Stormwater Reporting · RAL22/0021

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  Table 1 – Overall catchment peak flow discharging to existing pit in Picture Point Crescent


                                            Peak Flow (l/s)
        AEP
                  Pre-development        Post-Development              Difference

        39%               100                      120                    +20
         1%               260                      300                    +40

The existing kerb and channel on Picture Point Crescent is required to have capacity
equivalent to the 39% AEP storm event. The half road capacity up to the top of the kerb on the
development side of the road has been calculated to be 598 L/s using Manning’s equation.
The peak 39% AEP discharge from the post-development catchment is 120 L/s. Therefore, the
road has adequate capacity to cater for the increase in stormwater discharge from the
development. The half road width is only required to cater for the 39% AEP stormwater event
however it does have the capacity to cater for the 1% AEP discharge from this catchment. It is
important to note that the discharge to the Picture Point Crescent kerb and channel must be
limited to 25 L/s per lot with any excess flow to discharge toward the north of the property (as
explained later in this report).

The kerb and channel then discharges into the existing grated pit in the road way. It is noted
that the existing pit and pipe stormwater network is required to achieve a capacity of 39% AEP
discharge of the incoming catchment. Based on the GIS data from Noosa Council Mapping
system and using Manning’s equation the capacity of the downstream pipe was calculated.
The downstream pipe is a 300mm RCP at 25% grade which has a capacity of approximately
522 L/s. In the post-development case, the proposed 39% AEP discharge to this pipe is 120
L/s. Therefore, the existing 300mm RCP pipe has more than enough capacity to capture for
the increase in stormwater discharge due to the proposed development. Any flow that does
not get captured by the stormwater pit will flow through the overland flow path above the
stormwater alignment between 35 and 37 Picture Point Crescent, as it does in the pre-
development case.

Therefore, the proposed development will have a non-worsening effect on the peak stormwater
discharge to Picture Point Crescent and the surrounding properties.


Stormwater Discharge to Noosa Drive

The proposed ground areas (including the future driveways) will discharge to the proposed
infiltration trench provided for each lot respectively. The infiltration trench will allow the
stormwater runoff to discharge into the natural sand as on-site disposal. In extreme rainfall
events, when the infiltration trench reaches its maximum capacity, any excess stormwater will
discharge as sheet flow over the northern boundary due to the proposed level spreader
(concrete edging) which forms part of the infiltration trench.

The proposed design is consistent with the recommendations in Tectonics Geotechnical
Investigation Report No. 22511-001-Rev0 provided in Appendix C. In the report they also
determined that the proposed design would not have any impact on slope stability provided
their recommendations in the report were implemented. Further, Tectonic has determined that
the sandy soil encountered on-site was consistent with clean fine to medium grained sand
which have a design coefficient of permeability of 1 x 10-4 m/s. Details of the proposed
infiltration system are shown in Appendix A.