Flood terms and definitions
The following list of flood terms and definitions are provided to support consistent communication about flooding.
|The area of land draining to a particular site. It is related to a specific location and includes the catchment of the main waterway as well as the tributary streams.
|Flooding due to prolonged or intense rainfall (e.g. severe thunderstorms, monsoonal rains in the tropics, tropical cyclones). Types of catchment flooding include riverine, local overland and groundwater flooding.
|Flooding due to tidal or storm-driven coastal events, including storm surges in lower coastal waterways. This can be exacerbated by wind-wave generation from storm events.
|Defined flood event (DFE)
|The flood event selected for management of flood hazard to new development. This is generally determined in floodplain management studies and incorporated in floodplain management plans. Selection of DFEs should be based on an understanding of flood behaviour and the associated likelihood and consequences of flooding. It should also take into account the social, economic, environmental and cultural consequences associated with floods of different severities. Different DFEs may be chosen for the basis for reducing flood risk to different types of development.
|Defined Flood Level
Defined flood level (DFL) is the level to which it is reasonably expected flood waters may rise.
|Effective Warning Time
|The effective warning time available to flood-prone community is equal to the time between the delivery of an official warning to prepare for imminent flooding and the loss of evacuation routes due to flooding. The effective warning time is typically used for people to self-evacuate, to move farm equipment, move stock, raise furniture, and transport their possessions.
|Existing Flood Risk
|The risk an existing community is exposed to as a result of its location on the floodplain.
|A flood that is sudden and occurs with little warning. It is often caused by sudden local or nearby heavy rainfall. It is generally not possible to issue details flood warnings for flash flooding. However, generalised warnings may be possible.
|Flooding is a natural phenomenon that occurs when water covers land that is normally dry. It may result from coastal of catchment flooding, or a combination of both.
|An appreciation of the likely effects of flooding, and a knowledge of the relevant flood warning, response and evacuation procedures. In communities without a high degree of flood awareness, flood warnings are liable to be ignored or misunderstood, and residents are often confused about what they should do, when to evacuate, what to take with them and where it should be taken.
|A term used to describe the pattern, and nature of a flood in terms of characteristics such as levels, velocities and flows.
|The Bureau uses a three tiered classification scheme that defines flooding as minor, moderate, or major at key river height stations. Each classification is defined by water level that causes certain impacts upstream and downstream of the station. These levels have been determined based on standard descriptions of flood effects, historical data, and relevant local information. The classifications are revised from time to time by the Bureau’s partner agencies and affected communities.
|Areas where a significant flow of flood water occurs. These areas typically flow continuously from the upper reaches of waterways and flow paths within the catchment to the outlet during a flood. These flows often align with naturally defined channels and are areas that, even if only partially blocked by changes in topography or development, cause a significant redistribution of flood flow or a significant increase in flood levels. They are often, but not necessarily, areas of deeper flow or areas where higher velocities occur.
|The tangible (direct and indirect) and intangible costs (financial, opportunity costs, clean-up) of flooding. Tangible costs are quantified in monetary terms (e.g. damage to goods and possessions, loss of income or services in the flood aftermath). Intangible damages are difficult to quantify in monetary terms and include the increased level of physical, emotional and psychological health problems suffered by flood-affected people that are attributed to a flooding episode.
|Education that raises awareness of the flood problem, to help individuals understand how to manage themselves and their property in response to flood warnings and in a flood event. It invokes a state of flood readiness.
|Flood emergency management
|Emergency management is a range of measures to manage risks to communities and the environment. In the flood context, it may include measures to prevent, prepare for, respond to and recover from flooding.
|Flood emergency management plan
|A step-by-step sequence of previously agreed roles, responsibilities, functions, actions and management arrangements for the conduct of a single or series of connected emergency operations. The objective is to ensure a coordinator response by all agencies having responsibilities and functions in emergencies.
|Flood emergency response planning
|The process to inform the development of flood emergency response plans by providing advice on the variation of hazardous conditions to people, vehicles and buildings within the floodplain.
|The remainder of the flood extent for the particular event once flood conveyance and storage areas are accounted for. Developing flood-fringe areas is unlikely to significantly alter flood behaviour, beyond the broader impact of changes to run-off because of urbanisation within the catchment. However, other flood-related constraints may exist in flood-fringe areas.
|Potential loss of life, injury and economic loss cause by future flood events. The degree of hazard varies with the severity of flooding and affected by flood behaviour (extent, depth, velocity, isolation, rate of rise of floodwaters, duration), topography and emergency management.
|A practical tool for identifying most at risk to flooding. They provide a visual representation of possible flood consequences (i.e. extent, depth, and velocity of flood waters at different gauge heights) and can be used to raise flood awareness in a community by delineating the areas at risk of flooding.
|Flood planning area
|The area of land below the flood planning level, and is thus subject to flood-related development controls.
|Flood planning level (FPL)
|The FPL is a combination of the defined flood levels (derived from significant historical flood events or floods of specific annual exceedance probabilities) and freeboards selected for floodplain management purposes, as determined in management studies and incorporated in management plans.
|Flood proofing of buildings
|A combination of measures incorporated in the design, construction and alteration of individual buildings or structures that are subject to flooding, to reduce structural damage and potentially, in some cases, reduce contents damage.
|An ability to react within the effective warning time
|The potential risk of flooding to people, their social setting, and their built and natural environment. The degree of risk varies with circumstances across the full range of floods. Flood risk is divided into three types – existing, future and residual. It is assessed considering both likelihood of flooding and the consequences of flood.
Handbook 7: Managing the Floodplain: A Guide to Best Practice in Flood Risk Management in Australia (Australian Disaster Resilience) pg. 86 (PDF)
|Flood risk management
|The process applied to enable a community to become as resilient as practicable to floods. This is achieved through planning and preparing for, responding to and recovering from flooding. It requires a coordinated, multi-disciplinary approach across all levels of government and between agencies with different responsibilities. It also requires the support of a range of non-government organisations and industry professionals in a wide range of activities and fields, and the active engagement of the community.
|A qualitative indication of the ‘size’ of a flood and its hazard potential. Severity varies inversely with likelihood of occurrence (i.e. the greater the likelihood of occurrence, the more frequently an event will occur, but the less severe it will be).
|Flood storage areas
|The parts of the floodplain that are important for temporary storage of floodwaters during a flood passage. The extent and behaviour of flood storage areas may change with flood severity, and loss of flood storage can increase the severity of flood impacts by reducing natural flood attenuation. Hence, it is necessary to investigate a range of flood sizes before defining flood storage areas.
|A comprehensive technical investigation of flood behaviour. It defines the nature of flood hazard across the floodplain by providing information on the extent, level and velocity of floodwaters, and on the distribution of flood flows. The flood study forms the basis for subsequent management studies and needs to take into account a full range of flood events up to and including the probable maximum flood.
|Messaging distributed to potential at-risk communities to inform of an emerging flood threat. Effective warning is crucial to support a community's ability to respond to a flood event. Effective flood warnings require effective messages disseminated via an effective system.
What is a Flood Watch?
The Bureau issues a Flood Watch to provide early advice of a developing situation that may lead to flooding. A Flood Watch is not a warning of imminent flooding.
A Flood Watch provides information about a developing weather situation including forecast rainfall totals, catchments at risk of flooding, and indicative severity where required. The product also provides links to weather warnings, other Bureau flood-related products, and contact details and information of relevant emergency services.
Although there is uncertainty attached to a Flood Watch, its early dissemination can help individuals and communities to be better prepared should flooding eventuate. A Flood Watch may discuss possible snowmelt, local flooding or tidal impacts but a Flood Watch will not be issued solely on the basis of these phenomena.
When is a Flood Watch issued?
A Flood Watch is generally issued up to four days in advance of the expected onset of flooding. A Flood Watch can be issued before, during and after the rainfall has occurred, depending on the level of maturity of the flood warning systems and services, and flood impact information made available from the local emergency services or state agency.
Flood Watches are updated at least daily and finalised once all areas are covered by flood warnings or the risk of flooding has passed.
|An area of land this is subject to inundation by floods up to and including the probable maximum flood event – that is flood-prone land.
|Floodplain management plan
A management plan developed in accordance with the principles and guidelines usually includes both written and diagrammatic information describing how particular areas of flood-prone land are to be used and managed to achieve defined objectives. It outlines the recommended ways to manage the flood risk associated with the use of the floodplain for various purposes. It represents the considered opinion of the local community and the floodplain management entity on how best to manage the floodplain, including consideration of flood risk in strategic land-use planning to facilitate development of the community.
It fosters flood warning, response, evacuation, clean-up and recovery in the onset and aftermath of a flood, and suggests an organisational structure for the integrated management for existing, future and residual flood risks. Plans need to be reviewed regularly to assess progress and to consider the consequences of any changed circumstances that have arisen since the last review.
|Land susceptible of flooding by the probably maximum flood event. Flood-prone land is synonymous with the floodplain. Floodplain management plans should encompass all flood-prone land rather than being restricted to areas affected by defined flood events.
|The rate of flow of water measured in volumes per unit time.
|Flow conveyance areas
Those areas of the floodplain where a significant flow of water occurs during floods. They are often aligned with naturally defined channels. Flow conveyance paths are areas that, even if only partially blocked would cause a significant redistribution of flood flow or a significant increase in flood levels. They are often, but not necessarily, areas of deeper flow or areas where higher velocities occur, and can also include areas where significant storage of floodwater occurs.Each flood has a flow conveyance area and the extent and flood behaviour within a flow conveyance areas may change with flood severity. This is because areas that are benign for small floods may experience much greater and more hazardous flows during larger floods.
The height above the DFE or design flood used, in consideration of local and design factors, to provide reasonable certainty that the risk of exposure selecting in deciding on a particular DFE or design floods is actually provided. It is a factor of safety typically used in relation to the setting of floor levels, levee crest levels and so on. Freeboard compensates for a range of factors including wave action, localised hydraulic behaviour and levee settlement, all of which increase water levels or reduce the level of protection provided by levees. Freeboard should not be relied upon to provide protection for flood events larger than the relevant defined flood event of a design flood.Freeboard is included in the flood planning level and therefore used in the derivation of the flood planning area.
|The measure of likelihood expressed as the number of occurrences of a specified event in a given time. Typically measured in terms of Annual Exceedance Probability (AEP). For example, a flood with the frequency of 1% (1 in 100) AEP has a 1% chance of occurring in any given year.
|Future flood risk
|The risk that new development within a community is exposed to as a result of developing on the floodplain.
|A source of potential harm or situation with a potential to cause loss.
|A flood that has actually occurred in the past.
A computer model that uses data about the flow in streams and the terrain of a particular area to estimate flood heights, velocities and flow over time. In order to do this, the hydraulic model solves the equations for the conservations of mass and momentum/energy.The main aim of hydraulic modelling is to describe the details of the main water level and velocity characteristics of the hydrologically derived flood flows. An appropriate set-up and calibrated hydraulic model can be used to not only describe the details of flood flows and their distribution throughout a river and floodplain system, but also predict the likely impacts that any changes to that system may have on these flows.
The study of water flow in waterways; in particular, the evaluation of flow parameters such as water level, extent and velocity.
Hydraulic computations are usually carried out to determine flood characteristics such as:
Handbook 7: Managing the Floodplain: A Guide to Best Practice in Flood Risk Management in Australia (Australian Disaster Resilience) pg. 87
A graph that shows how the flow or stage (flood level) at any particular location varies with time during a flood.Estimates of the hydrograph are required where it is necessary to obtain an estimate of flood volume as well as peak discharge.
Handbook 7: Managing the Floodplain: A Guide to Best Practice in Flood Risk Management in Australia (Australian Disaster Resilience) pg. 87
|The study of rainfall and runoff process including the evaluation of peak flows, flow volumes and the derivation of hydrographs for a range of floods.
A computer model that uses rainfall data and estimates of the proportion of the rainfall which turn into runoff and the time which the runoff from each part of the catchment takes to flow into the stream to estimate flow in the stream over time.
Simplified mathematical conceptualisations to represent these complex spatially and temporally distributed hydrological processes. These hydrological models are calibrated to measurements of data on variables such as rainfall, evaporation, and flow.
|The term given to the study of the rainfall and runoff process, in particular the evaluation of peak flows, flow volumes and the derivation of hydrographs for a range of floods.
|A risk that, following understanding of the likelihood and consequences of flooding, is so high that it requires consideration of implementation of treatments or actions to improve understanding, avoid, transfer or reduce the risk.
|Flooding can isolate parts of the landscape and cut-off evacuation routes to flood-free land. This can result in dangerous situations, because people may see the need to cross floodwaters to access services, employment or family members. Many flood fatalities result for the interactions of people, often in vehicles with floodwaters.
The process by which the use and land is managed for the benefit of the whole community. Land use planning approaches that anticipate likely risk and vulnerability of the population can reduce the potential impact of future disaster events. Responsible land use planning can therefore prevent or reduce the likelihood of hazards impacting on communities and for this reason; land-use-planning is considered an effective preventative measure to mitigate the impact of floods.Land-use planning decisions need to adequately consider the availability of existing infrastructure and any impacts on it, along with potential costs and locations for infrastructure to service future needs.
|Likelihood of occurrence
|The likelihood that a specified event will occur (see also Frequency). Likelihood of flooding is expressed as the number of occurrences of a specified event in a given time. Typically measured in terms of Annual Exceedance Probability (AEP). For example, a flood with the frequency of 1% (1 in 100) AEP has a 1% chance of occurring in any given year.
|Local overland flooding
|Inundation by local runoff on its way to a waterway, rather than overbank flow from a stream, river, estuary, lake or dam. Can be considered synonymous with stormwater flooding.
|Minor Flood (gauge classification)
|Causes inconvenience and low-lying areas next to watercourses are inundated. Minor roads may be closed and low-level bridges submerged. In urban areas inundation may affect some backyards and buildings below the floor level as well as bicycle and pedestrian paths. In rural areas removal of stock and equipment may be required.
|Moderate Flooding (gauge classification)
|In addition to the above, the area of inundation is more substantial. Main traffic routes may be affected. Some buildings may be affected above the floor level. Evacuation of flood-affected areas may be required. In rural areas removal of stock is required.
|Major Flooding (gauge classification)
|In addition to the above, extensive rural areas and/or urban areas are inundated. Many buildings may be affected above the floor level. Properties and towns are likely to be isolated and major rail and traffic routes closed. Evacuation of flood-affected areas may be required. Utility services may be impacted.
|Monte Carlo Analysis
A Monte Carlo analysis, or probability simulation, is a technique used to understand the impact of risk and uncertainty in flood models.Monte Carlo techniques can be used to derive expected probability quantiles of selected flood characteristics arising from the join interaction of many factors, but the defensibility of these estimates rests upon the representativeness of the inputs and the correct treatment of correlations which may be present.
Under the Planning Act 2016, each local government planning scheme needs to set out integrated state, regional and local planning and development assessment policies for an entire local government area.
Planning schemes needs to address the community’s expectations and needs by providing for development to occur in appropriate places and in a suitable form.
|A statistical measure of the expected chance of flooding. It is the likelihood of a specific outcomes, as measured by the ratio of specific outcomes to the total number of possible outcomes.
|Probable maximum flood (PMF)
|The hypothetical flood estimate relevant to specific catchment whose magnitude is such there is negligible chance of it being exceeded. It represents a notional upper limit of flood magnitude and no attempt is made to assign a probability of exceedance to such an event. It is also used to define the extent of flood-prone land. The PMF causes the largest scale of flood emergency and is also therefore often used for emergency management planning.
|Probable maximum precipitation (PMP)
|The PMP is the greatest depth of precipitation for a given duration meteorologically possible over a given size storm area at a particular location at a particular time of the year, with no allowance made for long-term climatic trends. It is the primary input to probably maximum flood estimation.
The rate at which rain falls, typically measured in millimetres per hour (mm/h). Rainfall intensity varies throughout a storm in accordance with the temporal pattern of the storm.
|Rate of rise
|The speed at which floodwaters rise over time. A rapid rate of rise can lead to people evacuating being overtaken or cut off by rising floodwaters. It is often associated with high velocities but it can be an issue if access routes are affected by flooding.
|Residual flood risk
|The risk a community is exposed to that is not being remedied through establishment risk treatment processes. In simple terms, for a community, it is the total risk to that community, less any measure in place to reduce that risk.
|The systematic use of available information to determine how often specified (flood) events occur and the magnitude of their likely consequences. Flood risk analysis is normally undertaken as part of a floodplain management plan. The floodplain management plan reflects the adopted means of managing flood risk.
|The systematic application of management policies, procedures and practices to the tasks of identifying analysing, assessing, mitigating and monitoring risk.
|Inundation of normally dry land occurring when water overflows the natural or artificial banks of a stream, river, estuary, lake or dam. Riverine flooding generally excludes watercourses constructed with pipes or artificial channels considered as stormwater channels.
|During a flood, rainfall is converted to runoff and is transferred through a network of flow path to the catchment outlet. These flow paths include overland flow on hill slopes, down tributaries, across floodplains, through natural and artificial storages and along main streams.
|The increase in coastal water levels above predicted astronomical tide level (i.e. tidal anomaly) resulting from a range of location dependent factors including the invested barometer effect, wind and water set-up and astronomical tidal waves, together with any other factors that increase tidal water level.
|Is the inundation by local runoff caused by heavier than usual rainfall. It can be caused by local runoff exceeding the capacity of an urban stormwater drainage systems, flow overland on the way to waterways or by the backwater effects of mainstream flooding causing urban stormwater drainage systems to overflow.
|The static or dynamic water level at the downstream boundary of the flood model.
The variation of rainfall intensity with time during a rainfall event.
|The measures that might be feasible for the treatment of existing, future and residual flood risk at particular locations within the floodplain. Preparation of a treatment plan requires a detailed evaluation of floodplain management options.
|Velocity of floodwater
|The speed of floodwaters, measured in metres per second (m/s).
The characteristics and circumstances of a community, system or asset that make it susceptible to the damaging effects of a hazard.
(QRA Reference: CM QRATF/20/756)