Figure 13: Exile Bay Land Use Map

Figure 13: Exile Bay Land Use Map

7. HYDRAULIC MODEL

7.1 Overview

Hydraulic models are used to produce flood depths, levels and velocities across the study area, based on the inflow hydrographs output from the hydrologic model. The current study has established a hydraulic model for Exile Bay based on the TUFLOW software. TUFLOW is a hydrodynamic modelling program that represents the floodplain as a grid of cells and resolves flow behaviour using a finite difference method.

The TUFLOW software used in the current study is the 64-bit version 2016-03-AE. A 2 m grid cell size was adopted on that basis that; it is fit for purpose, the smallest grid cell that can be reasonably utilised given the available topographic data and achieves reasonable runtimes. The hydraulic model extent and the model schematisation is shown in Figure 14. The sensitivity of the model results to the adopted values and boundary conditions assumptions has been investigated in Section 10.

7.2 Model Topography

The TUFLOW hydraulic model uses the 1 m raster product from LiDAR data described in Section 3.3.1.

7.2.1 Selection of Cell Size

TUFLOW uses a matrix of square (grid) cells as the computational structure. The selection of model cell size needs to be sufficiently small to adequately represent hydraulic behaviour yet large enough to minimise run times. Based on this criterion a 2 m cell size was adopted.

The cell size is also dependent on the resolution of input data. Given the LiDAR is ~1.44 m2 resolution, a resolution of 2 m was the smallest possible cell size. Furthermore, cell sizes of less than 2 m would cause issues with the shallow water equations used by TUFLOW.

7.3 Boundary Conditions 7.3.1 Inflows

Flow hydrographs generated in the hydrologic DRAINS model were applied at the downstream of each sub-catchment within the TUFLOW hydraulic model. Where possible, these inflows were applied to pit locations at the downstream of each sub-catchment.

7.3.2 Downstream Boundary

For all model iterations, it was necessary to define the downstream water level. The downstream model boundary is located at Exile Bay an inlet of the Parramatta River.

For calibration of the November and October 2018 events, the NSW Tide Charts for 2018 were used to derive the tidal level at the time of each event.

For the design flood modelling, the effects of both overland flow and mainstream flooding in Exile Bay must be considered (see Section 2.2). These mechanisms can occur independently or at the same time. OEH (Reference 18) recommend specific combinations between catchment flooding and ocean water levels for design flood modelling. These recommendations have been incorporated into the