Yorkshire Philosophical Society

Event Information

“What are Sustainable Drainage Solutions (SuDS), and their environmental benefits for cities like York?”

Kevin Smalley BSc (Hons) MIET, Technical Director, Arcadis 

Joint Lecture with IET North Yorkshire

(Lecture originally programmed for October 1st 2024.)

With climate change increasing the regularity and intensity of storms and heavy downpours we see the impact firsthand with flooding of homes and businesses and the impact on our river health from inundated sewer systems.

Our current combined sewers are a legacy of a time before these climactic events and new thinking is being applied to how we can better manage water and prevent these impacts.

This talk will explore the consequences of legacy systems and some of the new tools and techniques being applied to manage storm water more effectively and mitigate the devastating effects flooding and pollution can cause.

7pm in the Tempest Anderson Lecture Theatre in the Yorkshire Museum.

Member’s report

The wide-ranging lecture held jointly with IET Yorkshire covered many aspects including surface water flooding, sewage flooding within buildings and external sewage flooding. The ‘big picture’ is that 4.6 million properties were at risk of flooding in 2024 – an increase from 2.8 million 10 years ago. In 2024, there were 7625 flood alerts and warnings and spills from CSOs (Combined Sewage Outflows, in which wastewater and storm water are combined) totalled 3.6 million hours with an average duration of 8 hours. Without strong action the future situation will get worse. Climate change is predicted to increase the rainfall by 20% by 2070. There has also been a 30% increase in urbanisation in the past 30 years which is continuing at a fast rate.

Combined sewers are a major problem. They were first seen in the Victorian era and were standard practice in the UK until the second half of the 20th century when new foul and storm water systems were separated. In combined sewers a controlling device prevents the sewage works from being overloaded and minimises the risk of flooding homes by diverting excess flows into rivers. There are about 14,000 CSOs in England, totalling 500,000 km of pipework only 1% of which have been replaced since 1991. Interestingly, in 2023 all the CSOs in England were fitted with event duration monitors and the data is available to the general public (eg. on the Yorkshire Water website). There is also a regulatory obligation to reduce pollution events by 30% every five years. Amazingly, 30% of pollution events involve mis-connections between the waste water and storm water systems and there is no legal obligation to remedy these.

Relevant legislation includes the European Flood Directive of 2007 which led to the Flood Water Management Act of 2010 and subsequent changes to UK planning policy (which now removes the automatic right for developers to use the sewage systems). The legislation changes have recently led the water industry to publish its National Storm Overflow Plan for England (2024) which will lead to the biggest program in the world for reducing overflows at a cost of £11 billion and with a target to reduce spills by 4 million per year. Objectives are to increase the capacity of sewage treatment plants, install storm overflow tanks, replace concrete with grass and ponds and increase the size of critical pipework. A major focus is to introduce ‘sustainable drainage systems’ (SuDS) by managing storm water as close as possible to its source, mimicking natural drainage, slowing water flow by natural permeable surfaces and planting trees. The aim is to even out peak flows and to give time for water to reach the aquifers. Work already underway includes a new wetland in Norfolk, installation of sewer smart sensors to monitor the performance of the sewer network and building a sustainable drainage system for London. Proposed future work includes building more storage within sewers, upgrading the capacity of sewage works, installing smart technology using sensors, reducing the volume of rainwater entering sewers and treating spills before they enter the main water body eg by the use of reed beds. Smaller local initiatives include ponds and wetlands, swales and detention basins, permeable paving, use of water butts and soakaways.

A major UK initiative is the Mansfield Project, basically a pilot study for retrofitting the SuDS approach to existing infrastructure – it is being paid for by the Severn Trent Water Authority at a cost of £76 million. Mansfield is an ideal test location as it is in a geological bowl which makes drainage difficult. The philosophy is to develop standard solutions to common problems – this gives a faster and cheaper delivery and leads to easier training of new staff. All stakeholders now work in the same offices – this has proved critical in keeping a cohesive and focused approach.

Of more local interest, in 2019 beavers were introduced in Cropton Forest on the North York Moors. Their dams are already making a discernible reduction in downstream flooding which has caused £7 million in damage to homes and businesses over the last 20 years.

Rod Leonard