Most of Switzerland’s large dams were built from 1940-70, mainly for the purpose of hydropower. As Schwager et al discuss in their paper on ‘Swiss Dam Safety Regulation”, in recent decades an increasing number of smaller dams have also been built to protect against natural hazards such as flood and debris flow. However, as Swiss dam safety regulations were developed for larger dams serving the hydropower industry, various adaptations are now need for these natural hazard protection dams (NHPD).
Under Swiss law a retaining dam structure is any installation designed to retain water, debris, sediment, ice, or snow – whether on a permanent or temporary basis. About 50 NHPDs have been built across Switzerland since 2000 and they have been subjected to the same safety directives as water retaining facilities in case they fulfil the size criteria, or have a particular risk potential in case of collapse of the retaining facility.
As the authors highlight in their paper there are fundamental differences between NHPDs and conventional water retaining ones. These include that NHPDs are built for civil and infrastructure protection when conventional ones are used mainly for hydropower production. NHPD reservoirs are seldom impounded and when they are, only for hours or days (or in the case of debris and snow possibly weeks to months), whereas conventional ones are impounded for most of the year with water.
This potentially leads to ‘unproportionate safety requirements’ for NHPDs and there has been a call to revisit and, where appropriate, add or modify safety regulations for this specific type of smaller dams. Aspects that require further research and potentially updates to Swiss dam safety regulations include:
- Specific load cases and load combinations for NHPDs, such as avalanche impact forces.
- Determination of the downstream inundation zone in case of (partial) failure (e.g. breach mechanisms, remobilisation of retained debris, vulnerability of downstream objects).
- Specific monitoring and maintenance concepts.
- Emergency planning.
A risk-based approach is considered to be the most appropriate and reliable option to develop a proportionate safety assessment for these natural hazard protection dams.
AAR
One of the other challenges facing Swiss dams as they age includes alkali aggregate reaction (AAR) when certain aggregates in concrete react with the alkali pore solution, causing expansion and cracking. Schwager et al claim the majority of large Swiss concrete dams were built without much knowledge about the reactivity of such aggregates, and the Swiss Committee on Dams estimates that 35-45% of the country’s concrete dams are affected. However, recent findings indicate that this could be even higher.
Although previous research has focused on understanding and diagnosing the chemical and micromechanical processes of AAR and possible modelling approaches have been published, no comprehensive framework has yet been established to reliably determine the effects of AAR on dam safety.
Schwager et al say that in order to build such a framework, the following questions need to be answered:
- How does concrete with AAR behave under environmental conditions (such as under the influence of temperature, moisture, alkali content in concrete, stresses and creep in concrete)?
- How can the behaviour of a dam with AAR be predicted on the basis of model calculations?
- How does the evolution of damage due to AAR affect the safety of the entire dam (eg how is the resistance of a dam against static and seismic loading affected by AAR)?
To answer these questions, the Swiss Federal Office of Energy (SFOE) is initiating in-depth research and is also planning to evaluate the above in a few well instrumented case studies of dams, establishing a database on the subject.
Seismic
Switzerland has one of the oldest portfolios of dams in the world which, at an average age of 72 years old, were mostly not designed for earthquake loading – or at least not for modern day seismic standards.
Various reviews have taken place in recent years and the dam safety directive was revised in 2021 to include more seismic hazard analysis. These revisions aim to follow the regulatory philosophy that reduced uncertainty due to in-depth investigations may allow for less conservative model assumptions. Since the publication of the new directive, a comprehensive review of seismic hazards for all federal dams confirmed the expected increase of the hazard (in terms of spectral acceleration) is about 75% of dams.
SFOE is also working to update safety assessments to the state-of-the-art, such as accounting for foundation, mass and radiation of energy at the model boundaries; plus consideration of nonlinear behaviour of material, block and dam-foundation joints, etc. It is also working on a manual on state-of-the-art seismic modelling of dams, in particular regarding the assumptions for non-linear FEM analyses, which will support dam owners and their engineers.
The Contra Dam supports a 105MW power station on the Verzasca River in the Val Verzasca, Switzerland
Reference
Swiss dam safety regulation: Framework, recent changes and future perspectives by M.V. Schwager, A. Askarinejad, B. Friedli, P.W. Oberender, A.J. Pachoud & L. Pfister. Role of Dams and Reservoirs in a Successful Energy Transition – Boes, Droz & Leroy (Eds). Proceedings of the 12th ICOLD European Club Symposium 2023 (ECS 2023, Interlaken, Switzerland, 5-8 September 2023). https://doi.org/10.1201/ 9781003440420
