Appendix E - Stormwater Management Plan

4.4.4.2 GENERIC NODE

        A generic treatment node was used for the proposed treatment system for the Aged Care Facility. The following
        treatment efficiencies were modelled.

                     Table 4-3:     Generic Node Treatment Removal Efficiency (Aged Care Facility)
                                                           Treatment Removal
                                            Pollutant
                                                               Efficiency
                                          TSS                     90%
                                          TP                      60%
                                          TN                      45%
                                          GP                      90%

        The future proposed stormwater quality treatment system for the Aged Care Facility must achieve this pollutant
        removal efficiency or greater. This is to be demonstrated in future Development Applications for the Aged Care
        Facility.

4.4.4.3 POND NODE

A separate MUSIC model was configured, including the WCQP to determine Hydraulic Residence Time (HRT), which is
an important feature of the pond required for avoiding outbreaks of algae (as discussed below). Table 4-4 indicates the
pond node configuration adopted in MUSIC. Further details of the WQCP are indicated on the Drawing No. 16-001014-
402 in Appendix G.

                                          Table 4-4:    Pond Node Configuration

                                         Pond Parameter                Central Basin
                                  Surface Area (m²)                        1,940
                                  Extended Detention Depth (m)              0.40
                                  Permanent Pool Volume (m³)               1,840
                                  Notional Detention Time (hrs)             48.4
                                  Equivalent Pipe Diameter (mm)              55

        Since the Issue A report only the permanent pool volume has been changed from 1,845m2 to 1,840m2. As this is
        such a minor change the previous HRT analysis of the pond is considered to be still applicable (i.e. no update to
        the modelling is required).

Hydraulic Residence Time/Turnover Period

An important factor in determining pond sustainability is the turnover period, or Hydraulic Residence Time (HRT). Ponds
with high residence times are considered to be at risk of algal bloom and poor water quality.

Water By Design’s Urban Lakes Discussion Paper-Managing the Risk of Cyanobacterial Blooms dated Febuary 2012
recommends 10 to 15 days Hydraulic Residence Time for a lake. Other Queensland publication recommend 20 days
maximum hydraulic retention time. As such, we have recommended a Performance Indicator of a maximum Hydraulic
Residence Time of 15 days for the Central Basin / WQCP.

Using a simplistic approach as detailed in WSUD Engineering Principles – Stormwater (2005), the 20th percentile
Hydraulic Residence Time of 12 days was determined over a 30-year period (1973-2003) based on available data. Refer
to Table 4-5 below.




16-001014-01B | STORMWATER MANAGEMENT PLAN                        COOROY RETIREMENT COMMUNITY, TRADING POST
(SWMP) | PALM LAKE RESORTS                                                                    ROAD, COOROY            | 20