Sponge cities are the flood prevention action the UK needs now

First published in the January edition of The Water Report.

Extreme weather and severe flooding events are becoming more frequent globally, and our towns and cities are especially vulnerable to its impact. WRc Principal Consultant Nick Orman makes the point that rethinking our approach to development and flood planning could transform the way this threat is managed.

Flooding in Valencia last year and the UK this year has highlighted the vulnerability of our cities to this catastrophic effect of climate change. The threat posed by river and surface water flooding can be devastating. In the summer of 2021, serious surface water flooding affected many London boroughs, fortunately without any loss of life but with considerable economic impact. We know these risks are there and that our cities are increasingly vulnerable.

One reason why surface water flooding is so devastating in cities is that they are densely covered with hard, impermeable surfaces – roofs, roads and paved areas – that do not allow rainwater to sink into the ground as it did before the cities were built. When extreme rainfall does come, drainage systems become full and streets turn into channels for huge volumes of water. Any obstacles in the natural path of the floodwater, such as buildings, bridges or vehicles, can be impacted by the flow of water.

The larger the city area, the more water can accumulate along a street and the greater the potential for damage. Underground spaces, such as basements, car parks and underground railways, could be inundated, as was the case in Valencia in October 2024 and London in 2021.

What is a sponge city?

For more than 40 years, practitioners have been exploring a move towards a different way of planning cities. The novel approach involves creating green spaces, both large and small, to allow rainwater to pond and infiltrate the ground as near as possible to where it falls, reducing and slowing the flow on pavements and roads. The idea is often referred to as source control, sustainable drainage systems (SuDS) or blue green infrastructure, and more recently the term ‘sponge cities’ has been coined in China to describe its widespread application.

Inevitably, there will be times when there is too much rain in a short time for it to infiltrate the ground, so water needs managed routes to leave a city in a controlled manner.

This vision cannot be achieved overnight. But because it will take time doesn’t mean we shouldn’t make a start now. If we look back at how our cities have changed over the last 30 years, it is clear that considered redevelopment has a big impact, but there is still much more to be done to better manage water in urban settings.

Short-, medium- and long-term goals

There are actions we can take now that will contribute towards our goal in the short, medium and long term. No matter how small or large, creating more green space across our cities and adapting the green spaces we have more intentionally to have a drainage function will start to have a noticeable impact. Grasses, trees and shrubs hold water and help draw it down into the ground – building ‘rain’ gardens along pavements, roads and paved areas will reduce surface water build-up in heavy rain and help reduce overheating. A tree-lined street can be as much as 4–6°C cooler in a heatwave than one that has no vegetation, and reducing the volume of water entering the network from road run-off will reduce the potential of sewage spills into waterbodies.

Following the 2021 floods, the Mayor of London commissioned the London Climate Resilience Review, which reported in July this year. In its evidence, Thames Water proposed a Sustainable Drainage Solution (SuDS) first approach using nature-based systems as a means of mitigating future flood risk. In its subsequent Drainage and Wastewater Management Plan, Thames Water proposed that 7000 hectares of land in London should be made into spongy green surfaces over the next 25 years. The timescale of this single project demonstrates the need for a long-term view on this issue.

But, above all, it will require a radical culture change across many professional disciplines. There should be a collaborative and interdisciplinary approach to drainage. Drainage and highway design could play their part if we managed rainwater on the surface rather than solely in pipes. Urban planning should incorporate the management of surface water flow routes at the masterplan stage and include the necessary long-term changes for flood resilience in local development frameworks. At a landscape level, emphasis should be on creating green spaces that also have a drainage function; realising the importance of considering drainage at an early stage, not as an afterthought, is paramount to achieving this. Finally, we may all need to learn to live with the idea that some hard surface must be given up if we are to make our cities more flood resilient. We need to recognise that a shift in how we do things is needed before the onset of more significant climate change impacts. The old ideas worked in the past but should evolve so that infrastructure serves the present and the future.

If we can all work together to achieve this transformation then we can create more resilient cities – and not just to flooding. To do this, we must create city-wide multi-agency strategies and implement them now. But keeping to these strategies in the face of our various professional, political, personal and cultural questions will require iron discipline, as, to use a phrase coined by Peter Drucker, an American management consultant, “culture eats strategy for breakfast”.

Nick Orman is a principal consultant at WRc, a specialist provider of sustainable water and wastewater solutions and research. He has 45 years’ experience as a civil engineer and is a specialist in the planning, design and maintenance of urban drainage systems. Nick is a Chartered Civil Engineer and Chartered Environmentalist and is a member of both the Institution of Civil Engineers and the Chartered Institution of Water and Environmental Management (CIWEM). In November 2024, Nick was the winner of the 2024 WaPUG Prize from CIWEM for his significant contribution to the development of urban drainage practice.

Created by potrace 1.16, written by Peter Selinger 2001-2019

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Nick Orman

Principal Consultant (Wastewater Networks)

Nick is a Principal Consultant at WRc with over 35 years' experience in the water sector. He is a Chartered Civil Engineer and Chartered Water and Environmental Manager. His specialist areas include sewer inspection technologies, sewer deterioration mechanisms, sewer collapse analysis, sewer hydraulic modelling and cost analysis of sewer flooding schemes. Nick was a major contributor to the SRM Sewer Risk Management website and is the Technical Lead on the revision of the SRM Sewer Renovation Design Guide. He has been involved in the drafting or updating of many of the guidance documents relating to sewerage, examples include Sewers for Adoption, the Civil Engineering Specification for the Water Industry and the Manual for Sewer Condition Classification.

2025-01-19 14:00:00