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Extreme groundwater surges in fractured and karstified carbonate aquifers


Najib Kenza

Mediterranean regions are known to be vulnerable to flood hazards: river flood, urban flood, flash flood. However, groundwater floodings are seldom accounted for. Water table rising resulting from extreme rainfall events can generate disastrous damages, particularly in fractured and karstified carbonate aquifers, that are largely widespread in the Mediterranean basin. Hydrodynamic conditions prevailing in these heterogeneous formations containing fractures and conductive drains, can involve high and fast water table risings. Surface or underground buildings can then be exposed to pressure rises or infiltrations likely to damage their foundations.
The focus is put on the assessment and the prevention of risks associated to extreme groundwater surges resulting from intensive precipitations, especially within the framework of building protection. The developed methodology firstly consists in estimating the T-year hydraulic head, given a reference return period T in accordance with safety standards. It also aims at defining the rainfall events that generate these extreme events. This first part provides reliable informations to help decision-makers to quantify water table rising risk.
Regarding the risk prevention, an underground gravitational drainage system made of artificial drains is proposed. A hydrodynamic model, well suited for fractured and karstified reservoirs, is then used to simulate the drainage of the aquifer zones prone to water table risings. This model, based on the dual-porosity concept, allows drains and outlets to be taken into account. The simulation gives the highest hydraulic head within the study area and the transient flow rate in the drains intersecting conductive fractures, for a given recharge. The simulation results are finally used to calibrate the drainage device. This methodology was applied and validated in a karstic Mediterranean basin.