MODELING BASED CHARACTERIZATION OF GLACIERS AND GLACIAL LAKE OUTBURST FLOODS (GLOF) USING EARTH OBSERVATION TECHNIQUES

Abstract

The Himalayan belt is known for its ice cover over the high-altitude regions, consisting of a total of more than 12000 glaciers. It has been observed that the sensitivity and vulnerability of the glaciers to climate change have led to its significant recession. Thinning and retreat of the glaciers have led to the formation of numerous high-altitude lakes in the Himalaya, most often dammed by glacial deposits. Several pieces of evidence have shown the growth of the existing lakes associated with the retreat of the parent glacier. The increase in the volume of these lakes can affect the integrity of the damming material, thereby increasing the chances of its failure. The growth of these lakes at higher elevations, therefore, increases the risk of the low-lying areas to catastrophic glacial lake outburst flood (GLOF) events. GLOF events are associated with the sudden discharge of a large volume of water along steep and narrow channels, with potential flow energy that can cause great damage to humans and infrastructure in the downstream region. GLOF events have mostly been reported extremely catastrophic in the Himalaya and are one of the primary concerns of the mountain communities due to the threat it imposes on them. This thesis is aimed at providing a synoptic view of the potential GLOF’s in the central Indian Himalaya (Uttarakhand and Sikkim). This is achieved by field and modeling based analysis of the potentially critical lakes in the region. Also, the integration of remote-sensing and hydrodynamic modeling enabled hazard assessment of the inaccessible lakes located at higher elevation. The preliminary data required for the analysis of GLOF hazard is a glacial lake inventory. The study presents a new updated glacial lake inventory for the central Himalaya. A series of hydrodynamic simulations were performed to model different moraine-breach scenarios based on different modes of failure (overtopping & piping) and breach parameters (breach width and failure time). The study performs dynamic routing of GLOF hydrographs along the main channels to evaluate the potential impact in the downstream region. This is achieved by characterizing the potential GLOF events in term of its hydraulic properties like flow velocity (ms-1), flow depth (m), inundation area (m2), peak discharge (m3s-1) and time of peak (in min). The growth of an existing proglacial lake and the formation of new lakes depends on the topography of the glacier bed. Accurate mapping of the glacier-bed topography enables evaluation of the potential lake formation sites in the future. The present study is aimed to evaluate the future GLOF potential of the existing proglacial lakes by mapping its maximum Page ii extent. This is achieved by spatially distributed glacier ice-thickness modeling, which in turn is employed to map depressions present on the glacier bed. The future hazard assessment of the lakes is performed by dynamic modeling considering the future volume of the proglacial lakes. Overall, this thesis is an attempt to characterize different potential extreme GLOF events (present and future) and to understand the complex flow hydraulics during its propagation along a given flow channel. It involves identification of critical glacial lakes in Uttarakhand and Sikkim Himalaya and the detailed risk assessments of four potentially critical lakes to evaluate its impact in the downstream region. The sensitivity of the hydrodynamic model is analyzed by performing a series of GLOF-scenario modeling on a potentially hazardous lake. The present study includes five different case studies of potential GLOF assessment considering four potentially critical lakes spread across the state of Uttarakhand and Sikkim. Based on the accessibility, the integration of field- and modeling assessment of the Satopanth lake located in the Alaknanda basin, Uttarakhand is undertaken. Field mapping of the lake and its associated moraine was performed using accurate DGPS points. Here, a potential GLOF event is coupled with a 100-year return period flood to evaluate a combined impact on a hydropower dam site located downstream. The potential flood event resulted in a peak discharge of 2612 m3s-1 that arrived at the dam site 38 minutes after the initiation of the breach event. The study also undertakes a bathymetry-based hydrodynamic study of the South Lhonak lake located in the Teesta basin, Sikkim. The lake is one of the largest lakes in the state of Sikkim which has shown exponential growth in the past decade. A detailed potential-GLOF assessment of the lake is performed based on the lake bottom topography. Different moraine-failure mechanism (overtopping and piping) is modeled for scenario evaluation. Further, one- and two-dimensional hydrodynamic routing is performed to evaluate the impact of a potential GLOF along the given flow channel till it reaches a hydropower dam site at Chungthang town located 62.3 km downstream of the lake. An overtopping failure of the lake resulted in a peak flood of 3828.08 m3s-1 calculated at the Chungthang town, which has a potential to inundate a total area of 55,000 m2 of the existing settlements where flow depth and velocity reach up to 8 m and 9 ms-1 respectively. The potential future-GLOF assessment was carried out on two lakes namely the Dhauliganga lake and the South Lhonak lake. The Dhauliganga lake is the largest and the highest proglacial lake located in the Dhauliganga basin, Uttarakhand. The maximum extent of the lake is mapped using an ice-thickness based approach. The future volume of the lake is calculated to evaluate a Page iii potential future-GLOF event of the lake. The potential impact on a hydropower dam site located 72 km downstream is assessed using one-dimensional hydraulic routing. At the dam site, a maximum discharge of 1686 m3s-1 is calculated where the peak discharge is reached within 98 min (1.6 hr) after the initiation of the moraine-breach event. Peak discharge of 1595 m3s-1 (Tf= 0.50 hr) and 1489 m3s-1 (Tf= 0.75 hr) is recorded at the dam site with a time of peak recorded at 103 min (1.7 hr) and 111 min (1.8 hr) respectively. Similar, evaluation is performed on the South Lhonak lake, Sikkim to examine the future potential of the lake. Three different potential moraine-breach scenarios based on varied time of moraine failure (Tf=1 hr, 2hr, and 3hr) is evaluated. The GLOF hydrograph Tf =1.0 hr produced the maximum peak discharge of 8021 m3s-1. The peak discharge decreases to 7076 m3s-1 and 6462 m3s-1 for Tf=2.0 hr and Tf=3.0 hr respectively. At the hydropower dam (Chungthang town) a maximum discharge of 4801 m3s-1 is calculated where the peak discharge is reached within 124 min (2.06 hr) after the initiation of the moraine-breach event. Peak discharge of 4677 m3s-1 (Tf= 2.0 hr) and 4653 m3s-1 (Tf= 3.0 hr) is recorded at the dam site with a time of peak of 142 min (2.4 hr) and 156 min (2.6 hr) respectively. The sensitivity of the hydrodynamic model to the input parameters and channel characteristics is evaluated by performing a series of hydrodynamic simulations of potential GLOF on the Safed lake located in the Goriganga basin, in the state of Uttarakhand. The lake have grown double its size in a span of 48 years from 1968 to 2016. The potential GLOF of the lake is evaluated by a modeling a moraine-breach event in which breach parameters (breach width and formation time) were calculated using empirical relations. For sensitivity analysis, a series of hydrodynamic GLOF routing is performed for the same lake for varied Manning’s N, breach width and formation time to evaluate the sensitivity of the model. The sensitivity of the model results to channel properties like slope and top-width is evaluated along the flow channel. The breach hydrograph is most sensitive to the breach formation time (Tf) when compared to the width of the breach (Bw). It is observed that the flow velocity is more sensitive to Manning’s N as compared to flow depth. Also, flow velocities have shown a linear increase as the slope of the channel increases (R2=0.78). Flow depth and velocity vary with the top-width of the channel. The flow velocity vs. top width has shown a correlation coefficient of 0.83 and that of depth vs. top width is 0.93.