Floods generally occur as a result of bad weather ( for example : storms, cyclones, tornadoes or thunderstorms, heavy or persistent rain), melting snow or ice, or the sudden discharge of water.
These events lead to a rise in the water level of major riverbeds or to an accumulation of flow water, to the resurgence of underground rivers or to thaws in low-lying areas adjoining stream banks or river estuaries. In mountainous areas, landslides, earthquakes, and avalanches can cause the overflowing or rupture of hydraulic dams or cause unstable blockages at certain points on rivers, and thus threaten or destroy people and the infrastructure located in the lower parts of valleys.
Bursting or overflowing of hydraulic dams, as well as seismic/tidal waves are special cases which require specific protective measures.
Damage caused by floods can vary greatly according to the nature of the phenomenon at the origin of the flood. They can affect whole regions or certain limited sectors. Moreover, their impact will vary according to whether the event is sudden and violent or a steady but relatively slow rise of flood waters which will inundate a vast area, especially plains situated near or at the mouth of rivers.
The danger is obviously highest in mountainous areas or steep valleys where depressions will be totally levelled or momentarily drowned then covered in debris carried by the flood water (rocks, sand, gravel); this will result in the destruction or burial of all or some of the dwellings, of the economic infrastructure and of vital installations (energy grids, communications and drinking water). This also happens when torrential rainstorms occur in arid regions and deserts where the water cannot penetrate the impermeable soil and, depending on the topography, runs off as surface water destroying homes and settlements over a wide area.
AIl types of floods have grave consequences, mainly because of their secondary effects. Localised flood, or one involving a small rise in water level can lead to large-scale damage, to the interruption or destruction of communication routes (road, rail, etc.) or the loss of infrastructure and damage to the environment (economy, supplies, crops).
2. Preventive and protective measures
As opposed to other natural phenomenon (landslides) or the causes of certain floods (tidal waves, bad weather), the risk of flood from rising water levels is predictable, although it is difficult to know when it will occur. It is therefore possible to determine the areas at risk.. This is also true in the case of hydraulic dam bursts or overflows since their consequences can be calculated from the volume of water held, the slope and topography of the valley, and the absorptive capacity of the rivers located below.
Long-term preventive and protective measures will essentially consist in regulating the layout of the areas at risk by legislation calling for a systematic appraisal of potential natural dangers, and taking these into account when defining areas that can be built on, and when granting planning permission. These preventive measures will, as far as possible, be completed by an obligation to replant and maintain forests and vegetation, and by the control of upland water streams and rivers. To this end, it can only be recommended that a national hydrological or geological institute responsible for studying these problems and coordinating preventive measures be set up.
At the community level general preventive measures can be summarised as follows:
- Permanent monitoring of the risk of flooding (establishment of a network to measure the water levels of streams/rivers).
- Setting up one or several information and flood warning centres for the population especially with regard to large capacity hydraulic dams.
- Issuing regulations banning building, residing in, and access to identified risk zones and implementing specific protective systems such as alarm signals.
- Building and developing infrastructure that will prevent, avoid or limit floods and protect the population.
- Planning the evacuation of the population likely to be at risk, and instructing them on how to behave in case of a flood.
- Forming well trained and equipped management and rescue teams.
- Lowering water levels in hydraulic dams and increasing the flow of rivers ) preventive measures.
3. Intervention and rescue measures
As soon as a major flood danger is identified, and after an assessment of the seriousness of the situation, the local, regional or national authorities decide on the protective measures to be taken, as dictated by the probable evolution of the situation. It is up to them to instigate the preventive and protective measures planned, notably the alarm and warning services, close ( the area under threat, instruct the population on how to behave, order if necessary the evacuation of all or some inhabitants, domestic animals and cultural objects and valuable goods from the potential flood area.
The management of protection, search, rescue and assistance measures (supplies, medical assistance evacuation) falls to the political authorities and their executive organs in charge of the coordinated deployment of the available civilian and military means of intervention. These endeavour to collaborate, to the extent possible, with the personnel responsible for security firms, rescue organisations from neighbouring areas and cross-border emergency rescue al assistance teams.
If the national capacity for prevention and protection is proven to be insufficient, especially terms of environmental protection (pollution by hydrocarbons or toxic products) the government of the stricken state can call for international emergency assistance by contacting the United Nations Department of Humanitarian Affairs (UNDHA).
4. Instructions for the population
4.1 In case of potential danger from flooding
- Avoid as far as possible living in an area at risk from flooding.
- Conform to the laws on land development and environmental protection, especially with regard to building restrictions and technical security measures intended for installation substances and waste material likely to cause major damage to man and the environment in the event of a disaster.
- Find out the highest levels reached by floods in the past and the speed at which the water rose. Keep up-to-date with the protective measures to be taken immediately and, in particular, obey the specific prescriptions on buildings and other installations.
- Know the alarm signals and behavioural rules, especially the evacuation zones and routes.
- Always keep an emergency kit ready for the family, as well as enough supplies of food and drink to last the estimated length of the emergency situation.
- Plan how to reinforce the building and neighbouring structures, and to protect vital installations from floods (electricity, gas, water, heating, evacuation of waste water).
- Take into account the risk of flooding when fitting the interior of the house, especially by placing furniture, valuable goods or those susceptible to water damage, as well as pollutants, on higher levels together with reserves of food, drinking water, a portable radio and a torch.
- Foresee a reserve of materials that are useful in case of a flood, for example, bricks, plaster, cement, sand, jute or plastic bags, textiles, ropes, tools, etc.
4.2 During a disaster
- Keep calm, do not panic.
- Warn neighbours and give priority to helping the handicapped, children and the elderly.
- Keep informed of the danger and how it may evolve, listen to the radio but do not use the telephone without good reason (do not overload the lines).
- Switch off electricity, gas and central heating. If there is enough time move valuable or delicate objects and pollutant products to the higher levels of the strongest parts of the building.
- Implement the measures planned for the immediate protection of people and the environment (if possible untie and set free animals from stables and other such buildings).
- If obliged to leave your house, lock it securely and make your way on foot to the evacuation zone using the route indicated by the authorities (especially in cases of hydraulic dam bursts).
- Take with you only what is strictly necessary (emergency baggage, identity and personal papers, medicines).
- Do not cross flooded areas on foot or in a vehicle. If necessary secure yourself by holding onto ropes or cables.
- Collaborate with public safety bodies and the services helping the homeless.
4.3 After the flood
- Keep calm, do not panic.
- Check and see if there any injured people in the vicinity and, if possible, help them.
- Listen to the radio but do not use the telephone unnecessarily.
- Collaborate with official rescue and assistance services helping the victims.
- Collaborate in the identification of bodies.
- Make yourself available to help with rehabilitation work.