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Extreme groundwater surges: hazard assessment and risk prevention

Congress: 2008
Author(s): NAJIB, K.; JOURDE, H. and PISTRE, S.

Keyword(s): extreme groundwater surge, fractured aquifer, risk prevention,
AbstractGreat attention is devoted to surface floods such as river, urban, coastal or flash floods, due to their socio-economic and environmental impacts. However, floodings from groundwater are seldom accounted for. Most of the studies are actually dedicated to the analysis of floods due to direct overflow of surface water following heavy rainfall events. This study focuses on groundwater floodings due to water table rises that are induced by extreme rainfall events. This special kind of flooding can also cause devastating damages, particularly in fractured and karstified carbonate aquifers. Indeed, hydrodynamic conditions prevailing in these heterogeneous formations containing fractures and conductive drains can involve strong groundwater surges. This phenomenon, characterized by its suddenness and its rudeness, can lead to the flooding of basements and tunnels, and damage road networks and building foundations. The present work contributes to the assessment and the prevention of risks associated to extreme groundwater surges resulting from intensive precipitations, especially within the framework of building protection. Firstly, a methodology devoted to groundwater flooding hazard assessment is developed. It consists in the predetermination (statistical prediction) of the T-year hydraulic head that characterizes the T-year groundwater surge, given a reference return period T in accordance with safety standards. The developed methodology also aims at defining the rainfall events that cause these extreme events. Then, in order to prevent resulting damages, an underground gravitational drainage system made of artificial drains is put forward. 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 rises. This model, based on the dual continua approach, 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 obtained results are finally used to calibrate the drainage device. The proposed approach finally results in a new methodology devoted to the characterization of extreme groundwater surge events. It consists in an effective an operational tool for both diagnosis and prevention that can provide reliable informations to guide decision-makers. This methodology was applied and validated in a karstic Mediterranean basin.
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