Ice in the Environment: Proceedings of the 16th IAHR International Symposium on IceDunedin, New Zealand, 2nd–6th December 2002International Association of Hydraulic Engineering and Research

METEOROLOGICAL AND HYDROLOGICAL CONDITIONSRELATED TO FRAZIL PROBLEMS IN HOKKAIDO

H. Kasai1, M. Yamazaki1 and K. Hirayama2

ABSTRACTThe cold region river produces various ice formations during winter period. Typical frazilice problems have occurred in the hydropower stations and water supplies in Hokkaido.We have investigated the relationship between meteorological factors and frazil problemsat the three observation sites, the Asahikawa City Water Services, the Setose PowerStation, and the Niupugawa Power Station. We can classify representative two types ofweather conditions from these data. One is Clear weather type with radiative cooling,calm wind, and cold night for a few clear sky days. The standard of the daily minimum airtemperature is 15 °C below zero. The other is Blizzard type with strong wind, heavy snow,and low air temperature for a few days due to a growth of low atmospheric pressure and apassage of cold front from west to east in Hokkaido.

In this paper, meteorological and hydrological conditions related to frazil ice problemsoccurred recently in Hokkaido are discussed.

INTRODUCTIONIn our recent study (Yamazaki et al., 1996, 1998), growth and decay of anchor ice can beobserved in the rivers of Hokkaido particularly at the beginning and the ending of thefreezing season in winter. We developed a predictive method for river discharge changescaused by anchor ice dams based on the air temperature and weather condition.

In winter, frazil, anchor ice and ice cover, grow and decay simultaneously almost everyday in freezing rivers. The positive balance between their growth and decay accumulatesand consequently forms anchor ice dams and solid ice covers. Although river ice isformed every day, frazil ice causes few problems for those hydraulic power stations andmunicipal water services.

For the three problems that occurred at the intake station of Asahikawa City WaterServices in 2001, we noticed the weather and meteorological conditions when they hadhappened. Among those intake troubles, their meteorological conditions were totally

1 Hokkaido Electric Power Co., Inc., 2-1, Tsuishikari, Ebetsu, Hokkaido, 067-0033, Japan. Tel.:

+81-11-385-6553, fax: +81-11-385-7553, e-mail: hideo-k@epmail.hepco.co.jp2 Iwate University, 4-3-5, Ueda, Morioka, Iwate, 020-8551, Japan

different from one to the other. A Clear weather type problem was recorded on January 9,2001 when a cold wave settled down persistently over Hokkaido and clear weather lastedfor several days. A Blizzard type was recorded on February 3 and 11, 2001 when thegrowth of an atmospheric depression and the passage of a cold front through Hokkaidocaused a severe snowstorm, which continuously decreased the air temperature.

Based on these two different meteorological types, we discuss the relations between frazilice problems and meteorological conditions. This verification was conducted at theSetose Power Station and the Niuppugawa Power Station of Hokkaido Electric Power Co.,where their meteorological observation records were made available.

FORMATION OF FRAZIL IN HOKKAIDOHokkaido and the northern part of Honshu Island belong to the Subarctic Zone. Heavysnowfalls and the cold weather in those districts can be explained by the existence of theSiberian continent and the Sea of Japan. Temperatures drop extremely in winter inMongolia and Siberia, which increases the weight of the atmospheric air and forms astable, high atmospheric pressure. This high pressure blows out cold waves in the easterndirection toward the Sea of Japan. The blowing cold air absorbs aqueous vapor, whichevaporates from the relatively warm surface of the Sea of Japan. This air producessnowfall on the western side of Japan’s central mountains and dry air on its eastern side.

Hokkaido is a major island located at the northernmost part of Japan and it is situated atlatitude 42-45° N as shown in Figure 1. In comparison with the northern districts offoreign countries, Hokkaido has a larger amount of seasonal solar radiation in winter witha wide diurnal change of temperature.

The locations of our observations were the Niuppugawa Power Station, the Setose PowerStation and Asahikawa City, which are all situated in the central part of Hokkaido. In thisarea, the air temperature drops sharply due to the radiation cooling effect at night while itrises due to the solar radiation in the daytime. The difference between the daily maximumand minimum temperatures falls in a range of 10–20 °C. As the highest temperatures ofJanuary and February seldom exceed the freezing point, all rivers in the area are generallyfrozen especially at their gentle slope sections. Many rivers in Hokkaido experiencing ice

Figure 1: Map of Hokkaido Island, and location of observation sites

processes originate in the mountains, are short and generally steeply sloped, and have icecovers that are discontinuous. The daily fluctuations in water temperature and frazilproduction with the resulting channel storage and release of water cause unsteady riverflows (Hirayama et al., 2002).

METEOROLOGICAL EFFECTS ON RIVER ICEFrazil Ice Problems in the Asahikawa City Water ServicesThe frazil ice problems experienced at the Asahikawa City Water Services were causedby two kinds of meteorological phenomena, which are totally different from one to theother. The trouble recorded on January 9, 2001 can be classified into the Clear weathertype, in which a cold wave settled down persistently over Hokkaido and Clear weatherlasted for several days. The other one on February 3 and 11, 2001 is a frazil formation ofthe Blizzard type, in which the growth of an atmospheric depression and the passage of acold front through Hokkaido caused a severe snowstorm, which continuously decreasedthe air temperature.

For both Clear weather and Blizzard types, Figures 2(a) and 2(b) show the airtemperatures, wind speeds and snowfalls, which were recorded by the AsahikawaMeteorological Observatory. Figures 3(a) and 3(b) show the weather charts of thosetypes.

In the Clear weather type of January 9, 2001, a high atmospheric pressure of anexcessively cold front advanced southwards from the Siberia continent and settled downover Hokkaido. This high pressure encouraged the radiation cooling effect, resulting inthe daily minimum temperature of 23 °C below zero. However, in the Blizzard type ofFebruary 3, 2001, a low atmospheric pressure and a cold front grew in the Sea of Okhotskafter their passing through the Sea of Japan and Hokkaido. This was a representativedistribution of atmospheric pressures around Japan in winter: The high pressure area liesto the west and the low pressure area to the east.

(b). Blizzard type on February 1-7, 2001

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(a) Clear weather type on January 4-10, 2001

Figure 2: Air temperature, Snowfall and Wind velocity in the Clear weather type andBlizzard type in Asahikawa City, 2001

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(b) Blizzard type at 9:00 a.m.,February 2

(a) Clear weather type at 9:00 a.m.,January 9

Figure 3: Weather charts in the Clear weather type and Blizzard type in winter of 2001

Table 1: Initiative date of frazil ice problem and meteorological data at the Setose P.S.and the Asahikawa city water services

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Frazil ice problems at the Setose Power StationSevere frazil ice problems were not experienced at the Setose Power Station for 12 yearsafter the start of its operation in 1979. However, along with the sand sedimentation in thereservoir, frazil problems have happened especially at the tail race of its branch waterchannel almost every year since 1991. For the last 12 years from 1991 to 2002, the powerstation suffered intake trouble due to frazil formation 17 times or 59 days in total. Aninvestigation was made on the meteorological conditions when those frazil ice problemshad taken place. Its results were categorized into the Clear weather and Blizzard types asshown in Table 1.

In the Clear weather type, similar to those shown in Figure 3(a), frazil problems tookplace after the continuation of unusually low temperatures for several days. The dailyminimum temperature in this period should be at least 15 °C below zero. In the Blizzardtype on the other hand, frazil problems happened due to the combination of the snowfallwith strong winds and abnormally low temperatures.

Observation results at the Niuppugawa Power StationIce fences were installed in the Penke-Niupu River in 1992, where the NiuppugawaPower Station is located, in order to protect the power station from flowing frazil slush(Mineta et al., 1994). Thereafter, meteorological and hydrological conditions wereobserved there in winter from 1992 to 1995. In addition, for a period from December 28,1993 to March 7, 1994, the physical changes of the ice fences and the ice formation onboth upstream and downstream of the fences were video-taped. Figure 4 shows thedevelopment of anchor ice and the formation process of ice covers, which were bothvideo-taped, together with the air temperatures, water temperatures, wind speeds andsnowfalls. The figure indicates the close relationship between air temperature and riverdischarge.

In the Clear weather type (of January 1, 7 and 14, 1994), the river flow diurnally changedto a large degree. In the Blizzard type (of January 4, 5, 6, 1994), the river dischargeshowed little diurnal change but decreased continuously. Figures 5(a) and 5(b) show theweather charts of these two types.

When the weather is clear, a large amount of frazil is formed at night and it becomesactive. This frazil contributes to the growth of anchor ice and the rise of river water,resulting in channel storage. In the daytime when the quantity of heat by direct solarradiation prevails, frazil stops growing and it becomes passive, resulting in the decay ofanchor ice and the increase in river flows.

At the time of a snowstorm, frazil does not show a noticeable change of its state frombeing active to passive. Strong winds with snowfall keep frazil active for a long period oftime, continuously building up the channel storage. Examination of the videotapesenabled us to verify that the snow falling above anchor ice narrowed the river width. Itmay be considered that the growth of anchor ice and the snowfall jointly encourage theformation of ice covers.

Figure 4: Variation of River discharge, Air temp., Snowfall, Water temp., and Windvelocity at the Niupugawa P.S. in winter of 1993-94. (Wind velocity data were takenfrom the Asahikawa Meteorological Observatory)

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ANALYSIS OF FRAZIL FORMATION AND DECAYFlowing frazil slush is less adhesive when it is in the passive state and it may seldomcause a serious intake problem. In contrast with this, the frazil that remains in the activestate for a long period of time will flocculate and adhere to submerged structures. Thisadhesion imposes a small risk of intake troubles. The active state of frazil means that riverwater is super cooled. Though it depends on the flow velocity, water depth, bed materialsand ice cover formation, activation of frazil is generally governed by such meteorologicalconditions where the existence of a cold wave supports a large amount of heat to removefrom the water surface into the atmospheric air.

There is something common observed between the Clear weather and Blizzard types offrazil troubles. Warm weather increases the air temperature 3 or 5 days before theoccurrence of a frazil trouble. Increase in the air temperature widens the area of iceopening or reduces the size of anchor ice, thereby decreasing flow resistance in the river.Besides, in both types, a strong cold wave settles down over Hokkaido for several days,which hinders the rise of temperatures in the daytime. The frazil activated in thenighttime is normally passive in the daytime by solar radiation heat. However, it canremain active even in the daytime when the air temperature is excessively low or snow isfalling with strong winds. Frazil formation of these types follows the processes below anddevelops into the formation of ice covers.

Clear weather type: the process of frazil formation consists of the growth of anchor ice onthe riverbed, an increase in the water level, growth into anchor icedams, decrease in the river width, and the growth of solid ice covers.This process repeats until the river is totally frozen and closed withice covers. Until that time, surface frazil slush can be observed.Frazil trouble can occur at an intake when the frazil remains to beactive due to low air temperatures in the daytime.

Blizzard type: the process of frazil formation consists of the snowfall above anchorice, a decrease in the river width, increase in the water level, snowblowing into the open channels in river, and the growth of solid ice

(a) Clear weather type, on January 13 (b) Blizzard type, on January 4

Figure 5: Weather charts in the Clear weather type and Blizzard type in winter of 1994

covers. Frazil trouble occurs at an intake when frazil remains to beactive due to persistently low air temperatures in the daytime.

The water level in the river rises due to the formation of anchor ice dams and hanging icein the Clear weather type. However, it takes place in the Blizzard type when snowfall inriver channels reduces the cross-sectional area of river flow. In both cases, temporarychannel storage reduces the downward flow and makes its water depth shallow, whichmay facilitate the frazil formation.

CONCLUSIONSFrazil ice problems caused by two different meteorological effects are summarizedbelow.

Clear weather type:1. Active frazil is formed by strong radiation cooling in the nighttime when the weather

is clear. The daily minimum temperature should be at least 15 °C below zero.

2. Anchor ice often develops at locations where the bed gradient changes and associatesstep pools. When the weather is clear in the daytime, anchor ice occasionally lifts offthe riverbed and flows downwards.

3. When the weather is clear, radiation heat in the daytime melts ice covers or anchor ice.This allows temporarily stored river water to down-flow through the channels,occasionally causing large oscillations in river discharge. This event particularlyhappens at the beginning of the freezing season and in the thawing season. Withwide-open channels, a large quantity of frazil slush in active state flows downwards,which may cause frazil ice problems.

Blizzard type:1. When a cold front is in the Sea of Japan to the west of Hokkaido, air temperatures rise

in every part of Hokkaido. This increase in temperature partly melts ice covers anddecays anchor ice dams, reducing flow resistance in the river.

2. When a low atmospheric pressure develops on the northern side of Hokkaido or in theSea of Okhotsk, air temperatures sharply goes down in Hokkaido. With a cold waveadvancing southwards from the Siberia continent, heavy snowfall with strong windskeeps frazil in active state for several days.

3. A massive snowfall piles up above anchor ice, which reduces the river width andraises its water level. Continuation of the active state of frazil and the closure of riverchannels for a long period of time may cause frazil ice problems.

REFERENCESYamazaki, M., Hirayama, K., Sakai, S., Sasamoto, M., Kiyohara, M. and Takiguchi, H.

Formation of frazil and anchor ice. In Proceedings of the 13th IAHR Symposium onIce, Beijing (1996) 488–496.

Yamazaki, M., Hirai, Y., Hasegawa, K., and Hirayama, K. Anchor ice formation anddischarge change on a cold region river. In Proceedings of the 14th IAHR Symposiumon Ice, Potsdam, NY (1998) 77–84.

Hirayama, K., Yamazaki, M. and Shen, H.T. Aspects of river ice hydrology in Japan.Journal of Hydrology Processes 16(4): 891–904 (2002).

Mineta, M., Yamazaki, M. and Hirayama, K. A field study of ice control structure on riverrapids. In Proceedings of the 12th IAHR Symposium on Ice, Trondheim, Norway(1994) 139–148.