16-001014-01A (Swmp)

                                          Table 4-3:   Pond Node Configuration

                                        Pond Parameter                  Central Basin
                                Surface Area (m²)                            1,940
                                Extended Detention Depth (m)                  0.40
                                Permanent Pool Volume (m³)                   1,845
                                Notional Detention Time (hrs)                 48.4
                                Equivalent Pipe Diameter (mm)                  55


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 14 days was determined over a 30-year period (1973-2003) based on available data. Refer
to Table 4-4 below.

                  Table 4-4:    20th Percentile Hydraulic Residence Time Results (Simplistic Approach)
                Parameter                                                   30-Year Period (1973-2003)
                Impervious Area (ha)*                                                   8.8
                Lake Volume (m3)                                                       1,845
                Pond Area (m2)                                                         1,940
                All Year Rainfall (mm)                                                  678
                Catchment Inflow (m3)                                                 59,956
                Yearly Evaporation (mm)                                                1,629
                Net Evaporation (m3)                                                     1,844
                Net Inflow (ML)                                                           58
                Net Yearly Inflow / Lake Volume Ratio                                     31
                Probabilistic Residence Time (No. of days)                                12
         *The catchment impervious area to the central pond is consistent with MUSIC modelling areas and Fi values.

A continuous modelling approach was also undertaken using daily output from MUSIC modelling over the same 30-year
period. Using the more detailed approach, the 20th percentile HRT was determined to be 9 days, which also achieves the
required objective.

Therefore based on both the simplistic and continuous modelling approaches the Hydraulic Residence Time for the pond
will be below 15 days. This demonstrates that the required performance objective will be met.

If the water quality of the pond is found to be poor, recirculation of flows through the WQCP could be provided to increase
the turnover of water within the pond.

4.4.4.3 PROPRIERTY GROSS POLLUTANT TRAP (GPT)

A propriety (GPT) is proposed upstream of large catchments (above 5ha) or significant areas of car parking areas before
discharging to the proposed Bioretention Basins. While GPT’s are proposed upstream of the Central Basin, Southern


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