Water Industry News

Trapped Sediment Affecting Global Dam Storage Capacity

A new report has revealed that trapped sediment is causing problems for many of the dams around the world and affecting their storage capacity, which experts are now predicting will have an impact across various aspects of society, including water supply, power generation and irrigation.


The study, carried out by the United Nations University’s Institute for Water, Environment and Health, indicates that trapped sediment has seen around 50,000 large dams robbed of between 13 and 19 per cent of their combined original storage capacity… and total losses are expected to reach between 23 and 28 per cent come 2050.


The UK, Ireland, Panama, Japan and the Seychelles were identified as being the countries likely to see the highest water storage losses over the next few decades of between 35 and 50 per cent of their original capacities. In contrast, Guinea, Niger, Ethiopia, Cambodia and Bhutan will be those nations least affected.


Over time, river sediment collects behind dam barriers and this is a problem that’s often ignored… but, because of the risks now posed to global water storage infrastructure, this has become a significant challenge, one that will need to be tackled with a long-term sediment management strategy.


Potential solutions


There are various potential solutions that could be implemented to help address the situation, with the report authors suggesting that their estimates could be improved upon through frequent bathymetry surveys and consistent monitoring of sediment transport.


Bypassing could also prove to be beneficial, where flow is diverted downstream via a separate channel to help mitigate the risks of high-flow events where sediment concentration can be particularly high. These channels, when operating at optimal levels, can reduce sedimentation by between 80 and 90 per cent.


Other options include enhancing dam height to help recover storage losses as a result of sedimentation, although it was noted that this should only be considered as a course of action following careful assessment of the dam’s structural strength.


Dredging could be another potential solution, although it can be expensive and is only a temporary measure. Sediment flushing can be more cost-effective, but it can also cause problems further downstream.


Finally, one slowly emerging practice is complete dam removal, where rivers are returned to their natural state and natural river sediment transport is reestablished.


Co-author of the report Dr Vladimir Smakhtin commented on the findings, saying: “Sedimentation is a serious issue that endangers the sustainability of future water supplies for many.


“It stimulates downstream flooding causing erosion, impacting wildlife habitats and coastal populations. And abrasive sediments can damage hydroelectric turbines and other dam components and mechanisms, decreasing their efficiency and increasing maintenance costs.”


Fellow report author Dr Duminda Perera made further comments, saying that new dams currently under construction or those still in the planning stages won’t offset storage losses to sedimentation and, as such, this new paper sounds the alarm on this increasingly important global water challenge, which has potentially significant implications for developments.


He went on to add: “Clearly, this study’s results need to be interpreted by local authorities with consideration given to local specifics and factors. What is most important to underline is the disturbing overall magnitude of water storage losses due to sedimentation. This adds to the list of world water development issues we need to address with resolve.”


Ageing water infrastructure


It’s not just sediment that’s a growing cause for concern where dams and water infrastructure are concerned.


Further research from the UN shows that by 2050 most of the world’s population will live downstream of dams built in the 20th century, which are becoming increasingly expensive to maintain and, as such, will likely lead to a rise in the decommissioning of such structures.


Signs of ageing include increasing dam failure, increased reservoir sedimentation, progressively increasing costs of dam repair and maintenance and a loss of functionality and effectiveness.


This report went on to note that well designed, constructed and maintained dams can reach 100 years of service with ease, but it’s likely that we will start to see a rise in decommissioning (particularly in the US and Europe at the moment) as practical and economic limitations means that upgrades aren’t carried out.


Commenting, Dr Smakhtin said: “Underlined is the fact that the rising frequency and severity of flooding and other extreme environmental events can overwhelm a dam’s design limits and accelerate a dam’s ageing process. Designs about decommissioning, therefore, need to be taken in the context of a changing climate.”


The report further noted that it is unlikely that the world will see another extensive dam-building revolution as was witnessed in the middle of the 20th century, but the dams that were constructed back then will inevitably now start showing their age.


Dam construction pace has fallen dramatically over the last 40 years, continuing to decline because the best places for these structures around the world have been diminishing progressively because almost 50 per cent of river volume globally is already regulated or has been fragmented by dams.


Furthermore, there are growing concerns about the environmental and social impacts of dams, while innovation is ongoing in finding different types of water storage and nature-based solutions, as well as energy production alternatives beyond hydropower.


The report concluded, stating that it is not an easy process to decommission dams and it will be important to share lessons on different experiences around the world. Failure to share knowledge and reflect upon regional and national policies and practices could have an impact on how water storage infrastructure is managed as it ages over time.


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