Slushflow activity in the Rana district, North...

Avalanche Formation, Movement and Effects (Proceedings of the Davos Symposium, September 1986). IAHS Publ. no. 162,1987.

Slushflow activity in the Rana district, North Norway

ERIK HESTNES & FRODE SANDERSEN Norwegian Geotechnical Institute, PO Box 40 Taasen, N-0801 Oslo 8, Norway

ABSTRACT Slushflows are a major natural hazard in Norway. Objective criteria to identify hazard areas and methods for slushflow prediction are sought. By analysing sixteen slushflow situations in Rana, basic interrelationships between meteorological conditions, snowpack properties and geomorphic features of slushflow terrain are established. Districts subjected to high cyclonic activity during winter are most susceptible to slushflow hazard. Sloping rock surfaces in drainage channels are the most frequent starting zones. The probability for specific slushflows to occur can be calculated.

Activité d'écoulement de neige mouillée dans le district de Rana, Norvège du nord RESUME L'écoulement de neige mouillée constitue un danger important en Norvège. Des critères objectifs pour identifier les zones de danger et des méthodes de prévision pour l'écoulement de neige mouillée sont requis. Par l'analyse de 16 écoulements de neige mouillée dans le district de Rana, des interrelations fondamentales ont pu être établies entre conditions météorologiques, paramètres de tassement de neige et caractéristiques géo-morphologiques du terrain d'écoulement de neige mouillée. Les districts soumis à de grandes activités cycloniques pendant l'hiver sont les plus sujets aux écoulements de neige mouillée. Les surfaces rocheuses inclinées dans des canaux de drainage représentent les zones de déclenchement les plus fréquentes. La probabilité de déclenchement pour des écoulements spécifiques peut aussi être calculée.

INTRODUCTION

Rapid mass movement of water-saturated snow, usually known as slushflow or slush avalanche, is a major natural hazard in Norway. Available information indicates that slushflows impose on everyday life in the same countries as snow avalanches do, and often to the same degree. Despite this, slushflows have not received the emphasis given to snow avalanches. The main reason for this may be that slushflows normally are classified as snow avalanches or floods, according to the character of the deposit in the run out zones.

Geomorphic features of slushflow terrain have been evaluated by 317

318 Erik Hestnes & Frode Sandersen

FIG.l Rana district, North Norway. (Printed by permission of the Norwegian Mapping Authority.)

Slushflow activity in the Rana district, North Norway 319

Hestnes (1985) and Nyberg (1985). Snowpack characteristics and meteorological conditions related to slushflow release are discussed by Hestnes (1985), Hestnes et al. (1987), Nyberg (1985) and Onesti (1985, 1987).

To elucidate inter-relationships between these factors, i.e. meteorological conditions, snowpack characteristics and geomorphic features, 16 slushflow situations in the Rana district, North Norway, have been analysed (Fig.l). The accidents in Sjânesheia, Rana January 1981, serve as a practical example to introduce different aspects of slushflow hazard (Fig.2-3).

Rana district is well suited to detailed investigations of conditions preferential to initiate slushflows. Frequency of slushflows imposing on everyday life is high, historical evidence of slushflow activity is well documented and the Norwegian Meteorological Institute (NMI) has three meteorological stations located within the district.

The main sources of information regarding the investigated events have been local newspapers and police, road and railway authorities. People involved and other eyewitnesses have also contributed. The geomorphic and vegetational characteristics of drainage basins and avalanche paths were evaluated by interpretation of aerial photographs, comprehensive field work, and by use of vegetation survey maps (Scale 1:5000) and vegetation classification index. The winter and current weather conditions and corresponding development of snowpack were analysed using data from the local meteorological stations.

THE SLUSHFLOW EVENTS IN SJÂNESHEIA, 27-28 JANUARY 1981

During the evening and night of January 27-28, 1981, heavy rainfall and rapid snowmelt released seven slushflows in Sjânesheia, 5 km southwest of the town Mo, Rana district (Fig.1-2). Five people were killed. Two houses, four cottages and several boat-houses were destroyed, and four cars badly damaged. The main road and railway through North Norway were closed for 2 and 4 days respectively. There were twenty other houses in the hazard area.

The first slushflow was reported at 19:00 on January 27. It crossed the main road and railway in a path approx 25 m wide and closed both. Queues of cars, waiting for the road to be opened, formed on both sides of the flow track. At 22:00 a second slushflow occurred 250 m west of the first one. Across the road the flow path was 50 m wide. Four cars were struck by the snowmasses, 3 people were killed and 5 injured. By this time the road authorities decided to close off the road and remove all people and cars from the area. Between 02:15 and 02:45 on January 28, five new slushflows occurred. The first and smallest hit the leader of the rescue team just finishing its work, but did no harm. A few minutes later, two huge slushflows came down destroying two houses, four cottages and several boat-houses. Two people living in one of the houses were buried in the snowmasses and found dead several hours later. These two slushflows covered the road and railroad over a total distance of 350 m (Fig.2). About 15 minutes later, another two slushflows occurred in the southern part of the area, damaging two additional houses. The road


FIG.2 Slushflows on a sparsely forested hillside. Sjânesheia, Rana, January 27-28, 1981. The two huge slushflows in the center of the photo killed 5 people. Arrows indicate location of crown surfaces. (Photography by 0.1. Tysnes.)

was not opened until 10:00 on January 29, when the rain had subsided. The railroad stayed closed for two more days due to damage to the tracks.

Recordings of the preceding weather conditions from the météorologie station located 2.5 km southwest of the hazard area, show that November and December 1980 were unusually cold and dry in the Rana district. The mean temperatures for these months were -3.5°C and -6.8°C, respectively, which is approx 4.0°C below the standard normal (1931-60). The precipitation was 74% and 95% of the normal. The cold weather continued until late January 1981. The snowpack, which by this time was about 80 cm deep, consisted of coarse grained cohesionless snow, probably with depth hoar at the base (Fig.3).

A sudden change in the weather situation took place on January 24, with warm and moist air coming in from the southwest. The temperature rose to 4.4°C, and 40 mm of rain was registered during a 24 hour period. The warm front was followed by cold weather and temperatures below zero, creating an ice crust on top of the snow-pack. Eighteen centimetres of new snow accumulated during the following two days, bringing the total snow height to 93 cm. A new warm front entered the area on the morning of January 27. Temperature rose from below zero to 5.9°C at 13:00. This sudden change in temperature was accompanied by heavy rain and high wind. At 19:00 of January 27, the temperature was 6.4°C, and during the preceding twenty-four hours 68.6 mm of rain was recorded. In the following twelve hours an additional 46.3 mm rain was recorded, of which most fell within the first six hours. The snow height decreased from 93 cm to 55 cm between 07:00 on January 27 to 07:00 on January 28.


After these catastrophic events, the safety of the remaining houses was questioned. Long-time residents of the district were interviewed and old newspapers examined. It was stated that slush-flows of even greater magnitude than those recently experienced had occurred during the last century. The last slushflow of comparable size took place in 1943, four years before the first house was erected in the area. Most of the houses were constructed after 1964.

With this information, detailed investigation of the slushflow hazard was undertaken. The conclusion was that all the remaining houses were exposed to slushflow hazard. Alternative defense structures for protecting the houses were proposed by the Norwegian Geo-technical Institute. These included construction of various types of concrete deflecting and retaining structures as well as earth fills. The height of the proposed structures was 4 to 5 m. The total costs were calculated to be NOK 7.65 mill (USD 1 mill). The total value of the houses and properties was estimated to about NOK 8.0 mill. After negotiations, it was decided to abandon the area except for one house, which was protected by a deflecting dam at a cost of NOK 50,000. Eighteen houses have been removed. The road authorities have built bars on both sides of the hazard area, and guards, staying in the area during periods of heavy winter rainfall and snowmelt, will close the road if necessary.

NERDAL SNOW HEIGHT

cm 100

PRECIPITATION

60 • Rain * Snow J Sleet «°

(Snow and rain

22 Time 2 0

•<-Slushflow

Precipitation mean monthly max. daily max.

Snow height mean monthly max. daily max.

Temp, monthly mean

NOV 30Yrs

111 304 71 12 28 78

0.6

1980 82

11.0 16

40 -3.6

30Yrs 136 507 58 27 85

103 -2.8

1980 147

,14.1 51

83 -6.8

30Yr8 135 353

90.8

1981 353

90.8

43 139 165

-4.6

88

107 -5.1

WIND

Speed m/sec. Direction Time Date

3 7

21

3 3 3 1 0 0 0 1 7 7 0 12 12 11 e-o^o ^ s S S • ^»s* ®^* / - • "

01 07 13 19 1 1

3 1 ,7 3 3 4 4 7 8 9 14 13 1315 1511 15 7 7 0 -«\ o o \\~o—o 9 o o o a o 0 0 O jo—o ©

010713 19

25 29

FIG.3 Winter and current weather conditions leading to the catastrophic slushflow situation of January 27-28, 1981. Daily records from the NMI observation station 7940 Nerdal, 30 m a.s.l.


METHODS OF INVESTIGATION

The investigated slushflow situations occurred during the 30-year period 1957-86. Sixteen reference events were selected, one within each situation. Different slushflow paths were singled out (Fig.l). The events are listed chronologically in Table 1.

During some of the slushflow situations more slushflows may have occurred in the same path within short intervals. The reference events at the sites is, however, the first slushflow causing damage or closing road. In case the location of crown surface was unknown, the most likely location was chosen based on field investigation and previous research (Hestnes 1985).

Slushflows are frequent in many of the investigated paths, in others only one or two events were reported during the sixteen slushflow situations dealt with.

Note that the study does not cover all the slushflow situations in the Rana district, during the last 30 years.

The meteorological stations used in the analysis are Nerdal in Rana, Mo in Rana and Nord-Rana (Fig.l). Nerdal is a synoptic weather station recording 4 times a day, in operation since 1966. Mo was operated as a synoptic station until 1965, since then only precipitation data have been recorded. Nord-Rana has daily records of precipitation since 1895.

The sixteen slushflow situations have been analysed both regarding the effect of the winter weather and current weather situations. • The winter weather controls the contemporary ongoing changes in texture and structure of the winter snowpack.

• The current weather is the weather condition conducive to slushflow release.

Normally there are a couple of days of overlap between the periods. Early winter- situations may sometimes almost coincide (Hestnes et al. 1987).

The properties of the snowpack at the end of the 'winter weather' period are essential to slushflow formation and downslope propagation (Hestnes et al. 1987). The relative rates of formation and discharge of free water in snowpack during the current period are the critical conditions for slushflow release. The temporal and spatial variations in water content depend on the intensity of water supply, hydrological condition of snowpack, as well as infiltration at the ground surface (Hestnes 1985, Hestnes et al. 1987, Nyberg 1985, Onesti 1985 & 1987).

Interpretation of snowpack development, snow metamorphism and hydrological properties of snowpack are mainly based on literature summarized by Colbeck (1978, 1982) and Kattelmann (1984).

Water supply to snowpack originates from two sources: rainfall and snowmelt. The amount of water received per unit area at the crown surface before slushflow release, is estimated.

The rainfall at the actual slushflow sites is assessed to be the recorded amount at the nearest NMI station, but might be somewhat higher. Interpolation based on current meteorological observations are used to evaluate the total amount by the time of release.


Estimation of snowmelt during rainfall is discussed by Kattelmann (1985). Our calculations are primarily based on an energy balance model outlined by Harstveit (1984). According to his work, the net radiation and the turbulent fluxes of sensible and latent heat are the significant energy fluxes feeding the energy consumptions in snowmelt in maritime regions and wind-exposed sites.

Air temperature, humidity and wind speed in the starting zones were extrapolated from the records at Nerdal, which are supposed to be fairly valid. Factors influencing wind speed have been taken into account; i.e. relief, aspect, local topography and vegetation (Hestnes et al. 1987).

Wind speed 2 m above ground is calculated from the predicted speed at 10 meter level, assuming a logarithmic wind speed profile (Harstveit 1984). A correction index related to surface roughness is applied in starting zones where wind speed is affected by vegetation or local terrain features.

The major landforms of the district, and the geomorphic and vege-tational characteristics of the drainage basins of the 16 slushflows were analysed, as well as the morphology of starting zones and tracks opened in snow cover. Landforms interfere with the wind systems and thus affect the distribution and intensity of precipitation and snowmelt.

Quantitative and qualitative aspects of catchment area, starting zone and slushflow tracks, as related to their location, release and motion, are discussed by Hestnes (1985) and Nyberg (1985).

The vegetation survey maps and classification index obtained from the local Forest Service, have detailed information on vegetation distribution, type, density, height, etc. They were useful tools in interpreting wind speed and radiative exchanges at the ground surface (Hestnes et al. 1987).

Major morphological types of starting zones and tracks are discussed by Hestnes (1985) and Hestnes et al. (1987). The morphology of the investigated slushflows are summarized according to this classification.

METEOROLOGICAL CONDITIONS AND SNOWPACK PROPERTIES

A selection of weather and snowpack data characterizing the pre-slushflow conditions, is listed chronologically in Table 1. It is organized in two main groups, parameters related to the winter and the current weather situations, respectively.

The slushflows occurred during the period November to April, yielding a time span variation in winter periods of 21-149 days. The meteorological conditions of these periods revealed the same main characteristics as documented from other parts of Norway (Hestnes 1985). However, in all cases this maritime district had mean winter temperatures fairly below normal, snow height correspondingly higher, and frozen ground by the time of slushflow release. Winter situations characterized by changes between snowfall, rain, and cold and dry weather were also more frequent, and showed a greater variety in both frequency and duration of the periods with air temperature


Table 1 Weather, snowpack and snowmelt parameters

No

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

Date

57 04 05 58 02 01 61 02 17 66 03 20

67 01 16 68 04 26 71 01 10 73 03 10

76 02 17 77 03 09 79 02 26 80 11 30

81 01 28 82 01 13 82 03 26 83 12 03

Time

1900 1300 1800 0100

1430 0700 1300 1100

0600 2130 0745 2200

0245 2200 0700 0700

days

74 70

105 121

23 149 63

121

88 106 102 24

67 67

132 21

WINTER WEATHER

Sw cm

58 64 54 31

17 91 34 92

98 68 60 21

63 33 22 39

Tm

°C -5 .7 - 7 . 8 -5 .4 -9 .9

- 4 . 7 -5 .2 -5 .3 -4 .0

-4 .1 -6 .4 -7 .1 -7 .0

-7 .1 -7 .3 -4 .6 -6 .3

V° days

4 0 3 4

1 11

1 0

4 1 1 0

1 1

15 0

V° days

38 17 42 13

9 50

5 14

34 26 17

2

19 16 60

6

days

6 7 7 5

9 15 10

7

8 13

3 15

7 7 5

12

Ss cm

55 105 110 20

13 166 57

180

155 91 91

6

91 40 20 18

C U

INITIAL PERIOD

S X

cm

55 144 152

65

41 166 129 194

163 138

98 4.5

97 92 20 89

SS

mm

42 53 22

24

94 32

21 54 16 34

21 35

78

RR

mm

30 48 52

48 23

3

13 6

20

53

41

B R E N

m mm

45

5

175 15

5 15

5

55

SS

mm

8 12

6

10

7

3

T

RR

mm

24 57 64

100

64 58 66 28

39 36 36 20

91 22 65 32

W E

LAST

MW

mm

25 30

5 20

30 30 30 10

35 10 20

5

25 5

50 25

A T H E R

24 HOURS

T m

°C 4.6 4.1 2.5 3.7

0.1 5.1 5.2 3.2

5.6 4.3 4.5

-0 .2

5.7 - 1 . 5

5.4 4.1

M s

m7s

3.6 8.0 4.1 4.5

9.0 8.4 8.0 5.3

12.4 3.4

11.0 4.9

13.4 4 .8 9.1 5.0

S f cm

10 107 105

30

17 116

59 171

110 108

70 45

55 90

3 42

SS

mm

42 61 34

30

94 32

21 54 16 44

21 42

81

TOTAL

RR

mm

54 105 116 100

64 106 89 31

52 42 56 20

144 22

106 32

MW

mm

70 30 10 20

30 205

45 10

•40 25 25

5

25 5

105 25

S -Snow height mean; T -Air temperature mean; T >0-Min.temperature above 0°C; T>0-Max.temperature above 0°C; MW-Meltwater;

S x f-Snow height (s tar t , max, f i n a l ) ; SS-Precipitation as snow; RR-Precipitation as rain; W -Windspeed mean.

above 0°C. Mean winter temperatures along with the number of days of maximum and minimum temperature exceeding 0°C, weighed against the snow height and length of period, display an indication of the composition of the winter snowpack (Table 1). It is stressed, however, that the listed data lack detailed meteorological information essential in the snowpack evaluation.

The snowpack at the end of the winter periods turned out to be of three types: • Weak cohesionless snowpack of coarse grains, sometimes with depth hoar at the base

• Stratified snowpacks of different compositions, often with crusts of snow or ice

• Cohesionless new snow on icy ground, sometimes resting on a relatively thin snowpack of coarse grains, or stratified snowpack with an icy crust on top.

The length of the current weather situation varied from 4 to 15 days. According to Hestnes et al. (1987) the meteorological conditions of the last 24 hrs are vital to slushflow release. Hence, data related to snowpack and rate of formation of free water are divided in two groups (Table 1).

Snow height at the opening of the 'initial periods' showed remarkable differences. Considerable snowfall is a striking feature of the first part of these periods, except for 3 typical spring thaw situations. These latter situations distinguish on the other hand, intense snowmelt.

Common characteristics of the slushflow situations taking place early in the winter are short total length of the pre-slushflow period with a long overlapping current period, and relatively large amount of new snow deposited on thin snowpack with a crust of ice. This corresponds with the third group mentioned above.

The quoted rainfall of the spring thaw situations appeared as an almost steady drizzle during the 'initial period'. Besides, the


largest amounts are recorded in those four cases when an initial warm front passed prior to the disastrous one (cfr. Sjânesheia 1981, Fig.3).

The warm fronts connected to the slushflow situations entered the area between 6 and 36 hrs before the release of the investigated slushflows. Periods shorter than 24 hrs, normally have snow listed in the first column of the 'last 24 hrs' (Table 1). Low daily mean temperature is the best indicator of the shortest periods.

The major part of the rain always fell within the 'last 24 hrs', sometimes close to the recorded daily maximum of the month, other times far from it. Meltwater contributed to the water supply in all cases. The meltwater calculations are based on the actual length of the melting periods. Thus, the mean air temperature and wind speed in Table 1 are sometimes lower than the values used in the calculations. Net radiation constitutes 20 to 50 per cent of the energy budget for the spring break-up situations. Besides, a small contribution is estimated in 8 other cases despite latitude, overcast sky and albedo of approx 50 per cent. The calculated meltwater is adjusted to the nearest 5 mm. In many cases the values may be less accurate. The wind direction was from the southwest in all situations except one.

The total amount of precipitation and meltwater received during the current period are listed in Table 1. From the last columns of the table it is obvious that intensive water supply is essential to bring about instability in the snowpack, and that meltwater is an important part of the water budget. There are, however, remarkable variations both in estimated water supply and accumulated new snow. Five main combinations of winter snowpack and current weather conditions are found to characterize the slushflow situations: • Ripe snow/Spring break-up. Coarse grained or stratified winter snow supplied with extensive rain and meltwater during the current period. (Situation no. 1, 6 & 15.)

• New snow (early winter). Varying amounts of cohesionless dry new snow of current period, resting on icy surface. Abrupt change to intense pouring rain and snowmelt. (Situation no. 5, 7, 12 & 16.)

• Coarse grained snowpack. Weak cohesionless winter snowpack of coarse grains. Large variety of current weather characteristics. (Situation no. 4, 10, 11, 13 & 14.)

• Stratified snowpack I. Relatively stable winter snowpack. Current period characterized by intense snowfall and large amounts of rain. (Situation no. 2 & 3.)

• Stratified snowpack II. Relatively stable winter snowpack. Current period characterized by some snowfall and moderate supply of rain and meltwater. (Situation no. 8 & 9.)

The sixteen slushflow situations in Rana, thus reveal the same variation in snowpack properties and current weather conditions as found by Hestnes et al. (1987).

GEOMORPHIC FEATURES OF THE SLUSHFLOW SITES

The main direction of the fiord and valley systems in the Rana district is SW to NE. A second system runs roughly perpendicular


to the first one, dividing the landscape into numerous mountain ridges and hills. The elevation of these varies between 350 and 800 metres (Fig.l). The vegetation cover above 300 to 400 metres is sparse.

The examined slushflow sites are located in areas with very different environmental characteristics, and the drainage basins vary widely in size, shape and geomorphic configuration (Fig.l). Field observations regarding catchment areas, starting zones and tracks are summarized in Table 2.

The catchment areas of the slushflows in Sjânesheia and vicinity are facing WNW to N (Fig.l). The areas are rather steep, with average inclination between 18° and 28°. The elevation of crown surfaces varied from 30 to 250 metres, and the upper limit along the mountain ridge between 440 and 710 metres. The other catchment areas are slightly sloping ENE to SE, with average inclination between 5° and 15°. The variation in elevation of crown surface and upper limit of the basin at these sites was 165 to 300 metres and 345 to 530 metres, respectively.

The variations in size and shape of the catchment areas are very prominent. However, in this respect there are no regional differences. Sparsely forested impediment, bogs and rock surface are the dominant ground conditions. Dense forest occurs in the lower parts of some of the areas.

All starting zones were located in small streams and brooks. The majority of the drainage channels were periodically dry, and some had an almost invisible course. The main geomorphic characteristics of the zones are listed in Table 2. The crown surfaces were normally located either at sloping rock surfaces or at local drops in inclination associated with irregularities in ground conditions.

A bare rock surface is the dominant ground condition of the starting zones and tracks of the seven slushflow sites located on

Table 2 Geomorphic features and flow path morphology

3 O

=3 33

No

1 2 3 4

5 6 7 8

9 10 11 1Z

13 14 15 16

C A T C H M E N T A R E A

O +J O

0) QJ CTl r~ > S ul - ^ OJ C .— i-

*C l/l _J " - UJ t_>

km^ km ip° m m

N340 0.13 1.2 24 710 170 N 65 0.2 1.0 5 345 260 N 75 0.3 1.2 12 450 195 N340 0.12 1.0 24 710 250

N330 0.15 0.8 28 660 Z30 N 75 0.4 1.6 7 500 300 N120 0.55 1.3 13 500 190 N135 1.6 2.3 8 530 200

N300 1.2 1.3 18 640 215 N 65 1.15 2.5 5 480 285 N310 0.2 1.0 23 580 150 N 75 0.5 1.7 10 440 150

N 25 0.17 1.2 19 440 30 N 90 0.1 0.8 15 375 165 N340 0.05 0.8 28 680 250 N335 0.4 1.4 22 715 150

STARTING ZONE

LOCATION

-a -a c m o r a s- • . - E O 1 - S-Q . CU QJ

X . . . X . . X .

X . .

X . .

. X .

. X .

. X .

. . X

. X .

. X .

. X .

. X . X . . . X . . . X

GROUND

III., nil . . X . X . . . . . X . . . X .

. . X .

. . X .

. X . . X . . .

. X X .

X . . X

T R A C K

ENVIRONS

ill . X

X X . . X . X .

X

. X

. X

x . x . X X . .

. X

X X . .

GR0U D

lit X . . . X . X . . X . .

X X X

X . X

. X X

. X X

. X .

Ill X . X

X X X . X

X . X

. X .

. X .

FLOW P A T H M O R P H O L O G Y

sz

h X . X . X . X .

X .

X .

TRACK

llUïïIBîi . X . . . X X . . X . . X . X X . . . X . . . X

X . . . X

. X . X . . X X . X . .

X . X . . X

X . . . .

. X . . . .

dr. - drainage; SZ - starting zone; mod. - moderate; grad. - gradient


sparsely forested hillsides. An incoherent cover of grass and turf provides roothold for the trees. The slushflows originating on forested hillsides and in open forest with grass and bush vegetation, revealed a greater variety in ground conditions both in the starting zones and along the track. Six of these continued on to cultivated land in the lower part of the track (Table 2).

The inclination of the slushflow tracks varied locally and often abruptly in smaller or larger steps, and the direction of the main drainage channel changed many times down most of the tracks. The average inclination of the slushflow paths, i.e. from the crown to the bottom boundary of the main accumulation of slush, has not been evaluated due to the fact that most of the slushflows entered into the fiord or rivers.

Linear (channel-like) starting zones with almost point-like apexes opened in snow cover at all of the slushflow sites, except three (Table 2). The length of crown surface was sometimes less than 2 metres. Scar-like zones were formed in a drained depression and at two smooth sloping rock surfaces. The length of crown for these was about 15 and 5 metres, respectively.

Channelled or channelled-undefined were the dominant morphology of the upper part of all tracks, except two. Half of them changed character downslope as they merged into areas of lower or alternating gradient. Open slope features were primarily a phenomena of the lower part of tracks. Recurrence of these morphological features down a track implies classification as stepwise reduction of gradient and/or alternating gradient (Table 2). Tracks including these flow path morphologies, revealed these features for the whole or main part of the tracks.

Enlargement and confinement of the slushflow tracks were mainly due to local changes in topography, snow conditions and man made influences.

SLUSHFLOW CHARACTERISTICS IN RANA

The high frequency of slushflows in the Rana area is due to weather and ground conditions. The district is exposed to high cyclonic activity during the winter. The cyclonic centers normally pass almost parallel to the coastline in a northeastern direction. Heavy rainfall and snowmelt occur when warm fronts of high temperature, moist air and high wind enter the area. This may happen any time during the winter. Bare rock surface and thin soil cover are the conducive ground conditions. In addition, frozen ground is normal from the early beginning of the winter. These factors restrain infiltration of water to the ground.

There is no significant difference in slushflow occurrence within the area, despite the variation in topography and aspect. However, wind exposed localities are observed to respond earlier than other sites with similar ground conditions, due to higher input of precipitation and meltwater. An orographic effect, caused by the steep hillsides facing the main wind direction, may contribute to some additional precipitation in the northern part (Fig.l).

Slushflows are released when shear stress of the snowpack exceeds

328 Erik. Hestnes & Frode Sandersen

shear strength, due to basal accumulation of water. Location of starting zones is conditioned on snowpack properties, ground condition and water supply (Hestnes et al. 1987). This explains why slushflows were observed to release at different elevations within the drainage basins. The observations of release at successively higher levels along some of the drainage channels prove that the shear strength was exceeded at different locations within short intervals. They also indicate that size and inclination of catchment area is of minor importance to the location of starting zones. Sloping rock surfaces with abundant supplies of water were typical for the starting zones with the highest frequencies. Slushflows were least frequent in drainage channels of low inclination. There were significant variations in the geomorphic features of the individual tracks, but the overall variation was limited, due to the ground conditions of the area.

The texture and structure of snowpack is found to be fairly identical throughout the area. The snow cover is, however, somewhat higher in the northern part than in the southwest. The snowpack properties characterizing the investigated slushflow situations reveal the same variation as found elsewhere in Norway.

The critical amount of water supply is related to the ground conditions, and depth and strength properties of the snowpack. In a couple of situations, water-saturation of the snowpack may have taken place above impervious icy layers and not at the base.

Cohesionless new snow and coarse grained snow were identified as particularly critical qualities. The spring thaw situations revealed a complex interaction between water supply, collapse of snowpack and opening of individual drainage systems. Thus, the number of slushflows occurring during two of these situations were limited. Stratified snowpacks are most stable. Nevertheless, intense snowfall and rain on these substrata were twice observed to cause numerous slushflows. The slushflow situations no. 8 and 9 were characterized by catastrophic break-up of some drainage channels.

Finally, it can be stated that the investigated slushflows in Rana revealed all the major morphological flow path characteristics described by Hestnes (1985), except bowl-like starting zones.

CONCLUSIONS

Basal accumulation of water in snowpack occurs when water supply exceeds discharge. The hydrological conditions of the snowpack 'and infiltration capacity at the base are the critical conditions. Both rainfall and snowmelt are normally essential contributors. Net radiation accounts for a significant part of the energy budget in spring break-up situations.

Adding water to snowpack implies reduction of cohesion, and free water will create pore water pressure. Slushflow will occur when shear stress exceeds shear strength. New snow and coarse grained snow are most liable to start flowing, and tend to propagate down-slope. Stratified snow is less susceptible to release and to be swept along.

Slushflows are primarily released in drainage channels. Sloping


rock surfaces are the normal location of starting zones in flow paths of high slushflow frequency. Slushflow size is closely related to snow condition, not to location or inclination of starting zones.

Districts exposed to high cyclonic activity during winter are most liable to slushflow hazard. This includes both West and North Norway. Slushflows released during spring break-up are also common. They primarily affect inhabited areas in North Norway.

The documented inter-relationships between meteorological conditions, snowpack properties and flow path characteristics implies that it is possible to calculate the probability for specific slush-flows to occur. However, the prediction has to be based on data related to the actual paths (cfr. Bakkeh0i 1987).

ACKNOWLEDGEMENT

This research was sponsored by the Royal Norwegian Council for Scientific and Industrial Research, the Norwegian Pool for Natural Perils, the National Fund for Natural Disaster Assistance and the Norwegian Geotechnical Institute. This support is gratefully acknowledged.

The authors are also much indebted to the Norwegian Meteorological Institute, which put climatic data at our disposal, to S. Bakkehoi for helpful suggestions concerning the data evaluation, and to G.K. Hestnes and P. Hestnes who did the tedious preparation of data and library work.

REFERENCES

Bakkeh0i, S. (1987) Snow avalanche prediction using a probabilistic method. IAHS Publ. no. 162.

Colbeck, S.C. (1978) The physical aspects of water flow through snow. Advances in Hydroscience 11, 165-206.

Colbeck, S.C. (1982) An overview of seasonal snow metamorphism. Rev. of Geophys. and Space Phys. 20 (1), 45-61.

Harstveit, K. (1984) Snowmelt modelling and energy exchange between the atmosphere and a melting snow cover. Univ. of Bergen, Geophys. Inst., Sci. Report 4.

Hestnes, E. (1985) A contribution to the prediction of slush avalanches. Ann. of Glac. 6, 1-4.

Hestnes, E., Andresen, L., Bakkehoi, S., & Sandersen, F. (1987) Meteorological significance to slushflow release. Norw. Geotech. Inst., Report 58200-5.

Kattelmann, R. (1984) Wet slab instability. International Snow Science Workshop (Aspen, Colorado, Oct. 1984). Proc, 102-108.

Kattelmann, R. (1985) Temperature indices of snowmelt during rainfall. Western Snow Conference (Boulder, Colorado, Apr. 1985). Proc, 152-155,

Nyberg, R. (1985) Debris flow and slush avalanches in Northern Swedish Lappland. Lunds Univ. Geogr. Inst., Medd., Avh. XCVII.

Onesti, L.J. (1985) Meteorological conditions that initiate slush-flows in the Central Brooks Range, Alaska. Ann. of Glac. 6, 23-25.

Onesti, L.J. (1987) Slush avalanche release mechanism: A Scenario. IAHS Publ. no. 162.


DISCUSSION

M. de Quervain Is the meltwater production by rain of a temperature > 0° negligible?

E. Hestnes Yes, the rain droplets have just minor importance to meltwater production.

E. Hanausek At what inclination did the slush avalanches start and how steep where the paths and runout zones?

E. Hestnes The inclination in the starting zones varied between 4.5° and 40.5°, but only exceptionally exceeded30°. The average inclination from the crown surface to the bottom boundary of the main accumulation of slush, varied between 5° and 20°. The runout zones of the Investigated slush flows have not been defined. However, those who didn't enter into the fjord and rivers had their lower boundary on horizontal or slightly inclined terrain.

K. Hutter I wonder whether you have some knowledge about the flow behavior of such slush avalanches. Are there any indications in the deposits or along the track that the mass of snow is essentially rigid and slides along a sliding plane? Or does the avalanche flow? Does the motion consist of sliding and differential creep? I know from experiments done by T. Davies that debris flow behaves very much like the second case. Stated yet another way: Does the water in a slush avalanche play a dynamic role or simply passively go along with the snow particles?

E. Hestnes Our observations of slush flow tracks and deposits clearly state that the water plays a dynamic role. However, the behavior of the flow may differ from event to event according to the geomorphic conditions in the starting zone and the inclination and roughness of the flow path.

Slushflow activity in the Rana district, North...

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