Urbenville is affected by flooding from two sources; Tooloom Creek to the East of the township and local catchments which run through the town as overland flows towards Tooloom Creek.

Upstream of Urbenville, Tooloom creek has a catchment area of 170.9 square kilometres. It connects with Boomi Creek downstream of the town which has a catchment area of 114.8 square kilometres. Local catchments that drain through the town to Tooloom Creek have a catchment size of 2.3 square kilometres.

For the Flood Study a hydraulic flood model was developed of the catchment and creek. Tooloom Creek has been modelled up to five kilometres North of Woodenbong and on the downstream past the Tooloom Falls, Boomi Creek has been modelled up to Brumby Plains Road.

Photo 1: Local Catchment.

Photo 2: Tooloom Catchment.

The Tooloom Creek and Boomi Creek valleys are subject to flooding causing road closures and isolating the community. Recent flooding occurred in 2010, 2011, 2013, 2016 and 2017. Roads were closed and the town isolated, however no properties were reported to have flooded above habitable floor levels. Properties along Tooloom Street in low lying areas were surrounded by flood waters.

It has been some time since Tooloom Creek and Boomi Creek have caused a major flood. Flooding in 1950 and 1954 is thought to be the highest on record although there is limited available observed data for these floods. These floods are estimated to be between a one per cent and five per cent Annual Exceedance Probability flood. Information has been gathered from the residents in the form of a questionnaire as part of the community consultation part of this study. This has been used in understanding the flood behaviour of the town.

Click here to watch the video.

Flood modelling is developed using two models:

1. A rainfall runoff routing (or hydrology) model which takes rainfall depths over time and converts this to river, channel and overland flow rates.
2. A 2-Dimensional hydraulic flood model which simulates the movement of water over the ground surface and through the drainage network, to output flood behaviour estimations including flood levels, depths, velocities and hazard.

Photo 1: Flood Modelling.

To develop the flows for the flood model rainfall patterns were used which predict the rainfall for different sized storm events; for example, a one per cent Annual Exceedance Probability storm.

This rainfall information was developed by the Bureau of Metrology for all of Australia. The rainfall runoff models consider the catchment area, slope, vegetation cover, how much impervious surfaces, soil infiltration etc.

To test the models predicted flood behaviour accurately, rainfall from a number of gauges in and surrounding the catchment was used. The model results were compared with observed flood behaviour including from information provided by the community.

Photo 1: Rainfall Gauge Locations.

A large section of Tooloom Creek and Boomi Creek were modelled. A more detailed refined model was made for Urbenville as seen in the image for this section.

The general terrain was developed from LiDAR. This provides the ground surface elevation in two metres grid resolution.

The flood model also includes drainage structures and features that would affect flood water behaviour. These include:

• bridges;
• roads;
• culverts;
• Council’s Stormwater Network; and
• drainage channels.

Sensitivity analysis was undertaken on model parameters to understand the effect on predicted flood behaviour and ensure that the modelling produced robust results.

Photo 1: 2-Dimensional Flood Modelling.

Model calibration and validation is undertaken to check that the flood models produce robust flood predictions.

Model results were compared against real data observed during past flooding events including the information provided by the community in response to the questionnaire and from Council and the local State Emergency Services. Model parameters are adjusted so that the flood model results are similar to the known flood behaviour.

The January 2008 and February 2010 events were used for model validation. Through statistical analysis of the rainfall the flood event for the January 2008 was estimated to be about a five per cent Annual Exceedance Probabilities (AEP) event. That is a flood with a five per cent chance of occurring in any one year. The February 2010 event was expected to be rarer than a 63.2 per cent AEP with a short duration of rainfall.

The only rainfall gauge in the local catchment is in Urbenville and records only 24-hour rainfall totals. Rainfall gauges and RADAR data from surrounding catchments were used to supplement this data.

In general the flood model proved to be accurate in representing what was seen by the community, with the main flood behaviour being recreated. Where depths were recorded the flood model gave results that were a reasonable approximation of the observed depths.

Click here to watch the video. The video shows the modelled simulation of the 2008 flood event.

Photo 1: Comparison of Observed Depths and Modelled Depths.

Once the flood model was validated against observed flood behaviour, design flood events were run through the model.

Design events are estimations of the flood behaviour for a range of Annual Exceedance Probabilities (AEP) Return Periods. The modelling considered the 20 per cent AEP (1-in-5 chance of occurring in a year), five per cent AEP (a 1-in-20 chance of occurring each year), one per cent AEP (a 1-in-100 chance of occurring in a year), 0.2 per cent AEP (a 1-in-500 chance of occurring in a year), and the Probable Maximum Flood (PMF) which is the largest conceivable flood.

You can download the flood maps in the Downloads and Links section of the website to see the predicted extent of flooding for the ranges of flood frequencies that have been considered.

Click here to watch the video and see the change in predicted flood events for a range of probability floods.

Urbenville is subject to flooding from both overland flows from the local catchments and mainstream flooding from Tooloom Creek. An overland flow path from the local catchments runs alongside Urbenville Road and south of Stephen Street before crossing Tooloom Street. Sheet flow occurs through the town from Beaury Street moving South through the town towards Tooloom Creek. This occurs particularly between Welch and Urben Streets.

In the animation local catchment flooding is seen in the shallow flows from the hills surrounding Urbenville flow through the town and joining Tooloom Creek. The critical duration of local catchment flooding is quite short, typically around one hour. It is what is considered as flash flooding.

Click here to watch the video.

Flooding from Tooloom Creek is typically longer duration given the larger catchment area. Due to the winding nature of Tooloom Creek and the downstream confluence with Boomi Creek, floodwaters exceed the creek channel and spill into the floodplain. Floodwaters affect properties along Tooloom Street and inundate the campground and showground in the 20 per cent Annual Exceedance Probability and larger. Tooloom Street and Clarence Way are flooded cutting access into the town.

Click here to watch the video.

Flood hazard is based on Guidelines from the Australian Institute for Disaster Resilience and Australian Rainfall and Runoff. Flood hazard increases as flood depth and flood velocity increases. Research has shown that it can take as little as 300 millimetres of water to make a small car float.

Areas of significant hazard occur near Tooloom Creek and the overland flow path from local catchment flow. In the one per cent Annual Exceedance Probability flood there are areas of high hazard on Tooloom Street and Clarence Way making them unsafe for vehicles.

Photo 1: 100-Year Hazard.

Flood Emergency Response Classification of Communities is defined to assist in managing flood evacuation and response. It informs local emergency response how where residents may be inundated or become isolated with no form of evacuation. During a large flood, local respondents such as the New South Wales’ State Emergency Services, are able to prioritise areas that need to be evacuated.

Within Urbenville the majority residential properties area classified as being within a Rising Road Access Area. This is an area where evacuation can take place to higher ground by vehicle or foot as the floodwaters advance. However, once on higher ground people may become isolated with no places to shelter or access to supplies. More details on the meaning of each classification can be found within the Flood Study Report.

Photo 1: Flood Emergency Response Classifications.

Photo 2: Flood Emergency Response Classifications Urbenville.