Intra-annual dynamics of ice-rich riverbank erosion in the central Lena Delta

• Main Authors: Stettner, Samuel, Lantuit, Hugues, Bartsch, Annett, Heim, Birgit, Grosse, Guido, Roth, Achim, Widhalm, Barbara
• Format: Conference Object
• Language: unknown
• Published: 2016
• Subjects: Arctic; The Sentinel; Ice; lena delta; lena river; permafrost; Thermokarst; Tundra
• Online Access: https://epic.awi.de/id/eprint/41195/; https://epic.awi.de/id/eprint/41195/1/2016_ICOP_S1-3_talk_stettner.pdf; https://hdl.handle.net/10013/epic.48137; https://hdl.handle.net/10013/epic.48137.d001

Arctic warming accelerates the rapid degradation of ice- and organic-rich permafrost landscapes through thermokarst and thermal-erosion. These processes lead to the retreat of ice-rich coasts, riverbanks, lake shorelines, to surface subsidence and gullying. The subsequent reactivation of ancient carbon previously stored in the eroded ice- and organic-rich sediments could have tremendous impact on the carbon cycle from regional to global scale. Yet, information at high temporal and spatial resolution is often lacking to describe the rates and the timing of permafrost degradation. Synthetic aperture radar (SAR), which operates independently of atmospheric distortions, is particularly valuable to alleviate these issues because of its potential for high temporal resolution monitoring in a region where cloud cover often limits the use of optical satellite imagery. In this study, we used SAR data to investigate the spatiotemporal dynamic of a rapidly degrading ice- and organic-rich up to 50-m-high and 2000-m long riverbank in the central Lena Delta. Our main objectives were to 1) assess the applicability of synthetic aperture radar (SAR) satellite data for high-temporal resolution monitoring of rapidly eroding riverbanks and 2) to identify the seasonal timing of ice-rich permafrost riverbank erosion. We analyzed a unique time-series of high-spatial and temporal SAR images from the German TerraSAR-X (TSX) satellite, operating in X-band wavelength, as well as very high resolution optical satellite imagery and in-situ time-lapse data. We processed 77 HH- polarized SAR backscatter images with acquisition dates between August 2012 and October 2015. The imagery was first pre-processed using the Sentinel-1 toolbox from the European Space Agency. We then applied a thresholding to better identify the transition line from undisturbed tundra surface to the actively eroding cliff we refer to as cliff top line. We then calculated cliff top retreat rates and finally compared these with environmental baseline data to identify the main driving factors of riverbank retreat. Visual interpretation of the TSX time-series showed that the cliff of the riverbank is only visible in the months June to October. Annual erosion rates were in the same range when comparing the optical reference with the SAR datasets. The in-situ time-lapse data for the summer of 2015 showed similar results for the intra-annual erosion compared to the SAR derived results. Based on the SAR dataset we detected mostly constant erosion rates at our test site throughout the thawing period for the years 2013, 2014 and 2015. Our results show that the cliff-top at the test site retreats constantly over the thawing season rather than event driven (i.e. through the spring peak discharge only). The studied cliff top is protected from spring flood events by sandbanks in front of the riverbank. However, runoff caused by permafrost thaw, precipitation and flooding will degrade the protecting sand banks and consequently will lead to a reconnection of the cliff system to the Lena River System, even when water level is lower towards the end of the thawing season. We conclude that x-band backscatter time-series are valuable for monitoring rapid permafrost degradation with high spatial and temporal resolution. Our results indicate that cliff top erosion of ice-rich riverbanks takes place constantly over the thawing period and is not event driven.