Page 104 - Bexley SuDS DESIGN & EVALUATION

Detailed Design 9.6.8 Sizing flow control openings 9.6.8.1 Approach 1 methodology

The following methodologies for sizing flow
An orifice opening will deliver variable
outflow rates as the severity of rainfall
controls are intended for use by those with
increases, producing and storing more runoff.
knowledge of hydraulic calculations. Careful
As the depth of stored water increases the
consideration should always be given to the
selection of equations and coefficients.
gravitational pressure forces more flow
Section 6.4.3.6 outlines two approaches for
through the opening - sometimes referred to
as the ‘driving head’ of water stored.
the control of flow, summarised as follows:
Approach 1 – Variable control
The following steps outline the process of
calculating the opening size of an orifice flow
Non Statutory Technical Standard S2 allows control to meet the requirements of NSTS S2:
for varying the outflow rate for the 1 in 1 year
and 1 in 100 year greenfield runoff rates for 1. Establish the controlled outflow (or
the respective rainfall events. Greenfield runoff) rates for the 1 in 1 year
and 1 in 100 year rainfall event.
Approach 2 - Qbar method
2. Define the first, lower orifice invert. A
Where the design requirements for volume
reasonable starting point is to set the
control (S3) cannot be achieved then all invert at the base (or slightly below the
runoff from the site for the 1 in 100 year event
base) of storage.
including CCA should be discharged at a
maximum Qbar rate (or equivalent) for the 3. Calculate the maximum storage depth for
your SuDS component, based on its
99 development. A lower flow control threshold
of 2 l/sec/ha is acceptable to enable catchment, for the 1 in 100 year event and
reasonable drain down times.

It is noted that the maximum Qbar rate is
only reached when the SuDS component is
full and the design head reached.

relationship between driving head and
flow through an orifice flow control

Head

required 1 in 1 flow rate

Graph comparing required flow rates
and the variable flow rate through a
simple orifice as head increases. Flow

London Borough of Bexley SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates