REPS - Updated SWMP

  4.1.4       Existing Detention Basin – Climate Change
An additional assessment was completed of the above assessment of the existing detention basin using the updated IFD values to account
for climate change. From this the IFD values increased by 20%.

A comparison of the existing and mitigated peak flows at the Site outlet is presented in Table 4.7. The results show that a minor increase in
peak flows is expected in 63%, 50%, 20%, 2% and 1% AEP, with reductions in the 10% and 5% AEP events. These results are consistent with
those presented in Table 4.6, with the update to IFD effectively increases the magnitude of e ach event compared to the previous assessment.
As such, it is considered that these minor increases would not result in actionable nuisance, and the outcome remains consistent with the
approved SMP.

Further to this, the gap flow from the townhouse site (refer 4.3) is approx. 0.13 m3/s in the 1% AEP, which would report south downs
Starboard Street (away from basin) and improve results further in the 2% and 1% AEP events.

TABLE 4.7: PEAK DISCHARGE COMPARISON – CLIMATE CHANGE EXISTING VS MITIGATED SCENARIOS


 Scenario               63.2% AEP        50% AEP          20% AEP          10% AEP         5% AEP           2% AEP           1% AEP

 Existing (m3/s)        0.78             0.93             1.43             1.79            2.12             2.48             2.78

 Mitigated (m3/s)       0.83             0.99             1.45             1.72            2.10             2.59             2.94

 Difference (m3/s)      0.05             0.06             0.01             -0.07           -0.02            0.11             0.15




4.2         Townhouse Site
An assessment completed of the existing stormwater drainage network to determine if it can convey the additional flow from the Townhouse
site and achieve compliance with the QUDM drainage criteria. It was found that with the increase in flows that some of the minor storm
(10% AEP) drainage criteria became exceeded. The original drainage design completed by TOD Consulting identified that the capacity of the
stormwater network from Catchment 1/3 (refer Figure 2.3) is 0.24m3/s in the 10% AEP, from which any additional flow generated in the 10%
AEP would result in in non-compliance with the minor drainage design criteria.

As such, a detention tank is proposed to be installed within the townhouse site to ensure the 10% AEP peak flows are detained back to the
development intensity that the drainage system was designed upon. The existing drainage network assumed the contributing catchment of
the original development had an imperviousness of 50%. With the Townhouse site increasing the imperviousness of this catchment to 90%,
the detention tank has therefore been sized to mitigate flows back to the original design intent.


  4.2.1       Catchment
The catchment reporting into the existing drainage network is catchment 1/3, which is 0.687 ha, of which 0.506 ha is the townhouse site. An
excerpt of the catchment plan from the original design drawing is detailed in Figure 2.3.




73 EUMUNDI - NOOSA ROAD, NOOSAVILLE I BBNE00529_0002_0001-REP-001-2                                                                       16