Flooding: holding back the risks ?

To protect themselves from storms, floods and high waters, humans have always built protective dykes. For this reason, there are around 10,000 km of protective dykes in France, linked to other linear constructions (roads, railway tracks, canals, etc.) and natural structures (dunes, buttes, etc.) performing a similar role. However, a poorly maintained or run-down structure will no longer be able to resist the various stresses involved, including those of hydraulic origin. Monitoring and maintenance must therefore be integrated into the safety and reliability of these structures.

Around 40 billion euros per year are spent in Europe on flood prevention, considered to be one of the main natural hazards in the European Union, and on post-flood recovery measures. This is a significant cost and raises the question of how to understand and improve these types of structures. Unfortunately, studying dykes is difficult : in contrast to dams, these structures are long (sometimes several tens of kilometres), old (some date from the Middle Ages) and not at all homogeneous. So how can it be done ?

In accordance with the European Flood Directive of 2007 [1], FloodProBE, a European research project, was launched in 2009 bringing together 14 partners, including Irstea.

FloodProBE: 14 partners and 4 years of research

FloodProBE [2] ("Flood protection of the built environment") uses mixed approaches to manage flood risks, with particular focus on understanding vulnerability, resistance to flooding and adapting buildings, infrastructure or protective dykes by proposing cost-effective solutions. Over 75 % of damage caused by floods occurs in urban areas.

Irstea used its position as a national leader in dyke evaluation to provide expertise to the European project, concentrating its activities on the “Reliability of Urban Flood Defences” Work Package. Researchers concentrated on understanding degradation and resistance mechanisms and establishing specifications and recommendations for using surveillance methods and diagnostic tests in order to qualify risks of failure.

Improving diagnostic methods

Using a general overview of all methods used to evaluate the reliability of structures, specification documents, recommendations for surveillance and a framework for diagnostic methods were set up across Europe. Not only does the evaluation of dyke performance require a large amount of data linked to the type of failure that can occur (external/internal erosion, sliding etc.), in some situations, these failures can follow on from each other. It is therefore difficult to characterise failures using equations or to perform precise evaluations. The researchers worked towards finding other methods that combined several classical processes (deterministic, probabilistic or empirical calculations, formal or informal expert analyses, uncertainty integration) in order to overcome the lack of knowledge.

In total, 4 years of work resulted in improvements to : 

  • Internal erosion. Phenomena within this group are often the root cause of dyke degradation. Understanding the various processes and their possible sequencing is key to improving management and diagnostic tools for dykes.
  • Structural discontinuity. In the past, dykes were divided into sections for the purposes of evaluation. However, geometry, materials and coverings can change, or other structures (canals and various networks, buildings etc.) can be included within the dykes, creating transitions within the structure which create specific and localised degradation. Researchers therefore focused on these transitions : what happens with the various types of transition, regardless of the cause, and how should they be evaluated ?
  • Cover planting on dykes. Cover planting on dykes is recommended as it provides improved resistance. However, as with the dykes themselves, it must be maintained. This project looked at formalising procedures linked to the evaluation of cover planting.
  • High-performance surveillance methods. Specifications for the use of geophysics and airborne lasers (Lidar) for the surveillance of existing dykes were drawn up to define, perform and interpret this type of method.
  • A framework for diagnostic methodology. A comparison and summary of current diagnostic methods facilitated the creation of a general methodological framework taking into account all the available data for these structures, as well as the provision of advice for using GIS-based tools to manage and process the data.

This research involved several research units at Irstea: Hydraulic Structures and Hydrology (OHAX) in Aix-en-Provence, Land Use, Environment, Remote Sensing and Spatial Information (TETIS) in Montpellier, Hydrology and Hydraulics (HHLY) in Lyon and Mediterranean Ecosystems and Risks (EMAX) in Aix-en-Provence.

What next ?

FloodProBE, together with the first international guide for dykes (see table), provides a general overview of good practices when evaluating flood risks for built-up environments and managing, designing and diagnosing dykes. FloodProBE also provides appropriate knowledge for the design and vulnerability evaluation of critical networks and infrastructure in flood zones. It is up to structure managers and design offices to make use of the results and methods which have been provided for them.

What about new legislation in France? The Regulation on Hydraulic Structures of 2007, relating to the safety of hydraulic structures and to the permanent technical committee for dams and hydraulic structures, requires the inspection and maintenance of these structures. Results from the research will enhance this document and may even be used to draw up the framework of a new regulation.… 

FOCUS  / International Levee Handbook

The International Levee Handbook (ILH) is the first international reference document on managing and designing dykes. It is the result of a collaborative project involving over 100 experts from around the world, namely French, American and British/Irish researchers.

Five years were needed to define the knowledge of levees and to draft this publication designed for managers and technicians. Irstea was heavily involved in writing and coordinating the publication.

Available for purchase as a hard copy but also as a free download

A French version will be available during 2014.

Consult the FloodProBE project website

For further information

[1] The European Flood Directive of 2007 invites Member States to reduce the negative consequences of flooding on human health, the environment, heritage sites and economic activity. It is based on 3 major aims: a preliminary evaluation of flood risks at a catchment area scale, establishing flood zone maps and drawing up a flood risk management plan.

[2] The European FloodProBE project (2009-2013) was undertaken as part of the 7th Research and Development Framework Programme (RDFP), the main community research funding tool for the European Union.