Att 1 - Engineering - RFI Reponse · MCU21/0117.01

                                                                                        23098 - SBSMP/4
                                                                                                 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              Q2 (m3/s)    Q5 (m3/s)   Q10 (m3/s) Q20 (m3/s) Q50 (m3/s) Q100 (m3/s)

        Pre development                 0.045       0.068       0.083       0.099      0.125       0.143
        Post development                0.062       0.094       0.116       0.139      0.176       0.199

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:

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 DRAINS Modelling Assessment
In order to ensure the proposed development will not discharge stormwater runoff greater than
pre-development flow rates, a detailed detention analysis was undertaken to determine what
detention mitigation is required for the proposed development to meet the requirements outlined
in Noosa Shire Council’s planning scheme policy. The IL-CL hydrological model has been used to
calculate the peak discharges from the site under the existing (pre-development) and proposed
(post development) conditions.




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