What is a ‘flood control lot’?

A flood control lot is a property that has the potential to be affected by a 1 in 100 year flood event. The flood control lots are defined based on the findings of the Leichhardt Flood Study. Details can be found in Section 12 of the Leichhardt Flood Study

What is a “1 in 100 year” flood?

A 1 in 100 year flood occurs on average once every 100 years. This event can also be described as the 1% AEP flood event, which means there is a 1% chance of a flood of this size occurring at a particular location in any given year. This does not mean that if a location floods one year, it will definitely not flood for the next 99 years. Nor does it mean that if it has not flooded for 99 years that it will necessarily flood the next year. Some parts of Australia have received more than one 1 in 100 year flood within a decade of each other. 

What is the difference between a Flood Study and a Flood Risk Management Study and Plan?

A Flood Study provides technical information on the likelihood and characteristics of flooding within a catchment. It provides detailed information about the nature of flood risk including the distribution, extent, depths, levels and velocity of floodwaters for a range of storm events, including a 100 year flood. 

A floodplain risk management study and plan analyses flood behaviour and identifies and prioritises options to help protect people and property through better planning, emergency management and infrastructure works.  

How are flood risks managed?

There are three main ways of managing flood risk:

  • emergency management
  • property modification
  • flood modification
Emergency management measures seek to help people at risk during a flood. This includes sharing information with the State Emergency Service, flood warning systems and identifying evacuation routes and locations of emergency refuge centres.  

Property modification measures consider the way in which land is used, the buildings that are on it and the extent of flooding on a property. These measures are typically addressed in the Local Environmental Plans and Development Control Plans. The Leichhardt Flood Risk Management Study and Plan investigates the policies and planning controls of the former Leichhardt Council area and makes recommendations on how they could be amended to better manage flooding. 

Flood modification measures change the behaviour of flood waters with structural methods. This includes stormwater upgrades, additional underground pipelines and introducing detention basins and levees. 

How were the flood mitigation options in the draft plan identified?

The flood mitigations options were identified by Cardno in consultation with Council staff and the State Emergency Services. The options were identified by: 

  • Determining locations where flooding occurs in the 100 year ARI event in the Leichhardt Flood Study (Cardno 2014);
  • Developing a preliminary list of flood mitigation options for each of these locations; 
  • Creating a short list of options by identifying those likely to be most effective and feasible to implement; and
  • Modelling shortlisted options to determine the option would result in a beneficial reduction to flooding

How have the flood mitigation options been assessed and prioritised?

All of the options were assessed for their economic, social and environmental benefits. The economic benefit is determined by comparing the potential financial saving that the option would provide in a flood event with the cost to implement it. The outcome of this was combined with expected social and environmental benefits to provide an overall ranking to compare all options against each other. The outcomes of this multi-criteria assessment can be found in section 4 of the plan.

The draft plan shows a new pipeline within my property – what does this mean?

The draft plan only shows new pipelines within private properties where there is already an existing pipeline. Enlarging or replacing existing pipelines is only shown where it is considered to provide a significant reduction in flooding. The draft plan is a concept so the exact route of any new pipeline isn’t known – this would be determined as part of a detailed analysis.  In some cases it may be possible to undertake work when a property owner is developing the land.  Only in cases where it is unavoidable will properties be considered for acquisition.  

The draft plan shows a detention basin in a nearby park – what is a detention basin?

Detention basins are areas where stormwater is temporarily stored during large storm events. The water is then slowly released at a controlled rate so that downstream areas are not flooded. Detention basins are usually located in public parks and are constructed by forming low mounding around the edges to force the water to pond temporarily within the park. This won’t have any negative impact to people using the park and its facilities.  

When will Council implement the flood management options identified through the plan?

The draft Leichhardt Flood Risk Management Plan does not recommend a specific works plan for implementation. Instead the Plan recommends a series of structural and property modification measures to be implemented when the opportunity arises.  

The Plan does recommend emergency response measures should be implemented as the first planned risk management action.  

Will the adoption of a Flood Risk Management Study and Plan result in a more expensive insurance premium for my property?

Council provides the Insurance Council of Australia (ICA) with the most recent flood data available to help it determine the insurance costs for properties in flood affected areas. While this allows insurance companies to set premiums based on the actual flood risk, Council ultimately has no control over how they do this.