Appendix D - Engineering Assessment · OPW26/0090

The results of the Rational Method analysis are summarised in Tables 1, 2, and 3. Full Rational Method
calculations are also attached to this letter.

Table 1 Pre-Development Rational Method

                               Area [ha]                   C10             Minor Storm          Major Storm
                                                                         Discharge [m3/s]     Discharge [m3/s]
    Subject Site                           0.1710                 0.76               0.053                0.157

Table 2 Post-Development Rational Method

                               Area [ha]                   C10             Minor Storm          Major Storm
                                                                         Discharge [m3/s]     Discharge [m3/s]
    Lot 1 (Vacant)                         0.0540                 0.70               0.015                0.046
    Lot 1 (Developed)                      0.0540                 0.85               0.019                0.054
    Lot 2                                  0.1170                 0.88               0.042                0.118


Table 3 Combined Post-Development Discharges

                          Pre-Development           Post-Development       Difference             Difference
                               [m3/s]                     [m3/s]             [m3/s]                   [-]
    Subject site Minor                0.053                      0.061              0.008                  +15%
    Storm (Developed)
    Subject site Major                      0.157                0.172               0.015                 +10%
    Storm (Developed)

The results above indicate that the proposed kerb adaptor is a suitable arrangement for LPD1. As the
proposed Reconfiguring a Lot application introduces no new impervious areas to the site, no further
stormwater quantity controls (i.e. detention tanks) are recommended at this stage1.

A review of the Low-Density Residential Zone Code and the Low-Density Housing Code confirms that
stormwater detention is not required to meet the relevant Performance Outcomes. No on-site detention to
maintain pre-development discharge rates is anticipated for the future development of Lot 1, unless
conditioned by Council as part of the Development Approval.

Stormwater Quality
The State Planning Policy proposes triggers for stormwater quality management that apply to ‘high-risk’
developments. The criteria include one or more of the following:
    - A Material Change of Use for an urban purpose which involves greater than 2,500m2 of land that:
            o will result in an impervious area greater than 25% of the net developable area; or
            o will result in six (6) or more dwellings.
    - A Reconfiguration of a Lot for urban purposes that involves a land area greater than 2,500m2 and
       will result in six (6) or more lots; or
    - Operational works for urban purposes that involve disturbing more than 2,500m2 of land.



1
 The minor increase in combined discharges from Lot 1 (Vacant) and Lot 2 is due to assumptions regarding C10 values
rather than a material change to catchment hydrology.


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