Diagnosis and Adaptation of Levee Systems

Riverbanks and coastlines have always been ideal locations for human settlements, both for vital needs and for a wide range of activities. To protect these areas from river flooding or marine submersion, "diking systems" have been built: an organized set of dykes, generally combined with other hydraulic structures or facilities (protective walls, closing structures, retention devices, pumping stations, etc.).

However, the protection offered by these systems is limited: when natural phenomena exceed a certain intensity, the structures allow water to pass through, either by overflow, or through a breach in the event of accidental rupture. In either case, the result can be a worsening of the hazard, particularly through the acceleration of hydraulic phenomena. As the positive effect of reducing the frequency of flooding produced by the diking system is more immediately perceptible than its potential negative effects, this often produces a false impression of safety.

The development of "protection systems" over the centuries has therefore modified the expression of the hazard in dyked areas, but without eliminating it. Moreover, in more recent times, the exposure of stakeholders has increased significantly: the growing attractiveness of seaside and river banks and, above all, the increasing scarcity of available land on these sites have led to the siting of vulnerable stakeholders in areas located at low altitudes or in "basins" subject to rapid submersion phenomena.

Beyond this, flood risk management in dyked areas is subject to many uncertainties: hydraulic stresses, morphological changes in the environment and changes in the condition of structures can only be imperfectly known.

We can therefore draw two conclusions: on the one hand, an increase in vulnerability due to a concentration of stakes on sites exposed to a strong hazard, and on the other, protection whose principle is equivocal (since in certain cases, the hazard can be aggravated) and whose performance is subject to multiple uncertainties.

It therefore appears necessary to improve the safety of diking systems by :

• understanding how these systems work and the risks of failure;

• regular assessment of their performance and residual risks in relation to identified natural phenomena;

• maintenance, reinforcement or adaptation work required to achieve the desired performance.

After providing a few basic elements of knowledge, notably through a historical perspective, this article deals with the diagnosis and risk analysis of diking systems, an essential approach for informing management and...