Attachment 4 - Stormwater Management Plan by O2 · MCU13/0251

6.7.          Site Hydrology
Using the data provided above, Rational Method calculations have been undertaken for the site to
determine the peak flow rates associated with the standard ARI events. A summary of the Rational Method
calculations are provided in Table 11 for the pre and post development scenarios. The Rational Method
calculations have been used for comparison against the results of the hydrologic modelling.
Table 11 – Site Hydrology

                           Q1          Q2           Q5              Q10               Q20             Q50           Q100
 Catchment                 3           3            3                3                 3               3             3
                         (m /s)      (m /s)       (m /s)           (m /s)            (m /s)          (m /s)        (m /s)

 Pre Development

 Site                    0.10         0.14           0.19            0.22            0.26            0.32          0.37

 Post Development (unmitigated)

 Site                    0.14         0.19           0.26            0.31            0.36            0.45          0.50


6.8.          Hydrologic Modelling
The XP-SWMM runoff-routing model been has used to estimate design flood discharges within the site.
The model represents the sub-catchments as a network of nodes linked to a 1D drainage network. Each
node is defined by its pervious (undeveloped) and impervious (developed) area, fraction impervious and
average catchment slope. A fraction impervious of 0% has been applied to the undeveloped portion and
100% to the developed portion. The net rainfall is routed through the network after appropriate losses
(initial and continuing) and roughness factors are applied, resulting in a surface runoff hydrograph for each
sub-catchment. Hydrologic modelling has been undertaken using the Laurenson Runoff Routing Method.

6.8.1.        Rational Method Comparison
The peak discharges provided by the hydrologic model have been compared against those obtained from
the Rational Method calculations to ensure the model is providing reasonable results. The results of this
comparison are provided in Table 12.
Table 12 – Rational Method comparison

                                    Q1          Q2            Q5             Q10             Q20          Q50          Q100
 Catchment                          3           3             3               3               3            3            3
                                  (m /s)      (m /s)        (m /s)          (m /s)          (m /s)       (m /s)       (m /s)
 Pre Development
 Site – Rational Method           0.10        0.14          0.19            0.22              0.26          0.32       0.37
 Site – XP-SWMM                   0.10        0.14          0.19            0.22              0.26          0.31       0.35
 Post Development
 Site – Rational Method           0.14        0.19          0.26            0.31              0.36          0.45       0.50
 Site – XP-SWMM                   0.20        0.26          0.29            0.32              0.37          0.43       0.47



The hydrologic model is providing results within a reasonable range of the Rational Method calculations
and is therefore considered appropriate for use in this analysis.

The XP-SWMM model was analysed for the 20, 25, 30, 45, 60, 90 and 120 minute storm events, with the 25
minute storm producing the peak flow rate for the majority of events.

STORMWATER MANAGEMENT PLAN – PEREGIAN VILLAGE MARKET                                                                      Page 15

Scanlon Property Group
R002616