The Sau reservoir has become the main visual reference of the drought in the internal basins of Catalonia. The old church of Sant Romà de Sau, whose remains are visible in the center of the reservoir every time the rains are scarce, is undoubtedly the most photogenic work in the area but the central element, the heart of this reservoir in the regions of Barcelona and Girona, is located about two kilometers from the Romanesque bell tower.
Plugging the River Ter, the 335,000 cubic meters of concrete that form 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 mass, a network of narrow corridors that total just over a kilometer in length facilitate access to some of the equipment for controlling the water and monitoring the integrity of this work that has been in service for 60 years.
The desert area that surrounds Sant Romà and the exposed walls of the dam are two of the signs of a situation that is approaching an emergency. “Sau is now experiencing the worst drought episode 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 Sau reservoir, at the foot of the dam.
Morilla has worked in this facility for more than 30 years and knows precisely its history, operation, security systems and curious details. “The staircase visible in the front part of the wall has 478 steps,” this specialized technician from the Catalan Water Agency (ACA) details from memory.
Under this stepped 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 hallway that allows you to walk to the innermost part. of the construction. Once in the internal center, another corridor – somewhat larger and more rectilinear – extends perpendicularly following the direction of the drainage channels. “From this intersection to the exit, where you can see the light, there is a thickness of 60 meters of concrete; Towards the other side we have a three-meter-wide wall until we reach the water of the reservoir,” 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 back to 1931 and although later, before construction was carried out (1947-1963), some improvements were introduced, operational experience has revealed some technical drawbacks. The hallways, for example, are so narrow that they prevent the passage of vehicles to transport heavy equipment such as pumps or valves, so any movement must be done manually or in small vans.
Another obstacle is that the three levels of hallways are not connected internally (they are accessed through three different points). “In other larger and more modern dams, for example, the galleries are interconnected with an elevator; here they didn’t even leave a gap,” explains Morilla. Certainly, the Sau dam accumulates 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 Les Masies de Roda i 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 reservoir was being built, cement was very scarce and, sometimes, the trucks that came with material from Pobla de Segur for the reservoir were stopped in Vic and their burden ended in the reconstruction of the seminary.”
All in all, the Sau dam is a completely safe and insured work thanks to constant maintenance work, equipment renewal and incorporation of new surveillance and protection systems.
“If it is monitored and maintained well, the concrete resists without problems, and this is what we are seeing in the walls of Sau; As far as the valves, motors and gates equipment is concerned, they have been completely renewed,” explains Morilla, pointing out, for example, the four drainage gates at the bottom of the reservoir that have been metal-coated in recent years to prevent wear. of the concrete by the passage of pressurized water.
Inside this solid structure there is equipment that allows detailed monitoring of possible water leaks, cracks, subsoil pressures or movement of the concrete blocks. In addition, the technicians who supervise the operation of the dam maintain a detailed schedule to verify that all equipment is working correctly.
Although it is not necessary in normal operating situations, for example, technicians enter the heart of the dam periodically to activate each and every one of the valves to certify the good condition of the mechanism. “They could be started remotely but we come here in person to see and hear how they work,” explains Morilla with something resembling devotion to his work.
Among the most advanced security 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 the rock at the bottom and the sides. “From my mobile phone, through an application, I can see at all times if there is any change due to a leak or pressure; so we could act immediately,” explains Morilla.
The Sau reservoir is now at 12.8% of its capacity and in recent days it has been releasing about 6 cubic meters per second, water that circulates to the Susqueda reservoir for subsequent treatment and supply to the water network. drinking water in the regions of Girona and Barcelona.
If there are no significant rain episodes, the level of Sau will continue to fall and could approach historical lows like that of 1990 or 2008. The current drought, that is, the period of precipitation deficit, is more marked in the Ter basin 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 Tordera and Prat de Llobregat desalination plants that allow obtaining drinking water and thus reducing dependence on reservoirs.
The current drought that affects the Sau reservoir (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 inflow of water 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. remembering one of the most complex episodes he has experienced while working in the Sau reservoir.
Storm Gloria was devastating in many areas but resulted in more than acceptable results in terms of flood risk management. Reservoirs like Sau’s draw attention when there is a lot or little water, but their correct management and maintenance require constant work.
