1909-13367 SRA referral response approved docs

Table 4 – Time of concentration summary

                                 Time of Concentration                    Adopted Parameters
          Method Used
                                       (Minutes)


                                                                               Figure 4.5

                                                          L = 4640m; Fall = 49m, delta = varies between 2 and 3

                                                                             t= 145 minutes
   Figures 4.5/4.6 QUDM 2017              153
                                                                               Figure 4.6

                                                                           L = 50m, n = 0.035

                                                                         S = 18%, t= 8.0 minutes


    Bransby-Williams (Equ 4.9                                           L = 4.6km; A = 386.98Ha
                                          166
           QUDM 2017)                                                         Se = 0.562%


The lengths of each catchment used were taken as the longest flow path from the top of the external
catchment to the discharge point at the end of the study area. The lengths and the slopes of the bed
were determined from LIDAR information.

Equal area slope was determined using a longitudinal section of the creek bed.

Using the above times of concentration the rational method was used to calculate an estimate of peak
flow.

Below is a list of the parameters that were used to determine the peak flow of the external catchments
based on the time of concentrations detailed above.


          C10 combined = 0.74

          I100 2.55hrs = 61.8mm/hr

          I100 2.77hrs = 59.2mm/hr

          The above parameters have been used to calculate the below peak catchment flows.

          1% AEP Peak flow2.55 hours = (1.2 x 0.74 x 0.0618m/hr x 3,869,800m2)/3600 = 58.99m3/s

          1% AEP Peak flow2.77 hours = (1.2 x 0.74 x 0.0592m/hr x 3,869,800m 2)/3600 = 56.51m 3/s

The 1% peak flows calculated using the rational method are approximately 0.8% (2.77 hour tc) and
5.3% (2.55 hour tc) greater than the 1% peak flow as determined using the Laurenson’s method. This
difference has been attributed to the runoff routing within Laurensons method combined with the
detention in the 1D channels which the rational method does not consider.

3.1.9 2D M odel lin g p a ra m e t e rs

The external catchment and development site flows have been modelled in 2D to determine the existing
flow conditions, peak water levels, maximum flow velocities and extent of flow widths adjacent to the
development site.




                                                 Flood Study                                                      14
S19-007                                   122 & 144 Myall St, Cooroy