Approved Documents

3.0 PRE & POST DEVELOPMENT STORMWATER DRAINAGE

The Rational Method Formula was used to calculate the stormwater runoff flows through
and off the site for the Pre-Development 100 year ARI event. The Rational Method
formula is as follows.

Q=C1A/360

Where Q = Peak Flow Rate (m*/s)
C = Runoff Co-efficient
A = Area of Catchment (ha)
{ = Rainfall Intensity (mm/hr)

3.1 Catchments

The catchment in the pre-developed scenario is based on the natural contours as
opposed to the developed scenario where the catchments are divided to suit the
proposed development flow characteristics. The catchment areas are shown in Figure 2
and Figure 3 for the pre-developed scenario and post-developed scenario respectively.

3.2 Pre — Development Flows

The catchment area for the pre-development scenario is 5.46ha, refer Figure 2. For the
purpose of this scenario the catchment is assumed to be open space/parks and hence
from Table 5.04.1 in QUDM (Queensland Urban Drainage Manual) a value of 0.00 was
adopted for the Fraction Impervious.

For the pre-development scenario the site topography is relatively flat and therefore a time
of concentration of 15 minutes was adopted from Table 5.05.1 in QUDM. A Table of the
Rainfall Intensities assumed for the proposed development site are shown in Appendix A.

Table 1 Catchment Parameters — Pre-Development Scenario

 

 

 

Catchment | Area | Fract.| t, I Cio | Croo | Qtoo

Scenario (ha) | Imp. | (mins) | (mm/hr) (m’/s)
Pre-

Development 5.46 | 0.00 15 212 0.70 0.84 2.70

 

 

 

 

 

 

 

 

 

3.3. Post - Development Flows

The stormwater system adopted for the proposed site includes an underground
stormwater pipe network with 3 outlets to the north and north-western boundaries. It also
includes an underground infiltration system consisting of perforated FRC pipes collecting
Qzo roofwater runoff and infiltrating this into a sand layer located at approximately RL
1.0m AHD to RL 2.5m AHD before being discharged into an existing melaleuca swamp
located downstream. The underground infiltration system is not connected to the
underground stormwater system. Therefore, all runoff captured in the infiltration system
will be infiltrated and treated via the sand layer. There are also various points where
overland flow will be discharged from the site.

The proposed development site was modelled using the computer modelling program XP
- SWMM. The model includes both an underground pipe network and a surface network

 

Noosa Domain Retirement Village, Noosaville 5 Site Based Stormwater Management Plan
M1717 Revision B 16 August, 2006