Experience in diverting and containing lava flow by barriers constructed from in situ material during the 2021 Geldingardalir volcanic eruption

Abstract The 2021 Geldingadalir eruption in the Mt Fagradalsfjall Volcanic System within the Reykjanes Peninsula volcanic zone was the first eruption on the peninsula in about 800 years. Prior to the eruption, increased earthquake activity and signs of magma intrusion indicated a volcanic threat to populated areas and important infrastructures. Preliminary design principles were developed for protection works comprising lava barriers, earthen diversion barriers, or dams, respectively, for diversion or delaying lava flow. During the 2021 event, three dams were constructed from in situ earth material, along with a 300-m-long diversion barrier, accompanied by a short 35-m diversion barrier. The barriers constructed reflected the site conditions, available material, and equipment. The article describes the dam construction and the experience in securing the construction site, as well as diverting, containing, and delaying lava flow by the different barriers, for potentially reducing/delaying downstream effects. The importance of considering the lava type in the design of lava barriers is highlighted. ʻAʻā lava thickened considerably behind a barrier with influx of fresh lava under chilled outer crust, while sheets of pāhoehoe lava accumulated and eventually overtopped or bypassed it. Generally, pāhoehoe lava creeping slowly over barriers did not cause erosion or damage. However, pāhoehoe lava could cause failure on a downstream slope in the case of weak dam tops of loose material. The dams delayed lava flow, one of them by up to 16 days, and the diversion barriers diverted lava effectively. The lessons learned were valuable for constructing lava defences in subsequent eruptions closer to populated areas, starting in 2023.