Stephen G. Evans

Recent climatic change and catastrophic geomorphic processes in mountain environments

Climatic warming during the last 100-150 years has resulted in a significant glacier ice loss from mountainous areas of the world. Certain natural processes which pose hazards to people and development in these areas have accelerated as a result of this recent deglaciation. These include glacier avalanches, landslides and slope instability caused by glacier debuttressing, and outburst floods from moraine- and glacier-dammed lakes. In addition, changes in sediment and water supply induced by climatic warming and glacier retreat have altered channel and floodplain patterns of rivers draining high mountain ranges. The perturbation of natural processes operating in mountain environments, caused by recent climatic warming, ranges from tens of decades for moraine-dam failures to hundreds of years or more for landslides. The recognition that climatic change as modest as that of the last century can perturb natural alpine processes has important implications for hazard assessment and future development in mountains. Even so, these effects are probably at least an order of magnitude smaller than those associated with late Pleistocene deglaciation ca. 15,000 to 10,000 years ago.

A review of catastrophic drainage of moraine-dammed lakes in British Columbia

Abstract Moraine-dammed lakes are common in the high mountains of British Columbia. Most of these lakes formed when valley and cirque glaciers retreated from advanced positions achieved during the Little Ice Age. Many moraine dams in British Columbia are susceptible to failure because they are steep-sided, have relatively low width-to-height ratios, comprise loose, poorly sorted sediment, and may contain ice cores or interstitial ice. In addition, the lakes commonly are bordered by steep slopes that are prone to snow and ice avalanches and rockfalls. Moraine dams generally fail by overtopping and incision. The triggering event may be a heavy rainstorm, or an avalanche or rockfall that generates waves that overtop the dam. The dam can also be overtopped by an influx of water caused by sudden drainage of an upstream ice-dammed lake (jokulhlaup). Melting of moraine ice cores and piping are other possible failure mechanisms. Failures of moraine dams in British Columbia produce destructive floods orders of magnitude larger than normal streamflows. Most outburst floods are characterized by an exponential increase in discharge, followed by an abrupt drop to background levels when the water supply is exhausted. Peak discharges are controlled by dam characteristics, the volume of water in the reservoir, failure mechanisms, and downstream topography and sediment availability. For the same potential energy at the dam site, floods from moraine-dammed lakes have higher peak discharges than floods from glacier-dammed lakes. The floodwaters may mobilize large amounts of sediment as they travel down steep valleys, producing highly mobile debris flows. Such flows have larger discharges and greater destructive impact than the floods from which they form. Moraine dam failures in British Columbia and elsewhere are most frequent following extended periods of cool climate when large lateral and end moraines are built. A period of protracted warming is required to trap lakes behind moraines and create conditions that lead to dam failure. This sequence of events occurred only a few times during the Holocene Epoch, most notably during the last several centuries. Glaciers built large moraines during the Little Ice Age, mainly during the 1700s and 1800s, and lakes formed behind these moraines when climate warmed in the 1900s. Twentieth-century climate warming is also responsible for recent moraine dam failures in mountains throughout the world. Warming from the late 1800s until about 1940 and again from 1965 to today destabilized moraine dams with interstitial or core ice. The warming also forced glaciers to retreat, prompting ice avalanches, landslides, and jokulhlaups that have destroyed some moraine dams.

Entrainment of debris in rock avalanches: An analysis of a long run-out mechanism

Many rock avalanches entrain and liquefy saturated soil from their paths. Evidence for this includes mud displaced from the margins of rock avalanche deposits, substrate material smeared along the base of deposits, extrusion of liquefied soil upward through the deposits, and increases of volume. A hypothesis first suggested in 1881 and since reinforced by several authors suggests that entrainment of substrate material increases mobility. Although the process has been discussed in the literature for more than 100 years, few detailed and quantitative descriptions exist. The main purpose of this paper is to describe two recent cases from British Columbia, Canada, where rockslides entrained substrate on a very large scale, influencing the character of the events. Estimated volume balance curves, based on detailed field mapping, are provided for both cases. Dynamic analyses are carried out using a numerical model and using the same set of rheological parameters. The mechanism of material entrainment and displacement is discussed. The data suggest that rapid rock failures entraining very large quantities of saturated substrate material represent a special type of landslide, transitional between rock avalanche and debris avalanche. Many rock avalanches can thus be seen as end members of a continuum of phenomena involving rock failure followed by interaction with saturated substrate.

Catastrophic Landslides: Effects, Occurrence, and Mechanisms

>This volume documents further advances in our knowledge of catastrophic landslides since the pioneering compilations of the late 1970s by Barry Voight. It provides a worldwide survey of catastrophic landslide events written by leading authorities. Catastrophic Landslides begins by drawing upon South America to dramatically illustrate the impact of these phenomena on human populations. The occurrence of catastrophic landslides, including site-specific insights, is shown through six events of the past 20 years. Several other chapters focus on the mechanisms involved with catastrophic landsides both in relation to geologic factors in a particular geographic area as well as to specific geologic processes.

Extensive deformations of rock slopes in southern Coast Mountains, southwest British Columbia, Canada

Extensive deformations of mountain slopes occur in crystalline intrusive rocks in the southern Coast Mountains of British Columbia. Typical morphological evidence of slope movement includes extensive systems of tension cracks, grabens, and antislope scarps (collectively referred to here as linears). These landforms involve displacements along penetrative joints observed in surface exposures. Kinematic tests on rock-structural data indicate that the observed patterns of linears are generally consistent with the feasible gravitational movements along the dominant discontinuities. Most sites indicate sliding as the most likely initial mode of movement, followed by or accompanied by toppling and toppling-induced sliding movements, and do not support the view that linears are the traces of active faults.

LANDSLIDES FROM MASSIVE ROCK SLOPE FAILURE AND ASSOCIATED PHENOMENA

Landslides from massive rock slope failure (MRSF) are a major geological hazard in many parts of the world. Hazard assessment is made difficult by a variety of complex initial failure processes and unpredictable post-failure behaviour, which includes transformation of movement mechanism, substantial changes in volume, and changes in the characteristics of the moving mass. Initial failure mechanisms are strongly influenced by geology and topography. Massive rock slope failure includes rockslides, rock avalanches, catastrophic spreads and rockfalls. Catastrophic debris flows can also be triggered by massive rock slope failure. Volcanoes are particularly prone to massive rock slope failure and can experience very large scale sector collapse or much smaller partial collapse. Both these types of failures may be transformed into lahars which can travel over 100 km from their source. MRSF deposits give insight into fragmentation and emplacement processes. Slow mountain slope deformation presents problems in interpretation of origin and movement mechanism. The identification of thresholds for the catastrophic failure of a slow moving rock slope is a key question in hazard assessment. Advances have been made in the analysis and modeling of initial failure and post-failure behaviour. However, these studies have been retrodictive in nature and their true predictive potential for hazard assessment remains uncertain yet promising.

The Formation and Behaviour of Natural and Artificial Rockslide Dams; Implications for Engineering Performance and Hazard Management

The formation and behaviour of natural and artificial rockslide dams are reviewed to update the well-known work of Costa and Schuster [1]. Rockslide dams block surface drainage to form upstream lakes. They may occur naturally due to landslides or as a result of engineered rock slope failure. As evidenced by the 2010 Hunza event (Pakistan), the stability of rockslide dams is a major consideration in landslide risk assessment in mountain terrain, particularly with respect to the possibility of a destructive downstream flood resulting from a breach of the dam. The damming of a river by a rockslide may require immediate engineering response to mitigate the hazard. However, failure by breaching is less frequent than long-term stability. These issues are examined with reference to nine case histories of rockslide dams and rockslide-dammed lakes; Gohna (1894), Rio Barrancas (1914), Condor-Sencca (1945), Mayunmarca (1974), La Josefina (1993), Tsao-Ling (1999), Yigong (2000), Tangjiashan (2008), and the Hunza (2010). The case histories also illustrate the utility of digital terrain data (especially the SRTM-3 data set obtained in February 2000) and remote sensing imagery to obtain accurate estimates of the impoundment volumes and other geomorphic data on rockslide-dammed lakes. Methods of estimating peak breach discharge and downstream flood effects exist but are still largely empirical in nature. Measures to mitigate hazard associated with rockslide-dammed lakes include the construction of a spillway over the rockslide debris, a by-pass tunnels through the abutments of the debris dam, the implementation of dam and lake-level monitoring and failure warning systems to mitigate downstream damage. A review of some well-documented examples show that these measures have had been applied with mixed success in the past. Natural rockslide dams are commonly used for foundations for conventional constructed dams. Artificial rockslide dams are created by rock slope failure induced by large-scale explosion (blast-fill dams). The largest blast-fill dam yet constructed is the Medeo Dam, a debris flow retention structure near Alma-Ata, Kazakhstan. Rockslide dams and their geomorphic effects may create an important legacy in the landscape through massive accumulations of lake sediments, impact on river channels, and effects on river long-profiles.

Satellite Monitoring of Pakistan's Rockslide‐Dammed Lake Gojal

On 4 January 2010, a rockslide 1200 meters long, 350 meters wide, and 125 meters high dammed the Hunza River in Attabad, northern Pakistan, and formed Lake Gojal. The initial mass movement of rock killed 20 people and submerged several villages and 22 kilometers of the strategic Karakoram Highway linking Pakistan and China. Tens of thousands of people were displaced or cut off from overland connection with the rest of the country. On 29 May, the lake overflow began to pour through a spillway excavated by Pakistani authorities. On approximately 20 July, the lake attained a maximum depth of 119 meters and a torrent at least 9 meters deep issued over the spillway, according to Pakistans National Disaster Management Authority (NDMA). To date, the natural dam is holding and eroding slowly. However, the threat of a catastrophic outburst flood remains.

Monitoring of Bashkara Glacier lakes (Central Caucasus, Russia) and modelling of their potential outburst

Glacier lakes pose threat to downstream settlements and infrastructure. In recent decades the number and area of lakes have been growing at an accelerating rate due to worldwide glacier shrinkage. In the Russian Caucasus this process is understudied. We present results obtained during a 12-year (1999–2010) continuous field monitoring of the Bashkara proglacial lakes group, which we identified as the place with the highest GLOF risk in the region. Recession of the parent Bashkara Glacier was the main driver of the rapid expansion of the lower Lake Lapa. The upper Lake Bashkara has not been enlarging, but its water level has shown significant inter- and intra-annual fluctuations. The lake outburst probability has increased in recent years, and in 2008 we observed surface overflow over the moraine dam. Taking into account that in the late 1950s lake outbursts at this site led to large-scale glacial debris flows, we have simulated a potential outburst using River and FLO-2D software and carried out hazard zonation. An early warning system has been designed and established at Lake Bashkara, and measures to mitigate risk have been proposed. Rapid change of proglacial lakes requires regular monitoring in ‘hot spot’ areas where the GLOF hazard is high and is dynamically changing.

Extreme weather and landslide initiation in coastal British Columbia

Abstract More frequent more intense storms predicted by climate models for the Pacific Northwest of North America could increase the regional landslide hazard. The impacts of one such storm are examined on Vancouver Island, British Columbia, during which 626 mapped landslides occurred, encompassing >5 km2 total area and generating >1.5 × 106 m3 of sediment. The relationship between rainfall intensity, air temperature and wind speed obtained from mesoscale numerical weather modelling is examined relative to landslide incidence within steep terrain. A critical onset of rainfall intensity between 80 and 100 mm in 24 h that results in a rapid increase in landslides with increasing precipitation is demonstrated. The argument is presented that this result is more useful for landslide management decisions than a minimum threshold. The component of wind-driven rain was calculated, and results indicated that wind caused increased concentrations of rainfall associated with the occurrence of landslides. Approximately half the landslides studied were not related to rainfall alone, but to rain on snow, and we argue that wind played a crucial role. This often neglected component of hydrological analysis remains a major challenge as the role of snow transition zones and a warming climate in coastal mountain watersheds is considered.