Spain has one of the largest stocks of big dams in the world. With roughly 1,200 large dams on the register — those over 15 metres high or holding more than 100,000 cubic metres — the country ranks fifth globally, behind only China, the United States, India and Japan. That dense network of reservoirs plays a crucial role in water management, urban supply, irrigation, hydropower and, of particular relevance here, flood protection.

This article looks at how reservoirs work as flood defences, at the safety mechanisms built into dams, at the regulations that govern them and at the critical decisions operators face when torrential rain is on the way.

Flood attenuation: the reservoir as a shield

The most important role a reservoir plays against flooding is flood attenuation — in Spanish, laminación de avenidas. The idea, fundamental in hydrological engineering, is to reduce the peak discharge of a flood by temporarily storing part of the incoming volume.

Put simply: when a flood wave reaches a reservoir with spare capacity, the reservoir holds part of the water back and releases it downstream more gradually. The peak is shaved off, and the hydrograph flattens and stretches out in time. What would have been a peak of 2,000 m³/s for three hours can become a more manageable 800 m³/s over eight hours.

Flood storage allowance

To attenuate a flood, a reservoir needs empty space: the flood storage allowance, the volume between the normal maximum operating level and the maximum flood level the reservoir may reach during an extraordinary event.

Managing that allowance is one of the most delicate decisions in reservoir operation. Keeping a large empty volume improves attenuation capacity but reduces the water available for supply and irrigation. In a Mediterranean country under chronic water stress, that balance is particularly hard to strike.

Key point: The operating rules of many Spanish reservoirs set seasonal allowances: more empty volume during the wet season in autumn and winter, less during the summer low-flow period, when storing water becomes the priority. These rules are reviewed periodically in the light of experience and of how the climate is changing.

Outlet works: how the water is released

Dams have several mechanisms for managing water during a flood, each with a specific function.

Spillways

Spillways are the structures designed to evacuate surplus water once the reservoir rises above a given level. They are a dam’s primary safety feature against overtopping. There are several types:

  • Free-overflow spillways, without gates. Water spills over a crest once it reaches a certain level. They need no human intervention, working passively by gravity, which makes them the most reliable option.
  • Gated spillways. Movable gates — radial or Tainter gates, for example — allow the released discharge to be regulated. They offer more operational flexibility but depend on the gates working correctly at the critical moment.
  • Tunnel and shaft spillways, including the morning glory type. These are used where the topography rules out a conventional spillway: water enters through a vertical funnel and is carried by tunnel to a point downstream of the dam.

Bottom outlets

These are conduits at the base of the dam that allow the reservoir to be drawn down or emptied. They are used to:

  • Lower the reservoir quickly ahead of a forecast flood, in what is known as a preventive release.
  • Maintain a minimum environmental flow downstream.
  • Carry out maintenance and flush out sediment.
  • Handle emergencies requiring the reservoir to be emptied.

Mid-level intakes and outlets

Located partway up the dam, these draw water for supply or irrigation, and can also contribute to releases during a flood episode.

Why maintenance matters: Spillway gates and bottom outlets have to work when they are needed, which is precisely during the most extreme episodes. Preventive maintenance, periodic testing and backup power for the operating machinery are critical elements of dam safety.

The regulatory framework

Dam safety in Spain is governed by the Technical Regulation on the Safety of Dams and Reservoirs, approved by the Order of 12 March 1996 and complemented by the Basic Guideline for Civil Protection Planning against Flood Risk. This framework establishes:

  • Dam classification. Every large dam is classified as category A, B or C according to the potential risk should it fail. Category A dams — those that could affect urban areas or cause fatalities — face the strictest requirements.
  • Operating rules. Every dam must have approved rules governing reservoir levels, release criteria and the procedures to follow during a flood.
  • Safety reviews. Mandatory periodic inspections assess the structural condition of the dam, the functioning of the outlet works and whether the safety criteria remain adequate.
  • Dam emergency plans. Compulsory for category A and B dams, they set out emergency procedures, including the areas that would flood in the event of failure and how long the flood wave would take to arrive.

Dam emergency plans

These plans are critical documents defining what to do when a dam enters an emergency situation. They set out four scenarios.

Scenario 0: pre-emergency conditions

Circumstances arise that pose no immediate danger but call for heightened vigilance: a rapidly rising reservoir, forecasts of very intense rain, minor anomalies in the instrumentation. Monitoring is stepped up and those responsible are alerted.

Scenario 1: exceptional situation

Phenomena occur that could compromise the safety of the dam if not corrected: the reservoir rising above its normal maximum level, abnormal seepage, gate failure. Communication protocols with Civil Protection are activated.

Scenario 2: risk of failure

There is a real risk that the dam will fail or suffer serious damage. Every warning mechanism is activated, including notification of the population downstream and possible evacuation of the flood zones.

Scenario 3: dam failure

The dam has failed, or failure is imminent and unavoidable. Full evacuation of the affected areas defined in the plan’s flood maps is carried out.

What the plans model: The flood maps in a dam emergency plan model which areas would be inundated, and how quickly, if the dam failed. At some category A dams the breach wave can reach towns a few kilometres downstream within minutes, which is why the warning systems must work with maximum speed and reliability.

The decision to release water: a genuine dilemma

One of the most complex and contested decisions in flood management is when and how much water to release. The operator regularly faces a dilemma.

Preventive release

If the forecast points to intense rain, the reservoir level can be lowered in advance to create more attenuation volume. The problem is forecast uncertainty: if less rain falls than expected, water has been «thrown away» in a country that may need it. If more falls, the preventive release will have been insufficient.

Releasing during the flood

Once the flood is under way and the reservoir is filling, water has to be released to keep it from overtopping the dam crest, which could cause failure in embankment dams. Releases during a flood must be managed so that the outflow never exceeds the inflow: the point is for the reservoir to reduce the peak, not add to it.

Coordinating reservoirs in series

On rivers with several reservoirs in series — the Tagus, with Entrepeñas and Buendía, or the chain of reservoirs in the Ebro basin — releases must be coordinated so that flows from several reservoirs do not converge downstream at the same moment. That coordination is the responsibility of the river basin authorities and is managed from the SAIH control rooms.

Three reservoirs and what they do

Tous (Júcar, Valencia)

After the original dam failed in 1982, a new dam was built at Tous and completed in 1995. It stands 115 metres high, holds 379 cubic hectometres and was designed with particular emphasis on flood attenuation. Its gated spillway can discharge up to 10,000 m³/s. Since it was built it has successfully attenuated multiple Júcar floods, protecting the Ribera districts of Valencia.

Santillana (Manzanares, Madrid)

Sitting in the upper Manzanares basin a few kilometres from urban Madrid, the Santillana reservoir (91 cubic hectometres) plays a vital role in regulating floods that could affect the capital. Its operating rules require flood storage allowances during the wet season to guarantee attenuation capacity.

Buendía (Guadiela and Tagus, Guadalajara and Cuenca)

With a capacity of 1,639 cubic hectometres, Buendía is one of Spain’s largest reservoirs. Together with Entrepeñas (835 cubic hectometres) it forms the headwater system of the Tagus, which regulates flows for the Tagus-Segura transfer while also managing floods in the upper Tagus. The balance between storage and flood protection is a recurring theme in Spanish water policy.

When a reservoir is not enough

It is important to be clear that reservoirs cannot eliminate flood risk. Their limitations include:

  • Finite capacity. In exceptional floods the available storage may simply not be enough. The dam spills and attenuation falls away.
  • Lateral inflows. Downstream of the dam, unregulated tributaries can contribute significant flows that never pass through the reservoir.
  • Basins with no reservoirs. Many Mediterranean coastal rivers and ravines have no regulating reservoir at all, leaving them unprotected.
  • Reservoirs already full. In particularly wet periods, reservoirs may be at capacity when the next flood arrives, with no attenuation left to give.
  • False sense of security. A reservoir upstream can create a perception of safety that encourages building on flood-prone land which, in an extraordinary flood, remains at risk.
Building on floodplains: Urban growth in recent decades has occupied extensive floodplains downstream of dams, often justified on the grounds that «the reservoir protects us». Recent disasters show that this protection has limits, and that land-use planning is the first line of defence against flooding.

How WhatAWeather tracks reservoirs

WhatAWeather brings together reservoir data to show how much regulation capacity is available in each basin. The information includes:

  • Current level and percentage full relative to maximum capacity.
  • Recent trend, with charts showing how the level has moved over recent days and weeks.
  • Comparison with the historical average, to judge whether the reservoir is above or below normal for the time of year.
  • Risk context. When high reservoir levels coincide with forecasts of intense rain, WhatAWeather flags the possibility that the reservoir will have to release significant volumes.

The network covered includes the main reservoirs of every Spanish basin, more than a hundred under continuous monitoring, alongside a selection of international reservoirs.

The future: smarter dams

Reservoir management is moving towards more dynamic, adaptive models:

  • Real-time optimisation. Mathematical models that adjust releases as the weather forecast updates, maximising attenuation and useful storage at the same time.
  • Automated monitoring. Sensors that continuously track the structural condition of the dam — deformation, seepage, pore pressures — and raise alarms at any anomaly.
  • Probabilistic forecasting. Instead of a single deterministic forecast, ensembles of scenarios are used to quantify uncertainty and base release decisions on probabilities.
  • Digital twins. Three-dimensional numerical models of the dam and its reservoir that make it possible to simulate emergency scenarios and rehearse operating decisions before acting on the real structure.

Conclusion

Spain’s reservoirs are fundamental infrastructure for water management and flood protection, and their attenuation capacity has averted countless disasters over recent decades. They are not, however, an absolute solution: they have physical limits, they need rigorous maintenance, and their effectiveness depends on intelligent, well-coordinated operation.

With climate change bringing more intense extreme rainfall and longer droughts, managing reservoirs becomes harder still. Tools such as WhatAWeather help make that management transparent, letting anyone check the state of the reservoirs in their basin in real time and understand better how this infrastructure works to protect them.