Introduction
The Risk of flooding can be increased by the development of land. Surface water drainage design is an important consideration in alleviating this. Rainwater runoff will need to be collected and discharged into soakaways or some form of attenuation or sustainable urban drainage system (SUDs) to prevent localised flooding.
Flood Risk Categorisation
One of the first steps in identifying whether a site is at risk of flooding is to commission a desktop environmental risk report (Groundsure, Landmark or similar). This will search various datasets relating to flooding and provide a flood risk categorisation for any given site.
The Environment Agency is the government body who manages flood risk in the UK. Land is divided into 4 flood zones which categorise the risk of flooding:
Flood Zone 1 (Low Probability)
In these flood zones, there are few restrictions to development. An exception applies for development land over 1 hectare. In these instances, a flood risk assessment will be required as part of any planning application for development.
Flood Zone 2 (Medium Probability)
“Land identified as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% to 0.1%); or between a 1 in 200 and 1 in 1000 (0.5% to 0.1%) annual probability of sea flooding”.
Flood Zone 3a (High Probability)
“Land identified as having a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater (>0.5%) annual probability of sea flooding”.
Any development planned in flood zones 2 and 3a will require a flood risk assessment as part of any planning application. Applications in flood zone 3a will also need details of suitable mitigation measures to remove or reduce the risk of flooding.
Flood Zone 3b – ‘The Functional Floodplain’
“Land where water has to flow or be stored in times of flood. Identified as a flood zone having a 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood or at another probability determined by the local planning authority and the Environment Agency (including water conveyance routes).
Flood zone 3b development will require a flood risk assessment as part of any planning application & only ‘water compatible’ uses and essential infrastructure projects will be permitted in this zone”.
Source – The Environment Agency
Flood Risk Assessment (FRA)
A flood risk assessment will usually be prepared by an environmental engineer specialising in this field of work. As a minimum it should include the following:
- An assessment of the flood risk and estimated flood level for the site with contingency to allow for future climate change
- Details of all plans and designs clearly showing planned finished floor levels and their relation to maximum flood levels
- Details of flood defences and flood mitigation measures.
An FRA will set out the approach to mitigate the flood risk identified using the following approach:
1 – Avoidance
The site layout should be designed to protect the most vulnerable buildings by locating these within the areas of lowest flood risk on the site. Ground floor and basement areas should be designed so that these are the areas least likely to be affected by flooding e.g storage, garages etc
2 – Control
Solutions can be deployed to control how flooding affects a site. Examples include localised flood defence works and water catchment zones, ponds etc.
3 – Mitigate
Buildings within a site can be designed to incorporate flood resistant construction to further mitigate the risk of damage occurring in the event of a flood event.
4 – Management of residual risks
Despite the deployment of flood mitigation works such as those described above, there will inevitably be residual risks remaining. Further management of flood risk in those circumstances is necessary and will include solutions such as early warning systems and the creation of safe pedestrian flood escape routes.
Final Thoughts
As climate change continues to cause an increase in the number of flooding events in the UK, the careful design and management of rainwater drainage systems will become increasingly vital. It is important to be fully versed of the exact risk flooding presents to a site and expert advice should be sought in how best to control and mitigate those risks.
Darren Frias-Robles FRICS is a Chartered Surveyor (Fellow of the RICS) with over 25 years of experience in development and construction. Darren is owner and Managing Director of Whitefox – Chartered Surveyors www.whitefoxgroup.co.uk
