The major problem that threatens the lifespan of key Malaga reservoir: a third of it is filled with silt

In addition to the emergency work, the Junta de Andalucía is already planning future hydrological work to resolve the issue at the dam in Almogía

Chus Heredia

Malaga

Tuesday, 25 March 2025, 10:50

This is a well-known, well-studied and common phenomenon with reservoirs. The accumulation of sediment affects the discharge facilities and the reservoir itself, reducing its storage capacity. All this significantly alters the useful life of the infrastructure. We always talk about many years ahead but, in this field, planning is key. Casasola, which links with the Campanillas river in Almogía, is not the only one with this problem in Malaga, but it is the clearest example in the province.

Sediment build-up

The regional ministry of agriculture is already considering two scenarios: in the short term some emergency work; in the long term, a complete hydrological fix of this siltation. According to reliable sources, when the reservoir is full, as is the case following the train of storms that rumbled through the province, it has a 30% siltation rate. This means that a third of its contents are solids and debris that are washed down into the reservoir (reeds, mud, clay and more).

Many solutions are possible: dykes, reforestation, emptying of the reservoir and removal of the water deposits. The action is not imminent, but it is already on the table.

It is a problem that also affects the Guadalhorce system, but to a lesser extent. Much less so, due to the type of reservoir basin, in La Concepción, near Marbella. This is why bathymetries are often carried out to provide a physical representation of the bottom of these reservoirs.

Bathymetries and capacity

Casasola was inaugurated in 2000 with a capacity of 23.64 million cubic metres and now has a filling limit of 21.72, although this can vary slightly, as we have seen in recent days. This is the capacity set by the first upper spillway, the central one, located at the lowest level. The reservoir holds more than 25 million cubic metres.

In fact, a bathymetry of the reservoir bed will be carried out in the near future to try to solve the problem of the bottom outlet getting so clogged up with sediment, which has prevented Casasola from draining properly during the rush of rainfall from Storm Laurence and the preceding three, named storms.

The Junta started emergency work with special dredgers, called clamshell dredgers because of their resemblance to these shellfish, to excavate the sediment. The two 'Dana' storms in autumn had already damaged the discharge system. Unfortunately, the excessive turbidity of the water made it impossible for divers to work in such unsafe conditions, so they could not pinpoint the precise location of the blockage.

First work interrupted

Then the work was halted by the train of storms this spring, with no let-up. The reservoir doubled its level in just a few days. An attempt was made to start up the valve that pumps the water out. It was sort of working, but there was clearly a physical blockage preventing the process from running smoothly.

There is another solution that has just been contracted: a new drainage outlet pipe, which will be a permanent feature. It will be ready before Easter and will allow three million cubic metres a week to be discharged when necessary in a controlled manner.

Staying below the safe level

The important thing now is to keep the dam below its set safety level, where it begins to relieve water pressure from above. Then, by lowering its level later, the task of unblocking all the silt and sediment can be carried out with a greater guarantee of success.

The sluice gates are opened periodically in reservoirs such as Casasola, Limonero or Guadalhorce. It is necessary to keep the outflow channels clear. However, sometimes these require heavy investment, such as was done three years ago for the Conde de Guadalhorce reservoir.

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