Flood Investigation & Stormwater Quantity Management Plan

Flood Investigation & Stormwater Quantity Management Plan
Proposed Residential Development at Doonella - Stage C

                              Table 3.5: Adopted WBNM Loss Values & Lag Parameter
                         Initial Loss (mm)             Continuing Loss                 Lag Parameter (C)
                                                          (mm/hr)
                                 25                             2.5                            1.8
                                          Note: Initial impervious loss of 1mm also adopted.

3.2.4    WBNM Calibration
In order to calibrate the WBNM Queensland Rational Method calculations were undertaken to determine peak flows at
the downstream site boundary. For the calibration the catchment has been assumed to be in an undeveloped
condition, therefore the calculations and WBNM have been configured with an impervious fraction of zero. The
Rational Method calculations have been estimated for a range of design storm events including the 2, 10, 20, 50 and
100 year ARI in accordance with Council’s DDPSP as well as QUDM (2007).
Time of Concentration
The time of concentration (tc) is an input parameter to derive relevant rainfall intensity to adopt in the Rational Method
calculation. Given the size of the site catchment and condition of the main flow path the tc has been determined
following a methodology consistent with the procedure for ‘(c) Bushland catchments to small to allow the formation of a
creek with defined bed and banks.’ as per Section 4.06.3 of QUDM (2007). The following travel time components were
calculated to derive the tc:
    •    Friend’s Equation (Section 4.06 of QUDM). This is the preferred QUDM method to derive overland sheet
         flow time, based on existing site slopes, surface roughness and length of flow path; and
    •    Concentrated overland flow time based on assumed channel flow velocity of 1m/s within the
         environmental corridor
Rainfall Data
Intensity Frequency Duration (IFD) data for the Rational Method calculations were sourced from Noosa Shire Council’s
Rainfall Intensity Chart as per Council’s DDPSP.
Runoff Coefficients
As sated in Section 3.2.4 the fraction impervious for the calibration of the WBNM has been set to zero. The C10 value
for the site catchment has therefore been set to 0.7 in accordance with Table 4.05.3(b) of QUDM (2007). Frequency
Factors (Fy) from Table 4.05.2 of QUDM (2007) were then applied to determine the coefficients for the remaining
design storms.
Peak Flows
Peak flows from the WBNM model configured for the calibration (zero percentage impervious for each sub-catchment)
and the Queensland Rational Method calculations are presented in Table 3.6. Detailed Rational Method calculations
are presented in Appendix A.
                                      Table 3.6: Peak Flows for Calibration of WBNM
                                                                         Peak Flow (m3/s)
                   ARI
                                                               QRM                             WBNM
                   2                                           0.98                             0.84
                   5                                           1.36                             1.33
                   10                                          1.61                             1.64
                   20                                          1.92                             2.08
                   50                                          2.53                             2.48
                   100                                         2.85                             2.93


B09182.W-02A                                                                                                         Page 5