0
Anonymous Posted 21 years ago
Essay & Composition Writing
Language review
Hello everybody, I need a native English speaker to review a paper I wrote. It is 4 pages long, and all comments to improve the language will be welcome, as some parts are unclear.
Many thanks,
Bjossi
here is the text:
1. INTRODUCTION
Verne and Torryuis defined slush flows as “a mudflow-like flowage of water-saturated snow”, according to their observations in northeast Greenland. Other definitions mentioned a dense snow avalanche with at least 25 % of water content, or a rapid mass-movement of water-saturated snow. The water content within the snow-pack is therefore primordial to form the slush and release the flow, that is composed of water saturated snow, rock debris and mud. Since more than 300 years, this specific process has been recognized in a large range of arctic and subarctic areas, as a supraglacial dynamic, a hillside dynamic, and a fluvial/torrential dynamic, caused by snowmelt and/or rainfall. It has also been described in lower latitudes mountainous environments, as it is expected in all areas that encounter seasonal snow-cover; their occurrence is nevertheless more seldom in such environments.
From the literature, slush-flow characteristics could be inventoried. Favourable terrain for slush-flow occurrence is drainage basin and depression or gullies in mountain slopes. It is expected at locations where pond water can easily accumulate and where water input is higher than the output. Accordingly, the starting zone is often a stream channel, or a shallow depression, but it could be an open slope as well. Therefore, the slush-flow track can be a channel, or an open slope, or alternatively defined and undefined. A slush flow could be released even on very gentle slopes, as the slope angle is not the main triggering factor.
From a geomorphic point of view, slush flows are a significant agent, responsible of erosion, entrainment, transportation and deposition of large amount of material.
Due to its content in water, slush flows are expected to occur in the springtime, during the break-up period and accelerated snowmelt correlated to solar radiation. It could also happen during the winter months, when the snow covered area experiences warm-front intrusion and rainfall.
This paper focus on meteorological conditions that initiate hillslope slush flows, and emphasizes its geomorphologic impact. Furthermore, it discusses its triggering and geomorphic characteristics, compared with those reported from other areas. The selected study area is one of the three communities of northwestern Norway where slush flows have been reported during the last century.
Following the recommendations suggested by Sonz, we use the term of “slush flow” as we are here studying a small-scale and confined drainage basin, instead of “slush avalanche”, that should be only applied to major event, travelling several hundreds of meters (up to 2 km).
2. STUDY AREA
The Dale area is located on the western shore of the Bíldudalsvogur bay, in the southern part of the Dalefjord. Its topography consists in the extensive summit plateau of the Dalefjal mountain, at 460 m a.s.l., and of a 300 m high headwall, that faces SE. The headwall is deeply carved with two 400 m wide gullies, the A-gil gully to the south and the B-gil gully to the north. Sediments transferred from the gullies have formed large colluvial fans below them, which can be followed down to the present shoreline. The A-gil gully is especially studied here, as the B-gil gully encountered several man-made changes that considerably modified its surface. Between the two large basins several smaller gullies occur, named the M-gil gullies, with slope gradients vary from 20° to 65°. The Dale village (c. 300 inhabitants) is located on the narrow rim of land, along the shoreline (Fig. 2).
A subpolar oceanic climate, very variable with frequent temperature and precipitation changes, characterize the area. Mean annual air temperature is 1°C, and annual precipitation amount is 90 mm.
The Dale area is also prone to debris-flow and snow-avalanche activity.
3. METHODS
Meteorological data, i.e. air temperature, precipitation, wind speed and wind direction were analysed from the closest meteorological stations, during the days previous to the events, in order to highlight the initiation factors of the slushes. To assess the slush-flow geomorphologic impact, the most recent landforms were studied. A long profile of the A-gil cone (in the main axis of the cone) and cross profiles of the slush-flow deposits were drawn up using a tape and a Suunto inclinometer (precision of 0.5°), to highlight the microtopography of the fan and the slush-flow landforms features distribution. Measurements of clast size (length of the a-axis) were carried out along the talus cone at each station, which occurs at 10 m interval along the cross profiles. In addition, vegetation cover and lichenometric measurements were made both on the debris particles and between them, in order to identify the effects of the most recent slush-flow events on debris transfer and fan-surface erosion. For geomorphological mapping, a set of photographs (both ground photographs and aerial photographs) complemented the field investigations. Land survey data and historical sources were used as well.
4. THE HISTORY OF SLUSH-FLOW ACTIVITY IN THE DALE AREA
Since the beginning of the twentieth century, at least 10 slush flows have been reported from the Dale valley, from both the A-gil and the B-gil catchments: in 1925, 1975 (several slushes), 1981, 1991 (3 slushes), and 1994 (2 slushes). It is though uncertain if more slushes have occurred during heavy snowmelt seasons, since local population give slush events different appellations. Here we join the documentation issue raised by Hestnes.
According to the documented slush-flow history, only few months recorded slush-flow activity: 70 % of the events occurred in January, 50 % in February, 15 % in March and 15 % in May. Slush-flow activity has often been a serious threat to the village, causing heavy damages but no human lives have been lost there.
· The slush flow on the 6th of February in 1925 from the B-gil gully passed the local school building; the school principal was caught in the flow and carried out to the sea, where he was rescued.
· On the 22nd of January in 1975, several slush flows came down the B-gil gully and caused severe damages on properties. That same day 4 people where killed by two slush flows in the town of Patreksfjörður close to the Dale area.
· On the 10th of May in 1981 slush flow occurred in the same path as the 1925 event.
· At around 20:00 on the 28th of January, in 1991, a slush flow came down the B-gil gully. The flow was diverted to the south by the upslope deflector and the main part of the flow stopped about 75 m above the school building, but the water body reached the fjord. At 21:45, another slush flow is released from the A-gil gully. It flowed down the debris cone and the main part of the slush stops on the main road, but the water body reached the sea. At around 22:00, a second flow was released from A-gil gully. This flow was not as big as the former one and stopped on the talus cone.
· On the 14th of March in 1994, around 01:45, a slush flow fell down the A-gil gully. This flow had a shorter runout distance than the one in 1991 and stopped below the main road. Another smaller flow occurred around 03.21, showing similar pattern as the year before.
5. METEOROLOGICAL CONDITIONS RELATED TO SLUSH-FLOW RELEASE
The meteorological data from the surrounding climatic stations, on the highland plateau, the village and the valley exhibit the origin of the rapid input of water to the snow pack, which is definitely the main triggering factor contributing to slush-flow event.
· The slush flow in 1925 was clearly triggered by rapid snowmelt, as no precipitation was recorded during 10 days prior to the release. Despite the lack of air temperature data at this time, it is assumed that increasing air temperature and accumulation of solar radiation are involved in the formation of the slush. Secondly, the slush flows that occurred in 1975, 1981, 1991 and 1994 were triggered by snowmelt and rainfall during cyclonic activity.
· 1975: Previous to the 1975 event few freeze-thaw cycles occurred ending in a two days heavy precipitation prior to the release, reducing considerably the snow cohesion.
· 1981: In late April and early May 1981, the slush flow was released after 11 days of cool weather that did not encountered overnight freezing. But heavy precipitation were recorded on May 1st (65 mm), small amounts of precipitation fall the days after, and on May the 10th 11.5 mm of rain were recorded, triggering the slush flow.
· 1991: Prior to the January 1991 event, 16 days with overnight freeze occurred. During those two weeks, sleet, rain or snow were recorded every day. The slush flow occurred after a 3-4 days negative temperature period and a 40.5 mm rainfall: The cooling on January 28th was quick, as the temperature gain more than 20°C overnight. From the 18th of January the northerly wind shifted to significantly stronger south wind (up to 45 m/s), accelerating snow transformation and weakening the snow pack cohesion.
· 1994: Rain combined with a sudden temperature rise is involved too in the 1994 slush event, as heavy precipitation were recorded on March 13th and 14th, with 24 and 26 mm respectively, occurring after an intense frost period that lasted for more than 15 days. The southerly wind could also have been a significant triggering factor.
According to these data, the duration of thawing period spanned from few hours (1975 and 1991) up to more than 17days (1981), and the cumulative records in the rainfall gauges from the closest weather stations range from 45 to 232mm during this time. The maximal air temperature reached 14°C after relative intense frost periods. The wind plays an important roll on the snowmelt. Especially, southerly winds directly accelerated heat transfer between the snow and the atmosphere, coursing snowmelt. Strong southerly wind is clearly related to the slush-flow release in 1991. Therefore, the production of running water in the snow pack through thaw and/or rainfall is effective and significant, and it is obvious that intense frost previous to milder meteorological conditions accelerate snow metamorphism and weaken its cohesion; moreover the wind is proven to be relevant assistant.
All mentioned meteorological factors are very conducive to rapid snowmelt, thus to slush formation. However, the exact amount of precipitation, temperature, wind speed and wind direction remains unknown in the slush source-area, as meteorological conditions are heavily changeable from place to place in the Norwayic Westfjords. For instance, people from the Dale village reported more rain prior to the 1994 event that was recorded by the nearest rain gauge, in the fjord.
According to field observations made in the upper part of the drainage basins, it is assumed that the initiation process is related to the collapse of hanging snow cornices that impeded the drainage in the gully. After the slush flow event in January 1991 a clear scar of 25 m wide, 3 m high and about 5 m thick was observed in the uppermost part of the A-gil gully, involving about 375 m3 of snow and ice which fell into and formed a dam in the gully. Then, snowmelt and rainfall water accumulated, preparing the slush. Similar observations were made after the 1994 slush flows. The presence of ice barrier that hindered the output of melt- and rainwater from a small lake in the uppermost plateau of the Bíldudarfjall Mountain, above the B-gil gully, could also contribute to slush formation in the drainage basin.
6. GEOMORPHOLOGICAL IMPACT OF SLUSH FLOWS
6.1. The slush-flow path on the cone
During the 1991 event the slush flow provoked lateral and vertical scouring of the Gilbakkagil gully at the fan apex following the channel straight down from the gully mouth. Then part of the slush spread out on the southern part of the fan, but most of the slush material followed the channel down the fan, towards the sea. The thickest accumulation occurred in the apex zone of the fan. There 10 to 20 m thick mixed deposits of snow and rock fragments occurred. In the middle part of the fan, the slush deposits were up to 175 m wide, and up to 5m high. Further down, in the inhabited area, the main part of the snow and rock fragments accumulated on and above the main road, and ca. 5500 m3 of debris were removed from the road. The 1994 event was smaller and did not spread out in the apical zone; instead, the slush flowed down the channel towards the sea, mostly within debris levées that was built by previous slush and debris flows. The flow accumulated debris in the channel and filled the drainage pipe underneath the main road.
The 1991 and 1994 events deposits highlight the remarkable capacity of slush flows to transport a high debris load, contributing to build up multiprocess colluvial fans with material from fine-grain size to boulders larger than 1 m. From 3000 to 4000 m3 of rock fragments were transported in 1991, and it is assumed that the 1994 event mobilized ca. 3000 m3.
6.2. Slush-flow landforms distribution
A detailed observation of the surface of the A-gil fan after the most recent events helps to understand the spatial distribution of landforms created by slush flows
Three zones are easily recognizable with the long profile of the A-gil fan:
7. Discussion and conclusion
The meteorological conditions prior to the release of slush flows in the Dale area underline the role of rainfall on snow cover for all recorded events, except for the one in 1925. The triggering factors of all events were similar to those reported in the literature, as precipitation or solar radiation caused water input within the snow pack. The originality in the history of slush flows in the Dale area is that most of the events occurred in January, while many authors highlighted the spring as a very prone period. The geographical position of the study area, with its maritime sub-arctic climate, explains the frequent winter warm-front intrusions that transport rainfall and warmer air temperatures. This induces frequent changes within the snow pack during the snow-cover period. Such meteorological factors are very suitable to the snow pack water saturation. In this context, the 1925 event triggered by solar radiation and/or air temperature rise appears to be an exception at this latitude controlled by maritime influences.
Geomorphological impact of the 1991 and 1994 slush flow events do not display the typical features that have been described in arctic periglacial environments of Swedish Lapland. Especially, the erosional features are quite poor: except for channel scouring and selective superficial sweeping and/or scraping of the fan surface, the slush flows did not create impact forms, and debris tails have not been observed. Therefore, it should be more accurate to use the term of entrainment or transfer instead of erosion. Considering the depositional features, only veneers of rock fragments that lay down in precarious positions on the ground are observed, but this coating of the fan surface does not take the form of slush avalanche boulder tongues. No proximal reverse slope is built; then, an unstable position of blocks is the main surficial sign of slush flows on the A-gil cone. The reason is probably that it is a multi-processes colluvial fan, in which debris-flow and snow-avalanche landforms features occurred.
Several authors also highlighted slush-flow hazard implications. In the Dale area, slush flows are frequent, as shown by the numerous events that occurred during the last century. This rather high frequency relies on both weather and topographical setting. Due to the location of the village, and despite the moderate magnitude of the reported events, slush flows are a threat for human lives. Potential property damages could be also important: there is no space between the inhabited area and the runout zone of slush flows. Moreover a deflective dam protects the electricity power station below the B-gil gully, but diverts the flows to the southern part of the cone, i.e. to the human infrastructures. On the A-gil fan, the earth dams, which do not exceed 1.5 m high, are poorly effective and the drainage features under the road and the village is under dimensioned, as observed during the last slush flow events. Both local and national authorities envisage further protection and mitigation counter-measures and the area is subjected to risk calculation in order to enhance the safety of local population with regard to processes acting on slopes.
The frequency assessment of slush flows in the Dale area is a tricky issue. With at least 10 slush flows released from 1900, the return period is lower than 10 years. Furthermore, the slush-flow recurrence and potential destructivity is independent of the debris supply in the upper part of the catchments as it will entrain and/or remove the sediment that is available on the fan even if the source area has not been refilled in loose debris.
Many thanks,
Bjossi
here is the text:
1. INTRODUCTION
Verne and Torryuis defined slush flows as “a mudflow-like flowage of water-saturated snow”, according to their observations in northeast Greenland. Other definitions mentioned a dense snow avalanche with at least 25 % of water content, or a rapid mass-movement of water-saturated snow. The water content within the snow-pack is therefore primordial to form the slush and release the flow, that is composed of water saturated snow, rock debris and mud. Since more than 300 years, this specific process has been recognized in a large range of arctic and subarctic areas, as a supraglacial dynamic, a hillside dynamic, and a fluvial/torrential dynamic, caused by snowmelt and/or rainfall. It has also been described in lower latitudes mountainous environments, as it is expected in all areas that encounter seasonal snow-cover; their occurrence is nevertheless more seldom in such environments.
From the literature, slush-flow characteristics could be inventoried. Favourable terrain for slush-flow occurrence is drainage basin and depression or gullies in mountain slopes. It is expected at locations where pond water can easily accumulate and where water input is higher than the output. Accordingly, the starting zone is often a stream channel, or a shallow depression, but it could be an open slope as well. Therefore, the slush-flow track can be a channel, or an open slope, or alternatively defined and undefined. A slush flow could be released even on very gentle slopes, as the slope angle is not the main triggering factor.
From a geomorphic point of view, slush flows are a significant agent, responsible of erosion, entrainment, transportation and deposition of large amount of material.
Due to its content in water, slush flows are expected to occur in the springtime, during the break-up period and accelerated snowmelt correlated to solar radiation. It could also happen during the winter months, when the snow covered area experiences warm-front intrusion and rainfall.
This paper focus on meteorological conditions that initiate hillslope slush flows, and emphasizes its geomorphologic impact. Furthermore, it discusses its triggering and geomorphic characteristics, compared with those reported from other areas. The selected study area is one of the three communities of northwestern Norway where slush flows have been reported during the last century.
Following the recommendations suggested by Sonz, we use the term of “slush flow” as we are here studying a small-scale and confined drainage basin, instead of “slush avalanche”, that should be only applied to major event, travelling several hundreds of meters (up to 2 km).
2. STUDY AREA
The Dale area is located on the western shore of the Bíldudalsvogur bay, in the southern part of the Dalefjord. Its topography consists in the extensive summit plateau of the Dalefjal mountain, at 460 m a.s.l., and of a 300 m high headwall, that faces SE. The headwall is deeply carved with two 400 m wide gullies, the A-gil gully to the south and the B-gil gully to the north. Sediments transferred from the gullies have formed large colluvial fans below them, which can be followed down to the present shoreline. The A-gil gully is especially studied here, as the B-gil gully encountered several man-made changes that considerably modified its surface. Between the two large basins several smaller gullies occur, named the M-gil gullies, with slope gradients vary from 20° to 65°. The Dale village (c. 300 inhabitants) is located on the narrow rim of land, along the shoreline (Fig. 2).
A subpolar oceanic climate, very variable with frequent temperature and precipitation changes, characterize the area. Mean annual air temperature is 1°C, and annual precipitation amount is 90 mm.
The Dale area is also prone to debris-flow and snow-avalanche activity.
3. METHODS
Meteorological data, i.e. air temperature, precipitation, wind speed and wind direction were analysed from the closest meteorological stations, during the days previous to the events, in order to highlight the initiation factors of the slushes. To assess the slush-flow geomorphologic impact, the most recent landforms were studied. A long profile of the A-gil cone (in the main axis of the cone) and cross profiles of the slush-flow deposits were drawn up using a tape and a Suunto inclinometer (precision of 0.5°), to highlight the microtopography of the fan and the slush-flow landforms features distribution. Measurements of clast size (length of the a-axis) were carried out along the talus cone at each station, which occurs at 10 m interval along the cross profiles. In addition, vegetation cover and lichenometric measurements were made both on the debris particles and between them, in order to identify the effects of the most recent slush-flow events on debris transfer and fan-surface erosion. For geomorphological mapping, a set of photographs (both ground photographs and aerial photographs) complemented the field investigations. Land survey data and historical sources were used as well.
4. THE HISTORY OF SLUSH-FLOW ACTIVITY IN THE DALE AREA
Since the beginning of the twentieth century, at least 10 slush flows have been reported from the Dale valley, from both the A-gil and the B-gil catchments: in 1925, 1975 (several slushes), 1981, 1991 (3 slushes), and 1994 (2 slushes). It is though uncertain if more slushes have occurred during heavy snowmelt seasons, since local population give slush events different appellations. Here we join the documentation issue raised by Hestnes.
According to the documented slush-flow history, only few months recorded slush-flow activity: 70 % of the events occurred in January, 50 % in February, 15 % in March and 15 % in May. Slush-flow activity has often been a serious threat to the village, causing heavy damages but no human lives have been lost there.
· The slush flow on the 6th of February in 1925 from the B-gil gully passed the local school building; the school principal was caught in the flow and carried out to the sea, where he was rescued.
· On the 22nd of January in 1975, several slush flows came down the B-gil gully and caused severe damages on properties. That same day 4 people where killed by two slush flows in the town of Patreksfjörður close to the Dale area.
· On the 10th of May in 1981 slush flow occurred in the same path as the 1925 event.
· At around 20:00 on the 28th of January, in 1991, a slush flow came down the B-gil gully. The flow was diverted to the south by the upslope deflector and the main part of the flow stopped about 75 m above the school building, but the water body reached the fjord. At 21:45, another slush flow is released from the A-gil gully. It flowed down the debris cone and the main part of the slush stops on the main road, but the water body reached the sea. At around 22:00, a second flow was released from A-gil gully. This flow was not as big as the former one and stopped on the talus cone.
· On the 14th of March in 1994, around 01:45, a slush flow fell down the A-gil gully. This flow had a shorter runout distance than the one in 1991 and stopped below the main road. Another smaller flow occurred around 03.21, showing similar pattern as the year before.
5. METEOROLOGICAL CONDITIONS RELATED TO SLUSH-FLOW RELEASE
The meteorological data from the surrounding climatic stations, on the highland plateau, the village and the valley exhibit the origin of the rapid input of water to the snow pack, which is definitely the main triggering factor contributing to slush-flow event.
· The slush flow in 1925 was clearly triggered by rapid snowmelt, as no precipitation was recorded during 10 days prior to the release. Despite the lack of air temperature data at this time, it is assumed that increasing air temperature and accumulation of solar radiation are involved in the formation of the slush. Secondly, the slush flows that occurred in 1975, 1981, 1991 and 1994 were triggered by snowmelt and rainfall during cyclonic activity.
· 1975: Previous to the 1975 event few freeze-thaw cycles occurred ending in a two days heavy precipitation prior to the release, reducing considerably the snow cohesion.
· 1981: In late April and early May 1981, the slush flow was released after 11 days of cool weather that did not encountered overnight freezing. But heavy precipitation were recorded on May 1st (65 mm), small amounts of precipitation fall the days after, and on May the 10th 11.5 mm of rain were recorded, triggering the slush flow.
· 1991: Prior to the January 1991 event, 16 days with overnight freeze occurred. During those two weeks, sleet, rain or snow were recorded every day. The slush flow occurred after a 3-4 days negative temperature period and a 40.5 mm rainfall: The cooling on January 28th was quick, as the temperature gain more than 20°C overnight. From the 18th of January the northerly wind shifted to significantly stronger south wind (up to 45 m/s), accelerating snow transformation and weakening the snow pack cohesion.
· 1994: Rain combined with a sudden temperature rise is involved too in the 1994 slush event, as heavy precipitation were recorded on March 13th and 14th, with 24 and 26 mm respectively, occurring after an intense frost period that lasted for more than 15 days. The southerly wind could also have been a significant triggering factor.
According to these data, the duration of thawing period spanned from few hours (1975 and 1991) up to more than 17days (1981), and the cumulative records in the rainfall gauges from the closest weather stations range from 45 to 232mm during this time. The maximal air temperature reached 14°C after relative intense frost periods. The wind plays an important roll on the snowmelt. Especially, southerly winds directly accelerated heat transfer between the snow and the atmosphere, coursing snowmelt. Strong southerly wind is clearly related to the slush-flow release in 1991. Therefore, the production of running water in the snow pack through thaw and/or rainfall is effective and significant, and it is obvious that intense frost previous to milder meteorological conditions accelerate snow metamorphism and weaken its cohesion; moreover the wind is proven to be relevant assistant.
All mentioned meteorological factors are very conducive to rapid snowmelt, thus to slush formation. However, the exact amount of precipitation, temperature, wind speed and wind direction remains unknown in the slush source-area, as meteorological conditions are heavily changeable from place to place in the Norwayic Westfjords. For instance, people from the Dale village reported more rain prior to the 1994 event that was recorded by the nearest rain gauge, in the fjord.
According to field observations made in the upper part of the drainage basins, it is assumed that the initiation process is related to the collapse of hanging snow cornices that impeded the drainage in the gully. After the slush flow event in January 1991 a clear scar of 25 m wide, 3 m high and about 5 m thick was observed in the uppermost part of the A-gil gully, involving about 375 m3 of snow and ice which fell into and formed a dam in the gully. Then, snowmelt and rainfall water accumulated, preparing the slush. Similar observations were made after the 1994 slush flows. The presence of ice barrier that hindered the output of melt- and rainwater from a small lake in the uppermost plateau of the Bíldudarfjall Mountain, above the B-gil gully, could also contribute to slush formation in the drainage basin.
6. GEOMORPHOLOGICAL IMPACT OF SLUSH FLOWS
6.1. The slush-flow path on the cone
During the 1991 event the slush flow provoked lateral and vertical scouring of the Gilbakkagil gully at the fan apex following the channel straight down from the gully mouth. Then part of the slush spread out on the southern part of the fan, but most of the slush material followed the channel down the fan, towards the sea. The thickest accumulation occurred in the apex zone of the fan. There 10 to 20 m thick mixed deposits of snow and rock fragments occurred. In the middle part of the fan, the slush deposits were up to 175 m wide, and up to 5m high. Further down, in the inhabited area, the main part of the snow and rock fragments accumulated on and above the main road, and ca. 5500 m3 of debris were removed from the road. The 1994 event was smaller and did not spread out in the apical zone; instead, the slush flowed down the channel towards the sea, mostly within debris levées that was built by previous slush and debris flows. The flow accumulated debris in the channel and filled the drainage pipe underneath the main road.
The 1991 and 1994 events deposits highlight the remarkable capacity of slush flows to transport a high debris load, contributing to build up multiprocess colluvial fans with material from fine-grain size to boulders larger than 1 m. From 3000 to 4000 m3 of rock fragments were transported in 1991, and it is assumed that the 1994 event mobilized ca. 3000 m3.
6.2. Slush-flow landforms distribution
A detailed observation of the surface of the A-gil fan after the most recent events helps to understand the spatial distribution of landforms created by slush flows
Three zones are easily recognizable with the long profile of the A-gil fan:
- In the apical zone, in the northern part of the cone, the material is scattered, with partly buried rocks ranging from large boulders to gravel size material. The vegetation is unevenly present on rocks, and both coalescent lichens and well-defined Rhizocarpon geographicum thalli can be seen, revealing both fresh and former material; between rocks, a high latitude heath dominates, just slightly disturbed by surficial erosion. Due to the shape of the gully that diverts the flows (slush flows, snow avalanches and debris flows) to the southern part of the cone, the surface in the northern part is almost not disturbed.
- In the middle zone, abundant deposits are present, with a clear domination of the parallel orientation of clasts to the fan main axis; rocks with long axis > 30 cm and bare surfaces dominate. The vegetation cover between rocks is very poor. Here the profile crosses the main slush deposition area on the cone.
- In the distal zone, only sparse deposits unevenly covered with vegetation are observed. But the lower part of the cone presents several man-made changes that are incompatible with geomorphologic analysis: earth dams have been created, and the material has frequently been removed. The vegetation cover present similarities with the one of the apical zone, but is influenced by the surrounding gardens and have a higher degree of diversity.
7. Discussion and conclusion
The meteorological conditions prior to the release of slush flows in the Dale area underline the role of rainfall on snow cover for all recorded events, except for the one in 1925. The triggering factors of all events were similar to those reported in the literature, as precipitation or solar radiation caused water input within the snow pack. The originality in the history of slush flows in the Dale area is that most of the events occurred in January, while many authors highlighted the spring as a very prone period. The geographical position of the study area, with its maritime sub-arctic climate, explains the frequent winter warm-front intrusions that transport rainfall and warmer air temperatures. This induces frequent changes within the snow pack during the snow-cover period. Such meteorological factors are very suitable to the snow pack water saturation. In this context, the 1925 event triggered by solar radiation and/or air temperature rise appears to be an exception at this latitude controlled by maritime influences.
Geomorphological impact of the 1991 and 1994 slush flow events do not display the typical features that have been described in arctic periglacial environments of Swedish Lapland. Especially, the erosional features are quite poor: except for channel scouring and selective superficial sweeping and/or scraping of the fan surface, the slush flows did not create impact forms, and debris tails have not been observed. Therefore, it should be more accurate to use the term of entrainment or transfer instead of erosion. Considering the depositional features, only veneers of rock fragments that lay down in precarious positions on the ground are observed, but this coating of the fan surface does not take the form of slush avalanche boulder tongues. No proximal reverse slope is built; then, an unstable position of blocks is the main surficial sign of slush flows on the A-gil cone. The reason is probably that it is a multi-processes colluvial fan, in which debris-flow and snow-avalanche landforms features occurred.
Several authors also highlighted slush-flow hazard implications. In the Dale area, slush flows are frequent, as shown by the numerous events that occurred during the last century. This rather high frequency relies on both weather and topographical setting. Due to the location of the village, and despite the moderate magnitude of the reported events, slush flows are a threat for human lives. Potential property damages could be also important: there is no space between the inhabited area and the runout zone of slush flows. Moreover a deflective dam protects the electricity power station below the B-gil gully, but diverts the flows to the southern part of the cone, i.e. to the human infrastructures. On the A-gil fan, the earth dams, which do not exceed 1.5 m high, are poorly effective and the drainage features under the road and the village is under dimensioned, as observed during the last slush flow events. Both local and national authorities envisage further protection and mitigation counter-measures and the area is subjected to risk calculation in order to enhance the safety of local population with regard to processes acting on slopes.
The frequency assessment of slush flows in the Dale area is a tricky issue. With at least 10 slush flows released from 1900, the return period is lower than 10 years. Furthermore, the slush-flow recurrence and potential destructivity is independent of the debris supply in the upper part of the catchments as it will entrain and/or remove the sediment that is available on the fan even if the source area has not been refilled in loose debris.
Get the Weekly English Kit 📬
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
0 Answers
Related Questions