Information Response - Email 1 of 2 MCU16/0153 & REC16/0041

                                      STORMWATER MANAGEMENT PLAN
Client: Thomco (No 2087) Pty Ltd                             Location: 215 David Low Way, Peregian Beach

flow and to assist in design of an appropriate stormwater outlet from the site. Pre and Post Developed
peak flow rates are presented in Table 6 for the 39%, 10% and 1% AEP storm events

Table 6 - Rational Flow Calculations
                                                                           Rainfall
                                                                                          Peak Flow
     Scenario              AEP ARI        Tc (mins)        C Factor       Intensity
                                                                                            (m3/s)
                                                                           (mm/hr)
                            39% 2yr          13             0.595            117             0.225
    Existing
                           10% 10yr          13              0.7             157             0.356
   Catchment
                           1% 100yr          13              0.84            223             0.606
                            39% 2yr          13             0.714            117             0.270
   Developed
                           10% 10yr          13              0.84            157             0.427
   Catchment
                           1% 100yr          13              1.0             223             0.722

It can be seen that the development will result in peak stormwater runoff increases of between
0.045m3/s (in a 39% AEP event) and 0.116m3s (in a 1% AEP event). This represents an
approximately 20% increase in peak flows from the existing scenario.

5.3 Incidental Detention
The inclusion of bioretention basins which capture the runoff from all impervious surfaces on the
site, does provide an incidental detention capacity during very small, frequent rain events. It is
generally accepted that the initial losses of rainfall falling on a pervious surface will be in the range
of 2mm to 10mm (depending on antecedent moisture conditions and soil profile), therefore in very
minor, frequent events it is expected that most rainfall runoff from a site will be from impervious
areas. With approximately 24m3 of storage provided within bioretention basins (at the extended
detention depth of 300mm), this equates to approximately 3mm of the initial rainfall, from impervious
areas, during a rain event being captured and slowly discharged through the filtration device.

5.4 Stormwater Outlet Arrangement
The primary concern for an increase in peak flows leaving this site would be localised scouring at
the stormwater outlet location. The outlet design needs to ensure that flow velocities at the outlet are
reduced and not significantly concentrated at the property boundary. Stormwater flows should be
returned to a ‘sheet flow’ scenario leaving the site. The stormwater outlets from this site have been
designed to satisfy this philosophy and preliminary design drawings can be found in Appendix B.

5.5 Ecologist Feedback
Advice from an Ecologist (JWA Ecological Consultants), was sought regarding discharge of
stormwater from the site in the manner described above. JWA were of the opinion that if stormwater
peak flows are increased but outlet treatment is provided which controls velocity and ensures sheet
flow from the site, the development is unlikely to represent a significant impact on the hydrology of
the receiving environment. The advice from JWA can be found in Appendix C.

5.6 Mitigation Recommendation
It is not recommended that any specific detention infrastructure be introduced to the development to
mitigate peak flows from the development site. The incidental detention capacity of the bioretention
and the proposed design of the stormwater outlets are believed to be sufficient treatment, from a
stormwater quantity perspective, to mitigate the effects of the increase in impervious area on the
receiving environment. This treatment will also ensure that an actionable nuisance is not created
with the development of the site.




Covey Associates Pty Ltd                              11                                Project No: 162042
Consulting Engineers                                                                     100949Rpt Issue B