M2677 - IR response

        ii.        Rational method calculations are based on the entire site discharging to a single point, where-as the
                   catchment plans in Appendix F identify that the existing and developed site will contain a number of sub-
                   catchments and discharge locations. The rational method calculations should be revised for each sub-
                   catchment in order to provide comparison to the hydrologic model outputs.
        iii.       Provide further detail on the hydrologic model assumptions, such as initial and continuing losses, source of IFD
                   data and temporal patterns adopted.
        iv.        Clarify how the downstream drainage network has been accounted for in the model and the potential for the
                   network capacity to affect the basin outlet characteristics. Consideration should be made to including the
                   downstream pipe network within the easement into the model (to the connection with the street network).

Response
A revised Stormwater Management Plan has been prepared by Morgan Consulting Engineers (Refer to Attachment A),
which includes a revised stormwater detention strategy. Morgan Consulting Engineers have prepared the following
responses to Council items i to iv:
   i.   From site investigations, it was determined that pre-development stormwater runoff from Catchment A
        appeared to generally flow towards the south eastern corner of the site and concentrate across the boundary
        adjacent to Lot 46 SP132578. Stormwater discharge exceeding the 5%AEP event in the post development
        scenario will be dispersed over the length of the bioretention/detention basin wall (32m approximate) adjacent
        to the property boundaries to ensure that discharge from the basin is not concentrated across the neighbouring
        properties. Additionally, surface runoff exceeding the 5%AEP event from the landscaping areas located to the
        rear of the proposed buildings will sheet flow across the boundaries at less than pre-development flow.

               It is not feasible to detain up to 1% AEP in the detention basin as not all flows up to this storm event will be able
               to make its way into the detention basin due to some roof slopes and landscaping areas sloping away from the
               internal driveway.

               However, the updated detention basin design over mitigates post development flows up to 5%AEP to ensure
               events greater than 5%AEP will be less than pre-development. Furthermore, the detention design allows for a
               120mm freeboard at the 5%AEP event which will withhold flows in excess of the 5%AEP event. Therefore, based
               on the above, we can confirm that stormwater discharges onto the neighbouring land will not concentrate nor
               worsen for all events up to and including the 1%AEP event.

  ii.          Refer to the updated Site Based Stormwater Management Plan (20169- SBSMP/2) outlining the amended
               Rational Method Calculations to suit the sub-catchments shown in the Concept Stormwater Catchment Plan
               (20169/SK20-B)

 iii.          Refer to the updated Site Based Stormwater Management Plan (20169-SBSMP/2) outlining further detail on the
               detention modelling inputs.

 iv.           Refer to the updated Site Based Stormwater Management Plan (20169- SBSMP/2) outlining the considerations
               made to the capacity of the downstream pipe network.