In-Situ Bathymetry and Volume Estimation of Glacial Lakes in Western Himalaya

In recent years, glacial lake outburst flood (GLOF) hazards in the Himalayan region have garnered considerable attention. The expansion of glacial lakes and the corresponding increase in volume play major roles in the initiation of GLOFs. Due to the lack of systematic assessments and the challenges associated with conducting field surveys, communities living downstream face significant risks from potential GLOFs. Accurate volume estimation of glacial lakes is crucial for calculating outburst flood peak discharge and simulating flood evolution. However, in-situ bathymetry-derived volume estimations are limited to only a few glacial lakes. Consequently, earlier studies have relied on volume-area empirical relations, which have shown substantial discrepancies. In this study, we surveyed four glacial lakes—Kya Tso Lake (KTL), Panchi Nala Lake (PNL), Gepang Gath Lake (GGL), and Samudri Tapu Lake (STL)—located in the Chandrabhaga basin, western Himalaya. Among these, GGL and STL are reported as potentially dangerous glacial lakes (PDGLs) due to their rapid expansion and risk of future bursts. In-situ depth measurements were conducted using an echo sounder mounted on an unmanned surface vehicle (USV) and portable inflatable kayak. The lake basin morphologies were modelled using triangulated irregular networks (TINs). We compared the bathymetry-derived lake volumes with volumes estimated using commonly used empirical equations. The results revealed maximum depths of 16 m, 10 m, 46 m, and 59 m for KTL, PNL, GGL, and STL, with corresponding storage capacities of 0.89 × 10⁶ m³, 0.44 × 10⁶ m³, 24.12 × 10⁶ m³, and 24.69 × 10⁶ m³, respectively. Substantial discrepancies (± 47-309%) were observed between volumes derived using existing empirical equations and those obtained through in-situ bathymetry for all four lakes. None of the commonly used equations produced similar volume with in-situ observations. Despite several challenges during the USV survey, like noise from variable sound penetration under different turbidity, limited telemetry, wind, boat speed relative to water depth, and floating ice on lake, this study provides valuable in-situ bathymetric data for future modelling and hazard assessment of rapidly expanding PDGLs in the region. The present study emphasizes the need for more robust, in-situ-based bathymetric datasets of glacial lakes to develop an empirical equation with better applicability.