Moraine dam failure and Glacial Lake Outburst Floods present one of the greatest threats to people and property in mountainous regions. In the last several centuries, many of the present moraine-dammed lakes were created by advances and retreats of valley glaciers. Moraine dams pose hazards because of their composition and location. Moraine dams are usually located downslope from steep crevassed glaciers and vertical rock slopes, and located upslope from steep canyons with easily erodible materials. Overtopping and breaching of the dam by waves generated by avalanches or glacier calving is the most common failure mechanism. The result of the dam failure is catastrophic downstream flooding. Some of the largest downstream flood peaks are produced from moraine-dam failures. Dam characteristics and failure mechanisms affect the flood peak (Costa and Schuster 1988).
End and lateral moraines are created from material pushed and piled from glacier movement along with till released when a glacier melts and recedes. End and lateral moraines form at the glacier margin. Till is a heterogeneous accumulation of unsorted soil, rock, and other material released from the glacier ice. End moraines are formed at the farthest limit reached by a glacier. Lateral moraines are formed along the sides of a glacier. Both terminal and lateral moraines may act as dams. A moraine-dammed glacial lake is formed as the glacier recedes from the end moraine in a mountain valley. The moraine acts as an unstable dam to water melting from the glacier.
Climate is important to the creation and stability of moraine dams. Many moraine dams and moraine-dammed lakes formed when glaciers retreated at the end of the Little Ice Age (Clague 2003; Hambrey and Alean 2004). The warming of the climate is resulting in the recession of glaciers and glacial lake formation in mountain ranges around the world. As glaciers continue to melt, more lakes are created and the size of the lakes is increasing. Failure of moraine dams is becoming increasingly frequent. Failures have occurred in the Himalayas, European Alps, North American Rockies, Andes, Tien Shan Mountains, and Pamirs (Reynolds 2006). The frequency and magnitude of moraine dam failures and Glacial Lake Outburst Floods will continue to increase with the current and continued scenario of global warming (RGSL 2003).
Most moraine dams and moraine-dammed glacial lakes have a history of cyclic outburst activity. The cyclic outburst activity ends when the moraine dam is damaged by the outburst floods to the point that it can no longer hold water. As glaciers retreat, the cyclic outburst activity may travel up glacier to newly formed moraine dams and moraine-dammed lakes (Clague 2003).
The unstable nature of moraine dams greatly increases the chances of dam failure. Many factors contribute to the unstable nature of the moraine dam. The moraine dam may contain stagnant glacier ice. Moraine dams are typically narrow and high reducing structural strength. Large lake volume above the moraine dam increases pressure against the dam. Limited freeboard between the crest of the moraine dam and the lake level reduces the height of waves needed to overtop the moraine dam (Reynolds Geo-Sciences Ltd 2003). Since moraine dams have steep slopes and contain loose, poorly sorted sediment, moraine dams are highly susceptible to failure (Clague 2003).
Moraine dam failure may be caused from one of several triggers. Large waves caused from calving glaciers or ice or rock avalanches into the lake may overtop the moraine dam. Settlement within the dam due to voids left during deposit may reduce the freeboard or create passageways for piping to occur. Melting of stagnant glacier ice in the moraine dam may also reduce the freeboard or create passageways for piping to occur. Catastrophic glacial drainage may raise the lake level quickly and overtop the dam (RGSL 2003; Hambrey and Alean 2004). Earthquakes may also cause moraine dam failure (Clague 2003).
The sudden failure of a moraine dam on a glacial lake can release a very large amount of water called a Glacial Lake Outburst Flood. Glacial Lake Outburst Floods threaten people and property in mountainous regions around the world. The released water may destroy property hundreds of kilometers from the source. Although Glacial Lake Outburst Floods have occurred around the world, outburst floods in the Andes, European Alps, and Himalayas have a history of property destruction and loss of human life due to settlement. As remote mountain valleys become settled, more people and property are put in danger (Clague 2003).
During the last century, human activities including settlement, hydropower production, forestry, mining, and wilderness tourism in mountainous areas have increased conflict with glacial hazards including moraine dam failures and Glacial Lake Outburst Floods (RGSL 2003). Between 1940 and 1970, one major Glacial Lake Outburst Flood was reported per decade in the Himalayas. In the 1990s, one major outburst flood was reported every three years. By 2010, Glacial Lake Outburst Floods will cause more then 1 million dollars per year in damage in the Himalayas (Richardson and Reynolds 2000). In December of 1941, the worst ever recorded Glacial Lake Outburst Flood disaster occurred in the town of Huaraz in the Cordillera Blanca, Peru. The town was destroyed and at least 6000 people died (Hambrey and Alean 2004).
Glacial Lake Outburst Floods increase to peak flow then gradually or abruptly decrease to normal levels once the water source is exhausted. Outburst flood peak flow is directly related to lake volume, dam height and width, dam material composition, failure mechanism, downstream topography, and sediment availability. When abundant sediment is available below the dam, outburst floods may transform into debris flows. Slopes greater then 10 to 15 degrees are required for debris flows to form and be sustained. Glacial Lake Outburst Floods alter the river floodplain by eroding, transporting, and depositing large amounts of sediment. The alteration of the river floodplain may occur tens of kilometers away from the water source (Claque 2003).
In 2002, Hubbard Glacier created a moraine dam across the entrance to Russell Fjord creating a moraine-dammed lake. Hubbard Glacier is the largest calving glacier on the North American continent. Russell Fjord is perpendicular to the flow of Hubbard Glacier in Disenchantment Bay. The moraine dam was created from glacier advance instead of retreat and across a different valley than the one the glacier occupied. The moraine dam failed and the Glacial Lake Outburst Flood was the largest ever recorded for a moraine-dammed lake. The 2002 peak discharge was 52,393 m³/s (Trabant, March, and Thomas 2003). In 1986, glacier advance across the entrance to Russell Fjord created an ice-dammed lake (Hambrey and Alean 2004). The ice dam also failed creating the largest glacier outburst flood ever recorded. The 1986 peak discharge was 112,433 m³/s (Trabant, March, and Thomas 2003).
In 1997, the moraine dam impounding Queen Bess Lake in the southern Coast Mountains of British Columbia failed and produced a Glacial Lake Outburst Flood. The dam was overtopped and breached by a wave created from an ice avalanche from Diadem Glacier into the lake. The flood was recorded at Bute Inlet over 100 km away. The moraine-dammed lake was created by the retreat of Diadem Glacier out of the lake bed and up a steep slope (Clague 2006). When the Queen Bess Lake moraine dam failed, 6.5 million m³ of water was released and 280,000 m³ of material was eroded from the dam. The amount of material eroded from the dam was limited by the presence of a bedrock sill. Queen Bess Lake still holds 12 million m³ of water. Based on downstream evidence, the flood consisted of two distinct phases - the initial overtopping of the dam then the failure of the dam (Evans et al. 2002).
In 1983, the Nostetoku Lake moraine dam in the southern Coast Mountains of British Columbia failed. A wave generated by an ice avalanche from Cumberland Glacier into the lake overtopped and breached the dam. When the dam failed, 6.5 million m³ of water from the lake was released and 1.5 million m³ of material was eroded from the dam (Evans et al. 2002).
Engineering and societal techniques are the main methods for reducing the hazards from moraine dam failures and Glacial Lake Outburst Floods. Engineering techniques in mountainous regions are technically and logistically challenging. Engineering techniques involve lowering the glacial lake level, strengthening the moraine dam, removing or restraining trigger mechanisms, and installing protective measures downstream from the lake. Lowering the lake level not only reduces the risk of dam failure, but also reduces the magnitude of the hazard (RGSL 2003). Engineering techniques must be carefully planned or premature failure of the moraine dam could be triggered by unintentional damage to the dam. In October of 1950, a Glacial Lake Outburst Flood was caused from engineering work at Cañon del Pato in Peru. The resulting flood caused 200-500 deaths (Hambrey and Alean 2004).
The lake level can be lowered with tunnels through rock, culverts or open spillways through the dam crest, siphons or pumps, or raising the height of the dam. The dam can be strengthened with placement of rock armor on the slopes, concrete or masonry construction, and soil nailing or jet grouting techniques. Removing or restraining trigger mechanisms include stabilization of adjacent slopes and controlled smaller avalanches. Downstream protective measures include check dams, flood diversion works, and energy dissipation baffles. Societal techniques include relocating settlements or not allowing settlement in dangerous areas. Early warning systems and education of the local population are also options (RGSL 2003).
Even with hazard reducing techniques, Glacial Lake Outburst Floods caused from moraine-dam failures in mountainous regions will continue to occur. Based on global warming, Glacial Lake Outburst Floods will not only continue but will increase in frequency. Although engineering techniques may reduce the hazard, societal techniques may provide the best solution. If the local population understands the potential hazard, hopefully they will make better decisions on where to settle and frequent.
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