RFI - Engineering - Stormwater

On-site Underground Detention Storage Orifice Plate Sizing
for Deemed-to-comply Detention
October 2020
This fact sheet describes the steps that
should be taken to size an orifice plate for
on-site underground detention when
using deemed to comply detention
storage. The standardised orifice plate
design will help to ensure post-
development flows match pre-
development conditions for a range of            The outlet pipe to which the orifice plate is
storm events.                                    installed should be sized to convey the 10%
                                                 AEP (at 2100) peak flows. This is to allow for
An orifice plate is required to ensure that      potential future upgrade of the receiving
peak flows being discharged from an              drainage network. Allowance for blockage of
underground storage tank can be attenuated       the underground stormwater network and
for a range of storm events. Without an          surcharge of the underground storage
orifice plate, outflows would be controlled by   should be considered in the siting and
the capacity of the outlet pipe. As the outlet   design of the underground detention storage.
pipe will typically be sized to convey the 10%   The overland flow path is required to have
Annual Exceedance Probability (AEP) event        sufficient capacity so that surrounding floor
(at 2100) peak flows, the post-development       levels will not be inundated if there is
discharges will likely exceed pre-               complete blockage of the underground
development peak flows for smaller AEPs if       stormwater drainage network during the 1 in
an orifice plate or another form of staged       100 AEP (at 2100) event.
discharge control is not installed.
                                                 The stormwater drainage upstream of the
Existing drainage networks, particularly in      detention storage should be designed so that
older established urban areas, were              the 1 in 100 AEP peak flow can be conveyed
generally sized to convey the 39% AEP peak       to the underground detention storage.
flows. Therefore discharging at higher than
pre-development rates during smaller storm       To the maximum degree practical, the entire
events could cause local flooding issues for     site should be drained to the detention
surrounding areas. Control of peak flows         storage. Where this is not possible, the
during smaller events is also necessary in       equivalent storage for the entire site should
order to reduce the risk of increased scour      be provided and the outlet should be
and erosion in receiving waterways.              reduced in size to achieve the same overall
                                                 site discharge. A maximum of 20% of the
Having an outlet pipe sized to convey the        developed portion of the site is allowed to
smaller 39% AEP peak flow, without an            bypass the detention storage.
orifice plate, so that outflows would be
restricted for all storm events, would require   The depth of the required underground
more detention storage volume than               detention storage is flexible in order to allow
proposed by Table 2 of the Flooding and          for a range of proprietary detention storage
Stormwater Management Guidelines (SCC,           products to be utilised. Pre-treatment of
2018).                                           flows entering the tank should be provided
                                                 by a Gross Pollutant Trap (GPT) or gully pit
                                                 baskets.




                                                  Fact Sheet On-Site Detention Orifice Plate Sizing