平成 31 年 4 月 10 日（水）
国総研 水害研究室 板垣
生江
仏国「PLAN DE PREVENTION DES RISQUES NATURELS D'INONDATIONCOMMUNE DE BOUJAN-SUR-LIBRON
Rapport de présentation」（ PLAN FOR PREVENTING NATURAL RISKS OF FLOODING
COMMUNE OF BOUJAN-SUR-LIBRON
Introduction Report）仮英訳
原文：http://www.boujansurlibron.com/wpcontent/uploads/2018/05/PPRI_rapport_presentation.pdf（参照：2019. 03. 29）＊この資料は Google 翻訳によって、仏語から英語に機械英訳がなされています。一部の単語については辞書で調べて補足をいたしました。
表紙

Freedom, Equality, Fraternity
France Republic
Prefect of Herault

Departmental Direction of
Territories and Sea Service Water,
Risks and Nature

PLAN FOR PREVENTING NATURAL RISKS OF FLOODING
COMMUNE OF BOUJAN-SUR-LIBRON
Introduction report
Procedure

Prescription

Public Inquiry

Approval

Preparation

06/12/2011from 07/03/2016 to

31/05/2016

07/04/2016
13頁
TABLE OF CONTENTSPART ONE: GENERAL PRINCIPLES OF PPR AND RISK OF FLOOD ..................... 10
1. INTRODUCTION .......................................................................................................... 10
1.1 GENERAL FINDINGS ................................................................................................ 10
1.2 WHY A NATIONAL POLICY FOR NATURAL HAZARD PREVENTION? .......... 10
1.3 THE GLOBAL STATE PREVENTION APPROACH TO NATURAL RISKS ............ 11
1.4 CHRONOLOGY OF LEGISLATION CONCERNING RISK PREVENTION ............ 11
1.5 PURPOSE OF THE PRESENTATION REPORT ........................................................ 14
2 APPROACH TO THE PREPARATION OF A NATURAL FLOOD RISK PREVENTION
PLAN .....................................................................................................................................15
2.1 WHAT IS A NATURAL HAZARD PREVENTION PLAN? ........................................ 15
2.1.1 What does the Flood Natural Disaster Prevention Plan (PPRI) contain? .................. 16
2.1.2 What are the phases of developing a PPR? ................................................................. 18
2.2 IMPACT OF PPR .......................................................................................................... 19
2.2.1 Scope of the PPR ......................................................................................................... 19
2.2.2 Penalties for non-compliance with the provisions of this PPR ................................... 19
2.2.3 Effects of PPR .............................................................................................................. 20
3 METHODOLOGY AND DEFINITIONS ........................................................................ 22
3.1 APPROACH TO EXTENSION OF THE MAIN TERMS USED IN RISKS ................ 22
3.2 GENERAL PRESENTATION OF RISK FLOOD ......................................................... 24
3.2.1 The presence of water: the hazard ............................................................................... 24
3.2.2 The presence of the man: the stakes ........................................................................... 25
3.3 PROCESS FOR FLOODING AND FLOODING ......................................................... 25
3.3.1 Definition and types of floods ..................................................................................... 25
3.3.2 The formation of floods and floods ............................................................................. 26
3.4 FACTORS AGGRAVING RISKS .................................................................................. 27
3.5 THE CONSEQUENCES OF THE FLOOD ................................................................. 28
3.6 REFERENCE EVENTS IN THE PLAN TO PREVENT NATURAL RISKS OF
FLOODING ..........................................................................................................................28
3.6.1 Descriptive parameters of the hazard .......................................................................... 29
3.6.2 The qualification of the hazard .................................................................................... 29
3.7 DEFINING THE ISSUES .............................................................................................. 30
3.7.1 REGULATORY ZONING ......................................................................................... 31
3.7.2 Areas at risk ................................................................................................................. 31
3.7.3 Areas not directly at risk .............................................................................................. 31
2

4 THE MEASURES PRESCRIBED BY THE PPR .............................................................. 34
4.1 PREVENTION, PROTECTION AND SAFEGUARD MEASURES ............................ 34
4.1.1 Control of rain runoff .................................................................................................. 34
4.1.2 Protection of densely urbanized places ....................................................................... 35(4 頁)
4.1.3 Preventive information ................................................................................................ 35
4.1.4 Safeguard measures ..................................................................................................... 35
4.2 MITIGATION MEASURES ...........................................................................................36
4.2.1 Definition .................................................................................................................... 36
4.2.2 Objectives .................................................................................................................... 36
4.2.3 Measures applicable to existing properties .................................................................. 37
4.3 REFERENCES AND RESOURCES .............................................................................. 37
SECOND PART: THE NATURAL RISK PREVENTION PLAN FLOODING OF THE
COMMUNITY OF BOUJAN-SUR LIBRON ....................................................................38
1. THE LIBRON .................................................................................................................. 40
1.1. GENERAL PRESENTATION OF THE RIVER BASIN .............................................. 40
1.1.1. Geographical features ................................................................................................. 40
1.1.2. Weather conditions...................................................................................................... 41
1.1.2.1. Type of climate ........................................................................................................ 41
1.1.2.2. Rainfall ................................................. ................................................................... 41
1.1.3. Geological and geomorphological description of the basin ........................................ 41
1.2. HYDROGEOMORPHOLOGICAL ANALYSIS .......................................................... 45
1.2.1. Section 1: From the source to the RD13 (1 km) ……………………………………… 47
1.2.2. Section 2: From the RD13 departmental road to Laurens (3.4 km) …………………47
1.2.3. Section 3: The crossing of Laurens (800 m) ............................................................. 48
1.2.4. Section 4: From Laurens to the downstream side of Castle of Grezan (2.3 km) ... 49
1.2.5. Section 5: From Castle of Grezan to Lieuran-les Beziers (13.2 km) ..................... 50
1.2.6. Section 6: From Lieuran-les Beziers to Boujan-sur-Libron (7.7 km) ........................52
1.2.7. Section 7: From Boujan-sur-Libron to Vias (13.9 km) .............................................. 54
1.2.8. Section 8: From Vias to the Mediterranean (1.8 km) ................................................ 55
1.2.9. Main tributary LIBRON Boujan-sur-Libron: the Creek Ardaillou .............................56
1.3. HISTORICAL ANALYSIS ON BOUJAN-SUR LIBRON .......................................... 56
1.4. HYDROLOGICAL ANALYSIS .................................................................................... 61
1.4.1. Synthesis of documents or previous studies ............................................................... 61
1.4.1.1. Hydraulic Study of the Libron Valley - Sogreah - 1968 .......................................... 61
1.4.1.2. Hydraulic development scheme of the Libron de Laurens valley at RN9 - BRL and3

BCEOM - 1988 .....................................................................................................................61
1.4.1.3. Statistical treatment of data from the la Courtade, Béziers rainfall station –BCEOM- 1997...................................................................................................................... 62
1.4.1.4. Hydraulic study of the A75 motorway - Pezenas / Beziers - Hydratec section 2004 ...................................................................................................................................... 62
1.4.1.5. Flooding areas of the Libron in the Boujan-sur Libron crossing - Hydraulic study Aqua Conseil / ENTECH - April 2010 ................................................................................. 63
1.4.1.6. Summaries of the results of previous studies ........................................................... 63
1.4.2. Existing rainfall and hydrometric data ........................................................................ 64
1.4.3. Hydrology of watersheds greater than 10 km2 ........................................................... 66
1.4.3.1. Methodology retained .............................................................................................. 66(5 頁)
1.4.3.2. Validation of results ................................................................................................ .66
1.4.4. Centennial flow of the Libron taken into account in the ppri .....................................67
1.5. HYDRAULIC MODELING UNDER THE PPRI ........................................................ 68
1.5.1. Model used ................................................................................................................. 68
1.5.2. Calibration of models ................................................................................................. 68
1.5.3. Modeling of flows ....................................................................................................... 68
2. CARTOGRAPHIC RESULTS ..........................................................................................69
2.1. MODELED SECTORS ................................................................................................. 69
2.2. NON-MODELED SECTORS .......................................................................................69
3. REGULATION ................................................................................................................ 70
3.1. CONSTRUCTION OF THE REGULATORY CARD ................................................. 70
3.1.1. Hazards........................................................................................................................ 70
3.1.2. Challenges................................................................................................................... 70
3.1.3. Regulatory zoning ....................................................................................................... 72
3.1.3.1. Crossing grid of hazards and issues ......................................................................... 72
3.1.3.2. Scope........................................................................................................................ 72
4. BIBLIOGRAPHY ............................................................................................................. 74
5. USEFUL LINKS ............................................................................................................... 74
6頁
LIST OF ACRONYMS AND ABBREVIATIONSAZI: Flood Zone Atlas
CAR: Regional Administrative Committee
CATNAT: Natural disasters regime4CD: Departmental Council
CNPF: National Center for Forest Property
CR: Regional Council
DDRM: Departmental Dossier on Major Risks
DDTM: Departmental Direction of the Territories and the Sea
DI: Flood Directive
DICRIM: Communal Information Document on Major Risks
DREAL: Regional Directorate for the Environment, Planning and Housing
DUP: Declaration of Public Utility
EAIP: Envelope Approached to Potential Floods
EPCI: Public Institution of Intercommunal Cooperation
EPRI: Preliminary Flood Risk Assessment
ERP: Institution Receiving Public
FPRNM: Major Natural Risk Prevention Fund
HLL: Light Houses of Leisure
IAL: Information Buyers Tenants
NGF: General Leveling of France
PCS: Municipal Protection Plan
PHE: Higher Waters
PLU: Local Urbanism Plan
PLUI: Local Local Urbanism Plan
POS: Land Use Plan
PPR: Risk Prevention Plan
PPRI: Flood Risk Prevention Plan
RSD: Departmental Health Regulations
SAGE: Diagram of Development and Management of Waters
SDAGE: Master Plan of Development and Management of Waters
SLGRI: Local Flood Risk Management Strategy
SNGRI: National Flood Risk Management Strategy
SPC: Flood Forecasting Service
TN: Natural Land
TRI: Important Flood Risk Territory
７頁
Glossary
Alea: probability of occurrence of a given natural phenomenon, intensity and occurrence in5

a given territory. The hazard is low, moderate, strong or very strong, depending on thewater depth, the flow velocity and the submersion time compared to the reference
phenomenon.
Landing: alluvium (sediment such as sand, silt, clay, silt, gravel) transported by runningwater, and settling in the stream bed or accumulating at break points.
Watershed: territory drained by a watercourse and its tributaries.
Cofferdam: removable flood barrier.
Flood expansion field: non-urbanized or poorly urbanized sector allowing the temporarystorage of flood waters.
Change of destination: transformation of a surface to change its use.
Change of destination and reduction of vulnerability: in the regulation, it is sometimes
indicated that work is allowed provided that it does not increase vulnerability. Will beconsidered a change of destination increasing vulnerability, a transformation that increases
the risk, such as the transformation of a rebate into housing.
Article R 123-9 of the urban planning code distinguishes nine classes of buildings groupedin this document into three classes according to their vulnerability:a / housing, hotel accommodation, buildings and installations required for public
services or of collective interest including sleeping quarters at night, b / office,commerce, crafts, industry, constructions and installations necessary for public services
or of collective interest not including overnight accommodation, c / farm buildings orforest, buildings warehouse function (by extension garage, shed, shed, annex),
constructions and installations necessary for public services or of collective interest
strictly assigned to agricultural, forestry or warehouse uses.
The following hierarchy, in descending order of vulnerability, can be proposed: a> b> c
For example, the transformation of a commercial re-launch from an office to a home is inline with increasing vulnerability, while the transformation of a home into a home reduces
this vulnerability. The distinction of building types is based on the vulnerability to flood riskof the people occupying them, and is part of the management of the crisis with a view topotential evacuation.
To note :6in terms of vulnerability, hotel-type accommodation is comparable to housing,while a restaurant is a commercial activity.

the transformation of housing into multiple dwellings increases vulnerability.

NGF rating: altimetric level of a land or submersion level, attached to the General Levelingof France (IGN 69).
PHE rating (highest water level): NGF rating reached by the reference flood.
Flood: a rapid and temporary increase in the flow of a watercourse that results in an increasein the water level and its flow velocity.
８頁
Reference flood: it serves as the basis for the development of the PPRI and corresponds tothe calculated centennial flood or the highest known historical event, whichever is greater.
Centennial Crude: A statistical flood that has a 1 in 100 chance of occurring each year.
Crude except it: raw determined by hydrogeomorphological method, likely to occupy theentire major bed of the watercourse.
Historical flood: highest known flood.
Flow rate: volume of water passing at a given point in one second (expressed in m3 / s).
Footprint: trace on the ground or vertical projection of the volume of the construction,
including overhangs and overhangs.
Issues: people, goods, activities, means, heritage likely to be affected by a natural
phenomenon.
Equipment of general interest: infrastructure or superstructure intended for a public service(drinking water supply including boreholes, sewerage, sewage treatment, networks, publicpassenger transport equipment, protective dike close to densely urbanized places, ...). Are
not considered as equipment of general interest the equipments receiving from the public,
even carried by a collectivity and / or destined to a public use (swimming pool, gymnasium,7school building, ...) nor the operations of urbanization when well even they would have been
the subject of a declaration of public utility.
Extension: increase of the footprint and / or floor area.
Water Height: The difference between the PHE rating and the TN rating.
Hydrogeomorphology: study of the hydraulic functioning of a watercourse by analysis andinterpretation of the valley structure (photo-interpretation then field observations).
Flooding: temporary submersion by water of land that is not submerged in normal times.
This concept covers floods caused by river floods, mountain streams and intermittent
Mediterranean rivers, as well as floods from the sea in coastal areas.
Mitigation: action to mitigate the vulnerability of existing assets.
Modification of construction: transformation of all or part of the existing surface, without
increase in footprint or floor area. This assumes not to touch either the building volume or
the floor area, otherwise the project will be expanded.
Open: any surface through which water can enter a building (door, window, bay windows,etc.).
Habitable floor: all living spaces or arranged to accommodate commercial, craft or industrial
activities. It excludes warehouses, garages, forestry or agricultural exploitations.
9頁
Risk Prevention Plan: a document worthy of public utility, it is annexed to the Local Town
Planning Plan in order to guide the urban development of the municipality outside the floodzones. It aims to reduce damage during disasters (natural or technological) by limiting
urbanization in risk areas and reducing the vulnerability of already urbanized areas. It is thestate's essential tool for risk prevention.
For example, there are:
-The Flood Risk Prevention Plan (PPRI)-The Forest Fire Risk Prevention Plan (PPRIF)-The Ground Movement Risk Prevention Plan (PPRMT): landslides, falls of blocks and8landslides, shrinkage-swellings of clay, subsidence or collapse of cavities, mudslides.
Prescriptions: local rules to be applied to a construction to limit risk and / or vulnerability.
Prevention: all the measures to be implemented to prevent, if not reduce, the impact of aforeseeable natural phenomenon on people and property.
Project: any new construction, including extensions, but also existing intervention projects
such as changes or changes of destination.
Property: all contiguous parcels belonging to the same owner.
Floor area: floor area closed and covered under a ceiling height exceeding 1.80 m.
TN (natural land): natural land before works.
Vulnerability: potential consequences of the impact of a hazard on issues (populations,buildings, infrastructure, etc.). An essential concept in crisis management determining the
likely reactions of populations, their ability to cope with the crisis, evacuation needs, etc.
Zone refuge: habitable covered floor level accessible directly from the interior of thebuilding above the reference elevation and provided with access to the roof for evacuation.

9

10 頁
PART ONE: GENERAL PRINCIPLES OF PPR AND RISK OF FLOODING
1. Introduction
1.1 GENERAL FINDINGSWith 17 million people potentially exposed to flood risk, 9 million jobs exposed to
overflowing watercourses and more than 18,000 vulnerable communes, France is exposed to
the natural risks of flooding. The storm Xynthia of 2010, the floods of the Var of spring 2010
and autumn 2012 and more recently the succession of bad weather and unusual floods from
September to November 2014 and August to September 2015 in the Hérault havedramatically recalled.
In the Languedoc Roussillon region, about three-quarters of municipalities are subject to therisk of flooding, and 25% of the population are potentially impacted. The proven risks
represent an average financial cost of € 500 million, paid each year by the insurances toindemnify the damages. Thus, 97% of the municipalities of Languedoc-Roussillon havebeen declared at least once in a state of natural disaster since 1982 for flooding by overflow
of rivers, runoff or mudslide.
1.2 WHY A NATIONAL POLICY FOR NATURAL RISK PREVENTION?
For many decades, the coastal plains have been the site of massive population concentration.
Indeed, the presence of rivers and the sea has long conditioned the development of multiple
activities, from the supply of drinking water, to industrial processes, through crafts ornavigation.
During the nineteenth and twentieth centuries, industrial development led to a proliferationof installations in these sectors. This development also reached its climax during the Thirty
Glorious (1945-1975) with the completion of large industrial settlements and the expansionof agglomerations, both strongly attracted by land easily developable.
Large river and maritime developments have, on the other hand, developed the illusion of
total control of flood risk. This has been further strengthened by a period of hydrologicalrest for nearly three decades. As a result, industrial and commercial zones as well as
suburban housing estates have largely invaded floodplains and coastlines without any
particular precaution due to numerous economic, social, land and/or political pressures.
However, in the early 1990s in France and in the 2000s on the south-east quarter, a series ofcatastrophic floods reminded people and public authorities of the existence of a long10

forgotten risk (Nimes in 1988, Vaison-la-Romaine in 1992, flood of 1999 on Aude, Gard in
2002, Rhône in 2003, etc.).
Watercourses have too often been developed, dyked, covered or deviated, thus increasing
the vulnerability of populations, goods and activities in these submersible areas.
11 頁
1.3 THE GLOBAL STATE PREVENTION APPROACH TO NATURAL RISKS
Since 1935 and the submersible surface plans, the State policy has moved towards areinforcement of the prevention of natural risks: the law of July 13, 1982, reinforced by thatof July 22, 1987 relating to the organization of the security has put preventive information at
the heart of the prevention policy and has introduced Risk Exposure Plans (PERs).
Following the catastrophic floods of the late 1980s and early 1990s (Grand-Bornand in
1987, Nimes in 1988, Vaison-la Romaine in 1992), the State decided to reinforce its overall
forecasting policy and prevention of flood risks, by the law of 2 February 1995, byintroducing the Natural Risk Prevention Plans (PPRN), then that of 30 July 2003.
It will also be specified, that even if the State and the communes have responsibilities in thisarea, every citizen also has the duty to protect themselves and diminish their own
vulnerability. The objective of this policy is of course to ensure the safety of people andproperty by trying to anticipate the best natural phenomena while allowing sustainable
development of territories.
1.4 CHRONOLOGY OF LEGISLATION CONCERNING RISK PREVENTION
Among the regulatory arsenal relating to the protection of the environment and natural
hazards, it is useful - and without pretending to be exhaustive - to cite the main steps:


The Act of 13 July 1982 (codified in Articles L. 125-1 and following of the InsuranceCode) on "compensation for victims of natural disasters" set the objective ofcompensating victims on the basis of the principle national solidarity. Thus, a claim iscovered under the guarantee of "natural catastrophes" from the moment when thenatural agent is the determining cause and that it presents an abnormal intensity. Thisguarantee will be put into play only if the goods affected are covered by a "damage"insurance contract and if the state of natural disaster has been confirmed by aninterministerial decree. This law is also at the origin of the development of Natural
Hazard Exposure Plans (implementing decree of 3 May 1984) whose objectives were to11

prohibit the construction of new buildings in the most exposed areas and to prescribe
special measures for new construction in less exposed areas.
The law of 22 July 1987 (as amended by Law No. 95-101 of 2 February 1995 - Article
16 and codified in Article R.125-11 of the Environment Code) on "the organization of
security the protection of the forest against fire and the prevention of major risks'
stipulates that all citizens have a right to information on the major risks to which they
are exposed as well as on the safeguarding measures (means of protect yourself)(Articles L.125-2 of the Environmental Code). To do this, several documents of an
informative nature (not opposable to third parties) have been elaborated:• The Departmental Major Hazard Files (DDRM), developed by the State, aim to identify ineach department, major risks per municipality. They explain the phenomena and present
the general safeguarding measures.
• The Transmission of Information to Mayors (TIM), carried out by the Prefect. It consists
in sending to the mayors the information necessary for the establishment of thecommunal information document on major risks drawn up by the mayor.
12 頁• The Communal Information Document on Major Hazards (DICRIM) is drawn up by themayor. This informative document aims to supplement the information acquired byparticular measures taken in the municipality under the police power of the mayor.
➢ The law of 3 January 1992, also known as the "Water Law", Article 16 (Article L.211-1
and following and L.214-1 and following Of the Environmental Code) relating to thepreservation of aquatic ecosystems, to the management of water resources. This law
tends to promote a political will of global management of the resource (SDAGE, SAGE)
and in particular, the installation of compensatory measures to the urbanization in order
to limit the effects of the waterproofing of the grounds.
➢ The Law of February 2, 1995, known as the "Barnier Law" (articles L.562-1 and R.562-1
of the Environment Code) on the strengthening of the protection of the environment,
encourages public authorities, and in particular municipalities, to specify theirdevelopment projects and to avoid an uncontrolled extension of urbanization. This text
emphasizes the need to maintain rivers and aquatic environments, but also the need to12further develop public consultation (consultation). The Barnier law is at the origin of thecreation of a special funding fund: the Fund for Prevention of Major Natural Risks(FPRNM), which makes it possible to finance, within the limits of its resources, theprotection of densely urbanized places and possibly the expropriation of highly exposed
property. This fund is funded by a levy on the product of premiums or additional
contributions relating to the guarantee against the risk of natural disasters, provided forin Article L.125-2 of the Insurance Code. This law also saw the establishment of Natural
Risk Prevention Plans (PPRN), following an application decree dated October 5, 1995.
➢ The law of 30 July 2003 known as the "Bachelot law" on the prevention of technologicaland natural risks and the repair of damages was the subject of a first bill after the
explosion of the AZF plant in Toulouse September 21, 2001. This project was completed
only after a "natural risks" component to respond to the shortcomings and dysfunctions
also noted in the prevention of natural hazards during the floods in the south of France.
France in September 2002. The law is based on five guiding principles:• Strengthening information and consultation around major risks: The mayors ofcommunes covered by a prescribed or approved PPRN must issue periodic information
to the population at least once every two years about the natural hazards and thepreventive measures implemented to deal with them.
• The development of a conscience, a memory and an appropriation of the risk:
Obligation since the decree of March 14th, 2005 to inventory and to materialize themarkers of floods, in an essential objective of visibility and sensitization of the public
the level reached by the highest known waters (PHEC).
• Controlling urbanization in risk areas• Information on risks at source: Following the decree of 15 February 2005, notaries are
obliged to mention to purchasers and tenants the flood character of property; this is theIAL, Information Buyers tenants.
13 頁
Article L. 125-5 of the Environmental Code provides that purchasers or tenants of realestate located in areas covered by a Technological Risk Prevention Plan (PPRT) or a Plan
for Prevention of Natural Risks foreseeable (PPR), prescribed or approved, or in areas of13

seismicity, be informed by the seller or lessor of the existence of the risks.
This information is issued with the assistance of the competent State services, based on theinformation brought to the attention of the mayor by the State representative in thedepartment.
General information on the obligation to provide information is available on the website of
the Prefecture of Hérault.
• Improvement of compensation conditions for the victims: Increased possibilities to use theresources of the FPRNM to finance the expropriation of goods exposed to certain naturalrisks that seriously threaten human lives.
➢ The law of August 13, 2004 on the modernization of civil security and its implementing
decree of September 13, 2005, aim to broaden the action taken by the government interms of prevention of natural hazards. It is a matter of civil security, everyone's business(need to inculcate and sensitize children from an early age to the prevention of the risksof everyday life), to give priority to local (the objective is to give the population all theuseful instructions in the event of a major accident and to allow each municipality to fully
support the action of the rescue services through the communal plans of safeguard(PCS) replacing the plans emergency and rescue, and to stabilize the institution of fireand rescue services within the department (this bill creates a national fire and rescue
representatives of the State, local elected officials, firefighters and departmental fire andrescue services (SDIS) and to encourage solidarity (as soon as the situation imposes thereinforcement of external means in the affected department, the state will play national
solidarity).
➢ Directive 2007/60 / EC of the European Parliament and of the Council of 23 October2007 on the evaluation and management of flood risks, known as the "Flood Directive".
It aims to reduce the potential consequences associated with floods with a view tocompetitiveness, attractiveness and sustainable development of flood-prone areas.
To implement this renewed flood risk management policy, the French State has chosen to
rely on national and territorial actions:• a national strategy for flood risk management, provided for by Article L. 566-4 of theEnvironment Code, which brings together the provisions in force to make sense of national14policy and to display priorities;• Flood risk management plans (PGRI), provided for by Article L. 566-7 of theEnvironment Code, drawn up at the scale of the river basin district (scale of development of
the SDAGE).
The ambition is to achieve an integrated flood risk management policy in each territory,
shared by all stakeholders.
14 頁
To do this, the State first mapped the theoretical flood hazard on a large scale and thenmade a crossover with the impacted issues. Based on the analysis of this inventory, priority
areas have been identified to prevent floods. In these sectors, flood risk prevention actions
will have to be implemented.
3 territories with significant risk of flooding (TRI) were identified in Hérault and a flood risk
mapping was carried out for each IRR for 3 types of events: low probability (extreme
events), average (centennial), strong➢ TRI of Béziers-Agde, gathering 16 municipalities,➢ TRI of Sète, gathering 7 communes,➢ TRI of Montpellier, Lunel, Mauguio, Palavas extending over 49 communes including
39 in the Hérault,
The cartography of the TRI carried out which is not intended to replace the hazard maps
flood risk prevention plans (PPRI), when they exist on the territory allows to improve andhomogenize the knowledge of the risk of flooding in the most exposed sectors.
In the end, a Flood Risk Management Plan (PGRI) for the Rhône-Mediterranean basin will
be developed for each IRR in local strategies (SLGRI).
NB: for further information on the implementation of the flood directive on the RhoneMediterranean district, it is advisable to refer to the website http://www.rhonemediterranee.eaufrance.fr/➢ The law of 12 July 2010 on the national commitment for the environment known as"Grenelle 2 ", transposes the Flood Directive into French law and amends certain
provisions of the Environment Code (Articles L 562-1 and following) Concerning the15

preparation, modification and revision of Risk Prevention Plans.
NB: for more information on the various legislative supports (laws, decrees, circulars), it isadvisable to refer to the website https://www.legifrance.gouv.fr/.
To take into account the local specificities and harmonize the approaches in LanguedocRoussillon, the "Guide for the elaboration of PPRI in Languedoc Roussillon" validated by
the Regional Administrative Committee (RAC) by the Regional Prefect in June 2003, sets
out the general principles of thresholds, hazards and zoning,
1.5 PURPOSE OF THE PRESENTATION REPORTThe introductory report is a document that states:• the objectives of the PPR and the reasons for its development,• the principles of PPR development and its content,• natural phenomena known and taken into account,• how to qualify the hazard and define the stakes,• the objectives sought for the prevention of risks,• the choice of zoning and the applicable preventive measures,• the reasons for the settlement inherent in each zone,• application to the commune of BOUJAN-SUR-LIBRON (climatological, hydrographic
and geomorphological context).

16

15 頁
2 Approach to develop a natural flood risk prevention plan
2.1 WHAT IS A NATURAL HAZARD PREVENTION PLAN?
Developed at the initiative and under the responsibility of the State, in consultation with themunicipalities concerned, the PPR is a tool for decision support. This regulatory document
makes it possible to locate, characterize and predict the effects of foreseeable natural risks
with the dual aim of informing and raising public awareness, and to direct communal
development towards risk-free areas with a view to reducing the vulnerability of people andproperty through preventive measures.
Risk prevention plans (PPR) can deal with one or more types of risks, and can be spreadover one or more municipalities. At the beginning of 2013, more than 7,500 PPR had been
approved and more than 3,600 prescribed in France. They are part of a global risk
prevention policy of which they are the preferred tool.
The main lever of the RPP is the control of the occupation and the planning of the territory.
Other preventive actions, carried out under the responsibility of the State, local authorities
and private individuals, complete the system: preventive information, crisis preparation andmanagement, forecasting and warning .....
PPR are governed by Articles L.562-1 and following of the French Environment Code.
Article L.562-1 provides in particular that:"I.- The State develops and implements plans for the prevention of foreseeable natural risks
such as floods, landslides, avalanches, forest fires, earthquakes, volcanic eruptions, storms or
cyclones.
II.- The purpose of these plans is, where necessary:No. 1 To delimit the zones exposed to the risks, taking into account the nature and theintensity of the risk incurred, to prohibit there any type of construction, work, development
or exploitation agricultural, forest, artisanal, in particular in order not to aggravate the risk
to human lives or, in the case where constructions, structures, developments or agricultural,
forestry, craft, commercial or industrial exploitations may be authorized, prescribe theconditions in which they must be made, used or exploited;
No.2 to delimit zones which are not directly exposed to risks but where constructions,17

works, developments or agricultural, forestry, artisanal, commercial or industrial
exploitations could aggravate risks or provoke new ones and provide for measuresprohibition or requirements as provided for in No.1
No.3 To define the prevention, protection and safeguard measures that must be taken, inthe zones mentioned in No.1 and No.2 , by the public authorities within the scope of theircompetences, as well as those which may be the responsibility of individuals ;No.4 To define, in the zones mentioned in No.1 and No.2, the measures relating to thedevelopment, use or exploitation of constructions, works, spaces put in cultivation orplanted existing on the date of Plan approval that must be made by the owners, operators orusers.
16 頁
III.- The realization of the measures envisaged in No.3 and No. 4 of the II can be madeobligatory according to the nature and the intensity of the risk within a period of five years,
being able to be reduced in case of urgency. Failing compliance within the prescribedperiod, the prefect may, after formal notice not followed by effect, order the realization ofthese measures at the expense of the owner, the operator or the user.
IV.- The preventive measures envisaged in the No.3 and No.4 of the II, concerning thewooded grounds, when they impose rules of management and forestry exploitation or therealization of work of prevention concerning the wooded spaces put in charge public andprivate forest owners and operators are taken in accordance with Title II of Book III andBook IV of the Forest Code.
V.- Preventive works imposed pursuant to No.4 II to property built or developed inaccordance with the provisions of the Town Planning Code before the approval of the planand charged to the owners, operators or users cannot bear only on limited facilities.
VI. - Flood risk prevention plans are compatible or made compatible with the provisions of
the flood risk management plan defined in Article L. 566-7.
VII. - Decrees in Council of State define as necessary the methods of qualification of thehazards and the risks, the general rules of prohibition, limitation and framing of theconstructions, prescription of works of reduction of the vulnerability, as well as informing
the population, in areas exposed to the risks defined by the plans for the prevention of18

foreseeable natural risks. "2.1.1 WHAT DOES THE NATURAL FLOOD PREVENTION P LAN CONTAIN (PPRI)?
Article R.562-3 of the Environment Code states that the project plan file includes:


Note a presentation note indicating the geographical area concerned, the nature of thenatural phenomena taken into account and their possible consequences, taking into
account the state of knowledge;

one or more graphic documents delimiting the zones mentioned in No.1 and No.2 of II
of Article L.562-1;

a regulation specifying, as needed:(a) the prohibition measures and the requirements applicable in each of these zones under
No,1 and No.2 of II of Article L.562-1;(b) the prevention, protection and safeguard measures mentioned in No.3 of II of Article
L.562-1 and measures relating to the development, use or exploitation of buildings,
works, structures and spaces planted or planted existing at the date of approval of theplan, mentioned in No.4 of this same II. The regulation shall mention, whereappropriate, those measures whose implementation is obligatory and the deadline fixed
for it.
(17 頁)
Graphic documents include:• the hazard map developed from hydrogeomorphological analysis and modeling of thereference hazard,• the map of regulatory zoning obtained by crossing the hazard with the exposed issues,
making it possible to establish the red, blue and gray zonation conventionally encounteredin PPR
18 頁
2.1.2 WHAT ARE THE PHASES OF DEVELOPING A PPR?
The preparation of PPR is conducted under the authority of the departmental prefect. The
latter then designates the deconcentrated service of the State which will be responsible for
instructing the project.

19

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19 頁
2.2 IMPACT OF PPR
2.2.1 SCOPE OF PPR
Once approved and published, the PPR is worthlessness of public utility. In municipalities
with a PLU or PLUI, this servitude must be attached within three months. All regulatory
measures defined by the PPR must be respected. These are essential for all existing and new
constructions, installations and activities.
20

Existing goods and activities prior to the publication of this natural hazard prevention plan
continue to benefit from the general guarantee scheme provided for by law.
For goods and activities created after publication, compliance with the provisions of the
PPR conditions the possibility for the insured to benefit from compensation for material
damage directly caused by the abnormal intensity of a natural agent, provided that the statusof natural disaster is established by interministerial order.
The preventive measures prescribed by the PPR regulations and their conditions ofexecution are the responsibility of the contracting authority and the contractor responsible
for the constructions, works and installations concerned.
In addition to the provisions imposed on new projects, the PPR also imposes measures,
known as mitigation measures, on existing assets, in order to reduce their vulnerability.
2.2.2 PENALTIES FOR NON-COMPLIANCE WITH THE PROVISIONS OF THIS PPRIn the case of measures imposed by a PPR and integrated in the PLU or PLUI, pursuant to
Article L. 480-4 of the Urban Planning Code:

Natural persons recognized as liable may incur a fine of between € 1,200 and an
amount that may not exceed € 6,000 per m² of built area, demolished or renderedunusable in the case of construction of a floor area, or 300 000 € in other cases. In theevent of a second offense, in addition to the penalty fine, a sentence of imprisonmentof 6 months may be imposed.



Pursuant to Articles 131-38 and 131-39 of the Penal Code, legal persons can incur afine of not more than five times the amount incurred by natural persons, as well as
permanent or temporary prohibition of activities, temporary placement under judicial
supervision, permanent or temporary closure of the establishment in question,
permanent or temporary exclusion from public contracts and publication of thedecision pronounced. A compliance of places or works with the PPR may finally beordered by the court.

In the case of measures imposed by a PPR for the reduction of vulnerability of persons,21

pursuant to Article 223-1 of the Penal Code:

Defective natural persons may be found guilty of deliberately violating a particularsafety or prudential obligation imposed by the regulation, directly exposing others to
an immediate risk of death or injury, and this title one year imprisonment and 15,000€ fine.

(20 頁)


Legal persons are liable for the same offense, in accordance with Article 223-2 of thePenal Code, to a fine not exceeding five times that incurred by natural persons, as wellas the definitive prohibition or temporary activities, temporary placement underjudicial supervision and publication of the decision pronounced.

In case of occurrence of a disaster causing damage to persons, pursuant to Articles 222-6,
222-19 and 222-20 of the Criminal Code:

Defective natural persons may be convicted by reason of the mere negligence or
manifestly deliberate violation of a particular safety or security obligation imposed by
the regulation, of homicide or unintentional injury, and as such may incur one to three
years' imprisonment and a fine of € 15,000 to € 45,000, depending on the seriousnessof the damage and the offense.



Legal persons incur a fine of not more than five times the amount of the fines incurredby natural persons for the same offenses, as well as the permanent or temporary
prohibition of activities, temporary placement under judicial supervision, thepublication of the decision pronounced and, in the case of manslaughter, the definitiveor temporary closure of the establishment in question.

Article L.125-6 of the Insurance Code provides for the possibility for insurance companies
but also for the prefect or president of the Central Reinsurance Fund, to refer the central
charging office for the application of special allowances on the amount of natural disasterindemnities (franchise surcharges) up to 25 times the basic deductible amount forresidential goods and up to 30% the amount of direct non-insurable material damage(instead of 10%) or 25 times the minimum of the basic exemption for goods for professionaluse.
When a PPR exists, the Insurance Code specifies that there is no possible derogation fromthe guarantee obligation for "goods and activities existing prior to the publication of this22

plan", except for those whose compliance with measures made compulsory by this plan has
not been carried out by the owner, the operator or the user. In this case, the insurers are notrequired to indemnify or insure the property built and the activities carried out in violationof the current PPR rules.
2.2.3 EFFECTS OF PPRPreventive information
The general prevention, protection and safeguard measures referred to in the regulation
concern the preservation of human life through protective devices, passive provisions,preventive information and the maintenance of existing structures.
Since the "Risk" law of July 30, 2003 (strengthening information and consultation on majorrisks), all mayors whose municipalities are covered by a prescribed or approved PPR mustissue at least once every two years from the population periodic information on natural
hazards. This procedure will have to be supplemented by an obligation to inform all theadministrators every year by means of a relay left to the free choice of the municipality(municipal bulletin, public meeting, diffusion of a brochure) mandatory and recommended
measures for future projects and for the existing frame.
21 頁
Municipal plan of safeguard (PCS)
Beyond the effects of the provisions issued in the regulation for new projects and forexisting properties, the approval of the PPR makes it obligatory to draw up a communal
safeguard plan (PCS), in accordance with Article 13 of the Law No. 2004-811 of 13 August
2004 on the modernization of civil security. Pursuant to article 8 of decree No. 2005-1156 of
13 September 2005 relative to the communal plan of safeguard and taken in application ofarticle 13 of the law No. 2004-811, the commune must realize its PCS in a two years fromthe date of approval by the Prefect of the PPR Department.
Article 13 of the law No.2004-811 specifies that "the communal plan of safeguard gathers all
the documents of communal competence contributing to the preventive information and theprotection of the population. It determines, based on the known risks, the immediate
measures of safeguard and protection of the people, fixes the organization necessary for the
diffusion of the alert and the safety instructions, identifies the available means and defines
the implementation of the measures accompaniment and support of the population ".

23

The municipal plan of safeguard is stopped by the mayor of the commune and its
implementation falls of each mayor on the territory of his commune.
The municipal plan of safeguard is adapted to the means which the commune has. He
understands :the municipal information document on major risks provided for in Article 3 (III) of
the abovementioned Decree of 11 October 1990,



diagnosis of local risks and vulnerabilities,the organization providing protection and support to the population, which specifies
the internal measures taken by the municipality to be able to alert and inform the
population at any time and to receive an alert from the authorities. These provisions
include, in particular, an operational directory and a regulation for the use of thedifferent means of alert that may be implemented,



the procedures for implementing the municipal civil security reserve when the latter
was set up pursuant to Articles L. 1424-8-1 to L. 1424-8-8 of the General Code ofTerritorial Collectivities.

It is eventually completed by:

the organization of the communal command post set up by the mayor if necessary,the actions to be carried out by the municipal technical and administrative services,

if applicable, the appointment of the deputy mayor or municipal councilor responsible
for civil security issues,(22 頁)
an inventory of the municipality's own resources or that can be provided by private
persons located in the municipality. This inventory includes the means of transport,
accommodation and food for the population. This system may be supplemented by aninventory of the means likely to be made available by the intercommunal
establishment of which the municipality is a member,24

the specific measures to be taken to deal with the foreseeable consequences on the
territory of the municipality of the risks identified,

the methods of exercise to test the communal plan for safeguarding and training
actors,a census of the measures already taken in the field of civil security by any public or
private person established on the territory of the municipality,

how to take into account the people who volunteer for the victims,

provisions ensuring the continuity of daily life until the return to normal.

25

22 頁
3 Methodology and Definitions
3.1 APPROACH TO EXTENSION OF KEY TERMS EMPLOYED IN RISKS
The risk is often defined in the specialized literature, as being the result of the crossing of
the hazard and the stakes.
We thus have:
ALEA x ISSUES = RISKS
23 頁
Hazard is the manifestation of a natural phenomenon (potentially damaging) of occurrence
and intensity.

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）23 頁より作成。
The issues presented correspond to all people and property (human, socio-economic and /or heritage issues) likely to be affected by a natural phenomenon.

26

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）23 頁より作成。
The risk is the potential for brutal, random and / or massive damage following a natural
event, the effects of which may involve human lives and cause significant damage. The
term "risk" is therefore used only if issues (present in the area) can potentially be affectedby a hazard (possible damage).

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）23 頁より作成。
24 頁
3.2 GENERAL PRESENTATION OF THE RISK FLOOD
The flood risk is thus the consequence of two components: the presence of the hazard(water) as well as that of the man (the stakes).
27

3.2.1 THE PRESENCE OF WATER: The FTAA
On the national territory, the majority of rivers (rivers, streams) have a morphology that isorganized into three beds (see figure below):

The minor bed (L1) which is constituted by the ordinary bed of the watercourse, forthe low water flow or for the frequent floods (annual floods: T1)



The middle bed (L2), in certain climates, we can identify an average bed. For floodsof 1 to 10 years, the flood submerges the lands bordering the river and extends into
the middle bed. It corresponds to the alluvial space ordinarily occupied by theriparian forest, on which medium floods flow (T2)

The major bed (L3) which includes the low areas located on either side of the minor
bed, over a distance that ranges from a few meters to several kilometers. Its limit isthat of exceptional floods (T3). We distinguish the flow zones, in the vicinity of theminor bed or flood channels, where the current has a high speed, and the areas of
expansion of flood or water storage, where speeds are low. This storage isfundamental, because it allows the rolling of the flood (reduction of the flow and the
speed of rise of water downstream).

Outside of the floodplain, the risk of fluvial flooding is zero (which does not exclude
the risk of flooding by rain runoff, particularly in urbanized areas). On the maps, we
distinguish the ancient alluvial terraces, which no longer participate in the floods, but
are witness to the disappearing hydraulic or climatic conditions. Their characteristics
make it possible to consider a redeployment of sensitive land occupations outside theflood zones.

28

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）24 頁より作成。
(25 頁)
This distinction of topographic beds of the river is possible by the hydrogeomorphologicalapproach, recognized and developed since 1996, which aims at the study of the hydraulicfunctioning by analysis of the structure of the valleys. By various techniques such as
photo-interpretation, photogrammetry and field observation, this is a method ofinterpretation of the natural terrain that identifies the structuring elements of thewatershed likely to modify the flow of flood.
In densely populated urban area where the stakes are high, this approach may becomplementary studies such as wired hydraulic modeling (or bidirectional) that is tomodel the Centennial calculated flow if high historic flood. Through this method, waterlevels, velocities and water flow directions can be established for a reference flood throughprofiles across the stream or successive traps. The crossing of these two criteria makes it
possible to obtain the representative cartography of the different degrees of hazard.
3.2.2 THE PRESENCE OF MAN: THE CHALLENGES
By settling in the bed major, the man has settled in the river itself. But this occupation has
a twofold consequence: it creates the risk by exposing people and property to floods andaggravates the hazard by changing the flow conditions of the water.
With regard to the risk of flooding, the issues to be taken into account are two
types:- areas with little or no urbanization,29

- urban areas defined on the basis of the existing physical reality.
With the exception of existing campsites, areas with little or no urbanization are
inherently vulnerable to human and economic vulnerability as few properties and people
are exposed to them. However, to the extent that these areas are likely to allow the
expansion of the flood and slow down dynamic flows, it should not open them tourbanization. On the other hand, it is essential not to expose in the flood zone new humanand economic issues.
Urbanized areas include urban centers, communication routes, activities, sensitive or
strategic equipment for crisis management.
3.3 PROCESS FOR FLOODS AND INUNDATIONS
3.3.1 DEFINITION AND TYPES OF FLOODS"Inundations" and "floods" are frequently confusing terms. But the latter have very
different characteristics. Indeed, a flood does not cause systematic inundations and viceversa!
(26 頁)
Flood is a rapid and temporary increase in the flow of a watercourse beyond a certain
threshold. It is described from three parameters: the flow, the water level and the speed of
the current. These parameters are conditioned by precipitation, the state of the watershed
and the characteristics of the watercourse (depth, width of the valley). These naturalfeatures may be aggravated by the presence of human activities. Depending on theimportance of flow rates, a flood can be contained in the minor bed or overflow into the
middle or major bed.
Inundation is a rapid or slow submersion of an area outside the minor stream bed. There
are several types of Inundations:

We speak of plain inundation to designate the slow rise of water in the plain region.
It occurs when the river slowly leaves its minor bed and floods the plain for a
relatively long time. The river occupies its middle bed and possibly its major bed.



The torrential flood corresponds to the rapid rise (usually within six hours after
rainfall) of the water in the valleys and gorges following intense rains over a shortperiod.
30

Urban run-off flooding in urban and peri-urban areas, following intense and intense
thunderstorms leading to saturation of drainage systems and then dripping ontoimpervious soils.

3.3.2 FLOODS AND INUNDATIONS TRAINING
Different elements contribute to the formation and the increase of flows of a watercourse:

Mobilisable water, which may include snow or ice melt at a time of mild rains,repeated and prolonged rains or relatively short showers that may affect all small
catchments of a few square kilometers. This case does not concern, or only very
marginally, our Mediterranean rivers.



Runoff depends on the nature of the soil and its surface occupation. It corresponds
to the part of the water that has not been intercepted by the foliage, that has notevaporated and that has not been able to infiltrate, or that resurfaces after
infiltration (saturation phenomenon of the soil).

Concentration time is the time required for a drop of water with the longest
hydraulic path to travel to reach the outlet. It is therefore a function of the size andshape of the watershed, the topography and the land use.



Flood propagation (runoff) tends to congregate in a drainage axis where it forms aflood that spreads downstream. Propagation is further slowed down as the flow field
is wider and the slope is smaller.



Overflow occurs when there is propagation of a flow rate higher than that which canevacuate the minor bed.

(27 頁)
Our regions are obviously concerned by runoff, very strong in case of Cevennes episodes
where infiltration is very low given the diluvian nature of the rains. The low concentration
time makes the propagation fast and the prediction delicate.
Areas close to the coast (sea or pond) can also be flooded by the accumulation andinteraction of extreme physical phenomena (atmospheric depression, wind, waves, etc.).

31

3.4 FACTORS AGGRAVING RISKS
Aggravating factors are almost always related to human intervention. They result inparticular from:

The location of people and property in the floodplain: not only is the risk exposure
increased, but moreover, the waterproofing of soils due to urbanization promotes
runoff at the expense of infiltration and increases the intensity of the flows. Landuse also has an impact: the presence of vines (with rainwater drainage on slopes) or
cornfields rather than grasslands contributes to a faster flow and decreases the timeof water concentration towards the soil outlet.

Potential failure of protection devices (dams, dikes, merlons, embankments, ...): therole of these devices is limited. Their effectiveness and resistance depend on theirconstruction, management and maintenance, as well as the reference flood for which
they were designed. In addition, breaking or submerging a dike exposes thefloodplain to flooding more than if it was not protected. In the event of a break, forexample, the wave effect generated is all the more devastating. In addition, natural
structures such as dune ropes are not intended to serve as a protection structure anddo not fall under the regulations relating to the safety of hydraulic structures. Their
impact on the flows must be taken into account, but these cords cannot be
considered as protective works resistant to the reference storm.

Transport and dumping of unwanted products: Floods sometimes carry and thenleave polluting or dangerous products on their route, particularly in urban areas.
This is why it is essential that special precautions be taken regarding their storage.

The formation and breakage of ice jams: the floating materials carried by the current(trees, bushes, caravans, vehicles ...) accumulate upstream of the narrow passages tothe point of forming dams which greatly raise the level of the water and, in case of
breakage, cause a powerful and devastating wave downstream.

The raising of water upstream of the obstacles: the presence of bridges,
embankments or walls in the flow field causes an elevation of the water upstream
and on the sides which accentuates the consequences of the flood (increase of theduration of submersion, creation of eddies and currents, ...)

32

28 頁
3.5 THE CONSEQUENCES OF THE FLOODEndangering people: The danger is manifested by the risk of being swept away ordrowned due to the water level or the speed of flow, as well as the duration of theflood which can lead to the isolation of population centers. This is why it is essential
to have a warning system (flood announcement) and organize the evacuation of
populations especially if the time is very short, especially during fast floods or
torrential.

The interruption of communications: in the event of flooding, it is common for thecommunication routes (roads, railways, ...) to be cut, prohibiting the movement ofpeople, vehicles or even rescue services. In addition, underground or surface
networks (telephone, electricity, etc.) can be disturbed. However, all this can haveserious consequences on the dissemination of the alert, the evacuation of
populations, the organization of relief and the return to normal.



Damage to property and activities: The damage caused by floods can reach various
degrees, depending on whether the goods have simply been brought into contact withwater (traces of moisture on the walls, mud deposits) or that they have been exposed
to strong currents or flows (partial or complete destruction). Movable damages are
more common, especially in the basement and ground floor. Activities and theeconomy are also affected in case of damage to the equipment, agricultural losses,
production stoppage, impossibility to be supplied, ... In case of flood caused by thesea, the salinity of the water as well as marine sediments carried on normally emerged
land cause additional damage, particularly on agricultural land. On the seafront, themechanical effect of the breaking can cause significant material damage.

3.6 REFERENCE EVENTS IN THE FLOOD NATURAL RISK PREVENTION PLAN
Some small floods are frequent and do not lend or little to consequence. "Bigger" floods
are also rarer. The establishment of a well-documented historical chronicle makes it
possible to estimate, by statistical calculation, the probabilities of recrudescence of suchflood intensity in the years to come. This establishes the probability of occurrence (orfrequency) of a flood and its period of return. For example:
A decadal (or centennial) flood is a flood of such importance that it is likely to occur every
10 years (or 100 years) on average over a very long time. The 100-year flood is therefore
33the theoretical flood, which each year has a probability of 1% (one "chance" out of 100) to
occur.
As stated in the texts, the reference event taken into account in the context of a PPRI is
the calculated centennial flood or the highest known historical flood if it proves to besuperior.
Over a period of about thirty years (minimum life of a construction) the centennial flood
has about one in four possibilities to occur.
If this is indeed a theoretically infrequent flood, the centennial flood is a predictable eventthat must be taken into account at the scale of sustainable development of a municipality:
it is not in no case of a maximum flood, the occurrence of a higher flood can not be
excluded, but the reference flood remains sufficiently significant to serve as a basis for the
PPRI.
29 頁
3.6.1 THE DESCRIPTIVE PARAMETERS OF THE FAA.
The parameters primarily integrated in the study of the PPR hazard are those that make itpossible to apprehend the level of risk induced by a flood or a marine storm:The submersion height currently represents the factor best describing the risks forpeople (isolation, drowning) as well as for goods (damage) by direct action(degradation by water) or indirect (pressure, pollution, short -circuit, etc.). Thisparameter is, moreover, one of the most easily accessible by direct measurement(field survey) or hydraulic modeling. It is considered that heights of more than 50
cm are dangerous for people (see graph in 3.6.2). Beyond 100 cm of water, damage
to the building can be irreversible (destabilization of the building under pressure,soaked water, ...)

The flow velocity is conditioned by the slope of the bed and its roughness, for thefluvial hazard. It can reach several meters per second. The danger of the flow
depends on the height / velocity pair. For example, from 0.5 m / s, the speed of thecurrent becomes dangerous for the man, with a risk of being carried away by thewatercourse or to be hurt by objects carried with high speed. The flow velocity alsocharacterizes the risk of transporting light or unsealed objects as well as the risk of34

gullying of embankments or embankments. It is clear that, in the case of a dikebreak, this parameter becomes preponderant over the first tens of meters. In thecase of marine submersion the flow velocity is considered to be less than 0.5 m / s.
The submergence time corresponds to the duration of isolation of people or thedysfunction of an activity. When this time is important, health problems can occur,
the water is often dirty, contaminated by sewers and a high degree of salinity in caseof marine submersion. For fast kinetic fluvial floods, characteristic of Mediterranean
climates, submergence time is not a parameter studied because of the rapid descentof the waters after the event.

3.6.2 THE QUALIFICATION OF THE FAA
It is determined by two distinct methods, depending on whether one is in an urban
environment (hydrogeomorphology and wired or racked hydraulic modeling) or in anatural environment (hydrogeomorphology).
Depending on the values of the parameters studied, it results in flood zones of"moderate", "strong" and "residual" hazard.
Is classified as a "strong" hazard zone, an area flooded by the reference flood, and whosewater depth is greater than 0.5 m or the speed is greater than 0.5 m / s.
Is classified in zone of hazard "moderate", a zone by the flood of reference, and whose
height of water is strictly lower than 0,5 m and the speed of flow is strictly lower 0,5 m / s.
It is classified as a "residual" hazard zone, an area that is not flooded by the reference
flood, but that is likely to be mobilized for a higher flood.
(30 頁)The threshold of 0.5 m is explained by the fact that the risk for people starts from thisheight of water:➢ from this value, it has been shown by feedback from past floods, that an untrained
adult and, even more so, a child, an elderly person or a person with reduced mobility
encounters serious difficulties in displacements, reinforced by the total disappearance ofthe relief (sidewalks, ditches, open manholes, etc.) and increased stress,35➢ In addition to the difficulties of movement of people, this limit of 0.5 m of water
characterizes a threshold for the movement of vehicles: a car can start to float from 0.3 m
of water and can be washed away from 0 , 5 m by the current as weak as it is,➢ A height of 0.5 m of water is also the limit of movement of conventional emergency
response vehicles.

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）23 頁より作成。
Travel limits in case of flood
The limit of the speed parameter is more complex, depending on the location of thebuildings, the height of dikes, their constitution, etc.
3.7 DEFINITION OF ISSUES
The issues are based on the current land cover analysis (review of current urbanization,
location of sensitive, strategic, vulnerable, etc.). They make it possible to define the"natural" flood zone (moderate stakes) and the "urbanized" flood zone (strong stakes).
(31 頁)The moderate stakes cover the non-urbanized zones at the date of the elaboration of thisplan and thus include, the agricultural zones, the natural zones, the forest areas, according36

to the terms of the article R.123-4 of the code of the urban planning and urbanization
areas not yet built.
Strong issues cover urbanized areas and areas to be urbanized already developed.
The delineation of urban areas (high stakes) is included in the hazard mapping of the
PPRI.
At this stage, the aim is to meet the twofold objective set by state policy: to define andprotect urbanized flood zones on the one hand, and to preserve non-urbanized areas on
the other, particularly for the conservation of the field. flood expansion.
3.7.1 REGULATORY ZONING
Article L. 562-1 of the Environmental Code defines two main types of zones:➢ areas directly exposed to risks, hereinafter referred to as "danger zones",➢ areas not directly exposed to risks, hereinafter referred to as "precautionary zones".
3.7.2 AREAS AT RISK
Qualified in the PPR of danger zones, these are the areas exposed to a strong hazard, andin which most of the facilities are therefore prohibited.
These danger zones consist of:

the Ru Rouge urban zone, flood areas subject to high hazard, where the stakes arehigh (urban areas),Natural Red Zone Rn, flood areas subject to a high hazard where the stakes are low(natural areas).

They serve two purposes:


not to increase the population, the buildings and the risks by allowing, however, aminimal evolution of the building in urban area to favor the continuity of life andthe urban renewal (all red zones),enable urban development that takes into account risk exposure by taking care not37

to increase vulnerability (urban reds).
3.7.3 ZONES NOT DIRECTLY AT RISK
Areas qualified as precautionary in the PPR, they correspond to the whole of themunicipal territory which is not located in zone of danger.
These are therefore areas where constructions, works, developments or agricultural,
forestry, artisanal, commercial or industrial exploitations could aggravate risks or provoke
new ones.
(32 頁)
They cover areas of moderate hazard and non-floodable areas by the reference flood.
They have several objectives:preserve areas of non-urbanized flood expansions,

prohibit any project likely to aggravate the existing risk or to provoke new ones,

prohibit any construction promoting isolation of persons and / or inaccessible torescue,

to allow a reasoned and adapted urban development in urban area of moderate
hazard,

allow urban development taking into account sea level changes due to global
warming,allow urban development taking into account the potential risk in the event of aflood higher than the reference flood,

allow urban development of non-floodable areas without aggravating the floodabilityof flood-prone areas.

They consist of:

the Blue Bu zone, flood areas subjected to a moderate hazard, where the stakes arehigh (urban areas),

Red Precaution Zone Rp, flood areas subject to moderate hazard, where the stakes
are low (natural areas),Precautionary zones Z1 and Z2, areas not flooded by the reference flood, composedof the residual hazard zone Z1, potentially flooded by an exceptional flood, and Z2
zero hazard zone, which concerns the rest of the municipal territory, not subject toeither the reference flood or the exceptional flood.

38

The following table and diagram illustrate these classifications of zones, resulting
from the crossing of the hazard and the considered issues.
AleastrongModerateissuesStrong (urban

Moderate

areas)(natural areas)

Flood for the reference

Red Ru danger

Red Rn danger

flood

zone

zone

Flood for the reference

Buoyant Blue

Precautionary

flood

Precaution

area Red Rp

Zone
Exceptional

Hydrogeomorphological
limit

No

Precautionary zone
Z1

Beyond thePrecautionary zonehydrogeomorphological

Z2

limit
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34 頁
4 THE MEASURES PRESCRIBED BY THE PPR
The PPRI Regulation incorporates prevention, protection and safeguard measures andmeasures on the existing which are briefly described below.
4.1 PREVENTION, PROTECTION AND SAFEGUARD MEASURES
These collective or specific measures, established by Article L. 562-1 II 3 ° of theEnvironmental Code, aim to preserve human lives by actions on the phenomena or thevulnerability of goods and people.
Some of these measures belong to the public authorities within the scope of theircompetences, others are the responsibility of the individuals.
They aim to reduce the impact of a phenomenon on people and property, to improve
people's and elected officials' knowledge and perception of risk and to anticipate the crisis.
To this end, several provisions can be made such as:• carrying out specific studies on hazards (hydrology, hydraulic modeling,hydrogeomorphology, atlas of flood zones, etc.),• setting up a monitoring and announcement system,• the development of a crisis management plan at the municipal level, the PCS, or even at
the inter-communal level,• implementation of public information meetings on risks, elaboration of information
documents such as DICRIM, etc.
4.1.1 CONTROL OF RAIN FLOWS
The control of rainwater, including in the face of exceptional events of centennial
occurrence, is a major challenge for the protection of inhabited areas. This rainwater
management is the responsibility of the municipality. If it is not already realized, themunicipality will have to establish a zoning of stormwater treatment, in accordance with thearticle L.2224-10 3 ° of the General Code of the Territorial Collectivities, within five years
as from the approval of the PPRI.
41

In accordance with article 35 of the law No. 92-3 on water (codified in article L.2224-8 of
the general code of the territorial collectivities), the communes or their groupings mustdelimit the zones where measures must be taken to limit the waterproofing of soils and tocontrol flow and runoff of storm water and runoff and areas where it is necessary to provide
facilities for collection, possible storage, and as needed, rainwater treatment.
In application of the SDAGE Rhone - Mediterranean, measures to limit runoff must be
absolutely favored: limitation of waterproofing, retention on the plot and storm water
storage devices (retention basins, valleys, tank roads, ...)
35 頁
4.1.2 PROTECTION OF URBANIZED DENSITY PLACESIn accordance with Article L.221-7 of the Environmental Code, local authorities or theirgroup may, as part of a declaration of general interest, study and undertake flood
protection works. In application of the SDAGE Rhone - Mediterranean, this work must be
limited to the protection of densely urbanized areas. They must be subject, in theframework of authorization procedures related to the application of the Water Act, to a
sufficiently comprehensive analysis to make it possible to understand their impact
upstream and downstream, both in terms of hydraulics and the preservation of aquatic
environments. Works that leave rivers with the greatest freedom should be preferred tonarrow embankments bordering the minor bed.
If protection work is in most cases conceivable, it should be borne in mind that these
protections remain in all cases limited. The occurrence of a flood exceeding the projectflood cannot be ruled out.
When the basin is the subject of a flood prevention action plan (PAPI), the State is likely tocontribute to the financing of such work as part of the fund for the prevention of major
natural hazards (FPRNM dit Barnier fonds).
Existing dams protecting important stakes will be subject to rigorous management,
maintenance, regular inspections, and if necessary, work of reinforcement, enhancement,
etc.
4.1.3 PREVENTIVE INFORMATION
Article L125-1 of the Environment Code states that "Citizens have the right to information42

on the major risks to which they are subject in certain areas of the territory and on thesafeguarding measures that concern them. This right applies to technological risks andforeseeable natural risks.
The mayor must issue at least once every two years to the population periodic information
on natural hazards. This procedure must be supplemented by an obligation to inform all
citizens on a yearly basis through a relay left to the free choice of the municipality(municipal bulletin, public meeting, distribution of a brochure, exhibition, ...) on
mandatory measures and recommended for projects and existing buildings.
4.1.4 SAFEGUARD MEASURES
The mayor, by his powers of police, must elaborate a communal plan of safeguard (PCS),in accordance with article 13 of the law No.2004-811 of August 13, 2004 relating to themodernization of the civil security, within a two years from the date of approval of the
PPR. This article specifies that "the municipal plan of safeguard regroups all documents ofmunicipal competence contributing to preventive information and the protection of the
population. It determines, based on the known risks, the immediate measures of safeguard
and protection of the people, fixes the organization necessary for the diffusion of the alert
and the safety instructions, identifies the available means and defines the implementationof the measures accompaniment and support of the population. He may designate thedeputy mayor or the municipal councilor responsible for civil security matters.
36 頁
The following provisions are made mandatory for communities in the context of theprevention, protection and safeguarding of existing and future buildings:


the approval of the Flood Risk Prevention Plan opens a period of 2 years during
which the town hall must draw up a Municipal Protection Plan (see above),

Owners or managers, public or private, protection dikes in highly urbanized sectors
must comply with the requirements of the current regulations on the safety of
hydraulic structures (Decree No. 2007-1735 of 11 December 2007 on safety
hydraulic works and decree No. 2015-526 of May 12, 2015, relating to the rules
applicable to structures built or developed to prevent floods and the safety rules ofhydraulic structures, applicable at the date of approval of the PPRI),

43



According to their characteristics and the protected population, dikes and protection
works for the protection of urbanized places must be the object of their owner of acomplete diagnosis, a thorough technical visit, a report of inspection and report ofmonitoring at a frequency of 1 to 5 years.

4.2 MITIGATION MEASURES
These measures, introduced by Article L. 562-1 II 4 ° of the Environment Code, haveresulted in the drafting of a specific part of the regulation attached to this PPRI file, inwhich all mandatory measures are detailed.
4.2.1 DEFINITION
The mitigation measures concern individuals (owners, operators, users) and apply to their
existing property.
4.2.2 OBJECTIVESOf very different natures, these measures pursue three objectives which allow tohierarchize them:Ensure the safety of people (adaptation of goods or activities in order to reduce thevulnerability of people: safe haven, consolidation works of protection works),

Reduce the vulnerability of buildings (limit material damage and economic damage),

Facilitate the return to normal (adapt the goods to facilitate the return to normalwhen the event occurred: choice of materials resistant to water, etc., mitigate thepsychological trauma related to a flood by facilitating the waiting for rescue orrecession, and possible evacuation in conditions of comfort and safety satisfactory).

37 頁
4.2.3 MEASURES APPLICABLE TO EXISTING GOODS
A diagnosis (or self-diagnosis) must first be developed by the owners, communities,businesses and individuals, to know their vulnerability and thus determine the necessary
measures to reduce it. This diagnosis must establish the height of water likely to invade thebuilding in the event of flood similar to that taken by reference by the PPRI.
For property built or developed in accordance with the provisions of the Urban Planning
Code and prior to approval of this PPR, work relating to certain individual building
measures is now mandatory. They are imposed only within the limit of 10% of the market
value or estimated value of the property considered at the date of approval of the plan44

(Article R562-5 of the Environment Code). These mandatory measures are described in
the regulations of this PPRI.
Unless otherwise specified in the regulation, the implementation of these provisions must
be carried out as soon as possible and within a maximum period of 5 years from theapproval of this plan (pursuant to article L.562 -1 III of the Environmental Code, according
to the terms of its implementing decree).
If these measures are not implemented within the prescribed time limits, the prefect may
impose the implementation of these measures at the expense of the owner, the operator or
the user.
Since the law of 30 July 2003 on the prevention of technological and natural risks and therepair of damage, all the work of safety of persons and reduction of the vulnerability of thebuildings prescribed by an approved PPR can benefit from a State subsidy. This grant fromthe Fund for Prevention of Major Natural Risks, known as the Barnier Fund, aims toencourage the implementation of these measures and concerns:• individuals (residential property) up to 40%,• companies with fewer than 20 employees (professional use) up to 20%.
4.3 REFERENCES AND RESOURCESPortal for the prevention of major risks:
http://www.prim.net

Risk prevention portal of MEDDE:
http://www.developpement-durable.gouv.fr/Enjeux-et-principes.html

Risk component of MEDDE - DGPR:
http://www.developpement-durable.gouv.fr/-Risques-naturels-et-ouvrages-.html

Information portal on natural and technological risks:
http://www.georisques.gouv.fr/

Website of the Rhone Mediterranean Basin Water Information System:
http://www.rhone-mediterranee.eaufrance.fr

Website of the state services in the Hérault:
http://www.herault.gouv.fr/

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38 頁
SECOND PART: THE NATURAL RISK PREVENTION PLAN FLOODING OF THE
COMMUNITY OF BOUJAN-SUR-LIBRON
The watershed of Libron covers 16 municipalities of the department: Faugères,Caussiniojouls, Cabrerolles, Laurens, Autignac, Magalas, Puissalicon, Puimisson, Pailhes,
Lieuran-Les-Beziers, Corneilhan, Bassan, Boujan-Sur-Libron, Montblanc, Beziers andVias.
In 2010, in response to the preparation of Flood Risk Prevention Plans (PPRi), the Frenchgovernment commissioned the engineering consultancy EGIS Eau to carry out a study of
the areas flooded by overflowing water courses. Water, excluding rain runoff, on theperimeter of the communes of the watershed, scale relevant for the risk analysis.
The mission consisted of two phases. A first phase whose objective was to understand thefunctioning of the rivers present in all the communes of the watershed and to determine,for the realization of the second phase, the sectors requiring a more in-depth study with theimplementation of place of a hydraulic model to characterize the hazard.
The knowledge of the functioning of the rivers relied mainly on the collection of historical
data and hydrogeomorphological analysis:- the historical data identified and analyzed emanated from different sources: datatransmitted by the communes, data from the testimonies of residents, data owned by the
DDTM or data present in existing studies,- the hydrogeomorphological analysis allowed to determine the maximum expansion fieldof the flood and to retranscribe the hydrodynamism of the watercourse (minor bed, middle
bed, major bed, secondary flow axes ...).
The meeting of the communes, the field expertise and the hydrogeomorphological analysis
made it possible to determine the sectors treated by hydraulic modeling, the choice of themodeled sectors depending notably on the stakes located in the hydrogeomorphologicalflood envelope.
For sectors subject to modeling, a hydrological analysis has been carried out in order todetermine the characteristic flows and the reference flow rate to be taken into account(centennial flow rate or flow rate of a historical flood in the event that the latter is greaterthan the centennial flow).
46

The municipal territory of Boujan-sur-Libron is mainly drained by the Libron and its
tributary Ardaillou.

47

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40 頁
1. THE LIBRON
1.1. GENERAL PRESENTATION OF THE RIVER BASIN
1.1.1. GEOGRAPHIC CHARACTERISTICS
With a total area of 236 km², the Libron catchment area is oriented North-Northwest to
Southeast. Its length is of the order of 40 km. The highest point of the watershed is located500 m above sea level. With a relatively elongated shape, its slope is very steep up to
Laurens (greater than 2%) and softens over the rest of the linear (approximately 0.4%) to
the outlet at sea from the river at Vias. It is also from Laurens he takes the name of Libron
and is fed by several tributaries.

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Watershed of Libron
In its upstream end, the Libron crosses natural formations composed mainly of leafy trees(green oak, pubescent ...). Part of the rain is therefore intercepted by this relatively dense
vegetation.
But very quickly the vegetation becomes less present. Only the middle bed of the Libron is
occupied by dense vegetation, constituting the riparian forest of the river. The space is then49

occupied by agricultural activities, mainly vineyards and orchards.
41 頁
1.1.2. WEATHER CONDITIONS
1.1.2.1. TYPE OF CLIMATEThe watershed is subject to a Mediterranean climate characterized by its summer drought.
Summers are hot and winters are mild and lightly watered. The rainfall pattern is bimodal
with rainfall distributed mainly in autumn and spring. However, there are important interannual variations between very dry years and very rainy years. The most intense rainfall
events usually occur in autumn. These are brief and violent thunderstorms favored by the
upstream reliefs that block the air masses concentrating on certain sectors very large
quantities of water. Significant rainfall can also occur in winter, as evidenced by the floodsof January 1994 and 1995, or spring (floods of May 1992, 1999 and 2008), or even insummer, during which time nearly 10% of Extreme rainfall (rains> 100 mm / day) wasrecorded in the Hérault for the period 1958-2012.
1.1.2.2. RAINFALL
It is relatively homogeneous throughout the watershed:

in the upper basin, the annual rainfall is between 800 and 1000 mm,

on the average valley and low plain, annual values are of the order of 600 to 700 mm(Source: Météo France)

1.1.3. GEOLOGICAL AND GEOMORPHOLOGICAL DESCRIPTION OF THE BASIN
The upstream part of the Libron watershed extends to the eastern end of the southernslopes of the Montagne Noire, within a formation called the scales of Cabrières.
At this point the limestone substrate dates from the Devonian. The stratigraphic series are:
a dolomitic series,

limestones with siliceous intercalations,versicolored limestones, which are massive and composed of very fine grains of veryvariable color,

nodular limestones.

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42 頁

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Until Laurens, the Libron flows mainly in a formation of Flysch. These are syn-orogenic
rock formations (formed at the same time as the mountain ranges) formed by alternating
sandstone benches at the base and schists upward from the formation. They are formed bydissolving rock fragments torn from mountains in peripheral seas: they are turbidites.
Downstream of Laurens, and as far as Lieuran-les-Beziers, the Libron incises a substrate51

composed of marine molasse (succession of sandstone at the base and clays at the top)
dating from Middle Miocene.
43 頁
In its downstream section, the Libron abuts volcanic outcrops much less weatherable. A
million years ago, the sea covered the entire littoral zone. The earth's crust, a little less thickat this place, lets out some gases and incandescent materials. And then, it is the explosion:
the chimney of the volcano at 100-500 meters under the surface is invaded by the sea waterthat heats up and vaporizes creating a gigantic geyser. It is in a cloud of steam that lava
fragments associated with a cloud of dust rise more than 1,000 meters away. Slag, lava dust
exploded, agglomerate under the stratifying water found in both Cap d'Agde and Vias where
they are called "Bosc cinerites ".
These volcanic reliefs have the impact of a narrowing of the alluvial plain to the right ofVias, playing a role of natural lock. The significant extension of the plain immediately
upstream of this tightening would also be one of the consequences of this geological context.
The geology in the Libron watershed is characterized by a predominance of limestone. This
type of substrate is particularly alterable (by dissolution) and permeable, the waterinterfering in the subsoil through numerous cracks. This partly explains the absence of
exotic flows, in favor of inferred-flow (underground flows). The volcanic rocks in its
downstream part are on the contrary very little alterable and constitute natural obstacles
determining the route of the Libron. In addition, these formations are characterized bysignificant impermeability, favoring runoff.

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45 頁
1.2. HYDROGEOMORPHOLOGICAL ANALYSIS
From its source to its outlet in the Mediterranean, 8 homogeneous sections can be observedon the line of about 44 km traveled by the Libron.
Naubine Creek, Gournautuc Creek, and Mas d'Azil Creek, Mayroune Creek, Badeaussou
Creek, Riels Creek, Sèbe Creek, del Basth and Combes, Vinassac Creek, Garenne Creek,
Aire Creek and Ardaillou Creek are the major tributaries of the Libron and have also beenstudied.

54

46 頁

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作成。
Hydrogeomorphological approach - watercourses analyzed
47 頁
1.2.1. SECTION No. 1: FROM THE SOURCE TO RD13 DEPARTMENT ROUTE (1KM)
The Libron takes shape at 265 m altitude in the Regional Natural Park of Haut Languedoc;
it is called Sauvanès and flows into a very narrow V valley where the slopes are very
pronounced. The slopes structure the alluvial plain and bring more or less importantmaterials into the bed.
At only one kilometer from its source, the Libron already encounters significant
anthropogenic obstacles. Two huge infrastructure embankments support the RD13 road andthe railway. The Libron is then forced to borrow a nozzle, certainly undersized for an
extreme flood. The flows are thus strongly disturbed from the upstream.

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）

Busload of the Libron from its upstream part (upstream / downstream)
1.2.2. SECTION 2: FROM THE RD13 DEPARTMENTAL ROUTE TO LAURENS (3.4
KM)
Up to Laurens, a succession of slope breaks alternates between limited overflows and small
areas of expansion. Globally, the plain of Libron widens significantly, fed by thecontributions of many small tributaries. The minor bed is well marked, may be very wide inplaces, but still remains dry in low water regime.

56

（著作権の保護のため、挿入されている写真を削除しています。
）

Libron minor bed in an area with poorly maintained slopes significant hold with formationof a small natural water reservoir and a desiccation crust（48 頁）
About 1.5 km upstream of Laurens, the Libron changes radically orientation, blocked by amassive dolomitic barrier (The Causse). It then goes to the South-West by describing
meanders inscribed before incising again to the village of Laurens.
Crossings of the Libron are of the type rafting on this section. Only a municipal road and an
embankment can disturb the flow.
Few issues are identified in this sector. Some recent constructions on the right bank,resulting from the urban development of Laurens, are localized in bed major.

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）

Recent dwellings in floodplains upstream of Laurens
1.2.3. SECTION 3: LAURENS CROSSING (800 M)
The first urban area crossed is at the confluence of Naubine, Libron and a small tributaryleft bank. These streams are in the form of fully artificial channels. Their concrete bed
promotes the rapid evacuation of floodwater which allows to reduce slightly the risk at thevillage level. On the other hand, floods will be more devastating downstream of thisanthropised sector; in fact, these developments induce downstream higher velocities(because of a reduced roughness) and influx of water in greater quantities.
This relatively long concreted linear also has morphological impacts. The Libron's tendency
57

towards an equilibrium profile leads to regressive erosion upstream of the anthropised
section, and progressive erosion downstream.
The streams are completely integrated into the urban landscape. The crossing structures aresubmersible and the minor beds act as parking (photo left below). The banks are artificial on
most of the linear (rip-rap or retaining walls).
49 頁
Many buildings are very vulnerable and limit overflows on the left bank, carrying them tothe right bank. These homes are the first victims of the waves, which can be devastating anddeadly as evidenced by past events ("In the village of Laurens, eight houses collapsed"
September 26, 1857- Floods in France from the sixth century to the present day M.
Champion, Paris, Dunod, 1858-1864). Some of them have been recently renovated andtheir first floor is now habitable (photo right below). The risk remains very important toLaurens given the multiplicity of issues present around the minor bed.

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）

Minor bed serving as parking. Houses on the left bank very vulnerable.
Immediately downstream of the sector, the bed section is diminished by the presence ofgardens and outbuildings established on embankments. These developments still disturb
the flow, before the Libron flows more naturally. Once this urban crossing completed, an
average bed stands out very clearly and the plain manages to be structured.
1.2.4. SECTION 4: FROM LAURENS TO THE BEGINNING OF GRÉZAN CASTLE(2.3 KM)
On this stretch the Libron valley is relatively well structured, with a clear demarcation of thedifferent beds. The appearance of an average bed and bank erosion areas testify to asustained hydrodynamism, resulting in part from the phenomena induced by the crossing ofLaurens but also the presence of fine and mobilisable material. The route of the Libron isrelatively straight, the casing being less restrictive.
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The Libron perceives the contributions of the Gournautuc creek, an important tributary on
the right bank. The confluence zone is disturbed by the presence of the RD909 roadembankment which blocks the valley of this tributary at its outlet.
The stakes are very limited on this section. The presence of activities in major beds could
lead to the mobilization of a significant amount of materials. Some vineyards are also
vulnerable, some are bordered by low walls.
50 頁
1.2.5. SECTION 5: FROM THE GRÉZAN CASTLE TO LIEURAN-LES-BÉZIERS (13.2
KM)
This homogeneous section is relatively long; it represents a linear of about 13 km. It ischaracterized by an alternation of sinuous paths (weakening of the slope) and straight lines(when the slope increases slightly). Thus, on certain sectors, the Libron describes meanders,sometimes inscribed, sometimes free, sometimes even intersected. These intersections favor
an increase of the slope, which induces a more rectilinear layout. Old terraces line the main
bed and show the past activity of the watercourse. These formations tend to evolve
downstream, whether registered or free, as illustrated in the figure below.

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Evolutionary dynamics of a free meander(Source: Landform shapes, DERRUAUX, 1996)

59

At Puimisson's right, the different meander phases are clearly observable（著作権の保護のため、挿入されている写真を削除しています。
）

1: Before crossover = overflow point and discharge arm that tend to short the meander.
Erosion begins to increase inside the meander. In the long run the Libron will borrow what
is today a preferential axis. Note that here the meander has already been intersected. It
passed the limit of the current major bed, the former flow channel of Libron being today adischarge arm.
51 頁
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）

2: Meandering cut = the action of the man accelerated things in this place by therecalibration of the new bed and the reduction of the old bed (embankment). But the Libron
would naturally cross this meander. The plots in the lobe are very vulnerable, judging by the
number and size of pebbles on the ground.

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）

3: The old channel is in the form of a cut meander called Ox-Bow (a form of beef collar). It
is reactivated during extreme events.
Hydrodynamics is very strong throughout the section. The presence of overflow points,
outburst arms, bank erosion ... testify to intense hydrodynamic activity in extreme
60

conditions. The average bed is very clear, well demarcated from the major bed by slopes
several meters high (or even decametric in places). The dynamics of the Libron is very
intense on this section.
In addition to the erosive dynamics, several accumulation zones are identifiable. Landings
are formed where the slope decreases. These banks, if they are difficult to mobilize, can haveconsequences on the flows: modification of the layout of the minor bed, facilitatedoverflows, increased risks in case of stakes ...
52 頁（著作権の保護のため、挿入されている写真を削除しています。
）

Bank erosion, landing formation and diversion of the Libron bed
The stakes are few on this stretch. In the municipality of Magalas, at the Villa Sainte-Pauleright, two houses on the left bank are located at the edge of the middle bed and are thus
vulnerable. Several other recent constructions are at the edge of a flood zone, locatedastride an old Libron terrace and its current alluvial plain.
The Magalas wastewater treatment plant is also vulnerable, located in a confluence zone
where hydrodynamism is extreme during floods. The lagoon basin is also located in a floodzone but it was built on embankment.
Finally some drilling along the section is threatened, either directly by the overflows of the
Libron, or indirectly by bank erosion and jeopardizing the works. Shore comforting was
also necessary to limit the damage.
Upon arrival at Lieuran-les-Béziers, the Libron is less winding. A large expansion zone
allows the water to be stored during extreme floods, and then the stream cuts 500 m
upstream of the village, indicating a specific increase in the slope.
1.2.6. SECTION 6: FROM LIEURAN-LES-BÉZIERS TO BOUJAN-SUR-LIBRON (7.761

KM)
On this stretch the slopes are becoming weaker. The Libron then roams on an alluvial floor
growing wider, reaching more than 400 m in places, and describes light meanders free.
Upstream of Boujan-sur-Libron, it turns abruptly towards the East thanks to a more binding
geological change.
Hydrodynamics remains very strong during floods. A minor / medium bed is delimited byhigh slopes all along the linear.
On this section the stakes are relatively numerous. Much of the village of Lieuranles-Béziers
is very vulnerable, both by the overflows of the Libron and by the contributions of atributary left bank Creek Riels. At this point, the alluvial plain widens abruptly but remains
unclear, because of its interlocking in the formation of colluvium. Many homes as well as
businesses and public buildings (town hall, local municipal, ...) are installed in flood zone on
the left bank. At the village square, most dwellings have cofferdams more than 50 cm high,indicating frequent floods.
53 頁
On the right bank of the recent constructions, installed on embankments, as well as older
buildings are also present in bed major. The crossing of the Libron is a bridge with several
large arches, unlike the vast majority of works previously encountered which were
submersibles or rafts. The importance of this one lets imagine the heights of water that canbe reached at Lieuran-les-Béziers.

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Lieuran-les-Béziers Bridge and stakes located on the left bankAt the exit of the village, towards the road RD15, several buildings are also vulnerable. Someof them are flooded by a small tributary left bank channeled, whose flows are strongly
disturbed by the presence of a transverse SNCF embankment. The football stadium and thetennis courts are present upstream of the same infrastructure embankment, at the62confluence with the small tributary which is again blocked by the embankment of the RD15.
The anthropogenic elements disturbing the flow begin to be very numerous within an
alluvial plain more and more wide and flat. It is also a big break with the previous sectors
upstream, and we will find this type of disruptive development until the outlet of the Libron.
At the exit of this urban area, an equestrian center and two properties are vulnerable. Two
purification stations are too; one of them is located at the confluence between the Libron
and the Vinassac, while the other seems less vulnerable, in limit of major bed.
The overflows can extend up to the area of the Castle of Ribaute, without affecting thebuildings, built on the edge of the flood zone.
At the Arnoye domain, the physical context limits the overflows, which are then reported on
the right bank. Only the lowest buildings in this area are vulnerable.
At the entrance to Boujan-sur-Libron, on the left bank, corporate buildings and dwellings
have recently been built at the foot of a steep slope, to which the overflows of the Libron
extend. To the right of this sector is the cooperative cellar, which is also vulnerable. The risk
is increased in this sector by the presence of a huge transverse embankment, formerly built
for crossing the Libron. Today the infrastructure is no longer used and buildings come tooccupy the unsinkable space. The impact of this development is far from negligible. The
section is greatly reduced and the overflows are larger upstream, making buildings recently
installed on the right bank even more vulnerable.
54 頁
Globally, few issues are identified in Boujan-sur-Libron, the urban extension being mainly
realized on the other side of the village. Nevertheless some sports facilities (stadium, tennis
courts, sports facilities ...) have settled in the alluvial plain and new buildings are beginningto build (example: the cultural and sports center of the town hall under construction).
It is at the end of this section that the Libron begins to be dammed; the overflows are thenlimited by very high artificial slopes and its route is then completely anthropised. The risk
remains nevertheless very present, in particular during break of dike.
1.2.7. SECTION No. 7: FROM BOUJAN-SUR-LIBRON TO VIAS (13.9 KM)
63

One of the characteristics of this section is the progressive widening of the alluvial plain of
Libron, which reaches in places impressive proportions; overflows can indeed extend over akilometer or even nearly 3 kilometers at Saint-Privat.
From the RN9 road, the Libron receives inputs from the Ardaillou stream and the Baume
stream, two very important tributaries on the left bank that drain a significant area of thewatershed and participate in the expansion of the Libron.
About 6 kilometers from its mouth, the Libron changes direction again. It then heads south
to the sea, blocked in the east by reliefs where settled the Castle of the Gardie and the city ofVias. These formations, of volcanic nature and therefore little alterable, leave no otheralternative to the Libron; the only outlet to reach the sea is then between Vias andMadeilhan (eastern end of the formation of Terres Nègres). These basaltic formations also
form a tightening of the valley, which limits the overflows at this level but also is one of thecauses of the extent of overflows upstream of this sector (dam effect).
The extremely low slopes on this section explain the extent of flooding in this downstream
sector, combined with an alluvial floor which is also very flat and has a very large drained
catchment area. Many anthropogenic works are intended to contain the vagaries of the
Libron (lateral and transversal dikes, embankments, channelization of major rivers, etc.).
Nevertheless, traces of current or past floods are still visible. Many relief arms (left photo
below), major bed depressions, erosion of banks or dykes ... show an increased flood
intensity.
Many overflow points are present, mainly located on the right bank and identifiable by aweakening of the dike of Libron. Testimonies report that during breaks in this dike to theright of these overflow points, there may be a wave that takes the land and layer the vines ortears the fruit trees (photo right below). Overflows can also occur where the dyke isnaturally cut, at confluences with some small streams (Montimas, Les Redonnières...) that
do not allow a continuity of its lateral dike. These areas may be the weak spots where
overflows will occur during extreme events.

64

55 頁
（著作権の保護のため、挿入されている写真を削除しています。
）

Secondary flow shaft and auxiliary discharge arm（著作権の保護のため、挿入されている写真を削除しています。
）

Orchard affected by a dike break (trees washed away)
The alluvial plain is very important in this downstream section, but the stakes are very
limited. They are mostly represented by crops: vines, orchards, cornfields or sunflowers ...
Some buildings are vulnerable; the lower part of the domain of Saint-Jeandu- Libron, thedomain of Saint-Bauzille, Saint-Privat, La Jourdane, and the buildings at the place called
Médeilhan. Some of them are nevertheless elevated, or protected by a dike.
1.2.8. SECTION 8: FROM VIAS TO THE MEDITERRANEAN (1.8 KM)
This downstream section corresponds to the Libron outlet. Its alluvial plain is then confusedwith that of Hérault and Orb, making this part of the territory very vulnerable. The sector
being very flat, and the mechanical actions of the sea being felt, the "major bed" extends on
a broad band about 2 kilometers since the littoral. Water levels can be important locally.
The Libron is still dammed up to its outlet. This arrangement makes it possible toconcentrate the flows and to evacuate the rainwater more quickly towards the sea. Before
the construction of these lateral dams and the creation of an artificial outlet, the Libron was
blocked by an important dune cord; the waters were then stored behind this natural barrier,forming lagoons, and then heading west to the old outlet east of Portiragnes Plage. The
waters were evacuated to the sea by what is now called the old Grau du Libron, now a largehydromorphic area.

65

The Midi Canal and its embankment strongly disrupt the flow. A consequent structure,
made up of several locks, allows the Libron to cross the structure without damaging it(photo below), and limits the sediment input upstream of the structure. The stakes are very
numerous in this plain. downstream, mainly related to tourism activity.
56 頁（著作権の保護のため、挿入されている写真を削除しています。
）

Work for the crossing of the Midi Canal
1.2.9. MAIN TRIBUTARY OF THE LIBRON ON BOUJAN-SUR-LIBRON: THE CREEKOF ARDAILLOUThe Ardaillou stream has its source in the village of Bassan and flows south to open on theleft bank of the Libron. The slopes are generally weak and the valley remains open on all the
linear. The Ardaillou is in the form of a very deep bed (soft formations) and its alluvial plainis structured quickly, fed by many tributaries. Its major bed is relatively large mid-term, theoverflows may spread over the entire alluvial floor. Hydrodynamism is particularly marked in
the downstream section, downstream of the RN9 road. The plain is very clear in this sector,
bounded by steep slopes on the left bank and by very incised colluvium (embankment morethan 3 m high) on the right bank.
1.3. HISTORICAL ANALYSIS ON BOUJAN-SUR-LIBRONIn addition to the information collected from the commune, the analysis of various historical
documents and hydraulic studies attest to the historical floods of Libron occurring in 1907,
1956, 1964, 1996, known as the most important of memory of man and 1999. In addition tothese historical floods, that of November 2014, although not the subject of recognition of
the state of natural disaster in the town of Boujan-sur-Libron.
In general, in case of flood, the Libron regularly overflows in the vineyards. During the floodof 1907, the water rose to the village houses on the right bank of the Libron. A house,located behind the stadiums, had between 50 and 60 cm of water (cellar on the groundfloor).
66

57 頁"[...] The floods are sudden and easily reach 6 meters and even 8 meters above the bed of
the torrent. We will quote in modern times that of 1856, which caused 60,000 francs ofdamage to Lieuran. More recently, in 1907, the Libron caused 80,000 francs of damage toLaurens and 60,000 to Boujan. On that date he took the metal bridge of the Béziers
railway at Méze, which was 31 meters long, however.
All the roads of the properties downstream of Boujan were covered, and some of them
resisted the flooding. The castle of Saint-Jean-de-Libron was surrounded by water,
although 600 meters separate the constructions of the river. The whole plain of Jourdanne
and Vias was flooded. In October 1913, a new flood carried the national road from Béziers
to Pézenas in two different points. One of the levees that channels the Libron around the
Jourdanne was covered by water and half carried away. Our wadi creates an artificial bed
in the vineyards of the Count of Cassagne. The last houses of Vias were invaded by thewaters. [...] >> Geography Bulletin of Languedoc-Le Libron - J. COULOUMA Doctor of
Pharmacy, Bachelor of Science, 1929

It is a landscape of desolation that stands at the Domaine de Libouriac: the vines aredestroyed and the mutes cut※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）57 頁より作成。
Extract from the collection "The floods of 1907 in the Hérault" (Source: IPSEAU 2010
study)67

58 頁
The Little Southern - September 27, 1907
"Béziers, September 26, 1907
At the start of Béziers, the ground is almost dry, it
has more little around our city, but in Boujan the
Libron overflowed, the plains of Libouriac andBoujan are submerged, a thick mud covers thevines, everything is almost lost, the mostexperienced is our friend M. Castelbon deBeauxhostes, our friendly Maecenas. The
population is on the embankment along therailroad track, many owners lament, it is a new
ruin for our valiant farmers. On the road, a cart
harnessed with three horses was swept away by
the torrent, the driver was able to save himself ona tree, the three horses died. >>The Flash – November 9, 1907<< A BOUJAN
Boujan, November 8, 1907
On the night of Wednesday to Thursday, a stormof unheard-of violence unleashed on our locality,
quickly transforming our streets into real torrents
and flooding, in the low quarters of the village, apart of the houses.
As a result of this torrential rain, the smallest
streams have swollen and overflowed.
The Libron is out of bed and pours silty waters into
the riparian lands. The plain is no more than animmense lake, the stumps are entirely covered by
the liquid layer.
The railway bridge of the Local Interest, built on
the Libron, was demolished by the waters. Trains
do not circulate. We are also told that, for a
distance of about 80 kilometers, the road to
Pézenas has been cut off by the waters. This
morning, the rain continues to fall, the thunderresumes its terrible bearings and the weather
remains threatening. >><<The road to Pezenas ravaged
The Libron ravaged the road to Pézenas, near the
Domaine of Libouriac, over a length of 80 meters,
the path was dug to a great depth, with huge
excavations. The bridge was taken away and the
Libron has created a new >>68

The Lightning - November 12, 1907<<A BOLJAN
Boujan, November 11th.
Last Saturday, around 6. from the night, the Libron isstill out of bed for the second time, causing further
damage. It is anxiously queried when this period of
bad weather ends.
We need a correction to our article last Saturday. It ison a course of about 80 meters, and not 80 kilometers,
as it was said by mistake, that the road to Pezénas was
cut near the area of Libouriac, by the storm of water
fell on the night of November 6th or 7th. Our readers
had already corrected themselves. >>※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）58 頁より作成。
Articles of the Little Southern and Lightning - September 1907

59 頁
During the floods of 1964, some damage affect the infrastructures, breaks of bridges anddikes are noted.
In Boujan-sur-Libron
Here again, we had not seen it since 1907.
The water went up to the cooperative cellar.
The pillars of the old railway bridge sunk 6
meters deep left like straw fetuses. In thevineyards, the stumps were torn out by
hundreds and buried in the silt. Among themost affected properties are those of MM.
Gaujal and Castelbon de Beauxhostes. At theArnoy estate, the Guilhem family had to be
evacuated, the waters reaching the first floorof their home.
The Free Midi - Friday, October 2, 1964

The Free Midi - Friday, October 2, 1964
In Boujan-sur-Libron
Here again, we have never seen this since 1907. The
water has risen to the cooperative cellar,
The pillars of the old bridge of road sunk at 6 meters
deep are gone like straw fetuses.
In the vineyards, the stumps were ripped out by
hundreds and spread under the silt.
Among the most affected properties are those of MM.
Gaujal and Castelbon de Beauxhostes.
At the Arnoy estate, the Guilhem family had to be
evacuated, the waters reaching the first floor of their
home.
TAKEN IN THE OVERFLOW OF LIBRONROUTE OF PÉZENAS
A motorist and his family had to spend two hours
on a tree and a truck driver on the roof of his cabin

69

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）59 頁より作成。
Article of the Free Midi of 02/10/1964
60 頁
TORRENTS OF WATER ON BITERROISThe Marseillaise - Friday, October 2, 1964
A 5 ton truck carried away[...]
On the road Béziers - Pézenas a heavy weight was
carried away in a vineyard while four motorists had
to take refuge in a tree where the firemen came toclear them [...]
A doctor from Donzère-Mondragon, Dr. Jean
Magnen has known tragic hours not far from
Béziers. On the road Pézenas-Béziers his<<404>> was stopped near Libron-Séjour, theengine drowned.
The flooded Libron was breaking on the road. The
car was dragged by the waves. Fortunately, she
stopped against a tree on the edge of the low side.
The flood became so large that the water reached
the inside of the car, threatening to drag it into thetorrent. His occupants, the doctor, his wife and twochildren, a boy and a girl from 5 to 7 years, wereable to operate the sunroof and take refuge on the
bodywork. It was then that the driver of a truckcompany Laurent de Perpignan, tried to help themby pushing the car with his truck. He was in turnimmobilized. The water reached the roof of the car.
The doctor and his family had to take refuge in the
branches of the tree where the firemen came torescue them as soon as the alert was given. [..] >>※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）60 頁より作成。
Article from the newspaper La Marseillaise, 02/10/1964
During the flood of 1996, the water reached the RD15, north of the village, on the rightbank of the Libron. There was about 10 cm of water on the road. The road was cut for about
10 days. The water was spread over a width of 500 meters. On the left bank, it reaches thedump The white bridge has never been submerged. Further downstream, on the right bank,the waters of the Libron arrived in the midst of vineyards. There were ravines in farmland,
vineyards and fields.

70

The flood of 1999 is described as more modest than that of 1996, the Libron invading only
agricultural land.
The following flood markings have been identified in the municipality of Boujan-sur-Libron:
Location

Raw dates

Mazets30/09/1964 ? - Mazet 1

PHE DDE

42.92

upstream

04/09/1964 ? - Mazet 2

PHE DDE

42.66

30/09/1964 ? - Mazet 2

PHE CG

42,96

RD15E2

SourceRating (m NGF)

Hérault
Downstream

29/01/1996 ? - Mazet 2

PHE CG Hérault

41.96

29/01/1996

PHE CG Hérault

42.42

treatment
plant
CD15E2(right bank)※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）60 頁より作成。
61 頁
Also worth mentioning outside but close to the municipal area of Boujan-sur-Libron:• A benchmark of the 1996 flood (source: testimony of M COSTA, Director of SIGAL)
is located at the intersection of the RD15 and the Servian road, on the tree trunk, at acoastline of about 45, 50 m NGF. The high level is explained in particular by the study
carried out by the presence of the embankment of the old railway which blocked theflows.


Flood marks on the pumping station building of the Domaine of Libouriac (communeof Béziers).

The various decrees of natural disaster recorded on the commune since 1982, year of
establishment of the CATNAT system, are presented in the table below.
Type of disaster

Beginning on

End

Stop ofOn the JO of

Storm

06/11/1982

10/11/1982

18/11/1982

19/11/1982

Floods and mudslides

13/10/1986

17/10/1986

27/01/1987

14/02/1987

Floods and mudslides

13/12/1987

14/12/1987

07/04/1988

21/04/1988

Landslide and ground

28/10/1993

01/11/1993

08/03/1994

24/03/1994

71

collapse
Floods and mudslides

28/10/1993

03/11/1993

08/03/1994

24/03/1994

Floods and mudslides

17/10/1994

28/10/1994

12/01/1995

31/01/1995

Floods and mudslides

28/01/1996

30/01/1996

02/02/1996

03/02/1996※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）61 頁より作成。
1.4. HYDROLOGICAL ANALYSIS
1.4.1. SYNTHESIS OF PREVIOUS DOCUMENTS OR STUDIES
1.4.1.1. HYDRAULIC STUDY OF THE LIBRON VALLEY - SOGREAH – 1968
The purpose of this study is an analysis of flood flow conditions, an estimation of thecharacteristic flood flows and capacities of the minor bed, and proposals for hydraulicdevelopment.
The characteristic flows were estimated by comparison with the department's watersheds(Orb, Lez, Lirou, Bérange), taking into account the rainfall intensities observed on each ofthem.
The period of return of the flood of 1964 is estimated at 100 years, even superior to Magalasand Lieuran.
1.4.1.2. LAURENS LIBRARY VALLEY HYDRAULIC LAYOUT DIAGRAM – RN9 - BRL
AND BCEOM – 1988
The study area is the Libron watershed. The linear studied extends from Laurens to the sea,
about 45 km.
The rainfall data used are the available data for the Laurens and Béziers substations. At the
Béziers substation, only the decennial daily rainfall is known, the other values are
determined by applying the same ratios as those of Laurens.
62 頁
Characteristic peak flows were determined using three methods: the Crupedix method, therational method using the gradex method to estimate the centennial flow rate, and the Soil
Conservation Service (SCS) method. The results obtained were compared and criticized in72

order to retain the most representative hydrographs of the hydraulic functioning of thewatershed.
The method chosen is the SCS method, which takes better account of the damping effects ofthe flood wave especially downstream of Magalas. The hydrographs obtained for the
upstream sub-basin (up to Magalas) and the downstream sub-basin (up to the sea) werecomposed and flattened from upstream to downstream to take into account the flood peakattenuation. Time offsets have also been applied to best represent flood propagation andconcomitances on the watershed.
The flood of 1964 was analyzed during this study. The previous method was applied withthe characteristics of the rainy episode of September 30, 1964. The period of return of thisflood is therefore of the order of 100 years in Magalas and between 50 and 100 years on thedownstream basin where the rains were less strong. The flows are close to those of the
Sogreah study from 1968 to Magalas, but much lower downstream. The attenuation of thepeak flow seems more in line with reality, particularly given the very long shape of thewatershed.
1.4.1.3. STATISTICAL TREATMENT OF THE DATA OF THE PLUVIOGRAPHICSTATION OF BÉZIERS LA COURTADE - BCEOM - 1997
The purpose of this study is the statistical treatment of rainfall data from the Béziers la
Courtade substation. Between 1980 and 1994, only 8 years of observations are complete.
Despite the small size of the exploitable samples, statistical adjustments of this station were
calculated and compared with those of the Montpellier Bel Air stations (in service from1920 to 1971) and Montpellier Fréjorgues (in service since 1957) on different time steps( 30 minutes to 48 hours) and return periods.
The conclusions of the analysis of Béziers la Courtade are as follows:• the observation period of the post is too low to draw specific conclusions about therainfall in the Béziers region,• the distortion within the sample makes adjustment by usual statistical laws difficult forperiods of less than 12 hours,• in conclusion, the quantiles calculated on this station do not allow to revise upwards thequantiles usually taken into account (quantiles of Montpellier Bel Air) in thehydrological calculations on the Béziers region.
731.4.1.4. HYDRAULIC STUDY OF THE A75 MOTORWAY - PÉZENAS / BÉZIERSSECTION - HYDRATEC - 2004
The purpose of this study is to design hydraulic structures for crossing the sevenwatercourses crossed by the future A75 between the Pezenas exit and the Béziers entrance,
including the Libron and Ardaillou. Since none of the streams are equipped with a flow
measurement station, flow estimates are made using empirical formulas.
The rainfall data used are those of the Montpellier-Fréjorgues substation for small time
steps, corrected by the ratio of daily rainfall on the Pézenas, Servian and Gabian substations(+25%).
63 頁
The concentration times selected are the average of the results of the empirical formulas.
The runoff coefficients are 0.6 for the decadal flood and 0.8 for the centennial flood.
The centennial flow is calculated by the rational method and the gradex method. The
proposed flow is 460 m3 / s. This value is greater than the value of the 1988 BRL study, but
it is retained on the one hand to be placed in the high value of the gradex method range, in asafer way, on the other hand because it is consistent with the flows obtained on neighboringwatersheds, notably the Thongue, whose values are close to those of the Sogreah study.
1.4.1.5. FLOODING ZONES OF THE LIBRON IN THE CROSSING OF BOUJAN-SURLIBRON - HYDRAULIC STUDY - AQUA CONSEIL / ENTECH - APRIL 2010
This study, conducted by the municipality of Boujan-sur-Libron, aims to determine the flow
conditions and flood areas of the Libron to the right of Boujan-sur-Libron.
It is based on a hydrological study to determine the volumes and flows of the 100-year flood(rainfall-flow modeling) and on hydraulic modeling of the Libron valley at the right of thestudy area.
Topographic surveys were conducted as part of this study (cross-sections) in addition to
those conducted as part of the PPRi study. The modeling of this sector was carried out using
HEC-RAS software. The centennial flow of the Libron to the right of Boujan-sur-Libron isestimated at 473 m3 / s.
1.4.1.6. SUMMARIES OF RESULTS OF PREVIOUS STUDIES74

Watershed

Study nameQ10

method(m³/s)
LibronAqua Conseil

Q100

methodFlood
1962(m³/s)

Boujan

216

SCS

473

SCS

HYDRATEC

Downstream of

177.8

rational

373

rational

2004

Boujan215.2

crupedix468

gradex

94.6

SOCOSE460

gradex/ ENTECH2010

BRL 1988

Boujan

394

SCS

335

Sogreah 1968highway

295

SCS

280

230

460

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）63 頁より作成。
64 頁
1.4.2. EXISTING PLUVIOMETRIC AND HYDROMETRIC DATA
Several rain stations are located in or near the Libron catchment area and provide daily
rainfall values. Statistical adjustments give daily rainfall values for characteristic return
periods of 5, 10, 50 and 100 years:
PluvioSourceYears of

P5

P10

P50

P100

1970-2008

109.6

136.4

203.2

235

1955-2001

106.3

127.2

175.3

196.5

1990-2008

106.9

120.7

142.4

148.8

1961-1998

130

150

197

216

1961-1981

133

158

215

239

1961-1998

108

127

168

186

measurement
Béziers

Météo France (GEV law)
2010

St Genies

Météo France (GEV law)2010

MurvielMétéo France (GEV law)
2010

Bédarieux

MF synthesis (renewallaw) 1999

Cazouls

MF synthesis (renewallaw) 1999

ServianMF synthesis (renewallaw) 1999

GabianPOS Lieuran (Gumbel)> 25 years old

2001 study

1948
75

144

221

Pézènes

POS Lieuran (Gumbel)> 25 years old

2001 study

1966

164

204

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）64 頁より作成。
Characteristic daily rainfall values for each sub watershed concerned by the modeling were
calculated by performing a weighted average of the values of the nearby rainfall gauges,
according to the distances and altitudes of the stations.
These stations do not provide data for durations of less than 24 hours. Since theconcentration times of the watersheds studied are well below 24 h, we used rainfall datafrom the Montpellier Fréjorgues station to calculate the Montana coefficients over therequired time steps. For each watershed studied, these coefficients were weighted by theratio between the daily rainfall of the watershed concerned and that of Montpellier, in orderto obtain values representative of the sector considered.
Montana formula: h = a * t 1-b, with:h = height corresponding to the time step considered, in mm
t = no time in minutes
Rain duration from 6

Rain duration from 1 h to 6 hRain period from 6 am to 24 pm

Duration of

Duration ofminutes to 1 hour
Durationabof returnab

returnab

return

10 years

4.913

0.4

10 years

16.502

0.691

10 years

22.0110.748

100 years

7.043

0.385

100 years

26.8230.702

100 years

34.2

0.754

※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）64 頁より作成。
Coefficients of Montana calculated at Montpellier Fréjorgues station
No hydrometric station is present in the Libron catchment area. It is therefore not possible
to directly calculate reference flows by statistical analysis of hydrometric data. Only the
Magalas station (St Paul) was commissioned between 1970 and 1973. Unfortunately, theresults are unreliable (questionable validated flows) and the series remains too small toperform a statistical analysis.
65 頁
76

Empirical formulas were used to determine return flow rates Q5 (5 years), Q10 (10 years),
Q50 (50 years), and Q100 (100 years) at each confluence point. For the sake of consistency,
the methodology used here will be similar to that of the hydrological studies carried out
under other PPRI in the department (Lirou watershed, Thau lagoon watershed, etc.)（著作権の保護のため、挿入されている写真を削除しています。
）

Location of the subwatershed studied on Boujan-sur-Libron
66 頁
1.4.3. HYDROLOGY OF WATERSHEDS UP TO 10 KM 2
1.4.3.1. METHODOLOGY RETAINEDThe 5 and 10 year return period rates were calculated using the Crupedix method and the 50
and 100 year return period rates using the gradex method.
The gradex method is based on the assumption that for high return period rainfall, the basin
is saturated with water and any supplemental rainfall causes an equal flow supplement. Thismethod requires the determination of a pivot point, which corresponds to the return periodin years of the value of the precipitation from which the flow deficit no longer increases. The
pivot point will be taken equal to 10 years.
Decadal flow is calculated using the Crupedix method. From this pivot point, the gradex
method is applied. The gradex, a gradient of the extreme values of the rainfall heights, istaken for a duration equal to the base time, the duration corresponding to the average timeof the direct runoff hydrographs. Since this base time is not known for the small watersheds
studied (no hydrometric measurements), we will evaluate it with the Socose formula.
Name

Area

Slope

Q10

Q50

Q100

Specific

Exceptional

Q previous

BV

(km²)

(m/m)

(m3/s)(m3/s)(m3/s)

Q100(m3 / s)studies (m3(m3 / s/ s)

/ km²)
Libron

93.9

0.018

162

401.6

502.9

77

5.4

905.2

473/460/394

toBoujan※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）66 頁より作成。
1.4.3.2. VALIDATION OF RESULTS
The specific flows obtained were related to the flows calculated in the neighboringcatchment areas (Lirou, Haute Vallée de l'Hérault), for equivalent catchment areas andsimilar landforms.
67 頁※（COMMUNE DE BOUJAN-SUR-LIBRON, Préfecture de l'Hérault 2011）67 頁より作成。
Specific centennial flows for catchments> 10km²
1.4.4. CENTENNIAL DEBIT OF THE LIBRON TAKEN INTO ACCOUNT IN PPR I
As part of the hydraulic study "Floodplains of Libron in the crossing of Boujan on Libron,
hydraulic study, April 2010, Aqua Council / ENTECH", a topographic survey and ahydraulic modeling were carried out on this sector for the centennial flood , using the HEC
RAS model.

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The centennial flow taken into account is 473 m3 / s, which is significantly higher than thatof the 1988 BRL study (Q100 = 394 m3 / s, Q1964 = 325 m3 / s), but close to the centennial
flow rate of the study. Hydratec 2004 downstream from Boujan (Q100 = 460 m3 / s) andconsistent with the specific flows of neighboring watersheds.
This value is therefore consistent as a centennial rate. However, the water levels obtained in
the Aqua Conseil / ENTECH study prove to be significantly lower than the existing PHEs inthe municipality, particularly at the RD15E Bridge. The flood zone could therefore beunderestimated compared to the levels reached in 1964 and 1996.
Thus, for the sake of consistency with the rest of the watershed and due to the inaccuraciesof the historical flood lanes identified, the calculated centennial flow being greater than theflow of the historical floods, the PPRi in the municipality of Boujan-sur-Libron was carriedout by modeling the 100-year flood. The peak discharge of the 100-year hydrographretained under the PPRi is 502.9 m3 / s.
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1.5. HYDRAULIC MODELING ACHIEVED UNDER PPRI
1.5.1. MODEL USEDThe model used in this study is Infoworks RS. It is a mathematical simulation software that
reproduces and analyzes the functioning of rivers, canals, rivers, floodplains and estuaries.
Infoworks RS is used to model flow and water levels in a river, over a long period of time orat a small time scale. It was developed by Wallingford Software.
1.5.2. SETTING MODELSIn the absence of a hydrometric station on rivers, historical flood data cannot be used tocalibrate hydraulic models. Calibration of the models was therefore carried out thanks to thefield expertise which made it possible to estimate the roughness coefficients.
The roughness coefficients retained on the Libron are generally 15 in a minor bed andcomprised between 10 and 15 in a major bed consisting essentially of vine-type field /culture.
1.5.3. MODELING FLOWS
Modeling of the reference flood (100-year flood with a 100% Q100 flow rate of 502.9 m3 /79s) and field expertise enabled mapping of the flood zones for the reference flood. It allows
the drawing of iso-heights and iso-velocities for this same flood with as a result the
cartography of the hazard.
The calculated odds are infrequently lower than those recorded for the historical floods of1964 and 1996. The EGIS WATER study carried out in the framework of the PPRI explains
it by accidents and ice jams which occurred during these floods, which raised the waterlevels locally(pillars of the old railway bridge carried, breakages of embankments, ice jams).
These phenomena are not quantifiable and therefore not taken into account in modeling.
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2. Map Results
The mapping of the hazard is carried out differently depending on whether one is on amodeled area (implementation of a model), or on a sector where the flood zone has been
determined by the hydrogeomorphological approach supplemented by modelizations
hydraulics to the right of the stakes strong.
2.1. MODELED SECTORSThe mapping of the hazard on the modeled sectors is based on the following hazard grid,
resulting from the regional Languedoc-Roussillon doctrine of PPRI development.

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On the modeled sectors, one thus sees appearing in the envelope of the centennial floodzones in moderate hazard and areas in strong hazard.
80Beyond the envelope of the centennial flood but in the maximum extent of the flood definedby the hydrogeomorphological approach, the zone is in residual hazard.
2.2. NON-MODELED SECTORSFor the Libron catchment area, hydraulic modeling has been carried out in the areas ofstrong stakes. These vulnerable zones have in the past, often as a result of major floods,
experienced changes in the bed (rectifications or recalibrations of the minor bed change theflow conditions) to limit the risks resulting in the presence of a residual hazard sometimes
therefore.
The residual hazard is by definition the hazard that is greater than the reference hazard.
In addition, when the reference flood is a 100-year recurrence flood, based on a statisticalanalysis, there may be no consistency with the so-called rare flood determined by thehydrogeomorphological method. This method does not allow to precisely distinguish therecurrence of different ranges of floods. It only indicates different levels of hazards byfocusing on the dynamics of flood flows.
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Conditions Explaining the Presence of a Residual
Hazard Several problem areas have been the subject of hydraulic modeling. The modelingwas carried out for flow rates determined from rainfall data. These data made it possible tomodel the response of the watercourse to these rains and, coupled with topographic data, tocarry out a zoning of the hazard. The modeled flows are then generally lower than thehydrogeomorphological flows. When accurate information on the water depths reachedduring historical floods were available (flood markings) the models were calibrated on thesereferences.
Despite all the hydrogeomorphological envelope is in many sectors superior to thesecalculated hazards. This exceptional hazard corresponds to the maximum morphogenic
flood, which has occurred several times during the last tens of thousands of years(contemporary period at the scale of the watercourse). This difference can be explained byvarious factors:• human interventions on the conditions of water flows: acceleration of the flow rates
by concreteization of the bed which limit the heights of water,• surface embankments rendering some areas unsinkable,81• retention ponds to store runoff water,• dams or thresholds having a significant impact on the flows and their dynamics,• a tributary of a problem area that is part of the study area but has not been modeled.
On the other hand, some small tributaries have a valley with a flat bottom or on the contrarya very steep valley with steep slopes (torrential facies of the upstream parts). During heavy
rainfall, the creek overflows over the entire functional alluvial plain. In fact, the hazard inthese sectors will be considered strong.
3. Regulation
3.1. CONSTRUCTION OF THE REGULATORY CARD
3.1.1. HAZARDS
According to the methodology described in the first part of this report and explained above,the mapping distinguishes the high hazard sectors, the moderate hazard sectors, the residual
hazard sectors and the non-hazard sectors.
3.1.2. CHALLENGES
According to the methodology described in the first part of this report, the issues taken into
account in the municipality are of two types:•

areas with little or no urbanization,•

Urbanized areas defined on the basis of the existing physical reality.

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In the town of Boujan-sur-Libron, several issues were identified near the Libron, during
meetings with the commune and the recognition of land. From upstream to downstream:•

the cooperative cellar, located on the right bank, behind the D15 road,•the urban zone, made up of private dwellings, on the right bank, accessible by the
Old Station road,•sports facilities: Domenech sports stadium, bowling alley, tennis court, arena,
located on the right bank of the Libron,•a picnic area, on the left bank,•a detached house, on the left bank,•

The dump, on the left bank,•

The treatment plant on the left bank.

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（著作権の保護のため、挿入されている写真を削除しています。
）

Vines on the right bank of the Libron, downstream of the RD15E2（著作権の保護のため、挿入されている写真を削除しています。
）

Bridge RD15E2 and picnic area on the left bank
3.1.3. REGULATORY ZONING
Regulatory zoning is one of the vectors of risk prevention policy, which must guide urban
development outside risky sectors and reduce the vulnerability of existing or futurebuildings.
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The zoning must aim in particular to:•prohibit or limit very strictly construction in hazardous areas,•in urban areas, do not aggravate the stakes in areas of high risk.

By crossing the level of hazard and the nature of the issues, one obtains an estimate of therisk and the determination of zones of constraint useful to define the regulatory zoning.
3.1.3.1. CROSSING GRID OF THE FATE AND ISSUES
Alea
StrongModerate
ResidualissuesFlood by the reference flood.
Flood by the reference flood.

Strong (urban

Moderate

areas)(natural areas)

Red Ru danger

Red Rn danger

zone

zone

Buoyant Blue

Precautionary

Precaution Zone

area Red Rp

Hydrogeomorphological limit

Precautionary zone83

of the flood zone.
No

Z1

Beyond thePrecautionary zonehydrogeomorphological limit

Z2of the flood zone.
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3.1.3.2. SCOPE
As a preamble, it should be noted that the purpose of this paragraph is to syntheticallyexplain the principles that governed the drafting of the Boujan-Sur-Libron PPRIRegulation, to which the reader is invited to refer in order to know in an exhaustive manner
the rules applicable to each zone.
The urban planning rules applicable to new projects and to existing building modifications
are mandatory and apply imperatively to new projects, to any use or occupation of theground, as well as to the management of existing properties.
For each of the red, blue, gray and white areas, a body of rules has been established.
The regulation consists of several chapters relating to the different zones.
These chapters have two parts:•

ARE PROHIBITED that indicates prohibited activities and occupations,•

ARE ALLOWED to specify under which conditions activities and occupations may
be permitted.

In each of these chapters, the rules are intended to meet the main objectives, which
motivated the drafting of these requirements:•

safeguarding the inhabitants•

protection of existing goods

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Thus, according to the intensity of the hazards and the situation with regard to the stakes, 6
regulatory zones are distinguished. The prevention principles used are as follows:➢ Zone Rn, flood zone of high hazard in sector with moderate stake (non-urbanizedsector):84

Due to the danger, it is important not to introduce new issues (population, activities, ...).
The general principle associated in the regulation is the prohibition of any new
construction.
➢ The Ru zone, a flood zone of high hazard in a sector with high stakes (urbanized sector):
Due to the danger, it is important not to introduce new issues (population, activities, ...) byallowing a minimal evolution of the existing building to promote continuity of life andurban renewal.
The general principle associated in the regulation is the prohibition of any new
construction.
➢ The Rp zone, flood zone of moderate hazard and moderate stakes (non-urbanizedsectors):
This flood expansion zone should be preserved and any project that could aggravate orcause new projects should be prohibited. The general principle associated with theregulation is the prohibition of all new construction, but with provisions to ensure themaintenance and moderate development of agricultural development or construction.
➢ The Bu zone, moderate hazard flood zone in high-stakes sector (urban sectors):
In view of the existing urbanization, urban development that takes risk exposure into
account should be allowed through the implementation of constructive arrangements. The
general principle associated with the regulation is the possibility of carrying out new
developments and projects, excluding institutions of a strategic or vulnerable nature, undercertain requirements and conditions, including floor level.
➢ Zone Z1, zone not subject to the reference flood but potentially flooded by an exceptional
flood:
It is necessary to allow an urban development taking into account the exposure to risks,
generated by a flood higher than the reference flood, through the implementation of
constructive provisions.
The general principle associated in the by-law is the possibility of making new
developments and projects, with the exception of buildings of a strategic or vulnerable
character, under certain prescriptions and conditions of floor level (minimum 50 cm above
the natural terrain) , and subject to compensate for the waterproofing of soils so as not toaggravate the risk downstream.

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➢ Zone Z2, zone not subject to either the reference flood or an exceptional flood:
All new works and projects are authorized subject to compensating for the waterproofing ofsoils so as not to aggravate the risk downstream.
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4. BibliographyGuide for the elaboration of PPR in Languedoc-Roussillon - June 2003



Study defining the floodplain areas of the Libron catchment area - Boujan-Sur-Libron
Commune - EGIS Water - December 2012

5. Useful links
Website of the state services in the Hérault: http://www.herault.gouv.fr/
Website of the DREAL: http: //www.languedoc-roussillon.developpement-durable.gouv.fr/

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