Glossary F


Inability to achieve a defined performance threshold (response given loading). "Catastrophic" failure describes the situation where the consequences are immediate and severe, whereas "prognostic" failure describes the situation where the consequences only grow to a significant level when additional loading has been applied and/or time has elapsed.

Source: FLOODsite


Engineered sand filters designed to remove pollutants from runoff.

Source: Tourbier/White (2007)

Filter Drains

Linear drains consisting of trenches filled with a permeable material, often with a perforated pipe in the base of the trench to assist drainage, to store and conduct water; they may also permit infiltration.

Source: Tourbier/White (2007)

Filter Strips

Vegetated areas of gently sloping ground designed to drain water evenly off impermeable areas and to filter out silt and other particulates.

Source: Tourbier/White (2007)

Zones used to filter out impurities in water.

Source: FLOWS, Cambridgeshire County Council (2006)


Flood Liaison and Advice Groups, originally named Flood Appraisal Groups (FAGs)) essentially emerged through National Planning and Policy Guidance 7 (NPPG7, 1995). The primary focus of FLAGs is in coordinating the interests of planning and water authorities. Attendees at FLAGs are ‘key stakeholders’ involved directly in flood mitigation and are local authorities. Other professionals involved in flooding may attend.

FLAGs can be seen as the CPD (Continuing Professional Development) and a key deliverer of capacity for the organisations involved in flood risk management. The purpose of their existence is to expand current knowledge of floods and flooding events from understanding the technical aspects of flooding to the likely impacts an event may have and to ensure that any information is shared amongst the professional stakeholders.

Source: Ashley et al, 2007


A temporary condition of partial or complete inundation of normally dry land areas. Floods arise from an overflow of inland water or tidal waters or the unusual accumulation and runoff of surface waters from any source. Sometimes flood events involve abnormal, flood-related erosion and undermining of shorelines. Flood also means inundation from mud flows caused by accumulations of water on or under the ground.

Source: Munich Re - 1997

Temporary covering of land by water as a result of surface waters (still or flowing) escaping from their normal confines or as a result of heavy precipitation.

Source: Leopold et al,1964

Flooding is a natural and recurring event for a river or stream. Statistically, streams will equal or exceed the mean annual flood once every 2.33 years.


A temporary covering of land by water outside its normal confines.

Source: FLOODsite

Flood control (measure)

A structural intervention to limit flooding and so an example of a risk management measure.

Source: FLOWS

Flood damage

damage to receptors (buildings, infrastructure, goods), production and intangibles (life, cultural and ecological assets) caused by a flood.

Source: FLOODsite

Flood defence structure

Structure designed to provide protection against floods. Synonymous to: Flood protection structure, Flood control structure, Flood protection works, Flood control works, Flood mitigation works.

Source: FLOWS

Flood event management

a sequence of four activities:

  • detection of the likelihood of a flood forming (hydro-meteorology),
  • forecasting of future river flow conditions from the hydro-meteorological observations,
  • warning issued to the appropriate authorities and the public on the extent, severity and timing of the flood, and
  • response to the emergency by the public and the authorities.

Source: FLOODsite

Flood forecasting system

A system designed to forecast flood levels before they occur.

Source: FLOODsite

Flood hazard map

map with the predicted or documented extent of flooding, with or without an indication of the flood probability.

Source: FLOWS

Flood level

water level during a flood:

Source: FLOODsite

Flood management measures

Actions that are taken to reduce either the probability of flooding or the consequences of flooding or some combination of the two.

Source: FLOODsite

Flood management

Sum of all operational activities to be taken before, during and after an event as well as political and administrative decisions that are aimed at preventing or mitigating a flood event.

Source: FLOWS

Flood mitigation

Measures taken in order to protect people or property from the damaging effect of water.

Synonymical: Flood damage mitigation, Flood control, Flood protection, Flood defence.

Source: UNESCO/WMO Gloss. Hydrology, 1992

Flood mitigation measures

Mitigation measures are planned actions or structures that will be triggered if a certain risk become real and it is treated as a issue that must be resolved. Mitigation plans take into account contingencies and preventive measures as well protective measures that must be put in place to avoid the realisation of the risk. Related to floods mitigation measures are methods of reducing the effects of floods. These methods may be structural solutions (e.g. reservoirs, levees) or non-structural (e.g. land-use planning, early warning systems).

Source: FLOWS

Flood mitigation shema

General description of the possible measures to be taken in order to guarantee appropriate flood safety in particular area.

Source: FLOWS

Flood peak

highest water level recorded in the river during a flood.

Source: FLOODsite


Area of land adjacent to and formed by a stream that is covered by water in case of flood.

Source: UNESCO/WMO Gloss. Hydrology, 1992

All land adjacent to a watercourse or the coast over which water flows in time of flood or would flow but for the presence of flood defences where they exist.

Source: Lincolnshire County Council, SFRA, 2006

Floodplain (1% and 0.1%) – This identifies land which is potentially vulnerable to floods of a particular magnitude. Replacing older terms relating to 1 in 100 and 1 in 1000 year floods, the 1% and 0.1% floodplains refer to land which in any one year has that likelihood of flooding.

Source: FLOWS – Cambridgeshire County Council, Matthew Hunt

part of alluvial plain that would be naturally flooded in the absence of engineered interventions.

Source: FLOODsite

Flood prevention

actions to prevent the occurrence of an extreme discharge peak.

Source: FLOODsite

Flood proofing

Measures to seal off buildings from floods by constructional changes at or near by the property, a kind of retrofitting measures.

Source: FLOWS

Flood protection measures

to protect a certain area from inundation (using dikes etc).

Source: FLOODsite

Flood risk management

Continuous and holistic societal analysis, assessment and mitigation of flood risk.

Source: Schanze et al (2005a,b)

According to context, either action taken to mitigate risk, or the complete process of risk assessment, option appraisal and risk mitigation.

Source: HR Wallingford (2002)

Continuous and holistic societal analysis, assessment and mitigation of flood risk.

Source: FLOODsite

Flood risk mapping

In contrast to hazard maps, risk maps do not only show where and what kind of damage may occur but they do also quantify economic losses considering the exposure and vulnerability of material values. Risk maps do either show a classification of losses, mostly directly expressed as a monetary value, or they show different zones of risk - as a product from hazard, exposure and vulnerability.

Visualising the results of risk assessment on a map, showing the levels of expected losses which can be anticipated in specific areas, during a particular time period, as a result of a particular flood event describes the process or activity of flood risk mapping.

Source: FLOWS

Flood risk zoning

delineation of areas with different possibilities and limitations for investments, based on flood hazard maps.

Source: FLOODsite

Flood warning system (FWS)

A system designed to warn members of the public of the potential of imminent flooding. Typically linked to a flood forecasting system.

Source: FLOODsite

Flooding system

In the broadest terms, a system may be described as the social and physical domain within which risks arise and are managed. An understanding of the way a system behaves and, in particular, the mechanisms by which it may fail, is an essential aspect of understanding risk. This is true for an organisational system like flood warning, as well as for a more physical system, such as a series of flood defences protecting a flood plain.

Source: FLOODsite


Barriers made of concrete or masonry, a kind of flood proofing measures.

Source: FLOWS

Focus Groups

A qualitative research technique whereby carefully selected groups (8 to 12 participants) are brought together under skilled presenters (researchers) to exchange views on defined subjects and to bring forward the most possible number of views from a group. It provides knowledge and understanding as opposed to measuring quotas. Because of this it gives insight in how people think and why, but not to what extent the different views are shared by others.

Source: FLOWS


The propensity of a particular defence or system to fail under a given load condition. Typically expressed as a fragility function curve relating load to probability of failure. Combined with descriptors of decay/deterioration, fragility functions enable future performance to be described.

Source: FLOODsite


Specific number of events, often related to a timeframe (in the case of Return Period this is usually expressed in years).

Source: TUHH

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