Ian Joyner, Associate Director and lead on Flood Risk
To little fanfare, and amongst a host of other announcements today, the Environment Agency released updated climate change allowances for flood risk and new development in England, bringing their guidance in line with UKCP09 climate change scenarios.
However evidence-based and necessary this new advice may be, the changes in river flows under certain scenarios mark a significant departure from the existing 20% increase in a flood flow with an annual chance of 1 in 100 (1%), currently used to plan for new development.
The change, which broadly appears to apply to all developments not already submitted for planning, means that the existing well-resourced stock of Environment Agency climate change simulations are now outdated, leading to a short-term need for developers to undertake (and the Environment Agency to review) their own climate change modelling.
Furthermore, the disconnect between the online Flood Map for Planning (which sets the Flood Zone extents based on present day risk) and the climate change flood events (to which buildings and mitigation measures are designed) has increased following this change. This could make planning more challenging at the edge of Flood Zones.
A fairly significant change then, which will require careful thought from regulators, developers and their advisors.
Arbitrary percentage increases in peak flows bring about their own problems (where flood volume is important for example) and there are likely to be theoretical limits in very urban catchments where an increase in storminess instead leads to an increase in surface flooding across a catchment where sewers do not have sufficient capacity to convey this increased flow directly to rivers.
It could also be argued that the change, whilst bringing requirements in line with current climate change scenarios, does nothing to address the main criticism levelled at flood risk management over this winter; that climate change is already influencing river flows in the present day and this is not factored into current estimates. Addressing this would require a more fundamental look at the way pooled hydrological data recorded in the (relatively recent) past is used to predict future extremes.