Risk associated with flood-prone areas

The "Risk associated with flood-prone areas" fact sheet identifies the regions where there is a higher risk of flooding, as well as the measures taken or to be taken to decrease their occurrence and their consequences for human health, the environment, heritage and economic activities.


Floods are extreme and temporary natural phenomena caused by moderate and permanent rainfall or by sudden and high-intensity rainfall. The drainage of the flows originated by this excessive precipitation causes an increase in water speeds and a rise in water levels, which result in the dangerousness of the flood, leading rivers to overflow their natural beds and flood their banks and neighbouring lands.  The combination between dangerousness and the consequences for human health, the environment, heritage and economic activities defines the risk associated with flood-prone areas.

Finding out more about the most significant floods and their impacts on human health, the environment, economic activities and heritage allows outlining a series of measures that will lead to a reduction of the detrimental consequences of floods. Reducing these impacts or consequences will allow decreasing the risk associated with flood-prone areas.

In 1996, following the severe floods that struck Portugal, the Ministry of the Environment launched baseline studies for implementing a National Flood Surveillance and Warning System, to reduce the vulnerability of the populations, infrastructures and the environment when faced with these extreme phenomena. These hydrological and hydraulic studies identified the affected areas and the most reliable technical means (sensors, telecommunications and computer systems) for the operationalisation of a Flood Surveillance and Warning System (SVAC), which is the information system used by the Reservoir Management Commission (the permanent body responsible for carrying out and monitoring the management of reservoirs in the event of flooding, created by Decree-Law no. 21/98, which brings together all the necessary information, namely meteorological and hydrometric information and information regarding the status and operation of the reservoirs. This system was subsequently updated, with the incorporation of new features and objectives, thus becoming the Portuguese Water Resource Surveillance and Warning System (SVARH).

Directive 2007/60/EC corroborates most of the work developed by the Portuguese administration in the field of flood risk management over the last decade, with significant effects on the decrease of vulnerability. It was transposed into Decree-Law no. 115/2010 and aims at assessing and managing the risks of flooding to reduce the consequences associated with floods which are harmful to human health (human losses), the environment, cultural heritage (infrastructures) and economic activities.

The risk associated with flood-prone areas results from a combination of dangerousness and the impacts of flooding in the identified exposed elements. Therefore, the impacts of flooding are systematised for the following areas:

  • Human health, represented by the potentially affected population;
  • The environment, represented by the water bodies, protected areas outlined as part of the Water Law (drinking water catchments, areas designated as sensitive, areas designated as vulnerable, bathing waters, Habitats Directive and Birds Directive and protected areas - Natura 2000 Network sites) and the National Network of Protected Areas and RAMSAR;
  • Natural mineral waters are only identified considering that water resource protection measures represent an added value to those specific resources;
  • Cultural heritage, represented by World Heritage, National Monument, Public or Municipal Interest Property and Archaeological Sites;
  • Infrastructures, represented by sensitive buildings (hospitals, nursing homes, nurseries, kindergartens, schools, buildings for storing or processing hazardous substances, infrastructures for managing effluents and for storing or transforming waste, and buildings relevant for managing emergencies, fire stations, facilities of the security forces and the armed forces, the Red Cross, national command and district commands of relief operations and municipal civil defence services), road and railway infrastructures, public water supply infrastructures and waste and wastewater treatment infrastructures;
  • Economic activities, represented by agriculture, tourism, facilities with Integrated Pollution Prevention and Control and facilities covered by the major accident prevention scheme (Seveso), among others.

The methodology used in preparing the maps concerning floods was based on historical hydrometeorological data stored in the National Water Resource Information System, on the present land occupation and on the historical records of damage, and was developed to be applicable to other areas subject to assessment in the 2nd cycle of Directive 2007/60/EC .

The flood-prone area maps are associated with recurrence periods of 20, 100 and 1000 years, making it possible to identify the extent of the flooded area, as well as the water heights and speeds that are reached.

Risk Matrix

    Flood Intensity      
    1 2 3 4 5    
Consequences 1 I I L L M     Non-existent/Negligible
2 I L M M H     Low
3 L M M H H     Medium
4 L M H H VH     High
5 M H H VH VH     Very High

Flood risk maps correspond to the same areas characterised in the flood-prone area maps, to which the above mentioned risk matrix was applied. In this matrix, the flood intensity / flood dangerousness and their respective consequences are assessed according to the criteria identified in the tables below.

Table for assessing flood intensity / flood dangerousness

Flood dangerousness
RI=dx(v+0,5) Degree of flood threat Risk Description (considering population only)
<0,75 1- Non-existent (I) -
0,75-1,25 2- Low (L) Caution
1,25-2,5 3- Medium (M) Danger to some people
2,5-7 4- High (H) Danger to most people
>7 5- Very High (VH) Danger to the entire population

d- Depth (m)
v- Velocity (m/s)


Table for assessing consequences depending on the exposed elements

Consequence Criterion (description)
  • Continuous urban fabric
  • Discontinuous urban fabric
  • Industries covered by the Seveso and the Integrated Pollution Prevention and Control (IPPC) Directives
  • Commercial units
  • Airports
  • Camping sites
  • Discontinuous sparse urban fabric
  • Renewable energy production infrastructures
  • Non-renewable energy production infrastructures
  • Infrastructures for the abstraction, treatment and supply of water for consumption
  • Waste and wastewater treatment infrastructures
  • Cultural facilities and historic areas (world heritage, national interest monument, public interest properties):
  • Public and private facilities (sensitive buildings): fire stations, substations, State administration, education, health, security and justice
  • Industries (not included in the High consequence class)
  • Agricultural facilities
  • Public and private facilities (not included in the High consequence class)
  • Road and railway networks and associated areas
  • Sea and river port terminals
  • Aerodromes
  • Leisure facilities (not included in the High consequence class)
  • Greenhouses and nurseries, including forest nurseries
  • Landfills, dumpsites and scrap yards
  • Historic (municipal) areas and archaeological sites
  • Shipyards and dry docks
  • Marinas and fishing docks
  • Open-cast mines
  • Golf courses and other sports facilities
  • Areas under construction
  • Abandoned areas in artificialised territories
  • Aquaculture
  • Non-perennial irrigated crops
  • Parking facilities and common land
  • Parks and gardens
  • Cemeteries
  • Quarries
  • Water bodies
  • Wetlands
  • Forestry areas
  • Agricultural areas (not included in the Medium and Low consequence class)
  • Protected areas or water bodies designated under the Directives on Birds and Habitats, Bathing Waters and Perimeters for Protecting water for human consumption, sensitive areas and vulnerable areas


The programme of measures is one of the most important elements of the Flood Risk Management Plan, considering that it outlines the technically and economically feasible actions that allow achieving a reduction of the risk of flooding by decreasing its potential detrimental consequences to human health, economic activities, cultural heritage and the environment.

It is also important to provide a framework for the origin of the different sources of funding to be mobilised for implementing the programme of measures, such as the state's general budget, the water resource protection fund, sums to be generated by the users, community funds, among others. In this respect, ensuring an alignment between the relative financial effort, i.e., the financial effort of each of the sources of funding, of the programmes of measures in Portugal and other Member States, namely Spain, is considered to be particularly relevant.

The programme of measures includes four different types of measures:

  • Protection: Structural and non-structural solutions resulting in a reduction of the flood drainage flow or height;
  • Preparation: Forecasting, warning and emergency planning systems and public awareness-raising actions;
  • Recovery and learning: Restoration of normality (hydrographic network and society) following the occurrence and assessment of improvements to be introduced in future practices;
  • Prevention: Spatial planning and land use policies (including supervision) and infrastructure relocation policies.

This fact sheet concerns mainland Portugal and shall be updated every 6 years, following the preparation of the Flood Risk Management Plans.

Objectives and targets: 
  • Raise awareness to flood risk and action strategies among the population and the social and economic agents;
  • Improve knowledge and forecasting abilities for an appropriate flood risk management;
  • Improve spatial planning and the management of exposure to flood-prone areas;
  • Improve the resilience and reduce the vulnerability of elements located in flood-prone areas;
  • Contribute to improving or maintaining the water bodies in good condition.
Progress analysis:
Last update: 
Monday, 5 June, 2017