Att 1 - RFI Amended SWMP

                                                                                        23098 - SBSMP/3
                                                                                                Page 9




5.1.5 Peak Flows
Refer to Table 5-3 for a summary of the predicted minor and major discharge rates for pre and
post development conditions and Appendix E for the Rational Method calculations.

                               Table 5-3 – Pre and Post Development Peak Flows

    Property State       Q1 (m3/s)    Q2 (m3/s)    Q5 (m3/s)   Q10 (m3/s) Q20 (m3/s) Q50 (m3/s) Q100 (m3/s)

  Pre development          0.037        0.044        0.066      0.081       0.097      0.122       0.140
  Post development         0.051        0.073        0.110      0.132       0.156      0.194       0.218

As shown in Table 5-3, an increase in impervious surface areas will consequently increase
stormwater runoff peak flow rates from the proposed development.
Hence, it is proposed to utilise a series of on-site above ground detention tanks to mitigate the
stormwater runoff of the proposed development prior to discharging to the Lawful Point of
Discharge, northeast of the site.

Refer to Appendix E for the Rational Method Calculations, Appendix F for the Concept Site Levels
and Drainage Plans and Section 6 for the Stormwater Quality Management Plan.


5.2 Proposed Development Drainage
The proposed development drainage intent is as follows (refer to Appendix F for the Concept Site
Levels and Drainage Plan):

Development Catchment:

    1. Runoff from the roof catchments will flow via downpipes to aboveground detention tanks
       combined with rainwater reuse for mitigation prior to discharging to the lawful point of
       discharge, northeast of the site.
    2. Runoff from the ground catchment will be directed to the bioretention basin and level
       spreader located in the north eastern corner of the site prior to discharging to the lawful
       point of discharge, northeast of the site.
External Catchment:

    1. Runoff from the external ground catchment, southwest of the site, will be collected by a
       positive drainage system and directed to the lawful point of discharge, northeast of the
       site.

Refer to Appendix F for the Concept Site Levels and Drainage Plans.


5.3 XPSTORM Modelling
5.3.1 Intensity Frequency Distribution
The IFD data and temporal patterns used for the XPSTORM modelling has been obtained from the
Bureau of Meteorology for the following coordinates:
    •   Latitude -26.4181
    •   Longitude 152.9128

Refer to Appendix G for a copy of the BOM IFD data table.


Site Based Stormwater Management Plan
Proposed Townhouse Development at 5 Pearl Street, Cooroy