Floods

Contents:

• Flood Definitions
• Nature, causes and effects of floods s
• Flood prevention measures

1 Definition of floods

Flooding is the temporary submersion of land by water that is not normally submerged.

a flood can have various origins and occur:

• when there is an exceptional rise in the level of the water table closest to the ground (known as the water table): depending on the topography, more or less extensive areas may be affected, sometimes for long periods of time;
• when a watercourse overflows its usual bed (described as a minor bed for a river, a channel for a torrent), the rise in water level being more or less rapid depending on the size and gradient of the catchment area, the nature of the soil, the state of the vegetation cover, etc. The curve representing the evolution of the flow as a function of time at a given point is called the flood hydrograph: the slowly rising floods of rivers and large rivers can be contrasted with the rapidly rising floods of torrential rivers and especially torrents which, moreover, because of their steep slope, have a significant erosive power, pulling the materials from the mountains before depositing them on their torrential cone;
• when rainwater or snowmelt flows over the surface in an unorganized manner before reaching a natural or artificial outfall.

From this very basic approach, a first typology of floods can be drawn up:

• the slow floods:
• flooding by rising water table,
• the floods of the plains;
• flash floods:
• the torrential floods of torrential rivers and torrents,
• flooding by storm runoff.

As for floods due to rainwater from sewerage systems, whether or not they are included in the general phenomenon known as flooding depends on the perpetrators and even on regulations.

On the operational level of flood forecasting and preparation for crisis management, the Ministry in charge of the Environment qualifies as flash floods the floods occurring on catchment areas whose response time is between 2 hours (time below which only very specific local devices allow anticipation) and half a dozen hours (time beyond which one enters the field of conventional flood forecasting).

To these floods caused directly or indirectly by rainfall, must be added the case of marine submersion resulting from the temporary or permanent rise in sea level; it is dealt with in a specific sheet.

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2 Nature, causes, and effects of floods

Flood risk is the leading natural risk in France in terms of the number of municipalities concerned (nearly half of the municipalities, to varying degrees, including 300 conurbations), the extent of flood-prone areas (5% of the metropolitan territory), in the light of the report on the Preliminary Flood Risk Assessment (EPRI) 2011 whose objective is to assess the potential risks of extreme floods, which then induces the transparency of hydraulic works), the populations residing in these areas (according to the Observatoire des territoires, 9% of the population in 2006 for events lasting approximately one hundred years ; according to the EPRI 2011, 16.8 million permanent residents in metropolitan France and at least 9 million jobs), its frequency and the extent of the damage it causes. As a result, there are many different types of damage, including victims (dead, injured) for the most serious disasters and always direct or indirect damage; moreover, it is the largest item of compensation for natural disasters (55 per cent of the total, ahead of drought 41 per cent).

Some meteorological events can have particularly heavy impacts; by way of illustration, the following report details the material damage caused by the December 2003 floods in 24 departments of the large south-eastern quarter of France; in addition to the human toll (7 deaths; 27 000 evacuations), the initial estimate of 1.092 billion euros should be supplemented by the costs of intervention, indirect impacts, etc.

The risk of flooding is directly linked to rainfall: summer thunderstorms which cause violent and localised rainfall; autumn thunderstorm disturbances, particularly on the Mediterranean coast, but whose effects can be felt throughout the southern half of the country; oceanic rainfall which causes flooding in winter and spring, especially in the north and west of France; sudden snowmelt, which sometimes has an amplifying role, particularly if prolonged and intense rainfall occurs. Catchment areas, depending on their size, can respond by floods of various types depending on the intensity, duration and distribution of this rainfall.

The risk can be amplified according to the slope of the catchment area and its plant cover, which accelerate or slow down runoff, according to the absorption and infiltration capacities of the soil (which in turn feeds the groundwater) and, above all, according to the action of man, which modifies the conditions of runoff ... or settles in particularly vulnerable areas.

Specific phenomena, often difficult to predict, can also greatly increase the level of risk locally, whether natural (e.g. glacial break-up) or man-made (e.g. break-up of dikes, etc.).

2.1 Upwelling floods

To accompany the provision, on a specific site, of maps of the tablecloth rise per commune (see § 3.1 below), the Bureau de recherches géologiques et minières (BRGM) provides a comprehensive presentation of the phenomenon and its causes, the main consequences for buildings and infrastructures, and the precautions to be taken in areas that are a priori sensitive; it also provides a typology of the corresponding floods and analyses the behaviour of the two main types of aquifers, i.e. the aquifers in sedimentary formations and the aquifers contained in the cracks and fractures in the hard basement rocks.

2.2 Lowland flooding

Lowland rivers are subject to slow flooding, which usually allows for the warning of floods and the evacuation of people at risk. Nevertheless, the safety of people is sometimes compromised, most often by failure to respect the instructions or by ignorance of the risk, in particular that induced by the speed in the so-called flow areas (it is estimated for example that for a child the limit of movement is 50 cm of water or a current speed of less than 50 cm/s).

In view of the areas affected, these floods often have very serious economic consequences, especially as the floods can last for several days or even weeks, causing considerable damage to property, major disruption to activities, health disorders and sometimes serious psychological damage.

During a flood, any watercourse may abandon its ordinary (or minor) bed, whose capacity is generally limited to return period flood flows of the order of 1 to 5 years, to occupy all or part of the major bed at the bottom of the valley, which constitutes a so-called expansion zone and contributes, through its water storage capacity, to a certain rolling of the flows for the downstream. When the highest waters retreat, they leave traces, called flood leashes: marks on the walls, waste hanging on branches, etc.

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2.3 Torrential flooding

As much as the distinction based on flood dynamics (time between rainfall and rising water levels), what differentiates torrential floods from floods in lowland rivers - usually referred to as "liquid" floods - is the coarse solid load that accompanies the flows and significantly aggravates their impact on exposed people and property.

As soon as the slope gets steeper, the flows become more and more loaded:

• first of all with a displacement of the materials (from a few mm to a few tens of centimetres) by rolling movements, sliding on the bottom or by a succession of small jumps ;
• movements already more en masse in the case of so-called hyper-concentrated flows;
• Finally, in the case of abundant loose materials and steep slopes (generally greater than 30% in the starting zone), torrential lava flows in successive gusts and conveys materials with a large grain size ranging from clays to metric-sized blocks; their high density, which can approach 2, makes them capable of transporting large blocks in near-flotation.

River types, solid transport mechanisms and flood types / L. Besson and M. Meunier 1995

While fine materials transported by suspension are generally not very damaging, the same cannot be said of sediments transported by carriage or by torrential lava flows, which can also cause bed swelling and subsequent wandering or obstruct the course of the river downstream; moreover, the flows have a significant erosive capacity, causing scouring and bank (or even slope) slides.

Finally, the transport of wood and various debris by flotation can significantly worsen the consequences of torrential floods by obstructing the river beds or structures, particularly crossing structures, and causing them to drift towards unusual areas. Dams made up of the accumulation of floating materials (ice jams) are always liable to break and cause devastating debris flows downstream, which are difficult to predict.

Torrent du Manival in the Grésivaudan valley (eastern slope of the Chartreuse massif) © S. Gominet

Torrents are located at the head of the catchment area, where the slopes are steepest (>6%): with very irregular, often intermittent flows, the reaction of a torrent to precipitation varies according to the intensity, duration and timing of the rainfall, the geological nature and sensitivity to erosion of the land crossed, and the state of the plant or forest cover in the catchment area. The transported material accumulates on the alluvial cone due to a sudden reduction in slope, and, in the absence of development, the entire cone can be swept away by the successive floods.

Immediately downstream, torrential rivers constitute an intermediate category of watercourse between torrents and rivers; with slopes lower than torrents (a few %), they can still be the seat of hyper-concentrated flows ; rapid floods and sudden overflows, intense scouring and/or solid additions can be observed in both the minor and major riverbeds - these opposite phenomena can follow one another on the same section, during the same flood, and cause the minor riverbed to wander throughout the major riverbed -, creating ice jams, particularly in the case of insufficiently dimensioned or poorly designed structures.

Note the special case of torrential rivers for their upstream section and torrents when their watersheds are partially glacial: these rivers can be subject to sudden variations in flow and solid transport depending on the hazards specific to the life of glaciers (see fact sheet RN 5: Snow, avalanches and glacial hazards).

The guide recently published by the Ministry in charge of the Environment in the series Construire en montagne (see § 3.6 below), in particular its chapters 1 and 2, further explains the phenomena linked to torrential floods and their differences with floods in lowland areas; it illustrates them with numerous examples, often of disasters.

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2.4 Rainfall runoff

Runoff is an unorganized flow of water over a watershed as a result of rainfall or a sudden melting of the snowpack. It lasts until it encounters a river, a sewer system or a marsh. Of natural or/and anthropogenic origin, it is often accompanied by erosive phenomena either generalized or more concentrated (for example in the form of mudflows).

A fact sheet in the Memento is devoted to it in view of the importance of this risk and the possibilities for action at local level.

2.5 What about climate change?

The impact of climate change must be assessed for the 2 components of floods: the liquid element component on the one hand, and the solid transport component on the other.

With regard to recent floods, most experts consider that the exceptional character too often attributed to them is in fact the result of a lack of knowledge of historical floods and an underestimation of the peak flows taken into account (taking into account, for example, hydrological series, where they exist, or meteorological series that are too short). The increase in damage is essentially the result of the considerable increase in vulnerability in recent decades, as the possible impact of climate change cannot generally be distinguished from the confidence interval or margin of uncertainty that should accompany any determination of flood flow.

Observations made following certain torrent floods whose catchment areas partly concern high altitudes, particularly in Switzerland and in the Northern Alps, suggest that new sources of materials have been mobilised to increase solid transport, for example due to the local disappearance of snowdrifts or the beginning of thawing of the permafrost (ground remaining permanently frozen, although the surface layer may thaw in summer and then refreeze during the cold period).

One of the difficulties in making predictions for the future is that the response of a river to identical rainfall events can be very different if the initial conditions differ: geological nature of the land, surface condition of the slopes, state of the vegetation cover, initial saturation state of the soil, amount of snow cover, anthropogenic factors (including in particular land use patterns) also differ.

On the other hand, it is easier to imagine that the volumes that can be mobilised, mainly in the form of torrential lava, or even as bedloads, by torrents whose catchment areas are partly at altitude, will most likely be increased by the availability of morainal-type materials, slope scree, or even materials resulting from possible slope destabilisation due to surface thawing or melting of the permafrost and, depending on the sensitivity of the soil, the retreat of glaciers.

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3 Flood prevention measures

Water governance initially focused mainly on the resolution of conflicts relating to the use of water and thus to its good and fair distribution or free flow; other interests have gradually been added such as the piscicultural value of water, the protection of populations against floods by major works, etc. The governance of water has been based on the resolution of conflicts relating to the use of water and thus to its good and fair distribution or free flow.

Water policy has thus become a global policy, bringing together numerous objectives with the aim of ensuring balanced and sustainable management of the resource (Article L.211-1 of the Environment Code); for this purpose, a structuring of governance has been set up, especially at the level of basins (with the Water Agencies, the Water Development and Management Master Plans (SDAGE), etc.) and sub-basins (with the Water Development and Management Plans (SAGE), etc.) in order to involve the many potential stakeholders in the decisions and coordinate their actions. It is within this general framework, which is increasingly defined at European level, that flood prevention policy fits.

Flood prevention is, as with most risks, event-based and has been built essentially on the feedback from successive disasters, which have been relatively numerous over the last few decades.

Today, the implementation of the 2007 European directive on the assessment and management of flood risks is underway, with a final deadline of 2015 (articles L.566-1 / 13 of the Environment Code); in addition, developed after broad consultation following the Xynthia storm on 28/02/2010 (53 dead) and the Var floods on 15/06/2010 (26 dead and missing), the rapid submersion plan (PSR) has identified 4 priority areas for action, namely : controlling urbanisation in high-risk areas; improving monitoring, forecasting and warning systems; improving the reliability of dikes; developing a genuine risk culture (notably through communal safeguard plans (PCS)).

3.1 Knowledge of the phenomena, hazards and issues related to floods

The documentation of past events, including floods of a historical nature, was relatively scattered and therefore sometimes ignored, except in special cases (e.g. in the case of floods in some basins of large rivers where the concern is long-standing or, in the case of torrential floods, in the 11 Alpine and Pyrenean departments with (inter)departmental Mountain Land Restoration (RTM) services). Therefore, in the framework of the implementation of the Flood Directive, the National Institute for Research in Science and Technology for the Environment and Agriculture (Irstea, ex Cemagref) was commissioned to draw up a methodological guide on the collection of historical data and to develop a specific database on past floods.

In addition, for metropolitan France, Météo France provides on a specific site maps (up to the departmental level) of the most intense rainfall events recorded since 1958, with the corresponding statistics, as well as a record of many previous significant events. For its part, the Central Reinsurance Fund (CCR) presents for the most significant floods a summary description of the event and a map of the municipalities impacted with the cost ranges corresponding to the compensation paid.

A prospective view of the flood risk has been established for flooding phenomena that may occur through rising water tables or river overflow.

Floodplain Atlases (FZA) are documents of knowledge that serve as a reminder of the existence and consequences of historical floods and present the hazard characteristics of the highest flood (100-year flood or historical flood); the delimitation of these flood-prone areas is based on the so-called "hydrogeomorphological" method which studies the natural functioning of watercourses by analysing the structure of the valleys; the areas thus identified are potentially floodable, in the natural state of the watercourse, with more or less significant intensities depending on the type of area described.

The atlases of flood zones, some of which are accessible on the Cartorisque site and almost all of which are accessible on the sites of the regional directorates for the environment, development and housing (DREAL) or the prefectures (departmental directorates for territories (and the sea) (DDT(M)), were intended to be enriched as knowledge evolved. In the absence of more elaborate maps, they constitute a reference tool for State services and for local authorities in the various tasks for which they are responsible (bringing to light, application of soil law, etc.).

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3.2 Flood Monitoring, Forecasting, Vigilance, and Alerting

Several bulletins of vigilance concerning floods are issued to the attention of prefectures, services, elected officials and the general public for civil protection purposes:

• On the major rivers identified by regulation as being monitored by the State, the Central Hydrometeorological and Flood Forecasting Support Service (SCHAPI) draws up theflood forecasting map, accompanied by a national information bulletin; in addition, each of the 22 flood forecasting services (SPCs) provides a more detailed map, together with an information bulletin, from which it is possible to obtain, in particular, precise information on real-time observations and references to the various measuring stations. It should be noted that the SPC does not limit itself to providing the level of vigilance per river section, i.e. the level of risk of flooding in the next 24 hours; it also provides forecasts of changes in water levels at reference stations during floods.
• The meteorological vigilance map covers the entire territory and draws attention to the possibility of the following phenomena occurring per department:
• Flooding, even in the absence of local rainfall, particularly in areas crossed by large rivers covered by the Vigicrues system,
• Rain-flooding, in the event of intense rainfall that is expected to bring a very large quantity of water over a short period of time (from one hour to one day),
• Thunderstorms, reflecting a generalised thunderstorm situation, capable of generating gusts of wind, intense rain (thus flooding on small catchment areas) and sometimes hail, waterspout or tornado phenomena (see sheet RN1: Storms, cyclones, tornadoes).

These different maps are updated at least twice a day with, when a department is classified in orange or red, a specific monitoring procedure and the distribution of a vigilance bulletin describing the phenomenon and providing, in view of its foreseeable consequences, appropriate behavioural advice.

It is from the orange level that the prefect of the zone or department implements an alert system for mayors, General Councils and operational services.

This forecast is ill-suited to the implementation of flood prevention covering floods in small catchment areas: Indeed, flood vigilance only concerns about 20,000 km of rivers, i.e. only a part of the rivers presenting risks of floods and floods (the total length of a river is about 500,000 km); in the absence of such a system, the capacity to anticipate crisis management is uncertain, the use of the meteorological vigilance map often being difficult to interpret, and even unsuitable for flash floods. This is why a certain number of local authorities have set up local risk forecasting, often with the support of external service providers (sewerage network managers, etc.): this forecast is based in particular on the use of data from complementary measurement stations (rainfall, flow) and real-time radar images.

Also, the recent ISP provides for the strengthening of monitoring, forecasting and warning tools by :

• the gradual introduction by Météo France, with the support of the flood forecasting network, of an initial warning service signalling the exceptional nature of cumulative intense rainfall observed at the sub-departmental level ("APIC chain (warning of intense rainfall at the commune level)", which, with the current radar network, enables more than 75% of the communes in mainland France to subscribe to this service free of charge in the near future);
• the renewal and extension, in particular for mountainous areas that still lacked them, of the hydro-meteorological radar and ground rain gauge networks, in order to better cover, in a sustainable way, the territories particularly concerned by runoff or flash floods (the Météo France network of rain gauges and radars is described on the Extreme Rainfall website under the heading "Our approach" (see § 3.1));
• further consolidation and extension of the network monitored by the State for flood forecasting ;
• support and cooperation with local authorities that have, or wish to have, a specific warning system, in line with the SPC networks;
• feasibility analysis by the SCHAPI-SPC network in liaison with Météo France, then the deployment, in partnership with local authorities and their potential service providers, of a warning service on the possibility of flash floods, taking into account their hydrological consequences.

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3.3 Preventive education and information

In addition to the general actions carried out by the State on an identical basis with the necessary adaptations to the nature of the risk, there are many actions carried out by organizations, especially basin organizations, national or local associations. Among them, the European Center for Flood Risk Prevention (CEPRI) has set up a network for the exchange and capitalization of know-how for local authorities and is carrying out or participating in the implementation of methodological and technical studies on the vulnerability of people and goods to the risk of (slow) floods at the level of a territory.

In addition, the presence of flood markers contributes locally to raising awareness and maintaining the collective memory: the mayor, with the support of the State services, must in fact, in the parts of his municipality exposed to the risk of flooding, make an inventory of the existing flood markers, complete, if necessary, this network to take into account the historical floods and finally establish new markers following any new exceptional flood.

3.4 Taking flood risks into account in development and urban planning

Two different approaches are required for slow flooding and for the rush and scour/alluvium hazard.

In the first case, the hydraulic characteristics can be described using two or three parameters: height, speed and, if necessary, the duration of submersion. The expert makes extensive use of modelling based on the laws of river hydraulics (which takes little or no account of the concentration of the flow during a flood).

In the second case, the solid charge can no longer be neglected given the strong interactions between the liquid phase, the solid phase and the bed geometry. The specificities to be taken into account are numerous and both the variability of the phenomena and the levels of uncertainty are much higher than previously. Therefore, the expert will have a more naturalistic and multi-criteria approach: he will try, depending on the history, the analysis of the site and its predisposition to risk as well as, if necessary, the results of various models, to assess both the levels of foreseeable intensity (flow height, size and concentration of sediments, scouring, floating, torrential lava) and the probabilities of occurrence.

The various methodological guides specialised in flood risk prevention plans (PPRI) help to understand how, depending on its nature, to map the hazard and take it into account when drawing up a PPRI; the same principles can be used to draw up a local urban plan (PLU) by directly integrating the hazard or to examine an urban planning file in the absence of any regulatory document or in the event of new data (events, expert reports).

As the PPR guide on torrential floods has not yet been published, it is recommended to refer to chapters 3, 6 and 7 of the Construction guide on this topic (see § 3.5 below).

3.5 Vulnerability reduction at the level of

If the foreseeable risk of torrential or fast-rising floods (the only cases of flooding retained by the legislator along with that of marine submersion) does not appear acceptable with regard to the safety of people, it is advisable to resort to relocation measures by amicable settlement or, failing that, by expropriation.

Reducing the vulnerability of the existing infrastructure (buildings, networks, maintenance of activity) and the proper rehabilitation of neighbourhoods damaged by flooding are very important issues at the national level.

For any project in a zone of acceptable risk, builders would do well to familiarise themselves with the various existing guides, in particular those published by the Ministry in charge of the Environment and by CEPRI; they could also usefully complete their documentation, particularly in terms of structural dimensioning, by reading the recommendations booklet made available to builders, policyholders and also to the (Swiss) authorities by the Association of Swiss Cantonal Fire Insurance Companies (VKF/AEAI) to enable them to take individual precautions, whether for existing buildings or those to be built.

3.6 Collective protection devices

The protection devices are part of more global policies, carried out at the level of homogeneous management units, such as those defined in multi-objective programmes such as large rivers and river contracts, in specialised flood programmes such as Flood Prevention Action Programmes (PAPI) or PSR or in RTM programmes.

Measures must be differentiated according to the types of floods, taking into account, for example, the nature or predictability of the phenomenon; regulations are often complex and funding varied (State, local authorities, water agencies, etc.).

Moreover, in view of changes in knowledge or regulations, experience gained, etc., the questioning of the effectiveness of a protective device can never be excluded.

The protection strategy is based on measures that can be complementary, for example combining temporary and permanent defence or civil and biological engineering:

• Temporary defence, in particular:
• restriction or prohibition of traffic, evacuation of vehicles outside the flood zone,
• sheltering of the population either on site (in their living or working places) or by evacuation, preferably to suitably equipped and serviced locations,
• installation of mobile protection devices, often makeshift (sandbags, cofferdams, panels, etc.) and sometimes specially designed (opening safety screens, concrete element or trestle systems for raising dikes, etc.).
• Permanent Defense :
• active protection by preservation and development of flood expansion areas and/or creation of retention/regulation works, by revegetation works, by torrential correction works on the slopes, in ravines and torrents, by correct stowage of various tanks and street furniture,
• passive protection near the stakes by developing beds, by dyking, by creating beaches of deposits and traps for ice jams, by creating routes with less damage, by setting up specific warning systems.

In order to remain operational, any permanent defence system must be subject to regular close monitoring and then, if necessary, to recommended maintenance. The state of the bed and banks of streams, torrents, ditches, drains, etc. must be the subject of particular vigilance.

To date, only works built to prevent floods and submersions are subject to specific regulations, depending on their potential dangerousness estimated in view of their characteristics (for example, for high dikes and protected population - see various references below); the law specifies that the manager cannot be held liable for damage that the work has not prevented, provided that it has been designed, operated and maintained in accordance with the rules of the art and in compliance with legal and regulatory obligations (Article L. 568-8-1 of the Environment Code).

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3.7 Preparedness for critical flood situations

The nature and diversity of the situations faced by public authorities, local actors and the population can be understood on the basis of feedback from past events: in the case of large-scale disasters, these are carried out at the request of the State by specialised inspection missions (see the various reports in the appendix); for more limited events, they are carried out, where appropriate, on local initiative. One of the actions carried out within the framework of the European Interreg III A Alcotra PRINAT project "Creation of the Natural Risk Centre in Mountain Areas" (2003-2007) consisted in comparing the points of view of different actors (local authorities, services, associations, etc.) who have been involved in crisis management due to torrential floods; as such, it offers various lessons and avenues for reflection.

With regard to the municipalities likely to have to manage a crisis situation, it can only be recommended to the mayors concerned to establish and maintain a PCS, whether it is compulsory or not: indeed, crisis management is prepared outside a crisis period, in particular through simulation exercises, in order to be able to respond when the time comes to all aspects of such an episode and in all its phases.

In addition, particular attention to the state of maintenance of watercourses and other water flow routes, the establishment of periodic relations with the managers of protection works concerning the municipality (e.g. dikes, RTM works), participation in the setting up, if necessary, of a local risk forecast, etc., the effective exercise of his police powers, finally, are as many assets for a mayor anxious to limit the consequences of a possible flood.

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Floods Extract from the Mayor's MENTO Last update: 27 June 2012

Jean-Pierre Requillart