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

INFLUENCE OF URBAN FEATURES ON INTRA-URBAN NOCTURNAL COOLING: AN EMPIRICAL APPROACH USING CLIMATIC MAPPING

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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

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)

Mesure de l'évapotranspiration de différentes toitures végétalisées

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Sensitivity analysis of the soil parameters of a green roof for the TEB-GREENROOF model (v7.3)

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