Callaghan and Toth IWMP Report revised

1.0   INTRODUCTION AND SUMMARY

This report describes a preliminary assessment of a stormwater detention volume for the mitigation of
peak rates of stormwater runoff from a 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.

The eastern boundary of the 4.05 hectare site borders a tributary of Wooroi Creek, and stormwater runoff
from the entire site discharges to that tributary. The 0.51 hectare stormwater catchment area for the
proposed extension consists of four subareas, as outlined in red on Figure 1 below.




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 32 percent. The proposed additional buildings and parking spaces will increase the
design catchment’s impervious fraction to 71 percent.

Urbs modelling of the pre- and post-development scenarios has indicated that the proposed extension will
increase the peak runoff rates from the design catchment by 25 to 43 percent across the range of ARI from
1 year to 100 years. The modelling has further indicated that complete mitigation of the increase will
require a detention volume of 80 cubic metres if the difference in critical storm durations between the pre-
and post-development scenarios is ignored, or 89 cubic metres for complete mitigation at like-for-like
durations. Those detention volumes should be slightly reduced at the detailed design phase, using the
latest rainfall intensity data and with an allowance for the biopod storages and the infiltration through the
porous floor of the detention units. An arrangement of underground detention cells, based on the ecoAid

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