FloodproBE Project WP 3: Reliability of Urban Flood Defences. D3.2: Rapid and cost-effective dike condition assessment methods: geophysics and remote sensing | Projet FloodproBE WP 3: Fiabilité des digues urbaines. D3.2 : Méthodes d'investigation rapides et performantes pour le diagnostic des digues : géophysique et télédétection

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE

English. The recent and dramatic floods of the last years in Europe (Windstorm Xynthia, February 2010) and United-States (Hurricane Katrina, August 2005) have shown the vulnerability of flood defence systems composed of man-made structures (as levees, walls, etc.) and natural structures (as dunes, etc.). The first key point for avoiding these dramatic damages and the high cost of a failure and its consequences lies in the knowledge of the safety level of the protection system. Identifying weak points of the system is the most important but the most difficult issue. Most of the levees are old structures, built several centuries ago, then rebuilt, modified, heightened several times, with some materials that do not necessarily match the original conception of the structure. Other factors introduce weaknesses in a levee: (i) trees, roots, burrows or nests could modify the structure of the levee and reduce its mechanical properties; (ii) particular geological formations and their evolution could also threaten the dike, as it occurred in the city of Orléans, France, where levees have collapsed in karstic areas. In urban context, the levees present many other singularities, such as embedded networks, canalisations, human constructions like houses and walls. Due to all these factors, levees have to be considered as heterogeneous structures. Considering the stretch of hundreds of kilometres and the heterogeneity of the structures, rapid, cost-effective and reliable techniques for assessing and surveying the defence system must be carried out. This report refers to the question of assessing embankment levees safety. The first part briefly presents a synthesis of the global approach related to diagnosis. The second part focuses on the contribution of geophysical methods; guidelines are issued from the conclusions of an International Workshop on Geophysics held in Paris in March 2011. This chapter contains guidelines on application to urban areas for managers to implement and integrate geophysical investigation results into the asset support system. If focuses on technical, practical and economical features such as geophysical method applicability, reliability, rapidity, limitations (particularly in urban areas) and cost-effectiveness. Approaches based on method combination and comprising overall investigation followed by detailed investigation phases are confirmed. Slingram (electromagnetic induction) profiling and Electrical Resistivity Tomography are among the most preferred methods. However, all other methods can play important and specific roles, depending on the stakeholder requirements and the asset features and setting. Temporal approaches have proved powerful tools for weak zone detection and monitoring and should be more widely used in the near future. The third part is dedicated to remote sensing and more specifically to the helicopter borne LiDAR (Light Detection and Ranging) technology, which provides extremely accurate topographic data at a highly efficient rate. In support of a real case study (“Val d’Orléans” Pilot Site), a methodology is developed for performing an helicopter borne survey and for using remote sensing LiDAR data and high-resolution aerial imagery – acquired in “dry conditions” (e.g. not in a flood context) - to contribute efficiently to a rural or urban flood defense structure diagnostic or assessment.