Page 105 - Newham SuDS DESIGN & EVALUATION GUIDE

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opening size until the 1 in 1 year event is
the 1 in 100 flow rate - for example this
Detailed Design 9.6.8 Sizing flow control openings 9.6.8.1 Approach 1 methodology 4. Make a note of the calculated opening 8. Re-run the calculations for the 1 in 100 Detailed Design
attenuated to the 1 in 1 discharge rate and
may be 350mm for a permeable
make a note of the resulting maximum
An orifice opening will deliver variable
The following methodologies for sizing flow
pavement or up to 600mm for basins.
storage depth.
outflow rates as the severity of rainfall
controls are intended for use by those with
knowledge of hydraulic calculations. Careful
increases, producing and storing more runoff.
size to achieve the 1 in 100 flow rate at
year event based on the changed
consideration should always be given to the
As the depth of stored water increases the
this storage depth.
opening. The maximum flow rate will now
selection of equations and coefficients.
gravitational pressure forces more flow
5. Based on the same storage component
be below the allowable discharge rate
through the opening - sometimes referred to
Section 6.4.3.6 outlines two approaches for
design and flow control opening, calculate
resulting in more storage than is
as the ‘driving head’ of water stored.
the control of flow, summarised as follows:
– make a note of the maximum storage
opening may be placed above the 1 in 1
The following steps outline the process of
Approach 1 – Variable control
depth and maximum flow rate. Note that
storage depth noted in step 7. Add a
calculating the opening size of an orifice flow how a 1 in 1 year rainfall event will behave necessary. To overcome this, a second
the volume and therefore driving head will
Non Statutory Technical Standard S2 allows control to meet the requirements of NSTS S2: be significantly smaller for the 1 in 1 year second opening so that it’s lower most
for varying the outflow rate for the 1 in 1 year rainfall event and therefore the flow rate point (invert) is at or above the 1 in 1
storage depth and recalculate the storage
and 1 in 100 year greenfield runoff rates for 1. Establish the controlled outflow (or through the orifice will be significantly behavior in a 1 in 100 event. Adjust the
the respective rainfall events. Greenfield runoff) rates for the 1 in 1 year lower. opening size and height above the 1 in 100
and 1 in 100 year rainfall event. 6. If the calculated maximum flow is less storage depth until the 1 in 100 flow rate is
Approach 2 - Qbar method achieved at the maximum storage depth
2. Define the first, lower orifice invert. A that the 1 in 1 year control rate then the
Where the design requirements for volume opening does not need changing. for the 1 in 100 event.Bo
reasonable starting point is to set the
control (S3) cannot be achieved then all invert at the base (or slightly below the 7. If the calculated maximum flow for the 1 in
runoff from the site for the 1 in 100 year event 1 event is larger than the 1 in 1 year control Design Notes:
base) of storage.
including CCA should be discharged at a rate then reduce the opening size and Both the 1 in 1 and 1 in 100 discharge
maximum Qbar rate (or equivalent) for the 3. Calculate the maximum storage depth for recalculate based on the 1 in 1 event being rates can be achieved by any
your SuDS component, based on its mindful that the 1 in 100 year scenario will
99 development. A lower flow control threshold have to be reconsidered. Amend the combination of the following: 100
of 2 l/sec/ha is acceptable to enable catchment, for the 1 in 100 year event and ■ Adjusting the depth of each defined
reasonable drain down times. storage tier by adjusting the area and
therefore volume of each tier
It is noted that the maximum Qbar rate is ■
only reached when the SuDS component is required 1 in 100 flow rate Incorporating one or more additional
openings
full and the design head reached.
Other options can be explored where
there is difficulty in matching outflow
relationship between driving head and flow rates derived by area drained rates for both the 1 in 1 year and 1 in 100
flow through an orifice flow control and respective growth curve year flows:
■ Try different types of openings such
Head as rectangular and v-notch weirs.

■ Store for a different return period – it
is not necessary to store for the 1 in
100 year return period in every sub-
required 1 in 1 flow rate catchment. The final discharge from
the site must meet requirements of
Graph comparing required flow rates
and the variable flow rate through a NSTS.
simple orifice as head increases. Flow

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