Caractérisation du fonctionnement thermo-hydrique in situ d'une toiture végétalisée extensive

International audienceThere are several issues related to the development of green roofs : a better understanding and estimation of their thermic and hydric performances as well as the strong necessity to develop innovations. This study is primarily based on the monitoring of a large-scale in situ green-roof that instrumented with temperature sensors, capacitive tensi-ometers and dielectric sensors. This experimental device was completed by a weather station monitoring few microcli-matic parameters (ambient temperature, humidity, wind speed and direction). The analysis of the results has lead to a better understanding of the seasonal behavior of the extensive green roof (EGR). Indeed, EGR contribution for thermal insulation has been estimated and reached maximum reduction of temperature of 24°C in summer and a temperature gain of 5°C in winter. In spring, the EGR succeeded in storing almost the whole rainfall. The substrate appeared to play an important role on the coupled thermo-hydric performances of the EGR and needed further characterization.Il existe aujourd'hui plusieurs enjeux liés au développement des toitures végétalisées à la fois pour une meilleure connaissance et évaluation de leurs propriétés (e.g. performance énergétique des bâtiments équipés, rétention en eau), mais aussi sur un réel besoin d'innovation. Ces travaux s'appuient sur le monitoring d'une toiture végétalisée équipée de grande taille, mise en place sur un bâtiment. Elle est instrumentée à l'aide de capteurs de température, de tensiomètres capacitifs et de sondes diélectriques. Ce dispositif expérimental est complété d'une station météorologique mesurant les paramètres microclimatiques (température extérieure, hygrométrie, vitesse et direction du vent). L'analyse des résultats des expériences in situ ont permis de dégager des résultats sur le comportement saisonnier de la toiture végétalisée (TVE). En effet, il est possible d'estimer la contribution de ce type de système à l'isolation thermique dans des conditions climatiques extérieures qui varient en hiver avec un gain de 5°C et en été avec une réduction de 24°C. Le suivi des données a aussi démontré qu'au printemps la TVE pouvait stocker l'essentiel des précipitations. La contribution forte du substrat au fonctionnement thermo-hydrique est ensuite évoquée ainsi que la nécessité de mieux caractériser ce couplage

How to evaluate nature-based solutions performance for microclimate, water and soil management issues – Available tools and methods from Nature4Cities European project results

International audienceIn the context of climate change, Nature-Based Solutions (NBSs), a recently developed concept, are increasingly considered as part of the adaptation strategies of the cities. Studies using expert models and methods (EMM) receive a great deal of scientific attention. Considering EMM increasing use, this study aims to perform an analysis of the reported evaluation results, reflecting the capability of the EMM to accurately tackle urban challenges identified within the EU Nature4Cities project. Then, we propose a set of indicators and recommendations about sixteen EMM to be used by funders, researchers and practitioners when evaluating the performance of NBSs. The coupling of the different components (climate, water and soil) is not a simple matter. The analysis relies on the definition of the range of the reported metrics and on the investigation of the relationship between the various indices, applied for the EMM evaluation. Secondly, the study assesses the existing EMM, indicating the potential of NBSs: (i) to reduce urban heat island, (ii) to limit surface warming, (iii) to increase the thermal comfort of people, (iv) to limit the overheating and runoff of surfaces due to impervious areas, (v) to increase water retention during stormy episodes, (vi) to improve storm water quality at the outlet of the sustainable urban drainage systems, (vii) to promote the filtration and epuration of storm water runoff in soil and (viii) to be a support for vegetation. The analysis reveals that EMM can be considered as helpful tools for urban microclimate, urban soil and water management analysis, provided their limitations and characteristics are taken into account by the user when choosing tools and interpreting results (e.g. application scale). With regard to the performance of NBSs, the most commonly used indicators clearly depend on the scale of the project

Experimental Comparative Study between Conventional and Green Parking Lots: Analysis of Subsurface Thermal Behavior under Warm and Dry Summer Conditions

Green infrastructure has a role to play in climate change adaptation strategies in cities. Alternative urban spaces should be designed considering new requirements in terms of urban microclimate and thermal comfort. Pervious pavements such as green parking lots can contribute to this goal through solar evaporative cooling. However, the cooling benefits of such systems remain under debate during dry and warm periods. The aim of this study was to compare experimentally the thermal behavior of different parking lot types (PLTs) with vegetated urban soil. Four parking lots were instrumented, with temperature probes buried at different depths. Underground temperatures were measured during summer 2019, and the hottest days of the period were analyzed. Results show that the less mineral used in the surface coating, the less it warms up. The temperature difference at the upper layer can reach 10 °C between mineral and non-mineral PLTs. PLTs can be grouped into three types: (i) high surface temperature during daytime and nighttime, important heat transfer toward the sublayers, and low time shift (asphalt system); (ii) high (resp. low) surface temperature during daytime (resp. nighttime), weak heat transfer toward the sublayers, and important time shift (paved stone system); and (iii) low surface temperature during daytime and nighttime, weak heat transfer toward the sublayers, and important time shift (vegetation and substrate system, wood chips system, vegetated urban soil). The results of this study underline that pervious pavements demonstrate thermal benefits under warm and dry summer conditions compared to conventional parking lot solutions. The results also indicate that the hygrothermal properties of urban materials are crucial for urban heat island mitigation

Fonctionnement hydrique d'un Technosol superficiel - application à une toiture végétalisée

The sealing in cities highly degrades the buffer and filter functions of soils which generates and/or emphasizes major environmental issues (e.g. urban heat island, flooding, pollution of the runoff water). Among other technologies, advances in green roof engineering provide solutions for the management of urban rainwater. Indeed, green roofs can highly contribute to water regulation service by delaying run-off peaks and decreasing water fluxes to storm water collection network. The purpose of this work is to quantify and model the hydric performances of such an urban Technosol by taking into account the seasonal variations and the aging of the green roof. Physic and hydric measurements were conducted on the green roof constituents. Then, water fluxes and meteorological parameters were monitored in four green roofs parcels – including two with an innovative water storage structure – both at the lab and the building scales. Finally, the hydrodynamics of green roofs was modeled and numerically investigated with HYDRUS-1D in the framework of the Richards equations and the van Genuchten-Mualem model that describe unsaturated flows. As a result: i) the water flows inside these complex porous media were physically characterized, ii) the hydric performances of different parcels over three years, under Lorraine climate, were evaluated, iii) the model approach reached to a good description of the hydraulic behavior at the lab-scale but tends to underestimate in situ water fluxes. Beyond that, this work can provide a robust approach to simulate water transfer in green roofs under different climates or situations and may also contribute to further technological developmentL’imperméabilisation des sols en ville génère des problématiques aigües au niveau du cycle de l’eau urbaine : dégradation de la qualité des eaux de ruissellement, saturation des réseaux de collecte, risque d’inondation. Parmi différentes solutions, la construction de toitures végétalisées offre de nouvelles perspectives dans la gestion de ces eaux pluviales urbaines. De telles structures jouent en effet un rôle de régulation hydrique en retardant les pics de débit lors des pluies d’orage et plus globalement en diminuant les flux envoyés vers les réseaux. L’objectif de cette thèse est de quantifier et de modéliser les performances hydriques ce type de Technosol urbain, en intégrant à la fois les variations saisonnières et le vieillissement de la toiture végétalisée. Le travail repose en premier lieu sur une caractérisation physique et hydrique des constituants des toitures, à travers une démarche pour partie originale de transposition des méthodes développées sur les sols. Ensuite, un suivi expérimental (monitoring des flux et paramètres météorologiques) de quatre modalités de toitures – dont deux équipées d’une structure innovante de stockage d’eau – a été effectué à deux échelles : le laboratoire et le bâtiment. La modélisation et la simulation numérique du transport de l’eau a enfin été effectuée à l’aide du logiciel HYDRUS-1D, avec le formalisme des équations de Richards qui décrivent le transfert en conditions insaturées et la résolution de van Genuchten-Mualem. Les recherches ont permis de caractériser, sur une base physique robuste, les écoulements au sein de ces milieux poreux complexes. Une estimation des performances de différentes modalités de toitures au cours de trois années climatiques est proposée en contexte climatique Lorrain. La démarche de modélisation permet de décrire fidèlement les transferts à l’échelle du laboratoire mais tend à sous-estimer les flux in situ. À plus long terme, ces travaux permettent d’envisager aussi bien la simulation du comportement de toitures végétalisées sous d’autres climats, que des développements technologiques basées sur des nouvelles associations de constituants

Hydraulic behavior of a shallow Technosol - green roof application

L’imperméabilisation des sols en ville génère des problématiques aigües au niveau du cycle de l’eau urbaine : dégradation de la qualité des eaux de ruissellement, saturation des réseaux de collecte, risque d’inondation. Parmi différentes solutions, la construction de toitures végétalisées offre de nouvelles perspectives dans la gestion de ces eaux pluviales urbaines. De telles structures jouent en effet un rôle de régulation hydrique en retardant les pics de débit lors des pluies d’orage et plus globalement en diminuant les flux envoyés vers les réseaux. L’objectif de cette thèse est de quantifier et de modéliser les performances hydriques ce type de Technosol urbain, en intégrant à la fois les variations saisonnières et le vieillissement de la toiture végétalisée. Le travail repose en premier lieu sur une caractérisation physique et hydrique des constituants des toitures, à travers une démarche pour partie originale de transposition des méthodes développées sur les sols. Ensuite, un suivi expérimental (monitoring des flux et paramètres météorologiques) de quatre modalités de toitures – dont deux équipées d’une structure innovante de stockage d’eau – a été effectué à deux échelles : le laboratoire et le bâtiment. La modélisation et la simulation numérique du transport de l’eau a enfin été effectuée à l’aide du logiciel HYDRUS-1D, avec le formalisme des équations de Richards qui décrivent le transfert en conditions insaturées et la résolution de van Genuchten-Mualem. Les recherches ont permis de caractériser, sur une base physique robuste, les écoulements au sein de ces milieux poreux complexes. Une estimation des performances de différentes modalités de toitures au cours de trois années climatiques est proposée en contexte climatique Lorrain. La démarche de modélisation permet de décrire fidèlement les transferts à l’échelle du laboratoire mais tend à sous-estimer les flux in situ. À plus long terme, ces travaux permettent d’envisager aussi bien la simulation du comportement de toitures végétalisées sous d’autres climats, que des développements technologiques basées sur des nouvelles associations de constituants.The sealing in cities highly degrades the buffer and filter functions of soils which generates and/or emphasizes major environmental issues (e.g. urban heat island, flooding, pollution of the runoff water). Among other technologies, advances in green roof engineering provide solutions for the management of urban rainwater. Indeed, green roofs can highly contribute to water regulation service by delaying run-off peaks and decreasing water fluxes to storm water collection network. The purpose of this work is to quantify and model the hydric performances of such an urban Technosol by taking into account the seasonal variations and the aging of the green roof. Physic and hydric measurements were conducted on the green roof constituents. Then, water fluxes and meteorological parameters were monitored in four green roofs parcels – including two with an innovative water storage structure – both at the lab and the building scales. Finally, the hydrodynamics of green roofs was modeled and numerically investigated with HYDRUS-1D in the framework of the Richards equations and the van Genuchten-Mualem model that describe unsaturated flows. As a result: i) the water flows inside these complex porous media were physically characterized, ii) the hydric performances of different parcels over three years, under Lorraine climate, were evaluated, iii) the model approach reached to a good description of the hydraulic behavior at the lab-scale but tends to underestimate in situ water fluxes. Beyond that, this work can provide a robust approach to simulate water transfer in green roofs under different climates or situations and may also contribute to further technological developmen

Hydraulic behavior of a shallow Technosol - green roof application

L’imperméabilisation des sols en ville génère des problématiques aigües au niveau du cycle de l’eau urbaine : dégradation de la qualité des eaux de ruissellement, saturation des réseaux de collecte, risque d’inondation. Parmi différentes solutions, la construction de toitures végétalisées offre de nouvelles perspectives dans la gestion de ces eaux pluviales urbaines. De telles structures jouent en effet un rôle de régulation hydrique en retardant les pics de débit lors des pluies d’orage et plus globalement en diminuant les flux envoyés vers les réseaux. L’objectif de cette thèse est de quantifier et de modéliser les performances hydriques ce type de Technosol urbain, en intégrant à la fois les variations saisonnières et le vieillissement de la toiture végétalisée. Le travail repose en premier lieu sur une caractérisation physique et hydrique des constituants des toitures, à travers une démarche pour partie originale de transposition des méthodes développées sur les sols. Ensuite, un suivi expérimental (monitoring des flux et paramètres météorologiques) de quatre modalités de toitures – dont deux équipées d’une structure innovante de stockage d’eau – a été effectué à deux échelles : le laboratoire et le bâtiment. La modélisation et la simulation numérique du transport de l’eau a enfin été effectuée à l’aide du logiciel HYDRUS-1D, avec le formalisme des équations de Richards qui décrivent le transfert en conditions insaturées et la résolution de van Genuchten-Mualem. Les recherches ont permis de caractériser, sur une base physique robuste, les écoulements au sein de ces milieux poreux complexes. Une estimation des performances de différentes modalités de toitures au cours de trois années climatiques est proposée en contexte climatique Lorrain. La démarche de modélisation permet de décrire fidèlement les transferts à l’échelle du laboratoire mais tend à sous-estimer les flux in situ. À plus long terme, ces travaux permettent d’envisager aussi bien la simulation du comportement de toitures végétalisées sous d’autres climats, que des développements technologiques basées sur des nouvelles associations de constituants.The sealing in cities highly degrades the buffer and filter functions of soils which generates and/or emphasizes major environmental issues (e.g. urban heat island, flooding, pollution of the runoff water). Among other technologies, advances in green roof engineering provide solutions for the management of urban rainwater. Indeed, green roofs can highly contribute to water regulation service by delaying run-off peaks and decreasing water fluxes to storm water collection network. The purpose of this work is to quantify and model the hydric performances of such an urban Technosol by taking into account the seasonal variations and the aging of the green roof. Physic and hydric measurements were conducted on the green roof constituents. Then, water fluxes and meteorological parameters were monitored in four green roofs parcels – including two with an innovative water storage structure – both at the lab and the building scales. Finally, the hydrodynamics of green roofs was modeled and numerically investigated with HYDRUS-1D in the framework of the Richards equations and the van Genuchten-Mualem model that describe unsaturated flows. As a result: i) the water flows inside these complex porous media were physically characterized, ii) the hydric performances of different parcels over three years, under Lorraine climate, were evaluated, iii) the model approach reached to a good description of the hydraulic behavior at the lab-scale but tends to underestimate in situ water fluxes. Beyond that, this work can provide a robust approach to simulate water transfer in green roofs under different climates or situations and may also contribute to further technological developmen

How lysimetric facility can contribute to monitor Technosols dynamics.

International audienceThe dynamic of water in soils is mainly controlled by a set of hydraulic properties that are characteristic of each type of soil and that reflect the architecture – more generally defined as soil structure - of such a specific porous medium. Structural changes are induced by external factors (e.g. climate, biology, human action) and are the result of pedogenetic processes that modify the solid phase and redistribute ions and particles. Consequently, changes in the poral volume and in the size and the connectivity of soil pores are observed that significantly influence regulating ecosystem services that can be provided. The temporal and spatial dynamics of these properties is complex to highlight and poorly studied, especially as the soil processes in natural soils are slow at human timescales. To question this crucial issue, we chose to focus our study on the dynamics of Technosols porosity as a result of seasonal climatic variations, vegetation and early pedogenic evolution – which kinetic is known to be much faster - (Lin, 2011; Séré et al., 2012). Our purpose is then to develop an original approach to characterize, in a continuous way, the evolution of soil’s structure. To do so, a natural soil and SUITMAs - from a Luvisol to a Spolic Garbic Technosol (Histic) -, within an anthropization gradient, have been studied. They have been studied under two treatments (with or without vegetation) in monitored 2 m3 lysimetric columns over a 3 to 6 years’ time sequence. Water balances have been performed as well as the monitoring of water transfer at different depths. Experimental data have been compared to a modelling approach that relied on the use of Hydrus 1D (Simunek et al., 2008). The results exhibit contrasted hydraulic behaviors that are mainly correlated to the age of the soils and the level of human influence. Only cyclic variations – for example on the amount of water that is stored (Figure) - were visible on natural and slightly anthropogenic soils that were attributed to seasonal factors (e.g. climate and vegetation). In addition to that cyclic changes, more drastic acyclic evolutions were observed on the Technosols that demonstrated their significant settlement and an evolution of the porosity due to their early pedogenesis (Figure). An inverse modelling approach led to the estimation of hydraulic parameters that confirmed that findings by highlighting an evolution of poral architecture with time

Experimental methods for estimating green roof evapotranspiration

International audienceGreen roofs (GR) are well­known for their hydrological performances. Their abilities to retain stormwater are mainly influenced by the initial water content of the substrate prior to a rain event. The evolution in time of the water content is driven by the evapotranspiration (ET) process. The main objectives of this study is to compare three green roof configurations on the ET flux. GRs are instrumented in order to measure thermal, hydrological are climatic variables. All measurements are done over one year. Our work focused on three methods: i) ET is measured by an ET chamber, developed by the Cerema, ii) ET is obtained from the residual calculation of the hydrological balance, iii) a thermal balance is solved to calculate ET. Results show that hourly ET measurements give a good estimation. During summer, ET values are up to 250W/m² while it is less than 10 W/m² in autumn. The values of evapotranspiration are strongly influenced by the LAI, albedo and emissivity of the vegetation. The local climate also affects ET. This study highlights the ability of GRs to participate in urban comfort at the building­scale and to mitigate urban heat island at urban scale

Use of a high capacity retention structure to improve extensive green roof rainwater discharge, World Green Infrastructures

International audienc

Green roof aging or Isolatic Technosol’s pedogenesis? Impact on hydrologic performances.

There is an urgent need to improve the water cycle balance in cities. Urbanization leads to sealed surfaces that are poorly covered with vegetation; consequently the amount of water that can infiltrate soils is limited compared to rural area (Lazzarin et al., 2005). Thus, run-off peaks during major rain events lead to high water volumes released in urban areas that require adapted strategies to avoid flooding. Among them, green roofs (GR) are now suggested to be one of the potential solutions for water management as this technique could provide pervious surfaces that contribute to storm water management (Mentens et al., 2006). In this framework, GR could be used as a control device which can store and release the rainwater with a delay. These hydrologic performances depend especially on the substrates properties (i.e. thickness, characteristics and proportion of its organic and mineral components) (Berndtsson, 2010). Up to now, it has been demonstrated that the averaged water storage capacity was about 40 to 80% of the total annual rainfall volume (Carter and Jackson, 2007; Moran and Smith, 2005). Apart from that, a significant evolution with time of the poral architecture of the substrates can be expected that would lead to changes in their hydraulic properties (Kutilek, 2004). This topic was yet poorly addressed by the GR scientific community. Mentens et al. mentioned, in their review (Mentens et al., 2006), the lack of influence of the age of green roofs on their hydrological performance. On the contrary, a study on a 5-years-old substrate showed that the water holding capacity has increased compared to a new one (Getter et al., 2007) and could be linked with the abundance and connectivity of the micropores and macropores. Evidences of the evolution with time of the composition (lower pH, higher organic carbon and total nitrogen contents) and structure (settling down) of different green roof substrates were also highlighted by Schrader and Boening (2006). Our work aimed at highlighting the relations between the evolution over time of a GR substrate and its hydrodynamic behaviour. It was based on in situ experimental GR plots installed in Tomblaine (north-east of France, under temperate climate). A sampling strategy was defined at different time steps considering the influences of soil cover (presence/absence of vegetation) and depth. Physical properties (bulk density, solid density, porosity, water retention curve, particles size distribution) and composition (concentrations in Corg and Ntot) were measured on all samples. Moreover, experiments were conducted on a laboratory setup (500 × 400 × 400 - H×h×l, in [mm]) to evaluate the hydrologic performances. Major results were obtained that showed evidences of an early pedogenesis: fine particles leaching from the surface, increase in Corg and Ntot in the upper layer, evolution of the poral architecture. It also appeared that this evolution could lead to a decrease of the water storage capacity of GR. Thus, such architectural green spaces as green roofs should also be considered as living soils, that can be classified as Isolatic Technosols (Andic, Drainic, Folic, Transportic) (IUSS, 2014)