Hybrid Drainage Plans

Where infiltration can be utilised hybrid solutions may be possible.

Where a site has some infiltration capacity, but rates are too slow to meet formal infiltration requirements for the site (1 x 10 -6 m/s), the LLFA may accept a well-designed hybrid solution, which uses low level infiltration for smaller storm events and a piped outfall for larger events. This maximises the capacity of the attenuation storage for larger infrequent events and allows for appropriate half drain times to be met for all events. An example of this would be where a site has demonstrated infiltration rates of 1 x 10 -7 m/s. Infiltration could be used for events up to the 1 in 5-year event assuming that 50% of the attenuation capacity of the storage feature is available 24 hours after a storm. Any event larger than the 1 in 5-year event would then discharge via a piped network at the 1 in 1-year greenfield rate.

Page updated: 17/02/2020

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