Floodplain management

Parts of the Ballina Shire have changed significantly since the last major flood event in the 1980s.  Because of our proximity to the coast, high tides also pose a significant flood threat, particularly when combined with floodwater from heavy rain. Sea level rise associated with climate change is also expected to result in more serious flooding in the future. This means it is important to plan for future possible events.

Ballina Shire Council is working to ensure it can do everything possible to make sure communities are aware of the danger and are prepared for a flood emergency, and importantly understand their own risks, hazards and tolerability to flooding.

In 2008, the Ballina Flood Study Update (BMT WBM) was completed to develop an improved understanding of existing and future flood risk in Ballina and surrounding communities. This included the development of updated flood models and flood inundation mapping for riverine, creek and ocean-driven design flood events.

In 2012, the Ballina Floodplain Risk Management Study (BMT WBM) was completed to identify and evaluate options available to manage riverine, creek and oceanic flood risk in the township of Ballina and its surrounding communities. This was followed by development of the Ballina Floodplain Risk Management Plan (BMT WBM) in accordance with the NSW Government Floodplain Risk Management Process.

One outcome of the Ballina Floodplain Risk Management Plan was to further investigate alternative flood protection measures, beyond the existing Council policy of filling within the floodplain. This resulted in the preparation of the Ballina Island and West Ballina Overland Flood Study and Flood Protection Feasibility Study and Plan (GHD). This study presented a strategic plan to mitigate localised existing and future flood risk in Ballina Island and West Ballina by providing practical information in regard to recommended floodplain management measures such as timing, priority, cost and responsibility, plus recommendations for further investigation.

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Policy Context

The responsibility for planning and management of flood prone lands in NSW rests with local government. The NSW Government provides assistance with state-wide policy and technical support. A Flood Prone Land Policy and a Floodplain Development Manual (NSW, 2005) forms the basis of floodplain management in NSW.

The objectives of the NSW Policy include:

• Reducing the impact of flooding and flood liability on existing developed areas by flood mitigation works and measures including ongoing emergency management measures, voluntary purchase and house raising programs, flood mitigation works, and development controls.
• Reducing the potential for flood losses in new development areas by the application of ecologically sensitive planning and development controls.

To support this policy, the NSW Floodplain Development Manual identifies four main stages in the floodplain risk management process:

1. Flood Study: Determines the nature and extent of flooding behaviour.
2. Floodplain Risk Management Study: Identifies, develops and compares various floodplain management options utilising the results of the Flood Study as well as assessment of social, economic, ecological and cultural issues.
3. Floodplain Risk Management Plan: Formalises outcomes of the previous studies and present the necessary information to enable relevant authorities to plan for the future.
4. Plan Implementation: Includes construction of structural floodplain management measures as well as incorporation of non-structural measures into existing Local Authority Environmental and Development Control Plans.

A focus of the recent Ballina Island and West Ballina Overland Flood Study and Flood Protection Feasibility Study and Plan has been on local overland flooding behaviour and some recommendations are made on this basis. Also some floodplain risk management measures to address regional flooding of the Richmond River, tidal inundation and storm surge are discussed in greater detail, and this project has included scope to assess in more detail major structural mitigation options, namely a ring levee system.

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Sources of Flooding

Richmond River flooding

The lower Richmond River flows in a north easterly direction adjacent to the coastline through a large rural floodplain. It turns east at its confluence with Emigrant Creek before passing Ballina and meeting North Creek near its outfall into the Pacific Ocean.

Richmond River flooding is caused by widespread storm systems with precipitation typically occurring over many days across the broader Richmond River catchment. These floods rise and fall relatively slowly at Ballina, with flood conditions lasting multiple days.

Flood water on the lower Richmond River travels in a north easterly direction towards Ballina, spilling out into the floodplain to the south and west of Ballina. These floodwaters also affect flooding on the local catchment creeks, especially in their lower reaches.

Significant regional and riverine flooding has been limited over recent times with the last major floods occurring during 2005 and the 1970’s and late 1980’s.

Creek flooding from Emigrant Creek, Maguires Creek and North Creek

Emigrant Creek flows in a southerly direction through Cumbalum before joining the Richmond River at West Ballina. Maguries Creek flows in a south easterly direction from Teven and joins Emigrant Creek on the lower floodplain. North Creek flows in a southerly direction from Ross Lane passing Ballina Nature Reserve on its right bank before passing through East Ballina and Ballina Island at its confluence with the Richmond River.

Flooding from these local creek catchments is caused by smaller storm systems with intense rainfall bursts typically lasting less than 12 hours. Flood waters rise and fall quickly with this form of flooding presenting a high hazard due to short warning times and fast flowing floodwater.

Ocean storm surge flooding

Ocean storm surge flooding caused by low pressure systems, strong onshore winds and storm wave conditions, which lead to high than usual ocean levels. This form of flooding is influenced by tides and will typically occur in combination with one or two high tides.

Ocean storm flooding dominates in the lower reach of the Richmond River and North Creek, thus affecting parts of Ballina Island and West Ballina. These areas comprise of concentrated urban development, which highlights the significance of this form of flooding in Ballina.

Overland flooding

In addition to the above sources of flooding, low lying residential and commercial areas surrounding Ballina are subject to overland flooding from local rainfall-runoff and backwater surcharge of the stormwater network during high tide events.

• Local runoff: 
  Over 60% of Ballina Island is a "trapped low point" where conveyance is achieved exclusively by piped drainage through surrounding higher ground. The centre of the island is characterised by a "bowl" formation with the lowest points of the island generally around Tamar Street, Grant Street and Moon Street. These unusual conditions demonstrate complex overland flooding characteristics which are largely driven by the interaction between rainfall runoff, surface storage, pipe capacity and tide oscillations. Historically, damages caused by overland flooding on Ballina Island and West Ballina usually occurs when continuous rainfall bursts in excess of 2 hours coincides with a high tide event, however there is little correlation with rainfall intensity alone and severity of observed overland flooding.
   
• Tidal backflow inundation: 

  Low lying areas around Ballina are known to experience inundation caused by tidal backflow through the stormwater drainage network during twice annual seasonal high astronomical tide conditions. This can and does occur without coincident local rainfall runoff or coincident riverine or creek flood events.

  Observations indicates that backflows from these “King Tides” tend to affect the road network to a greater degree than private properties, with potential saline water in road gutters (up to 300 mm deep) during a few periods during the year. 

  During such events, media releases are issued to warn the public of flood affected roads and inundated areas.

  It is noted that the highest astronomical tide recorded in Ballina (without coincident riverine flood) during the period January 1991 to February 2018 was 1.29 mAHD, occurring 3 January 2018. This is generally accepted by Council as the highest “King Tide” event on record.

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Frequently Asked Questions