Dynamics of the rapid topographic changes after recent moraine breach: a case study of the Djankuat catchment, Central Caucasus

In the Caucasus, there have been reported and documented seven recent moraine failures. Such events are common for highly unstable proglacial areas. Here we present a detailed reconstruction of the moraine failure in the Djankuat valley in the Central Caucasus. We then analyze the subsequent terrain dynamics of the breach and its surroundings.The event occurred on 1st July 2015 and led to the alteration of the Koyavgan creek stream. Koyavgan creek is the largest tributary of the Djankuat River that emerges from the small <0.5 km2 glacier in the cirque and further conjoins the stream from another small glacier Via-Tau. The total area of the watershed above the breach is 3.03 km2. Primarily the collapse was caused by intensive precipitation: 227&#160;mm during the seven preceding&#160;days. During the weekly period, the surface runoff was significantly increased due to the relatively large drainage area. It disintegrated the internal moraine sediments and led to a collapse. The buried ice underlying the moraine ridge had a crucial role in the event. The investigated section of the moraine on 60 m width was lying below the adjacent moraine edges before the event due to degradation of the ice core.&#160;To accomplish the established goal, we use satellite imagery in combination with repeated UAV surveys. Based on high-resolution stereo images we construct DEMs with a spatial resolution of 1 m to estimate the event.&#160;Since 2019 we have accomplished UAV monitoring: currently, five surveys have been conducted. Followingly, we obtained high-resolution DEMs with spatial resolution varying in diapason 4.7-9.5&#160;cm. We divided the area into several zones: the breach, the area of regressive erosion, the accumulative cone, and two sides of the adjacent lateral moraine. Finally, we conducted a quantitative analysis of the terrain dynamics for these sections.The&#160;total volume of eroded material during the breach event is&#160;105,000-146,300 m3. After the breaching, the area behind started to experience active regressive erosion: 80,000 m3 of the moraine was removed from the moment of the breach and for the following 2 years, then the rates slowed down for a value of 6,200 m3 denudation between 2019 and 2021. However, in the 2021-2022 interval, the dynamics switched to positive with 1540 m3 accumulation. The breach itself continued to erode after the event: 1310 m3 over 2019-2021 and 240 m3 over 2021-2022 periods. We could observe the stabilization of the terrain for these sections. The new-formed cone has a similar denudation tendency: 9880&#160;m3 in 2019-2021 and 6320 m3 in 2021-2022. The moraine breach area including the fan is crucial in terms of sediment delivery: during rainy periods (>30 mm) the Koyavgan creek contributes most sediments to the Djankuat River. And they are mainly associated with the denudation of the breach area, cone, and area of regressive erosion behind. Furthermore, it was proven that extreme hydrological events (1-12% of the annual events) that emerged from heavy rainfalls contribute to about 50% of total sediment yield.