Flood Investigation & Stormwater Quantity Management Plan

Flood Investigation & Stormwater Quantity Management Plan
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


B09182.W-02A                                                                                                      Page 9