We need smarter ways to store water

Every day we rely on dams for water supply, irrigation, hydropower and flood control but most of this infrastructure is very old. Alan Warren explores what steps can be taken to maintain these critical but ageing assets and ensure they continue to serve their function in the face of population growth and climate change.

Nearly 5000 years ago one of the earliest known dams was built at Sadd el-Kafara, Egypt, to provide a secure water supply for men and animals engaged in quarrying stone to build monuments for the pharaohs. It lasted only a few years before being breached by floodwater.

Today’s engineers face the same challenge as their ancient counterparts: how do you keep dams in service for as long as possible?

The average operational life of a dam is around 100 years. Dam building reached a peak in the 1970s, with more than 1500 large dams (>15m high) completed in 1976 alone (source: ICOLD World Register of Dams, 2020). It follows that a significant proportion of the world’s available reservoir storage will be lost over the next 50 years.

So, how can we best maintain water storage assets to prolong their effectiveness and safe operation?

Adapting assets to a changing world

Sedimentation is a significant factor in some parts of the world, causing most reservoirs to lose a significant fraction of their useable storage over their working life. At the same time, population growth and modern water consumption practices are increasing demands on water storage capacity.

These stressors are forcing planners and asset operators to seek ways of improving resilience of supply through dam raising, spillway modifications and other methods.

At Tarbela Dam in Pakistan, Mott MacDonald is involved in the design of major works to raise the elevation of the power intakes in response to sedimentation of the reservoir. This will extend the life of this nationally important dam for several decades.

Over time a number of other factors can affect the security of reservoir assets, including:

• Climate change – increasing the risk of dams being overtopped in extreme floods and other climate-related impacts on structural performance. The 2021 avalanche in Uttarakhand, India, which damaged two hydropower schemes and killed dozens of people, was a timely reminder of the threat posed by unstable geology probably exacerbated by climate change.
• Improved knowledge of structural performance and mechanisms of deterioration – leading to changes in design or evaluation and highlighting historical designs which may no longer be adequately safe.
• Changes in consequential risk through downstream development – which in turn affects the feasibility of undertaking proportionate safety improvement works.

In 2019 Toddbrook Reservoir in northern England almost failed during a flood event when spillwater damaged a concrete-lined auxiliary spillway which had been constructed 50 years earlier on the dam embankment rather than on natural ground at an abutment.

After 1500 residents living downstream had been evacuated, our engineers directed the emergency response needed to preserve the integrity of the dam.

The reservoir had to be emptied through flow diversion and pumping. Then the dam was stabilised by dropping bags of aggregate onto the damaged section using a heavy-lift helicopter.

The incident highlighted the challenges in retrofitting spillway facilities at very old dams and the importance of reviewing structures against current safety evaluation guidance. If Toddbrook was designed as a new dam today, it would look very different from the one that nearly failed.

Get smart with technology

New technologies present opportunities to evaluate assets in a way that has previously not been possible.

Geophysical surveys, for example, are now routinely used to evaluate the condition of structures and detect the presence of voiding, changes in substrate characteristics and seepage. This allows for better targeting of more detailed intrusive investigations and remedial works. It can provide confidence to adopt more measured, lower-carbon interventions rather than reverting to comprehensive replacement works.

New methods of monitoring and surveillance can bring greater insight and efficiencies in assessing performance. At high dams, drones have provided a step change in the quality of surveillance. Laser surveys and photogrammetry bring efficiency and accuracy in surveying surface and subsurface structures and can be useful in developing digital models for design applications.

Numerical modelling and geophysical surveys informed our design approach at remediating the spillway at Talybont Reservoir, Wales, where revetment and grouting works presented a substantial cost and carbon saving over more traditional concrete relining works.

Making the most of what we have

Creating new reservoir storage is becoming more difficult and challenging in light of society’s increased aversion to the physical, environmental and social impacts. New reservoirs are still being built, particularly for hydropower and flood control, but it is likely the rate of development will continue to decline in the decades ahead.

We must use available storage more flexibly to meet wider and sometimes competing demands on reservoirs.

One way is for water supply companies to use their reservoirs to store floodwater. Pilot projects are underway in the UK such as the one at Gorpley Reservoir in Yorkshire where Mott MacDonald Bentley, Mott MacDonald’s contracting arm, carried out modifications to facilitate its dual use.

The challenges in preserving water storage infrastructure vary from region to region, as underlined by a recent study by United Nations University Institute for Water, Environment and Health. This is where river basin transfers – across countries and across borders – can bring greater flexibility to reservoir operation, improving security of supply and drought resilience for millions of people.