RFI - P0478-26-001_45_Maple_Street_Cooroy_SWMP

4.2        Time of Concentration Inputs into DRAINS

The time of concentration (Tc) for each catchment has been determined using a combination of:

    1.   Roof inlet time (time for runoff to travel from impervious surfaces to the drainage system)

    2.   Pipe flow time (travel time through the underground drainage system)

Each component is broken down below for the catchment, as shown in the provided screenshot.

Total Area:

              1.   Site → 3,119m²

              2.   Upstream Catchment T 637 m²

              3.   Total site → 3,756 m² (0.376 ha)

Catchment: Impervious Areas

    •    Tc components:

              1.   Roof inlet time T 5 minutes

                            Based on standard inlet times from QUDM Table 4.6.2 for urban residential areas.

              2.   Time in pipe flow → 1.5 minutes

                            From QUDM Figure 4.5 – Flow Travel Times in Pipes and Channels

              3.   Kerb Travel Time → 0.5 minutes

                            Assume minor due to the limited surface flow distance

              4.   Total Time of Concentration (Tc): 7 minutes

Catchment: Grassed areas

    •    Tc components:

              1.   Overland sheet flow → 14 minutes

                            Assigned based on QUDM figure 4.4 – Overland Sheet Flow Times, using a 92m flow path
                             and 8.7% grade

              2.   Total Time of Concentration (Tc): 14 minutes

Calculation Methodology

    •    Roof inlet travel times were estimated using QUDM guidelines and empirical flow path analysis.

    •    Pipe flow times were determined using Manning’s equation for open channel and pipe flow, referencing
         the chart provided in the screenshot for additional validation.

    •    Catchment delineation and flow paths were confirmed using the proposed multiple dwelling development
         layout layout and area tool measurements.

Final Considerations

    •    One catchment has been adopted for the entire development.

    •    Time of concentration will be used in the hydrological modelling to determine peak flow rates and ensure
         adequate stormwater infrastructure design.

    •    All roofwater and driveways, where feasible, from the new development will be conveyed to the tank.

 45 MAPLE STREET, COOROY
                                          24 June 2026                                                 P a g e | 12
 Stormwater Management Plan
 Client Reference: P0478-26-001
 Prepared for Myall Rise PTY LTD