17 Bottlebrush Avenue, · MCU26/0001

The fraction impervious for the existing site was determined to be 38%, based on a manual measurement of the
exact impervious area to accurately represent existing site conditions in accordance with QUDM methodology.
This measured impervious area was input directly into the model to ensure realistic simulation of runoff generation.

Using QUDM Table 4.5.2, the C₁₀ was determined based on the 1-hour, 10 % AEP rainfall intensity of 74.9 mm/hr.
Applying this methodology, a C₁₀ value of 0.75 was derived, as summarised in Table 2-1.
Table 2-1: Rational Method key inputs

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

 Pre-Development                                0.051                 7                 7                0.38           0.75



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 2-2 presents the calculated pre-development flow rates for each of the assessed storm
events:
Table 2-2: Rational Method flow calculations

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

 Pre-Development Flow (m³/s)                            0.010      0.012      0.018   0.022     0.026     0.033        0.037



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.


2.3         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 Table 2-4.




 17 BOTTLEBRUSH AVENUE, NOOSA HEADS
                                                  7 January 2026                                                P a g e |8
 Stormwater Management Plan
 Client Reference: P0433-25-001
 Prepared for Made Capital PTY LTD