Boom or Burst: Flood Risk at the Property Level

Catherine Morgan
Monday, 9th February 2015

In this article, David Kelly explores the ways in which a holistic approach to  property rainwater drainage design is of vital importantance in building flood-resilient homes and communtities.

Like many countries around the world, the UK has witnessed record breaking rainfall in recent years.  In fact, data from the Met Office shows that five of the six wettest years on record have occurred since 2000.  The result is that the country has suffered the devastating consequences of some of the worst floods ever witnessed: with major damage and disruption to communities, businesses, and infrastructure.  While current rainfall events have become even more variable and unpredictable than ever before, projections of future rainfall show that, not only can the UK expect to be hit by more frequent and more intense rainfall, but the number of extreme rainfall events is due to increase overall.  It is clear to see that the effects of a changing climate, together with additional pressures such as urbanisation and population growth, will undoubtedly place significant additional stress on an already struggling drainage system.  This is particularly pertinent for drainage systems at the property level which are tasked with preventing water ingress to the building and avoiding localised flooding.

The operational performance of property rainwater drainage systems (comprising the roof drainage, surface drainage, and the underground receiving network) is vitally important not only because they help protect our buildings from rainwater damage, but as the most upstream inlet to the municipal drainage system, how well they deal with the rainwater falling within the property boundary, influences the operation and flood risk downstream.  Under-capacity of the property drainage system can lead to overtopping of the gutter, surcharging of the underground receiving network, surface flooding, and consequently, damage to the property.  Confounding the problem is the growing trend of homeowners paving over their front gardens to create parking spaces or low-maintenance gardens.  This practice of replacing grass and plants with impervious surfaces prevents the rainwater from infiltrating naturally into the soil, therefore, increasing surface runoff and increasing flood risk.   While recent planning changes have attempted to address this issue, by requiring only permeable materials be used for covering gardens, compliance and enforcement issues have shown these changes to have limited effect.

To ensure property rainwater drainage systems have the flexibility to cope with changing rainfall patterns and provide the resilience needed to protect against more extreme rainfall events, an overhaul of the current design methodology is needed.  The issue is that whilst current design standards suggest that engineers should consider the potential impacts of climate change, no guidance is provided on how this can be achieved.  Also, the design standards themselves are based on historic rainfall data and therefore provide outdated information on which to base designs.  For example, analysis of future trends of extreme rainfall shows that a current 100-year event for London may become a 25-year event by the 2080s.  Furthermore, the numerous standards1 that currently cover the design of different components of the property rainwater drainage system (each specifying different rainfall profiles, event durations, and return periods) produce a disjointed design methodology whereby whole system design performance is difficult, or even impossible, to achieve.

In reality, the necessary changes required to system design would constitute very simple adjustments or additions, such as: increasing gutter and pipe sizes to increase capacity; incorporating rainwater recycling systems to reduce flow to drains; and preserving, or increasing, permeable surfaces, such as green roofs, to minimise runoff from the property.

It is important to remember that within the context of a changing climate, where extreme rainfall events are projected to occur more frequently, unless the design of property rainwater drainage systems starts to look to the future by embedding climate change projections and adopting a more holistic design approach, every property in the country could be vulnerable to increased flood risk or water damage. 


David Kelly is Assistant Professor at the Centre of Excellence in Sustainable Building Design at Heriot-Watt University. His recent paper Property-based rainwater drainage design and the impacts of climate change was the basis for evidence he presented to the All Party Parliamentary Group for Excellence in the Built Environment (APPGEBE) at the Houses of Parliament in November 2014.



Roof drainage systems are covered by BS EN 12056-3:2000, siphonic roof drainage systems are covered by BS EN 8490:2007, and surface & local drainage systems are covered by BS EN 752-2:1997