Glen - REC13/0146 Flood Study BSM 30 April 2014. Email 2 Attachment 2

5.3     Model Calibration

The catchment runoff processes have been modelled with the “basic” variant of Version 3.5g of the Urbs
software, with the “Rational Method” procedure invoked so as to improve the routing simulation within
individual subareas, and with rainfalls converted to runoff via the initial/continuing loss procedure. The
operating parameters of the model which affect the outcome of the simulation are the storage lag
parameter (alpha), the non-linearity parameter (m), the initial rainfall loss (IL) and the continuing rainfall
loss (CL).

Ideally, the operating parameters should be calibrated to observed rainfall and runoff data in the subject
catchment, using either data from particular storm events or derived statistical data via a frequency curve
of recorded flows. In this instance, there is a streamgauge further downstream in the broader catchment
(at Cooran, where the contributing catchment area is 180 square kilometres, or about 120 times larger
than the subject catchment). This gauge has been recording flood flows for 60 years, but the accuracy of
its height/flow rating curve has been confirmed by current-metering to only just above the 1-in-2 year
flood height, which means that its usefulness for model calibration to flows of 5 year or greater severity
is very questionable.

The calibration has therefore reverted to an alternative process involving three stages:

- Calibration to Rational Method calculations of the peak flow rates from individual subareas and groups
  of subareas.

- Comparing the calibrated parameters from the first stage to the corresponding parameters from
  previous calibrations to other morphologically similar catchments with more highly rated streamgauge
  data.

- Comparing the predicted peak flow rates at particular locations in the modelled area to the
  corresponding flow rates in the 2002 L&T floodway model.

The Rational Method calibration has been applied to the peak flow rates entering the floodway model
near the upstream boundaries of the site and to the peak inflow rates from each of the subareas which
discharge to the floodway within the subject site. The relevant calculations are summarised in
Appendices B and C of this report, for the pre- and post-development scenarios respectively. An
acceptably close calibration to these calculated peak flow rates was achieved, as in Table 10 below and in
Table 11 on the following pages of this report, using an alpha parameter of 0.85 an m parameter of 0.8,
an initial rainfall loss of 16 mm, and a continuing rainfall loss of 2 mm/hour. The alpha parameter for
individual small subareas which discharge overland flow mostly as shallow sheet flow was increased by
arbitrarily applying reach factors of up to 3.

Table 10 :     URBS / Rational Method Peak Flow Calibration (m3/sec) For Pre-Development Scenario
              (Refer Figure 4 For Subarea Locations)
 Subareas       1 year        2 year        5 Year        10 Year        20 year        50 year       100 Year
   1–5         5.2 / 6.0     6.7 / 7.6    11.2/ 11.5     14.5 / 14.2   17.4 / 16.9    21.4 / 21.5    24.8 / 24.6
     7        0.29 / 0.29   0.36/ 0.35    0.54 / 0.52    0.64 / 0.63   0.78 / 0.75    0.96 / 0.95    1.10 / 1.10
     8        0.24 /0.25    0.30 /0.30    0.47 / 0.45    0.56 / 0.55   0.68 / 0.65    0.83 / 0.83    0.96 / 0.96
     9        0.21 / 0.23   0.26 / 0.27   0.40 / 0.40    0.49 / 0.49   0.60 / 0.59    0.74 / 0.74    0.85 / 0.85
  12 – 16      4.8 / 4.8     6.1 / 5.8     9.3 / 8.6     11.2 / 10.8   13.6 / 12.8    16.5 / 16.2    18.8 / 18.7
    17        0.19 / 0.22   0.24 / 0.26   0.39 / 0.38    0.48 / 0.48   0.59 / 0.57    0.72 / 0.71    0.82 / 0.83
    18        0.20 / 0.21   0.25 / 0.26   0.39 / 0.38    0.47 / 0.47   0.57 / 0.56    0.70 / 0.71    0.81 / 0.81
    19        0.19 / 0.21   0.23/ 0.25    0.37 / 0.37    0.46 / 0.46   0.56 / 0.54    0.69 / 0.70    0.79 / 0.79




      Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                            Page 10