Glacial lake outburst floods (GLOFs)

Explanation

Both statements are correct. Moraine dams — the natural barriers that hold back glacial lakes — are composed of loose debris (sand, silt, gravel, boulders) deposited by retreating glaciers. Unlike engineered earthen dams, they lack structural integrity, making them prone to sudden and catastrophic failure that can release millions of cubic metres of water in a short time. This is the core physical mechanism behind GLOFs. Regarding risk reduction, the NDMA's recommended approach follows a sequential framework: first, identify and map dangerous lakes using field observations, remote sensing (including Synthetic Aperture Radar imagery), and geotechnical assessment; then take structural measures such as controlled breaching, outlet construction, or pumping; and finally establish early-warning systems to protect lives and property downstream. Both the physical mechanism (moraine-dam failure) and the NDMA mitigation protocol (identify → structural intervention → early warning) are standard static-syllabus content on GLOF risk management.

Explanation

All three pairs are correctly matched. First, a moraine-dammed glacial lake forms when meltwater from a glacier accumulates behind a moraine — the ridge of rock, sediment, and debris that a glacier deposits at its margins or terminus; this is the classical GLOF formation mechanism in the Himalayas. Second, a bathymetric survey is the standard technique used to measure and map the depth and shape of the floor of a water body such as a glacial lake; it is essential for estimating a lake's water volume and therefore its potential flood hazard. Third, a cascading hazard describes the scenario — common in high-mountain environments — where one geophysical process (an ice or rock avalanche, a landslide, or a temporary river blockage) triggers one or more further destructive events, amplifying and complicating the overall disaster; this is why GLOF preparedness must account for multi-process hazard chains rather than treating outburst floods in isolation. Understanding these three concepts is central to the Himalayan GLOF risk framework.

Explanation

Statement 1 is correct. Moraine-dammed glacial lakes are inherently unstable because moraines — ridges of unsorted glacial debris — lack the structural integrity of bedrock. They are vulnerable to sudden breach when subjected to impact triggers such as ice avalanches falling into the lake (generating displacement waves) or intense rainfall saturating and weakening the moraine dam. This is a well-established principle in glaciology and hazard science. Statement 2 is incorrect. In the HKH region, ice avalanches are the primary trigger of GLOF events, accounting for approximately half of recorded occurrences, while intense rainfall accounts for roughly a quarter. Stating that rainfall is a more significant trigger than ice avalanches inverts the empirically documented relationship. Statement 3 is correct. The clustering of GLOF events during summer monsoon months is mechanistically explained by the simultaneous peak of two independent drivers: elevated temperatures that accelerate glacial melt and destabilise hanging ice masses (increasing avalanche probability), and high precipitation that both adds hydraulic load to proglacial lakes and weakens moraine dams. The convergence of these thermal and hydrological stressors during the same seasonal window amplifies GLOF risk significantly beyond what either driver would produce alone.

Explanation

Statement 1 is incorrect on two counts. Supraglacial lakes are not permanent — they are temporary bodies of meltwater that collect in surface depressions on a glacier or ice sheet. They also do not form in bedrock depressions at the base of a glacier; that description fits subglacial or proglacial settings. Supraglacial lakes are known to grow steadily as glaciers retreat and surface melting intensifies, making them dynamic rather than stable features. Statement 2 correctly describes moraine-dammed glacial lakes: when meltwater is impounded behind a moraine (an accumulation of glacially transported rock and debris), the resultant lake can outburst catastrophically if the dam is breached — either by overtopping, internal seepage, or an ice avalanche — releasing stored water very rapidly and causing destructive downstream flooding. This is the classic GLOF mechanism in the Himalayas. Statement 3 is also correct. The high altitude, remoteness, and extreme terrain of Himalayan glaciated zones make regular ground-based surveys hazardous and infrequent. Satellite remote sensing — especially multispectral and SAR imagery — allows near-continuous, repeated observation of lake surface areas, enabling timely detection of rapid changes that precede or follow GLOF events, which is central to disaster risk reduction in the region.