P0198-25-001_12_Gympie_Street_Tewantin_SWMP

2.2         Flow calculation using the Rational Method
The Rational Method was initially applied as a verification tool to cross-check outputs from the DRAINS
hydrological model, which was subsequently developed to assess both pre- and post-development flows.

The time of concentration (Tc) was determined using Friend’s Equation (refer to Figure 2-1) in accordance with
Table 4.6.4 of QUDM. This approach provides a reliable estimate of the travel time for runoff from the hydraulically
most distant point in the catchment to the outlet.

A maximum flow path length of 92.82 m was adopted, based on survey data collected by DSQ Surveyors Pty Ltd
in 2025 (refer to Figure 2-2).

The resulting Tc of 22 minutes was further validated against QUDM’s Figure 4.4 – Overland Sheet Flow Times,
which provides empirical guidance for various surface and slope conditions. This comparison confirmed the
calculated Tc to be consistent with expected values for the site’s topography and surface characteristics.

To refine the analysis, the following factors were incorporated to ensure accuracy and representativeness of the
stormwater design:

     •    Surface roughness and vegetation cover, which influence flow resistance and travel time

     •    Topographic gradients, confirming alignment between flow velocities and assumed conditions

     •    Site hydraulic connectivity, including potential depressions or ponding areas, verifying the
          appropriateness of the adopted Tc.




Figure 2-2 Max overland flow path




 12 GYMPIE STREET, TEWANTIN
 Stormwater Management Plan                                                                                      7
 Client Reference: P0198-25-001         4 November 2025
 Prepared for Novato Group