Green roofs and compact infiltration systems in Malta : a modelled assessment of runoff and peak-flow reduction for urban flash flood mitigation

Abstract

Flash flooding is a type of hazard characterised by intense rainfall over a relatively short period (Abegaz et al., 2024), significant economic and health harms (Alipour et al., 2020), and a pattern of accelerating increase in terms of frequency and intensity due to climate change. Additionally, urbanised areas are particularly vulnerable to hazards associated with these extreme weather events, due to their traits of impermeability that increase runoff generation. Within Malta, flood risks have also been intensified by the inconsistent implementation of adaptation measures, particularly the integration of rainwater cisterns into buildings. Although rainwater cisterns have long been required by law and form part of Malta’s traditional approach to rainwater management, periods of rapid development have seen these measures overlooked or inadequately implemented. Consequently, rainfall that could otherwise be stored or managed on site is often discharged directly onto streets, contributing to surface runoff and urban flooding. This study describes and compares the potential benefits of individual and combined green stormwater infrastructure (GSI) measures through their effectiveness in reducing flood impacts, especially flash flooding, in terms of reductions in total runoff contribution and peak flow. The individual and combined impacts of both green roof and compact infiltration system (CIS) GSI are explored. Green roof GSI refers to rooftops that utilise green space, incorporating vegetation and soil, to manage overflowing stormwater through absorption and retention. Alternatively, CISs are infiltration systems developed locally within Malta through a research project named GEO-INF conducted by Ing Marco Cremona, and are characterised by their compact size, low costs, and low maintenance (Green Stormwater Infrastructure Guidance Manual, 2022). These potential benefits in mitigation effectiveness are explored within an 8028 m2 study area within Birkirkara, Malta; a densely populated and heavily urbanised area known for flooding downstream. These GSI types are explored through modelled simulations in Stella Architect, based on Maltese weather data and GSI data from current GSI projects, with the express purpose of testing mitigation effectiveness across a variety of storms that emphasise current averages, increasing intensities, and increasing temporal scales. In total, four scenarios were modelled and tested for effectiveness, including no GSI implementation, green roof, CIS, and integrated green roof and CIS implementation. The results of this study show a substantial runoff contribution reduction across all scenarios featuring GSI, with the integrated green roof and CIS scenario mitigating approximately 100% of runoff contributions and peak flow rates across all storm scenarios. The CIS scenario showed complete mitigation in scenarios where its capacity was reached; however, in other scenarios, increased runoff and especially peak flow rates occurred. Green roofs showed greater resilience across all storms and held an 88% runoff reduction percentage against storm seven, as opposed to the 37% runoff reduction percentage exhibited by the CIS scenario in the same storm event. Finally, the GSI scenarios were analysed under varying roof area and CIS tank size configurations to find further optimisation opportunities, with significant tank size reductions being found to be effective for the combined green roof and CIS scenario. This study holds implications for the tradeoffs present across different GSI configurations, through the comparisons identified and the overall evidence for the effectiveness of alternatives and complements to conventional stormwater drainage infrastructure, including rainwater cisterns. Overall, this study addresses knowledge gaps relating to key GSI types that will be necessary to reduce the increasing impacts of flash flooding events, as they grow in intensity and frequency, due to climate change.