BsmFinal_1

1.0     INTRODUCTION AND SUMMARY

1.1     Introduction

Brisbane Stormwater Management (BSM) has been commissioned by Mr Mark Jones to address flooding
issues raised in an 8th December 2017 information request by Noosa Council in relation to Mr Jones’
proposed development of 11 Church Street, Pomona. The location of the site is defined in Figure 1 on
page 6 of this report. The development involves reconfiguring the site into four new lots as shown in
Figure 5 on page 10 of this report. The relevant part of Council’s information request is:

Issue

The site is affected by flooding and the extent of flooding is a critical element in determining if there is
sufficient building area within the lots and if safe access can be provided. In order to address the effects
of flooding the submitted plan indicates that the Defined Flood Level at the eastern boundary is RL 91.35
m. This level has been assumed to be the flood level across the site. It is noted that the Defined Flood
Level is the same as that indicated on a recent Flood Information Certificate issued by Council
(reference: CER17/2341 dated 10 August 2017). This certificate also included a flood level on the
western boundary (RL 91.85), a notation regarding the flow in the southern gully and an attached sketch
plan. The southern gully is noted as “Flow in this gully is not modelled so only backwater is shown.
Flood level and extent may not be correct”.

As the flooding in the southern gully is a critical element in the subject reconfiguration it must be
modelled and the potential impacts investigated. Please note that Council can provide access to its
existing flooding modelling in the area for a fee.

Information Required

1.  Provide suitable flood modelling for the southern gully. Provide amended plans that are based on
    the flood modelling for the site as a whole (northern and southern gullies) and include:
(a) Bearings and distances for the 1% AEP flood line with any areas below the flood line included in a
    “Drainage Easement”
(b) The area available within the lots above the 1% AEP flood line.

2.     Provide preliminary engineering designs of the access driveway to the house sites on Lots 2 & 3 and
       the associated stormwater system. The designs must be based on the flood modelling of the site and
       be in accordance with QUDM providing for a safe access for pedestrians and vehicles.

1.2     Summary

BSM’s contribution to the required information comprises the flood modelling, including the definition
of the relevant flood levels and flood boundaries together with the calculation of suitable culvert sizes
and depth-velocity parameters on the critical access points. The conversion of the calculated flood
boundaries to “bearings and distances”, the calculation of flood-free areas and the preliminary design of
the access driveways and associated stormwater systems are being undertaken by others.

The calculated flood boundaries and the recommended culvert sizes are shown in Figure 5 on page 10 of
this report. The culverts have been dimensioned to reduce upstream affluxes to practically zero (i.e. a
maximum of 0.01 metre, or “within the accuracy of the modelling”. Downstream impacts are practically
zero. The relevant design flood levels are summarised in Tables 4 and 5 on pages 11 and 12. The highest
depth-velocity parameter is 0.12 – i.e. safe.

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      Brisbane Stormwater Management Pty Ltd Document File: ChurchSt.Doc