obtained in an outdoor test facility on several green roof specimens, representing a variety of green roofs configurations. IHMORS was able to reasonably predict the soil moisture dynamics for all tested specimens. The specimens of 10 cm depth were the best simulated by the model, while some overestimation was observed during the model validation for the 5 and 20 cm depth specimens. The model was then used to estimate the number of days in which irrigation is needed, as well as analyze the water runoff control performance of all specimens. We related the amount of water retained by the substrate and depth, magnitude and intensity of precipitation event, and the initial substrate moisture. For all events, the lowest runoff coefficient was simulated for the 20 cm specimens. Our study showed the full potential of the model for estimating the water needs and the runoff control performances of different variants of green roofs.