Revised Civil Engineering Response Cover Letter

22.1 Sizing appears to be small given the land use change. This may be due to the low
initial loss values adopted for pervious surfaces in the model. Provide further information
of how rational method flows were calculated, including tc estimates. Provide, as a
comparison, detention sizing based on QUDM recommended initial pervious loss rate of
20mm.
22.2 Provide survey for the existing drainage system that the detention tank will connect
into (i.e. existing pit IL and pipe diameter and outfall IL).
22.3 The outlet structure from the detention tank is unclear. Is the final outlet an upstand
225 dia pipe as seems to be shown? If this is the case won’t the upstream orifices all be
drowned?
22.4 Regardless of the above, for whatever outlet pipe is selected for the detention tank,
demonstrate that the final outlet won’t drown the orifices and that the assumptions in the
model will be met (eg. The outlet pipe from the tank should be set down lower than the
lowest orifice so that it does not become drowned)
22.5 An access chamber to the detention basin outlet structure is shown and is supported.
However why is the lid on this chamber shown as a grate rather than a sealed lid? Won’t
the grate allow flows to bypass the detention tank?

RESPONSE
Item 21. From our analysis of Council assessment of Lot 5 Taylor Court, there appears
the need for a detention basin on the subject site to account for high peak stormwater
discharge to the downstream network. Unfortunately, we do not have the report or
correspondence to confirm the downstream basin provides detention of flows to this site
and from conversations with Council staff, are of the opinion it provides a water quality
function only. If Council has access to this drainage study and basin design report, we
would appreciate a copy. Despite this, it is acknowledged that any downstream basin
structure will provide some form of flow mitigation to runoff entering the basin.
Item 22.1. Rational method calculations were provided in Appendix C of the stormwater
management report 2123-SBSMP-01. Tc is calculated using the friends eq’n for overland
sheet flow from QUDM. Initial and continuing loses for the XP Storm model were then
adapted to these Rational method results as per Tables 4.2 an 4.3 in the report and Figure
4.1. The pre-development rational method values were consistent with the XP Storm
outputs however they were typically 5% less in magnitude so the existing case rational
method results were used as the base case to compare against the post development
(mitigated) XP Storm model results. This builds in a conservative 5% overdesign into the
detention model.
Item 22.2. It is recognised that The invert of the existing pit was not included in the original
site survey. Barlow Shelley attended site to measure the depth of the SW pit and confirm
it is 1.65m deep. This should be confirmed prior to construction but established that there
is sufficient depth to reliably connect the stormwater system to this existing pit structure
and outlet pipe.
Item 22.3. The hydraulic orifice controls are mirrored into the wall of the riser pipe. This
was not clear on the original plans so please refer to the updated drawing 2123/R07 (Rev
C).
Item 22.4. The outlet pipe from the tank exits the detention structure through the floor.
The lowest pipe orifice is the tank invert so the system cannot become permanently
drowned in function. There is a second maintenance access in the pipe riser and overflow
area for maintenance to clear this space of debris if required. In the event this area is not
maintained and a significant storm (or design storm grater than the 1% AEP) is
experienced on site. A weir to the south side of the bioretention basin provides overflow
redundancy so the drainage system inside the development site does not back up and
enter the units.



2123 – 3 Taylor Court, Cooroy                                                                 4
CL/cl