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

6.            Water Quantity Management

6.1.          Introduction
Hydrologic and hydraulic modelling has been undertaken of the site to establish the flow regime and peak
discharges at the outlet of the site to the west. The modelling has been undertaken in XP-SWMM for both
the pre and post development scenarios.

6.2.          Rational Method
The Rational Method formula was used to calculate the stormwater runoff flows through and off the site
for the pre and post development scenarios. The Rational Method formula is as follows:

Q = (2.78 x 10-3) C I A                                                         Equation 1 (QUDM, 2007)

          Where:
                    Q is the peak flow rate (m3/s)
                    C is the coefficient of runoff
                    A is the area of catchment (ha)
                    I is the rainfall intensity (mm/hr)

6.3.          Catchments
For both the pre and post development scenarios, it was assumed that the entire site discharges to the
west. The site has a total area of approximately 0.58ha.

The subject site was considered as a single catchment in both the pre and post development scenarios. The
pre and post development catchment plans and indicative flow directions are shown below in Figure 8 and
Figure 9 respectively.

6.4.          Coefficient of Runoff
Based on the 10 year 1 hour rainfall intensity and the fraction impervious in the pre and post development
scenarios, a runoff coefficient has been adopted in accordance with Section 4.05 of QUDM as summarised
below:

         Pre Development:           fi = 0.26        C10 = 0.74
         Post Development:          fi= 0.92         C10 = 0.88

Frequency factors have been applied in accordance with QUDM to determine runoff coefficients for other
ARI events.




STORMWATER MANAGEMENT PLAN – PEREGIAN VILLAGE MARKET                                                Page 12

Scanlon Property Group
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