USC FloodStudyUpdate Report - Flipbook - Page 263
development peak flood levels and pre-development peak flood levels. Impacts are to be
reported as indicated in Section 5.4.
5. If required, refine the design to ensure there are no increases in peak flows at the project
boundary, and no increases in peak flood levels (greater than ±0.01 m) upstream or
downstream of the site.
5.2. Model Parameters
In general, the model parameters and schematisation should be consistent with those outlined in Section 3
and 4 for any modifications made to the models. For XP-RAFTS, this includes:
Bx of 1.0,
Impervious fractions using the methodology summarised in Diagram 2,
Table 1,
Rainfall losses outlined in Table 4.
For TUFLOW, this includes:
Maintain a grid size of 3 m and model the whole catchment to Bringelly Road.
Represent the proposed landform within the TUFLOW model, including bulk earthworks, creek
corridors and detention basins.
Include key hydraulic structures such as basin outlets, cross drainage culverts, stormwater
network (if available). Parameters should be consistent with those adopted for the current
model (as outlined in Section 4.6 and 4.7).
Table 5.
Detention basin outlets should include an allowance for blockage. For basin outlets, all grated
pits should have a blockage factor of 25% and all headwall outlets should have 10% blockage
applied for design runs.
If there are numeric computational impacts arising from the adaptive time-step feature in HPC, it is
recommended that the control number factor be set to 0.8 for both the pre-development and postdevelopment scenarios.
For the purpose of the impact assessment, it should be assumed that:
Existing farm dams start full (initial water level is at the height of the lowest level of the dam
embankment crest), and
Existing and proposed detention basins start with a water level equal to the lowest outlet level.
The TUFLOW model should be run for a duration long enough such that the South Creek peak flow has
been passed at Bringelly Road, and that the peak water levels within basins and dams have been reached.
The XP-RAFTS model should be run for a duration that covers the TUFLOW model run time at a minimum.
If there is any deviation from the parameter values or schematisation methodology specified in this guide,
then these should be clearly documented and justified.
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118112: 201030_USC_Regional_Model_User_Guide_Final: 30 October 2020
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