RFI - Attachment 3 - Stormwater Management Plan

Table 3.2 – Critical Median Storm Event at culvert crossing

      AEP                         Characteristic                           Duration                            Storm
 1%                 Depth                                                    90                                 5328

 1%                 Depth x Velocity product                                 90                                5328

                                                                             120                               5323

 10%                Depth                                                    120                               5367

 10%                Depth x Velocity product                                 120                               5367

                                                                            270*                               5435


It is noted that temporal pattern 5435 for the 10% AEP 4.5 hour storm produces an embedded burst (where a small
period of the rainfall has an intensity rarer than the storm event as a whole), resulting in 100mm of rain occurring
within a 1 hour period. This rainfall is approximately equivalent to the 1% AEP rainfall intensity for this period
(between a 5% and 2% AEP storm under 2100 climate change conditions), and therefore this 1 hour burst would
govern the results and design based on this storm event. This is not considered to reflect of the 10% AEP
conditions as it would result in overdesign of the culvert being assessed so this temporal pattern has therefore
been excluded from the results.

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.


     3.2. Flood modelling results
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. Refer to Figure 3.3 – Flood
modelling points of interest which demonstrates these locations and Table 3.3 – Point of Interest Descriptions
describing each location.




Project number 21020                                                                                            Page 14 of 36
File RECOR Concept Stormwater Management Report - 21020.docx, 2022-09-05
Revision 0


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