RFI - Attachment B - Stormwater management Report · MCU20/0026

to an infiltration tank at the rear of the site, where it shall discharge via infiltration or surcharge and fall
across the site boundary in a non-worsening manner.

The catchment area remains unchanged at approximately 550m2. The total impervious area was
calculated to be approximately 360m2 which equates to 65% fraction impervious (fi=0.65).

3. 1       Sto rmw at e r De t en t io n & I nf i ltr at ion

Stormwater runoff from ground areas of the site shall be directed to a proposed infiltration tank and
discharged from site by infiltration or by surcharging and falling across the site boundary in a manner
that results in no increase in peak flow rate and total runoff volume from pre-development levels.

The Atlantis Flo-Tank Modular tank system was adopted as part of the stormwater design. An infiltration
rate of 0.6m/d was adopted based on the subsurface profile of similar developments in the Sunshine
Beach area. Refer to Table 1 and Appendix B for further details of the proposed system.
Table 1 - Detention system details

                         Description                                             Value

 Tank Model                                                           6 x Atlantis Flo-Tank - Triple

 Gross Volume (m3)                                                                2.20

 Detention Capacity (m 3)                                                         2.09

 Dimensions (m)                                                       2.06(L) x 0.97(W) x 1.31(H)

 Infiltration Rate (m/d)                                                           0.6

 Catchment Area to Tank (Ha)                                                      0.029

 Catchment Area Bypassing Tank (Ha)                                               0.026


3. 1       H yd ro log i c and H yd r aul i c M ode ll ing

The hydraulic and hydrologic modelling software package XPSTORM was used to model the pre-
developed and post-developed flow conditions for the site. XPSTORM provides a simulation of the
natural runoff processes and the hydraulic performance of stormwater infrastructure. An integrated
1D/2D hydraulic model was developed incorporating the TUFLOW engine. The TUFLOW engine
accurately represents site specific topography, defining flow paths, discharge, storage, and variation in
flow direction in addition to modelling flood hazard.
Hydrologic Modelling

The 10% AEP event was defined as the minor storm and the 1% AEP event was defined at the major
storm for the analysis. A number of storm durations including the 10 minute, 25 minute, and 1 hour
duration were simulated for each AEP for the pre developed and post developed model.
Rainfall data for each storm duration for the 10% and 1% AEP events was obtained from the ARR 2016
data hub. The Laurenson method was utilised to calculate runoff hydrographs with the following losses:

           Pervious areas – initial and continuing losses of 10mm and 3mm respectively

           Impervious areas – initial and continuing losses of 2mm and 0mm respectively



11 June 2020                                 Stormwater Management Report                                     3
S20-023                                    40 Park Crescent, Sunrise Beach