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Understanding floods: questions and answers

From December 2010 to January 2011, Queensland experienced widespread flooding, with three quarters of the state declared a disaster zone. Understanding Floods: Questions and Answers was prepared by the Queensland Floods Science, Engineering and Technology (SET) Panel, convened by the Queensland Chief Scientist, Dr Geoff Garrett AO, to explain in clear and simple language the fundamental concepts of floods, including flood causes, impacts, forecasting, and flood risk management now and in the future.


Introduction and Report Summary

Throughout Australia's long history, the flood-drought cycle has been a natural part of life, with periods of severe drought followed by extensive flooding playing an important and defining role in shaping the Australian landscape and how we live. Nature is unpredictable, and there will always be a risk of flood. However, we can reduce the risk of floods to communities, economies and environments by effectively communicating lessons from past floods and applying advancements in knowledge and technology. Here we provide a summary of answers to eight critical, underlying questions relevant to floods, covering: what we mean by a 'flood'; flood causes and consequences; how we forecast and warn about them; and how we can best plan for floods now and in the future. More

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Question 1 What is a flood?

When water inundates land that is normally dry, this is called a flood. Floods can be caused by a number of processes, but the dominant cause in Australia is rainfall. Floods are a natural process, but mankind's activities affect flooding. Floods occur at irregular intervals and vary in size, area of extent, and duration. More

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Question 2 What factors contribute to floods?

Rainfall is the most important factor in creating a flood, but there are many other contributing factors. When rain falls on a catchment, the amount of rainwater that reaches the waterways depends on the characteristics of the catchment, particularly its size, shape and land use. Some rainfall is ‘captured’ by soil and vegetation, and the remainder enters waterways as flow. River characteristics such as size and shape, the vegetation in and around the river, and the presence of structures in and adjacent to the waterway all affect the level of water in the waterway. More

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Question 3 What are the consequences of floods?

Floods impact on both individuals and communities, and have social, economic, and environmental consequences. The consequences of floods, both negative and positive, vary greatly depending on the location and extent of flooding, and the vulnerability and value of the natural and constructed environments they affect. More

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Question 4 How do we forecast floods?

Weather forecasts can provide advance warning of a flood, and seasonal forecasts can alert of a heightened chance of flooding in the coming months. However, forecasting river levels and flood extent is a complex process that is continually being improved. More

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Question 5 How do we communicate and warn about floods?

Flood warning systems turn forecasts into messages designed to reduce the negative impacts of floods. Warning systems should be accurate, timely and reliable. Prior community awareness of flood risk can make warnings more effective. Improving our warning systems could reduce social losses from floods. More

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Question 6 How do we estimate the chance of a flood occurring?

Understanding the chance of different sized floods occurring is important for managing flood risk. The chance of a flood event can be described using a variety of terms, but the preferred method is the Annual Exceedance Probability (AEP). A flood with a 1% AEP has a one in a hundred chance of being exceeded in any year. Currently, the 1% AEP event is designated as having an ‘acceptable’ risk for planning purposes nearly everywhere in Australia. However, good planning needs to consider more than just the 1% AEP flood. More

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Question 7 How do we manage flood risks?

Flood risk includes both the chance of an event taking place and its potential impact. Land use planning informed by floodplain management plans can reduce risk for new development areas. Flood risk is harder to manage in existing developed areas; however modification measures such as dams or levees can change the behaviour of floodwaters. Similarly, property modification measures can protect against harm caused by floods to individual buildings, and response modification measures help communities deal with floods. More

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Question 8 What does the future look like?

Australia's growing population and changing climate patterns imply that the characteristics of the floods we experience will change in the future. Better future land use planning and floodplain management can mitigate the impacts of flooding. Appropriate urban design can reduce the severity of flood impacts. Catchment and waterway revegetation can reduce the impact of flooding. Emerging technologies can improve our ability to predict and manage floods. More

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The glossary provides a summary of key floods terms and definitions. More

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Last updated:
15 March, 2013
Last reviewed:
12 July, 2011

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