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Detailed Design Approach 1 - worked example 9.7 Water quality Open water features should not receive flows Detailed Design
Rainfall picks up pollution from development
For the purpose of the example the following
surfaces. As runoff moves slowly through
directly from development without sufficient
rates are assumed:
•
1 in 1 year 3.5 l/s
SuDS components most pollution is removed
treatment.
•
1 in 100 year 11.1 l/s
through sedimentation, filtration and
■
Hydrocarbons remain in pond sediments
Depths of storage are assumed as 150mm and
bioremediation. Naturally occurring
for extended periods.
600mm for 1 in 1 year and 1 in 100 year return
processes in many SuDS components break
periods respectively.
1 in 1 year
the aquatic environment and add to
no build up or need for removal of this
65mm opening with 150mm depth of storage for 1 450mm additional storage - 422mm head for 2nd orifice down organic pollution, meaning that there is ■ Silts which carry heavy metals impact on
in 1 year, which provides 3.5 l/s outflow . 600mm – total depth of storage 1 in 100 year rainfall + CCA pollution over time. maintenance problems due to the build-
1 in 100 year 55mm dia. The NPPF sets an obligation on proposed up of toxic sediments.
65mm opening for 600mm depth of storage opening development to have no negative impact on The amenity and biodiversity value of ponds
provides outflow rate of 6.9 l/s. Allowable the environment and encourages provisioning and wetlands should be protected with
discharge is 11.1l/s. opportunities for biodiversity and habitat pollutants removed at source and along the
Therefore 11.1 – 6.9 = 4.2 l/s. The additional flow creation, not just in the wider landscape, but management train.
will be provided by an additional opening which 150mm 1in1 storage 65mm dia.
will only operate once the 1 in 1 year storage is opening within development.
utilised. Using source control and the management
Using an additional 55mm opening with invert train, SuDS delivers the requirements of
150mm above base invert of storage provides 4.2 NPPF by providing a controlled flow of clean NPPF Paragraphs 165 and 170
l/s outflow
water through the development.
101 102
9.6.8.2 Approach 2 methodology
1. Establish the Qbar rate for the flow
A single opening can also be sized to control location. The Qbar rate should be
discharge at Qbar for the 1 in 100 year + CCA proportional to the contributing
rainfall event. This does not meet the catchment.
requirements of NSTS S2 but can be 2. Define the maximum storage depth. For 9.7.1 The objectives of designing for water quality
considered to demonstrate S6 as more flow example 600mm could be adopted for
is held back on site for longer. the 1 in 100 year + CCA rainfall event. ■ Treat runoff to prevent negative impacts ■ Manage surface water runoff at or close
The Qbar (or Qmed) flow rate will occur Define the maximum storage depth. to the development’s landscape and to source and at or near the surface
biodiversity as well as receiving where possible to begin treatment quickly
whenever the storage volume is full and the 3. Define the orifice invert. A reasonable
design head is reached. This methodology is starting point is to set the invert at the watercourses and water bodies within the and maximise treatment through the
wider landscape. system.
simpler to apply than Approach 1 as there is base (or slightly below the base) of
only one target flow to be sized for, however, storage. ■ Design for interception losses to occur for Where water quantity design adopts a SuDS
it may also result in increased storage 4. Using the appropriate orifice equation most small rainfall events so that the most management train approach, as outlined in
volumes. establish the opening size which will polluted part of runoff is more effectively this document, water quality objectives are
The following steps outline the process of convey the required QBar flow rate at the held and treated on site. normally achieved by default, due to the
calculating the opening size of an orifice to defined 1 in 100 year head (depth of water number of components already limited in
discharge at Qbar rate. above the orifice). series.
London Borough of Newham SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates London Borough of Newham SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates
