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Special Issue "Hydrological Performance of Green Roofs"

A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Urban Water Management".

Deadline for manuscript submissions: closed (1 June 2018).

Special Issue Editors

Guest Editor
Dr. Ir. Klaas Metselaar

Assistant professor, Department of Environmental Sciences, Wageningen University
Website | E-Mail
Phone: 0031317485322
Interests: ecohydrology; soil physics; urban hydrometeorology; water quantity; water quality
Guest Editor
Dipl. Geoecol. Michael Richter

Environmentally Sound Urban and Infrastrucure Planning, HafenCity University, Hamburg, Germany
Website | E-Mail
Phone: +49 40 42827-5222
Interests: urban hydrology and ecology; urban climate change adaptation; sustainable urban drainage systems; stormwater management; nature-based solutions to climate change in urban areas; urban green infrastructures
Guest Editor
Dr. Ir. Petra Van den Berg

Managing Director, Netherlands Institute of Ecology (NIOO-KNAW)
Website | E-Mail
Phone: 0031317473566
Interests: green roofs; ecotechnology; sustainable construction and management; technology transfer; biophysics

Special Issue Information

Dear Colleagues,

Urban hydrology is characterized by short lag times, low peak reduction, and low evapotranspirative losses. Whereas the hydrological problems arising in cities were initially and easily transported toward the city boundaries, there is an increasing need to formulate persistent solutions "on-site", which challenges creativity in terms of efficient use of space, but also in terms of governance. Given that climate change is expected to lead to heavier and more frequent rainstorms, the process of city densification requires strenghtening the adaptive capacity of the built environment and stimulating the search for new means of closing water cycles and increasing retention times. One such solution in temperate climates is green roofs. It now seems generally accepted that green roofs successfully retain small rainfall events, but that for the less well-studied large rainfall events green roofs need to be designed for use in combination with other measures, or to be redesigned to allow for more storage of water (so-called blue-green roofs). At the same time, green roof design criteria, such as the substrate conductivity and retention need to be reconsidered to accomodate the objective of retaining and slowly releasing water. An open question is the effect of the type of roof vegetation on the storage emptying time and water quality. Roof water management may influence vegetation and the potential of roofs to contribute to biodiversity. Slowing down the urban water cycle may have consequences on the water quality of the urban open water ecosystems. Up-scaling of blue-green roofs for a synergistic impact on the scale of cities is a challenge in terms of technology as well as—again—governance. This Special Issue aims to present to what extent knowledge on blue-green roofs is available; to what extent model and/or engineering approaches are available, and to what extent experimental evidence supports and illustrates these changing requirements.

Dr. Ir. Klaas Metselaar
Dipl. Geoecol. Michael Richter
Dr. Ir. Petra van den Berg
Guest Editors

Manuscript Submission Information

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Keywords

  • (decentralised) stormwater management

  • extreme events

  • substrate properties

  • roof construction

  • attenuation

  • flow modeling, urban development

  • hydrologic performance

  • low-impact development

  • urban drainage

  • water sensitive urban design

  • urban ecology

Published Papers (10 papers)

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Research

Open AccessArticle
Monitoring and Modeling the Long-Term Rainfall-Runoff Response of the Jacob K. Javits Center Green Roof
Water 2018, 10(11), 1494; https://doi.org/10.3390/w10111494
Received: 15 June 2018 / Revised: 16 August 2018 / Accepted: 25 September 2018 / Published: 23 October 2018
Cited by 1 | PDF Full-text (5547 KB) | HTML Full-text | XML Full-text
Abstract
Drainage from the 27,316-m2 Jacob K. Javits Convention Center (JJCC) green roof was investigated in the field to quantify the system’s long-term rainfall-runoff response. The JJCC hosts one of the largest extensive green roofs in the United States. Utilizing four years of [...] Read more.
Drainage from the 27,316-m2 Jacob K. Javits Convention Center (JJCC) green roof was investigated in the field to quantify the system’s long-term rainfall-runoff response. The JJCC hosts one of the largest extensive green roofs in the United States. Utilizing four years of rooftop monitoring data collected using a weather station, custom designed and built drainage systems, three Parshall flumes equipped with pressure transducers, and weighing lysimeters, this study quantified the 25.4-mm-deep green roof’s ability to decrease the volume and peak rate of runoff. With parameters derived from the site, the Environmental Protection Agency Stormwater Management Model (EPA-SWMM) predicted event total runoff volume and event peak runoff rates to within +10% to −20% and +25% to −15% of the observations, respectively. The analysis further indicated that approximately 55% of the cumulative precipitation that fell on the JJCC extensive green roof during the monitoring period (warm weather months, June 2014–November 2017) was captured and retained. The average percent retained on an event-basis was 77%, and average event runoff coefficient was 0.7, implying a substantial reduction in the volume and rate of runoff generated from the roof compared to the pre-green roof condition, when most, if not all, of the precipitated water would have immediately resulted in runoff. Our research suggests that, on average, 96% of rainfall events 6.35 mm or less were retained within the green roof, whereas 27% of the total event volume was retained for events greater than 12.7 mm in depth. A sensitivity analysis suggests if the substrate depth were increased, better stormwater capture performance would be achieved, but only up 127 mm, whereas increased precipitation coupled with warmer temperatures as a result of climate change could decrease the performance by up to 5%, regardless of substrate depth. An equivalency analysis suggested that even shallow green roofs can significantly reduce the required stormwater detention volume that New York City requires on new development. This particular green roof appears to be more than 18 times as cost-effective as a subsurface cistern would be for managing an equivalent volume of stormwater in Midtown Manhattan. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessArticle
Evaporation from (Blue-)Green Roofs: Assessing the Benefits of a Storage and Capillary Irrigation System Based on Measurements and Modeling
Water 2018, 10(9), 1253; https://doi.org/10.3390/w10091253
Received: 12 July 2018 / Revised: 27 August 2018 / Accepted: 11 September 2018 / Published: 14 September 2018
Cited by 1 | PDF Full-text (5312 KB) | HTML Full-text | XML Full-text
Abstract
Worldwide cities are facing increasing temperatures due to climate change and increasing urban density. Green roofs are promoted as a climate adaptation measure to lower air temperatures and improve comfort in urban areas, especially during intensive dry and warm spells. However, there is [...] Read more.
Worldwide cities are facing increasing temperatures due to climate change and increasing urban density. Green roofs are promoted as a climate adaptation measure to lower air temperatures and improve comfort in urban areas, especially during intensive dry and warm spells. However, there is much debate on the effectiveness of this measure, because of a lack of fundamental knowledge about evaporation from different green roof systems. In this study, we investigate the water and energy balance of different roof types on a rooftop in Amsterdam, the Netherlands. Based on lysimeter measurements and modeling, we compared the water and energy balance of a conventional green roof with blue-green roofs equipped with a novel storage and capillary irrigation system. The roofs were covered either with Sedum or by grasses and herbs. Our measurements and modeling showed that conventional green roof systems (i.e., a Sedum cover and a few centimeters of substrate) have a low evaporation rate and due to a rapid decline in available moisture, a minor cooling effect. Roofs equipped with a storage and capillary irrigation system showed a remarkably large evaporation rate for Sedum species behaving as C3 plants during hot, dry periods. Covered with grasses and herbs, the evaporation rate was even larger. Precipitation storage and capillary irrigation strongly reduced the number of days with dry-out events. Implementing these systems therefore could lead to better cooling efficiencies in cities. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessArticle
Assessing the Hydrologic Performance of a Green Roof Retrofitting Scenario for a Small Urban Catchment
Water 2018, 10(8), 1052; https://doi.org/10.3390/w10081052
Received: 11 July 2018 / Revised: 2 August 2018 / Accepted: 6 August 2018 / Published: 8 August 2018
Cited by 2 | PDF Full-text (1177 KB) | HTML Full-text | XML Full-text
Abstract
In an existing urban environment, retrofitting low impact development (LID) solutions can provide an opportunity to address flooding and water quality problems. Taking into account the need to effectively estimate the impact of vegetated LIDs, particular attention has recently been given on the [...] Read more.
In an existing urban environment, retrofitting low impact development (LID) solutions can provide an opportunity to address flooding and water quality problems. Taking into account the need to effectively estimate the impact of vegetated LIDs, particular attention has recently been given on the evapotranspiration (ET) process that is responsible for the restoring of green roof water-holding capacity. The present study aims to develop a methodological approach to estimate the actual ET as climate input data in the United States Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) continuous simulation. The proposed approach is calibrated on a single green roof installation based on one-minute continuous simulations over 26 years of climate records. Then the calibrated methodological approach has been implemented to perform continuous simulation of a small urban catchment retrofitted with green roofs. Based on simulation results, the peak and volume reduction rate evaluated for the 1433 rainfall events are equal to 0.3 on average (with maximum values of 0.96 for peak and 0.86 for volume). In general, the adopted methodology indicates that the actual ET estimate is needed to suitably assess the hydrologic performance of vegetated LIDs mainly concerning the volume reduction index; furthermore, the methodology can be easily replicated for other vegetated LID applications. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessArticle
Effect of Green Roof Configuration and Hydrological Variables on Runoff Water Quantity and Quality
Water 2018, 10(7), 960; https://doi.org/10.3390/w10070960
Received: 2 June 2018 / Revised: 29 June 2018 / Accepted: 3 July 2018 / Published: 20 July 2018
Cited by 2 | PDF Full-text (3995 KB) | HTML Full-text | XML Full-text
Abstract
Green roofs (GRs) are a feasible solution for mitigating increased runoff volumes in urban areas. Though many studies have focused their analysis on the quantity and quality of GR runoff, with respect to the relevance of specific site conditions in GR performance, the [...] Read more.
Green roofs (GRs) are a feasible solution for mitigating increased runoff volumes in urban areas. Though many studies have focused their analysis on the quantity and quality of GR runoff, with respect to the relevance of specific site conditions in GR performance, the information gathered for the tropical Andes is not sufficient. This study assessed the hydrological performance and runoff water quality of 12 green roof modular systems located at the Universidad de los Andes campus (Bogotá, Colombia). Based on 223 rainfall events spanning a 3-year period, average rainfall retention was 85% (coefficient of variation = 29%). t-tests, the Welch Test, multiple linear regressions, and correlation analysis were performed in order to assess the potential effect of air temperature, substrate type, vegetation cover, relative humidity, antecedent dry weather period (ADWP), rainfall duration, and rainfall maximum intensity. In some cases, GR design variables (i.e., substrate type and vegetation cover) were found to be significant for describing rainfall retention efficiencies and, depending on the GR type, some hydrological variables were also correlated with rainfall retention. Rainfall and GR runoff from 12 rainfall events were also monitored for total Kjeldahl nitrogen (TKN), nitrates, nitrites, ammonia, total phosphorus (TP), phosphates, pH, total dissolved solids (TDS), total suspended solids (TSS), color, turbidity, biological oxygen demand (BOD), chemical oxygen demand (COD), total coliforms, metals (i.e., zinc, copper, nickel, lead, selenium, aluminum, barium, boron, calcium, strontium, iron, lithium, magnesium, manganese, potassium, sodium), and polyaromatic hydrocarbons (PAHs). The results obtained confirmed that GR systems have the ability to neutralize pH, but are a source of the rest of the aforementioned parameters, excluding PAHs (with concentrations below detection limits), ammonia, TSS, selenium and lithium, where differences with control cases (rainfall and plastic panel runoff) were not statistically significant. Substrate type, event size, and rainfall regime are relevant variables for explaining runoff water quality. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessFeature PaperArticle
A Longitudinal Microcosm Study on the Effects of Ageing on Potential Green Roof Hydrological Performance
Water 2018, 10(6), 784; https://doi.org/10.3390/w10060784
Received: 21 May 2018 / Revised: 5 June 2018 / Accepted: 11 June 2018 / Published: 14 June 2018
Cited by 2 | PDF Full-text (5508 KB) | HTML Full-text | XML Full-text
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessArticle
Detention and Retention Behavior of Four Extensive Green Roofs in Three Nordic Climate Zones
Water 2018, 10(6), 671; https://doi.org/10.3390/w10060671
Received: 26 April 2018 / Revised: 11 May 2018 / Accepted: 18 May 2018 / Published: 23 May 2018
Cited by 6 | PDF Full-text (2690 KB) | HTML Full-text | XML Full-text
Abstract
Climate change coupled with increasing urbanization has made extensive green roofs, both for retrofitting and new developments, an attractive way to bring nature back to cities, while managing stormwater. This study has investigated extensive green roof retention and detention performance based on 3–8 [...] Read more.
Climate change coupled with increasing urbanization has made extensive green roofs, both for retrofitting and new developments, an attractive way to bring nature back to cities, while managing stormwater. This study has investigated extensive green roof retention and detention performance based on 3–8 years of field data from four Norwegian locations representing typical cold and wet Nordic climates, also comparing several different commercial configurations. Accumulated retention was found to be 11–30% annually and 22–46% in May through October. The performance was found to be strongly dependent in evapotranspiration and less dependent on material storage capacities. Estimates for available storage capacities for precipitation events larger than 5 mm are given and can be useful for design purposes. Median observed peak attenuation compared to the precipitation ranged from 65–90% depending on locations and configurations. The event-based approach for evaluating detention was found to be challenging due to the nature of the precipitation in the studied locations. An alternative approach using flow duration curves based on the observed time series was tested and found to give valuable information on runoff patterns from green roofs and to be useful for evaluating green roof performance in relation to local requirements. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessArticle
Observation and Estimation of Evapotranspiration from an Irrigated Green Roof in a Rain-Scarce Environment
Water 2018, 10(3), 262; https://doi.org/10.3390/w10030262
Received: 19 December 2017 / Revised: 17 February 2018 / Accepted: 26 February 2018 / Published: 2 March 2018
Cited by 4 | PDF Full-text (9754 KB) | HTML Full-text | XML Full-text
Abstract
While the rain-driven evapotranspiration (ET) process has been well-studied in the humid climate, the mixed irrigation and rain-driven ET process is less understood for green roof implementations in dry regions, where empirical observations and model parameterizations are lacking. This paper presents an effort [...] Read more.
While the rain-driven evapotranspiration (ET) process has been well-studied in the humid climate, the mixed irrigation and rain-driven ET process is less understood for green roof implementations in dry regions, where empirical observations and model parameterizations are lacking. This paper presents an effort of monitoring and simulating the ET process for an irrigated green roof in a rain-scarce environment. Annual ET rates for three weighing lysimeter test units with non-vegetated, sedums, and grass covers were 2.01, 2.52, and 2.69 mm d−1, respectively. Simulations based on the three Penman–Monteith equation-derived models achieved accuracy within the reported range of previous studies. Compared to the humid climate, the overestimation of high ET rates by existing models is expected to cause a larger error in dry environments, where the enhanced ET process caused by repeated irrigations overlapped with hot, dry conditions often occurs during summer. The studied sedum species did not show significantly lower ET rates than native species, and could not effectively take advantage of the deep moisture storage. Therefore, native species, instead of the shallow-rooted species commonly recommended in humid climates, might be a better choice for green roofs in rain-scarce environments. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessArticle
Using a Hydrological Model to Simulate the Performance and Estimate the Runoff Coefficient of Green Roofs in Semiarid Climates
Water 2018, 10(2), 198; https://doi.org/10.3390/w10020198
Received: 30 December 2017 / Revised: 2 February 2018 / Accepted: 9 February 2018 / Published: 13 February 2018
Cited by 9 | PDF Full-text (2643 KB) | HTML Full-text | XML Full-text
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessArticle
Laboratory Tests of Substrate Physical Properties May Not Represent the Retention Capacity of Green Roof Substrates In Situ
Water 2017, 9(12), 920; https://doi.org/10.3390/w9120920
Received: 29 September 2017 / Revised: 22 November 2017 / Accepted: 23 November 2017 / Published: 27 November 2017
Cited by 2 | PDF Full-text (3692 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Green roofs can be used to reduce the volume of polluted stormwater that is generated by cities. Modelling rainfall retention is critical, but green roof water balance models often rely on the physical properties of substrates. In these models, substrate water holding capacity [...] Read more.
Green roofs can be used to reduce the volume of polluted stormwater that is generated by cities. Modelling rainfall retention is critical, but green roof water balance models often rely on the physical properties of substrates. In these models, substrate water holding capacity (WHC) determines the depth of water which can be stored before runoff is generated; whereas, the permanent wilting point (PWP) limits evapotranspiration. The WHC and PWP, as well as plant available water (PAW; where PAW = WHCPWP), as determined from laboratory tests, may not truly reflect how substrates perform on green roofs. We therefore ran a simulated rainfall experiment on green roof modules to (i) compare the rainfall retention of vegetated and non-vegetated substrates with different WHC and PAW, and (ii) relate retention to substrate storage capacity, as calculated from laboratory measures of WHC and PAW. We found that the PAW of a substrate is a better indicator of evapotranspiration and retention when compared with WHC. However, we also found that substrates always retained less water than their calculated storage capacity would suggest, most likely being due to their high permeability. Our results indicate that using laboratory-derived measures of WHC and PAW in green roof models may be over-estimating both evapotranspiration and rainfall retention. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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Open AccessFeature PaperArticle
Substrate Composition and Depth Affect Soil Moisture Behavior and Plant-Soil Relationship on Mediterranean Extensive Green Roofs
Water 2017, 9(11), 817; https://doi.org/10.3390/w9110817
Received: 31 August 2017 / Revised: 16 October 2017 / Accepted: 19 October 2017 / Published: 25 October 2017
Cited by 2 | PDF Full-text (3131 KB) | HTML Full-text | XML Full-text
Abstract
The Mediterranean basin is extremely vulnerable to climate change, and one of the areas most impacted by human water demand. Yet the green roofs increasingly created both for aesthetic reasons and to limit pollution and urban runoff are themselves very water-demanding. Successful green [...] Read more.
The Mediterranean basin is extremely vulnerable to climate change, and one of the areas most impacted by human water demand. Yet the green roofs increasingly created both for aesthetic reasons and to limit pollution and urban runoff are themselves very water-demanding. Successful green roof installation depends on the establishment of the vegetation, and the substrate is the key element: it conserves water, and provides the nutrients and physical support indispensable for plant growth. Since typical Mediterranean plant communities require no maintenance, this study seeks to develop techniques for creating maintenance- and watering-free horizontal green roofs for public or private buildings in a Mediterranean context. The innovative aspect of this study lies in creating two soil mixes, fine elements (clay and silt) and coarse elements (pebbles of all sizes), in two different thicknesses, to assess vegetation development. Monitoring of substrate moisture was carried out and coupled with local rainfall measurements during summer and autumn. As expected, substrate moisture is mainly influenced by substrate depth (the deeper, the moister) and composition (the finer the particles (clays and silts), the higher the moisture content). Vegetation cover impacts moisture to a lesser extent but is itself affected by the composition and depth of the substrates. These results are subsequently discussed with relation to the issue of sustainable green roofs in Mediterranean climates. Considering applications of our results, for an optimal colonization of a Mediterranean vegetation, a substrate thickness of 15 cm composed mainly of fine elements (75% clay-silt and 25% pebble-sand) would be recommended in green roofs. Full article
(This article belongs to the Special Issue Hydrological Performance of Green Roofs)
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