GLACIER LAKE OUTBURST FLOODS Lasafam Iturrizaga
Этот обзор для Энциклопедии написан нашей хорошей знакомой д-ром Лазафам Итурризагой из Университета Гёттинген, которая ещё аспиранткой профессора Matthias Kuhle ездила со мной на практику в горы Алтая (сентябрь 1995 год). А совсем недавно мы виделись с ней в Берне на Конгрессе ИНКВА. Я помещаю текст этой статьи полностью, он доступен в сети. Однако я позволил себе поменять две первых фотографии, поскольку у Лазафам они были очень плохого качества - она фотографировала ещё в ту экспедицию 1995 года на плёночную мыльницу. Содержание фотографий соответствует первоначальному, надеюсь Лазафам не будет в претензии. А если будет - напишет, и я верну её фото. Статья большая, делаю я её постепенно. А после статьи добавлю ещё синие ссылки на актуальные работы по этой теме. На фотографии Михаила Докукина: Лазафам на международной конференции по проблемам пульсируюшего ледника Колка. Итак.
"Encyclopedia of Snow, Ice and Glaciers" © Springer, 2011/Eds. Vijay P. Singh, Pratap Singh and Umesh K. Haritashya
GLACIER LAKE OUTBURST FLOODS
© Lasafam Iturrizaga
Department of Geography/High Mountain Geomorphology,
Institute of Geography, University of Göttingen,
Göttingen , Germany
Aluviones; Débâcles; Glacial outwash floods; Gletscherlauf (German); Jökulhlaup (Icelandic); Megafloods and superfloods (Quaternary large-scaled glacier floods)
Glacier lake outburst floods (GLOFs) refer to sudden and in some cases cyclic release of meltwater from a glacierdammed or moraine-dammed lake, which can result in a catastrophic flood. Thorarinsson (1939) introduced the term “jökulhlaup” for glacial floods due to the cataclysmic drainage of subglacial lakes in Iceland. It was originally referred to outburst floods triggered by volcanic activity and has been subsequently transferred to a variety of other types of glacial floods. It has become a widely used synonym for describing catastrophic glacial floods in general. The size of glacial lakes varies considerably and the lakes may hold up to tens of millions of cubic meters of water. Glacier lake outbursts produce flows of water that may be an order of magnitude greater than average rainfall-derived peak flows. Geomorphological impacts and damages of infrastructures can occur up to tens to hundreds of kilometers downstream. Their competence and capacity is high enough to transport large amounts of debris, so that they may occur as hyperconcentrated flow or even debris flow. Catastrophic dam failure may release the reservoir water over a time span of hours to days. Peak flows as high as 30,000 m3 have been recorded for moraine-dammed lakes, but they are much higher for glacier-dammed lakes. In case the discharge exceeds a flood volume of about >106 m3 or peak discharge >106 m3/s (Martini et al., 2002; Korup and Tweed 2007) glacier lake outbursts are termed as megafloods. These extremely large paleofloods, which occurred mainly during the end of the Pleistocene, were also termed as superfloods (Rudoy, 2002). Glacier lake outbursts may be caused by different kinds of dam failures depending on the type of natural barrier (Clague and Evans, 1994; Tweed and Russell, 1999). Floods resulting from glacier lake outbursts may pose a severe threat to settlements and their infrastructure. As the spatial interconnection of glacier areas and sedttlement zones has generally increased in the last decades, new concepts and technologies in hazard assessment, evaluation, and mitigation have been developed (Richardson and Reynolds, 2000a, b; Huggel et al., 2004; Kääb et al., 2005a, b; Allen et al., 2009).
Glacier lake outbursts have contributed to the largest flood events worldwide and caused large-scaled geomorphological landscape transformations with specific landform assemblages. Glacier lakes generally occur in all glaciated landscapes in different varieties and with specific trigger mechanisms in terms of their catastrophic outbursts. The appearance of glacier lakes and associated floods provide landscapes to study Pleistocene, Holocene, and historical glaciation extent and is therefore an important paleoclimatic key indicator for reconstructing landscape environments and past climate. Moreover, the formation of glacier lakes and subsequent outbursts provide an obvious indicator for glacier dynamics and fluctuations. A striking shift in the distribution pattern of glacier lakes has occurred in the course of the general worldwide glacier recession during the twentieth century. In many glacierized areas, the glacier wastage produced an increase in the number of glacier lakes and lead consequently to an aggravation of the glacier hazard situation of the more and more densely populated glacier forefields and forelands. This is especially true for glaciers that are exploited economically in terms of irrigation and hydropower. Therefore, research on the controlling factors on the development of glacier lakes and glacier lake outbursts in regard to hazard mitigation has increased exponentially in the last decade. Nevertheless, in some regions the glacier ecession has caused the disappearance of glacier lakes and produced a decline of glacial flood inundations. Extensive review papers have been written on different topics of glacier lakes and outbursts (Blachut and Ballanytyne, 1976; Tweed and Russell, 1999; Herget, 2003) providing preferentially case examples from the higher latitudes. This entry gives an overview of the main types of glacier lakes with a special emphasis on lakes in high mountain regions, i.e., on floods occurring in more confined valleys. Glacial lakes are generally impounded either by a glacier or by a moraine or they occur in proglacial depressions caused by glacial overdeepening in the bedrock or sediment. The source area of the lake water may be the meltwater of the damming glacier or the adjacent glaciers. At temperate glaciers, large amounts of meltwater originate as well from basal melting (Benn and Evans, 1998). The lake size ranges from several square meters up to a million of square kilometers. In principle, three different types of lakes are distinguished: (a) glacier-dammed lakes, (b) moraine-dammed lakes, and (c) internal glacial lakes. Internal glacial lakes (englacial and subglacial) have been discussed elsewhere in this encyclopedia.
Worldwide distribution of glacier lakes prone to lake outbursts
Recent glacier-dammed, moraine-dammed, and internal glacial lakes arise at different scales in the glacierized landscapes of all geographical latitudes. In this regard, regional individual distribution patterns are recognizable (Figure 1). They are a response of the glacier type and therefore of the climatic-controlled nourishment conditions of the glacier, the glacier size, the topographical conditions, and the debris transfer system. In the Tropics, moraine-dammed lakes occur especially in the Peruvian Andes in the Cordillera Blanca (Huascaran Massif ) (8–10°S) (Ames, 1998). In particular the small, clean-ice glaciers have shrinked rapidly in the last decades due to the fact that they are subject to a all-season ablation regime in the lower glacier parts (Kaser and Osmaston, 2002). In the course of the twentieth century, some of the most devastating glacier lake outbursts have occurred in this region (Carey, 2005; Vilímek et al., 2005). In the inner Tropics, North of 8°S glacier Lakes are mostly absent. The topographical preconditions on the steep-sided volcanoes with their small mountaintop Glaciations prove to be unfavorable for the formation of moraine and glacier-dammed lakes. A further distribution area of moraine-dammed lakes is located in subtropical latitude between 28–34°N in the Himalaya Range, especially in Nepal, Bhutan, and China.
The Environmental Programme of the United Nations (UNEP) and the International Centre of Integrated Mountain Development (ICIMOD) have monitored 2,323 glacier lakes in Nepal (Mool et al., 2002a, b). In contrary, in the westward adjacent Karakoram (35–36°N) glacierdammed lakes are the main type of glacier lakes, mainly generated by advancing tributary glaciers (Hewitt, 1982; Iturrizaga, 2005a, b, c). High-magnitude glacier floods occurred especially at the end of the Little Ice Age until the 1930s. Even nowadays, glacier lake outbursts happen, but to a far lesser extent. In themid latitudes, such as in the European Alps or the Canadian Rockies with a comparative smaller glacierized area and shorter glaciers, moraine-dammed lakes and numerous new cirque lakes have formed in overdeepened rock and sediment basins (e.g., Trift glacier, Swiss Alps). Another principal distribution area of glacier-dammed lakes is located in the high latitudes (about 60°N) in Alaska (e.g., Abyss Lake) (Post and Mayo, 1971; Stone, 1963; Sturm et al., 1987). The often dendritic trunk glaciers impound the meltwater of their retreating tributary glaciers. Further ice-dammed lakes can be found in Greenland (Russell, 2007) and to a lesser extent in Norway (Howarth, 1968; Knudsen and Theakstone, 1988; Russell, 1994; Breien et al., 2008).
In regard to the hypsometric distribution, glacierdammed lakes occur from sea level in polar regions up to the high altitudes of the extreme high mountain areas. The largest lakes are located mostly up to 1,500 m below the equilibrium altitude line (ELA), where the meltwater of extensive catchment areas may be impounded by the natural dams. The moraine-dammed lakes arise commonly in the lower part of the ablation area of the glaciers. Some of the highest moraine-dammed lakes appear in the Chinese Himalayas at 5,700 m a.s.l. (Longbasaba and Pida lakes, Xin et al., 2008).
Fig. 1 Recent worldwide distribution of glacier lakes and potential glacier lake outbursts areas as well as key localities of Quaternary megafloods. Internal glacier lakes (supraglacial, englacial, subglacial) may exist at all glaciers causing catastrophic floods.
The different thermal regimes of the glaciers (cold and temperate glaciers) have different characteristics for lake formation and corresponding outburst mechanisms.
Quaternary glacier-dammed lakes (megafloods)
Glacier lake outbursts originating from glacier-dammed lakes have contributed to the largest and geomorphologically most significant flood events in the Quaternary (O’Connor and Costa, 2004). During the outbursts of megafloods, gigantic Landscape transformations have taken place in only a few hours or days similar to volcanic eruptions, tsunamis, or earthquakes (Rudoy, 2002). After an initial disagreement with the scientific main stream, the reconstruction of the Missoula Flood in North America by Bretz (1923) got widely accepted (Baker et al., 1988). The Missoula Flood area has become a key locality for the landform assemblages of cataclysmic floods. At the end of the Pleistocene before about 16 ka, a southern ice lobe of the Laurentide Ice Sheet impounded during the Wisconsin-Glaciation the Clark Fork River. The maximum lake volume of the Lake Missoula amounted up to 2,184 km3 with a lake depth of 635 m at the ice dam (Baker and Bunker, 1985). Repetitive lake outbursts showed peak discharges in the order of about 17 × 106 m3/s (for comparison the Amazonas has a discharge of 370,000 m3/s, 1953). The Missoula Flood left behind characteristic landforms consisting of (a) erosional forms (Scabland topography, Bretz, 1923) (e.g., cataract complexes, dry falls, spillways, cavitations, P-forms, shorelines) as well as (b) depositional forms (expansion, longitudinal, point and pendant bars, eddy bars, slackwater deposits, and giant or mega ripples).
A Holocene proglacial lake formed at the southern margin of the Laurentide Ice Sheet before about 12 ka, the Lake Agassiz (Clarke et al., 2003), which covered an area of about 1 Mio. km2 with a water volume of 163,000 km3 (Teller et al., 2002). The lake drained catastrophically at 8.45 ka into the Hudson Bay. The glacier lake outbursts supposed to be one of the main trigger of the 8.2 ka cooling event. The incoming fresh water from the icedammed lake caused a desalinization of the North-Atlantic and lead to the subsequent collapse of the Gulf stream (Clarke et al., 2003). This event accelerated the debate of climate change.
Glacier lakes occurred during the early and middle Weichselian (90–80 ka) and (60–50 ka) at the ice front of the Scandinavian Ice Sheet in Western Russia as well as at the southern margin of the ice sheet, which covered parts of the Barents and Kara Lake (Grosswald, 1980; Mangerud et al., 2004) outreached by far the size of present glacial lakes. It is assumed, that their outbreak had a significant impact on the sea ice formation in the Arctic Ocean and on the regional limate.
A different type locality for the outbreak of glacierdammed lakes is situated in the Altai Mountains (Southern Siberia) (Baker, Benito, Rudoy, 1993; Rudoy, 2002; Carling et al., 2002). A glacier dam, 15 km long and several kilometers in idth, impounded repetitively the drainage of the Chuja and Kuray Basins and formed a glacial lake (607 km3) with a maximum depth of 650 m at the ice barrier.The last cataclysmic glacier lake outbursts happened 13 ka during ELA-depressions of 800–1,200 m. The lakes were still in existence about 5 ka. These glacial superfloods had a discharge of about 18 × 106 m3/s and boulders with a diameter of 11 m have been transported. The outburst landscape is similar to that of the Missoula Flood. Moreover, huge fluvial gravel dunes or megaripples (giant current ripple relief ) have been deposited in the Chuja Basin, which are up to 23 m high and 320 m long, and giant diluvial ramparts and terraces-bars (Photos 1 and 2). Interestingly, the landforms have been previously mistaken for glacial deposits.
Photo 1. Megaflood deposits in the Altai mountains (Southern Siberia). Giant ripples in the Kuray Basin .
Photo by Artyom Golovin, August 2010.
Photo 2. Megaflood deposits in the Altai mountains (Southern Siberia). Giant gravel terraces at the Katun River.
Photo by Alexei Rudoy, 8 July 2011.
The reconstruction of megafloods in North America and Siberia has been the base for interpreting landforms of paleofloods on Mars (Baker, 2001; Fairen and Dohm, 2003). In the extreme high mountain ranges, such as in High Asia, the number of glacier dams may have been higher than today during the deglaciation of the Last Glacial Maximum (LGM) and during Postglacial times. This is shown by expansive lake sediment sequences throughout the Karakoram valleys (Paffen et al., 1956; Owen, 1996; Cornwell, 1998), such as Lake Guricot in the Astor Valley (Scott, 1992), the Malungutti lake in the Shimshal valley (Iturrizaga, 2005c) and in the lower Chapursan valley (Iturrizaga, 2008). Already in the end of the nineteenth century, glacial lake outburst floods have been discussed as transport agent for the “Punjab erratic boulders” located in the Peshawar Basin at altitudes of only 300–400 m a.s.l. (Cornwell, 1998). As source area of these boulders has been considered a 750 m high ice dam, the Darel-Shatial-moraine in the Indus valley at about 900 m a.s.l. (Bürgisser et al., 1982; Burbank, 1983; Desio and Orombelli, 1983; Shroder et al., 1989; Cornwell, 1998), which would be one of the largest Pleistocene ice barriers in High Asia. In the basin of Chilas, megaripples have been identified (Shroder et al., 1989). However, not only ice-dammed lakes have produced high-magnitude floods in the Indus valley but also landslides, such as the Lichar landslide in 1841, close to the western side of the Nanga Parbat area, 30 km south of Bunji, which reached down to 300 km south of Attock (Shroder, 1993). This fact makes it rather difficult to unravel the source areas of specific flood deposits. In this regard, it has been argued that many of the dams considered as glacier dams, such as the 600 m high Katzarah glacier dam (Dainelli, 1922), were not stable enough to impound a lake of a size of over 170 km. It is doubtful that ice dams provide the conditions for the undisturbed deposition of lake sediments of the size of several hundred meters in thickness (Hewitt, 1998). According to this argument, the existence of rock avalanche barriers is more likely than glacier barriers. The lakes in High Asia have been much smaller in size than those described from Northern America and Northern Eurasia, but due to the high relief energies the outburst floods might have had a tremendous geomorphological impact.
Megafloods have been as well proposed for the outlet glaciers of the Pleistocene (LGM) south Tibetan ice sheet between Cho Oyu and Shisha Pangma (Kuhle, 2002). New field evidence showed further Holocene moraine dams, which may have caused giant (1,011 m3) lakes with a maximum depth of 680 m in the Tsangpo River gorge in Tibet (Montgomery et al., 2004) with peak discharges higher than the Missoula and Altai floods.
Glacier-dammed lakes may be impounded by the trunk and tributary glacier in different topographical constellations or by small lateroglacial ice lobes. They are also known as ice-dammed lakes in the literature (Blachut and Ballantyne, 1976; Tweed and Russell, 1999). However, glacier barriers of the subtropical high mountain regions are characterized by a high content of debris incorporated in the supraglacial and lateral moraines. This debris material plays a vital role in the mechanics of the glacier lake outbursts and during the subsequent flood events. Therefore, the initial water flood may transform into a hyperconcentrated flow and finally in some cases into a debris flow. For this reason, the superordinated term “glacier-dammed lake” is used, to which the ice-dammed lakes are subordinated. The latter occurs mainly in high latitudes in moderate to low mountain relief with comparatively little glacial sediment transfer. Glacier-dammed lakes are in most cases a reaction of glacier front oscillations and form generally during periods of glacier advance. On a wider time scale, they occur especially during the wastage of an ice-stream network in the transition to individual valley glaciers (Figure 2). Glacier-dammed lakes are at present time comparatively short-lived and survive often only several months or years. Therefore, lake sediments are rarely deposited. The lake basins fill up rather quickly. Subsequently, a catastrophic drainage may occur. They empty commonly in summer time at the period of highest meltwater discharge.
Types of glacier dams
Tweed and Russell (1999) have summarized the existing classifications of types of glacier-dammed lakes in regard to their topographical settings (Maag, 1969; Blachut and Ballantyne, 1976; Costa and Schuster, 1988) and added the subglacial calderas and englacial or subglacial water bodies as result of volcanic activity. In principle, the following main categories of glacier dams are differentiated representing the source areas for most of the present glacier lake outbursts.
Tributary glacier dam
A tributary glacier advances into the trunk valley and impounds the river converting it into a lake (Photos 3 and 4). The length of the cross-valley barrier amounts generally not more than 1–2 km with an absolute height of about 250 m. The lowest point of the glacier barrier is located in the contact zone of the glacier tongue and the adjacent valley flank. This is the locality, where the dam may fail in different ways. During a long-lasting glacier terminus position, the glacier may be framed by lateral moraines, which then are an important part of the glacier dam. In case of a glacier advance, the glacier tongue is commonly heavy crevassed. The lateral moraine acts as an additional and rather impermeable barrier.At present time, tributary glacier barriers are widely distributed in the Karakoram and Hindukush Mountains (Figure 3), which show the greatest glaciation with the longest valley glaciers outside of the polar region. The LGM-ice-stream network (Derbyshire et al., 1984; Kuhle, 2001) has disintegrated into numerous valley glaciers during the deglaciation, which are as long as 72 m (Siachen glacier).
Fig. 2. Disintegration of an ice-stream network (a) into individual valley glaciers and the transition
from main valley glacier-dammed lakes (b) to tributary glacier-dammed lakes (d). Stage (c) shows transitional forms of tributary glacier lakes and proglacial moraine-dammed lakes (double dam formation, cf. Figure 10).
Photo 3 Glacier dams in the Karakoram Mountains. The Khurdopin glacier (47 km long) has been notorious for glacier lake outburst in the twentieth century. The photo shows the 3 km long debris-covered glacier tongue and the Virjerab lake basin. The Virjerab river drains even today subglacially (Photo: L. Iturrizaga, 18.07.2001).
Photo 4 Multiple glacier dams in the Hindukush Mountains. The Chillinji glacier has blocked the Karambar valley about 100 years ago. The glacier tongue is surrounded by up to 250 m high lateral moraines. During dam failure the moraine material is transported with the flood waters causing in some cases hyperconcentrated flows. Most of the Karakoram and Hindukush glaciers are avalanche fed glaciers and highly dynamic (Photo: L. Iturrizaga, 09.2002).
In historical times, about 22 tributary glaciers formed glacier-dammed lakes in the upper Indus catchment area, from which 12 dams were responsible for outburst floods (Hewitt, 1998). Since 1826, 35 glacier lake outbursts have been monitored Hewitt, 1982), although this number only includes a small selection of past flood events. Two localities were notorious for glacier lake outbursts in the last two centuries: the tributary glaciers of the Shyok valley (East Karakoram) (Mason, 1935; Hewitt, 1982; Feng Qinghua, 1991) as well as side glaciers of the Shimshal valley (North West Karakoram) (Iturrizaga, 1997, 2005c) (Figure 3, Photo 3). The topographic setting is at the current stage of glaciation in the Hindukush–Karakoram region favorable for the formation of this dam type: Tributary glaciers with catchment areas of over 7,000 m in height descend down to low altitudes below 3,000 m into the glacier-free trunk valleys and block temporarily the main river. A remarkable large number of glaciers terminate at confluence positions. Among them occur a lot of white, transversal glaciers in N-aspect.
The Karakoram rivers show discharge rates of up to 1,000 m3/s (Ferguson, 1984). Therefore large-sized lakes, several kilometers in length, may be impounded in a short time period. Lake volumes of up to 3.3 _ 109 m3 are reported from prehistorical glacier dams (Hewitt, 1982). In this region, a seasonal pattern dominates the outburst chronologies. The failures of glacier-dammed lakes mostly occurred between July and August during the time of the highest discharge. Most of the dams fail periodically with irregular possible return intervals of about 1–2 years.The lakes often drain in successive years due to internal changes in the ice barrier itself. In some valleys, numerous tributary glaciers have formed glacier barriers in the same main valley, so that several glacier lakes may have existed synchronously (Figure 4). In this topographical constellation, an outburst of an upstream-located glacier lake may have triggered the sudden drainage of a lower glacier lake and therefore induced a glacier lake outburst cascade (Iturrizaga, 2005b). Moreover during those flood events, backwater lakes may form further downstream at valley constrictions (Figure 4). In the Karambar valley (Hindukush) at several locations, even about 100 km downward of the glacier dam, backwater lakes developed (Iturrizaga, 2006, Figure 5).
Glacier-dammed lakes are often formed rather spontaneously by surging glacier (Iturrizaga, 2011). In the Aconcagua Mountain Range (Argentinean Andes), the Grande del Nevado Glacier (8 km long), terminating at 3,165 m and flowing down from Cerro de Rio Blanco, has impounded the Rio Plomo and transformed it into a lake, measuring 60 _ 106 m3 in size (Helbig, 1935; Espizúa and Bengochea, 1990). The outbreak in January 1934 produced a flood wave with a run-out-distance of 150 km. In 1984/1985, the Grande del Nevado Glacier has advanced about 2.7 km in less than in 2 months leading to a lake with a surface of over 3 km2 (Espizua and Bengochea, 1990). Main valley glacier dam A main valley glacier barrier blocks the discharge of one or more of its surrounding glaciers in its catchment area. 1. This may be the case when a tributary glacier recedes from a dendritic main glacier (e.g., Tulsequah Glacier Glacier
Fig. 3. Distribution of glacier-dammed lakes in the Karakoram and E-Hindukush Mountains.
Lake, 195 m in depth, British Columbia, Marcus (1960), Inyltschik glacier, Merzbacher Lake, Tienshan with a depth of 130 m, a length of 4 km, and a width of about 1 km, Mayer et al., 2008). The main distribution area of the glacier dam type is located in Alaska, where over 750 glacier lakes have been monitored (Stone, 1963). The glacier tongues terminate often at sea level and are formed by tidewater glaciers. In case of potential glacier lake outbursts, the run-out-distances are comparatively little. The ice-dammed Lake George (65 km2), 50 m in depth, impounded by the Knik glacier, east of Anchorage in the Chugach Mountains, showed large annual outbreaks from 1918 to 1967 (Post and Mayo, 1971; O’Connor and Costa, 1993) as a result of downwasting of the ice arrier. Its lake basin is (Inner Lake George) created by an end moraine from the tributary glacier, named Colony glacier. The end moraines are deposited against the lateral margin of the ice barrier of the Knik glacier. A well-known example from the Alps for the main valley glacier dam is the Lake Märjelen (2,350 m a.s.l.), which was impounded by the longest glacier of the Alps, the Aletsch glacier, at its margin.
Fig. 4. Multiple glacier dams in the Karambar valley in the Hindukush Mountains.
Fig. 5. Glacier-dammed lake formation by small tributary glaciers in the upper Karambar valley (Hindukush).
It is a transitional form between a lake dammed by the main glacier and a lateroglacial lake. About 25 glacier lake outbursts occurred in-between 1816 and 1896 and devastated the Massa valley downvalley. 2. A main valley glacier barrier may also be formed by the advance of the main glacier, sealing off the discharge of the less or non-glaciated side valleys that show mostly lower catchment areas. This type of glacier barrier is at present rather rare. Lateroglacial glacier-dammed lakes At some glaciers, small ice lobes form at the glacier margin in times of irregular ice flow are known as icemarginal lakes. However, in order to define the lateral location and excluding the proglacial location, the term “lateroglacial” had been introduced (Iturrizaga, 2003, 2007). These ice lobes may in turn block the creeks of the lateroglacial valleys, which flow parallel to the glacier. These lakes mostly drain subglacially through the main glacier. Due to their small volume they do not generally generate high-magnitude floods, but nevertheless they are hazardous.
A special case are lakes that are caused by ice avalanches (Gietro Glacier, European Alps), which may reform as regenerated glaciers at the base of the mountain. In 1595, an outburst flood killed about 140 people in the Val de Bagnes (Tufnell, 1984).
Trigger mechanisms for dam failures of glacier-dammed lakes
Commonly glaciers experience a variety of trigger mechanism, such as fluctuations in ice-dam thickness and length and varying water supply. The initial dam failure often takes places by sub- and englacial gradual widening of the internal glacial conduits due to frictional heat and erosion (Nye, 1976) or by flotation of the ice dam (Thorarinsson, 1939). A variety of trigger mechanisms have been discussed. Most of the assumptions are rather theoretical for the individual glacier lake outburst as direct observations of dam failures are extremely seldom. Trigger mechanisms for dam failures have been mainly described on the base examples of glaciers of higher latitudes. The following listing provides an overview about the main causes of dam failure (Tweed and Russell, 1999):
Simple overspilling When the lake depth overtops the glacier barrier, supraglacial drainage may take place. Predestined for processes of overspilling of the dam are cold-based glaciers, which are frozen to their glacier bed and characterized by a dry and dense ice, so that they are more impermeable for water (Liestøl, 1956). Moreover, a high debris content of the glacier supposed to be favorable for increasing the density of the glacier and preventing flotation processes (Knight, 1999). Highly crevassed glaciers, which are very common among advancing glaciers, are unfavorable for this outburst mechanism. Overspill processes may be generated by mass movements falling into the glacier lake and causing subsequent displacement waves.
Hydrostatic flotation of the glacier dam
Hydrostatic flotation of the glacier dam is a function of the specific density of glacier ice and water and may be altered by the debris content in the glacier ice. The buoying effect of the ice takes place when the water column of the lake has reached about 90% of the height of the glacier dam (Thorarinsson, 1939). The presence of debris may influence this process, as it increases the density of the ice. Subglacial hollows, which develop under thin glaciers especially at obstacles, may be opened and act as major drainage pathways. After the uplift of the ice barrier, a progressive base water flow occurs at the glacier bed. The emptying of the lake is dependent on the transport capacity of the subglacial channel. The process of flotation was described in detail for the subglacial lake drainage at Grimsvötn in Iceland (Thorarinsson, 1953).
The GLEN mechanism (Glen, 1954) is based upon the assumption that the water pressure of the lake may deform the glacier ice and therefore creates or opens drainage channels at the base of the glacier through the ice, whereby frictional melting processes are involved. At a certain critical height of the water column, the horizontal stress exceeds the vertical stress by the amount of P.
P = (pw - pi) gh
where pw is density of the water, pi is density of the ice, g is gravitational acceleration, and h is height of the water column. The critical height has been controversially discussed, but generally it is assumed that a height of 150–200 m (Glen, 1954) is necessary for plastic deformation of the glacier ice to operate at the base of the glacier.
During the process of subaerial breach-widening, the glacier dam ruptures suddenly or is detached from the adjacent valley flank. Subaerial breach-widening is supposed to be a common outburst trigger in the European Alps (Häberli, 1983) and at glaciers in Alaska (Stone, 1963). It mainly occurs at tributary glaciers that have advanced into main valley. The melting may take place in the contact zone between rock and ice (Walder and Costa, 1996). After the dam break, typically a short, but high-magnitude flood is released from the lake. The break of the Chong Khumdum glacier in the Karakoram is a case example for a sudden break of the dam (Hewitt, 1982), even though the drainage has been initially started through subglacial drainage conduits. Subglacial volcanic activity
Subglacial volcanic activity may generate subglacial geothermal heat leading to a thinning of the ice dam or warm up of the lake water. Moreover, it may accelerate the melting of englacial and subglacial drainage tunnels. Avolcanic eruption destabilizes the glacier barrier or even may disrupt it completely (Björnsson, 2003; Magnús, 1997).
The mechanism of siphoning at glacier dams is based on changes in water pressure conditions in an existent suband englacial drainage system within the glacier barrier (Whalley, 1971) with that the glacier lake is connected. The siphoning process is launched, when the water pressure in the lake exceeds the water pressure of the drainage system. The difference in hydrostatic pressure results in
a kind of pull effect caused by the subglacial drainage system. Subsequently, the lake is emptied through the internal conduits in the glacier. At the end of the ablation area, the pressure in the englacial drainage channels is to be lesser than in the lake and the lake may be initiated to drain. It is assumed that the drainage terminates when the pressure conditions are inverted again.
As in all failures of natural dams, seismic activity in the form of earthquakes and faulting may be involved in the destabilization of the glacier barrier. However, there are few recent examples to underpin this obvious causal relation. As shown in many natural settings, a combination of the individual trigger mechanisms is highly probable.
Moreover, different trigger mechanisms could be responsible for repetitive lake outbursts from same glacier dam.
Moraine-dammed lakes are at their present extent a rather young geomorphological landform. In the textbooks of glacier science of the mid-twentieth century, they have been hardly mentioned as a type of glacial lake. Moraine dam failures resulted in devastating destructions of settlement areas, infrastructure, and loss of human life. Moraine-dammed lakes are impounded in-between the end moraine complex and the vanishing glacier tongue (Photos 5 and 6). In general, moraine-dammed lakes may be classified in two types that may occur (a) at steep hanging glaciers and (b) at more gently inclined valley glaciers (Figure 6). Smaller lakes are typically associated with shorter and steeper glaciers. Glacier shrinkage commonly takes place by thinning and subsequent ice frontal retreat. In the final stage, the glacier tongue may lose the contact to the lake. The majority of moraine-dammed lakes are retained by moraines of Neoglacial Times and the Little Ice Age. The horizontal extent of the lake is limited along cirque glaciers by the toe of the head wall. At valley glaciers, the maximum reported length for any lake in the Himalayas is about 3.3 km at the Lake Tsho Rolpa (Nepal). The terminal moraine complex consists of the end moraine, the laterofrontal moraine, and the lateral moraine (Figure 7). The end moraine may be the result of a single glacier advance or of several subsequent glacier advances. The inner slopes of the moraines are predominantly extremely oversteepened (up to 80°) and the outer slopes are mostly inclined more gently (20–35°). The absolute height of the end moraine reaches up to 200 m, whereby the actual dam height of the lake is much smaller. The moraine embracement is often intersected in a narrow and highly instable ridge. The composition of the sediment matrix is diamict. The higher the compaction and the smaller the porosity, the more stable is generally the dam. Glacier lakes in old tongue basins from pre-Neoglacial times are in principle rather stable.
Photo 5 Moraine-dammed lakes. Moraine-dammed lake at the Morsarjo¨ kull (Iceland) which is dammed by several end moraines. Glaciotectonic processes played a role in forming the depression upstrream of the end moraine complex (Photo: L. Iturrizaga, 09.07.2003).
Photo 6 Moraine-dammed lakes. Moraine-dammed lake basin at the base of the Kanchenjunga South Side (Sikkim Himalaya). The lake has outbursted and left behind the characterical V-shaped incision of the end moraine and a debris flow cone (Photo: L. Iturrizaga, 09.04.2002).
Dead ice cores and permafrost lenses may contribute significantly to the stability of the dam.Water outlets at the outer slopes of the lateral moraines, showing a lesser temperature than the glacier lake, may indicate the presence of ice lenses within the moraine (Lliboutry et al., 1977). Moraine dams may entirely be closed (Tap glacier, Himalaya) or they may also be markedly incised and show a permanent outlet (Müller glacier, New Zealand). The main distribution areas of hazardous morainedammed lakes are the Himalayas and the Andes. In Nepal, over 2,000 glacier lakes exist, of which about 20 have been classified as hazardous. The largest lakes are the Tsho Rolpa, Imja, Lower Barun, and Thulagi lakes, which are situated at an altitude between 4,000 and 5,000 m. The Tsho Rolpa (76 _ 106 m3) with a depth of 130 m and a width of 500 m, is located at an altitude of 4,580 m at the head of the Rolwaling valley and has formed during the last 40 years at the glacier tongue of the Trakarding Glacier (Everest, Nepal) (Figure 8) (Yamada, 1998; Reynolds, 1999). In the Rolwaling valley, the height of the valley floor decreases from 4,500 m to 1,700 m at the confluence with the Bhote Khosi over a vertical distance of only 20 km, providing a rather steep thalweg and high erosional flow velocities of potential floods.
The largest moraine-dammed lakes in the Himalayas are actually dammed by two moraine barriers (Figure 7): the end moraine of the parent glacier and the lateral moraine by a tributary glacier (Tsho Rolpa glacier lake, Imja glacier lake). In the Cordillera Blanca (Peru) four major lake outbursts occurred between 1938 and 1950, following the glacier ice losses of the 1920s and 1930s (Lliboutry
et al., 1977). The most devastating was the 1941 event, in which the Lake Palcacocha drained catastrophically. The event triggered a debris flow (8 × 106 m3) that destroyed one-third of the town Huaraz. Over 6,000 inhabitants lost their life.
Development of moraine-dammed lakes
The water supply of the moraine-dammed lakes takes place by supraglacial, englacial, and subglacial meltwater, by glacier calving, by tributary rivers, and by precipitation in the form of rain and snow. The amplitude of the lake level may vary several meters during the course of the year (3.4 +/- 0.7 m at Lago Paron in the Peruvian Andes, Lliboutry et al., 1977).
Fig. 6. Types of moraine-dammed lakes at short, steep hanging glaciers and gently sloped valley glaciers.
Fig. 7 Cross section of a moraine-dammed lake and potential triggers of lake outbursts A calving, B ice avalanches, C debris flows, D sudden meltwater drainage (sub-, en-, or supraglacial), E failure of the moraine dam, and F meltout of dead ice cores.
At valley glaciers, the morainedammed lakes possess mostly an oval to longated form and grow upvalley in longitudinal direction. The lowest point of the lake is situated in the upper third. Debriscovered, warm-based glaciers proved to be very favorable for the development of moraine-dammed lakes. The lake formation is linked with the surface gradient of the glacier (Reynolds, 2000). Larger supraglacial lakes may develop on valley glaciers up to a surface gradient of 2°. Important for the formation of a large supraglacial lakes is that glacier maintains a stable terminus position over a longer time period. Moreover, the presence of some stagnant ice is favorable. Especially the long valley glaciers are gradually downwasting and thinning and not responding with an immediate glacier front retreat on a negative mass balance.
The initial stages of moraine-dammed lakes at valley glaciers are generally in the form of small supraglacial ponds of a debris-covered glacier. They gradually coalesce to a larger unified lake over time. The formation of supraglacial lakes may begin at undulations or at obstacles at the underground or at crevasse zones (Blachut and Ballantyne, 1976). At first, a vertical expansion of the supraglacial depression occurs at the glacier surface until the shift toward a lateral lake expansion by retreat of the steep ice cliffs. During the enlargement of supraglacial lakes, differential melting und backwasting processes and topographical inversions play a major role, which may result in the development of glacier karst and sink holes (Clayton, 1964). Crucial for the survival of the lakes is that they are not connected to a subglacial or englacial drainage system. In the following, the lake expands horizontally and also inundates the end moraine. The growing of the supraglacial lake is subject to self-reinforcing processes with increasing vicinity to the end moraine. Due to its higher insolation absorption, the moraine material possesses a higher temperature than the glacier ice (Lliboutry et al., 1977). The direct contact to the lake water favors the Glacier
Fig. 8. The Tsho Rola Lake with a double dam formation of the former main valley glacier barrier of the Ripmo Shar glacier and the end moraine of the Trakarding glacier.
melting of the glacier tongue. The lake formation often starts 1–2 km upvalley of the end moraine, due to the fact that the glacier tongue area is heavily debris-covered. Therefore, the lakes are not that deep and their volume may be overestimated easily. Progressive enlargement of supraglacial lakes is well researched for glaciers of the Himalayas (Ageta et al., 2000; Benn et al., 2001; Chikita, 2008; Delisle et al., 2003; Fujita et al., 2008; Komori et al., 2003; Reynolds, 2000). In Nepal, glacier lakes increased between 33 and 71 m/a since their formation (Yamada, 1998). The surface area of Tsho Rolpa grew from about 0.23 km2 in 1959 to 0.62 km2 by 1972 and 1.3 km2 by 1990 (Yamada, 1998) (Figure 9). Imja Glacier lake increased from a few small ponds in the 1950s to a single body of water with a surface area greater than 0.5 km2 in 1984 (Watanabe et al., 1994). In general, the moraine-dammed lakes are not more than 150 m deep. During the lateral expansion of the lake, the lake water may undercut the inner slope of the lake and trigger mass movements into the lake. The Tsho Rolpa is classified as the most dangerous lake in Nepal. The Trakarding glacier recedes by 20 m/a and has increased its size by around eight times in the last decades.
Trigger mechanisms for lake outbursts of moraine-dammed lakes
The failure of moraine dams occurs through a variety of different mechanisms (Reynolds, 2000; Clague and Evans, 2000) (Figure 7).
1. A very common outburst mechanism is the generation of displacement waves generated by ice avalanches or rock fall into the moraine-dammed lake (Dig Sho outburst in Nepal in 1985, Vuichard and Zimmermann, 1987). Subsequently, the flood creates a characteristic V-shaped incision into the moraine (Photo 6) or surges through an existent outlet channel and widens it rapidly. The natural spillway may be enlarged subsequently by retrogressive erosion.
2. Glacier calving may induce displacement waves overtopping the moraine dam. At the glacier tongue, crevasses are located immediately behind the ice cliff and undercutting at the water line may accelerate the calving processes (e.g., Thulagi glacier, Nepal). Calving of glaciers into glacial lakes increases the lake size. Floating glacier tongues are considered to be highly dangerous. They can collapse catastrophically into the lake.
3. Wastage of ice cores increases the volume of the lake and poses a serious risk to dam stability (Richardson and Reynolds, 2000b). Average rates of subsidence due to melting of buried ice in the moraine dam at Imja glacier (Nepal) amounted up to 2.7 m/year (Watanabe et al., 1995).
4. Earthquakes may lead to a dam failure by mass movement and destabilization of the dam (1941 Palcacocha, Huascaran, Peru, Lliboutry et al., 1977).
5. The moraine dam may gradually become instable by seepage, piping, and the enlargement of drainage conduits in the moraine. This natural process is sometimes accelerated accidentally by artificial drainage measurements and leaking of the pipes (e.g., Tsho Rolpa, Grabs and Hanisch, 1993).
6. Catastrophic sub- or englacial drainage of the glacier tongue area into the moraine-dammed lake may lead to a sudden rise of the water level in the proglacial lake (Passu glacier, Karakoram Mountains, in January 2008).
Fig. 9. Development of the Tsho Rolpa glacier lake (Reynolds Geo-Sciences Ltd. (2000), Drawing: L. Iturrizaga.
7. Heavy rainfall or snow melt may increase the lake level abruptly and lead to an outburst (Clague and Evans, 2000).
Hazard potential by glacier lake outbursts
Among glacier hazards, glacier lake outburst floods possess the most far-reaching impact zone. The devastating effect itself is significantly dependent from the peak discharge, which is in general much higher than that of floods triggered by rainfall or snowmelt (Clague and Evans, 2000). General hydrology of such floods has been discussed in a separate article entitled Hydrology of jökulhlaup. From the historical record, glacier hazards are comparatively well described from the European Alps, where during the Little Ice Age (1600–1850) especially advancing tributary glaciers have dammed the main valley (e.g., impoundment of the Saaser Vispa by the Allalin glacier). In the Ötztal, the Vernagt glacier and Guslarferner sealed off the Rofental and impounded in 1848 a lake (3 × 106 m3) with a subsequent outburst flood (Hoinkes, 1969). In total, over 200 glacier lake outbursts have been recorded in the Alps. The general glacier shrinkage around the world during the twentieth century (Oerlemans, 2001) has lead to a remarkable shift in the glacial landscape systems and therefore in the glacial hazard potential (Evans and Clague, 1994; Häberli et al., 1999; Huggel, 2004; Kääb et al., 2005a, b; Baudo et al., 2007). The glacier retreat has become obvious in the accelerated formation of moraine-dammed lakes in the Himalayas since 1950s (Yamada, 1998; Kattelmann, 2003; Kattelmann and Watanabe, 1997) and the Peruvian Andes (Ames, 1998; Reynolds, 1992; Georges, 2004; Hubbard et al., 2005). With the augmented settlement density in high mountain regions, the hazard potential has increased in some areas. This is especially true for oasis settlements that are dependent on the irrigation of glacialmeltwater (Iturrizaga, 1997).
Glacier hazards have reached a wider recognition by the outbreak of the Dig Tsho August 4, 1985. The lake outburst has been triggered by an ice avalanche, falling into the lake and caused a 5 m high flood wave. This type of lake outburst may be one of the major hazards in high mountain areas in the future. A lot of glaciers are in the transition from small-sized valley glaciers, reaching just the foot of the mountain, to hanging glaciers. However, most of the settlements in the Himalayas are located at the slopes or even on mountain ridges, in a rather floodsave location. In contrary, in the Karakoram almost none moraine-dammed lake exist. The main hazards are generated by the outburst of glacier-dammed lakes, which may be several times larger than moraine-dammed lakes. Moreover, their formation and outburst is highly unpredictable. Glacier advances in this region would lead to an increase in glacier-dammed lakes and in turn to a higher hazard potential. A lot of the settlements are situated in flood-prone areas as the slopes are covered with talus accumulations, which are not suitable for housing. Moreover, the oasis settlements are in many cases directly located in the glacier forefields. Settlement loss by glacier lake outbursts amounts up to 300 m in width were reconstructed in the Shimshal valley (Iturrizaga, 1997, Photo 7).
In the Peruvian Andes (Cordillera Blanca), 100 new proglacial lakes have formed during the last century (Ames, 1998). In the 1930s only 30 lakes were monitored, whereas in the 1990s 138 lakes were counted (Ames, 1998; Kaser and Osmaston, 2002). Several glacier lake outbursts, partially triggered by earthquakes and involving subsequent ice avalanches, have seriously affected the settled mountain valley (Welsch and Kinzl, 1970; Patzelt, 1983). Richardson and Reynolds (2000b) have carried out an intensive evaluation with regard to dam stabilities as a base for measurements of technical prevention. The subsequent technical measurements maybe undertaken to mitigate the risk of the outburst of a morainedammed lake (Lliboutry et al., 1977; Grabs and Hanisch, 1993; Richardson and Reynolds, 2000b):
- Selective blasting of instable areas of the parent rock or of the overhanging glacier.
- Installation of mesh wires as protection against rock fall.
- Stabilization of the moraine wall by injection of a cement core.
- Installation of mesh wire at the outer slope of the moraine dam as protection against erosion.
- Artificial heightening of the moraine dam for avoiding the spillover of a displacement wave.
- Lowering of the lake level in order to decrease the pressure on the moraine dam, to reduce the water volume in case of an outburst.
- Drainage of the lake by (a) the stepwise deepening of the natural spillway channel and thus increasing the
Photo 7. The village of Shimshal has lost about 300 m of land by glacier lake outbursts of the Khurdopin glacier (Photo: L. Iturrizaga, 07.07.2001).
drainage rate, effusion of the channel with cement and paving with stones, (b) blasting of parts or the entire moraine dam in its initial stage as prevention measurement (evacuation of the inhabitants), (c) pipes through or below the moraine and stabilization of the pipes with cement, (d) drillings through the bedrock below the moraine dam. _ Method of lowering the lake level by a hydraulic syphon technique with plastic pipes. This method is cost efficient and easily transportable. The system uses the energy, which is produced by the hydraulic gradient between the higher inlet and the lower outlet. The pipes are located at the inlet several meters below the lake level and at the foot of the moraine dam. Leaking of the pipes may be serious problem in destabilizing the dam. In case of a glacier tongue that floats on the lake, the ice may become instable and produce a lake outburst. Downstream of the moraine dam, the common engineering protection measurements against floods may be installed. The technical opportunities to prevent an outburst of a glacial-dammed lake are rather limited and in most cases financial funding is as well a key restriction. In general, the hazard potential of glacial-dammed lakes by tributary glaciers has decreased due to the common tendency of glacier retreat. However, there has been substantial progress in hazard management of moraine-dammed lakes in regard to stabilization techniques and the application of hydraulic siphon techniques. Glacier lake monitoring and potential outburst risks are more and more monitored and evaluated by the analysis of satellite images (Huggel, 2004; Huggel et al., 2004; Quincey et al., 2007; Barry et al., 2008). Statistical remote sensing-based studies have been carried out for estimating the probability of catastrophic drainage from moraine-dammed lakes in southwestern British Columbia (McKillop and Clague, 2007). Flume tanks, which are artificial river channels, have been used to simulate natural river channels and get more insights into glacier lake outbursts sedimentation patterns (Rushmer, 2007). Carrivick (2006) and Herget (2005) have reconstructed computerbased large magnitude floods in two-dimensional (2D) and three-dimensional (3D) flows. These calculations may also be useful for hazard management of artificial dam failures. However, the outbursts of sub- and englacial water pockets, which represent in many mountain areas a serious hazard, are still rather unpredictable (Glacier du Trient). In the European Alps, about 60–70% of the glacier lakes are ice-marginal lakes and 30–40% are water pockets (Häberli, 1983). There are also indications that in the Karakoram Mountains the number of supraglacial lakes is increasing and related with that the danger of englacial lake outbursts (Iturrizaga, 2011). In Nepal, 34 of the existing lakes in the Dudh Khosi catchment area have been increased in size and 24 new lakes have been developed (Bajracharya and Mool, 2009). However, the lake size does not necessarily correspond with a higher risk, as the recent assessment for the Imja Glacier has shown (Watanabe et al., 2009).
Glacier lakes are indirectly a result of climatic changes expressed by glacier fluctuations. During the earth history glacier lakes have survived only a short time period, but glacier lake outbursts had a profound impact in shaping the landscape. The dominant type of glacial lakes may shift over time from glacier-dammed lakes to moraine-dammed lakes in different glaciation situations in dependence of the topographic setting. In the course of deglaciation, the types of glacier will change and therefore the type of glacial lakes. Glacier-dammed lakes occur mainly in times of glacier advance, but there are also some constellations in which they are the result of glacier retreat. Moraine-dammed lakes are mainly linked to glacier retreat regimes. During the Pleistocene glaciation, ice barriers of several hundred meters in height supposed to have existed. These large ice dams have no pendant at recent times. In general, the number of glaciers increases in the period of deglaciation from an ice-stream network to individual glaciers. Only some glaciers are prone to the formation of moraine-dammed lakes. Their size is in general much smaller than the glacier-dammed lakes. However, glacier lake outbursts from these small lakes can attain extremely high peak discharges by sudden dam failure. As a consequence of topographical and climatic conditions, glacier dynamics and the sediment transfer system glaciers are prone for the formation of moraine-dammed lakes. Therefore, a characteristic distribution pattern of glacier lake types can be recognized in individual mountain areas. In terms of natural hazards, sudden outbursts from small lakes with high peak discharges may have a more severe impact on human settlement than the drainage of large ice-dammed lakes. Not all glacier lake outbursts have to be necessarily released as water floods. They may also occur as debris flow with a high hazard potential.
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