Sustainable urban drainage: Retrofitting for improved flood mitigation in city centres

AIMThe aim of this research was to examine the potential for mitigation of predominantly pluvial flooding in CBDs through retrofitting of such systems. The research used two case study areas to explore the challenges and opportunities for retrofit of SUDS in different cities with varying climate zones, urban design and governmental regimes. The research sought to:a) Evaluate the potential to physically retrofit existing buildings and adjacent paved areasb) Explore the potential for run of reduction and therefore mitigation of associated flood damagec) Examine the costs and benefits of retrofitting SUDS and identify social and environmental benefits of sustainable drainage for commercial property and the business district.In order to provide information and guidance for surveyors in the emerging area of retrofit of SUDS and provide material to build capacity of surveyors to contribute to improved drainage and flood mitigation for commercial property.METHODOLOGYThe research commenced with a systematic literature review to identify the available information on the use and retrofit of SUDS to reduce flood risk. International databases of both academic and industry sources were accessed thereby creating a global picture of examples, together with available tools and guidance likely to be of benefit to the surveying community. Using information drawn from the literature, criteria were developed to determine the technical requirements that determine suitability of roofs and paving areas for retrofit including the position, orientation and location of the building, the roof pitch, weight limitations and ground conditions. Two detailed building databases were then developed for Melbourne, Australia and Newcastle, UK. The databases were compiled drawing on a wide range of sources, including existing commercial and publicly available databases, Google maps and visual inspections. The final building databases contained 526 commercial buildings in Melbourne and 507 commercial buildings in Newcastle. The technical criteria were used to identify those buildings which had the potential to be retrofitted and to estimate the potential for retrofit of permeable paving. A simple estimation method was applied to the data derived from the property databases and available land use data to assess the potential run-off reduction under three scenarios. For the Melbourne dataset a further run-off analysis was carried out on the behalf of the research by the University of Newcastle using a state of the art rainfall inundation model.RESULTS AND IMPLICATIONSThe most appropriate and well understood SUDS for retrofit in city centre locations are green roofs, permeable paving and rain gardens. Combination of these features into a ‘management train’ or ‘treatment train’ (a linked series of drainage techniques) can increase the attenuation potential of individual features. The evidence base for estimating the direct benefits of SUDS retrofit is rapidly developing. Both academic studies and city-wide assessments have been undertaken. For green roofs, benefits accrue to individual property owners in terms of reduced energy bills owing to the insulating properties of such roofs. The broader stakeholder community also derives benefits from green SUDS installation, such as roofs and rain gardens, some of which are less easily monetised. They include: biodiversity/wildlife habitat; improvements in water and air quality; and attenuation of the urban heat island effect. Green roofs also have a carbon sequestration function.The range of benefits of permeable paving are more limited and relate to stormwater management and water quality. However permeable paving may have a lifecycle cost advantage over other paving systems thereby representing a no regret option for renewal of urban paving.The consideration of the full range of benefits and their applicability from the perspective of actors engaged in the installation of measures is a recent development in the literature and has not yet been explored for commercial property owners and investors.The database analysis of buildings within two city centres revealed that the proportion with potential for retrofit of green roofs is fairly low (under conservative assumptions). Estimation of other surfaces potentially suitable for permeable retrofit in busy business districts also reveals that a large proportion of urban hard surface may be unsuitable for standard permeable paving systems.Estimation of the run off reduction potential for these two cases study areas can be seen to be realistically around 10-20%, however this can still make a significant contribution to stormwater reduction and peak attenuation. For both of the case study sites however it was seen to be necessary to consider the wider catchment area due to the presence of historic watercourses and topographical features