2008/0169 Response to information request - 71 Seaview Tce Sunshine Beach · 132008.169

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Outlet scour protection and energy dissipation structures, similar to Figure 7 will be provided. The 150mm outlet
pipes will not warrant a rock apron Sm long, a rock apron 2m in length will be more than sufficient to provide
effective scour protection. The rock apron will dissipate a great deal of energy from stormwater outflows and as
such discharge onto existing vegetation will not be harmful. The scour and energy dissipation capabilities of the
outlet structure are in accordance with Section 8.06 of QUDM and Healthy Waterways Guidelines. Partially buried
or depressed pipe outlets can also be utilised to dissipate velocity at these outlets. The Q100 flows also represent
a storm event that occurs on average once in every 100 years which is an extremely rare event. Final tank and
outlet size is to be determined at the operational works stage of the application.


Ideally the majority of all stormwater runoff will discharge to the storage system to reduce the number of outlets
from the site and scour possibilities due to the steep topography. The XP-SWMM model has assumed that runoff
from all impervious areas will discharge into the storage system. It is unlikely that all impervious area will be able
to discharge to the onsite detention system. Stormwater storage requirements for the site are not highly
dependent on the inflow to the storage device and a storage requirement of 60kL is reasonable if at least 50% of
the impervious area discharges to the storage system.


                                             fable 7 - Outflow from Storage System

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

             Inflow              0.068           0.091        0.125           0.146           0.174    0.212    0.234

      Allowable Outflow          0.048           0.065        0.089           0.103           0.124    0.156    0.180

        Actual Outflow           0.048           0.055        0.069           0.097           0.124    0.149    0.161

       Non- Worsening                YES          YES           YES            YES             YES      YES      YES



Detailed XP-SWMM Results are attached in Appendix D.

4.9   TREATMENT OF DRIVEWAY / CARPARK RUNOFF

All runoff from the driveway / carpark area will be directed to the north. All stormwater runoff from the driveway
/ carpark area will be captured and treated through a Fox Environmental Systems: Baffled Gross Pollutant Trap -
Model BPT600 (or approved equivalent). The device is to be located in the north-western corner of the driveway.
The GPT will capture silt, organic matter, floating debris and hydrocarbons attached to suspended solids.
Additional pits may be required upstream where additional treatment devices will also be required.


Runoff from the carpark will be directed to the proposed storage devices. A preliminary location of the GPT device
is shown in Figure 10 and the actual location will be determined at the operational works stage.




 SITE BASED STORMWATER MANAGEMENT PLAN - Revision D          ' 1 Seaview Terrace, Sunshine Beach                         13