The International Levee Handbook | Guide international sur les digues

français. L’ILH est un ambitieux projet international de guide couvrant l'ensemble des aspects relatifs aux digues de protection contre les inondations fluviales et les submersions marines.

anglais. Chapter 1 introduces the handbook and presents the motivation and process that led to its development. The chapter gives the reader an overview of its contents and explains how to use it. The chapter flow chart shows the conceptual links between the technical chapters that follow this introduction. It is repeated at the start of each chapter but is expanded to show more detail of the contents of that chapter. It indicates that the handbook is split into four major parts: - fundamentals – setting out what all users need to appreciate - managing levees – focusing on what managers of existing levees need to know - toolbox – providing detailed technical information (data equations etc) for use by all users - making changes – focusing on the needs of those involved in design and construction of new or improved levees. 1.1 Use of levees Levees are raised, predominantly earth, structures (also called dikes, digues or flood defence embankments) that are not reshaped under normal conditions by the action of waves and currents, whose primary objective is to provide protection against fluvial and coastal flood events along coasts, rivers and artificial waterways (Figure 1.1). Levees form part of flood defence systems that may also include flood walls, pumping stations, gates closure structures, natural features, and other associated structures. In many instances levees have been built up and extended over decades or sometimes centuries. Few of these were originally designed or constructed to modern standards and records of their construction and historical performance may not exist. Despite their apparent simplicity, levees can be surprisingly complex structures. They have generally been constructed by placing locally won fill material onto alluvial flood plains (with all their inherent natural variability). Unlike engineered structures, levees can be irregular in the standard and nature of their construction and can deteriorate markedly over time if they are not well maintained. Furthermore, levees are generally long linear structures that are part of an overall system. Such systems should be considered as chains that are only as strong as the weakest link. Evidence-based assessment, good design, effective adaptation, good inspection and routine maintenance are vital if levees (particularly those representing the weakest parts of levee systems) are to perform well on the occasions when they are loaded in storm or flood events. It should be noted that levees may stand for much of their lives without being loaded to their design capacity. This can create a false sense of security in the level of protection they will provide. 1.2 Background to the handbook Coastal and riverine flooding continues to produce devastating consequences, in both life and economic losses, around the world. With economic growth, urbanisation and the ensuing concentration of population and property, people are moving in increasing numbers to flood-prone areas in many countries. Where flood protection defences have been improved, have not been fully tested, or experience infrequent flooding, residents become complacent and less aware of the threat of floods. In such cases, they are hardly prepared for floods and by no means assured of proper actions to take, consequently suffering more serious damage once a flood occurs. Flood and storm events around the world continue to lead to critical flood defence failures resulting in tragic losses of life and the devastation of large areas. Also, levees have been severely tested by exceptional rainfall events. However, despite their critical importance in mitigating flood risk, interest and investment in levees has tended to be lower than in other critical water retaining infrastructure such as dams. In particular, in many countries, levees have lacked the legal and technical framework necessary to promote an appropriate level of performance. In September 2008, organisations from six countries (France, Germany, Ireland, the Netherlands, United Kingdom, and the United States of America) expressed a desire in principle to participate in an international project to learn from one another’s experiences and to share the effort to produce good practice guidance – The International Levee Handbook (ILH). That desire resulted in several international meetings and workshops, development of a scoping report and ultimately culminating in this handbook. The principal objective of the handbook is to provide a comprehensive and definitive guide to good practice in the evaluation, design, implementation, maintenance and management of levees. The handbook is a non-prescriptive reference and should be used in conjunction with other relevant national and international codes and manuals. It is not intended to be a prescriptive code of practice for decision making but should be regarded as an important document in decision support and for reference in the application of international codes and manuals. The handbook has been written by a core team of experts and practitioners from the full range of relevant disciplines drawn from the partner countries. The development of the handbook followed an agreed set of processes that was managed by a technical editorial team, and supported by an executive steering board drawn from national backing groups of the partner countries. Management support was provided by CIRIA (UK) who also prepared the resulting document for final publication. The document was made available to a broader international audience for review and comment during the development process. 1.3 Scope The handbook takes a risk, performance and systems based approach. Any levee will have a primary function of flood management or coastal defence to which performance objectives or standards will apply. All levees will also have various secondary functions, e.g. environmental, amenity, health and safety, access, which can impose significant performance requirements. The handbook also follows a tiered approach to all aspects of managing and maintaining a levee or levee system such that concepts are applicable to levees in both urban and rural settings. In drafting the handbook, the author teams considered the various management interventions that are needed to achieve the performance requirements of the levee or levee system over its whole life cycle. So the handbook addresses the assessment of existing riverine, coastal and estuarine flood protection levees (possibly for new or changed performance requirements), their adaptation or replacement, their operation and maintenance, as well as new design and removal. Consideration is also given to the fact that management interventions range from major construction projects carried out by external constructors through to routine maintenance by the involved authorities’ own work force. The handbook does not address levees constructed for purposes other than flood protection. Also, it does not cover the design of other water retaining structures. Associated structures are addressed because they influence the performance of a levee structure or its operation. The handbook also recognises the importance of structures that stabilise levees by managing riverine and coastal morphology such as beaches, dunes and groynes. Where necessary, reference to other management guidance is given for such structures. 1.4 Structure of the Handbook The handbook contains information that is useful for both existing and newly designed levees, however the structure of the handbook is such that existing levees are treated first followed by newly designed levees. Details about each chapter are presented in the rest of this section. Figure 1.2 presents a high level view of the handbook showing how each chapter contributes information to understanding a levee system as presented by the source-pathway-receptor conceptual model. 1.4.1 Use of the handbook The following features are designed to assist the reader in navigating the handbook: - diagram of general structure: Table 1.1 provides a layout of the structure and contents of the complete handbook. It also suggests a relationship between the main phases of a typical project - diagram of content relevance to different users: Table 1.2 presents an evaluation of the content from different users’ perspectives to assist the reader in finding information relevant to their needs - high-level contents list: this is given for the complete handbook at the start of the book - detailed contents list: at the start of each chapter there is a contents list for that chapter only - structure of each chapter: the front end of each chapter includes a detailed contents list for that chapter, an introductory box that describes what is included in that chapter, and a flow chart to demonstrate links with other chapters - where am I? each page tells the reader their current location in the handbook. The chapter number is shown on the navigation bar running down the outer edge of right-hand pages, and the chapter title is given on the page header - diagram of general structure: Table 1.1 provides a layout of the structure and contents of the complete handbook. It also suggests a relationship between the main phases of a typical project electronic version: the complete manual is available to download from CIRIA’s website: www.ciria.org 1.5 Target readership Potential users of the handbook include planners, developers, structure owners, asset managers, regulatory bodies engineers, risk analysts, designers, constructors, emergency planners and responders, environmental organisations, educational institutions and the public. The handbook is written to assist a technically competent practitioner with a broad (but not necessarily expert) knowledge of the field of application to arrive at the best approach for a particular levee or levee system. In this regard the handbook aims to provide information to support decision making rather than to direct it. The handbook will also seek to provide the intelligent client (i.e. a client with a technical background, but no particular specialist knowledge) with sufficient background information to understand the main issues and general procedures likely to be followed by an experienced practitioner. The handbook has been written to address two major viewpoints: 1 The manager of the operating authority’s physical structures who has the overall task of owning, maintaining, upgrading, adding to and disposing of its stock of flood or coastal levees. 2 The designer who will tend to focus on the need for, design of and implementation of improvements and new works. In addition, the handbook provides some useful information for constructors (or other organisations) that may be advising the manager or designer carrying out maintenance, or carrying out new construction work.