Reference Documents - MCU23 0070

   It is unclear how the proposed field inlet in catchment D3 will function during a 1% AEP event.
    Council recognise that this field inlet serves both the overflow from the detention tank and a small
    catchment area. Council is concerned that during events up to, and including, a 1% AEP event this
    pit may surcharge and direct additional runoff on to the footpath in the road reserve.
   The discharge rate of the RHS kerb adapter is unclear. Council is unlikely to support a discharge
    rate higher than 25 l/s during a 1% AEP event.

Information Required
13. Provide additional detail to address Council’s concerns. The additional detail must be prepared by a
suitably qualified person (RPEQ).
Note that the detail can be preliminary in nature but must be sufficiently detailed to demonstrate that
the stormwater system is conceptually functional.


RESPONSE:
The following responses are provided for each dot point raised:
   The ‘blue circle’ is a manhole/roofwater pit. It conceptually shows a junction at which drainage
    pipes change direction. The concept plans have been amended and further details can be provided
    during detailed design as required.
   The outflow from the tank is controlled via a 75mm diameter orifice. It limits 1% AEP site discharge
    to 25L/s as requested in dot point #5. An overflow weir is proposed at RL 33.11m AHD to allow
    overflow.
   No infiltration is proposed. All outflows are directed to Duke Street kerb and channel.
   Runoff from a majority of Catchment D3 will be collected by building’s roofwater system. The
    proposed field inlet in Catchment D3 has been amended to a roofwater pit and it can be provided
    with a bolt down cover so that it will not surcharge. The outflow pipe from the detention tank will
    operate under head. This is consistent with standard building hydraulic design.
   The discharge rate from site has been limited to 25L/s in a 1% AEP storm event. This has been
    achieved by increasing detention volume and limiting outflows via 75mm diameter orifice.
    Appropriate screens can be incorporated in detail design to minimise blockage of this orifice. Any
    additional requirements can be conditioned.




SITE BASED STORMWATER MANAGEMENT PLAN – Revision B                        36 & 38 DUKE STREET, SUNSHINE BEACH
                                                     P a g e |2