Response to Information Request 10-6-2025 Overland Flow Assessment · OPW25/0042

   –    Sensitivity 1 - Increased flow path roughness (005) – The proposed flow path through the development site is currently proposed to
        have a manicured grass base (n=0.035) with vegetated batters (n=0.06). The sensitivity assessment was completed assuming the
        base is also vegetated (n=0.06).

   –    Sensitivity 2 - Blocked Rapid Infiltration Pit (RIP) (005a) – Assumes the catchments on the north side of Serenity Close that report to
        approved on-site RIP’s are blocked with catchments Lot 4, Lot 5, 4A and 4B all report into Serenity Close. Lots 1 and 2 do not report
        to a RIP and would report to Serenity Close as currently does.

   –    Sensitivity 3 - Partially Blocked Rapid Infiltration Pit (RIP) (005b) – Assumes the catchments on the north side of Serenity Close that
        report to approved on-site RIP’s are partially blocked with catchments Lot 4, Lot 5, 4A and 4B all report into Serenity Close. Lots 1
        and 2 do not report to a RIP and would report to Serenity Close as currently does. For this scenario it is assumed that the RIP can
        manage the Q10 with excess flows reporting Serenity Close.

2.2.1.1 Topography
Model topography has been based upon 2015 Light Detection and Ranging (LiDAR) along with the detailed site survey. Design DEM has been
incorporated into the model to represent the proposed development.

The catchment has been modelled using a 0.5m hydraulic model grid size.

2.2.1.2 Boundaries
The inflow boundaries for the catchment have been read into the model using inflow polygons (2d_sa and 2d_bc) to represent inflow from
the WBNM hydrology model.

The outflow boundary for the model has been applied 70m downstream of the site, which is approximately 5m lower than the site outlet
and is considered suitable for this assessment. A normal depth rating curve has been adopted for the downstream with a water surface slope
of 0.02 m/m.

2.2.1.3 Roughness
The model roughness is represented by the Manning’s roughness coefficient, n, in the TUFLOW materials file. Manning’s coefficients have
been applied to reflect the land use within the catchment, assuming development conditions based on NSC zoning and aerial imagery.
Roughness coefficients are outlined in Table 2.4, while the spatial distribution of material types in the TUFLOW model extent is included in
FIGURE 2.3.

TABLE 2.4: ADOPTED MANNING’S ‘N’ ROUGHNESS

 Land Use                                                               Manning’s ‘n’

 Road                                                                   0.025

 Buildings                                                              0.3

 Vegetated Channel                                                      0.035

 Vegetation                                                             0.06

 Gravel/Grass                                                           0.065

 Dense Vegetation                                                       0.15

 Residential                                                            0.12


2.2.1.4 Structures
The stormwater network represented in the hydraulic model aligns with the stormwater plan and long-section developed by Alta Projects
and are presented in Appendix B:.




NOOSA CALILE I BBNE01676-0001-REP-001-1                                                                                                      7