Page 47 - Updated SuDS Design & Evaluation Guide-Newham V3 Spreads

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Concept Design 7.4.10 Infiltration The depth and location of infiltration tests 7.4.11 Managing runoff from site Some of the pre-development volume losses Concept Design

If the site does not infiltrate effectively over
After any allowances have been made for the
can be mimicked by using SuDS components
all return periods, then rainfall will leave the
should reflect where infiltration is proposed
potential to harvest runoff, the next
on site. Shallow features such as permeable
to demonstrate interception losses and
consideration in managing flows and volumes
site as runoff to a watercourse, the surface
ongoing losses (Long Term Storage). Other
water sewer or combined sewer. The
is to assess the ability of a site to infiltrate
pavements will require shallow infiltration
rainfall completely, partially, or discharge
greenfield flow rates from the site must be
tests.
methods such as rainwater harvesting will
calculated, and then attenuation volumes
largely as runoff.
further reduce the additional volume
Guidance exists which states that where
infiltration features are situated within 5m of
The ability of a site to infiltrate water should
foundations, the risk to the foundations
The approach to managing flows and
be evaluated considering:
Rainfall calculations are necessary, even at
volumes from developments - set out in the
should be considered. This is usually applied determined. generated by the development.
Concept Design stage, to gain an idea of
■ the nature of the soil geology and
as a general rule where infiltration within the volumes of runoff to be stored on site. NSTS - seeks to minimise the impact of the
capacity to infiltrate
5m offset from the foundation is not additional volume generated by development
■ the risk to stability of the ground where permitted. However, the guide was originally These calculations can also be used at the as well as control the rate of runoff to pre-
infiltration is proposed intended for point infiltration soakaways in Outline Design stage, but may need to be development patterns.
re-assessed at the Detail Design stage.
susceptible soils. SuDS design encourages
■ the risk of pollution to groundwater It allows a variable ‘greenfield rate’ of runoff
‘blanket infiltration’ features that are less New hard surfaces that are introduced
■ the depth of seasonal groundwater likely to affect soil conditions, as they mimic through development increase both the rate from development between the 1 in 1 and 1 in
grass surfaces around buildings. The distance and volume of runoff. This is because runoff 100 year return periods with the additional
■ the risk of unpredictable pathways being volume generated by the development
offset for infiltration will be at the flows more quickly from the site, and natural
taken by infiltrating water. allowed to discharge at a maximum of 2 litres
professional judgment of a suitably qualified volume losses do not happen as they did per second per hectare. This approach
engineer. before development.
41 (Approach 1) is now the preferred method 42
Infiltration will generally be possible if the Additional site investigations will be The additional rate of runoff is managed set out in the 2015 SuDS Manual. Managing
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infiltration rate is 1 x 10 ms (36mm/hr) or necessary to assess risks associated with through attenuation storage. flows and volumes to a single Qbar discharge
greater, subject to the soil and subsoil infiltration, and should follow guidance in the rate (Approach 2) may be acceptable if
retaining infiltration capacity following CIRIA SuDS Manual 2015, Chapter 25 p543. Approach 1 can be shown to be unachievable.
construction or site disturbance. Infiltration is
still viable on sites with lower infiltration BGS Infiltration SuDS map
rates, however additional storage capacity
would be required to allow time for flows to www.bgs.ac.uk
infiltrate.
See Section 7.4.13 for more info on
Measures must be taken to protect infiltration Using SuDS Close to Buildings
capacity during construction. Compaction of Flow rate calculations

soil layers may affect the ability of sites with www.susdrain.org
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infiltration rates lower than 1 x 10 to allow
water to soak into the ground. These sites are Risks Associated with Infiltration
particularly susceptible to damage due to Design Note:
construction activity. CIRIA SuDS Manual 2015, Chapter 25 The website www.uksuds.com provides estimation tools for the calculation of ‘greenfield
runoff rates’, ‘attenuation’ volumes and ‘long-term storage’ volume losses.

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