Eng DA RFI Response

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Requested Information:




Response
A DRAINS model has been built to quickly assess the increase in peak flow off site which has indicated
3L/s for the 10% AEP event and 1L/s for the 1% AEP event and does not consider any stormwater directed
to the infiltration trench though standard losses in line with ARR Data Hub figures have been modelled.
The catchments set up are summarised below:
    • Area – 3520m2 (for both)
    • Fi – 0.22 for existing & 0.44 for developed
    • Tc – 5 mins for impervious areas 9 minutes for pervious areas
This minor increase is considered to be a result of the larger pervious portion of the site dictating the
regime of the peak flow rate for both cases. The infiltration trenches are therefore considered adequate
to ensure no adverse impacts off site as the infiltration rate will more than offset the minor increase in
peak flow.

Any stormwater surcharging from the trenches will have adequate travel distance to the boundary to
ensure sheet flow will be achieved as both trenches are more than 10m from the rear boundary.


Requested Information:




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Requested Information:

Issue — Increase in Fraction Impervious & Stormwater Capacity

Section 3 of Westera Partners’ Stormwater Management Report details that detention of
stormwater is not required despite the proposed increase in fraction impervious from ~16% to
~37%. \nsufficient detail has been provided to demonstrate that the proposed infiltration
trenches have sufficient capacity and the necessary infiltration rate to limit the post-
development peak flow from exceeding the pre-development peak flow during events up to
and including the DFE.

Council is concerned that the development may adversely alter the existing drainage regime

of the site and cause downstream nuisance by failing to disperse flow back to sheet flow if the

infiltration pit surcharges during events up to and including the DFE.

Information Required

5. _ Provide additional detail by a suitably qualified representative of Westera Partners’ that
demonstrates:

a. the peak post-development flow will not exceed the peak pre-development flow in
events up to and including the DFE; and

b. how stormwater surcharging from the infiltration pit will be dispersed to sheet flow.

Response

A DRAINS model has been built to quickly assess the increase in peak flow off site which has indicated
3L/s for the 10% AEP event and 1L/s for the 1% AEP event and does not consider any stormwater directed
to the infiltration trench though standard losses in line with ARR Data Hub figures have been modelled.
The catchments set up are summarised below:

e =Area—3520m2 (for both)

e Fi-—0.22 for existing & 0.44 for developed

e Tc-—5 mins for impervious areas 9 minutes for pervious areas
This minor increase is considered to be a result of the larger pervious portion of the site dictating the
regime of the peak flow rate for both cases. The infiltration trenches are therefore considered adequate
to ensure no adverse impacts off site as the infiltration rate will more than offset the minor increase in
peak flow.

Any stormwater surcharging from the trenches will have adequate travel distance to the boundary to
ensure sheet flow will be achieved as both trenches are more than 10m from the rear boundary.

Requested Information:

Issue — Ground Area Bypassing Infiltration Trenches

Westera Partners’ Preliminary Stormwater Management Plan Sheet 1 of 2 (S23-043-SW01)
details a minimum catchment area of 138m? bypassing the infiltration trenches.

It is unclear how this catchment area bypassing the infiltration trenches will match the existing
drainage regime of the site (i.e., sheet flow) and avoid causing downstream nuisance.

Information Required

6. Provide comment by a suitably qualified representative of Westera Partners’ that
demonstrates how the catchment areas bypassing the infiltration trenches will follow the
existing drainage regime of the site (i.e., sheet flow) and will not cause nuisance
downstream.

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Response
The small ground level areas modelled as bypassing the infiltration trenches are located a minimum of
28.80m from the rear boundary. Further to this these areas are largely pervious so are not considered to
represent any risk of causing nuisance off site.

In summary the proposed stormwater outcome for the site is considered to be appropriate for the scale
of development and is in accordance with that of other similar developments in the region. The proposed
infiltration trenches will adequately offset the increase in impervious area from existing to developed case
and worsening to downstream property is therefore not expected to occur.

Should there be any other queries in this regard please do not hesitate to contact myself at the Sunshine
Coast office.

Yours faithfully,




Jared Hill CPEng | RPEQ 19891
Partner | Sunshine Coast Civil Manager
Westera Partners Pty Ltd




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Response

The small ground level areas modelled as bypassing the infiltration trenches are located a minimum of
28.80m from the rear boundary. Further to this these areas are largely pervious so are not considered to
represent any risk of causing nuisance off site.

In summary the proposed stormwater outcome for the site is considered to be appropriate for the scale
of development and is in accordance with that of other similar developments in the region. The proposed
infiltration trenches will adequately offset the increase in impervious area from existing to developed case
and worsening to downstream property is therefore not expected to occur.

Should there be any other queries in this regard please do not hesitate to contact myself at the Sunshine
Coast office.

Yours faithfully,

EE

Jared Hill cpEng | RPEQ 19891

Partner | Sunshine Coast Civil Manager
Westera Partners Pty Ltd