06/0210 - Response to request for information darted 14/03/06 [As Published] · 132006.210

1. INTRODUCTION

Brisbane Stormwater Management (BSM) has been commissioned to determine the 100 year flood
levels and flood boundaries on a property which is described as Lots 2 & 3 on RP 67407, at 80
Kauri Street and 17 Topaz Street Cooroy. The owner of the property, Mr W Hume, has an
application before Noosa Shire Council for construction of a duplex dwelling on the property.

The site location and the contributing catchment areas are shown on Figure 1 of this report. A
significant floodway crosses Topaz Street immediately to the west of the site. The site also adjoins
an overflow from the kerb at the junction of Kauri and Topaz streets during major storm events.
Officers of Noosa Shire Council have also queried the possibility of overflows to the subject
floodway from an adjoining catchment to the south, which is also marked on Figure 1.

2. OVERFLOWS FROM THE SOUTH

The adjoining catchment to the south joins with the subject floodway about 500 metres downstream
of Cooroy Mountain Road. The potential overflow points are at Cooroy Mountain Road itself, and
at a low-point in the catchment divide just upstream of Cooroy Mountain Road. A ground survey by
Max Watterson & Associates has found that the overflow level in the cutting of Cooroy Mountain
Road is at EL 107.22 m, while the low-point on the catchment divide just upstream of the road is at
EL 107.53 m. These overflow levels are marked on Figure 1 and the survey data itself is marked in
more detail on Figure 2.

The design of the nearby railway bridge, as shown on Queensland Rail’s Drawing Number $20656
(which is reproduced as page 14 of the Appendix hereto), was based on a 100 year design flood
level of EL 105.26 m. This level is also marked on Figures 1 and 2. Note that the potential
overflow points are downstream of this bridge.

Since the overflow levels are about two metres above Queensland Rail’s 100 year design flood
profile, there is suggested to be no possibility of overflow from the southern catchment to the
subject floodway during a 100 year storm.

3. DESIGN FLOW RATES

The contributing catchment areas are delineated on Figure 1. The contributing areas range from as
little as 2.7 hectares for the kerb overflow at the junction of Topaz and Kauri streets, to 17.8
hectares for the main floodway crossing Topaz Street, to 45.1 hectares just downstream of the
Cooroy Mountain Road crossing. Flow rates from catchments of this size can be adequately
predicted by the Rational Method, for which the appropriate calculations are shown on pages 1 to 7
in the Appendix hereto.

The rate of kerb overflow at the junction of Topaz Street and Kauri Street has been calculated by
subtracting the underground pipe capacity from the total flow at this point, as on pages 8 and 9 of
the Appendix.

The calculated 100 year peak flow rates are summarised on page 10 of the Appendix.

 

Brisbane Stormwater Management P/L Doc File Topaz.Rpt - 30 Oct 2006 Page 2