Recent guidance from the Environment Agency (EA) requires builders of new developments to produce flood risk assessments in line with new advice relating to increased risk of flooding because of climate change.
How is flood risk measured?
It is considered that climate change is creating an increased risk of flooding across the UK, with some areas affected more acutely than others.
The Environment Agency, hydrologists and consulting engineers have been making an allowance for climate change for a number of years when predicting flood risk from rivers, the sea and sewers.
River basins are modelled to assess their ability to convey the rainfall generated by predicted storm events. If the river basins are under capacity modelled, the extent or route of the resulting flood water is calculated, as shown on the flood maps that are published on the EA website. From a planning and development perspective, these flood maps are used to determine whether a site is positioned in high risk, medium risk or low risk flood zone. Prior to the new advice, a blanket figure of 20% increase in rainfall was added to the predicted storm event to allow for climate change.
A similar exercise is carried out to undertake the surface water drainage design, with a figure of 30% added to tackle increased water.
What exactly is changing?
Modelling under the new advice from the EA reflects the likelihood of greater increases of water volume when determining the likelihood of flooding.
River basin modelling will be regional:
- South East up to 105% increase
- South West up to 85% increase
- Midlands up to 70% increase
- North West up to 70% increase
- North East up to 50% increase
Surface water drainage design:
- 40% increase in peak rainfall intensity
Sea level rise
- 1.24m rise in South and East
- 1.02m rise in North-West and North East.
What does the new Environment Agency advice mean to developers?
Land allocated for development and site development will need to cater for the additional predicted water. This could mean:
- Larger flood plains, which result in less developable land.
- Larger volumes of surface water run-off being retained on site owing to increased rainfall intensity. (Mayer Brown has analysed a number of previous projects and in real terms attenuation volume increases by approximately 10%.)
- Increased height of finished floor levels of buildings.
- Greater sacrificial areas for flood compensation and drainage.
- Potential reduction in net developable area and site viability.
What should developers be doing now?
New developments should be designed so they are protected from flooding for the lifetime of the development, which generally extents 100 years plus into the future. In order to do this, new scheme proposals made after 19th February 2016 need to be assessed against the latest climate change predictions. This renders all previous analysis carried out by the EA and flood risk advisors as being superseded.
This in itself poses problems, because it is a major undertaking for the EA to update all river basin modelling and flood maps. In the meantime we are in a transitional stage where no climate change flood levels are being provided by the EA.
The Environment Agency states that the new standards should be applied to all future appraisals that are started (new) from March 2016 or are to be submitted for approval after 1st September 2016.
Work already in progress is also affected, so if you fall into that category, now is the time to take a second look at your flood risk assessment. If you do not, you could face large additional costs to complete your project.
Work already in progress should, as a minimum, be assessed to ensure that the new advice would not lead to different decisions. Even for substantially complete work, or that submitted for approval before September 2016, the EA states that if the new advice can be factored in, or the plan or investment decision tested against it without slowing completion or adding significantly to the cost, then this should be done.
As with any change in guidance to building development, it is important to seek good professional advice and ensure that all the latest restrictions have been taken into account. Attention to detail in times of change can ultimately save money in the future.