Ensuring reservoir safety and maintenance in the UK requires a variety of skills and diplomacy from the inspecting engineer. Chris Hoskins reports
Historically, concerns about reservoir safety followed several disasters and loss of life in the UK. This resulted in safety legislation through the Reservoirs (Safety Provisions) Act 1930 and the Reservoirs Act 1975. The current Act covers some 2600 reservoirs in the UK. It sets the legal framework for the design, construction, inspection and alteration or enlargement of reservoirs, and subsequently their abandonment or discontinuance. It is gratifying to note that since legislation was first introduced seventy years ago there have been no dam failures involving loss of life, despite some serious incidents.
The legislation can only be applied by qualified civil engineers appointed by the Secretary of State for the Environment after consultation with the Institution of Civil Engineers. The engineers are appointed to a series of panels with differing responsibilities to carry out specific functions, including inspections of each reservoir at intervals not exceeding ten years.
Most dams subject to the safety legislation are small, with only 22% greater than 15m in height (the international definition of a large reservoir). About 50% of the reservoirs are owned by water supply companies such as British Waterways, the Environment Agency and hydroelectric companies. These organisations own most of the dams in excess of 15m, with the remainder owned by individuals and organisations who each typically own one or two reservoirs. Of these reservoirs, over 80% impound watercourses with the remainder forming off-stream, non-impounding or service reservoirs. Only 20% of dams are of concrete or masonry construction. The rest are embankment dams of mainly earth construction.
The average age of dams in the UK is now in excess of 100 years, with approximately 70% built before 1900. Only about 8% have been built in the last 40 years using modern soil mechanics practice and large earth moving plants. These later dams were often built in less straightforward sites.
Most modern large embankment dams in the UK have relatively shallow slopes (typically 1V:3H or flatter), wide cores and cut-offs, and internal filter/drainage arrangements instead of zoned shoulders with narrow puddle clay cores and cut-off trenches. Few concrete dams have been built since the hydroelectric projects in Scotland in the 1950s, and the use of RCC has not proceeded to any extent in the UK.
An ageing dam population, often with little information about their construction details and history, provides the background against which inspecting engineers carry out their duties under
Dam safety in terms of the Act is solely related to the escape of water from the reservoir and its effect on persons or property. It is not directed at the integrity of the structure forming the reservoir as such (the word ‘dam’ does not appear in the Act) nor is it directed at maintenance or public/employee safety, although an inspecting engineer is likely to comment on these aspects in his report on the inspection.
The worst dam disaster in the UK was the failure of Dale dyke near Sheffield in 1864, which killed about 260 people. Recommendations that dams should be inspected regularly to minimise the risk of such failures were made at the subsequent enquiry. Concerns had already been raised after the collapse of Bilberry dam at Holmfirth in Yorkshire in 1852. In this instance, failure had occurred after a history of financial and engineering problems without action being taken. Attempts in the 1860s to introduce statutory controls for reservoirs over about 28,000m3 (only slightly greater than the amount introduced in legislation many years later), were unsuccessful and it required further failures in 1925 to generate action. In that year, Skelmorlie dam in Scotland and Eigiau dam in Wales both failed, killing five and sixteen people, respectively. At about the same time, failures occurred in Italy, the US and North Africa.
The Reservoirs (Safety Provisions) Act of 1930 introduced statutory controls on the creation and operation of reservoirs and finally invoked the proposals envisaged many years earlier.
The 1930 Act applied to all reservoirs in excess of 5M gallons (approximately 22,500m3), this being the size of the two reservoirs which had failed. This Act required an independent qualified civil engineer to be appointed to supervise construction and for periodic inspections at periods not exceeding ten years. The Act also required the undertakers to keep records of water levels and outflows, leakages, settlements of walls and other works and repairs; and to publicise the appointment of the inspecting engineer, the receipt of the report and accompanying certification. Whilst this Act worked well, it was realised that there were insufficient statutory powers to ensure compliance and that many reservoirs were not being inspected regularly and recommendations were not being carried out. More continuous supervision was also recognised as being necessary.
The 1975 Reservoirs Act was introduced to bring registration, enforcement and emergency powers for local authorities, continuous supervision of reservoirs and provisions for abandonment and discontinuance. Engineers are appointed to panels to carry out the statutory inspections and other duties by the Secretary of State for the Environment, Transport and the Regions after consultation with the Institution of Civil Engineers. In practical terms, this is achieved by peer review. Engineers are appointed to a panel for a five-year period and membership has to be re-applied for after this time. The panels comprise an all reservoirs panel (AR), a non-impounding panel (NIR) and a service reservoirs panel (SR). Only engineers on the AR panel are able to deal with floods and their passage through the reservoir. A fourth panel — supervising engineer — has more limited powers and can only carry out the supervising duties as defined by the Act. All members of the three other panels can also act as supervising engineer.
Dam engineering involves experience, theoretical studies, detective work and occasional lateral thinking. A dam engineer needs to have a wide knowledge of hydrology and hydraulics, structures, seismicity, environmental issues, risk appraisal and geotechnical aspects. Reservoir inspections require the ability to appreciate visual evidence and to recognise change. The observations and techniques complement the construction and operational records and monitoring observations (settlements, deformations, groundwater levels, seepages and drainage flows etc). This approach builds up a good picture of the dam’s condition and behaviour and assesses any inadequacies. Practical guides on dam engineering by the Department for the Environment, Transport and the Regions, the Building Research Establishment and the Construction Industries Research Association offer guidance to the inspecting engineer.
One of the major current concerns is the effects of ageing — particularly in respect of earth dams, the onset of seepage and the potential for internal erosion. The main safeguards against this are the visual inspection by the inspecting engineer and the surveillance and monitoring provisions directed by him during the statutory inspections. Many reservoirs owned by the water companies and other larger organisations are well documented and supervised, and frequently have monitoring systems installed. For many smaller reservoirs, the standards of records and monitoring are often poor and sometimes non-existent. Visual appraisal is then even more important and requires a very careful appreciation of all the available evidence.
In some situations, other sensory evidence is available, such as aural (cavities, leaks), touch (soft ground) or smell (associated with water movement), and can be useful indicators of the dam’s behaviour. These observations seek to identify both change from a known or recorded situation and the presence of undesirable or significant phenomena.
The external surfaces of an embankment and its abutments can often provide clues to the interior behaviour. Surface displacement on embankment dams can often be detected by sighting along the line of the embankment road or wave wall, and may indicate deeper movement. Uneven profiles, if not previously recorded, might also indicate movement. Cracks, especially in potentially critical positions, need to be investigated and the reasons for their presence determined. They might indicate a problem or might be the result of surface desiccation, settlement or other less significant causes. The upstream slope needs to be examined for signs of damage or distress. The downstream slope, the toe and the areas beyond need investigating for wet areas, springs, boils of eroded material, or depressions that might indicate seepage. Existing drainage features and the behaviour of vegetation and animals can also be good indicators of change and can indicate alterations in seepage patterns or volumes. A lot of vegetation will not tolerate waterlogged roots and will die or show distress upon saturation; rushes and reeds indicate wetter conditions. Also, burrowing animals will not normally dig in wet ground.
Evidence is affected by the weather, the seasons or even the time of day. Temperature will influence the size of cracks and an extreme period of weather can lead to fresh or rejuvenated cracking in large concrete structures. Rainfall can mask new seepage flows. Freezing conditions can help identify seepages as the issuing water will normally be warmer. Unusual lighting conditions such as a low angle sun can often show undulations, breaks in slope or other features which are not apparent in direct light. Sufficient photographs taken during the inspection will be invaluable both as a memory aid and as a future base reference for subsequent inspections if they are included in the report.
Concrete and masonry dams require slightly differing criteria. Evidence of overstressing or movements between adjacent monoliths and changes in cracking need to be reviewed carefully. Seepages within, through, or under the dam body must be considered particularly relative to waterstops, lift joints and any eroded or deposited material. Pressure relief holes and other drainage measures need to function effectively.
Appurtenant structures which could affect the continuing safe operation of the reservoir include the inlet/outlet works, scour arrangements, overflow structures and channels, and in some cases, the power outlets. Both the condition of the structure itself and all allied features, such as lifting arrangements and other mechanical and electrical equipment, need rigorous inspection. Adjacent sections of embankment or natural ground need to be considered and any potentially adverse features noted. Potential problems with obstruction from debris need consideration.
Monitoring needs to be robust, simple and repeatable. The increasing use of more advanced methods can be helpful but must not distract the observer from the significance and accuracy of what he is monitoring. Readings are frequently taken in note form for later plotting by others. Ideally, they need to be compared directly with previous readings to allow any changes or anomalous observations to be recognised. A mass of unplotted and unchecked data made available to an inspecting engineer is often of limited value. Where future monitoring is recommended, the inspecting engineer seeks to specify reliable simple methods that will enable long term observations to be gained. Measurements of seepage flows need to be considered with respect to time, reservoir levels and rainfall. If adequate records are available, they can be extremely useful in investigating seepage flows and can show whether a situation is developing insidiously, unchanged or improving with time. Where appropriate, seepages can be sampled to investigate whether they are carrying any suspended solids that might suggest erosion is taking place.
The inspecting engineer is required to review his findings in the light of the risks from extreme natural and man-made events. The natural events are primarily the passage of flood flow through impounding reservoirs, the effects of winds and waves on the dam, seismic events and possibly siltation. Guidance documents have been prepared for the first three issues.
Design flood inflow is assessed on the basis of the downstream risk. The flood needs to be considered jointly with waves and the potential effects of wave overtopping and also damage to the upstream slope of embankment dams. Where spillway capacity is inadequate and an unacceptable risk of overtopping is found to exist, measures are normally recommended to increase the safe passage of the flood water to increase the available flood storage or improve the resistance to overtopping of the dam.
Seismic effects on the dam and reservoir slopes have become more important in recent years. Guidance is available for assessing the potential effects on the dam, but experience has shown that most well- built embankment dams can withstand moderate earthquake shaking. Problems are more likely with particular types of fill dams, loose foundations, some designs of concrete dams or the appurtenant structures.
Human activities can impinge more directly on dam safety. Downstream development will require the re-categorisation of the dam for flood considerations. Badly considered excavations can directly affect the dam’s safety, whilst tree clearance in the catchment might affect the flood inflow.
The inspecting engineer is required to report his findings and recommendations. Certain requirements are laid down in the Statutory Instruments accompanying the Act. The standard report format would normally contain details of the dam; monitoring and other records; comment on the onsite inspection; appraisal of the floods and the adequacy of the overflow and associated works; seismic considerations; supervision requirements; the adequacy of the records; and, lastly, the findings and recommendations of the inspection.
Recommendations in the interests of safety (ie against the escape of water) are separated from other works, maintenance, monitoring, public/ personnel safety, records and supervision. The recommendations in the interests of safety have the power of law and need to be clear, concise and able to be certified as complete. Means of implementing the recommendations are often outlined outside the report and the inspecting engineer must be careful to recommend a solution that is realistic and easy to implement. He should not normally seek either to tie a reservoir owner to one solution when others are equally acceptable, or to require unrealistic measures.
An inspecting engineer cannot consider safety in isolation, although this is paramount. His recommendations in the interests of safety can have environmental or planning implications or affect other parties adjacent to or downstream of the dam. A suitable solution has to be developed that recognises the many factors which cannot readily be expressed directly, as well as being appropriate in economic scale and technical complexity for the problem. An engineer also needs to be mindful of potential problems in formulating his recommendations. Many privately owned reservoirs are owned by more than one party and might also have a public highway across the top of the dam. All parties are deemed responsible under the Act and any inappropriate works might affect the parties unreasonably (eg limit certain uses) or cause a disproportionate distribution of costs.
Some recommendations of inspecting engineers have been poorly formulated and current standards or approaches might be hampered by these earlier measures. Local authorities or conservation organisations need to be considered where possible, and recommendations framed accordingly.
Inspecting engineers have to make the best informed decisions on the basis of the available evidence and experience to assess the dam and the reservoir, and identify any inadequacies. They have to review all the scenarios for failure and define the implications of such an event. The inspecting engineer might need to make fundamental and often expensive recommendations to avoid such events whilst being mindful of all the other influences and the implications of this approach.
In coming to these important conclusions, inspecting engineers are often required to make engineering judgements on insufficient information. They are required to draw on a wide range of engineering skills, backed up by experience, intuition and lateral thinking. They are also required to have practical, environmental and economic knowledge and a capacity for explaining their views carefully to doubting owners. It is the application of these skills and responsibilities that make reservoir inspections a rewarding experience.