Attachment 6 - Stormwater Management Plan

R1767: NOOSA LANDFILL: CLEAN WATER DRAIN AND HES BASIN (SW5) – DESIGN REPORT & MAINTENANCE AND INSPECTION PLAN



4.5.                CATCHMENT LOSSES

4.5.1.           10 YEAR 24 HOUR TRADITIONAL BASIN
A sensitivity analysis of a range of Initial and Ongoing losses was undertaken for the water balance model. As the losses
associated with rainfall also affected the basin size (traditional basin size is calculated based on catchment size, design
rainfall depth and rainfall loss) it was found that the losses adopted had minimal effect on the percentage of annual
runoff treated with performance ranging from 83.4 – 85.5%.

4.5.2.           TYPE A HES BASIN
To ensure the HES basin model catchment runoff was consistent with the traditional basin model, the fraction impervious
in the source node was modified to ensure the total annual runoff was equivalent to the traditional basin model. This
was undertaken for a range of losses utilised in the traditional basin model for comparison purposes.

4.6.                BASIN OPERATION APPROACH

4.6.1.           10 YEAR 24 HOUR TRADITIONAL BASIN
For the purpose of the model, it was assumed that only runoff that was captured and had 5 consecutive days of no
runoff (i.e. minimal rainfall) would achieve treatment and release and could be considered treated. If further rainfall
occurred within the 5 succeeding days, then it was assumed the basin had not been dewatered and the basin would
continue to fill or overtop if that capacity had been reached. Any runoff that overtopped the spillway was considered to
be untreated.

4.6.2.           TYPE A HES BASIN
For the purpose of the model, an outlet pipe was sized in the MUSIC model based on the proposed decant outlet rate
in accordance with IECA (2018). All runoff discharging from the decant structure was considered treated with any runoff
overtopping the spillway to be considered untreated. This is in accordance with the design approach detailed in IECA
(2018).

4.7.                RESULTS

The results below show the percentage of annual runoff treated for a range of basin types sized in accordance with IECA
(2018).

Table 1 – Performance of 10 year 24 hour basin vs HES basins with various design events

                                                                                   Percentage of
                                                                                   Annual Runoff
                                                                                      Treated

  1 in 10 year 24 hour traditional basin                                                 85.2%

  1 in 1 year 24 hour Type A basin (HES)                                                 90.3%

  1 in 2 year 24 hour Type A basin (HES)                                                 93.0%

  1 in 5 year 24 hour Type A basin (HES)                                                 96.3%

Based on the results provided, a 1 in 1 year ARI Type A basin will exceed the performance of a 10 year 24 hour traditional
sediment basin and also achieve the 90% requirement of annual runoff treatment specified in the Environmental
Authority.



5                HYDROLOGIC & HYDRAULIC MODELLING
Hydrologic and hydraulic modelling has been carried out using the XP-Storm software. Drain sections have been
modelled as per the design plans for the 10 % AEP and 1 % AEP events. The layout of the model is illustrated as Figure
1, showing the clean and dirty water drains.




Environment. Engineering. Education.                                                                                PAGE 3