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

Reaction mechanisms of metal-metal bonded carbonyls. Part VI. Reactions of μ-carbonyl-μ-diphenylgermanediyl-bis(tricarbonylcobalt) with carbon monoxide, triphenylphosphine, and tri-n-butylphosphine

The reversible 'ring-opening' reaction of the complex [(OC)3Co(μ-GePh2)(μ-CO)Co(CO)3], (I), with carbon monoxide in decalin to form (μ-GePh2){Co(CO)4}2, (II), proceeds by a path first order in [Complex] and [CO], and the reverse reaction is first order only in [Complex]. Activation and equilibrium parameters have been obtained. Reaction with triphenylphosphine forms the complex (μ-GePh2){Co(CO)3L}2, (III; L = PPh3), probably via [(OC)3Co(μ-GePh2) (μ-CO)Co(CO)2PPh3], produced in a rate-determining CO-dissociative process and subsequently attacked by a second phosphine molecule in a rapid ring-opening reaction. Bimolecular attack by triphenylphosphine also occurs and leads directly to the complex (μ-GePh2){Co(CO)3L}{Co(CO)4}, (IV; L = PPh3). Reaction of the latter with triphenylphosphine produces complex (III; L = PPh3) by a process first order only in [Complex]. Reaction of complex (II) with triphenylphosphine proceeds via rate-determining formation of (I) which then reacts rapidly with the phosphine as described above. Tri-n-butylphosphine can attack the complexes (II) and (IV; L = PBu3) by bimolecular processes. The mechanisms of these reactions are discussed in terms, especially, of relative rate constants for bimolecular attack by carbon monoxide and triphenylphosphine on the complexes or reactive intermediates involved