P0198-25-001_12_Gympie_Street_Tewantin_SWMP

Figure 2-3: Friend’s equation (QUDM, 2025)




The fraction impervious for the site was determined to be 12%, 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 74.3 mm/hr.
Applying this methodology, a C₁₀ value of 0.70 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                                 0.59                 22                22               0.12       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 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)                            0.065       0.079      0.119   0.146    0.175     0.220     0.252



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.




 12 GYMPIE STREET, TEWANTIN
 Stormwater Management Plan                                                                                                8
 Client Reference: P0198-25-001                   4 November 2025
 Prepared for Novato Group