RFI - P0478-26-001_45_Maple_Street_Cooroy_SWMP

Table 3-1: Rational Method Key Inputs

 Catchment                                     Area (ha)          Tc (min)       Actual Tc (min)      fi            C10

 Pre-Development                                0.3756              14                 14            0.08           0.70



To quantify the peak flow rates for various Annual Exceedance Probability (AEP), the Rational Method formula
was applied:




where:

     •    Q = Peak discharge (m³/s)

     •    C = Runoff coefficient

     •    I = Rainfall intensity (mm/hr)

     •    A = Catchment area (hectares)



The following Table 3-2 presents the calculated pre-development flow rates for each of the assessed storm
events:
Table 3-2: Rational Method Flow Calculations

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

 Pre-Development Flow (m³/s)                    0.050       0.059        0.090     0.110     0.130   0.165         0.188



These calculations establish the hydrological baseline for the site and form the foundation for further stormwater
management analysis. The results will be used to assess post-development runoff characteristics and determine
the necessary mitigation measures to ensure compliance with the QUDM and Noosa Shire Council stormwater
management requirements.


3.3         DRAINS Model and Comparison to Rational Method

The DRAINS model was configured using the appropriate Australian Rainfall and Runoff (ARR) parameters specific
to the site conditions. Inputs for each catchment were derived from the parameters discussed in previous sections,
ensuring consistency with site-specific hydrology and QUDM low residential density development criteria. The
general configuration of the DRAINS model is shown in Figure 3-4.




Figure 3-4: DRAINS model for existing catchment

The existing catchment was modelled based on survey data (refer to the Existing Layout Plan in Appendix C). This
information was used to determine an accurate impervious fraction for the pre-existing site through measurement
of existing impervious surfaces. The relevant DRAINS catchment inputs are detailed in Figure 3-5.




 45 MAPLE STREET, COOROY
                                                  24 June 2026                                              P a g e |8
 Stormwater Management Plan
 Client Reference: P0478-26-001
 Prepared for Myall Rise PTY LTD