Property-based rainwater drainage design and the impacts of climate change

Property-based rainwater drainage provision comprises a number of components broadly categorised as roof, surface or underground drainage. These systems are relied upon to prevent water ingress to the building and to avoid localised ponding or flooding. Recognition of the inherently unsteady response of the integrated network, in part driven by a particular rainfall event but also due to the transient nature of the fluid flows therein, is important in allowing an understanding of system performance under both current and future climate conditions. Codes and standards suggest that roof drainage systems, conventional or siphonic, are designed using a single rainfall intensity figure, representative of the most intense part of a longer storm, whereas runoff to surface and underground drainage is typically based on a single-peaked rainfall profile. Together with the difference in event duration typically adopted as part of the design process, this means that, unless a straightforward uplift factor is applied, then understanding the potential impacts of climate change on overall system performance can be difficult. Using numerical modelling techniques to analyse the performance of a case study site located in Edinburgh, this research identifies an appropriate common rainfall event from which a system-specific exceedance threshold is identified. Specification of this rainfall intensity facilitates an exploration of the impacts of climate change undertaken within the context of UKCP09 projections. Using gutter overtopping as an indicator of failure, this paper explores possible changes in the frequency of system under-capacity under varying future climate change scenarios. Practical application: Engineers are encouraged, in relevant codes and standards, to take account of the potential impacts of climate change when designing rainwater drainage systems for buildings. However, little guidance is offered therein on how to achieve this. This paper presents an example of UKCP09 climate change projections, specifically Weather Generator precipitation data, applied to a case study site that integrates roof, surface and underground drainage networks. Results illustrate the possible change in frequency of system failure, identified in this case as gutter overtopping.