Final Engineering services report _62 Lake Macdonald Drive · OPW25/0057

                                                                                       Flood Hazard Assessment
                                                              Intersection of Lake Macdonald Dr and Dianella Crt



4        HYDRAULIC MODELLING
4.1 Model setup
To determine the potential flooding conditions of the Lake Macdonald Dr and Dianella Crt
intersection a high-resolution HEC RAS (Version 6.5) 2D flood model was developed for the local
catchment and watercourse.

The terrain base for the flood model was a 2022 digital elevation model provided by Noosa Council
derived from aerial laser survey (LiDAR).

The flood model included the entire contributing local catchment area upstream of the subject site
and extended approximately 450 m downstream to a confluence with another minor tributary.

A 2D model grid spacing of 5 m was adopted to ensure the accuracy of the terrain representation
throughout the waterway corridor. The 5 m grid was tested against a 2m grid and produced
consistent results.

Breaklines were incorporated to improve the model’s representation of significant changes in
surface elevation, land use and flow paths.

4.1.1   Model boundaries
To maintain consistency with the Six Mile Creek flood model, tailwater boundary conditions and
upstream tributary flows were extracted from the model and applied to the high-resolution local
flood model.

The downstream boundary of the local flood model was established as a time varying water level
hydrograph. Upstream of the subject site time varying discharge hydrographs were also extracted
from the Six Mile Creek flood model at five reference locations and used as inflow boundaries in the
local model, see Map 3.

Design tailwater level and instream flow hydrographs were extracted from the Six Mile Creek flood
model results for the following design flood events:

       50% AEP (1 in 2 AEP)

       10% AEP (1 in 10 AEP)

       1% AEP (1 in 100 AEP)

       1% AEP (1 in 100 AEP) 2100

       0.2% AEP (1 in 500 AEP)

4.2 Future climate
As a part of the Six Mile Creek flood study, the Cooroy Creek flood model adopted a climate
change/variability rainfall allowance of a 20% increase in rainfall intensity at year 2100.

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