Satellite image analysis and frozen cylinder experiments on thermal erosion of periglacial fluvial islands

Frozen islands in the Lena River, Siberia, experience rates of fluvial thermal erosion exceeding 10 m/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 degrees 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.