The likelihood of a flood happening in any given location depends on a complicated series of interactions. It is affected by the capacity of watercourses, drains and sewers, and the ability of the land and its surroundings to absorb run-off water. Any changes to this delicate balance can have a huge bearing on a site’s risk of surface flooding, with construction one of the biggest potential sources of disruption.
According to the Environment Agency, surface water flooding poses a threat to more than three million properties across England. As the proportion of the UK assigned for development increases – and the area of land with capacity to soak up surface water decreases – there is a significant risk that this number will grow.
For local authorities, which are under intense pressure to champion development and deliver new homes in their regions, it’s vital to understand the impact their plans will have on flood risks and to take steps to reduce the impact.
The big picture
Currently, sites earmarked for development are considered in isolation, with developers only obliged to mitigate flood risks up to the ‘red line’ boundary of the land they own. But this approach often serves to increase flood risks on adjacent plots of land due to the higher volume of water leaving the site at surface level, placing extra pressure on neighbouring water infrastructure. Not only does this increase the risk to people and property, it serves to hamper future development by making otherwise viable areas unsuitable for building.
It’s vital that instead, councils start to work with key stakeholders to build a holistic picture of the intricate system of drainage that operates across their jurisdiction, identifying where potential problems lie. This will allow them to make recommendations or apply conditions to planning consents to influence proposals put forward by developers, so that the risks are managed.
A key outcome of this approach will be to help local authorities identify multiple sites that can be developed as one. This way, councils can create a masterplan that maximises the value and potential of their land, as well as mitigating potential flood risks upfront.
Turning the challenge into an asset
One of the main benefits of understanding the large-scale flood risk landscape in this way is that ‘blue-green’ infrastructure – features built to help absorb and channel surface water – can be incorporated into the overall planning strategy.
By championing or mandating these attractive but functional measures, such as ponds and wetlands, councils can mitigate risks while also creating spaces that allow animal and plant life to flourish, and in which residents can enjoy a connection with nature.
These assets can often act as the focal point of a new development, becoming spaces for recreation, biodiversity and social activity, that also boost a site’s resilience to flooding and other weather events.
Several local authorities across the UK have already begun to factor flood risk management into their development strategies, with great success.
A recent flood risk assessment and drainage strategy delivered for Middlesbrough Council, for example, covering two large greenfield and one brownfield site that had been earmarked for future development.
By modelling existing surface water pathways, our engineers established the minimum level of mitigation the site would need, recommending blue-green infrastructure such as ponds and wetlands, in order to prevent future flooding and protect neighbouring areas.
These findings will now underpin any future development on the site, ensuring appropriate measures are always taken to manage the risk.
Another example is a project we carried out for a local authority in Scotland, where the team identified the key hotspots and pathways across an extensive area of the council’s catchment to enable the development of an informed Surface Water Management Plan.
This work led to a sustainable drainage system (SuDS) being implemented, as well as a revision to council planning guidelines that attaches more rigorous conditions to any new schemes.
The result is that flood mitigation solutions, such as new ponds and watercourses, have now been incorporated into a series of approved developments, attractively landscaped into the designs and accessible to residents via waymarked paths. This has also created additional capacity in existing drainage systems to enable future development, without the need for disruptive sewer upgrades.
As these examples show, by developing masterplans that address flood risks, councils can work collaboratively with developers to ensure schemes are progressed that deliver maximum value, both financially and in quality-of-life terms for the people who will ultimately use them.
Nick Fraser is operations manager at engineering consultancy Sweco
