1714 IWMP_Report_Carramar_Version2 Oct 2018

1.0     INTRODUCTION AND SUMMARY

This revised report describes a preliminary assessment of a stormwater detention volume for the
mitigation of peak rates of stormwater runoff from the proposed extension of Cartwright Village at
Carramar, at 186 Cooroy – Noosa Road, Tewantin. A plan of the overall site, including the existing
facilities and the proposed extension, is shown in Figure 1 below below. The revised proposal has two
less villas than the previous proposal.

The eastern boundary of the 4 hectare site borders a tributary of Wooroi Creek, and stormwater runoff
from the entire site discharges to that tributary. The 0.70 hectare stormwater catchment area for the
proposed extension (reduced from the previous 0.82 hectare) is outlined in red on Figure 1.

                                                         SITE BDY


                                                                                           VILLA 4   VILLA 3




                                                                                                               WATER
                       EXISTING FACILITIES                                                 PROPOSED            HOLE

                                                                                           EXTENSION




                                                                                                                       EK
                                                                                                                   CRE
                                                             ENTRY
                                                                          LANDSCAPED
                                                                           PODIUM




                                                                VILLA 1          VILLA 2
                                  SITE BD                                                                      NORTH
                                         Y

                 STORMWATER CATCHMENT BOUNDARY
                 FOR POPOSED EXTENSION           COOROY - NOOSA ROAD


Figure 1:     Overall Site Plan

The design catchment is mostly open space at present, with a car-park and some pathways constituting
an impervious fraction of 23 percent. The proposed extension will consist of an entry facility and four
villas, with associated pathways and podium areas and an extension of the existing carpark under two of
the proposed villas, which will increase the catchment’s impervious fraction to 66 percent.

Urbs modelling of the pre- and post-development scenarios has indicated that the proposed extension
will increase the peak runoff rates from the site by 30 to 50 percent across the range of ARI from 1 year
to 100 years. The modelling has further indicated that complete mitigation of this increase will require a
detention volume of 92 cubic metres if the difference in critical storm durations between the pre- and
post-development scenarios is ignored; or as much as 124 cubic metres for complete mitigation at like-
for-like durations. These detention volumes could be slightly reduced at the detailed design phase.

An arrangement of underground detention cells, based on the ecoAid system, and providing a total
detention volume of 110 cubic metres, is shown in Figure 4 on page 12 of this report. A description of
the ecoAID system is shown in Appendix A hereto.


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      Brisbane Stormwater Management Pty Ltd Document File: Carramar.Doc