[caption id="attachment_36085" align="alignright" width="300"]Barrage de Malpasset -Panorama 0 Malpasset arch dam failure in France in 1959 (421 people lost their lives)[/caption] Fortunately, few catastrophic failures have occurred in the UK and Ireland in recent history. Since 1925, there has been no loss of life due to dam disasters (Charles, 2011). However, there have been many failures involving the breaching of embankments, along with many near misses and other serious incidents (Wright, 1994). Globally, however, there have been many dam failures where lives have been lost (Charles, 2011). Essential insights and lessons can be learned from past dam incidents, near misses and failures to assist dam professionals and managers to better understand and manage risks associated with raised reservoirs. Often, new good-practice guidance and legislation is developed following serious incidents with the aim of preventing these reoccurring in the future. There are a number of databases and reports detailing dam incidents and failures providing important references of information including:

  • Tedd (1992): The Building Research Establishment (BRE) national dams database;
  • International Commission of Large Dams (ICOLD) (1984), Lessons from dam incidents;
  • ICOLD (1995): Statistical analysis of dam failures;
  • Environment Agency (EA) (2008): Post-incident reporting for UK dams, 2007 Annual Report (and subsequent annual reports from 2008 onwards); and
  • Charles (2011): Lessons from historic dam incidents.

Threats and modes of failure

[caption id="attachment_36087" align="alignright" width="300"]Ulley Overflow failure of Ulley dam spillway in the UK in 2007[/caption] Of the 76 incidents recorded in the UK between 2004 and 2013, 32 were as result of external threats, principally flooding, and 33 were as a result of internal threats, principally internal erosion (Charles, 2011). Other threats include ageing, animal activity, poor design or construction, and mal-operation. Many dams in the UK and Ireland are over a century old, and are often subjected to extremes of weather in remote environments. These threats can lead to various modes of failure including:
  • Catastrophic overtopping – uncontrolled flow of water over the dam crest;
  • Dam breach – uncontrolled flow of water though the dam’s main structure;
  • Foundation failure – design loads exceeded or loss of foundation strength;
  • Instability of concrete dam – dam slides or overturns because design loads are exceeded;
  • Instability of embankment dam – design loads exceeded or reduction of fill material strength;
  • Structural failure of concrete dam – design loads exceeded or due to deterioration over time;
  • Structural failure of embankment dam – design loads exceeded or due to deterioration of fill and/or core materials over time and/or increased permeability;
  • Overflow failure – overflow is unable to contain flow during flooding;
  • Uncontrolled flow due to appurtenant works failure – change in load conditions or deterioration over time resulting in an uncontrolled release of water;
  • Uncontrolled seepage – uncontrolled passage of water through, under or around a dam.
Some other notable threats and causes of incidents include:
  • Rapid drawdown of a reservoir causing slope instability of an embankment dam;
  • Human error, for example, maloperation of overflow gates or valves on outlet pipelines;
  • Leakage due to unknown pipes within or under a dam;
  • Burrowing animals at embankments;
  • Poor design and construction.

Reducing risks of dam failure

[caption id="attachment_36088" align="alignright" width="300"]Oroville dam spillway failure in the USA in 2017 Oroville dam spillway failure in the USA in 2017[/caption] Dam owners and managers are responsible for constructing and maintaining safe dams. It is vital that owners do not become complacent during periods without dam failures. There will most likely be future flood events due to increasing extremes of weather that will test the safety of many dams. While an experienced dam engineer may be able to detect early signs of potential problems during routine inspections, the owner should be aware of changes in the behaviour and appearance of their dams and should contact a dam engineer if they see any signs of leakage, movement or deterioration in the dam and its associated structures. Early identification of defects such as leaks, cracks, slips and erosion are important to prevent these from developing into bigger problems. Frequent surveillance visits by dam owners and managers are essential to watch for these defects. Once a problem has been found, the speed of response and, in particular, the capacity to draw down the reservoir level to take pressure off the dam is a key factor of reducing the risks (Charles, 2005). As well as undertaking routine surveillance, routine maintenance of dams and associated structures is also an important ongoing process to prevent dam failures. This should include ongoing and timely maintenance and monitoring of:
  • Upstream slopes – replacement and repair of wave protection systems; removal of shrubs and woody vegetation;
  • Downstream slopes – repair of erosion caused by burrowing animals, vehicle damage or other cause; maintaining grass length to enable easy observation of changes; maintaining grass cover to prevent erosion caused by overtopping;
  • Dam crest – repair of any damage to the crest and wave wall;
  • Spillway channels – replacement of deteriorating joint sealant between slabs; replacement of missing blocks; patch repairs; vegetation clearance; removal of debris at the weir;
  • Pressure relief drainage, underdrainage and other drains – clearing blocked drains; CCTV survey to check condition of drains; monitoring of flows to check for abnormal changes;
  • Valve towers – repair of any damage to the structures, ladders and platforms; monitoring of any leakage into the structure; collection of leakage to prevent it wetting any metalwork;
  • Tunnels/conduits – repair of any damage to the structures; clearance and monitoring of any silt on the tunnel/conduit floor; monitoring of any leakage into the tunnel/conduits; CCTV surveys if people are unable to enter;
  • Pipelines – routine painting to prevent corrosion; repair and/or replacement of any deteriorated sections;
  • Valves and gates on inlets, outlets and overflows – regular testing under pressurised conditions; timely repair and/or replacement of any non-operational key valves;
  • Instrumentation – repair any faulty instrumentation;
  • Access road – repair of ruts and potholes; repair damaged bridges.
There is much guidance to assist owners and managers with the proper management of their dams to effectively reduce the risks of dam failure. These include the Environment Agency’s Guide to Risk Assessment for Reservoir Safety Management (EA 2013), which gives a robust framework, supported by evidence, to assess the risks associated with any reservoir.

Summary

Dam safety is about managing dams to an acceptable level of safety to minimise any risk to life, property, essential services and the environment. There are many records documenting case histories of dam incidents and failures, which have led to improved guidance and technical understanding, and ultimately to safer dams. ICOLD (1995) found evidence that the percentage of dam failures has been falling over the last four decades, with less than 0.5% of failures occurring of the 12,138 large dams built between 1951 and 1986. However, that is still 59 failures around the world. Long-term concerns for improving dam safety include climate change, ageing dams and deterioration. With the growing need for increased water resources for water supply, hydropower, flood mitigation, industry and agriculture, dams will continue to be built and raised in height. To ensure the statistic for dam failures continues to fall and to prevent further serious dam incidents occurring such as the overflow failure of Oroville dam in the USA earlier this year, excellent standards of dam safety management are vital. Author: Tracey Williamson BEng CEng FICE, associate director, ARUP References: Charles J A (2005). 'Use of incident reporting and data collection in enhancing reservoir safety.' Dams and Reservoirs, Vol. 15, No. 3, pp 35, Thomas Telford, London. Charles J A, Tedd P, and Warren A (2011). 'Lessons Learnt from Dam Incidents.' Report SC080046. Environment Agency, Bristol. EA (2008). 'Post-incident reporting for UK dams, 2007 Annual Report.' www.publications.environment-agency.gov.uk EA (2011). 'Modes of dam failure, and monitoring and measuring techniques.' Report SC080048. Environment Agency, Bristol. EA (2013). 'Guide to Risk Assessment for Reservoir Safety Management.' Report SC090001/S. Environment Agency, Bristol. ICOLD (1984). 'Lessons from dam incidents.' ICOLD, Paris. ICOLD (1995). 'Statistical analysis of dam failures.' ICOLD, Paris. Tedd P, Holton I R and Charles J A (1992). 'The BRE dams database. Water Resources and River Engineering.' Proceedings of 7th Conference of British Dam Society, pp 403-410, Thomas Telford, London. Wright C E (1994). 'UK reservoir failures and safety legislation.' Dams and Reservoirs, Vol. 4, N0. 3, pp 20-21, Thomas Telford, London.