What makes floods occur

They can bring welcome relief for people and ecosystems suffering from prolonged drought, but also are estimated to be the most costly natural disaster in Australia. Flooding occurs most commonly from heavy rainfall when natural watercourses do not have the capacity to carry excess water. However, floods are not always caused by heavy rainfall.

In coastal areas, water inundation can be caused by a storm surge as a result of a tropical cyclone, a tsunami or a high tide coinciding with higher than normal river levels. If a dam fails, triggered for example by an earthquake, the downstream area will flood, even in dry weather conditions.

Riverine flooding occurs in relatively low-lying areas adjacent to streams and rivers. In the extensive flat inland regions of Australia, floods may spread over thousands of square kilometres and last several weeks, with flood warnings sometimes issued months in advance. In the mountain and coastal regions of Australia, flooding can happen rapidly with a warning of only a few hours in some cases. The Great Dividing Range which extends along the length of eastern Australia provides a natural separation between the longer and slower westerly flowing rivers and the shorter, faster easterly flowing coastal rivers.

In some cases, natural blockages at river mouths, including storm surge and high tides, can also cause localised flooding of estuaries and coastal lake systems.

Flash flooding, otherwise known as overland flooding, can occur almost anywhere there is a relatively short, intense burst of rainfall such as during a thunderstorm. As a result, the drainage system has insufficient capacity or time to cope with the downpour.

Although flash floods are generally localised, they pose a significant threat because of their unpredictability and normally short duration. Just because an area has not flooded in the past, does not mean it will not flood in future. Similarly, just because an area flooded during one event, does not mean that it will necessarily flood during the next one.

The Bureau of Meteorology maintains the Australia Rainfall and River Conditions which contains up-to-date rainfall and river information for all catchments within Australia. The most costly summer for floods in Australia was , with extensive flooding in the Lockyer Valley, Ipswich and Brisbane in January There were 35 deaths and 20, people were made homeless. A comparison of disaster cost estimates in Australia. International J of DRR. Report with AGD.

Geoscience Australia is committed to support Australia's capability to managing the impact of natural hazards, including flood. Geoscience Australia:. The Australian Flood Risk Information Portal the portal enables flood information, currently held by different sources, to be accessible from a single online location. The portal includes a database of flood study information and metadata the Australian Flood Studies Database.

The portal provides access to authoritative flood maps and flood studies, as well as information about surface water observations derived from the analysis of satellite imagery. The portal incorporates tools that enable users to search, display and retrieve information.

The data management tools and standards that have been developed for the portal will enable data custodians to standardise their data and upload it to the portal, or to make compliant data accessible via web services. The information in the portal has been provided by local, state and territories governments. Contributing more flood studies will significantly enhance its value. It will improve our understanding of flood behaviour on a regional scale, which is essential because floods do not respect jurisdictional boundaries.

The Australian Flood Risk Information Portal the portal hosts data and tools that allow public discovery, visualisation and retrieval of flood studies, flood maps, satellite derived water observations and other related information from a central location. The portal is focussed on providing access to flood information from authoritative sources.

The main catalogue of information available through the portal is the Australian Flood Studies Database. Information in the portal can be searched and accessed by all users, but only registered users can add flood studies and associated maps to the database. A registered user would typically have been responsible for the creation or publication of flood hazard information, and might include state authorities, local councils, consultants and authorised data custodians.

Data custodians can share their flood study information with other users either by adding data directly into the portal's data catalogue, the Australian Flood Studies Database, or via web services.

Data Entry Information Information on entering data into the portal, including how to login or register to use the Data Entry Application.

This is mostly the case in large urban areas — think Houston and Los Angeles. When heavy rains strike, the basins used to drain them cannot always handle the load. Rain is not always the culprit when it comes to flooding. Storm surges related to hurricanes and other storms can lead to significant flooding, as can tsunamis that are sometimes caused by underwater earthquakes.

Given modern technology, we often know about storm surges and tsunamis before they arrive, but this is not always the case. For example, in , an earthquake off the coast of Indonesia created a tsunami that gave little warning before coming ashore. Flooding often occurs when there is fast runoff into lakes, rivers and other reservoirs. This is often the case with rivers and other channels that feature steep sides. It is a similar issue to having a lack of vegetation, which is explained in more detail below.

Vegetation can help slow runoff and prevent flooding. When there is a lack of vegetation, however, there is little to stop water from running off. This can be a bit of a conundrum after a drought. While area residents likely welcome the rain, the lack of vegetation after the drought can cause flash flooding. This does not always happen given that basins and reservoirs are close to empty, but it can occur in cases of extreme rains following long periods of drought. A winter of heavy snow and other precipitation can lead to a spring of flooding.

After all, that snow and ice have to go somewhere when they melt. These conditions typically come from storms at sea like:. Tropical cyclones. Higher-than-average tides. Another type of flooding that is often associated with coastal flooding is storm surge. What is Storm Surge? Storm surge is an abnormal rise in water level in coastal areas over and above the regular astronomical tide.

Causes of Storm Surge. Storm surge is always a result meteorological storms that cause higher than normal tides on the coast. There are three parts of a storm that create this surge. Low atmospheric pressure. Learn about storm surge from one of our expert meteorologists, Andrew Rosenthal in the below snippet from our weather safety warmup webinar on hurricanes :. Storm surge is an extremely dangerous form of flooding.

It can flood large coastal areas at the same time. It can also cause flooding very quickly. Extreme flooding occurs when storm surge happens at the same time as high tide. This can cause storm tides to reach over 2o feet! Our meteorologists always stress that storm surge is the most dangerous aspect of any tropical system. It poses the most threat to both life and property. For example, storm surge took the lives directly and indirectly of over 1, people during Hurricane Katrina.

The next type of flood on our list is an inland flood. Some organizations refer to inland flooding as urban flooding. A flash flood is also a type of inland flood.

What is an Inland Flood? An inland flood is flooding that occurs inland or not in a coastal area. Therefore, coastal flooding and storm surge are not inland floods. Causes of Inland Flooding. Rainfall is almost always to blame for inland floods. Rain causes inland flooding in two ways. It can happen with steady rainfall over several days or it can happen because of a short and intense period of rainfall.

Snowmelt also causes inland floods, although rainfall is a more common cause. Another way inland flooding happens is when water ways get blocked by debris, ice, or dams. More on Inland Floods.

The following urban features can create urban flooding or make inland floods worse:. Paved roads and streets. Low-capacity drainage equipment. Dense buildings. Low amounts of green space. The most well-known and deadly type of flood is a flash flood. What is a Flash Flood? A flash flood is flooding that begins within 6 hours, and often within 3 hours, of heavy rainfall or other cause. What Causes Flash Floods? Most flash floods happen after extremely intense rainfall from severe thunderstorms over a short period of time normally 6 hours or less.

There are two key elements to determine is flash flooding is likely:. Rainfall rate. Rainfall duration. Flash floods also happen when damns break, when levees fail, or when an ice jam releases a large amount of water.

Dangers of Flash Floods. Flash flooding is the 1 severe weather killer in the United States. Flash floods are extremely powerful. They have enough force to roll boulders, tear trees from the ground, destroy buildings and bridges, and scour out new channels. This type of flood is characterized by raging torrents that rip through river beds, urban streets, or canyons, wiping out everything in their paths.

With heights reaching 30 feet, flash floods can completely cover communities. Another reason why flash flooding is so dangerous is that it can happen with little to no warning. This is especially true when dams or levees break. They also recommend having a family or business disaster plan ready in the case of a flash flood. If you read through the above section on types of floods, you might just want to skip down to flooding effects.

As we mentioned above, there are plenty of different causes of flooding. While different flood types typically have different causes, most floods are caused by one of the following activities.

Heavy rainfall is the simplest cause of flooding. This can result in floods like flash flooding. Overflowing rivers are another cause of floods. As we mentioned before, river flooding can happen when there is debris in the river or dams that block the flow of the water. Speaking of dams, broken dams are another cause of flooding. Older infrastructure can fail when heavy rains come and water levels rise. When dams break, they unleash torrents of water on unsuspecting households.

This is part of what happened when Hurricane Katrina hit New Orleans in Storm surge and tsunamis also cause flooding. Storm surges from hurricanes and other tropical systems can cause sea levels to rise and cover normally dry coastal areas in several feet of water. Tsunamis on the other hand are giant waves caused by earthquakes or underwater volcanic eruptions.

As these waves move inland, they build height and can push a lot of water inland in coastal areas. Channels with steep banks are also to blame for flooding. Flooding often occurs when there is fast runoff into lakes, rivers, and other basins.

This is often the case with rivers and other channels that feature steep sides. A lack of vegetation can cause flooding. Vegetation can help slow runoff and prevent flooding. When there is a lack of vegetation, there is little to stop water from running off and overflowing river banks and streams. Melting snow and ice is another common reason for flooding.