RESEARCH ARTICLE Satellite image analysis and frozen cylinder experiments on thermal erosion of periglacial fluvial islands Laure Dupeyrat 1 | Benoît Hurault 1 | François Costard 1 | Chiara Marmo 1 | Emmanuele Gautier 2 1 GEOPS, Université Paris‐Sud, CNRS, Université Paris‐Saclay, Orsay, France 2 Université Paris‐Sorbonne, UMR8591, Paris, France Correspondence Laure Dupeyrat, GEOPS‐Géosciences Paris Sud, Université Paris‐Sud, CNRS, Université Paris‐Saclay, Rue du 4 Belvédère, Bâtiment 504‐509, 91405 Orsay, France. Email: laure.dupeyrat@u‐psud.fr Abstract Frozen islands in the Lena River, Siberia, experience rates of fluvial thermal erosion exceeding 10m/year. The islands erode differentially, with rates of frontal retreat exceeding those on island sides. We define the erosion ratio (ER) between the front and sides to estimate this differential erosion. A GIS‐based study of 19 islands from 1967 to 2010 indicated average erosion rates of 19.7 and 3.7 m/year for the island heads and sides, respectively. The average ER over the period was 4.7. An analytical model of local thermal erosion for a frozen cylinder of sand in a turbulent water flow is proposed, assuming an ablation process. Thermal erosion of 19 frozen cylinders was measured for water flows of different temperature and velocity in a cold chamber. As observed in the field, frontal erosion always exceeded lateral erosion, with an average ER of 1.6. The ER decreased with increasing temperature from 5 to 15°C. The higher value of ER in the field may be due to interactions with neighboring islands and banks. An empirical law including phase change and the process of erosion is proposed, and validates our model compared with previous laws that do not account for erosion. The erosion process enhances heat transfer. KEYWORDS cylinder, islands, Lena, permafrost, thermal erosion 1 | INTRODUCTION Since 1970, numerous studies have investigated the stability of periglacial rivers in response to increased development of Arctic oil and gas resources. Although several factors affect river channel pat- terns, bank erosion is the most significant. 1 Periglacial channels undergo differential erosion controlled by bank material, ice content, vegetation, flow dynamics, bank height, 2 or channel morphology. 3 The morphology of the Lena River in Siberia is characterized by islands whose erosion modifies the river's pattern of flow. Periglacial rivers are affected by ice break‐up, which induces a sudden increase in water level, discharge and stream temperature. 4 This results in thermal ero- sion on the frozen river banks that can generate annual bank retreats of up to 40 m. 5 Thermal erosion has been studied in the field 5,6 and experimen- tally in the laboratory using water flow over a horizontal ice sheet, 7 frozen samples of sand and ice 5,8 or sloping frozen banks. 9 Using these laboratory configurations, empirical laws 5,7 and analytical 10,11 and numerical models 12 have been proposed. The relative influences of water discharge, temperature and sample composition have been quantified, and water stream temperature has been identified as the main control on thermal erosion. 8 Water flow in these experi- ments was tangential to the frozen sample, in the same way as for water flows along a river bank or island side. However, island heads are subject to water flow intersecting the bank at a high angle. Despite a relatively good understanding of the main parameters involved in the process of fluvial thermal erosion, 5,6,8 only a few studies report on the variability of the erosion rate during the flood season. 2,5 The purpose of this study was to quantify the difference in erosion rates at island heads and sides, and to investigate the effects of water temperature and velocity. First, we analyzed satellite images of islands in the Lena River to compare frontal and lateral erosion. Next, we used a two‐dimensional (2D) cylindrical model of local thermal erosion of a frozen cylinder in a turbulent water flow developed from a previous 1D Cartesian model. 8 This model was compared here to typical models of cylinders in a cross flow without erosion. Finally, we performed six series of repetitive measurements of local thermal erosion of frozen Received: 18 July 2016 Revised: 12 December 2017 Accepted: 22 December 2017 DOI: 10.1002/ppp.1973 100 Copyright © 2018 John Wiley & Sons, Ltd. Permafrost and Periglac Process. 2018;29:100–111. wileyonlinelibrary.com/journal/ppp