Achieving urban water supply and flood resilience using catchment scale rainwater management

Abstract

The increasing magnitude and frequency of extreme rainfall events coupled with a rapid growth in urban populations is leading to surface water flooding being recognised as one of the pre-eminent natural hazards impacting communities, properties and infrastructure. However, if managed effectively, urban storm water also represents a promising resource to enhance water supply resilience. This research investigates how catchment-scale rainwater management can be applied to achieve flood and drought resilience through capturing extreme rainfall and contributing to water demands at the property scale. The study investigates a case study in Pandon Dene, North East England, implemented in three stages: Firstly, evaluating the household water demand and supply from an individual rainwater harvesting system; Secondly, examining the effects of catchment scale rainwater management on urban flood resilience; And thirdly, assessing cost-effectiveness of strategies. Three types of household rainwater harvesting systems are considered in the performance evaluation, including a single storage tank, a passive system with regulated overflow and an active system with real time control. Household demand and supply is characterised using 15-minute resolution non-potable property scale water consumption, evaluated through continuous simulation over a 36-year period (1984-2019). Long-term continuous simulation enables quantification of water supply efficiency and overflow from the rainwater harvesting system at individual houses. Urban flood resilience from catchment scale application is evaluated using a rapid two-dimensional cellular automata flood model (CADDIES), with the estimated overflow from each of the household applied as a model input to examine strategy response to 36 historical rainfall events, ranging from a 1 in 1-year through to a 1 in 140-year return period. Cost effectiveness is evaluated using a rainwater system cost model and a GIS-based hazard impact assessment tool for flood damage mitigation. Analysis indicates that all three rainwater harvesting systems deliver water-saving benefits and stormwater benefits to varying degrees. The single storage tank and passive rainwater harvesting system are particularly effective to regulate more frequent (< 10-year) and moderate (< 30-year) storm events whereas the active system can effectively regulate larger (< 50-year) rainfall events. At the same time, results also indicate that a system primarily designed for water supply augmentation provides up to 64% of non-potable demand (toilet flushing) alongside 77% (median) reduction of stormwater peak runoff into the sewer system. Dual modes of resilience indicate that distributed catchment-scale rainwater management can deliver multi-functional, multi-benefit systems.