Watersheds, watersheds, marshes and coastlines are described from the documents gathered in the collection. Maps and analyses bear witness to the knowledge and development projects of their time.
The water regime in Vierville-sur-Mer and the associated risks
1 - Risks of groundwater rise (water)
2 - Wetland risks
3 - Risk of flooding by runoff
4 - Risks of marine submersion
5 - Quality of bathing water
The municipality is divided into two large watersheds:
- the Véret watershed, south of the municipal territory
- the coastal watershed, then divided into sub-basins.
The watershed line partially follows the RD 30 and then takes the way to Vaumicel.
- Aquifer systems: limestone layers of the Bajo-bathonian in the department of Calvados
The hydrographic network is not very present on the municipal territory. There is only one river, located in the southwestern part of the territory near the castle of Saint-Sever, the Fumichon.
(details) the Fumichon fountain, in Vacqueville. This source of the Fumichon is never dry and heads towards the Véret
The Fumichon, depicted in blue, between Vacqueville and the Véret, crossing the domain of Saint-Sever
Hydrography in the municipality
In blue the Fumichon and Véret rivers, neighbors of the Vierville/Louvières limit
The municipality of Vierville-sur-Mer is included in a ZRE (water distribution zone) under aquifer systems.
This implies that all surface or groundwater withdrawals, with the exception of those of less than 1000m3/year deemed domestic, are subject to authorization (A) or declaration (D) under the following conditions:
For sampling installations with a maximum capacity greater than 8m3/h, an authorization (A) must be obtained.
In other cases, a simple statement (D) is sufficient
These provisions are applicable to all underground withdrawals, regardless of their depth.
- reduce diffuse pollution of aquatic environments,
- reduce the pollution of aquatic environments by hazardous substances,
- reduce micro-biological pollution of environments,
- protect water catchments for current and future drinking water supply,
- protect and restore aquatic and wetlands,
- manage the scarcity of water resources,
The main rivers in the Seine-Normandie basin: Vire, Orne, Dives, Touques, Seine and its tributaries
1 - Flood risks by rasce of groundwater
A significant part of the municipal territory is affected by these ascents, a phenomenon that was the subject of a mapping on the occasion of the event of spring 2001, during which the water table largely rose.
Although the water table was close to the surface of the soil, no significant water table overflow was observed. The fact remains that this fact must be taken into account in terms of urban planning.
Five areas have been selected to represent the flood hazard as it is currently known. Are represented:
- in blue, the areas where the water table overflowed in 2001. Some mapped land remained flooded for several months below water heights close to the meter. These areas are not intended to be urbanized; embankments can be unstable in the same way as the buildings they support;
- in pink, the land where the water table flushes the ground during periods of very high water and sometimes in normal times. Groundwater is able to permanently flood all buried infrastructure and basements, making it difficult to control public health and safety (sewage networks in charge, water discharge on roads...). Damage to roads, networks and buildings can be considerable and damage management complex and expensive.
- in yellow, land likely to be sustainably saturated with water but at a greater depth than before (from 1 to 2.5 m). Building infrastructure can suffer significant and costly damage; basements are threatened with flooding.
- in green, land where the unsaturated area is less than 2.5 m deep. The hazard only concerns the deepest infrastructures (buildings, underground parking lots...) despite the cartographic inaccuracy specified above, the risk of flooding cannot be ruled out for deep basements.
- in colorless, sectors where the water table was probably quite far from the surface during the spring 2001 water table flood, i.e. more than 5 meters deep.
The risks of flooding by rising water table are located south of the RD514 and only concern the subsoil, more or less deeply. Plots whose basements are floodable are generally protected from any urbanization project, including zone 1AU. Part of the U zones being slightly affected by the risk of flooding of 0-1m in the basement (south and east of the village as well as the seafront), the PLU regulations prohibit any construction in the basement.
Groundwater in case of very high water
(2008 study)
Risks of water table rise (study around 1995)
Examples of water tables in 1995
Opposite, in January 1995, heavy rains caused water tables in Vierville, especially behind the castle, in the woods and in the so-called "Fossé Gras" terrain. Cellars were flooded.
To remedy this, in 1997, the municipality took over and improved the flow of water to the Vierville valley, along the Ash Hunt and through the Bricsart grass. Since then, groundwater rises no longer seem to occur, at least as long as rainfall is not exceptional.
January 1995, the field of Fossé Gras flooded. Below, in the wood of the castle, which runs along the Fossé Gras
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2 - Risks related to the presence of spaces predisposed to wetlands
Apart from flood zones, the DTTM has identified the areas predisposed to the presence of wetlands.
These wet corridors draw the spaces where the soils are supposed to be hydromorphic due to the presence of a layer of water very close to the surface. Resulting from a calculation, this map does not describe a terrain reality but a high probability of the presence of wet spaces.
Presence of wetlands
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3 - Flood risks par runoff
Only the Véret basin, which extends a little over the south of the territory of Vierville, could cause overflow floods.
During previous floods of the Véret and its tributary the Fumichon (which originates in Vacqueville), the flooded sectors were the fields and wooded areas along the Fumichon.
In addition, the municipal territory collects part of the rainwater from the territories set back from the coast.
Localized floods were found due to insufficient sewage regulation works upstream of RD 514.
Stainwater management
All urbanized sectors are equipped with a collection network: bused on all the roads of the village or in a ditch at the level of the agricultural area. The rainwater collection network will be extended in the village in the development sector of
Urbanization. In addition, rainwater management at the plot is imposed in the PLU regulation to allow their free flow.
Runoff problems are observed during heavy rainy episodes, especially in the descent from the village to the beach (rue de la Mer).
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4 - Risks of marine submersion - Areas below sea level in Vierville sur mer (2011 study)
Land located immediately south of the coastal road (RD517) is likely to be largely flooded in exceptional circumstances
- strong depression resulting in sea level rise
- high tide coefficient
- winds blow from the sea towards the land
- seabed configuration
- funnel effects of the ribs
We are talking here about centenal probability events (return period event of at least 100 years). and the level of surcharge can then reach more than one meter. See the table of surcharges according to the probable return time of the event in Le Havre and Cherbourg.
In addition, in these exceptional cases, the crossing of the dike by storm waves is foreseeable in Vierville, even in the absence of damage to the dike.
The houses behind the dike are generally located with a strong setback (30 meters imposed by the PLU regulation) and often in height on natural flats of the cliff several meters above the dike, which limits the risks. Nevertheless, some houses have sometimes been built for a long time very close to the dike and at its level.
Since the storms of 2008 and February 2010 (Xynthia), the administration in charge of coastal defenses (the DTTM today, Departmental Directorate of Territories and the Sea) has reviewed the risks of flooding by marine submersion
- On the one hand, the main areas below the level of the centenary tide (for example east of Arromanches) have been mapped: ZNM maps (Zones located below Sea Level). But Vierville does not seem to have been selected for a detailed study of the areas concerned on areas that are in fact quite limited, immediately behind the coastal road. This remains to be done.
This area is low, can sometimes be 3 meters below road level, the lowest parts being about 1 meter below the high sea level of open water. This low area is subject to groundwater rises during periods of heavy rain. It is sometimes filled.
- On the other hand, protective works and natural dune cordons have been identified in order to prohibit construction in an area 100 meters behind. See on the "Urban Planning" page the plans of this area for Vierville.
The protection zone, with the map at 1/25000 as a plan background.
The protection area, with photographic plan background
From the definition of low areas, the mapping of areas potentially submerged by one meter of water can be carried out, i.e. located more than one meter below the reference coast as well as potentially submerged areas in the perspective of sea level rise by the end of the century. Thus, three classes appear on the map:
Low Zones - 1m = territories located below the side of the reference tide -1m (figured in marine)
Low Zones = territories below the reference tide (figured in blue)
Low Zones + 1m = territories located below the reference tide +1m (figured in green)
The coast of Vierville has a linear of about 2 km of low coasts punctuated by areas whose topographic level is below that of the current high seas, which makes it particularly vulnerable to marine submersions, i.e. coastal risks. These areas are protected from direct impacts of the sea by a sea defense dike.
Risk management is primarily based on the qualification of the hazard, i.e. the intensity of a foreseeable and potentially dangerous natural phenomenon. For the risks of marine submersion, it is therefore a matter of identifying territories that have been submerged in the past or that could be submerged and of assessing the effects of these submersions on people and property.
The Z.N.M. atlas is the first step in the knowledge of the marine submersion ranity. It is still embryonic in Vierville sur mer, but it should be completed according to technological advances, in particular thanks to finer topographic data, hydraulic rupture models, a finer knowledge of the height and reliability of the dike.
- The "low zones -1m": these areas are located below the level of the centenary tide -1m. They are therefore potentially submersible by more than one meter of water (sooft or brach) in the event of a centennial event. These territories are therefore subject to a high risk and must be the subject of particular attention both in terms of planning, application of soil law and crisis management.
- The "low zones of side 0": these areas are located below the level of the centenary tide. They will therefore be potentially submersible or subject to hydraulic stresses in the event of a marine incursion - for coastal areas - but also flood episodes or flushing layers. In these areas, measures must be taken in terms of planning, application of soil law and crisis management.
- "Low areas +1m": these areas are located below the level of the centenary tide +1m. They could be subjected to submersion of marine or continental water in the short term during events of more than a hundred-year frequency and in the longer term - forecast at 100 years - to more frequent submersions due to rising sea levels (especially in the case of very low atmospheric pressures, or possible general rise in sea levels). In these areas, measures must be taken in terms of planning and application of soil law.
The "100m area" behind the dike could be subjected to violent and rapid submersion in the event of a break in the dike. In these areas, human lives are then in danger. They must be the subject of particular attention both in terms of control and maintenance of works, planning, application of soil law and crisis management.
Even in the absence of damage to the dike, it would be submerged by storm waves with exceptional sea level surcharges. In this case, the low areas behind the dike can be flooded. It has already happened and it can be repeated with much more magnitude in the case of events of centenary probability.
The Z.N.M. atlas is based on the data currently available, which have limitations of accuracy and interpretation, leading to uncertainties. Thus, the representation of these maps does not make it possible to understand the risk on the scale of a cadastral parcel. The reading scale of these cards is 1/25,000 (or 1 cm = 250 m).
The Z.N.M. atlas can be improved and is intended to evolve according to the improvement of scientific knowledge (including topographical surveys...) and historical (highest known waters).
In the sectors that will benefit from a Coastal Risk Prevention Plan (Coastal PPR), modeled hazards will replace the ZNM atlas. The old file did not provide for a PPR for the municipality of Vierville-sur-Mer at the time of its writing.
details Hydrographic map of the SHOM -extract-
The flood risk map
Card not confirmed in 2011, old, around 1999?
Risks of marine submersion: map around 2003, not confirmed by the 2011 ZNM map
Ancient events on the coast of Vierville
The storm of October 28, 1909, famous for the damage observed in Grandcamp, was also felt in Vierville where the Mathy family kept some photos:
1909, the boulevard de Cauvigny invaded by pebbles after the strong storm of Oct 1909, in front of the Mathy
The gardens of the flooded Parmentier and Mathy, Oct 1909
17 to January 31, 1995 (natural disaster declared by order of February 6, 1995) floods and mudslides
In January 1995, a very rainy period flooded the low grounds behind the road.
Above, Terrains Voisin, Lebrec, Mathy
These brutal ascents do not last, because the water ends up percoiling more or less quickly under the road towards the beach at low tide.
January 1995, rise of nappe in the Cordelle grounds, the flooded area extended over 40 meters and remained several days. This phenomenon has never been repeated since and until today (September 2011), the flow of water is very regular and quickly, probably by infiltration under the RD517 road which is built on an old natural dune cordon, quite permeable by nature.
(Natural disaster declared by order of July 17, 1996) floods and wave action
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25 to 29 December 1999
centenary storm
(Natural disaster declared by order of December 29, 1999, bad weather not specified)
The survey in Vierville of atmospheric pressure during the passage of the storm (probably centenary) of December 26, 1999
5 - Quality of bathing water
In the department of Calvados, with regard to classifications, for the second consecutive year, 100% of bathing waters comply with the 1976 European Directive: 13 in category A and 25 in category B. In addition, only 4 analyses were found to be non-compliant out of the 748 carried out.
The balance sheet is therefore satisfactory and reinforces the trend towards long-term improvement in bathing waters. The significant efforts made by communities in sanitation are the main explanation.
Quality of bathing water - 2008
Thus, the bathing waters are of good quality in the municipality of Vierville-sur-Mer. It should be noted that a study of watersheds is underway at the level of the community of municipalities, in particular for the management of bathing water quality
Quality of bathing waters on the Calvados coast
Recent studies by the ARS de Basse Normandie - Territorial Delegation of Calvados show that the
Quality of this bathing has seen a notable improvement, with the implementation of sanitation
Collective: alternating first sufficient quality and insufficient quality, this swimming then experienced a
Improvement from 2003 to achieve good quality in 2006, then the "EXCELLENT QUALITY" in
2007. The very good 2010 season reinforces the excellent quality.
The 2010 results are very good and give an A ranking for the 4th consecutive season.
This study also made it possible to simulate the application of the new 2006 directive. A
New European directive of February 15, 2006 was transcribed into French law in the Health Code
Public and ministerial orders of September 22 and 23, 2008. Implementation began with the
Census of bathing waters during the summer of 2007 and will continue gradually until 2015.
This new ranking will not be made until the end of the 2013 season. By applying the new criteria,
More severe, the results are often less favorable. The objective is above all to observe the evolution,
Which stands out better with this ranking, and also the road that remains to be done to achieve a quality at the
Less sufficient.
Thus, the bathing waters are of good quality in the municipality of Vierville-sur-Mer.
It should be noted that a study of watersheds is underway at the level of the community of municipalities,
In particular for bathing water quality management
Annex
Instructions for the use of municipal maps Areas below Sea Level Updated on: 10/02/2011
Method of development The reference defined by the Ministry of Ecology, Sustainable Development, Transport and Housing (M.E.D.D.T.L.) for marine submersions is an event of a return period of at least 100 years, that is to say, one in a hundred chance of occurring each year (reference range). The prerequisite for mapping marine submersion hazards is therefore the mapping of areas below the centenary recurrence tide defined by the Marine Hydrographic and Oceanographic Service (S.H.O.M.). This centenary rating does not take into account - in the current state - certain hydraulic phenomena, related to swell in particular. Anthropogenic structures, as well as natural dune cords, often have a protection function against marine submersion. In the event of ruin of these works, rapid and sometimes fatal submersion is possible at the right of the breach or the low point of the work. A safety zone of 100m is thus defined by the Ministry of Ecology as having to be taken into account behind each protection work against submersion and each dune cordon in order to take into account the rapid kinetics of submersion that could be caused by failure of the protection.
Thus, based on SHOM data, the DREAL of Lower Normandy has selected 24 ports in the region for each of which the centenary recurrence tide rating called reference rating was calculated. A division of the Lower Normand coast into 22 large submersion basins was then carried out. A port of reference was then associated with each land sector thus delimited, the coast of the port being retained as the reference hall of the basin. This rating is then projected on Digital Terrain Models (M.N.T.) - three-dimensional modeling of the Lower Normandy territory - to obtain the areas located below the centenary sea level or Low Zones.
Based on this definition of Low Zones, the national methodology provides for the mapping of areas potentially submerged by one meter of water, i.e. located more than one meter below the reference level as well as potentially submerged areas with a view to sea level rise by the end of the century. Thus, three classes appear on the map:
Low Zones - 1m = territories located below the side of the reference tide -1m (figured in marine)
Low Zones = territories below the reference tide (figured in blue)
Low Zones + 1m = territories located below the reference tide +1m (figured in green)
This general knowledge of the submersion hazard is then represented on a plan background - georeferenced digital images of the territory - SCAN 25® of the National Geographic Institute (IGN).
To go a little further
The Lower Normandy coast has a large linear of low coasts punctuated by maritime marshes whose topographic level is below that of the current high seas, which makes it particularly vulnerable to the phenomena of marine submersion and mobility of the coastline, i.e. coastal risks. Most of these marshes are protected from the direct impacts of the sea by natural dune cords or sea defense works. Human occupation, subsequent or prior to these structures, together with the safety of these protective elements, is the main issue in the management of coastal risks.
Risk management is based primarily on the qualification of the hazard, i.e. the intensity of a ph. The Z.N.M. atlas is the first step in the knowledge of the marine submersion hazard, it will be completed according to technological advances, in particular thanks to finer topographical data, rupture hydraulic models, a finer knowledge of the height and reliability of the works.
Although partial, this knowledge of coastal risks must be the subject of specific recommendations:
The "low zones -1m": these areas are located below the level of the centenary tide -1m. They are therefore potentially submersible by more than one meter of water (sooft or brach) in the event of a centennial event. The mapped territories are therefore subject to a high risk and must be the subject of particular attention both in terms of planning, application of land law and crisis management for existing issues. In coastal areas, human lives are in danger.
The "low zones": these areas are located below the level of the centenary tide. They will therefore be potentially submersible or subject to hydraulic stresses in the event of a marine incursion - for coastal areas - but also flood episodes or flushing layers. In these areas, measures must be taken in terms of planning, application of soil law and crisis management for existing issues. Predictable and potentially dangerous natural phenomenon For the risks of marine submersion, it is therefore a matter of identifying territories that have been submerged in the past or that could be submerged and evaluating the effects of these submersions on the issues (people, property...)
"Low areas +1m": these areas are located below the level of the centenary tide +1m. They could be subjected to short-term marine or continental water flooding during events of more than a hundred-year frequency and in the longer term - forecast at 100 years - to more frequent submersions due to rising sea levels. In these areas, measures must be taken in terms of planning and application of soil law
The "100m area behind a structure or a dune cordon": these areas are located in the immediate vicinity of a protective structure or a dune cordon. They could be subjected to violent and rapid submersion in the event of a failure of the protection system. In these areas, human lives are at risk. They will have to be the subject of particular attention both in terms of control and maintenance of works, planning, application of soil law and crisis management for existing issues.
Reference(s) used CAN 25;
Usage limit(s)
The three Lower Normandy departments are covered by M.N.T., owned by the General Council of Calvados for the department of Calvados, owned by DREAL for the departments of Manche and Orne. While the BD Topo® version "Country" of the IGN has a step of 25 m and an accuracy of +/- 2.5 m, the Low-Normand MNTs have a pitch of 20 m and inframetric precision.
The calculation of Zones below Sea Level (ZNM) is represented like the other risk atlases on the IGN SCAN 25®. The use of this plan background at 1/25,000 (1cm on the map represents 250m on the field) again limits the interpretation of the maps. The planimetric and altimetric precisions of the IGN support are good but do not allow a reading to the plot. Indeed, if an isolated object is precisely positioned, the building is often shifted so that priority objects (infrastructure for example) are better represented.
The precision of the mapping and positioning of structures and dune cords playing a protection role against submersion, although reliable, also varies according to the departments and the knowledge of these structures.
The Z.N.M. atlas is based on the most accurate data available at the time of its development (NTM, SCAN 25®, mapping of coastal protections). Nevertheless, these data have limitations of accuracy and interpretation. Thus, the representation of these mappings does not make it possible to understand the risk at the level of the parcel. The reading scale of these maps is therefore 1/25,000 (i.e. 1 cm = 250 m).
Legal constraints
The data relating to the M.N.T, the BD TOPO® (RGE), the SCAN 25® of the IGN, the work data of the Departmental Directorates of Territories and the Sea of Calvados (D.D.T.M. 14) and the Manche (D.D.T.M. 50), as well as the Statistics of the extreme sea levels of the Atlantic Channel Atlantic Channel of the S.H.O. are the subject of intellectual property or even moral rights. What to do if the mapping seems inconsistent with your field knowledge
As previously indicated, the mapping of Z.N.M. depends on relatively precise data but may occasionally present uncertainty. As for the various atlases produced by the D.R.E.A.L., the Z.N.M. atlas can be improved and is intended to evolve according to the improvement of scientific knowledge (more precise M.N.T., topographical surveys...) and historical (Higher Known Waters). In the sectors that will benefit from a Coastal Risk Prevention Plan (Coastal PPR), modeled hazards will replace the ZNM atlas.
This atlas therefore provides non-exhaustive but constantly improving information.
Contact:
Mathieu Morel - Major Natural Hazard Prevention Officer DREAL Basse-Normandie
Technological and Natural Risks Service
Natural Hazards and Basement Division 10 bld du GalVannier 14006 Caen