Page 106 - Hillingdon SuDS DESIGN & EVALUATION

Detailed Design Approach 1 - worked example

For the purpose of the example the following
rates are assumed:
•
1 in 1 year 3.5 l/s
•
1 in 100 year 11.1 l/s
Depths of storage are assumed as 150mm and
600mm for 1 in 1 year and 1 in 100 year return
periods respectively.
1 in 1 year
65mm opening with 150mm depth of storage for 1 450mm additional storage - 422mm head for 2nd orifice
in 1 year, which provides 3.5 l/s outflow . 600mm – total depth of storage 1 in 100 year rainfall + CCA
1 in 100 year 55mm dia.
65mm opening for 600mm depth of storage opening
provides outflow rate of 6.9 l/s. Allowable
discharge is 11.1l/s.
Therefore 11.1 – 6.9 = 4.2 l/s. The additional flow
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
utilised.
Using an additional 55mm opening with invert
150mm above base invert of storage provides 4.2
l/s outflow

101
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
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.

The Qbar (or Qmed) flow rate will occur Define the maximum storage depth.
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
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.
it may also result in increased storage 4. Using the appropriate orifice equation
volumes. establish the opening size which will
The following steps outline the process of convey the required QBar flow rate at the
calculating the opening size of an orifice to defined 1 in 100 year head (depth of water

discharge at Qbar rate. above the orifice).

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