Résumé:
This research focuses on the Upper Rhumel basin (1130 km2), controlled by the Hammam Grouz dam at Oued Athménia (Constantinois region in eastern Algeria). This basin of the semi-arid High Plains contains three main hydrogeological formations whose lithological and hydrodynamic conditions are varied: the surface layer of the mio-plio-quaternary, the aquifer of the Eocene and the aquifer of the Cretaceous. The cartography from the piezometric measurement campaigns showed a general drainage of the underground system towards the Oued Rhumel and two main directions of flow South West-North East and North East-South West respectively characterizing a convergent layer in the centre and divergent in the extreme West. Chemistry, the study of quality and the ACP have made it possible to highlight a high probability of mixing water and therefore a relationship between the surface aquifer and the deep aquifère (karstic). The Djebel Grouz massif, the site of the dam dyke, contains karst formations that cause remarkable
leakage flows to the springs, partly thermal (Hammam Grouz springs), which emerge immediately downstream from the dam.
The hydrological functioning of the dam (capacity: 45 hm3), is studied through the monthly and daily regulation balance data of the period 1987-2015, provided by the ANBT (National Agency of Dams and Transfers). These sets of information have made it possible to assess the significant water supply deficit of the basin, resulting in a low rate of annual regulation (30% on average).
The variability of the rains and reserves of the Hammam Grouz dam was assessed using the wavelet consistency analysis which showed a correlation between the two signals exceeding 50% (and below 70%) on all modes of variability. Two major events linked to the exceptional rainy inputs of the hydrological years 2002/2003 and 2014/2015 marked the functioning of the system resulting in large volumes of water leakage, hence the appearance of large sinkholes (vortex) within the reservoir. The leakage volumes are estimated on the basis of measurements of the flows from the sources on the one hand, and the abnormal removal of water reserves
from the dam on the other hand. The results of the three consecutive years of comparative analysis (2002/2003, 2003/2004 and 2004/2005) of the inputs (or ""tributary"") to the dam with those measured at the gauging station, located upstream,
updated the leakage volumes to 7.04 hm3, 32.2 hm3 and 26.2 hm3 respectively. These results show that the leakage rates are much higher than those measured by the ANRH (National Agency for Hydraulic Resources) at the sources of the Hammam, emerging at the bottom of the dyke. Complementary tools and approaches have been implemented for the understanding of the phenomenon
of leaks affecting Hammam Grouz: electrical imaging (geoelectric tomography, seismic reflection and seismic refraction), the log of reconnaissance drilling carried out in the dam site, the interpretation of fracturing maps and multivariate analysis of the degree of fracturing of RQD rock. They have made it possible to identify the fracturing-karstification-leakage link of the dam (sources) following the discovery of a fairly dense and complex fracturing network (large faults, corridors, beams, tectonic nodes, etc.),
Despite the solutions recommended to preserve the tightness of the basin (injection veil, clogging of vortex), the problem of destocking the Hammam Grouz dam is far from being mastered. It reflects the complexity of setting up hydraulic developments in Constantine karst context.
