MEDYCYSS: The Multi-Scale Observatory of Flood and Underground Hydrodyamics in Karst Environment

The Multi-Scale Observatory of Flood and Underground Hydrodyamics in Karst Environment (MEDYCYSS – OSU OREME) was set up to respond to problems specific to Mediterranean regions because of their particular climatic, hydrologic, and hydrogeologic environments. It includes the Lez-Mosson-Coulazou basin and corresponds to a vast karst hydrosystem of which the principal outlet is the Source du Lez, which supplies Montpellier and its surroundings.

This goal of this observatory is characterize transfer processes associated with various entities (surface and underground) of Mediterranean karst hydrosystems, and more specifically, of promoting:

  • observation and characterization at diverse scales (local and regional) of underground hydrodynamics of karst aquifers subject to the actions of climate and anthropic forcing;
  • observation and understanding of the rapid hydrologic processes that take place during extreme floods in Mediterranean drainage basins that have a high karst component;
  • the use of coupled hydrodynamic/hydrochemical monitoring to observe and determine the risk of groundwater contamination.

For this region, records of qualitative and quantitative data on groundwater and surface water have been maintained since the 1950s, roughly continuously over time. Complementary measurements and monitoring were established by the HydroSciences Montpellier laboratory in 2005, by installing surface and underground instruments in the principal hydrographic basins that interact with the karst: the Lez, the Mosson, and the Coulazou. This initiative was begun following the extreme rainfall events of 2002 that greatly affected the Gard and Hérault. These extreme rainfall events can generate high water and large-scale floods that cause major damage in areas where urbanization and economic activities are commonly concentrated, areas that have continuously increased in size in recent years. In most cases, the basins of watercourses subject to flash flooding have a karstic portion typical of carbonate environments in the Mediterranean region. These carbonates also contain a large portion, the major portion in some areas, of available water resources. Extreme rainfall events thus add to the risk of groundwater resource contamination.

The MEDYCYSS observatory makes it possible to evaluate the various processes and particularly to characterize transfers of flow and material at various scales:

  • regional scale: network of piezometers (14 borings), perennial and temporary springs, sinkholes,
  • local scale: the Terrieu experimental site (22 borings 60 m deep, over an area of about 500 m2, located 5 km from the principal outlet of the Lez aquifer, the Source du Lez)
  • boring scale: a deep boring at Triadou (333 m) located near the Source du Lez and an temporary river. It was equipped in July 2014 with multi-level monitoring (Equipex CRITEX) and “Multi-level monitoring PMPS” (SolExperts ®), which conducts continuous P and T° monitoring at various depths and hydrochemical monitoring at 5 intervals isolated by obturators.

In addition to hydrodynamic monitoring conducted in borings and karst cavities, other monitoring includes meteorological (6 rain gauges and a meteorological station), hydrologic (9 measurement stations), and biogeochemical (hydrochemistry, isotopes, dissolved gas, and bacterial communities).