MCU21 0146 Stockwell_Information Request Response Letter · MCU21/0146

Information Required

11. Provide a revised SBSMP and revised preliminary engineering plans which address the
following issues:

a) The stormwater quantity analysis and detention design is required to address climate
change by demonstrating no increase in peak discharges for the 1%AEP at 2100

b) The proposal appears to rely on the extended detention depth within the bioretention
basins for mitigating peak flow impacts. This volume is generally excluded from such
analyses and a revised assessment which excludes the extended detention volume
is requested

c) The model results show almost no change between pre-development and developed
(unmitigated) peak flows from the site for the 1%AEP and 2%AEP events, even
though very significant changes in imperviousness occurs. Explain this result.

d) The development proposes only 14m3 of detention storge to mitigate the impacts,
which is a fraction of the size usually expected for this type of land use change and
site area. Explain this result in consideration of usual expected detention volume
sizing.

e) There is no comparison of the pre-development or developed (un-mitigated) model
results to rational method estimates. This would highlight the above discrepancy in
the 1% and 2%AEP model results. Provide this comparison.

f) Update the sketch designs in Appendix C to clearly show the detention system
referred to in the report

g) Provide design sections showing the interaction between bioretention and detention
systems/storages

h) The use of ensemble temporal patterns is not discussed, and results are provided for
a single duration rather than a range of durations. Provide updated results which show
no increase for each AEP for a range of durations and based on the median temporal
pattern results for each AEP/duration combination

i) Part of the site is proposed to discharge by kerb adaptor to Hoffman Drive. The
preference would be for no new kerb adaptors. Either remove connection by kerb
adaptor or justify the proposal in reference to the capacity of Hoffman Drive and the
peak flowrate proposed to be discharged by the kerb adaptors

j) Clearly identify on a catchment plan any existing external catchments which need to
be accommodated by the design (even is such catchment will ultimately be diverted
by future development of those sites)

k) Include analysis for the 1:1yr ARI (1EY) event in order to demonstrate compliance
with the waterway stability management design objective

Response

Refer to attached cover letter from engineer with updated drawings and report.

   

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