Appendix D - Flood Assessment

R1593: FLOOD ASSESSMENT – 9A JIRRIMA CRESCENT, COOROIBAH



4.2.1.           ROUTING                                                                 Table 3 – Summary of Rational Method Comparison – 1 % AEP

Hydrologic modelling has been undertaken using the Laurenson runoff routing                                              PEAK FLOW RATE (M³/S)
method. The Laurenson method requires the catchment to be divided into a pervious           CATCH ID                                                                 DIFFERENCE (M³/S)
(undeveloped) and an impervious (developed) portion. A fraction impervious of 0 %                               RATIONAL METHOD                  XP
has been applied to the undeveloped portion and 100 % to the developed portion                   1                      1.60                     1.87                         -14
and used to calculate the overall fraction impervious for each sub-catchment.
                                                                                                 2                      1.65                     1.73                         -4
4.2.2.           ROUGHNESS COEFFICIENTS
                                                                                                 3                      0.81                     1.01                        -20
The Manning’s roughness coefficients have been varied for the developed and                      4                      0.66                     0.76                         -12
undeveloped portions of the catchment and include considerations for the extent and
density of vegetation. The following values were applied.                                        5                      0.55                     0.50                         8

                                                                                                 6                      0.29                     0.34                         -13
       +      Developed catchment                          n = 0.015
       +      Undeveloped catchment                        n = 0.045 to 0.065                    7                      0.59                     0.55                         7

                                                                                                 8*                     1.41                     1.35                         4
4.2.3.           RAINFALL LOSSES
                                                                                                 9                      0.58                     0.56                         5
Initial Loss (IL) and Continuing Losses (CL) have been applied to the modelling. These
                                                                                                 10                     1.49                     1.39                         7
values have been varied for the developed and undeveloped portions of the
catchment as shown below:                                                                        11                     0.84                     0.72                         14

                                                                                                 12                     0.80                     0.91                         -12
       +      Developed catchment                          IL = 0 mm, CL = 0 mm/hr
       +      Undeveloped catchment                        IL = 10 mm, CL = 2.5 mm/hr            13                     0.27                     0.26                         5


4.3.             RATIONAL METHOD COMPARISON                                              NOTE:        * Rational Method comparison undertaken for total catchment rather than individual smaller
                                                                                                      sub-catchments

A comparison has been made between peak discharges provided by XP-STORM and
                                                                                         Although not formally a calibration process, it is an effective way to ensure roughness
those provided by the Rational Method calculations for the pre-development
                                                                                         coefficients and loss rates are reasonable in the absence of recorded data. As can be
scenario 1 % AEP critical duration event. This comparison is summarised in Table 3.
                                                                                         seen, the results of the hydrologic model compare well with those provided by the
                                                                                         Rational Method and are therefore appropriate for use as part of this assessment. A
                                                                                         negative value for difference indicates the XP-STORM peak flow rates are higher than
                                                                                         the Rational Method calculation.




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