The Sau marsh has become the main visual reference of the drought in the internal basins of Catalonia. The old church of Sant Romà de Sau, the remains of which are visible in the center of the reservoir whenever the rains are scarce, is undoubtedly the most photogenic element of the area, but the epicenter, the heart of This deposit in the districts of Barcelona and Girona is about two kilometers from the Romanesque bell tower.
Covering the River Ter, the 335,000 cubic meters of concrete that make up the Sau Dam are distributed in a kind of elongated inverted pyramid 75 meters high, 65 meters wide at the base and 260 meters long at the top . In the bowels of this mill, a network of narrow corridors that are a little more than a kilometer long facilitate access to some of the equipment for controlling the waters and monitoring the integrity of this work in service for 60 years.
The desert area that surrounds Sant Romà and the exposed walls of the dam are two signs of a situation approaching emergency. "Sau is now experiencing the worst episode of drought in its history, on three previous occasions it had dropped in level even more than now, but never for so long in a row", explains Sergi Morilla, technician in charge of the swamp, at the foot of the dam of Sau. Morilla has been working in this facility for more than 30 years and knows precisely its history, operation, security systems and curious details. "The staircase visible at the front of the wall has 478 steps", this specialized technician from the Catalan Water Agency (ACA) details from memory.
Under this staggered zigzag, which Morilla points out, at the bottom of the front of the large concrete wall, on the left bank of the river, a small metal door gives access to a narrow and curvilinear corridor that allows walking to the innermost part of the construction. Once in the inner center, another corridor – a little bigger and straighter – extends perpendicularly following the direction of the drainage channels. "From this crossing to the exit, where you can see the light, there is a thickness of 60 meters of concrete; on the other side we have a wall three meters wide until we reach the reservoir water", explains Sergi Morilla. The work has, inside, three separate networks of corridors, located respectively in the lower, middle and upper part.
The original design of the Sau dam dates from 1931 and although later, before construction was carried out (1947-1963), some improvements were introduced, operational experience has revealed some technical drawbacks. The corridors, for example, are so narrow that they prevent the passage of vehicles for the transfer of heavy equipment such as pumps or valves, which is why any movement has to be done manually or in small vans.
Another obstacle is that the three levels of corridors are not internally connected (they are accessed from three different points). "In other larger and more modern dams, for example, the galleries are interconnected by elevator; here they didn't even leave the hole", explains Morilla. Certainly, the Sau dam has its pluses and minuses, explains to La Vanguardia Joan Lagunas, author of History of the construction of the Pantà de Sau, a book of almost 500 pages published in 2015 by the town councils of the Masies de Roda and Vilanova de Sau in commemoration of the half century of life of this reservoir. Lagunas recalls, among the anecdotes collected in his book, that at the end of the 1940s, when the Sau swamp was being built, "cement was very scarce and, sometimes, the trucks that came with material from Pobla de Segur to at the reservoir they were stopped in Vic and their load ended at the reconstruction of the seminary".
All in all, the Sau dam is a safe and secure work thanks to constant maintenance, equipment renewal and the incorporation of new surveillance and protection systems. "If you keep an eye on it, the concrete resists without problems, in terms of the valve equipment, motors and sluice gates they have been completely renewed", Morilla explains, pointing out for example the four drainage sluices at the bottom of the reservoir that have been lined metallically in recent years to avoid concrete wear due to the passage of pressurized water.
Inside this solid structure, there are equipment that allow detailed monitoring of possible water leaks, cracks, subsoil pressure or movement of concrete blocks. In addition, the technicians who oversee the operation of the dam maintain a detailed schedule to verify that all equipment is working properly.
Although not necessary in normal operating situations, for example, technicians enter the heart of the dam periodically to start each and every valve to certify the good condition of the mechanism. "They could be started remotely, but we come to see and feel how they work", explains Morilla. Among the most advanced safety measures, the technician responsible for this installation highlights the network of piezometric level sensors, embedded in 118 small perforations along the corridors inside the dam, which reach down to the rock at the bottom and the sides. "From my mobile phone, through an application, I can see at any time if there is any change due to any leakage or pressure; so we could act immediately", he explains.
The reservoir is now at 12.8% of its capacity and in recent days it has been releasing about 6 cubic meters per second, a water that circulates to the Susqueda reservoir for subsequent treatment and supply to the network of drinking water in the regions of Girona and Barcelona. If there are no significant rain events, the Sau level will continue to fall and could approach historical lows such as those of 1990 or 2008. The current drought, that is, the period of rainfall deficit, is in the basin del Ter more marked and longer than that of the two previous droughts mentioned, but fortunately for the inhabitants of the Barcelona area, among others, there are now infrastructures such as the desalination plants of Tordera and El Prat de Llobregat that allow obtain drinking water and reduce dependence on reservoirs.
The current drought affecting the Sau marsh (and much of the rest of Catalonia) contrasts with episodes of flood risk such as the one experienced in January 2020 with the Gloria storm. "For a few hours, Sau reached 100% of its capacity and the water inflow was impressive, but the floodgates could not be opened in an uncontrolled manner because downstream there were also other contributions that endangered cities like Girona ”, says Sergi Morilla, recalling one of the most complex episodes he has experienced in Sau.
