RFI - Engineering - Stormwater

                                UNIT DEVELOPMENT
    24 KATHARINA STREET AND 10 NATASHA AVENUE
                                      NOOSA HEADS

TECHNICAL NOTE ON STORMWATER MANAGEMENT

Introduction

Tod Consulting have been engaged by Mark Bain Constructions Pty Ltd to prepare preliminary
civil engineering plans for a proposed 8 unit development at the above site. The scope includes
design of the driveway and turning areas, bulk earthworks and preliminary stormwater drainage
design. This technical note focuses on the stormwater drainage design and management.


Stormwater Quantity

The total site area is 2039m2 and currently contains two separate residential unit complexes. The
proposed redevelopment of the site increases the impervious area slightly, in the order of 250m2.
As such it is proposed to include a detention tank under the lower driveway area to mitigate any
increase in peak flows. A detailed assessment and sizing of the detention tank will be conducted
during the detailed design phase, however for the purpose of the preliminary design and MCU
development application we have adopted a tank size based on a Sunshine Coast Council
deemed to comply solution for medium density development – refer SCC fact sheet attached. As
outline in table 2 of this document the detention volume is based on 90m3 per Ha. In this case
based on the total site area of 2039m2 the detention tank would be 18,000 litres. We believe this
is a very conservative approach and the final tank size is likely to be smaller but have
demonstrated that an 18,000 litre tank can fit under the driveway servicing the Katharina Street
units.

Runoff from the unit complex will be discharged to Council’s drainage infrastructure via a new
manhole over the existing drainage pipe in Katharina Street as shown on the preliminary
drawings.


Stormwater Quality

The State Planning Policy 2017 specifically excludes the need for any water quality treatment for
developments under 2500m2, however it is noted the new 2020 Noosa Plan overrides the SPP
and requires water quality treatment to be addressed for developments of all sizes. With
developments under 2500m2 it is often difficult to provide sufficient space for stormwater
treatment devices and this is certainly the case with this development. Notwithstanding, we have
identified a section of the site landscaping that can be constructed as a bio filter area that will treat
run off from 75% of the site (upper catchment area). The upper catchment area is 1500m2 and
based on Healthy Waterways deemed to comply solutions for Stormwater Quality Treatment for
the North Coast region, the filter area would need to be at least 1.7% of the catchment area, or
26m2. Refer deemed to comply solution R5 for appendix D2 of the above document – extract
attached below. We have determined and a filter area in excess of the above area can be
constructed in the garden bed at the rear of the upper catchment, driveway area.


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