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

Upstream of Woodenbong, Tooloom creek has a catchment area of 112 square kilometres. Local catchments that drain through the town to Tooloom Creek have a catchment size of 8.4 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 valley is subject to flooding closing roads and isolating community. Recent flooding occurred in 2010, 2011, 2013, 2016 and 2017. Roads were closed and the town isolated however no properties were flooded above floor levels. In December 2010, properties along Richmond Street were flooding within their backyards.

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.

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 to understand the flood behaviour of the creeks. A more detailed refined model was made for Woodenbong 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 this is the information provided by the community in response to the questionnaire, information 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 event and December 2010 events were used for model validation. Through statistical analysis of the rainfall the flood event for the January 2008 event was estimated to be about a five per cent Annual Exceedance Probability (AEP) event, that is a 1-in-20 year Average Recurrence Interval event. That is a flood with a five per cent chance of occurring in any one year. The December 2010 event was estimated to an event which ranged between 20 per cent and 50 per cent AEP; that is a flood ranging between a 20 per cent to 50 per cent 1-in-chance of occurring each year.

The only rainfall gauge in the catchment is in Woodenbong 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 Probability (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.

Woodenbong is subject to flooding from both overland flows from the local catchments and mainstream flooding from Tooloom Creek. A major flow path from the local catchments runs behind Richmond Street and connects to Black Gully. Properties are protected from flooding in smaller events by a small levee but the levee can be overtopped in larger flood events, about five per cent Annual Exceedance Probability.

During heavy rainfall, sheet flows can occur through the streets of Woodenbong. The majority of these overland flows stays within the road drains and has depths less than 50 millimetres when it does breakout.

In the animation local catchment flooding is seen in the shallow flows through the town and joining Tooloom Creek and Black Gully. 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 longer given the larger catchment areas. Floodwaters in the one per cent AEP or a storm with a 1-in-100 chance of occurring in a year shown event affect the showground, campground and sporting fields within Woodenbong. Backwaters from Tooloom Creek travel up Black Gully effects properties on the northern end of Roseberry Street. Mount Lindsay Road is cut-off from floodwaters to the East and West of the town.

Click here to watch the video. Please note, the animation in the video shows the flooding that occurs from Tooloom Creek Catchment. The flooding is a longer duration, typically around 12 hours.

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 Black Gully and the flow path behind Richmond Street. In the one per cent Annual Exceedance Probability flood, there are areas of high hazard on the showground, campground and sporting fields, properties along Richmond Street within the flood extent is unsafe for vehicles and people. Where Mount Lindesay Road is flooded it is classified as unsafe for vehicles and people. There are areas of low hazard within the town from stormwater runoff, this is mostly confined to the road drains.

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.

The majority of the residential properties in Woodenbong town are classified as Indirectly Affected Area. This is an area that is typically not affected by flooding but possibly require resupply, rescue and evacuation if it was to become isolated or supplies were cut. 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 Woodenbong.