With this issue of Dams and Reservoirs being published in the winter it is appropriate to include two papers looking at the effect that ice forming on reservoirs can have on dams.
The paper by AECOM and Scottish Water summarises an investigation into the possible effect of ice on Scottish Water's concrete dams located at altitudes of 200 mAOD or greater. As well as identifying ways in which ice damage may occur, the investigation included a desk study, and the paper provides a comprehensive list of references and a bibliography, all of which will undoubtedly help other dam engineers if their reservoirs are prone to icing.
Although the first paper relates to the effect of ice on concrete dams, much of the information in that paper regarding ice itself may be useful for any dam safety analysis at ice-prone reservoirs. Denis Krutov's paper shows how fluctuating reservoir water levels under a layer of ice can damage the riprap on the face of an embankment dam by plucking out individual rocks. His field observations have led to the design of a system that should minimise such damage, with the system being explained in the paper with the aid of diagrams.
Although we have not had any reports of ice creating a dam safety incident in England, there are many other causes of serious incidents. Alan Warren's paper on the Environment Agency's mandatory post-incident reporting system for statutory reservoirs suggests that not all incidents are actually being reported, and in any case non-statutory reservoirs do not come within the remit of the system. A case study of the failure of one such reservoir is included to stress the necessity of including these in statutory reporting. The paper concludes with suggestions on how the current status of post-incident reporting could be developed throughout the UK.
The history of slips on the earth embankment of the Canal & River Trust's 200-year-old Elton Reservoir is recorded in a paper by Arcadis and the owner. After the most recent slip a number of investigative works were carried out, with the subsequent analyses culminating in the design, and then construction, of a number of deep counterfort drains. The paper is well illustrated, showing the problem, the analyses and the solution that was subsequently designed and constructed.
The ageing stock of the UK's dams and reservoirs means that many are not constructed in accordance with modern practice. An example is Scottish Water's Balgray Reservoir, where cast-iron draw-off and scour pipes were laid directly through the base of the embankment dam when it was constructed about 170 years ago. A statutory inspection raised concerns about the structural adequacy of these pipes and their ability to pass the required flow for drawdown purposes. The paper by a team from Mott MacDonald and Scottish Water explains how a structural investigation into the pipes was carried out to determine their condition which, in this case, allayed the concerns raised. The paper covers a range of methods that could be used for in situ pipes within a dam.
By their very nature, flood-detention reservoirs rarely fill, and cannot usually be tested at top water level prior to being used ‘in anger’. It is thus very important to ensure that the design considers all aspects of watertightness of the dam – which is typically an earth embankment. With flood-detention reservoirs being the most common type of reservoirs currently being designed in the UK, a paper on foundation cutoffs by a team from Jacobs is particularly appropriate. The paper describes a design decision flowchart which has been created, involving geological and geotechnical modelling, with each step carefully explained.
Our final paper in this issue is from a team in Italy, explaining the technical regulations for ensuring dam safety in that country, where a semi-probabilistic limit-state method of analysis for dam safety was recently introduced. The regulations define the external forces that must be considered – including ice loading. However, application of the regulations has proved problematic in practice, especially for existing dams, and the authors suggest possible amendments to ensure a consistent approach to dam safety in Italy.
