RECENT DEBRIS FLOW DYNAMICS IN SENGISJOK VALLEY (LOVOZERSKIYE TUNDRY, KOLA PENINSULA)

Postglacial debris flow landforms are widespread in the Lovozerskye Tundry. However, there is almost no data on the recent debris flow dynamics. In 2009–2010 and 2017–2018, we investigated debris flow topography and conditions of debris flow formation in the Sengisjok Valley on the western slopes of the Lovozerskye Tundry applying satellite imagery interpretation, field geomorphological survey, UAV survey, and 14C-dating of sediments. Footprints of debris flow activity were revealed along the entire valley: the modern valley bottom is completely reworked by the recent debris flows while in the transit zone, older debris flow terraces reach up to 20–30 m high. A debris flow fan has an area of about 0.48 km2, with only 17% occupied by the Late Holocene accumulation.

On July 3, 2017, we observed a high-energy debris flow. Following the long and snowy spring and cold June, the rapid snowmelt triggered the event as positive air temperatures had settled round-the-clock. The event started as a hydraulically driven slushflow when a snow dam at the threshold of the Sengisjavr Lake’s outflow was breached. In the transit zone, the slushflow gradually saturating with debris transformed into a typical debris flow with an estimated velocity of 3.4–4.1 m/s and discharge of 75–90 m3/s. Total debris flow volume can be assessed up to 320 000 m3 with the volume of a clastic load of up to 80 000 m3. The debris flow passed 6.5 km.

The timing of another two high-energy debris flows that occurred in the last 150 years has been determined. The estimated frequency of such flows is 1 event per 15–20 years. Snow dam breaches with the subsequent partial drainage of the Sengisjavr Lake are the key debris flow drivers in the Sengisjok Valley.