BsmFinal_1

applied to the dams and flowing channels, while values around 0.045 and 0.050 were applied to grassed
overbank areas and values of 0.100 to 0.150 were applied to timbered area.

The existing floodways through the site and downstream areas, with their assigned n values, are depicted
in the photographs in Figures 3 and 4 on pages 8 and 9 of this report.

2.4 Storm Rainfall Inputs And Infiltration

Since the model is basically an extension of a model from a previous flood study, the design rainfall
inputs were from the ARR87 IFD data as per that study. However, the inclusion of initial rainfall losses
from 20 to 30 mm across the range of ARI in the previous study, so as to achieve a Rational Method
calibration, was replaced by a universal initial loss of zero in this extended model. The previous
continuing loss rate of 2 mm/hr was maintained in the extended model. The critical storm durations for
peak flow rates and flood levels across the site were found to be 25 minutes and 1 hour in the southern
tributary, 1 hour in the main floodway, and 25 minutes in the northern tributary.

The allowance for climate change involved a 15 percent increase in rainfall intensities at a 60 year
horizon.

2.5 Rational Method Sanity Check

The model’s predictions of the 1, 10 and 100 year peak flow rates at model nodes N48, N54 and N56
were compared to Rational Method calculations of the same flow rates, as summarised in Table 1, below.
The XPStorm predictions are consistently higher than the Rational Method predictions, with the disparity
ranging from 8 to 10 percent at 100 year ARI, to as much as 40 percent at 1 year ARI.

Table 1: Comparison, Rational Method To XPStorm
  Location       1 Year ARI               10 Year ARI                      100 Year ARI
 (XPStorm) Rat Method. XPStorm Rat Method. XPStorm                   Rat Method. XPStorm
    N48        0.44        0.56        1.00        1.28                  1.81        1.98
    N54        2.81        3.93        6.05        6.56                  11.2        12.3
    N56        0.23        0.27        0.53        0.68                  0.94        1.03

2.6 Existing Flood Levels

The full complement of the XPStorm model’s existing flood levels through and downstream of the site,
assuming 20% blockage at all culverts, are summarised in Tables 2 to 4 on pages 10 and 11 of this report.




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   Brisbane Stormwater Management Pty Ltd Document File: ChurchSt.Doc