Verkís discussed fuse plugs at ECSMGE 2019
Pálmi Ragnar Pálmason and Kristín Martha Hákonardóttir, both engineers at Verkis consulting engineers, presented an overview on the design of fuse plugs in dams in Iceland at the European conference ECSMGE-2019 in Reykjavík, on soil mechanics and geotechnical engineering in September 2019.
Their talk was based on an article that was written by Pálmi, Kristín Martha, Fjóla Guðrún Sigtryggsdóttir, NTNU, Department of Civil and Environmental Engineering, Trondheim, Norway, Auður Atladóttir, Civil Engineer M.Sc. at Verkís, Hörn Hrafnsdóttir, Water Resources Engineer M.Sc. at Verkís and Ólöf Rós Káradóttir, Landsvirkjun, The National Power Company of Iceland, Reykjavík, Iceland. Fjóla and Ólöf worked at Verkís previously. Here is the article.
The need for a fuse plugs on a reservoir is mainly due to the potential of jökulhlaups being emptied into a reservoir. Such may vastly exceed the design flood for the pertinent dams. A fuse plug has been incorporated into many dams, as a mitigation measure not the least in order to control the potential damage of such floods.
The fuse plugs, generally comprise a dam section with a lower crest than other dams on the same reservoir. In the relevant flood event overtopping will thus first occur on the fuse plug, initiating marked erosion and subsequent breaching. The breached section must accommodate the passing of the flood, otherwise this will probably expand uncontrolably. In the case that a fuse plug adjoins the main dam, the lateral extent of the breaching should be limited by some measure, preferably a reinforced concrete wall and the flow has to be bypassed safely in a channel or by robust divertion dikes, downstream of the dam.
Fuse plugs are incorporated into most major dams on reservoirs fed by glacial rivers in Iceland. Eight fuse plugs have been constructed in the Þjórsá-Tungnaá catchment, and one in the Jökulsá á Brú catchment. The first fuse plug here in Iceland is presumably this on the left bank dike of the Búrfell dam, designed 1966 by the US Engineering firm Harza. The most recent fuse plugs were designed and reviewed in 2014 due to volcanic activity on the Bárðarbunga fissure swarm. Subglacial eruptions on this fissure swarm may result in jökulhlaups into the Hágöngulón reservoir, and subsequently into Kvíslaveita reservoir.
However, marked uncertainties exist in the design of fuse plugs: Thus, the empirical Froehlich breaching equations for breach width and the breach time may not apply fully here in Iceland. These do not consider directly the potential effect of the fill material used on the erosion; the throughflow capacity of the rockfill overlying the core, the effect of pilot channels for enhancing breach to note but a few. Neither is the effect of frost in the fuse plug fill nor state of the reservoir during the relevant flood considered.
To address a few of these uncertainties, Unnar Númi Almarsson, engineer at Verkís, has conducted experiments at the NTNU in Trondheim with Fjóla Guðrún Sigtryggsdóttir as his supervisor. The experiments will form a part of his master´s thesis.
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