Micro Drainage supplies industry standard sustainable drainage and flood
hazard software for the civil engineering and environmental sectors

Mandatory at all levels to ensure additional predicted volume of rainwater discharge caused by the new development, for a 1 in 100 year event of 6 hour duration including an allowance for climate change (PPS25, 2006), should be reduced by the following:

• Using infiltration; and/or
• Made available for use in the dwelling as a replacement for potable water in WC flushing or operating a washing machine.

Where this additional rainwater volume cannot be prevented from being discharged, for all events up to the 100 year return period, the peak discharge rate from the site should be reduced to:

Pre-development site’s estimated mean annual flood flow rate (Qbar); or

• 2l/s/ha; or
• Minimum flow rate (litres per second), based on good practice guidelines to prevent easy blockage, by ensuring the outlet throttle is not too small.

Check the "volume balance" using the ICP SUDS test in the Design Audit Wizard in Simulation APT, covered in training courses C, G and H.