1909-13367 SRA referral response approved docs

5   CO N CLU S IO N


The proposed development consists of 245 lots to be developed with relocatable dwellings.

This report has outlined how the proposed development site (and the proposed bridge across the
existing natural channel) will not result in any nuisance to neighbouring properties for design events
ranging from the 50% to 1% AEP.

Hydrologic modelling has been completed using the integrated 1D/2D modelling program XPStorm.
XPStorm uses Laurenson’s method to compute unit hydrographs based on various user inputs. The
critical storm (defined as the storm producing the highest median peak flow for each duration) was run
through the 2D model using the modelling parameters detailed above to establish a base case.
The base case identified that flows approaching the existing infrastructure under Myall St to the South-
west of the development overtop Myall St during the 50% AEP event and flow through the development
site through the natural channel.

The proposed bridge crossing has been designed to maintain the flow widths of the existing natural
channel to ensure that no adverse effects occur on neighbouring properties. No filling outside of the
bridge abutments is proposed within the existing channel.

The developed models have shown a decrease in peak water elevation upstream of the proposed
bridge.

Peak flows downstream of the development site have been reported to determine the effect of the
proposed bridge and development on the downstream flow condition. It was found for all cases that
peak flows downstream of the development have decreased. This is attributed to the modelling of the
stormwater detention system detailed in the separate SWMR by Westera Partners.

Afflux has been checked for all design events. No afflux has been reported within neighbouring
properties or the road reserve for events upto and including the 5% AEP. Afflux shown in the 2% and
1% is located within the development property and golf course and has been attributed to the proposed
VXO off of Myall St and the overflow from the proposed bioretention basins associated the proposed
development.

As peak water levels upstream of the proposed bridge and peak flows downstream of the bridge are
lower for each respective design event it is deemed that the bridge does not have any adverse impact
on the conveyance of flow through the site up to the 1% AEP design event. Additionally afflux mapping
of water elevation and flow velocity have shown that no additional nuisance will occur due to the
presence of the development.




                                              Flood Study                                            35
S19-007                                122 & 144 Myall St, Cooroy