Freshwater submarine springs: Role of a dam in submerged karst—investigations measurement and works in subterranean rivers of cassis—france) from 1964 to 2013

Abstract

The port-miou river, close to Cassis, in the Provence (France) is the unique subterranean and sub marine river which has been dammed with the objective of collecting fresh water. The river present a section of almost 22 m wide and 18 m high, between -21 and -3 m asl. The construction of the dam has been stepped in two phases: the first one in 1971 was a chicane dam with unique aim of preventing sea water to penetrate towards the upstream of the karst by flowing at the bottom of the conduct whilst the fresh water flows at the upper part of the conduct toward the see. The chicane dam allowed passing the floods of the subterranean river without changing the water head in the karstic conduct. The second was built in 1976-1977. It was a complete dam closing the karst with a surface spillway excavated in the rock. The sill of the spillway is at elevation +3 m asl. The floods were estimated at 100 m3/s. A flood measured at 45 m3/s has been experienced in January 1978. Continuous measurements have been proceeded with before the construction of the first dam till now, more than 40 years later. In parallel, the submarine exploration continued. In 1978, divers reached a large gallery 1,165 m from the see outlet, 30 m wide and 20 m high at -35 m asl. In 1981, the extreme visited point was at 1,365 m from the see outlet. New exploration allows reaching 2,200 and -82 m asl in an almost vertical large karstic shaft. Exploration resumes in 1992 after 10 years and progress dramatically grace two new diving technologies and also grace to the utmost experience of the diving team. In May 2012, an exceptional diver reach -223 m asl, in an horizontal gallery almost 25 m wide, 30 m below the base of the urgonian limestone. Measurements are continuous. The most important is the measure of the discharge with several methods to cross checks the values of the measures. The measures of salinity at various levels were also made continuously to exhibit the correlation between discharge and salinity. In parallel another submarine karstic river close to Cassis, the Bestouan River, was explored and analyzed with the same aim of collecting fresh water. The subject of the proposed paper is to present this unique experience with the team which did the job from the very beginning to the present day.