If They Go in Can They Get Out? Evacuation Modelling

S Molino¹, M Hossain², R Khadka², R Autar², and K Sanborn¹

¹ Molino Stewart Pty Ltd, Parramatta, NSW ² Liverpool City Council, NSW

The Georges River has a catchment area of 960 km2 and is heavily urbanised in its northern half. Liverpool local government area in southwestern straddles the Georges River, including much of its broad floodplain between its upstream and downstream gorges. In Liverpool, the Georges River wraps around the Moorebank Peninsula to the west, north and east. There are low lying floodplains all along most of this reach of the Georges River on both sides of the river. In this area, the river has several major tributaries, including Anzac Creek, Cabramatta Creek, and Brickmakers Creek (Figure 1).

Figure 1. Georges River study area.

There is a long recorded history of flooding on the Georges River. The largest recorded flood occurred in February 1873 and is estimated to be well above the 1% annual exceedance probability (AEP) event. Floods in April 1860, April 1887 and May 1889

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are estimated to be similar in magnitude to a 1% AEP (Bewsher Consulting, 2004). In February 2020, river levels reached their highest since 1988. While only about a 5% AEP event, low lying premises near the river had to evacuate, as they have done on many past occasions. Today, floods approaching 1% AEP levels, would require much larger scale evacuations; floods approaching the PMF would require evacuation of up to 8,000 premises.

The Greater Sydney Regional Plan has identified the Liverpool Collaboration Area as a major growth centre for Sydney, with a target of nearly 20,000 new dwellings and more than 15,000 new jobs by 2036. Planning proposals currently before Council already equate to 30,000 new dwellings. However, flooding has been recognised as a major factor that could potentially limit growth in the area. Molino Stewart was commissioned by Liverpool City Council to create a flood evacuation model for the Liverpool Collaboration Area and adjoining suburbs to help determine to what extent flood evacuation is a constraint to development.

In recent years sophisticated models for the estimation of loss of life in any flood event have been created. One of the most advanced of these was developed by British Columbia Hydro in Canada and commercialised as the Life Safety Model (LSM) by HR Wallingford in the UK. It is an agent-based model which integrates warning dissemination modelling, traffic flow modelling and Council’s existing two-dimensional Georges River TUFLOW flood model to determine where and when people need to evacuate and whether they can do so before roads are cut or buildings are flooded. This dynamic model represents: • the rise and spread of floodwaters, • the receipt of warning messages, • the response of occupants to the warning, • evacuation traffic flow, and • the fate of those who fail to evacuate before the arrival of floodwaters.

This study used LSM to model the required evacuation from the Georges River Probable Maximum Flood (PMF). PMF model outputs were used to undertake an emergency response classification of communities in accordance with NSW Department of Planning, Industry and Environment (DPIE) guidelines to identify the flood islands within the study area and those which have rising road access and overland escape routes. Subsectors were then defined by selecting areas with shared evacuation routes and flood risks, and thus responding to shared specific trigger level(s) (Figure 2). In total, 43 evacuation subsectors were identified, including area impacted by the PMF from Georges River as well as the study area’s creeks. Of these subsectors, 15 are primarily industrial, 26 are primarily residential, one is both industrial and residential, and one is an equestrian area.

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Figure 2. Subsectors identified in the study area.

A key input for the model is the number of vehicles that will evacuate from specific properties in each modelled scenario. Molino Stewart developed a methodology for calculating the number of vehicles that would need to evacuate from existing and future residential developments in the study area by integrating flood model data, 2016 Australian Bureau of Statistics Census data, cadastre data, aerial imagery, Google Street View, and proposed development data from Council. In addition, NSW State Emergency Service (NSWSES) and Infrastructure NSW (INSW) were consulted to develop a methodology to estimate the non-residential vehicles that would need to evacuate from the industrial and commercial areas at the same time as the residential areas based on Journey to Work Data released by Transport Performance and Analytics within Transport for NSW.

In addition to vehicles, a simplified, digitised road network including the number of lanes for each road is included in the model set up. To account for the very real possibility of local creek flooding during an evacuation from Georges River flooding, every road that crosses Anzac Creek, Brickmakers Creek, Cabramatta Creek and Maxwell’s Creek was cut in the model if it was flooded by a 1 in 500 AEP or more frequent event (Figure 3).

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Figure 3. Road cut points.

Evacuation triggers were identified for each subsector based on an analysis of flood and warning information. Based on the NSWSES’s Provision and Requirements for Flood Warning in , the Bureau of Meteorology has a target warning lead time of 12 hours for major flooding of the Georges River. Therefore, the model incorporated a 12-hour warning before either a subsector would be flooded, or its evacuation route will be cut. The model also accounted for the standard warning acceptance factor (one hour), warning lag factor (one hour), travel time depending on the road capacity, and the traffic safety factor as per the NSW Timeline Evacuation Model (Opper et al., 2009).

A baseline scenario has been modelled assuming population and dwelling numbers as of the 2016 census. This has identified flood islands in Chipping Norton, Moorebank, and Warwick Farm. Some are isolated in the 5% AEP flood and the number increases with the increasing magnitude of flooding. Most are low flood islands (or Flooded Isolated and Submerged) where the area is first isolated from flood-free land and then completely inundated as flood waters continue to rise.

It was also found that simultaneous minor flooding of Anzac Creek, Brickmakers Creek, Cabramatta Creek or Maxwell’s Creek would pose a major constraint to evacuation in the study area. Many roads crossing these creeks could not be relied upon as evacuation routes.

One area in Warwick Farm with residential and commercial properties could be cut off by local creek flooding blocking public roads before being inundated by the Georges

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River flooding. However, it may be possible to identify a flood free evacuation route through private roadways within the industrial area (Figure 4).

Figure 4. Road cut points in Warwick Farm with a possible flood-free route via private roads shown with white arrows.

In addition, the model identified a major bottleneck for evacuation traffic. The NSW SES’s draft evacuation strategy is to direct evacuees north along the M7 motorway which would take them on a flood free route out of the area and towards alternative accommodation and a mass evacuation centre at Homebush.

Both the and the M5, with three lanes of traffic each, have single-lane connectors onto the M7, which itself only has two lanes of traffic. Therefore, there is significant queueing at the entrance of the M7, which causes traffic to back up throughout the study area. In addition, the model found significant queuing caused by the single lane M5 on ramp via Heathcote Road, which is the primary evacuation route for the Moorebank peninsula from Nuwarra Road (Figure 5).

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Figure 5. Model output at Georges River PMF time step t = 5 hrs, with rising floodwaters shown in blue. Yellow lines show queuing traffic on the roads, light purple points show vehicles ready and waiting to evacuate, and dark purple points show where people are still sheltering in buildings.

The NSWSES is now reviewing this information to inform its regional emergency response plan for the Georges River. Additionally, Liverpool City Council, the NSWSES and DPIE are reviewing the draft modelling inputs and assumptions before determining what will be used to assess the impacts on infill development and planning proposals on evacuation capacity and options for ensuring that if more people are put into the area, they can safely get out during a flood.

References

Bewsher Consulting (2004). Cabramatta Creek Floodplain Management Study and Plan.

Opper, S., Cinque, P., and Davies, B. (2009). Timeline Modelling of Flood Evacuation Operations, Presented at First International Conference on Evacuation Modelling and Management, Den Haag, The Netherlands, 23-25 September 2009.

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