Reference Documents RAL21 0035

In accordance with QUDM, pipe blockage factors for all pipes have been applied as described in Table 10.4.1, with
the inlets blocked by a 20% blockage combined with a 25% bottom-up blockage within the chamber.


     3.1. Flood modelling results
The simulations were run for both the existing terrain and stormwater infrastructure, as well as the developed terrain
consisting of a proposed upgrade to the existing causeway. The resultant depths, velocities, depth velocity products
and surface height at each grid square within the active extent of the modelled terrain were recorded for each storm
event across all durations.

The resulting data for each storm was aggregated using asc_to_asc software to find the median storm event within
each duration, and then determine the maximum result of these medians across all durations with the critical result
obtained within each grid. This data has been filtered to remove isolated grid squares and flow paths and drainage
features disconnected from the subject channel for ease of review.

While every care has been taken in preparation of the TUFLOW model and the choice of the adopted parameters,
there is an inherent level of uncertainty in hydrological and hydraulic modelling due factors which may include any
of the following:

     •    The accuracy and resolution of the survey and lidar composite terrain model, and the interpretation of this
          information by the hydraulic model (calculations using a fixed grid).
     •    The nominated position of grid boundaries and grid sizes to represent surface conditions and heights may
          not capture all flow paths or alignment characteristics so localised behaviour can vary.
     •    Impacts of structures and localised drainage (roof connections, interallotment drainage, etc) that aren’t
          represented in the model resulting in differences in flow distribution.
     •    Dynamic changes in the existing or proposed topography due to erosion or deposition of soil or debris
          during a flood event which can lead to changes in the distribution of flow.
     •    Uncertainty in the rainfall pattern and catchment conditions prior to a flood. Antecedent moisture
          conditions prior to rainfall, initial detention storage levels at the beginning of rainfall runoff and the
          intensity and uniformity of the rainfall event itself all vary during flood events. Modelling attempts to
          reproduce a composite representation of these patterns through the ARR temporal patterns, however
          individual rainfall events may exhibit a differing temporal pattern than those modelled.
     •    Estimation of input parameters to the model such as runoff coefficients, Manning's roughness, and
          infiltration characteristics, which may vary within the catchment to a higher level of granularity than the
          modelled conditions.


A number of locations were selected upstream and downstream of the proposed culvert upgrade that would
provide an indicative representation of the conditions in the existing state, as well as conditions resulting from the
proposed road and culvert upgrade in order to assess the impacts of the development and afflux impacts to the
waterway. Refer to Figure 3.2 – Flood modelling points of interest which demonstrates these locations and Table
3.2 – Point of Interest Descriptions describing each location.




Project number 20220089                                                                                  Page 12 of 23
File 20220089 Jirrima Cr SWMP.docx, 2023-05-09 Revision 0


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