B07056.W-01A Complete - Stormwater [As Published] · 132007.2398

Concept Stormwater Management Plan
RSL Care – Cooroy Retirement Village



3.0        HYDROLOGICAL INVESTIGATION
In order to assess the impact of the proposed development and the effectiveness of mitigating the increase in
flows, a hydrological analysis was undertaken. This analysis involved the following investigation.
      •   Identifying the appropriate catchment for the site and land uses,
      •   Modelling of the existing catchment using DRAINS Modelling Software (Version 2006.01),
      •   Modelling of the catchment with the proposed allotment development and comparison to existing
          scenario results,
      •   Calibration of the developed scenario DRAINS Model to the Rational Method,

Design and modelling of concept detention basins to ensure developed peak discharges from the proposed
residential development do not exceed those from the existing scenario is undertaken in Section 5.0.

3.1                DRAINS Model Setup
The existing and developed scenario flow rates were analysed using the computer package DRAINS. DRAINS
analysis enables both the hydrological and hydraulic modelling of the site for numerous design storm events and
land uses. DRAINS calculates the full hydrographs of flows resulting from the specified rainfall hyetographs.
Design storm bursts from Australian Rainfall and Runoff 1987 are used for generation of hyetographs. This
section sets out the various assumptions and procedures used in developing the DRAINS modelling.

3.1.1              DRAINS Input Data

DRAINS uses an ILSAX type hydrological model. A soil type 3 was adopted to reflect slow infiltration rates. An
antecedent moisture condition (AMC) of 3, reflecting fairly frequent catchment wetting was also used. Paved and
supplementary areas had depression storage of 1 mm and grassed areas have storage of 5 mm.

Rainfall storms for modelling were entered into DRAINS using intensity values for the area interpolated from
Noosa Council Standard Drawing 9232 ‘Rainfall Intensity Curves Plateau (Tinbeerwah to Black Mountain)’.
Cooroy falls within Zone 3 – North East Coast Rainfall Zone (Figure 3.1 of ARR87). The IFD curve used is
presented in Appendix B.

Modelling was performed for the 2, 5, 10, 20, 50 and 100 year ARI storm events with storm durations of 5, 10,
15, 20, 25, 30, 45, 60, 90 and 120 minutes. Intensity values corresponding to these storm events and durations
are presented in Appendix B.

Further input data to DRAINS that is required for catchments are detailed in Sections 3.2.1 and 3.3.1.




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