Our Senior Flood Risk Consultant, Daniel Cook was one of the keynote speakers in the main theatre of this year’s Flood Expo at the Excel Centre in London. A full house listened as he detailed how new climate change allowances have affected flood zones and impacted on development schemes, with a clear and welcome shift toward more sustainable flood mitigation solutions.

Daniel opened the presentation considering “The New Normal”– how the increasing severity of flood incidents over a 7-year period (2008 to 2015), is part of a high frequency flooding trend period – with the associated rise in insurance claims, damage to buildings and disruption.

With this context, he outlined the significant planning obstacles that need to be overcome working with clients on planning submissions for development in relation to flood risk.

Determining Risk

It starts with resilient design, understanding that if existing defences fail how would design of building and site stand up to it? Designing in access and egress is also important.

The site location and how well modelled the nearby water course is will determine how easy it is to analyse the flood risk. Rural watercourses or EA main rivers in less populated areas are typically with less well modelled and can be more problematic.

Daniel, then outlined the flood zone categories and how they influence development permission. Flood maps for Zone 1 (lowest risk) can be misleading and developers could feel they have a freer rein. However, Councils will have predetermined Critical Drainage Areas which cannot manage storm water in what would normally be considered innocuous low risk areas. Adding new build development can easily exacerbate this. An FRA will be required to justify the development is sustainable.

Daniel, explained how Ambiental work with councils on behalf of clients on assessing flood zones and local plans, with forensic consideration on the highest risk zone 3a (1:100 year flood events) and 3b (1:20 yr event). Planning Authority approaches vary considerably and this is increasingly being driven by Lead Local Flood Authority (LLFA) intervention on local policy.

Climate Change Increasing Vulnerability

Aside from site location, flood zone, type of flooding and the local and national policies in play, the design and the use of the development and its specific vulnerability to flooding is now a key determinant, especially with the revision of climate change allowances for flow levels on rivers, February 2016.

Last year, 11 river catchments were modelled and their upper, middle and lower scale flow rates were revised, in some case dramatically upwards. This has significant impacts on flood extents, but especially on the type of development in particular its vulnerability.

Developments near rivers in the South East now have to consider flows of 105% at the upper level estimates of flood extent (not the original 20% uplift for climate change variation). This puts greater pressure on the available land dependant on the type of development being proposed. More highly Vulnerable developments are likely to be pushed further away from the river with this climate change approach. Developments still being constructed in these areas will be permitted provided suitable mitigation measures are provided to ensure the functionality, sustainability and safety of the development exceeds the risk of flooding.

Daniel emphasized that outline plans are now far more robust, with infiltration tests/ percolation tests being needed more regularly. Local and national policies drive a need for the tests and level of detail required. This can add significant costs to the project, especially if the development is small scale and could affect viability.

He then explained the latest Thames Breach data release which will have significant impacts on redevelopment along upstream section from the Barrier.  A site close to Deptford previously thought secure, is now well within one of 5679 newly modelled upriver breach locations that is now redrawing the breach flood risk map of Central London.  This is important due to the local policies of most London boroughs which interact with the Thames have specific policy set out to adhere to these breach flood levels.

Existing applications are now requiring review, as the EA have made an instant change in policy overnight without any caveats.

Towards More Sustainable Flood Management

Councils and LLFAs are increasingly focusing on SuDS solutions as part of the flood mitigation response. Daniel closed his presentation by outlining the core benefits that planning authorities are seeking – water quality (improvement), water quantity (management) and community amenity.

Since April 2015, planning rules have changed so that developments with 10 or more dwellings (which could include 1 block of flats with 10 units) require a drainage strategy. Some councils are becoming even more stringent. Daniel quoted Croydon and Elmbridge Councils requiring drainage strategies for straightforward house extensions.

Soil conditions are key to the drainage strategy. Infiltration of surface water close to the property may not be possible on clay soils and cannot be implemented within 5m of a building regardless.  Tanking is an appropriate, if less sustainable solution for many urban areas. However, much can also be achieved with simple rain gardens, grass roofs and permeable pavements.

While SuDS are not yet mandatory for minor development unless Local Policy says otherwise, it is clear that they are forming a central part of sustainable planning conditions by councils. Recognising this, together with resilient design and detailed flood risk assessment submissions, places developers on the front foot and balances housing and supporting infrastructure need with climate change.