Flooding is a complex subject with wide variety of terminology. This page covers some of the common terminology used when talking about flood risk and flood risk management. The definitions of these terms can differ slightly depending on the organisation, industry or country that have provided the information, and so only a general definition is provided here. For an introduction to flooding, please see the What is flooding? page.
Return periods are used to describe how often a flood event of a certain magnitude is likely to occur.
These return periods, or annual probabilities are sometimes presented as a one in a certain amount of years scenario. For instance, a one in 30, one in 50, or one in 100 year flood event or return period.
These scenarios refer to the size of the event, determined by it's height and flow. For instance, if an event is classified as a 'one in 100 year flood event' this means that, on average, a storm event of this magnitude will occur once in every 100 years. A common misconception is that only one event of this size will occur within the classified year period. However, because this is an average based on recorded data, it may occur more than once in 100 years, or not at all.
A better way to consider flood risk probability is to convert the return periods into percentages. This is called Annual Exceedance Probability. For instance, a one in 100 year flood event has a 1% chance of occurring in any one year.
For river and watercourse flooding, the likelihood of a flood event refers to the size of the event, based on the maximum height the river reached in meters and the amount of flow that was within the watercourse in cubic metres per second (m³/s). These are recorded by gauging stations, located on watercourses all over the country. Where long-term gauge and flood records exist, the probability of certain sized events, can be statistically calculated. However, as flood probability depends on previous and historic records of flooding, there will always been some uncertainty on the likelihood of a flood event, particularly if there is a limited history of flood records for an area. The likelihood of flood events is therefore variable and subject to change with fluctuations in rainfall.
Other weather and rainfall events are also often calculated as return periods. For rainfall events, these return periods are usually estimated by the amount of rainfall in mm for a specified duration, considering historic and past rainfall records and rainfall modelling of depth, duration and frequency.
Flood Risk Management uses a diverse range of terms for different flood alleviation strategies and solutions. Some of these are included with some general definitions below.
Attenuation means storing water to reduce the peak amount.
Blue-green infrastructure are measures that aim to naturally manage flood risk within urban environments. Blue infrastructure can include water retention basins, ponds, and wetlands, while green infrastructure increases natural woodlands, gardens, parks and open spaces. These can provide multi-functional benefits for people and the environment.
Drainage systems move water away from areas. Natural drainage systems are rivers, their tributaries and other watercourses that collect rainfall and water. However, drainage systems can also be man made such as agricultural drainage systems, highway drainage systems, or sewage or surface water systems. Natural systems can also be managed, for instance with culverts that are channels or pipes that carry watercourses below the level of the ground.
Flood schemes refer to schemes in communities or certain areas that are specific to reducing the flood problem. They can differ in size depending on how many people or properties they are intended to protect.
Natural Flood Management , sometimes referred to as Nature Based Solutions, are measures that work with natural processes to restore catchments, or store and hold back water.
Property Flood Resilience (PFR) are resistance measures that can be installed in properties to stop water from entering (up to a certain depth), or make it easier to recover from flooding when flood water has entered properties as resilience measures. PFR was previously called Property Level Protection, but the name change represents the variety in measures available to increase the resilience of properties. There is more information on the different types of PFR in the Resilience section of the website.