Appendix D - Flood Assessment

R1593: FLOOD ASSESSMENT – 9A JIRRIMA CRESCENT, COOROIBAH



4.4.             HYDRAULIC MODELLING                                                   4.4.2.    ROUGHNESS
                                                                                       Review of aerial photography was undertaken to determine the Manning’s roughness
The unsteady state, 1D/2D hydraulic model XP-STORM (incorporating TUFLOW as            coefficients and expected loss rates within the catchment. Based on this assessment,
the model engine) was used to analyse the flood behaviour of the study area. LIDAR     five land uses/roughness coefficients have been applied to the modelling as
data from DNRM was applied for the external catchments, while detailed site survey     summarised below. The default land use is Grass.
was used to represent the site. This data was converted by XP-STORM to create a
TIN for the study area, from which 1.25 m (2D) cells were generated. Flows are             +    Roads and driveways                      n = 0.015
transferred into the 2D domain, and flow overland as the topography dictates. Flood        +    Grass/lawns (default)                    n = 0.045
levels, discharge and velocity can be extracted from the model as functions of time        +    Mature Trees/Dense Vegetation            n = 0.080
at required locations.                                                                     +    Carports/sheds                           n = 1.000
                                                                                           +    Dams/water bodies                        n = 0.025
4.4.1.           BOUNDARY CONDITIONS
                                                                                       Dwellings have been modelled as inactive areas and therefore no flow can pass
Hydrographs generated by the hydrologic module of XP-STORM are applied as the
                                                                                       through the buildings. The XP model features are illustrated as Figure 6.
inputs to the 1D hydraulic model and as point flows inside the active 2D domain.

The downstream extent of the model has been extended downstream of the site, with      4.4.3.    1D ELEMENTS
the boundary location chosen to ensure it has no impact of the observed flood levels   Cross drainage for the roadways and under the proposed driveway have been
at the site or the proposed driveway crossing. Normal flow conditions are assumed      represented as 1D elements, with the inlet and outlet nodes linked to the active 2D
at the outlet.                                                                         domain. Inlet and exit losses of 0.5 and 0.7 respectively have been applied to the
                                                                                       culverts (in accordance with QUDM (2013) Section 7.16.9 (b)(ii). Culvert details are
                                                                                       provided in the civil plan set included as Appendix A.




Environment. Engineering. Education.                                                                                                                                 PAGE 6