Document: The causes and mechanisms of moraine-dammed lake failures
Section: 3. The causes and mechanisms of moraine-dammed lake failures > 3.1 The dynamic causes of moraine-dammed lake failures
Type: text

, which , along with hydrostatic pressure , can lead to the final rupture of the dam . A legend at the bottom clarifies the symbols : a white box with a gray outline is a " long - term cause " ; a light gray box is a " dynamic cause " ; a medium gray box is an " interphase " ; a dark gray box is a " final phase " . It also distinguishes between line types : a dashed line for " lakes without surface outflow " and a solid line for " lakes with surface outflow " . Fig . 1 The causes and mechanisms that underpin the process of moraine dam failure .

1988). The waves have a considerable impact on the stability of the moraine-dam (e.g. Kattelmann & Watanabe 1997; Clauge & Evans 2000). The slope movement into the lake and its related displacement wave may lead to two different mechanisms of dam failure. The first is that of immediate dam rupture following the impact of the displacement wave and this represents the most catastrophic scenario while the second is that of dam failure due to an increase in outflow channel discharge. The second mechanism is essentially caused by the increased water level within the lake which increases the outflow channel discharge and causes increased incision which then, in turn, increases the discharge and increases the incision. This "positive feedback" (Yamada 1998) continues until the outflow channel is either able to resist the incision due to structural changes or until the lake empties (Clauge & Evans 2000). However, if the displacement wave has the necessary energy and moraine dam is at the same time sufficiently resistant, flooding may result from dam overflow (Kershaw et al. 2005). This may happen without significant damage to the moraine dam and an example occurred on 22nd April 2002 as a result of rockfall into lake Safuna Alta in the Cordillera Blanca (Hubbard et al. 2005). It is possible for dam overflow and outflow channel incision caused by sudden rise of the water level to occur during the same flood event. Kershaw et al. (2005) presented stratigraphical and sedimentological evidence from the Queen Bess lake GLOF on 12th August 1997 that indicated both scenarios. The first phase of flooding was represented by dam overflow during which the bulk of the flood volume escaped (Clauge & Evans 2000) while the second phase was represented by dam incision and final failure. The time interval between these phases was in order of minutes (Evans et al. 2002).


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 3. The causes and mechanisms of moraine-dammed lake failures > 3.1 The dynamic causes of moraine-dammed lake failures
Type: text

Cordillera Blanca ( Hubbard et al . 2005 ) . It is possible for dam overflow and outflow channel incision caused by sudden rise of the water level to occur during the same flood event . Kershaw et al . ( 2005 ) presented stratigraphical and sedimentological evidence from the Queen Bess lake GLOF on 12th August 1997 that indicated both scenarios . The first phase of flooding was represented by dam overflow during which the bulk of the flood volume escaped ( Clauge & Evans 2000 ) while the second phase was represented by dam incision and final failure . The time interval between these phases was in order of minutes ( Evans et al . 2002 ) .

It is possible for an earthquake to directly initiate a moraine-dammed lake failure as the shock can cause sufficient damage to a dam for it to fail (Clauge & Evans 2000) while there is also the chance that the seismic activity may initiate slope movements. In some cases it is not known whether dam failure was caused directly by an earthquake or if it was caused by slope movements initiated by an earthquake and this is particularly difficult distinguish in the case of historical events (Strasser et al. 2008). There is also the chance that an earthquake could change the internal structure of the moraine dam causing internal erosion in the form of piping which may then lead to the emptying of the lake (Lliboutry et al. 1977). In contrast, changes in the internal structure of the dam may serve to reduce infiltration through the moraine by blocking underground outflow channels. This may be dangerous in instances where the lake does not have a surface outflow as there is then a continuous rise in lake water level. It was seen at lake Parón (Cordillera Blanca) when an earthquake in 1966 inhibited flow along underground channels and the water level rose steadily until next strong earthquake in 1970 when discharge turned into the values before 1966 event (Lliboutry et al. 1977).

The flood wave from an upstream lake can cause dam failure on a lake situated downstream. The flood wave can often transform easily into different types of flows because of its high erosion and transport potential (Cenderelli & Wohl 2001; Breien et al. 2008). It produces a displacement wave or a significant rise in the water level when entering the downstream lake and this has the same consequences as a slope movement into the lake. A dam failure may occur as a result of the direct impact of the displacement wave or following incision of the outflow channels. The overall flood volume often considerable in these cases as there are inputs from two lakes. This was seen in the Cordillera Blanca following the Palcacocha outburst flood which destroyed the downstream lake Jircacocha on 13th December 1941 (Vilímek et al. 2005b) (Figure 2). The catastrophic debris-flow incorporated water from both lakes and destroyed one-third of the city of Huaraz claiming about 6000 lives (Lliboutry et al. 1977). However, in some cases, the downstream lake may absorb the flood wave if it has a sufficiently large accommodation space. It is known that lake Parón in the Cordillera Blanca has been able to absorb outburst floods from two lakes, the first from lake Chacrucocha prior to 1950 and second from lake Artesoncocha in 1951 (Lliboutry et al. 1977; Carey et al. 2012).

data/('The causes and mechanisms of moraine-dammed lake failures', '.pdf')_extraction.md


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 3. The causes and mechanisms of moraine-dammed lake failures > 3.1 The dynamic causes of moraine-dammed lake failures
Type: text

13th December 1941 ( Vilímek et al . 2005b ) ( Figure 2 ) . The catastrophic debris - flow incorporated water from both lakes and destroyed one - third of the city of Huaraz claiming about 6000 lives ( Lliboutry et al . 1977 ) . However , in some cases , the downstream lake may absorb the flood wave if it has a sufficiently large accommodation space . It is known that lake Parón in the Cordillera Blanca has been able to absorb outburst floods from two lakes , the first from lake Chacrucocha prior to 1950 and second from lake Artesoncocha in 1951 ( Lliboutry et al . 1977 ; Carey et al . 2012 ) .

The blocking of underground outflow channels can be caused by four mechanisms: clogging by sediments brought into the lake by its tributaries; clogging by material brought into the lake during mass-movements; freezing of outflow channels (O'Connor et al. 2001); and blocking of outflow channels caused by the changing internal structure of dam due to an earthquake (Lliboutry et al. 1977). The water level of the lake starts to rise if the channels are blocked and this leads to the same mechanism of dam failure as occurs during intense rainfall or snowmelt - dam rupture caused when the lithostatic pressure exceeds that of hydrostatic pressure (Richardson & Reynolds 2000a). There will also be increased erosion in instances of dam overflow. However, unlike the rise in water level caused by intense rainfall or snowmelt, the blocking of underground outflow channels usually causes the water level to increase until the lake basin is full. This occurs unless the hydrostatic pressure exceeds the lithostatic pressure as blocked channels do not usually unblock spontaneously. The moraine dam failure at lake Zhangzhanbo in Tibet on 11th July 1981 was caused by blocking of underground outflow channels (Ding & Liu 1992; Yamada 1998).

It is also clear that intense rainfall or snowmelt will lead to a rise in the water level of the lake. This cause depends on many factors of which the variability and extremity of the precipitation are perhaps the most significant or, alternatively, the variability and extremity of air temperature in relation to snowmelt (Yamada 1998). If a lake has surface runoff the increasing water level may lead to an increase in the erosion of the outflow channels and to the cycle of "positive feedback" (see above). If a lake does not


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 3. The causes and mechanisms of moraine-dammed lake failures > 3.1 The dynamic causes of moraine-dammed lake failures
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Figure

Image /page/4/Picture/2 description: A black and white photograph of a mountainous landscape, identified by the caption as the former lake basin of Jircacocha in the Cordillera Blanca. The image shows a steep, rocky slope covered in dense, dark vegetation. At the base of the slope is a flatter, more barren area that appears to be a dried lakebed or floodplain, with a small channel winding through it. Two large, white, downward-pointing arrows are superimposed on the image, pointing towards the edges of this barren area. The caption below the image reads: "Fig. 2 The former lake basin of Jircacocha in the Cordillera Blanca. Its dam failed after the arrival of a flood wave from lake Palcacocha. The..."


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 3. The causes and mechanisms of moraine-dammed lake failures > 3.1 The dynamic causes of moraine-dammed lake failures
Type: text

Fig. 2 The former lake basin of Jircacocha in the Cordillera Blanca. Its dam failed after the arrival of a flood wave from lake Palcacocha. The horizontal line indicated by arrows shows the former water level.

have surface runoff then the most important factor is the dam freeboard which is defined as the vertical elevation between the lake level and the lowest point on the dam crest. The rise in lake water level can lead to two different mechanisms of dam destruction. The first mechanism is dam rupture due to the increased hydrostatic pressure. This happens when the hydrostatic pressure overcomes the lithostatic pressure, which keeps the components of the dam together (Richardson & Reynolds 2000a). The second is dam overflow from which the subsequent erosion may induce dam failure (Kattelmann & Watanabe 1997). The failure of a dam under the Dallier glacier in the Cascade Range provides an example of one that occurred as a result of intense rainfall or snowmelt (O'Connor et al. 2001). It is also well known that intense rainfall also a major trigger for slope movements, which may initiate moraine dam failure.


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 3. The causes and mechanisms of moraine-dammed lake failures > 3.2 The long-term causes of moraine-dammed lake failures
Type: text

The long-term causes of dam destruction are the ice melting of buried ice, the impact of hydrostatic pres-sure, and the effect of time. It is difficult to accurately

constrain which of these long-term issues ultimately leads to the destruction of the dam. This led Yamada (1998) to group these under the title of "self-destruction" due to the absence of an initial external dynamic event (Yamada 1998; Bajracharya et al. 2007b). The long-term causes also weaken the resistance of the dam to dynamic causes: while the impact of hydrostatic pressure and the effect of time have some influence on the moraine dam they are not often the main cause of dam destruction. The degradation of a moraine dam is a function of time and this function does not constitute a single process. It is, instead, a group of processes that may lead to the degradation of a dam over a protracted period and which affects the moraine slope stability, dam freeboard, internal structure etc. In combination with, for example, intense rainfall, the effect of time may lead to mass movements on moraine slopes (Awal et al. 2010). If a lake is associated with a surface outflow, its sequential erosion may cause the lake to empty, without a significant GLOF (Yamada 1998). If a lake is not associated with a surface outflow, the internal erosion of outflow channels through piping may also cause the lake to empty, without a significant GLOF (Clauge & Evans 2000; Haeberli et al. 2001).


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 3. The causes and mechanisms of moraine-dammed lake failures > 3.2 The long-term causes of moraine-dammed lake failures
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Figure

Image /page/5/Picture/2 description: A black and white photograph of a rugged, mountainous landscape. In the center, a waterfall or stream of meltwater flows down a dark, steep rock face through a curved groove. The water empties into a small pool at the bottom. The foreground and middle ground are covered with a chaotic pile of large, light-colored, broken rocks and debris. In the upper right, a dark, sheer rock wall rises, contrasting with the cloudy sky visible in the upper left. A partial caption at the bottom of the image reads, "Fig. 3 An example of..." with the rest of the text cut off.


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 3. The causes and mechanisms of moraine-dammed lake failures > 3.2 The long-term causes of moraine-dammed lake failures
Type: text

**Fig. 3** An example of an uncovered ice lens in the protruding basal moraine of the developing lake Llaca in the Cordillera Blanca.

The term "buried ice" describes an ice lens integrated into the body of the moraine dam (Figure 3). It is possible for this to represent up to 90% of the dam volume (Costa & Schuster 1988). The melting of this buried ice weakens the stability of the dam as it disrupts its structural integrity and may also decrease the dam freeboard (Richardson & Reynolds 2000b; Huggel et al. 2002). The disruption of the structural integrity of the dam enables rupture by hydrostatic pressure while it also decreases the ability of the dam to withstand other causes that would not normally represent a significant problem. For example, the long-term degradation of the moraine dam leads to a decrease in the freeboard which, in combination with moderate rainfall or snowmelt, may lead to dam destruction.

The hydrostatic pressure is the pressure exerted by the gravitational force acting on a water column at certain depth. The water dammed in a moraine-dammed lake affects the dam by this pressure and its long-term affect may lead to the dam failure (Yamada 1998). This cause becomes especially significant if the moraine dam is weakened by, for example, buried ice melting while deep lakes are more susceptible to rupture caused by hydrostatic pressure. The destruction caused by the systematic effect of hydrostatic pressure occurs when the moraine dam is no longer able to resist the hydrostatic pressure, i.e. the hydrostatic pressure exceeds the lithostatic pressure (Richardson & Reynolds 2000a). This may be caused by an increase in the lake water level or the protracted degradation of the moraine dam (Yamada 1998; Jaboyedoff et al. 2004 in Vilímek et al. 2005a). The increase in lake water level may be caused by intense rainfall or snowmelt or the blocking of underground outflow channels (Costa & Schuster 1988; Grabs & Hanish 1993; Janský et al. 2006). It is also possible to increase the hydrostatic pressure by basal ice melting and lake deepening (Watanabe & Rothacher 1996). In cases where dam degradation occurs as a result of buried ice melting, intense slope erosion, or changes in the internal structure of the dam, rupture by hydrostatic pressure may occur without a significant change in the pressure (Richardson & Reynolds 2000a). These all represent examples of "dam self-destruction" and may occur without a dynamic trigger. This happened in 1994 at lake Lugge Tsho in Bhutan (Watanabe & Rothacher 1996). The impact of hydrostatic pressure has a specific position in the categorisation of mechanisms of dam destruction as both dynamic and long-term causes can lead to dam rupture caused by hydrostatic pressure (Figure 1).


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis
Type: text

The comparative analysis investigates historical moraine-dammed lakes failures between 1900 and 2009 from three regions: the Cordillera Blanca of Peru, the North American Cordillera, and Himalaya. The basic characteristics of these regions are listed in Table 1. These regions are commonly characterised by glacier retreat at present which is leading to the formation and

Tab. 1 The basic characteristics of the studied regions.


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis
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Table

| Region                       | Mountains                                                             | Coordinates              | Highest peak(s)                                               | Climatic settings                                     |
|------------------------------|-----------------------------------------------------------------------|--------------------------|---------------------------------------------------------------|-------------------------------------------------------|
| Cordillera Blanca            | Cordillera Blanca (Peru) Cordillera Huayhuash (Peru)               | 8°–10° S 77°–78° W    | Huascarán (6768 m asl)                                     | Tropical High Mountain Climate – Wet & Dry Seasons |
| North American Cordillera | Coast Mountains (CAN) Rocky Mountains (CAN) Cascade Range (USA) | 30°–50° N 120°–135° W | Mt. Waddington (4019 m asl) Mt. Rainier (4392 m asl) | Temperate High Mountain Climatic Zone – 4 Seasons  |
| Himalaya                     | Himalaya (China, India, Nepal, Bhutan)                             | 27°–35° N 75°–95° E   | Mt. Everest/ Sagarmatha (8848 m asl)                       | Temperate High Mountain Climatic Zone – 4 Seasons  |

Region
Mountains
Coordinates
Highest peak(s)
Climatic settings


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis
Type: text

development of new potentially dangerous morainedammed lakes. The threat of GLOFs in these areas is real while the downstream valleys are often settled.


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis > 4.1 The spatial analysis
Type: text

It is important to assess whether there are differences regarding the causes of moraine-dammed lake failures across the three study regions. The slope movements have been divided into two subgroups for this purpose: the first incorporates slope movements in which the displaced material was dominated by solid-state water (ice falls, ice avalanches, and snow avalanches) while the second incorporates movements in which the displaced material was dominated by rock or liquid-state water (rockfalls, landslides, and various types of flows). It should be noted that the failure mechanisms for these two subgroups are identical. Furthermore, the concept of dam failure stemming from a combination of long-term causes without an evident dynamic cause ("dam self-destruction" (Yamada 1998) suggests that it will always be difficult to define the precise cause of failure (i.e. there is no distinguishing between the ice melting of buried ice, the impact of hydrostatic pressure, and the effect of

time). Indeed, unless a lake is continuously monitored, it is often difficult to determine the cause of a contemporary GLOF, and even more so in the case of historical events. Therefore, while a single cause may be the most commonly ascribed, there is a possibility that it in part reflects the ease with which it can be identified. The comparison of dynamic and long-term causes (Table 2) shows that, across all three regions, dynamic causes prevail over long-term causes by a ratio of 4:1. There are, however, significant regional differences. In the Cordillera Blanca all of the twenty dam failures resulted from a dynamic event while more than two-fifths resulted from a longterm cause in Himalaya ("dam self-destruction").

The most frequent cause of failure in three regions of interest was found to be slope movements in which the displaced material was dominated by solid-state water (ice falls, ice avalanches, and snow avalanches) (Figure 4). This finding corresponds with those of previous studies (e.g. Costa & Schuster 1988; Ding & Liu 1992; Clauge & Evans 2000; Jiang et al. 2004; Awal et al. 2010). The second and the third most frequent causes tend to show distinct regional patterns with the second most common cause being slope movements dominated by solid rock or water in the Cordillera Blanca of Peru, intensive rainfall

Tab. 2 The dynamic and long-term causes of moraine-dammed lake failures (60 instances where causes of the moraine dam failures are known).

4. The comparative analysis
4.1 The spatial analysis


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis > 4.1 The spatial analysis
Type: table
Table

| Region                    | Dynamic Causes   |       | Long-term Causes |      | Total number of events |
|---------------------------|------------------|-------|------------------|------|---------------------------|
|                           | Number of events | %     | Number of events | %    |                           |
| Cordillera Blanca         | 20               | 100.0 | 0                | 0.0  | 20                        |
| North American Cordillera | 10               | 90.9  | 1                | 10.1 | 11                        |
| Himalaya                  | 17               | 58.6  | 12               | 41.4 | 29                        |
| Total                     | 47               | 78.3  | 13               | 21.7 | 60                        |

Region
Dynamic Causes
Long-term Causes
Total number of events


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis > 4.1 The spatial analysis
Type: figure
Figure

Image /page/6/Figure/9 description: A black and white chart comparing data from three different mountain ranges using pie charts of varying sizes. The vertical axis is labeled 'number of events' with dashed lines at 11, 20, and 29. The horizontal axis labels the three pie charts: 'Northamerican Cordillera', 'Cordillera Blanca', and 'Himalaya'. The size of each pie chart corresponds to the number of events on the vertical axis. The first pie chart for 'Northamerican Cordillera' corresponds to 11 events and is divided into three slices: 54,5% (dark gray), 36,4% (medium gray), and 9,1% (light gray). The second pie chart for 'Cordillera Blanca' corresponds to 20 events and has four slices: 45,0% (dark gray), 35,0% (medium-dark gray), 15,0% (light gray), and 5,0% (medium-light gray). The third and largest pie chart for 'Himalaya' corresponds to 29 events and has four slices: 51,8% (dark gray), 41,4% (light gray), and two small slices both labeled 3,4%.


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis > 4.1 The spatial analysis
Type: text

- slope movement of material with dominance of solid state water (ice falls, ice and snow avalanches)
- slope movement of material with a dominance of rocks or liquid water (rock falls, landslides, various types of flows)
- intense rainfall / snow melting
- □ earthquake
- flood wave form lake situated upstream
- □ blocking of underground outflow tunnel □ selfdestruction

Fig. 4 A representation of the causes of moraine-dammed lake failures in the Cordillera Blanca, the North American Cordillera, and Himalaya.

or snowmelt in the North American Cordillera, and self-destruction in Himalaya. It is of interest to note that intensive rainfall or snowmelt is only recorded as a trigger in the North American Cordillera and it may be that this reflects the particular climatic setting of the region. The moraine-dammed lakes in the North American Cordillera are situated between 1400 and 2400 m asl (O'Connor et al. 2001) whereas in the Cordillera Blanca and Himalaya they generally occur between 4000 and 5000 m asl (Lliboutry et al. 1977; Yamada 1998). The intensity of the combined rainfall and snowmelt may be greater in the North American Cordillera during the summer due to these lower elevations. Thereafter, only one moraine-dammed lake failure in the North American Cordillera was attributed to dam self-destruction.

The trigger of dam self-destruction represents the second most frequent cause of moraine dam failures in Himalaya. This may reflect the significant volumes of buried ice that occur in many moraine dam bodies in this region (Yamada 1998; Bajracharya et al. 2007b). The melting of buried ice leads to degradation of moraine dam and this may lead to its failure, especially in combination with the affect of hydrostatic pressure impact, without a dynamic triggering event (see above). Thereafter, one moraine dam failure in the region was caused by the blocking of underground outflow channels and one by a slope movement into a lake in which the displaced material was dominated by rock or liquid-state water. The second most frequent cause of moraine-dammed lake failures in the Cordillera Blanca were slope movements into lakes in which the displaced material was dominated by rock. In fact, slope movements account for 80% of all moraine dam failures in this region while 15% could be attributed to an earthquake. Thereafter, one dam failure was caused by the propagation of a flood wave which resulted from the failure of an upstream dam.


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis > 4.2 The temporal analysis
Type: text

This section focuses on the temporal distribution of 66 moraine-dammed lake failures and GLOFs over months (Figure 5) and 75 moraine-dammed lake failures and GLOFs over years (Figure 6). The monthly differences in the distribution of GLOFs shown in Figure 5 clearly reflect the general climatic setting of each of the study regions. The temporal distribution of moraine dam failures is similar in the North American Cordillera and Himalaya as dam failures occur exclusively during the summer season from June to September. The lake water levels are higher at this time because the warmer temperatures melt glacial ice and, thereby, reduce the dam freeboard. In the Cordillera Blanca, with its alternating wet and dry seasons, moraine dam failures are more evenly distributed but cluster during the wet season from December to May. The lake water levels are again higher at this time because there is a considerable amount of precipitation while the temperatures are also slightly warmer than they are during the dry season. These data indicate that the moraine dam failures occur most commonly during the warmer times of the year and corroborate the notion the most frequent cause of moraine dam failures are dynamic slope movements in which the displaced material was dominated by solid-state water (ice falls, ice avalanches, and snow avalanches).

The annual distribution of moraine dam failures has been analysed using a dataset of seventy-five events

4. The comparative analysis
4.2 The temporal analysis


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis > 4.2 The temporal analysis
Type: figure
Figure

Image /page/7/Figure/8 description: A radar chart, also known as a spider or web chart, displaying data for three different mountain ranges over twelve months. The chart has twelve spokes, labeled with Roman numerals I through XII, representing the months of the year in a clockwise direction. The concentric circles indicate a scale from 0 at the center to 50 at the outermost circle, with increments of 10. A legend on the right identifies the three data series: 'Himalaya' is represented by a thick solid black line, 'Cordillera Blanca (Peru)' by a thick dashed black line, and 'Northamerican Cordillera' by a thin double line. The 'Himalaya' data peaks in months VII (value approx. 10) and VIII (value approx. 12). The 'Cordillera Blanca (Peru)' data shows its highest peak in month I (value approx. 20), with other high points in XII (approx. 15) and II (approx. 10). The 'Northamerican Cordillera' data peaks in month VII (value approx. 15) and is similar in shape to the Himalaya data but with slightly higher values in the peak months.


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 4. The comparative analysis > 4.2 The temporal analysis
Type: text

Fig. 5 The distribution of moraine dam failures plotted according to month (66 instances overall).

for which the year of failure is known. In the period between 1900 and 1924 only one moraine-dam failure was recorded - this occurred at lake Zhanlonba in China during 1902 (Ding & Liu 1992) (Figure 6). In the period between 1935 and 1944 eight moraine-dam failures were recorded in the Cordillera Blanca region. These events claimed thousands of lives and caused considerable damage to infrastructure as well as leading to a series of investigations into moraine-dammed lakes (Zapata 2002; Carey 2005). These investigations called for remedial work at thirty-five sites in the form of artificial dams, concrete outlets, tunnels, etc. (Reynolds 2003; Carey et al. 2012). It may also be one of the reasons for the decreased number of moraine dam failures in the following years. In the period between 1970 and 1974 five failures were recorded in the region. These were all triggered by the catastrophic earthquake that occurred in Cordillera Blanca on 31st May 1970. The next moraine dam failure did not occur for another twenty-seven years. This protracted period of stability is thought to reflect the extensive remedial works and rupture of the most unstable lake dams following the earthquake in 1970. The annual distribution of moraine dam failure in Himalaya is more regular with at least two failures in each five-year period between 1955 and 2004. There was a maximum of six failures in the five-year period between 1980 and 1984. The annual distribution of moraine dam failures in the North American Cordillera is also broadly constant with no peaks evident.


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 5. Discussion
Type: text

There are a number of published lists of morainedammed lake failures for specific regions such as those that have been compiled for the Cordillera Blanca (Zapata 2002), the North American Cordillera (Clague & Evans 2000; O'Connor et al. 2001), and Himalaya (Yamada 1998; Ives et al. 2010). These lists have been updated and supplemented with as much data as it was possible to attain. There is, however, a paucity of events both at the beginning and at the end of the period from 1900 to 2009. The paucity of events at the beginning of this period may reflect an absence of moraine-dammed lake failures at that time or, more likely, that such events were not recorded if the areas in which they occurred were uninhabited or if no significant damage occurred. The paucity of events at the end of this period may again reflect an absence of moraine-dammed lake failures but is more likely to reflect the amount of time it takes for the data to be processed and published. In a number of instances the cause of the moraine-dammed lake failure is not known with certainty and it is only possible to attribute a probable cause. It is, nonetheless, evident that the most common causes are slope movements in which the displaced material is dominated by solid-state water (ice falls, ice avalanches, and snow avalanches). This finding corresponds with those of previous studies (e.g. Costa & Schuster 1988; Ding & Liu 1992; Clauge & Evans 2000; Jiang et al. 2004; Awal et al. 2010).

Fig. 6 The distribution of moraine dam failures plotted according to year (75 instances overall).


Document: The causes and mechanisms of moraine-dammed lake failures
Section: 6. Conclusions
Type: text

There are eight main causes, of which five are characterised as dynamic and three as long-term, and these are associated with around twenty failure mechanisms. The dynamic causes are slope movements into the lake (e.g. icefalls, avalanches, rockfalls, landslides, and other types of flow), earthquakes, a flood wave from a lake situated upstream, blocking of underground outflow channels, and intensive rainfall or snowmelt. The long-term causes are the ice melting of buried ice, the impact of hydrostatic pressure, and the effect of time. It is, therefore, clear that GLOFs are associated closely to various types of natural hazard. The similarities and disparities between the particular causes were investigated as well as the temporal characteristics of events in the Cordillera Blanca of Peru, the North American Cordillera, and Himalaya. It was found that dynamic causes are around four times more common than long-term causes although significant regional differences are seen: in the Cordillera Blanca of Peru all dam failures resulted from a dynamic event while more than two-fifths resulted from a longterm cause in Himalaya. The most frequent causes of GLOFs from moraine-dammed lakes were found to be slope movements in which the displaced material was dominated by solid-state water (ice falls, ice avalanches, and snow avalanches). This accounted for around half of all events irrespective of the specific study area. The other causes tended to show distinct regional patterns with the second most common cause being slope movements dominated by solid rock or water in the Cordillera Blanca of Peru, intensive rainfall or snowmelt in the North American Cordillera, and self-destruction in Himalaya. The temporal distribution of moraine dam failures is similar in the North American Cordillera and Himalaya as dam failures occur exclusively during the summer season from June to September while in the Cordillera Blanca they are more evenly distributed but cluster during the wet season from December to May. These patterns clearly reflect the general climatic setting of each of the study regions. The annual distribution of these failures is broadly constant with no particular trends yet evident. The recognition of these regional differences is necessary in order to build optimal regionally focused methods for moraine-dammed lakes hazard assessment in the future.


Document: The formation and failure of natural dams
Section: ABSTRACT
Type: text

Of the numerous kinds of dams that form by natural processes, dams formed from landslides, glacial ice, and neoglacial moraines present the greatest threat to people and property. Landslide dams form in a wide range of physiographic settings. The most common types of mass movements that form landslide dams are rock and debris avalanches, rock and soil slumps and slides, and mud, debris, and earth flows. The most common initiation mechanisms for dam-forming landslides are excessive rainfall and snowmelt and earthquakes.

Landslide dams can be classified into six categories based on their relation with the valley floor. Type I dams (11 percent of the 81 landslide dams around the world that we were able to classify) do not reach from one valley side to the other. Type II dams (44 percent) span the entire valley flood, occasionally depositing material high up on opposite valley sides. Type III dams (41 percent) move considerable distances both upstream and downstream from the landslide failure. Type IV dams (1 percent) are rare and involve the contemporaneous failure of material from both sides of a valley. Type V dams (1 percent) also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river. Type VI dams (3 percent) involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side.

Many landslide dams fail shortly after formation. In our sample of 73 documented landslide-dam failures, 27 percent of the landslide dams failed less than 1 day after formation, and about 50 percent failed within 10 days. Overtopping is by far the most common cause of failure. The timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry, material characteristics of the blockage, rate of inflow to the impoundment, size and depth of the impoundment, bedrock control of flow, and engineering controls such as artificial spillways, diversions, tunnels, and planned breaching by blasting or conventional excavation.

Glacial-ice dams can produce at least nine kinds of ice-dammed lakes. The most dangerous are lakes formed in main valleys dammed by tributary glaciers. Failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam. Cold polar-ice dams generally drain supraglacially or marginally by downmelting of an outlet channel. Warmer temperate-ice dams tend to fail by sudden englacial or subglacial breaching and drainage.

data/('The formation and failure of natural dams', '.pdf')_extraction.md


Document: The formation and failure of natural dams
Section: ABSTRACT
Type: text

and depth of the impoundment , bedrock control of flow , and engineering controls such as artificial spillways , diversions , tunnels , and planned breaching by blasting or conventional excavation . Glacial - ice dams can produce at least nine kinds of ice - dammed lakes . The most dangerous are lakes formed in main valleys dammed by tributary glaciers . Failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam . Cold polar - ice dams generally drain supraglacially or marginally by downmelting of an outlet channel . Warmer temperate - ice dams tend to fail by sudden englacial or subglacial breaching and drainage .

Late-neoglacial moraine-dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries. These late-neoglacial dams pose hazards because (1) they are sufficiently young that vegetation has not stabilized their slopes, (2) many dam faces are steeper than the angle of repose, (3) these dams and lakes are immediately downslope from steep crevassed glaciers and near-vertical rock slopes, and (4) downstream from these dams are steep canyons with easily erodible materials that can be incorporated into the flow and increase flood peaks. The most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake, generated by icefalls, rockfalls, or snow or rock avalanches. Melting of ice-cores or frozen ground and piping and seepage are other possible failure mechanisms.

Natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam. Although data are few, for the same potential energy at the dam site, downstream flood peaks from the failure of glacier-ice dams are less than those from landslide, moraine, and constructed earth-fill and rock-fill dam failures. Moraine-dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than  $10^{11}$  joules. Differences in flood peaks of natural-dam failures appear to be controlled by dam characteristics and failure mechanisms.


Document: The formation and failure of natural dams
Section: INTRODUCTION
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There are many ways in which natural lakes and their dams can form in nature. A general but useful classification of lake basins was proposed by Davis (1882), who categorized lake basins as constructive, destructive, and obstructive. This report is concerned with only one of Davis' classifications, that of obstructive basins. Obstructive barriers include landslide dams, glacier dams, moraine dams, volcanic dams, fluviatile dams, eolian dams, coastal dams, and organic dams (table 1).

This investigation of the hazards associated with natural dams indicates that, despite the great variety of natural dams, only three kinds pose a widespread threat to people and property: landslide dams, glacier dams, and alpine moraine dams.

There are many case studies of individual natural-dam failures, but an integrated view of the phenomenon as a whole does not exist. This paper attempts to use information from a large number of individual case studies to draw comprehensive conclusions about this important natural process. The data base used for this investigation consists of information on approximately 225 natural dams whose formation and (or) failure has been documented in the literature or is know to us from our own field investigations. This data base is believed to be sufficiently large and comprehensive that the conclusions in this report will not radically change even though it is recognized that there are some reported events not known to us and far more events that have never been recorded or reported.


Document: The formation and failure of natural dams
Section: INTRODUCTION > Table 1. -- Types of obstructive dams
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[Most dangerous dams are underlined. This table is modified from Hutchinson (1957) and Schuster and Costa (1986a)]

INTRODUCTION
Table 1. -- Types of obstructive dams


Document: The formation and failure of natural dams
Section: INTRODUCTION > Table 1. -- Types of obstructive dams
Type: table
Table

| Type of dam                     | Example                                                              | Type of dam                               | Example                                                          |
|---------------------------------|----------------------------------------------------------------------|-------------------------------------------|------------------------------------------------------------------|
| Volcanic dams:                  |                                                                      | Fluviatile dams:                          |                                                                  |
| Volcanic peaks                  | Lake Nicaragua, Nicaragua (Hutchinson, 1957)                      | Tributary sediments                       | Lake Pepin, Minnesota-Wisconsin (Davis, 1882)                 |
| Lava flows                      | Snag Lake, California (Finch, 1937)                               | Main channel sediments (lateral lakes) | Lake Tung-ting, People's Republic of China (Hutchinson, 1957) |
| Pyroclastic flows               | Rio Magdalena Lake, Mexico (Silva and others, 1982)               | Alluvial fans                             | Lake Tulare, California (Hutchinson, 1957)                    |
| Landslide dams:                 |                                                                      | Deltas                                    | Blue Lakes, California (Davis, 1933)                          |
| Rock and debris avalanches      | Castle Lake, Washington (Meyer and others, 1986)                  | Levee deposits (oxbow lakes)              | Old River, Louisiana (Campti Quadrangle USGS 1:62,500, 1957)  |
| Slumps and slides               | Earthquake Lake, Montana (Hadley, 1964)                           | Eolian dams:                              |                                                                  |
| Mud, debris, and earth flows    | Lake San Cristobal, Colorado (Crandell and Varnes, 1961)          | Dunes                                     | Moses Lake, Washington (Russell, 1893)                        |
| Liquifaction of sensitive clays | Yamaska River Lake, Quebec, Canada (Clark, 1947)                  | Coastal dams:                             |                                                                  |
| Peatslides                      | Addergoole Bog Lake, Ireland (Ousley, 1788)                       | Bay-bars                                  | Freshwater Lagoon, Eureka, California (Cotton, 1941)          |
| Scree                           | Goatswater, United Kingdom (Marr, 1916)                           | Organic dams:                             |                                                                  |
| Glacial dams:                   |                                                                      | Logs and other vegetation                 | Lake Okeechobee, Florida (Hutchinson, 1957)                   |
| Ice                             | Gapshan Lake (Shyok River), Pakistan (Mason, 1929)                | Beaver dams                               | Beaver Lake, Montana (Hutchinson, 1957)                       |
| Moraine                         | Nostetuko Lake, British Columbia, Canada (Blown and Church, 1985) |                                           |                                                                  |
| Ice and snow avalanche          | Rio Plomo, Argentina (King, 1934)                                 |                                           |                                                                  |


Document: The formation and failure of natural dams
Section: INTRODUCTION > Table 1. -- Types of obstructive dams
Type: text

Natural dams have many economic benefits, including hydropower generation (Anderson, 1948; Adams, 1981) and recreation (Jones and others, 1985), but also can constitute serious hazards. A great deal of information is available about the types and characteristics of constructed dams and the consequences of their failure, but similar information about natural dams is almost nonexistent, despite the fact that natural dams are far more numerous than constructed dams.

Natural dams hold most of the records for size and flood magnitude following failure. The largest flood known to have occurred on the surface of the earth, the "Spokane flood," which carved the channeled scablands in eastern Washington State between 16,000 and 12,000 years ago, was the result of failure of a natural ice dam near Missoula. Montana (Baker, 1973). Another extraordinary documented flood was that caused by the overflow of Pleistocene Lake Bonneville at Red Rock Pass near Preston, Idaho (Malde, 1968), about 14,000 years ago (Scott and others, 1980). The flood was caused by the overtopping and failure of a natural alluvial-fan dam (Gilbert, 1878; Malde, 1968). In 1911 an earthquake in the Soviet Union triggered a rock avalanche (the Usoy landslide) with a volume of 2.0- to 2.5-billion cubic meters, which dammed the Murgab River. This landslide dam is about 550 meters high (Berg, 1950), and the impounded lake (Lake Sarez) overflows the dam (Hutchinson, 1957; Gasiev, 1984). The Usoy landslide dam is nearly three times as high as the world's largest constructed dam, the Nurek rock-fill dam, also in the Soviet Union, which has a height of 300 meters.

The purposes of this report are to (1) document the methods by which the most dangerous types of natural dams form, (2) determine what constitutes the stability or instability of the dams, (3) compare some of the physical dimensions of the dams and lakes, (4) document the mechanisms by which these natural dams fail, and (5) compare some of the characteristics of the resulting downstream flooding.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS
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Landslide dams are remarkably diverse in their formation, characteristics, and longevity. One of the first recorded flooding catastrophes due to a landslide dam occurred in central Java in A.D 1006 (Holmes, 1965, p. 485-487). Many landslide dams have failed catastrophically, causing major downstream flooding and loss of life. At least 2,423 people died in the 1933 flood caused by failure of the large Deixi landslide dam on the Min River in central China (Li and others, 1986). A graphic account of the consequences of the failure of a landslide dam in the Indus River valley, India, is provided by Mason (1929).


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Geomorphic Settings of Landslide Dams
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Landslide dams form most frequently where narrow steep valleys are bordered by high rugged mountains. This setting is common in geologically active areas where earthquakes, volcanoes, or glacially oversteepened slopes occur. Geologically active areas usually contain abundant landslide source materials, such as sheared and fractured or hydrothermally altered bedrock, and experience triggering mechanisms to initiate landslides. Steep narrow valleys require relatively small volumes of material to form dams; thus even small mass movements present a potential for forming landslide dams. Such dams are much less common in broad open valleys, but in areas where rivers have incised lacustrine or marine deposits, slides and slumps or quick-clay failures have formed landslide dams (Evans, 1984; Clark, 1947).

LANDSLIDE DAMS
Geomorphic Settings of Landslide Dams


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Types of Movements that Form Landslide Dams
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Study of 184 cases of landslide dams from the literature and the authors' experience has shown that a broad range of mass-movement types can cause landslide dams (fig. 1; table 2). However, most landslide dams are caused by avalanches, slumps and slides, and flows. Relatively few dams have been caused by slope failure in sensitive clays or rock and soil falls. The number of dams resulting from failure of sensitive clays is small because of their limited extent and common occurrence in areas of low relief. The number of dams resulting from falls is small because the volumes of material constituting failures of this type are small.

In general, the highest landslide dams form in steep-walled narrow valleys because there is little area for the landslide mass to spread out. Large-volume earth and rock slumps and slides and rock and debris avalanches are particularly likely to form high dams in narrow valleys because they occur on steep slopes and usually have high velocities that allow complete stream blockage before the material can be sluiced away.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Types of Movements that Form Landslide Dams
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Figure

Image /page/9/Figure/0 description: A black and white vertical bar chart displays the number of landslides by type. The y-axis, labeled "NUMBER OF LANDSLIDES," ranges from 0 to 50, with tick marks every 10 units. The x-axis lists six different types of landslides. The bars are arranged in descending order of frequency from left to right. The approximate values for each category are as follows: "Undifferentiated landslides" is approximately 49, "Rock and debris avalanches" is about 44, "Rock and soil slumps and slides" is around 43, "Mud, debris, and earth flows" is approximately 37, "Liquefaction" is about 9, and "Falls" is approximately 2.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Types of Movements that Form Landslide Dams
Type: figure
Figure: Figure 1.--Graph showing distribution of landslide dams by type of landslide, based on 183 cases from the literature and the authors' experience. Classification based on Varnes (1978)

Figure 1.--Graph showing distribution of landslide dams by type of landslide, based on 183 cases from the literature and the authors' experience. Classification based on Varnes (1978)


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Types of Movements that Form Landslide Dams
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Commonly, large landslide dams are caused by complex landslides that start as slumps or slides and transform into rock or debris avalanches. An outstanding example was the 2.5-cubic kilometer rockslide/debris avalanche (the world's largest historic landslide) associated with the 1980 major eruption of Mount St. Helens, Washington. This high-velocity landslide originated as a rockslide on the side of the volcano, then transformed into a debris avalanche and traveled 24 kilometers down the North Fork Toutle River valley, impounding or enlarging five large lakes, only three of which remain (Meyer and others, 1986).

If the basal failure zone of a landslide extends beneath the valley floor, upward movement of the streambed itself can alter gradients and form shallow lakes. In this situation, downstream flooding is not great because the lakes are small, outlet gradients are low, and the streambed material is not easily erodible.

Mud, debris, and earth flows form a significant percentage of the landslide dams reviewed. Most of these dams have been cause by relatively high-velocity debris flows issuing from tributary valleys to briefly block rivers in main valleys. Generally dams formed in this manner are not high, nor are they particularly resistant to erosion (Li and others, 1986). Thus they commonly overtop soon and breach rapidly. Other kinds of flows are much slower or may form long-lived dams. An example is the 6.5-kilometer Slumgullion earthflow that dammed the Lake Fork of the Gunnison River in Colorado about 700 years ago, impounding the 3-kilometer Lake San Cristobal (Crandell and Varnes, 1961).

[Dashes indicate that authors were unable to obtain significant data]


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Types of Movements that Form Landslide Dams
Type: table
Table: Table 2.--Well-documented examples of landslide dams formed by specific classes of landslides

| Landslide class and name                                                   | Year          | Dammed river                 | State/ country                            | Landslide volume (m3) | Blockage dimensions              |                         |                            | Lake dimensions         |                             | Dam failed?         | References                                                |
|-------------------------------------------------------------------------------|---------------|---------------------------------|----------------------------------------------|-----------------------------|----------------------------------|-------------------------|----------------------------|-------------------------|-----------------------------|------------------------|-----------------------------------------------------------|
|                                                                               |               |                                 |                                              |                             | Height (m)                    | Length (m)           | Width (m)               | Length (km)          | Volume (m3)              |                        |                                                           |
| Rock and debris avalanches                                             |               |                                 |                                              |                             |                                  |                         |                            |                         |                             |                        |                                                           |
| Usoy landslide                                                                | 1911          | Murgab River                 | Tadzhik- istan, U.S.S.R.               | 2.0-2.5x109                 | 301 (Bolt) 550 (Gasiev) | 1,000                   | 1,000                      | 53                      | --                          | Partial Failure     | Gasiev, 1984                                              |
| Tanggudong debris slide/avalanche                                          | 1967          | Yalong River                 | Sichuan, People's Republic of China | 68x106                      | 175                              | 650                     | 3,000                      | 53                      | 680x106                     | Yes                    | Li and others, 1986                                    |
| Mayumarca rock slide/debris avalanche                                   | 1974          | Mantaro River                | Peru                                         | 1.6x109                     | 170                              | 1,000                   | 3,800                      | 31                      | 670x106                     | Yes                    | Hutchinson and Kojan, 1975; Lee and Duncan, 1975 |
| Mount St. Helens rock slide/debris avalanche                            | 1980          | North Fork Toutle River   | State of Washing- ton, U.S.A        | 2.8x109                     | avg.=45                          | 800 (Spirit Lake) | 24x103 (Spirit Lake) | 5.5 (Spirit Lake) | 295x106 (Spirit Lake) | No (Spirit Lake) | Meyer and others, 1986                                 |
| Rock and soil slumps and slides                                        |               |                                 |                                              |                             |                                  |                         |                            |                         |                             |                        |                                                           |
| Deixi landslide                                                               | 1933          | Min River                       | Sichuan, People's Republic of China | 150x106                     | 255                              | 400                     | 1,300                      | 17                      | 400x106                     | Yes                    | Chang, 1934; Li and others, 1986                    |
| Lower Gros Ventre landslide                                                | 1925          | Gros Ventre River         | Wyoming, U.S.A.                           | 38x106                      | 70                               | 900                     | ~2,400                     | 6.5                     | 80x106                      | Yes                    | Emerson, 1925; Alden, 1928                             |
| Tsao-Ling rock slide                                                       | 1941- 42   | Chin-Shui- Chi River         | Taiwan                                       | 250x106 (two slides)     | 217                              | 1,300                   | 2,000                      | --                      | 157x106                     | Yes                    | Chang, 1984                                               |
| Cerro Condor- Sencca rock slide                                            | 1945          | Mantaro River                | Peru                                         | 5.6x106                     | 100                              | 250                     | 580                        | 21                      | 300x106                     | Yes                    | Snow, 1964                                                |
| Madison Canyon rock slide                                                  | 1959          | Madison River                | Montana, U.S.A.                           | 21x106                      | 60-70                            | 500                     | 1,600                      | 10                      | --                          | No                     | Hadley, 1964; Knight and Bennett, 1960              |
| Thistle earth slide                                                        | 1983          | Spanish Fork River           | Utah, U.S.A.                              | 22x106                      | ~60                              | 200                     | 600                        | 5                       | 78x106                      | No                     | Kaliser and Fleming, 1986                              |
| Landslide class and name                                                   | Year          | Dammed river                 | State/ country                            | Landslide volume (m3) | Blockage dimensions              |                         |                            | Lake dimensions         |                             | Dam failed?         | References                                                |
|                                                                               |               |                                 |                                              |                             | Height (m)                    | Length (m)           | Width (m)               | Length (km)          | Volume (m3)              |                        |                                                           |
| Mud, debris, and earth flows |               |                                 |                                              |                             |                                  |                         |                            |                         |                             |                        |                                                           |
| Slumgullion earth flow                                                     | 1200- 1300 | Lake Fork, Gunnison River | Colorado, U.S.A.                          | 50-100x106                  | 40 (estimate)                 | 500                     | 1,700                      | 3                       | --                          | No                     | Crandell and Varnes, 1961                              |
| Gupis debris flow                                                             | 1980          | Ghizar River                 | Pakistan                                     | --                          | 30                               | 200                     | 300                        | 5                       | --                          | No                     | Nash and others, 1985                                  |
| Polallie Creek debris flow                                                 | 1980          | East Fork, Hood River        | Oregon, U.S.A.                            | 70-100x103                  | 11                               | --                      | 230                        | --                      | 105x103                     | Yes                    | Gallino and Pierson, 198?                              |
| Sensitive-clay failures:     |               |                                 |                                              |                             |                                  |                         |                            |                         |                             |                        |                                                           |
|                                                                               | 1898          | Riviere Blance               | Quebec, Canada                            | 2.6x106                     | 8                                | 400                     | 3,200                      | --                      | --                          | Yes                    | Dawson, 1898                                              |
|                                                                               | 1945          | Yamaska River                | Quebec, Canada                            | 117x103                     | 3-4                              | 75                      | 425                        | --                      | --                          | Yes                    | Clark, 1947                                               |

Landslide class and name
Year
Dammed river
State/ country
Landslide volume (m3)
Blockage dimensions
Lake dimensions
Dam failed?
References


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Types of Movements that Form Landslide Dams
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Table 2.--Well-documented examples of landslide dams formed by specific classes of landslides -- Continued

Two sensitive-clay failures that caused landslide dams to form both occurred in shallow broad valleys in sensitive marine clays (table 2). The dams were low broad blockages that failed by overtopping within a few hours or days.

There are only two known reports of landslide dams formed by rock or earth falls. One was the 1943 "cliff fall" of the bank of the Grande Riviere du Chene in marine clay in Quebec (Clark, 1947); the other was a rock fall that impounded Lake Yashinkul in the central U.S.S.R in 1966 (Pushkarenko, 1982).


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Causes of Dam-forming Landslides
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The two most important processes in initiating dam-forming landslides are excessive precipitation (rainfall and snowmelt) and earthquakes (fig. 2). These account for 90 percent of the landslide dams investigated. Volcanic eruptions constitute the third most significant dam-forming process (about 8 percent). Other mechanisms, such as devegetation and stream undercutting and entrenchment, account for the remaining 2 percent.

LANDSLIDE DAMS
Causes of Dam-forming Landslides


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Causes of Dam-forming Landslides
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Figure

Image /page/12/Figure/0 description: A black and white vertical bar chart showing the number of events for different causes. The y-axis is labeled "NUMBER OF EVENTS" and ranges from 0 to 70, with tick marks every 10 units. The x-axis categories are labeled on the bars. The first bar, labeled "Rainstorms and snowmelt," shows approximately 65 events. The second bar, "Earthquakes," shows 50 events. The third bar, "Volcanic eruptions," shows 10 events. The fourth and final bar, "Others," shows approximately 3 events.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Causes of Dam-forming Landslides
Type: figure
Figure: Figure 2.--Graph showing causes of landslides that have formed dams, based on 128 cases from the literature and the authors' experience.

Figure 2.--Graph showing causes of landslides that have formed dams, based on 128 cases from the literature and the authors' experience.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Causes of Dam-forming Landslides
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Numerous landslide dams can be formed by a single rainstorm or earthquake. In 1889 in the Totsu River basin, Japan, heavy rainfall produced over 1,100 landslides in the upper 1,100 square kilometers of the watershed. About 250 of these landslides were more than 90 meters wide and 90 meters long and produced 53 landslide dams (Swanson and others, 1986). The 1929 Buller earthquake (magnitude 7.6) in the northwestern part of South Island, New Zealand, produced landslides that formed at least 11 landslide-dammed lakes (Adams, 1981). In 1783, an earthquake in Calabria, Italy, triggered mass movements that formed 215 landslide-dammed lakes (Cotecchia, 1978).

An unusual series of events forming and destroying a landslide dam has been documented for the Tsao-Ling landslide in central Taiwan (Chang, 1984). This case demonstrates just how complex natural-dam processes can be. In 1862, a major earthquake triggered a landslide that dammed the Chin-Shui-Chi River. In 1898 the natural dam failed for unknown reasons. In December 1941 a major earth quake formed another landslide dam 140 meters high at the same location. In August 1942 heavy rainfall caused reactivation of the landslide, and the natural dam increased in height from 140 to 217 meters. In May 1951 several days of intense rainfall led to the overtopping and failure of the natural dam. In the subsequent flood 154 people were killed and 564 homes damaged. On August 15, 1979, heavy rainfall again activated the landslide, which dammed the river with a natural barrier 90 meters high. Heavy precipitation continued, and 9 days later the landslide dam was overtopped and failed, causing severe downstream flooding.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Classification of Landslide Dams
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Landslide dams can be classified geomorphically with respect to their relations with the valley floor (Swanson and others, 1986; fig. 3). Type I dams are small with respect to the width of the valley floor and do not reach from one valley side to the other. Type II dams are larger and span the entire valley floor, occasionally depositing material high up on opposite valley sides. Type III dams fill the valley from side to side and move considerable distances upvalley and downvalley from the failure. These dams typically involve the largest volume of landslide material. Type IV landslide dams form by the contemporaneous failure of material from both sides of a valley. landslides can adjoin head-to-head in the middle of the valley, or they can juxtapose one another. Type V landslide dams form when the same landslide has multiple lobes of debris that extend across a valley floor and form two or more landslide dams in the same reach of river. landslide dams involve one or more failure surfaces that extend under the stream or river valley and emerge on the opposite valley side from These dams typically involve slow basal sliding and the landslide. slumping and form lakes by raising the elevation of the streambed, changing the local gradient of the stream.

Of the 184 landslide dams classified from around the world, the most common are Types II (44 percent) and III (41 percent). The other types are much more infrequent (Type I = 11 percent; Type IV = 1percent; and Type VI = 3 percent). Type I landslide-dam lakes commonly are small, low, and usually not hazardous. Type II landslide-dam lakes are larger and much more dangerous. Type III dams not only can form large hazardous lakes behind the obstruction, but also can block tributaries to the main valley, creating additional potentially dangerous lakes. This is the case at Mount St. Helens, Washington, as a result of the 1980 debris avalanche that dammed tributaries to the North Fork Toutle River. Type IV and V landslide-dam lakes can be hazardous if the valleys are narrow and the volumes of landslide materials are large enough to form high dams. Only one example of a type IV landslide dam is known, the Yinping landslide blockage of the Min River, Sichuan Province, China, which forms Da Lake (Li and others, 1986; fig. 4). Only one example of a Type V landslide dam is known, the Slide Lake rockfall-avalanche in Glacier National Park, Montana (Butler and others, 1986). Only a few examples of Type VI are known, from Japan (Swanson and others, 1986) and one from Colorado (Muddy Creek landslide). This type of landslide dam typically poses less threat of downstream flooding than the other types because there may never be a complete blockage. The stream may continue to flow over the dam debris, so that the likelihood of abrupt overtopping and rapid incision of the dam is unlikely. Water storage usually is small and the stream gradient not relatively steep.

LANDSLIDE DAMS
Classification of Landslide Dams


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Section: LANDSLIDE DAMS > Classification of Landslide Dams
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Figure

Image /page/14/Figure/0 description: A black and white diagram showing a classification of landslides into six types, labeled TYPE I through TYPE VI. Each type is illustrated with a cross-sectional view of geological layers.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Classification of Landslide Dams
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- TYPE I shows a narrow, vertical column of material that has fallen or slumped through a central wavy layer. The associated text is "Falls" and "Slumps".
- TYPE II depicts a wider vertical column that has collapsed. The text reads "Avalanches" and "Slumps/Slides".
- TYPE III illustrates a horizontal movement of material, like a flow or avalanche, from the right side. The text is "Flows" and "Avalanches".
- TYPE IV shows a vertical column that has dropped and been sheared along the central wavy layer. The text lists "Falls", "Slumps/Slides", and "Avalanches".
- TYPE V shows a mass of material that has flowed onto the surface from above. The text is "Falls" and "Avalanches".
- TYPE VI shows a large void or cavern above the central layer, with a "Failure surface" indicated below it, suggesting a collapse. The text is "Slumps/Slides".


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Classification of Landslide Dams
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Figure: Figure 3.--Diagrams showing classification of landslide dams. Mass-movement processes most likely to form particular landslide dams are listed in lower left corner of each category. (Modified from Swanson and others, 1986)

Figure 3.--Diagrams showing classification of landslide dams. Mass-movement processes most likely to form particular landslide dams are listed in lower left corner of each category. (Modified from Swanson and others, 1986)


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Classification of Landslide Dams
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Figure

Image /page/15/Picture/0 description: A black and white photograph captures a dramatic landscape of a river flowing through a deep, steep-sided gorge. The mountains are rugged and imposing. In the foreground, a lighter-colored, turbulent river flows into a larger, darker, and calmer body of water that winds its way through the canyon into the hazy distance. A road is visible carved into the mountainside on the right. A caption at the bottom of the image reads, "Figure 4. Photograph showing a section along the Min River, Szechwan Province, Republic of China."


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Classification of Landslide Dams
Type: figure
Figure: Figure 4.--Photograph view upstream along the Min River, Sichuan Province, Peoples Republic of China, showing remains of the 1933 earthquake-induced Yinping landslide dam. Xico Lake (foreground) was impounded by another landslide dam caused by the same earthquake.

Figure 4.--Photograph view upstream along the Min River, Sichuan Province, Peoples Republic of China, showing remains of the 1933 earthquake-induced Yinping landslide dam. Xico Lake (foreground) was impounded by another landslide dam caused by the same earthquake.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Modes of Failure of Landslide Dams
Type: text

A landslide dam in its natural state differs from a constructed embankment dam in that it is made up of a heterogeneous mass of poorly consolidated earth material and has no channelized spillway or other protected outlet. Because of the lack of a protected spillway, landslide dams commonly fail by overtopping, followed by breaching from erosion by the overflowing stream (fig. 5). In most documented cases, the breach has resulted from the fluvial erosion of the landslide material by headcutting originating at the toe of the dam and progressively moving upstream to the lake. When the headcut reaches the lake, breaching occurs (Lee and Duncan, 1975). The breach commonly does not erode down to the original river level. In this situation, smaller lakes can remain after dam failure. In some situations the landslide dam will not fail during first filling and overtopping. Subsequent failure can occur by surface erosion accompanying unusually large runoff periods. This was the case in the failure of the landslide dams at Gros Ventre, Wyoming, and Lake Elizabeth, Australia.

Because landslide dams have not undergone systematic compaction, they may be porous, and seepage through the dams potentially could lead to failure by internal erosion (piping). Seeps have been noted on the downstream faces of many landslide dams. Examples are the 1925 Lower Gros Ventre landslide in northwestern Wyoming, which failed in 1927 from overtopping (Alden, 1928), and the 1945 Cerro Condor-Sencca landslide dam in Peru, which failed in 1945, probably because of "violent" seepage and piping (Snow, 1964).

However, only two cases where a landslide dam actually has failed in this manner are known: the 1945 Cerro Condor-Sencca landslide dam and the 1966 breach of the landslide dam that impounded Lake Yaskinkul on the Isfayramsay River in the south-central U.S.S.R (Glazyrin and Reyzvikh, 1968). In these failures, piping and undermining caused collapse of the dam, followed by overtopping and breaching.

LANDSLIDE DAMS
Modes of Failure of Landslide Dams


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Modes of Failure of Landslide Dams
Type: figure
Figure

Image /page/16/Figure/1 description: A vertical bar chart showing the number of failures for different causes. The y-axis is labeled 'NUMBER OF FAILURES' and ranges from 0 to 60. The x-axis displays four categories. The bar for 'Overtopping' shows approximately 51 failures. The bar for 'Piping' shows approximately 2 failures. The bar for 'Slope failure' also shows approximately 2 failures. The last bar, labeled 'Physically controlled (did not fail)', shows a count of 15.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Modes of Failure of Landslide Dams
Type: figure
Figure: Figure 5.--Graph showing modes of failure of landslide dams, based on 55 failures from the literature and the authors' experience.

Figure 5.--Graph showing modes of failure of landslide dams, based on 55 failures from the literature and the authors' experience.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Modes of Failure of Landslide Dams
Type: text

A landslide dam with steep upstream or downstream faces is susceptible to slope failure. If the dam has a narrow cross section or if the slope failure is progressive, the crest may fail, leading to overtopping and breaching. However, nearly all upstream and downstream faces of landslide dams are at the angle of shallower, and the dams are much wider than failures are rare. A special type of slope failure involves lateral erosion of the dam by a stream or river (for example, Jackson Lake, Mount St. Helens, Washington, U.S. Geological Survey, Vancouver, Washington, unpublished data, 1985; Hole-in-the-Wall Gulch, Oregon, J. M. Geist, U.S. Forest Service, oral commun., 1986). One well-documented example exists in which failure of the downstream slope of a landslide dam may have contributed to overall failure: the 1945 Cerro Condor-Sencca blockage of the Mantaro River, Peru (Snow, 1964).


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Longevity of Landslide Dams
Type: text

Landslide-dammed lakes may last for several minutes or several thousand years, depending on many factors, including volume, texture, and sorting of the blockage material; rates of seepage through the blockage; and rates of sediment and water flow into the newly formed lake. Rapid assessment of the upstream and downstream flood potential after formation of a landslide dam is essential. Seventy-three examples of landslide-dam failures in which the time to failure is known are compiled in figure 6 and indicate how dangerous landslide dams can be. Twenty-seven percent of the dams failed within 1 day of formation; 41 percent failed within 1 week; about 50 percent failed within 10 days; 80 percent failed within 6 months, and 85 percent failed within 1 year of formation. It is important to note, however, that these percentages pertain only to landslide dams that have failed. Numerous other landslide dams have formed and not failed.

The three factors that seem to be most relevant to the longevity of a landslide dam are (1) volume and rate of inflow to the impoundment, (2) size and shape of the dam, and (3) geotechnical characteristics of the dam. In many cases the amount of flow in a stream and thus the rate at which a natural lake will fill is directly proportional to the size of the upstream drainage area. The duration of a landslide dam is apt to be short whenever a small landslide blocks a stream with a large drainage area (Swanson and others, 1985). Unless seepage through the dam equals the inflow, the dam can fill to overtopping. Rapid inflow behind a low, small landslide dam means that the new natural lake probably will fill quickly to overflowing, which may lead to failure of the landslide dam.

LANDSLIDE DAMS
Longevity of Landslide Dams


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Longevity of Landslide Dams
Type: figure
Figure

Image /page/17/Figure/3 description: A line graph showing the cumulative percentage of landslide dams that have failed over time. The y-axis is labeled "PERCENTAGE OF LANDSLIDE DAMS WHICH HAVE FAILED, BELOW INDICATED AGE" and ranges from 0 to 100. The x-axis is labeled "AGE OF DAM AT TIME OF FAILURE (days)" and ranges from 0 to 400. The graph shows a single curve that starts near the origin, rises steeply, and then flattens out. Several data points are annotated along the curve: 27 percent lasted ≤ 1 day, 41 percent lasted ≤ 1 week, Half failed within 10 days (at 50 percent), 56 percent lasted ≤ 1 month, 80 percent lasted ≤ 6 months, and 85 percent lasted ≤ 1 year.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Longevity of Landslide Dams
Type: figure
Figure: Figure 6.--Graph showing length of time before failure of landslide dams, based on 73 cases from the literature and the authors' experience.

Figure 6.--Graph showing length of time before failure of landslide dams, based on 73 cases from the literature and the authors' experience.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Longevity of Landslide Dams
Type: text

Landslide dams of a predominantly soft, low-density, fine-grained, or easily liquified sediment are hazardous. Landslide dams composed of these materials can be expected to have relatively low bulk densities and resistances to erosion. Failure following overtopping is common. Peak discharge from the overtopping failure of a landslide dam is dependent on cohesive strength and friction angle of the landslide material (Fread, 1985). If the dam materials are mostly saturated, shear strength may be low and the dam may not be able to withstand the increasing hydrostatic pressure resulting from the impoundment, or the dam may erode rapidly when overtopped.

Some landslide dams are more resistant to all types of failure mechanisms than others. The most important characteristic in preventing failure is resistance to erosion, either at the surface of the dam from surface-water runoff or inside the dam from piping and seepage. As might be expected, landslide dams consisting of large or cohesive particles resist failure better than dams containing large percentages of soil or soft rock. Landslide dams typically are much wider than constructed earth-fill and rock-fill dams and involve larger volumes of material. The shear bulk of many such blockages provides some degree of protection against failure by any mechanism.

Sorting of landslide sediment also is an important factor. Natural materials with D15/D85 ratio greater than 5 are susceptible to internal erosion by piping (Sherard, 1979). If the material forming the landslide dam is permeable, easily eroded sediment, such as sand with a small proportion of silt and clay, rising water levels behind the dam may force water through the permeable beds and lead to piping and erosion that could cause the dam to fail.

Slope stability of the upstream and downstream faces of a dam is inversely related to the steepness of these faces. Obviously, landslide dams with relatively gentle surface slopes are less susceptible to slope failure than those with steep slopes.

In some cases lakes have not overtopped their dams because lake inflow is less than losses due to seepage, evaporation, or withdrawals for irrigation. An example is Bitang Lake in Gansu Province, China, which has stabilized at a level considerably below the crest of the landslide dam that impounded it in 1961 (Li and others, 1986).

In a few cases landslide-dammed lakes have formed natural spillways across adjacent bedrock abutments which prevent overtopping and possible breaching of the dams. This occurs when the surface of the toe of the landslide dam is higher than the surface of the adjacent bedrock abutment. A well-known example of a natural bedrock spillway for a landslide-dammed lake is the outlet across the left abutment of the Slumgullion earthflow in southwestern Colorado (Schuster, 1985). This natural spillway has existed for 700 years.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Physical Measures to Improve the Stability of Landslide Dams
Type: text

Construction of control measures (most commonly spillways) has been attempted in recent years on many major landslide dams as soon as possible after formation. However in some cases overtopping has occurred before satisfactory control measures could be constructed. This happening is particularly common in outlying areas of mountainous topography where transporting heavy construction equipment to the site is difficult.

The most simple and most common method for stabilizing landslide dams has been construction of channelized spillways, either across adjacent bedrock abutments or directly over the landslide dams. A well-known example of a successful spillway across a landslide dam is the spillway constructed in 1959 by the U.S. Army Corps of Engineers on the Madison Canyon landslide, Montana (Harrison, 1974; fig. 7).


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Physical Measures to Improve the Stability of Landslide Dams
Type: figure
Figure

Image /page/19/Picture/1 description: A black and white, high-angle photograph shows a massive landslide in a mountain valley. The landslide originates from the forested mountain on the left and has spread across the valley, damming a river to form a lake in the background. The mountain on the right is in deep shadow. The landslide area is a wide expanse of bare earth and debris.


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Physical Measures to Improve the Stability of Landslide Dams
Type: figure
Figure: Figure 7.--Photograph of the landslide dam formed by the 1959 Madison Canyon landslide, Montana, and Earthquake Lake, which it impounds. (Photograph by J. R. Stacy, U.S. Geological Survey)

Figure 7.--Photograph of the landslide dam formed by the 1959 Madison Canyon landslide, Montana, and Earthquake Lake, which it impounds. (Photograph by J. R. Stacy, U.S. Geological Survey)


Document: The formation and failure of natural dams
Section: LANDSLIDE DAMS > Physical Measures to Improve the Stability of Landslide Dams
Type: text

Spillways excavated across landslide dams are not always successful in preventing dam failure and subsequent flooding because they sometimes are eroded rapidly by outflow waters. Such was the case in the 1976 landslide damming of the Rio Quemaya in Guatemala, where highway workers trenched the landslide dam to drain the lake. It drained too rapidly and caused a flood that swept away several people (Harp and others, 1981).

In a few cases, large-scale blasting has been used to excavate new river channels through landslide dams. This technique was used in 1981 to open a channel through the Zhouqu landslide dam on the Bailong River in Gansu Province, China. Other methods of stabilizing lake levels behind landslide dams and preventing overtopping include pipe and tunnel outlets and diversions. Both a pipe and a tunnel have been used to control discharge from Spirit Lake, Mount St. Helens, Washington (Sager and Chambers, 1986), and at Thistle, Utah (Kaliser and Fleming, 1986).


Document: The formation and failure of natural dams
Section: GLACIER DAMS
Type: text

Glacier dams are dams that impound water in, on, beneath, or behind masses of glacial ice. These dams can occur in any area covered by continental or valley glaciers. Other related natural dams not discussed include icefalls (Ballantyne and McCann, 1980), snow and ice avalanches (King, 1934), and snowbanks (Church, 1972).

Jokulhlaup ("glacier burst") is the Icelandic term for a flood caused by the sudden and usually catastrophic release of water impounded within or behind glacier ice (Thorarinsson, 1953). Some of the largest floods known to have occurred in the history of the Earth were the result of the failure of ice dams during the waning stages of the Pleistocene. The failure of ice dams has caused great loss of life and property damage in many places (table 3), including Iceland (Thorarinsson, 1939, 1953, 1957); northern India (Hewitt, 1982); Pakistan (Nash and others, 1985); Peru (Lliboutry and others, 1977); Norway (Aitkenhead, 1960); Alaska (Post and Mayo, 1971); Washington and Oregon (Richardson, 1968); Switzerland, Austria, France, and Italy (Eisbacher and Clague, 1984); and Canada (Clarke, 1982; Young, 1980).


Document: The formation and failure of natural dams
Section: GLACIER DAMS > Geomorphic Settings of Ice Dams
Type: text

Ice dams can form in any area covered by or adjacent to continental or alpine glaciers (fig. 8). Glacial ice may obstruct drainage and form lakes in numerous ways (table 4; fig. 9).

One of the most hazardous kinds of ice-dammed lakes is category G in table 4, lakes formed in main valleys dammed by tributary-valley glaciers. These lakes can be large, and the ice dam can be relatively small. These dams usually form as a result of glacial surges, in which the velocity of a tributary glacier temporarily can increase 10- to 100-fold (Meier and Post, 1969). In the upper Indus River valley of Pakistan at least 18 tributary glaciers have dammed major valleys (Hewitt, 1982). The Chong Khumdan glacier on the Shyok River, a large tributary of the Indus River, is a classic example of a glacier projecting into and blocking a major valley. Numerous disastrous floods have occurred from the temporary blocking and failure of the ice dams in the valley of the Shyok River; the most notable was the flood of 1929 (Mason, 1929; Gunn, 1930).

GLACIER DAMS
Geomorphic Settings of Ice Dams


Document: The formation and failure of natural dams
Section: GLACIER DAMS > Modes of Failure of Ice Dams
Type: text

Ice-dam failures are complicated phenomena, involving many sets of independent factors. Glacier dams often fail periodically, with return periods of 1 to 10 years. About 95 percent of the more than 50 jokulhlaups in the Alps occurred from June through September, with maxima in June and August (Tufuell, 1984). The failure of glacier dams can occur by erosion of a drainage channel under, through, or over the ice dam. Many ice dams fail by rapid drainage through englacial or subglacial tunnels (Gilbert, 1971). Such tunnels can be tens of kilometers long (Nye, 1976), and the outlet location can change by hundreds of kilometers from one failure to another (Sturm and Benson, 1985). The ice comprising these dams behaves plastically, deforming and flowing to reestablish a blockage of drainage by closing englacial or subglacial tunnels or by squeezing breaches shut. The lake then can reform by collecting runoff from the drainage area upstream, from surface runoff from the glacier, and from subglacial flow into the reservoir.

GLACIER DAMS
Modes of Failure of Ice Dams


Document: The formation and failure of natural dams
Section: GLACIER DAMS > Modes of Failure of Ice Dams
Type: table
Table: Table 3.--Well-documented examples of glacial-ice dams that have failed, producing jokulhlaups

| Lake name                     | Location                       | Year failed                 | Dam height (m) | Lake volume (m3) | Flood peak (m3/s) | Reference                               |
|-------------------------------|--------------------------------|--------------------------------|----------------------|------------------------|-------------------------|-----------------------------------------|
| Missoula                      | Montana, U.S.A.             | 16,000- 12,000 yrs. B.P. | 1,078                | 2,184,000 x 106        | 21.3 x 106              | Baker, 1973; Clarke and others, 1984 |
| Vatnsdalur                    | Iceland                        | 1898                           | 372                  | 120 x 106              | 3,000                   | Thorarinsson, 1939                      |
| Chong Kumdan (Shyok River) | India                          | 1929                           | 120                  | 1,350 x 106            | 22,650                  | Gunn, 1930; Hewitt, 1982             |
| Demmevatn                     | Norway                         | 1937                           | 406                  | 11.6 x 106             | 1,000                   | Clague and Mathews, 1973             |
| Graenalon                     | Iceland                        | 1939                           | 535                  | 1,500 x 106            | 5,000                   | Thorarinsson, 1939                      |
| Gorner                        | Switzerland                    | 1944                           | ?                    | >6 x 106               | 200                     | Haeberli, 1983                          |
| Gjanupsvatn                   | Iceland                        | 1951                           | 167                  | 20 x 106               | 370                     | Arnborg, 1955                           |
| Lake George                   | Alaska, U.S.A.              | 1958                           | 40                   | 1,730 x 106            | 10,100                  | Stone, 1963                             |
| Tulsequah                     | British Columbia, Canada | 1958                           | 210                  | 229 x 106              | 1,556                   | Marcus, 1960                            |
| Summit                        | British Columbia, Canada | 1965                           | 620                  | 251 x 106              | 3,260                   | Mathews, 1965                           |
| Ekalugad Valley            | Baffin Island, Canada    | 1967                           | 120                  | 4.8 x 106              | 200                     | Church, 1972                            |
| Strupvatnet                   | Norway                         | 1969                           | 186                  | 2.6 x 106              | 150                     | Whalley, 1971                           |
| Hazard Lake                   | Yukon, Canada               | 1978                           | 300                  | 19.6 x 106             | 640                     | Clarke, 1982                            |

Lake name
Location
Year failed
Dam height (m)
Lake volume (m3)
Flood peak (m3/s)
Reference


Document: The formation and failure of natural dams
Section: GLACIER DAMS > Modes of Failure of Ice Dams
Type: table
Table: Table 4.--Classification of ice-dammed lakes (from Blachut and Ballantyne, 1976)

| Type of Lake                                                                                | Example                                                                                |
|----------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------|
| A. Supraglacial                                                                                | Generally small and not hazardous                                                                                |
| B. Marginal-ponded                                                                                | Unnamed lake, Greenland (Sugden and others, 1985)                                                                  |
| C. Converging ice-stream-ponded                                                                          | Between Lake, Axel Heiberg Island, Canada (Maag, 1969)                                                             |
| D. Tributary stream valley-ponded                                                                        | Flood Lake, British Columbia, Canada (Clarke and Waldron, 1984)                                                    |
| E. Tributary glacier valley-ponded                                                                       | Lago Rico, Argentina (Nichols and Miller, 1952)                                                                    |
| F. Interglacial-ponded                                                                                | Tulsequah Lake, British Columbia, Canada (Marcus, 1960)                                                            |
| G. Dammed by tributary glacier                                                                           | Shyok River Lake, Upper Indus Valley, Pakistan (Gunn, 1930; Mason, 1929)                                        |
| H. Proglacial ice-dammed                                                                                | Generally small with valley glacier can be gigantic with continental ice sheets (e.g., Glacial Lake Agassiz) |
| I. Miscellaneous (ice-dammed craters on volcanoes) (large englacial or subglacial water bodies) | Grimsvotn, Iceland (Thorarinsson, 1953)                                                                            |


Document: The formation and failure of natural dams
Section: GLACIER DAMS > Modes of Failure of Ice Dams
Type: text

Another commonly-evoked failure mechanism is the hydrostatic flotation hypothesis of Thorarinsson (1953), whereby subglacial drainage becomes possible when hydrostatic pressure of water in an ice-dammed lake exceeds the ice-overburden pressure in an ice dam. This excess occurs when the depth of water behind an ice dam reaches 0.9 times the height of the ice dam. The story is not so simple as measuring the lake level behind an ice dam, because some ice-dammed lakes fail before filling to 0.9 times the ice-dam height, and most lakes continue to drain well after water levels decline below this critical level. Lifting of the ice dam only may be a triggering mechanism of failure, followed by other processes that allow continued drainage of the lake, such as erosion of drainage-ways under the ice by escaping lakewater, uneven settling of the ice dam after flotation, or formation of tension crevasses from flotation (Marcus, 1960).

Other mechanisms for failure of glacier dams include (1) slow plastic yielding of ice from hydrostatic-pressure differences between the lake and adjacent less-dense ice, (2) crack progression under combined shear stress from glacier flow and hydrostatic pressure, (3) water overflow erosion of a breach in the dam, (4) subglacial melting by volcanic heat, and (5) weakening of the ice dam by earthquakes (Post and Mayo, 1971).