New changes to Environment Agency climate change allowances – what are they?

The Environment Agency (EA) have published updated climate change allowances for Peak Rainfall Intensity – which can be found here.

The Peak Rainfall Intensity affects surface water flood risk and the surface water drainage design, including the amount of surface water storage required.  The climate change allowance is based upon the River Basin Catchment area and the development design lifetime.  Residential developments have a design lifetime of a minimum of 100 years and would need to use the upper end allowances, in accordance with the EA guidance. 

The most significant change is the increase in the climate change allowance. The map below shows the climate change allowance required for residential developments; areas subject to a 50% allowance are shown in red, areas subject to a 45% allowance are shown in pink, and areas subject to a 40% allowance are shown in orange.

Not all LLFAs have identified when the updated allowances will come into effect.  However, it should be assumed that any application which has not received a recommendation for approval from the LLFA could be subject to the updated climate change allowances.

Stomor have prepared the below tables, which demonstrate the potential increase in surface water storage volume required to cater for the 45% climate change allowance, compared with the current 40% allowance.  This example is based upon a 5l/s discharge rate for varying contributing areas in Suffolk, Essex and Kent.

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