Trench investigation to quantify debris flow activity for landslide hazard mapping in populated areas: Lessons learned from Gol, southern Norway

We here describe the results of stratigraphic and sedimentological examinations of debris flow deposits at Breidokk, Gol, southern Norway. The deposits are situated at the valley floor, below a steep slope with three large and several smaller debris flow channels incised into the thick till cover. The study area is populated and with abundant infrastructure such as roads, public and private buildings and other types of infrastructure, including underground water pipes and cables. Six, 10–15 m long and 1–3 m deep trenches were dug out with an excavator and examined. The sediments in the trenches consist of moraine-, glaciofluvial/fluvial- and debris flow deposits. The latter consist of matrix supported, unsorted, massive beds from 1 cm to more than 1 m in thickness, with clasts up to 80 cm in diameter. A total of 16 post glacial debris flow beds are identified in five of the six trenches, representing a minimum of eight individual debris flow events. This is probably an underestimation of the debris flow activity through postglacial times as the location of the trenches was in large determined by infrastructure and were not optimally placed for mapping all debris flow deposits in the area. Also, correlation between trenches proved difficult. A total of 37 radiocarbon ages of buried soil and other organic material situated above and below debris flow deposits, together with the sedimentological and stratigraphical interpretation, show that debris flow activity has prevailed throughout the Holocene, also within the last 1000 years. A possible increase in activity within the last 3–4000 years BP has been noted. This is important knowledge to aid in the interpretation of the Quaternary history of the area but also to determine the hazard zones.