Page 89 - Oxford City SuDS DESIGN & EVALUATION

9.5.5 Calculating storage requirements

Runoff rates and volumes can be managed water long enough to be discharged either
by either infiltration or controlled discharge. into the ground or through flow-controlled
discharge to a watercourse or sewer.
Infiltrating runoff through the soil into Detailed Design
underlying geology is the first preference. Sections 6.4.3.1 and 6.4.3.5 cover the basics
Where soil, geology or ground conditions do of infiltration and attenuation storage

not enable infiltration, then attenuating flows calculation and should be referred to prior to
and volumes to controlled discharge rates progressing with this section where
would be appropriate. calculation inputs are considered in more
detail.
Both infiltration and attenuation require
storage within the development to hold

9.5.5.1 Infiltration

There are two methods for calculating CIRIA 156 method
temporary storage for infiltration.

The CIRIA 156 method assumes that there
will be infiltration through the base and sides
of the structure on an ongoing basis. Factors
of safety ranging between 1.5 and 10
depending on the consequence of failure, 84

and the area draining to the infiltration Factor of safety applied
structure (see C753 Table 25.2), are allocated
to account for potentially reduced infiltration
over time.

The BRE 365 method assumes that the base
of the system, such as traditional soakaway, BRE 365 method
will silt up and therefore infiltration is only
calculated through the vertical sides. The
assumption of no infiltration through the
base is the equivalent of the factor of safety.

It is noted that various systems such as
permeable pavement are resilient to siltation. Assume no infiltration through the base

However, infiltration schemes are not
straight-forward and sites which are free
draining can quickly become compacted
during the construction phase.

Oxford City Council SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates