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Flood modelling of the Parramatta River in Sydney, Australia

The Flood Modeller 1D and 2D solvers have been applied to simulate combined fluvial and direct rainfall flooding in a sub-catchment of the Parramatta River in Sydney, Australia.

The work was conducted as part of a wider floodplain risk management plan for a proposed river bank development of a brownfield site for a multinational environmental services company. The aim was to evaluate the impact of site development on the surrounding water levels.

An initial phase of the project involved the development and calibration of a 1D model of the Parramatta River. This was used to evaluate the impacts of proposed bank-side regeneration works at the site. This work highlighted the need for a more complete modelling approach, and the requirement for a 1D-2D model of the site and surrounding sub-catchment.

The second phase of the project involved developing a fully linked 1D-2D model, allowing the modelling of both fluvial and direct rainfall events. The Flood Modeller 2D ADI solver was chosen to allow the use key features, such as the spill generator in the modelling toolbox, for its speed and stability, and for the ease in which direct rainfall can be modelled in the 2D domain.

Rainfall hyetographs were developed for the catchment using locally accepted methods in Sydney and existing hydrographs were used for the flow series in the river.

The 2D model was developed using available LiDAR and aerial imagery data, with more accurate site topographic data overlaid, where appropriate. The domain covers the entire surrounding sub-catchment. Buildings in the catchment have been modelled at ground level with a high roughness to prohibit flow through them, whilst enabling the storage of water within them.

The surface water drainage network in the area has not been included in the model as the intensity of events being analysed would render it ineffective.

A sensitivity analysis concluded that a 5 m grid size provided sufficient accuracy over the 100 ha catchment.

To analyse the impacts of both direct rainfall and fluvial flooding, the events were combined. The peak rainfall-runoff at the site was timed to coincide with the peak river levels adjacent to the site by adjusting the start of rainfall. The baseline model was evaluated for 20-year, 100-year and Probably Maximum Flow events, considering the joint probability of both direct rainfall and fluvial events and their respective impacts on flood levels at the site.

A design scenario model was developed, based on the site design drawings. This model was evaluated for the same events, and the results were compared. 2D flood maps of depth were used to analyse water levels on site and within the catchment. These maps were used to determine the impact of the design on water levels. They were also used to determine hazard values along proposed evacuation routes from the site to ensure that a safe exit route would be available in the event of a large flood.

Using the Flood Modeller 1D and 2D solvers, it has been possible to show that the proposed design can be employed without increasing flood levels in the catchment and without increasing downstream water levels in the river, while providing sufficient flood protection for the site.

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