Pre-Request Response Application 132001.210310.1

Flood Investigation & Stormwater Quantity Management Plan                                               BROWN
                                                                                                              S...nWhlD’
Proposed Residential Development at Doonella   -

                                                   Stage C



4.0       HYDRAULIC ANALYSIS

Hydraulic modelling was undertaken using the steady state component of the computer package HEC-RAS (version
4.0) to determine the 100 year ARI flood level and extents of flooding along the environmental corridor and confirm
proposed lot and road levels have appropriate freeboard or an acceptable level of flood immunity.

4.1       Assessment Criteria

The criteria for assessment of the environmental corridor are listed below:

      .   300mm of channel freeboard be provided to the proposed lots adjacent to environmental corridor, as per

          Table 7.03.1 of QUDM (2007);

      .   The depth and    velocity of flows overtopping Dodonaea Close are limited to the acceptable criteria for
          trafficability and access during the 100 year ARI event in accordance with Table 7.03.1 of QUDM (2007);
      .   All flows to remain within the environmental corridor. No overtopping of banks along the corridor or levees

          towards the downstream extent (between cross sections 121 to 90); and

      .
          Sensitivity Check for Manning’s n    =    0.15 to demonstrate that freeboards are adequate and no breeching of

          levee banks occur.


4.2       HEC RAS Analysis

The following sections discuss the methodology and assumptions made for the HEC RAS modelling.

4.2.1     Cross Sections

Survey of the site and the adjacent development detention basin was undertaken by RPS (formerly PMM Sunshine
Coast Pty Ltd). From the survey, 15 cross sections were extracted along the environmental corridor and input into

HEC-RAS.      The most downstream section (Section 60) was taken through the detention basin on the adjacent

property. The most upstream section (Section 348) was taken just downstream of Beckmans Road. Refer to Drawing
B09182.W.SK02 in Appendix D for locations of the cross sections.

4.2.2     Manning’s Roughness
Manning’s Roughness coefficients used in the HEC-RAS analysis are indicated below in Table 4.1. Roughness values
have been determined from site inspection and aerial photography in accordance with the Brisbane City Council

Natural Channel Guidelines (2003). Roughness values have been entered using the ’Horizontal Variation in n Values’

function in HEC-RAS according to the density of vegetation along each cross section.

                       Table 4.1: Manning’s Roughness Coefficients used in HEC.RAS Model

        Location on Cross Section                     Manning’s Roughness                    Description
                                                                n

                                                               0.05                 Beyond environmental corridor
        Left & Right Over Bank                                 0.06                       Rock boulder wall
                                                              0.035                        Block work wall
                                                               0.1                         Vegetated Areas
        Main Channel                                           0.06                       Rock boulder wall
                                                              0.035                        Block work wall



4.2.3     Boundary Conditions

The boundary conditions at the downstream extents of the model were entered as a ’Known Water Surface’ into

HEC RAS. The 100 year ARI water surface level (of RL 7.6m) was obtained from the JF&P (2008) report (refer




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