EagFirRsp

years and have thoroughly checked its calculations of peak runoff rates from individual subareas against
Rational Method calculations of the same runoff rates and found it to be a satisfactory method. In this instance
it has been applied equally to the pre- and post-development scenarios. Agreed with Premise that the method
can continue to be used.

Information Required

Ensure the stormwater quantity study employs methods that are consistent with one version of ARR. Apply
appropriate routing methods and parameters to the XPSTORM model. The Rational Method can be used as a
‘checking tool’ but not for calibration of numerical models. See relevant comments above.

Considering the length of the basin’s design life, ensure the freeboard provided is sufficient under a climate
change scenario to protect adjacent residential properties. Premise noted that for a 60 year lifespan (i.e. to
about 2080) the increase in the rainfall intensities should be 15 percent. BSM previously checked the spillway
for a PMF equal to 400 percent of the Q100 and found it to have sufficient capacity to accommodate the PMF.
Remodelling the system with 115% of the 100 year rainfall indicates that the extra runoff will utilize just 0.02
metre of the 0.30 metre freeboard.

Ensure only major flows exit the basin over the weir and that water exiting the basin in this manner flows to
the parkland and does not enter adjacent residential properties. Premise have clarified that they were not
seeking to restrict the basin overflows to major storms only - instead BSM can use whatever overflow
frequency they prefer. However, Premise were wanting to keep the overflows out of the adjacent residential
properties. The design of the basin inlet at the sag in Eagle Drive, with its accompanying filling of driveways
and verges, provides for complete capture of all flows up to AEP1% and thereby eliminates the existing
overflows from Eagle Drive to the properties adjoining the basin.

The underground pipe outlet for the basin substantially reduces the quantum of overland flows through the
park adjoining the Tait Street properties, as documented in Table 3 on the following pages of this response.
However the topography of the park is such that the existing major overflows through the park probably
spread out onto No 21 Tait Street and, even with the reduced overflows, some part of those overflows might
still continue to spread out onto that property albeit at a much reduced rate. If this is a critical issue, the
applicant could construct a low bund along the boundary between the park and No 21 Tait Street to confine
the overflow to the park.

Revision To Impact Mitigation

The revised modelling results in some changes to the basin configuration. The rectangular low-level orifice in
the outlet box is enlarged from 100 mm x 90 mm to 200 x 100 mm. The 1 metre wide weir in one side of the
outlet box is lowered by 0.005 metre (5 mm), to EL 4.86, and the other three sides are raised by 0.03 metre to
EL 5.14. The spillway in the embankment crest is raised by 0.02 metre, from EL 4.98 to EL 5.00. The revised
basin and its inlet/outlet details are shown in Figure 5B attached hereto. The 100 year storage volume is
increased from 510 cubic metres at a water level of EL 5.11 to 523 cubic metres at EL 5.14. The key impacts
with this arrangement are described hereafter for the following locations:

- Water levels at node N36, at the Eagle Drive sag.

- Overflow rates through the park, between nodes N34 and N22.

- Overflow rates across Tait Street, between nodes N22 and N17.




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BSM Doc File EagFirResp.Doc