The influence of substrate and vegetation on extensive green roof hydrological performance

The objective of this research was to investigate the hydrological processes occurring in extensive green roof systems through data collected during a continuous monitoring programme of different green roof configurations. Nine green roof test beds (TB) which vary systematically in their substrate composition and vegetation options have been monitored since April 2010 at the University of Sheffield, UK. Three green roof substrates were tested: two commercial substrates manufactured by Alumasc – Heather with Lavender (HLS) and Sedum Carpet (SCS) Substrate were considered alongside a Lightweight Expanded Clay Aggregate (LECA)-based substrate. Three vegetation treatments have been tested: a drought tolerant specie (sedum), a meadow flower mixture and a no vegetation option. Per event retention performance varied depending on the initial water content within the substrate and the characteristics of the rainfall event. Consistent behaviour was observed among the tested green roof configurations with respect to per event retention. Greater retention was associated with HLS and SCS substrates when compared with LECA. Vegetated configurations showed consistently higher retention performance. Sedum vegetation resulted in higher retention performance than Meadow Flower. This was particularly evident on the LECA substrate

Independent Validation of the SWMM Green Roof Module

Green roofs are a popular Sustainable Drainage Systems (SuDS) technology. They provide multiple benefits, amongst which the retention of rainfall and detention of runoff are of particular interest to stormwater engineers. The hydrological performance of green roofs has been represented in various models, including the Storm Water Management Model (SWMM). The latest version of SWMM includes a new LID green roof module, which makes it possible to model the hydrological performance of a green roof by directly defining the physical parameters of a green roof’s three layers. However, to date, no study has validated the capability of this module for representing the hydrological performance of an extensive green roof in response to actual rainfall events. In this study, data from a previously-monitored extensive green roof test bed has been utilised to validate the SWMM green roof module for both long-term (173 events over a year) and short-term (per-event) simulations. With only 0.357% difference between measured and modelled annual retention, the uncalibrated model provided good estimates of total annual retention, but the modelled runoff depths deviated significantly from the measured data at certain times (particularly during summer) in the year. Retention results improved (with the difference between modelled and measured annual retention decreasing to 0.169% and the Nash-Sutcliffe Model Efficiency (NSME) coefficient for per-event rainfall depth reaching 0.948) when reductions in actual evapotranspiration due to reduced substrate moisture availability during prolonged dry conditions were used to provide revised estimates of monthly ET. However, this aspect of the model’s performance is ultimately limited by the failure to account for the influence of substrate moisture on actual ET rates. With significant differences existing between measured and simulated runoff and NSME coefficients of below 0.5, the uncalibrated model failed to provide reasonable predictions of the green roof’s detention performance, although this was significantly improved through calibration. To precisely model the hydrological behaviour of an extensive green roof with a plastic board drainage layer, some of the modelling structures in SWMM green roof module require further refinement

Zeolite activation of organometallics: revisiting substitution kinetics of [Mo(CO)6] with chemisorbed PMe3 in dehydrated Na56Y zeolite

Reactions of [Mo(CO)6] under vacuum in α-cages of Na56Y zeolite fully loaded with chemisorbed PMe3 form cis-[Mo(CO)4(PMe3)2] but mixtures of [Mo(CO)5(PMe3)] and cis-[Mo(CO)4(PMe3)2] are formed under CO. Reactions under vacuum exhibit low enthalpic and very negative entropic factors (ΔH = 71.4 ± 3.5 kJ mol-1 and ΔS = -102 ± 11 J K-1 mol-1) compared with much higher enthalpic and positive entropic factors for CO dissociative reactions with P(n-Bu)3 in xylene observed elsewhere. Reaction at 66 °C under vacuum is ca.10³ times faster than spontaneous CO dissociative reactions in solution. Intrazeolite substitution is concluded to occur by a zeolite assisted mechanism in which two oxide ions in the cavity walls simultaneously displace two neighbouring CO ligands from the [Mo(CO)6]. This contrasts with even lower enthalpy, and more negative entropy values, for simultaneous displacement of three neighbouring CO ligands in thermal decarbonylation reactions. The α-cages behave as multidentate anionic zeolate ligands with varying numbers of O2- ions participating to create highly ordered transition states. These results emphasize the high degree to which such kinetic studies can reveal intimate details of the nature of these activating effects.Reações de [Mo(CO)6] sob vácuo em cavidades α do zeólito Na56Y totalmente carregadas com PMe3 quimissorvido produzem cis-[Mo(CO)4(PMe3)2], mas misturas de [Mo(CO)5(PMe3)] e cis-[Mo(CO)4(PMe3)2] são produzidas sob CO. Reações sob vácuo exibem fatores entálpicos baixos e fatores entrópicos muito negativos (ΔH = 71,4 ± 3,5 kJ mol-1 and ΔS = -102 ± 11 J K-1 mol-1), comparados com fatores entálpicos muito mais altos e fatores entrópicos positivos para reações de substituição dissociativa de CO por P(n-Bu)3 em xileno observadas em outros trabalhos. A reação a 66 °C sob vácuo é cerca de 10³ vezes mais rápida do que reações dissociativas espontâneas'de CO em solução. Conclui-se que a substituição intrazeólito ocorre por um mecanismo assistido por zeólito em que dois íons óxidos nas paredes da cavidade deslocam simultaneamente dois ligantes CO vizinhos do [Mo(CO)6]. Isso contrasta com valores de entalpia ainda menores e de entropia mais negativos para o deslocamento simultâneo de três ligantes CO vizinhos em reações de descarbonilação térmica. As cavidades α comportam-se como ligantes zeolato aniônicos multidentados, com números variáveis de íons O2- participando da criação de estados de transição altamente ordenados. Os resultados enfatizam o alto grau com que estes estudos cinéticos podem revelar detalhes íntimos da natureza dos efeitos ativadores.862871Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Enhancing the Retention Performance of a Small Urban Catchment by Green Roofs

In existing urban areas the drainage systems can be retrofitted in order to address flooding and water quality problems. In this study, the installation of green roofs is assumed as hypothetic retrofitting scenario according to a sustainable storm water mitigation strategy for a selected urban catchment. The modelling is undertaken using EPA SWMM; the simulations are performed over a continuous simulation of 26-years of rainfall records. The modelling results point out that the retrofitting scenario contributes to the storm water runoff mitigation mainly in terms of volume and peak reduction