17155 - Stormwater Quantity Assessment Letter - Rev 0 · OPW17/0612

                                                                            MORGAN CONSULTING
                                                                           E NGINEERS P TY LTD
                                                                                     16003/CG
                                                                                         Page 3




Table 1 summarises the input parameters used in the XP-RAFTS pre development model for
each of the catchments. Pervious and impervious areas were measured from the detailed
survey plan for the development. The detailed survey of the development was used to
determine the vectored slope while aerial photos and findings from the site visit were used
to determine the Manning’s n for each of the catchments.

      Table 1: XP-RAFTS Source Node Parameters

                                                   Pre Development
                            Parameter
                                                Pervious      Impervious
                                               Catchment      Catchment
                       Initial Loss (mm)           10             0
                        Continuous Loss
                                                   2.5            0
                            (mm/hr)
                         Manning's "n"            0.035         0.015
                        IFD Coefficient           Noosa         Noosa
                      Storage Coefficient          1.0           1.0


Table 2 summarises the pre development peak discharges calculated by XP-RAFTS for a
range of design storms.

      Table 2: XP-RAFTS Pre Development Peak Discharge
                                            XP-RAFTS Pre Development
                      ARI
                                                   Peak Discharge
                    (Years)
                                                        (m3/s)
                       1                       1.432 (1yr 25min storm)
                       2                       2.084 (2yr 25min storm)
                       5                       2.769 (5yr 25min storm)
                      10                      3.127 (10yr 25min storm)
                      20                      3.603 (20yr 25min storm)
                      50                      3.776 (50yr 25min storm)
                      100                    4.186 (100yr 25min storm)

Refer to the attached pre development peak flow rate hydrographs from XP-RAFTS.

       2.2 Post Development Unmitigated

The catchment details in the XP-RAFTS model were amended to represent the ultimate post
development conditions. Refer to the enclosed Post Development Catchment Plan for the
catchment details.

The model input parameters summarised in Table 1 have been used for the XP-RAFTS post
development model for each of the nodes as described above. Pervious and impervious areas
were measured from the detailed design plans for the site. The detailed design plans were
used to determine the vectored slope and the Manning’s n for each of the catchments.