ICE FORMATION AND BREAKUP IN STEEP STREAMS - River Ice

Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
ICE FORMATION AND BREAKUP IN STEEP STREAMS
Knut Alfredsen 1 , Morten Stickler 1 and Curtis Pennell 2
1 Department of hydraulic and environmental Engineering, NTNU, Trondheim, Norway
2 Department of fisheries and oceans, St.Johns, Newfoundland, Canada
ABSTRACT
This paper gives an overview of ice formation processes in two small and steep streams and uses
field data from the Stavilla river in Norway and the South West river in Newfoundland, Canada to
illustrated the processes. Both rivers have slope and velocity conditions that prevent a usual build
up of an ice cover by accumulation of drifting ice, and dominated by anchor ice formation and
creation of anchor ice dams. Both rivers will develop a permanent ice cover, but the formation
process takes longer time and is subjected to longer period of dynamic ice production which
provides an increased ice production and a longer period with unstable in-stream conditions.
KEY WORDS: Ice formation; anchor ice; steep streams; fish habitat.
INTROCUDTION
Ice formation and breakup processes in small and steep rivers are not frequently described in
literature even if they commonly occur in all areas with ice formation during winter time. In
Norway, small steep streams are frequent, and the formation processes linked to steep rivers are
important in understanding the river ice environment. During recent winter small stream ice break
up’s have caused flooding and damages to infrastructure several places in the middle part of
Norway. Knowledge on the trigging mechanisms and the impacts from such events on in-stream
habitat for the aquatic ecosystem is still not well known.
As to our knowledge there is no unique and simple definition of what characterizes a small, steep
stream. Tesaker, (1994) uses a limiting velocity of near 60 cm/s, where retrograde build up is
prevented and ice formation is controlled by dynamic phenomena. Further, Tesaker, (1996) also
found that an ice cover initiated by anchor ice formation most frequently occurred in rivers with a
slope between 0.004 and 0.02 in rapid streams. Kanavin, (1970) defines a slope of 0.002 where
border ice and frazil ice dominates the ice formation, and further states that in rivers with slopes
higher than this anchor ice damming is needed to stabilize the ice formation. He also shows that it
is common to have a combination of these in the same river, and that it is therefore difficult to
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Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
draw sharp boundaries between the processes. Further, Hirayama et al. (2002) found similar
results in which a slope limit of 0.001 defined border ice dominated flow, but with a transition
between retrograde build up and border ice down to about a slope of 0.0003.
In general we would expect ice growth in a steep river in a series of processes that overcomes the
velocity and slope factor that prevents the usual retrograde build up of the ice (Tesaker, 1994).
These can be summarized as follows:
- Border ice formation in slow flowing areas along the bank and in areas with emergent
boulders.
- Anchor ice formation and the accumulation of anchor ice and frazil into anchor ice dams.
This usually occurs in areas with large substrate and boulders, over natural weirs in the
river and in shallow or restricted areas of the stream. With prolonged cold conditions the
anchor ice dams may freeze and stabilize.
- Anchor ice dams create a back water effect and reduce the upstream velocity. This allows
surface ice formation, and also ice cover build up by accumulation of floating ice.
- With time the dams may drain leaving ice covers that span the river width or causing a
cracked and refrozen cover behind the dam. The reach develops a stable ice cover
- A stabilized reach may hold until mild weather and increased discharge break up the ice.
In steep stream ice run caused by collapsed dams is common also in winter with just small
fluctuations in temperature
Over the last years a research program has been undertaken as cooperation between researchers in
Canada and Norway to study winter habitat for juvenile Atlantic salmon. As a part of this, work
has been undertaken to study the ice processes in several small streams and how they impact on
habitat availability. This paper gives an overview of this work and shows ice dynamics of two
streams with comparative hydraulic features.
a)
b)
Figure 1 Study sites
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Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
METHODS
Study sites
The ice formation was studied at two different sites, the River Stavilla in central Norway and
South West River in Terra Nova, Newfoundland, Canada. In River Stavilla data was collected
during winter of 2003/04 and 2005/06 and in South West river during the winter of 2005/06.
The river Stavilla (Figure 1a) is located in central Norway south of Trondheim (62° 55’ N 10° 15’
E). The catchment is 172.4 km 2 and mean discharge of 4.4 m 3 /s. The study site is located in the
lower part of the river where it meets tributary Ila to form the Sokna river. The average gradient is
1.6 % composed of two different sections; a steep rapid section upstream (gradient 0.024) and a
gentle run downstream (average gradient 0.005). Substrate distribution had a mean size of 25 cm
± 15 SD. ,in addition to several emergent boulders, particularly in the upstream section.
The South West River (Figure 1b) is located in Terra Nova Park in the eastern part of
Newfoundland (48 34’N 53 50’W). The catchment is 36.7 km 2 and has a mean winter discharge
of 0.5 m3/s. The study area has a gradient of 0.02 and the substrate is composed of cobbles and
boulders.
Data collection
Both sites have been mapped using a total station. Data on geometry and substrate distribution
were collected during ice free conditions, and data on ice have been collected during winter field
campaigns. The water level was monitored before and after the ice formed with objective to
estimate the ice effects on the water level. Anchor ice was mapped both for location and thickness,
particularly on locations were anchor ice dams formed. The extent of border ice was mapped
during initial ice formation, and the ice cover development was mapped several times during the
winter. A time lapse camera system was installed to monitor ice development and to find the
timing of releases and initial growth. In Stavilla and South West River climate data was logged
every hour using a weather station (Campbell Scientific) collecting data every hour.
100
90
Waterlevel (cm)
80
70
60
50
40
30
14.11 28.11 12.12 26.12 09.01 23.01
10
5
Air Temperature (deg C)
0
-5
-10
-15
-20
-25
14.11 28.11 12.12 26.12 09.01 23.01
Figure 2 Temperature and waterlevel in Stavilla
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Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
RESULTS
Figure 2 shows air temperature and water level fluctuations for Stavilla for the winter 2005/06,
and figure 3 shows the air temperature for South West River.
15
10
Air temperature (deg C)
5
0
-5
-10
-15
-20
02.01 09.01 16.01 23.01 30.01 06.02
Figure 3 Temperature South West River
28-2 2-3
3-3 4-3
Figure 4 Growth of anchor ice dams in Stavilla
In Stavilla we saw border ice formation during the first cold days in late October (2003) and early
November (2005). During the first cold period in both years, five anchor ice dams was established
which all except for the lower dam was drained. In addition, anchor ice formed on large boulders
and on the river bottom of the lower part of the reach. The edges of the initial anchor ice dams
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Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
froze over, creating a constriction in the river channel which increased the border ice cover and
worked as a foundation for repeated anchor ice dam formation over the following days. We
observed an initial anchor ice thickness of 58 cm in 2003 and the highest dam reached nearly 80
cm after several repeated events. Eventually, the river froze over. During winter of 2003 the ice
cover lasted until spring. In the winter of 2005 the ice was removed from the river at two
occasions, the first on the 11 th of December and the second on the 2 nd of February (see Figure 2).
This lead to three periods of ice growth in Sokna this winter each produced a complete ice layer
following the same pattern as outlined above. The development of anchor ice dams over a period
in early March 2005 can be seen on photos in figure 4.
During freeze up ice conditions in South West River were highly dynamic causing repeated ice
runs several times before it stabilized in mid February. Anchor ice formed during cold nights and
during nights with snow fall. On these events, four larger anchor ice dams repeatedly established
reaching up to 60 cm in height on the same locations characterized by emergent boulders. During
mid-day anchor ice ceased and the dams broke and drained causing a”domino-effect” downstream.
During days with air temperature above -15 C the anchor remained in the river. In the beginning
of the freeze up surface ice growth occurred along the river margins and around boulders. After
the establishment of the anchor ice dams, the surface ice cover further developed on ponds behind
each dam.
During freeze up water level fluctuated repeatedly in relation to establishment and drainage of
anchor ice dam. During the freeze-up period, the water level in both reaches showed a “stair step”
effect due to the damming. Figure 5 show a water level measurement for Stavilla with anchor ice
dams in place, and a similar graph for South West River measured after one anchor ice event in
late January. Upstream anchor dams, velocities were reduced, promoting the development of a
stable ice cover. In South West River velocities and depths were measured in two sections at
positions where anchor ice dams were formed. Table 1 shows the mean and maximum values
measured in each section.
Table 1 Measurements at anchor ice dam locations. Velocities (v) in cm/s and depths (d) in cm.
With anchor ice dams
Without anchor ice dams
Mean v / d Max v / d Mean v / d Max v / d
28 / 45 33 / 50 39 / 33 46 / 38
37 / 43 53 / 48 60 / 29 76 / 30
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Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
South West River
Stavilla
1007.0
99.4
1006.5
99.2
Elevation (m)
1006.0
1005.5
Elevation (m)
99.0
98.8
1005.0
98.6
1004.5
4830 4840 4850 4860 4870 4880 4890 4900 4910 4920
East (m)
98.4
0 20 40 60
Distance from lower end (m)
Jan 23 - with anchor ice dams
Jan 25 - ice free conditions
Anchor ice dam
Water level with ice
Water level without ice
Position of anchor ice dams
Figure 5 Water levels with anchor ice dams
Figure 6 shows the velocity profiles together with a control profile at a location upstream of the
anchor ice areas which is not much influenced by the dams.
Velocity (cm/s)
80
70
60
50
40
30
20
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Position from left bank (m)
Velocity - with ice
Velocity - no ice
80
70
60
50
40
30
20
10
0
1.5 2.0 2.5 3.0 3.5 4.0
Position from left bank (m)
80
60
40
20
0
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Position from left bank (m)
Figure 6 Velcoity distribution with and without ice
As mentioned, break-up’s were observed during the study period. In Stavilla the break up during
winter of 2005/06 created steep wave fronts and periods with local jamming and water level rises.
At the present time no measurements on ice breakup have been carried out, but during February
2006 ice breakup in Stavilla measurement equipment located 1.5 meters over normal river level
was flooded due to damming caused by the ice jam that formed, so breakups are also very
important for the understanding of the small river environment.
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Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
DISCUSSION
There are currently not many studies reported in literature on ice formation and breakup in small
streams, and results from the studies done does not give clear boundaries for the processes
controlling the ice formation. In general the observations in both rivers in this study follow the
process of ice formation were retrograde buildup is prevents described by Tesaker (1994). The
difference is mainly that South West River is more unstable due to larger temperature fluctuations
than Stavilla and therefore showed more frequent anchor ice releases. The time to ice
development on stabilized anchor ice dams was thereby prolonged and it took longer time to
proceed beyond step 2 in Tesaker’s process description.
River slope and temperature is used as a method to differentiate between retrograde and anchor ice
controlled ice build up. Both study streams are well within all slope criteria’s as defined by
Tesaker (1998) and Kanavin (1970). Hirayama (1986 and 2002) defined an ice formation
classification based on temperature and river slope. Both study rivers fall within the type II ice
formation classification, which is dominated by border ice formation. This corresponds well with
the observations done at the two study sites.
Velocities measurements at South West River show a reduction in velocity after the ice dam was
in place, and in all cases the velocity was below the 0.6 m/s limit proposed by Tesaker as limit for
ice cover formation by accumulation. But both in South West River and in Stavilla it was
observed that anchor ice formation was needed to form an ice cover at sites with ice free water
velocities below 0.6 m/s, and this indicates that the velocity criteria may not be directly applicable
but should more be used as an average over longer river reaches.
CONCLUSION
Ice formation in steep rivers may significantly alter the physical conditions. During freeze up and
break up the conditions may become highly variable due to dynamic ice formation. A
combination of discharge and accumulated cooling controls the timing of the anchor ice formation.
In this study we found that anchor ice dams were favored on locations with shallow, natural weirs
and/or in areas with emergent boulder. During freezing events, the river was transformed into a
succession of small pools replacing the original riffle habitat in the reach. There was no
observation of flooding outside the channel margins due to the anchor ice dam formation in
neither in Stavilla nor in South West river. In the South West River observations of anchor ice
dam releases during the study period were mainly controlled by smaller variation in water
temperature. In Sokna we saw two ice runs over the period, both related to mild weather
conditions with rain and increased discharge. The ice formation in both reaches follows the same
pattern regarding positions of dams and ice cover formation each time, but timing of events,
duration of freeze up period and ice thickness varies with climate and discharge. More studies are
needed to fully understand the growth and break up of ice in small rivers and to develop methods
to predict and analyze the ice dynamics and to link this to ecosystem response.
ACKNOWLEDGEMENTS
The authors wish to thank all the people that participated in the field campaigns. Thanks to Jo
Halvard Halleraker at SINTEF Energy Research who provided the Sokna climate data.
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Proceedings oh the 18th IAHR International
Symposium on Ice (2006)
REFERENCES
Hirayama, K. 1986. Growth of ice cover in steep and small rivers. IAHR Ice Symposium, Iowa
City, pp 451 – 464.
Hirayama, K., M. Yamazaki & H. T. Shen, 2002. Aspects of river ice hydrology in Japan.
Hydrological processes 16: 891-904.
Kanavin, E., 1970. Islegging i sjøer og elver [Ice formation in lakes and rivers] (In Norwegian).
Oslo, Norwegian Water and Energy Directorate.
Tesaker, E., 1994. Ice formation in steep rivers. S. Løset & E. Tesaker. 12th International
Symposium on Ice, Trondheim, Norway: 630-638.
Tesaker, E., 1996. Interaction between ice and water flow in rapids. Proceedings of the IAHR Ice
Symposium, Bejing, China.
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