Field experiments of winter flow in natural rivers

Gee Tsang and Leslie Szucs Canada Centre for InZand Waters, Department of the Enuironment,

Burlington, Ontario and

G. Douglas VaZlee Ltd., ConsuZting Engineers,

Simeoe, Ontario

ABSTRACT: This paper describes the reactions of a natural water- course to the effects of ice cover formation, growth, and deterior- ation. The study, which spanned two winters, undertook a systematic approach to collecting various types of data in an effort to deter- mine some of the parameters that affect rivers in cold weather. Ob- servations were made on the river itself in the areas of staging, velocity distributions, and riverbed erosion, with and without an ice cover. In addition, the phenomena of ice cover formation and deterioration, and the behavior of frazil ice, were noted. The study indicates that more research is required before a better understand- ing of winter hydraulics can be achieved.

RESUME: On décrit la formation, le développement et la disparition des glaces d'une rivière. Cette étude, faite sur deux hivers, a été systématique et l'on a ainsi collationné différentes données dans le but de mettre en Evidence certains des paramètres affectant les rivières durant l'hiver. la rivière, la rgpartition des vitesses et 1'Erosion du lit dans le cas de la rivière gelée et dans celui d'une surface libre de glaces. On a noté, d'autre part, les caractéristiques de la formation et de la disparition des glaces et le comportement de la glace frazil. Les résultats des travaux montrent qu'il est nécessaire de faire plus de recherches si l'on veut mieux comprendre les aspects hydrauliques de l'hiver.

Les mesures ont porté sur les régimes de

INTRODUCTION

In winter, northern rivers are covered by ice, as a result of which, flow in the rivers changes from open-channel flow to a close approximation of a closed conduit flow. The transition above is gradual over a period of several days or longer. width and thickness of the ice, the cover may or may not rise or fall following the variation of the rate of discharge of the river. The loading of frazil ice, which comprises ice crystals formed in a tur- bulent flow when the water is temporarily supercooled, also affects the characteristics of flow and the formation of ice cover.

In spring, the ice cover of a river deteriorates and breaks up into ice floes of various sizes. The ice floes, going downstream with the main stream will pile up and form an ice dam, when blocked by some obstacles.

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Depending on the

The spring ice jamming of a river has been the

cause of many spring floods.

study ice problems in rivers in the 1920's. More recently, work on the fermation and evolution of ice cover on rivers was done by Pariset and Hauser [4]. Carey [5, 61 studied the formation of ripples and dunes on the underside of ice cover and the friction factors of ice- covered rivers. Michel [7] did work on the forming of frazil ice in turbulent, supercooled water. Carstens [8] and Tsang [9] did studies on the hydraulic effect of frazil ice on water flow. Numerous papers have been written reporting frazil ice problems encountered in hydro- electric installations and water treatment plants. There is also much documentation recording ice jamming occurrences. Recently Moore and Watson [lo] did field experiments on the prevention of ice jam- ming by artificially weakening the ice cover prior to spring break-up. A review of the present stage of the art on ice problems was done by H.G. Acres Ltd. of Canada [ll]. As a whole, our knowledge at pres- ent in the area of winter hydraulics is poor. Much work is needed for a better understanding of the problem.

Barnes [l, 2, 31 was perhaps the first person to systematically

This paper will study: 1. the effect of ice cover on river stage and velocity

distribution; 2. the formation, growth, and break-up of ice cover in a

natural water course; 3. the behavior of frazil ice in a river; and 4. the effect of ice cover on sediment transport.

DATA COLLECTION

The major portion of the field data used for the present study was collected during the winters of 1969-70 and 1970-71 from the Nottawasaga River in southwestern Ontario. Additional data were obtained from Peace River in Alberta'during the winter of 1966-67.

The Nottawasaga data were collected from a site near the town of Alliston. A weir across the stream about 2 miles upstream from the research site causes a difference in water level of about 5 feet. An open section free of ice cover always exists immediately down- stream of the weir and provides a source of frazil ice production under favourable weather conditions. The research section is straight for a length of about 600 ft and averages 80 ft in width. It has a silt and sandy bottom desirable for bed erosion studies. As the test section is downstream from a bend, some undesirable centripetal effect is felt by the flow in the test section.

Data were taken at three crossings in the test section: the bridge crossing, the first crossing, and the second crossing. At the bridge crossing, which is about 150 ft downstream from the en- trance bend, a bridge was constructed across the river. The bridge could be raised or lowered to facilitate data collection at differ- ent river stages. A fixed point on the bridge base was chosen as the origin of a Cartesian coordinate system. All measurements were referred to this coordinate system. The first and second crossings were 187 and 387 feet, respectively, from the bridge crossing.

The data collected indicated the velocity of flow, the contour of the upper and the lower surface of the ice cover, the elevation of the free water surface in the holes drilled through the ice cover, and the contour of the river bed. Other relevant data, such as the ice cover condition, air temperature, frazil ice loading in the river,

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etc. were also noted. from holes drilled through the ice cover at approximately 5-ft inter- vals. At the first and second crossings, data were collected from drilled holes at 10-ft intervals.

crossings for elevation measurement. Data were taken from the bridge or by wading in the water, where the ice cover was thin, and directly from the ice cover, where it grew thick.

The Peace River data were obtained from the Peace River near Peace Point, Alberta. Data were collected at two crossings about a mile apart. A detailed description of the site and the procedure for data collection were given by Bennett [12]. The data measured were refersed to the upper surface of the ice cover, which was assumed to be horizontal.

Data collection was difficult on the coldest days. Water would freeze in the bearings of the current meter when the meter was taken from one hole to another. immersing the current meter in a pail of water when it was not in the river. During the interval between two field measurements, a layer of ice several inches thick would form in the drill holes and the bottom part of the holes was subject to erosion. It was neces- sary to measure the ice cover thickness beyond the eroded part when the holes were re-opened.

Data at the bridge crossing were collected

A taut wire was tied cross the river at the first and second

This difficulty was overcome by quickly

TREATMENT OF EXPERIMENTAL DATA

Of the Nottawasaga data, only those collected in the winter of 1970-71 are used in the paper as they were more accurate and system- atic. Days on which the data were obtained are shown in Table 1. Other matters of interest on these days were also noted and shown in the table. From these data, diagrams showing the contours of the river bed, the upper a.nd lower surfaces of the ice cover, the veloc- ity distribution, and other interesting particulars were drawn. Figures 1 to 8 are part of these diagrams.

results from displacing the coordinate system used in data collection to a suitable place. For each crossing, the coordinate system is constant for all the diagrams, but the displacements for the three crossings are different.

the Calgary Office of Water Survey of Canada. this study are shown in Figure 9.

In Figures 1 to 8, the coordinate system at the three crossings

From the Peace River data, similar diagrams were prepared by The diagrams used in

EXPERIMENTAL FINDINGS i. Effect of Ice Cover on River Stage und VeZocity Distribution

On December 3, 1970, the Nottawasaga River was open and the velocity distribution at the three metering crossings is shown in Figure la. In Figure la, the three different figures of the rate of discharge at the three crossings, which were obtained from velocity integration over the cross-sectional area, show the degree of accur- acy of metering in this study. Figure la shows that the velocity distribution was as expected in a wide open channel (note that the vertical scale of the diagram has been exaggerated). At the bridge section, the centripetal effect of the upstream bend was noted, which pushed the current towards the right bank. to form (see Figs. lb, 2a, and 2b), the resistance in the ice-covered

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As the ice cover began

part of the channel increased and pushed the flow to the open-channel part in the middle. As the channel resistance increased, the flow slowed down and the water stage increased. Figures la, 2b, and 2c show that, although the rate of discharge on Dec. 3 was about twice that of Dec. 22 and about one third higher than that of Dec. 23, the stage on the two later days was, respectively, about 10 inches and 15 inches higher. As for the velocity - on Dec. 3 the average vel- ocity at the bridge crossing, as calculated from the rate of flow and the flow area was 1.23 ft/sec. On Dec. 23, although the rate of discharge was only reduced by the order of 25 percent, the average velocity was reduced to 0.65 ft/sec, a 50 percent reduction.

The additional resistance reduced the prominence of the centritp- eta1 force at the bridge section. On Dec. 3, when the channel was open, the fastest flowing region was close to the right bank. Accom- panying this was a large velocity gradient at the right bank and con- sequently a high shear at the border. As the ice cover developed, the fastest flowing region shifted away from the right bank. The high velocity gradient and shear thus were relaxed. According to the observation above, we may say that, for bank protection, it is desirable to maintain a part ice cover at the bank subject to erosion.

When the ice cover was completely bridged, the additional resis- tance was felt by the whole width of the river. The geometric shape of the velocity contours changed to a form more similar to that of an open-channel flow, except at the top where the additional resis- tance was present. This is evident when the velocity distributions at the bridge crossing in Figures la and 2c are compared. ence of the additional resistance, however, still diminished the effect of the centripetal force by the upstream bend. indicate that the distance from the fastest flowing region to the right bank is greater in the ice-covered case than in the open-chan- nel flow case.

effects of ice cover on flow during the break-up period was similar, as may be seen, for instance, from comparing the velocity distribu- tions at the bridge crossing on March 10 and March 15 (see Fig. 7). The rate of discharge was about the same on these two days. As the rate of discharge varied greatly during the break-up period, the effects of the ice cover on the flow were blurred by the influences of other parameters.

The pres-

The figures

The discussion above pertains to the freeze-up period. The

.. zz. Forming of the Ice Cover The formation of ice cover in the test river usually lasts about

a week to a month with or without intermittent thaws. ent season, on November 24, an ice sheet bordering the left bank of the entire test stretch and an íce sheet bordering the right bank of the downstream section of the test stretch were noted. By Nov- ember 25, the ice cover at the left bank had extended about 20 ft into the stream. However, on Dec. 3, after a warm spell, the whole ice sheet melted away and the river was clear of ice. As the weather turned cold again on Dec. 14, the ice cover once again formed, com- pletely covering the river by Dec. 29.

For the pres-

Formation of the ice cover consisted of two stages: (a) Formation of border ice - The ice cover first grew from the

left bank where the water was shallow and the velocity neg- ligible. The shallowness of the water indicated a small heat capacitance per unit area of heat transfer and the

7 75

Table 1: Dates of Data Collection and

November 12, 1970 River free of ice, sky overcast, rain fallen overnight, water cloudy, tree trunk in test section near the first crossing and caused local scour.

November 19, 1970

November 24, 1970 Water clear, sky slightly overcast.

Soft slush section existed at the bridge crossing. Slush extended 10 ft. out at the left bank (looking downstream) and 2 ft. out at the right bank. the slush disappeared over the course of the afternoon. About 12" snow fallen an Nov. 23. Slush accumulated in the neighbourhood of the roots of the fallen trunk at the first crossing.

6ky slightly overcast, temp. about 45'F, water clear.

Sky clear and sunny, temperature 28'F. Some of

December 3 1970

December 10, 1970 River high, overcast, temp.-35'F. At the second crossing, ice cover was about 4 ft. extendina out from the left bank. Thickness of ice ~~ ~

cover about 0.8 ft. December 14, 1970

5" snow fallen over the past two days. grew from left bank out. and second crossing because of high stage.

dercast, temp. 3ZoF., river low, ice cover extended from the left bank at the br-idgr crossing. ?felting took place on the baflk due to warm weather. No ice cover from the right bank. since Dec. 14. Flow slow. Sandy sediment covered the river bed Silt sediment washed downstream.

Temp. about 30'F. Ice cover Data could not be obtained from the first

December 18 1970

2" sribw fallen

December 19, 1970 Mild at about 38OF. Ice melted quite rapidly. Much ice that had farmed earlier broke away and flowed downstream

Cover in floes.

December 22 1970 T&p. about 2OoF. Velocity metering incomplete because insufficient time. The outer edge of the ice cover was ice slush of 1" thick (frazil ice)

Ice cover built up and extended further aut.

December 23, 1970 Temp. 5OF. River up from previous day. Water level fluctuated. Ice cover soft, but not slush. No data were obtained at the 1st and 2nd crossings because of high stage. formed except narrow openings.

River further up from yesterday. ice cover in the previous day hardened. the previous day and lodged in the river to form an ice bridge.

5" snow accumulated over the past 4 days. Dec. 24. Temp. 25'F. lowering in stage. 25' and low at 0-5'F. At the 1st crossing in the middle of the stream, a slush layer of 2' thick accbaulated under the ice cover. Insufficient time for velocity metering at the second crossing.

Complete ice cover

December 24, 1970 Soft and slush part of the

A floe of ice broke

December 29, 1970 Water level down from

Ice cover at the banks curved down due to Temp. in the past 4 days was high at about

Air pockets farmed under the ice aver.

December 30, 1970 River down from the previous day. an.

Ice heard cracking when walked Temp. 20'F. At the 1st crossing, the water between two ice

sheets observed the previous day froze to form one ice sheet

December 31, 1970 Temp.. 22'F. sagged about 1' but still adhere to the banks.

Ice heard cracking when walked on. The ice cover

January 5, 1971 Temp. 33'F. Dec. 31.

Rain fell the previous day. No snow fall since No velocity meterinn for the Ist crossing because of

time limitation.

Temp. 15OF. Over the past week, 3" snow fell. Ice was hear¿ cracking when walked on.

January 13, 1971

January 20 1971 +emp. 20°F. approximately over the past week. accumulation on the ice 4". Air Dockets under the ice cover

Total snow

were again noted.

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Other Relevant Information (Nottawasaga)

January 28, 1971 On the original surface of the ice, the accumulated snow now turned into a layer of slush with a hard surface. On the top of this hard surface, a 1" layer of snow lay. Temp. 5-10°F. 6" of snowstorm and drift on Jan. 26.

Temp. was about loop over the past week and 28' today. snow fall on Jan. 29.

February 3, 1971 Large

February 10, 1971 Several snow falls over the past week. gushed aut from the drilled holes and flooded the surrounding area until stopped by the surrounding snow. Temp, 32OF. The hydraulic gradient of the test section was surveyed today to

Temp, about 15'F. Water

be S = 0.00033. February 24, 1971

Temp. 25-30°F over the past week. On punching the holes, the water in the holes became cloudy with particles of dirt. Slush and snow were on the top of the original ice. Water again gushed out from the drilled hales.

Temp. 45'F, windy. in the hales over the last 3 days. in the formed ice.

Temp. 40°F.

February 27, 1971 Nose snow had turned to slush. 112" ice formed

Sandy particles were observed

Narch 3, 1971 Temp. about 30'F. some mow, not slush, remained in odd spots. Ice was along the

Previous snow and slush turned into ice. Only

shore. No sandy sediment was observed in the formed ice in the drilled holes, only twigs, dead weeds, etc.

March 10, 1971 Temp. 35'F. Temp. in the past week below 25OF. except near the banks. ice in the drilled holes. The lower part of the drilled holes began to melt away.

Temp. over the past week 40-50'F. all ice had melled. All snow had melted and became ice. No sediment was noticed in the water.

There was no slush There was no sandy particles in the formed

March 15, 1971 For 10 fe. from the right bank,

March 16, 1971 General breakup began. open water surface. Large amounts of twigs, leaves, etc., were observed in the drilled holes. The ice was still very solid on the left bank.

Ice broke up on the right bank, about 12'

March 17, 1971 Ice became much thinner in places overnight. A large longitudinal crack was seen close to the left bank. Ice cover became dangerous to stand on.

hernight temp. 22'F approx.

Channel almost open all along.

Temp. about 32'7.

Temp. 28'F, windy. More ice melted over the past two days. Water was dawn about a foot. 1"snow fell an the remainder of the ice.

Temp. 35'F, no wind.

March 18, 1971 Some disintegration occurred overnight.

March 19, 1971

March 20, 1971

March 24, 1971

Ice disintegration overnight was slight.

Narch 26, 1971 River slower and lower than March 24.

March 31, 1971 All ice at the banks had now melted away. The channel was completely clear. Temp. 4O0P. Water subsided during the day. Temp. over the past few days 35'F.

April 7, 1971 River was high, muddy, turbulent and boiling. Temp. 40-45'F.

April 16, 1971 Temp. 40-50'F. Sediment seen in the turbulent water. Bank eroded fast. The river level had been 1' higher as could be seen from the water mark on the shore.

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slow velocity indicated little turbulent mass transfer, and hence heat transfer, between the main stream and the water body. This led to a favourable situation for surface freez- ing. When the surface ice sheet first formed, interwoven needle ice crystals, fractions of an inch long, were noted. The water between the ice needles froze into finer crystal grains. The early border ice sheet, although spread wide, was thin and it could easily be broken by the pressure of a finger. strength of the border ice would be directional. As time progressed, the border ice thickened through heat loss by conduction across the ice cover. As ice is a poor conduc- tor, the thickening o€ the ice cover during this period was slow. The ice cover also grew laterally as the ice needles, which branched out of the ice sheet, slowed the neighbouring water down for surface freezing. This lateral growth, how- ever, was slow and limited to a certain distance into the main stream, where the fast flowing water prevented the con- tinuous growth of the needle ice crystals.

The form of crystalization indicated that the

(b) Growth of the ice cover by consolidation - As time prog- ressed, the continuing heat exchange between the atmosphere and the river reduced the temperature of the main stream to near the freezing point. During cold spells, large quant- ities of frazil ice were formed in the turbulent, tempor- arily supercooled parts of the stream. crystals agglomerated into frazil clusters not long after their formation. The frazil clusters, being lighter than water, floated to the surface of the main stream, where, except in rapid flowing regions, the velocity gradient was small; hence the frazil clusters were not transported to the lower part of the stream by turbulence. cluster and water mixture was fairly sluggish. It adhered to the existing border ice, freezing onto it. It also dragged down other frazil clusters passing by in the outer flow. A layer of frazil slush thus was formed as shown in Figure 2a. existing ice cover was the chief factor in the growth of the ice cover.

The fine frazil

The frazil

The freezing of the frazil slush layer to the

The viscosity and buoyancy of the frazil-water mixture sup- pressed the turbulent motion in the stream, on Nov. 25, the weather was windy and ripples were observed on the water surface in the middle of the main stream. The surface ripples stopped abruptly at the edge of the frazil- laden strips, which were several feet wide along the ice covers at the two banks. Thus from the bridge, one observed a rippled main stream sandwiched between two strips of smooth water, beyond which were the ice covers.

Although frazil ice was constantly formed in sub-freezing weather, most of it was produced during the night and in the early morning. could be seen from the ice cover where it was noted that the ice cover was composed of strips of different shading, each several to ten feet wide, presumably the result of different thickness and different snow accumulation on the

For instance,

The diurnal effect of frazil production

top.

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Besides sticking to the existing ice cover, frazil ice also stuck to other obstacles in the river. On Dec. 18, an ice patch was seen to develop independently in midstream a little down from the first crossing, presumably from the accumul- ation of frazil clusters around a submerged obstacle. The ice patch grew laterally and in its upstream edge by catch- ing the frazil slush in the main stream. On Dec. 22, the upstream edge of the ice patch, although not yet hardened, reached the first crossing as seen from Figure 2a. The lat- eral growth of this ice cover eventually met the ice covers from the two banks and formed a continuous ice sheet.

The ice covers from the two banks grew and met in the middle of the stream, first in places where the velocity was slower. The meeting of the two ice covers produced a long V-notch. The propagation of the V-notch upstream eventually closed the whole stream. As the ice covers spread, they also thick- ened. The complete ice cover, not long after its formation, is shown in Figure 2c.

This stage of ice cover formation was more than a simple consolidation of the frazil slush. In the frazil laden strips, the velocity was low, so that some needle ice crys- tals were formed. The melting and refreezing of the snow cover, the intermediate thawing and refreezing of the ice cover, all had their effects on the structure of the ice cover. As the strength of an ice cover is completely de- termined by its crystal structure, the process of freeze-up thus must decisively determine the manner of spring break-up.

iii. Growth and Deterioration of the Ice cover from Freeze-up to the Beginning of Break-up The ice cover continued to thicken after it was formed. This

thickening might be divided into two stages. roughly from Dec. 29 to Jan 20. During this period, the ice cover thickened from 4-6 inches to 10-12 inches as may be seen from Fig- ures 2c to 4b. The thickening of the ice cover was caused by (1) the deposition and freezing of frazil ice, which was produced in the upstream open parts of the stream and was in large quantity in the early part of the period, to the underside of the ice cover; (2) the accumulation and freezing of precipitation to the upperside of the ice cover; and (3) more water frozen to the ice cover by heat trans- fer across the ice cover. The order of importance of the above fac- tors is probably subject to the influences of meteorological para- meters and is a topic of further research. the progressively decreasing thickness of the ice cover from the bridge crossing to the second crossing, one may conjecture that the deposition and freezing of frazil ice to the underside of the ice cover was an important factor in thickening the ice cover. The above is based on the reasoning that for the test stretch, the loading of frazil ice per unit length of the stream would likely be the same. At the bridge crossing, the river is narrow so that areal concentra- -tion of frazil ice over the surface of the ics cover was high. Thus the thickening of the ice cover at the bridge crossing was fast. Conversely, the thickening of the ice cover at the second crossing was slow.

The first stage occurred

In this study, from

From Dec. 29 to .Jan. 5, the rate of discharge remained more or

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less steady. From Jan. 5 to Jan. 20, the rate of discharge decreased gradually. sagging of the ice cover as shown in Figure 4b. As the water level in the narrow parts of a river is more sensitive to changes in the rate of discharge, the sagging of the ice cover was greatest at the bridge crossing and least at the second crossing.

from Jan. 20 to the beginning of river break up. By Jan. 20, most of the river, except the part immediately downstream of the weir, had frozen. duced. periodic flooding and freezing of water over the ice cover, supplemen- ted by the melting and refreezing of the snow cover. The first flood- ing of the ice cover occurred sometime between Jan. 20 and Jan. 28. Following the sagging of the ice cover, small cracks appeared on the ice cover. Water seeped through the cracks and flooded the ice cover as shown in Figure Sa. The flooding water mixed with the accumulated snow on the ice cover and formed a slush. The ice cover thickened as the slush froze to it. The presence of accumulated snow on the ice cover was immaterial to the added thickness of the ice cover. How- ever, it affected the structure and consequently the strength of the ice cover as less needle ice crystals would form in a water-snow slush than in water alone.

Figures 4b, 5a, and 5b show that surface flooding and freezing were fast processes in thickening the ice cover. The thickness of the ice cover increased from 10-12 inches to 18-20 inches as the result of the first flooding and freezing. As there was more sagging at the bridge crossing, the increase in thickness of the ice cover at the bridge section was also greatest. Although the figures above cover a time span of two weeks, the process of flooding and freezing was like- ly completed within a few days. flooding and freezing occurred, the rate of flow decreased slightly from Jan. 20 to Jan. 28, and remained steady from Jan. 28 to Feb. 3. As the thickest ice cover is formed in the narrowest part of a river, nature seems to promote spring ice jamming.

It was observed on Feb. 10 (see Fig. 6a) that the rate of discharge was increasing. The seepage and uplifting of the ice cover were in- sufficient to relieve the pressure of the flow. ing out from the drilled holes and flooding the neighbouring area of the holes. The river flow was therefore under pressure. The second flooding and freezing increased the ice cover to about 2 feet as may be seen from Figure 6b. The slush on the ice cover as shown in Fig- ure 6b was from the melting of the accumulated snow rather than from surface flooding, as the rate of discharge from Feb. 10 to Feb. 18 remained more or less constant. slush layer indeed indicated this.

As the rate of discharge again increased from Feb. 18 to Mar. 3, further surface floodings on Feb. 24, Feb. 27, and Mar. 3 were noted. On these occasions, as the ice cover was lifted by the swelling river, the depth of the flooding water on the ice cover was small. The in- crease in thickness of the ice cover by surface flooding and freezing therefore would not be much greater than the increase in thickness by the melting and refreezing of the accumulated snow. such as on March 3, it appeared that the shallow layer of water was due to both surface flooding and melting of ice and snow.

Following the recession in the rate of discharge was the

The second stage of ice cover thickening took place approximately

The rate of frazil ice production therefore was greatly re- The thickening of the ice cover was now caused mainly by the

The water-snow slush froze sometime between Jan. 28 and Feb. 3.

In the period in which the first

The second flooding of the ice cover occurred around Feb. 10.

Water was seen gush-

The irregular upper surface of the

In fact, at times,

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As the flooding and freezing of water on top of an ice cover is an important factor in thickening the ice cover, the thickness of an ice cover and consequently the danger of spring ice jamming may be reduced by keeping the water surface in a river at a constant level. This, from an engineering point of view, is very significant. iv. Break-up of Ice cover

along the right bank and was eroded quickly from the lower part of the drilled holes. From Figure 7a it is seen that the right edge of the ice cover was also eroded from below. It is not difficult to explain the faster erosion of the underside of the ice cover at the right bank as the flow there was faster and hence gave a higher shear. It is, however, difficult to see why surface melting should also be- gin from the right bank. The erosion of the underside of the ice cover continued. By Mar. 15 (see Fig. 7b), the thickness of the ice cover has been reduced by several inches and an open water strip along the right bank of the river at the bridge crossing was seen.

on Mar. 15. By Mar. 16 (see Fig. 8a) the thickness of the ice cover was reduced to the order of a foot. The right bank part of the riv- er was open. flowed downstream. When stopped by the ice cover, an ice flow tend- ed to go under it by tilting up its end.

The thickness of the ice cover was further reduced so that it became dangerous to stand on. A large longitudinal crack was seen to develop along the left bank. It would be an interesting exercise in future studies to see whether the longitudinal crack is related to the joints of the diurnal strips of the ice cover. The peak of break-up was over by Mar. 18, and by Mar. 19 break-up had more or less come to an end. The test section of the river was open on Mar. 19 except for an ice strip that adhered to the left bank (see Fig. 8b). By March 31, all ice was gone from the river.

It was noted that near the end of break-up, a slow flow region was created under the residual ice cover due to local high resistance. The ice slush eroded from the upstream ice cover, when transported by the turbulence to this region, tended to deposit itself on the underside of the residual ice cover. When the deposition outweighed the melting of the ice, the residual ice cover thickened rather than thinned. It was observed that the residual ice cover on March 19 was several inches thicker than on March 17. One therefore should not estimate the thickness of an ice cover based on the thickness of the residual ice cover.

During the break-up period, the rate of discharge increased con- tinuously, from about 230 ft3/sec on March 10 to about 700 ft3/sec on March 19. The increase in the rate of discharge seemed to be a com- bined effect of the thawing of snow and ice and the rapid reduction in river resistance following the breaking away of the ice cover. The contribution by the second factor is supported by noting that the rate of discharge increased steadily from the onset of break-up and reached the maximum value when break-up was completed on March 19. The rate of discharge then decreased steadily from March 19 to March 31 before the spring flood in April, which reached a magnitude of 2500 ft3/sec.

This experiment shows that the break-up of a river was concen- trated in the short period of a couple of days accompanied by an increasing rate of discharge. This is the main reason for the fre-

A sign of thawing was first observed on Mar. 10, when ice melted

The erosion of the ice cover was greatly accelerated overnight

Ice floes began to break away from the ice cover and

Active break-up continued on Mar. 17.

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\

quent spring flooding caused by ice jamming. break-up period is different from flood routing at other times in that the resistance of the channel is no longer a constant. The run- off is also fairly uniformly contributed by the melting of ice and snow along a lengthy stretch of the river. Future hydrological study in this area is desirable.

Flood routing for the

v. Behavior of FraziZ Iee The role played by frazil ice in forming an ice cover has been

discussed earlier. After an ice cover is completed, frazil ice pro- duced in upstream open water sections continues to affect the flow. The modification of velocity distribution by the presence of frazil ice in water has been discussed by Tsang [9].

In this study, a great quantity of frazil ice was noted at the Nottawasaga site for several days immediately following the freeze- up, as shown in Figures 2c, 3a, and 3b. Most of the frazil ice accu- mulated under the ice cover in the middle of the stream at the first crossing, where the ice cover formed by frazil ice accumulation had developed earlier (see Fig. 2a). It appeared that the velocity of the flow in the middle of the stream was slowed down by the presence of this part of the ice cover. The slow velocity in turn encouraged the settling down (eg., floating to the top) of the frazil ice clus- ters and a further reduction in velocity. The double vortex secon- dary motion, which is known to exist in open channel flow1, helped to transport the frazil ice to the middle of the stream. The accm- ulation of frazil ice thus divided the main stream into two jets.

Similar behavior of frazil ice was also observed at the Peace River, although it is a much larger river. Figure 9 shows that in the Peace River frazil ice also accumulated under the ice cover in the middle of the stream where the depth was least and hence the vel- ocity would be lowest. The main stream was again parted in two.

posited on the underside of an ice cover behaves like bed sediment in a stream. Thus frazil ice accumulated on the underside of an ice cover would move like sand dunes. Our experiment did not support this belief. It is seen from Figures 2c and 3 that the accumulated frazil ice at the first crossing did not move downstream as a dune to the second crossing, which was only 200 ft away. The frazil ice seemed to stay where it had first accumulated until it was eroded away by the flowing water.

time, became hardened as shown in Figure 2c. the frazil ice nearly misled the data collectors into believing that an anchor ice had formed, until it was found that frazil slush was below the hardened frazil ice. the accumulated frazil pack, which was also noted by the Nater Survey of Canada personnel who collected the Peace River data, was termed "casing effect". It was believed that the "casing effect" was caused by surface hardening and consolidation of the frazil ice pack. It was observed in Peace River that the "casing effect" winter, not long after the complete ice cover was formed, as in the Nottawasaga case. winter and this was thought to be a result of a possible development

It has been the belief of some researchers that frazil ice de-

Part of the frazil ice, after clinging to the same spot for some This hardened part of

This hardening of the upper part of

began in early

Weakening of the "casing effect" was noted in mid-

See p. 469, "Open Channel Flow" by F.M. Henderson (McMillan Pub. Co. 19701, for instance.

782

of holes in the frazil pack or deterioration of the ice pack. was no experimental evidence to support the conjecture above. Nottawasaga study, the hardened part of the frazil pack cbserved on Dec. 29 disappeared the next day. "softening" process of the frazil pack accumulated under an ice cover should be an interesting topic for future research.

Although the accumulation of frazil ice under an ice cover was noted at Nottawasaga River for only three days immediately following formation of an ice cover, the accumulation of frazil ice under the ice cover at the Peace River site persisted during the whole season, from freeze-up to break-up. of frazil ice production in the upstream open-water sections.

The loading of frazil ice in a river definitely increases the difficulty of winter flow monitoring. Even when the river is still open, the summer rating curve may not be used because the loading of frazil ice changes the viscosity of the water and consequently vel- ocity distribution and flow regime. A simple stage discharge curve does not exist as the loading of fsazil ice in a river is not con- stant. The non-uniformity of frazil accumulation under the ice cover also makes the evaluation of the flow area difficult. An accurate winter flow record simply cannot be obtained before a better under- standing of frazil behavior is known.

There In the

The study of the "casing" and the

This was presumably due to the high rate

vi. Sediment Activity and Bed Erosion in Association with Winter Flow One of the aims of the present program was to study sediment

activity in winter flow. itional boundary, the ice cover, would encourage sediment transport. The Nottawasaga site was chosen for its sandy bottom and hence its sensitivity to sediment transport. To study sediment activity at the Nottawasaga site, the river bed contours at the three crossings on Dec. 18, 1970, were used as reference contouss. By superimposing contours of the river bed obtained on other days over the reference contours, the sediment activity was seen.

When a bed contour is superimposed on a reference contour, the area between the contour above the reference contour and the refer- ence contour comprises the sediment deposition. The area between the contour below the reference contour and the reference contour con- stitutes the amount of sediment scoured from the river bed. The sub- traction of the deposition from the scour gives the net erosion of bed material from the crossing. The curves showing the scour, dep- osition, and erosion at the three crossings are shown in Figure 10, where the discharge curve is also displayed.

scour and deposition curves together. Figure 10a shows that, during the later part of freeze-up and after an ice cover has just formed, both the deposition and the scour curves show great fluctuation, in- dicating a very active lateral sediment transport, the result of velocity redistribution discussed earlier. Accompanying this lat- eral activity was the dune movement as may be seen from Figure 10b. It should be noted all this sediment activity took place at more or less a constant rate of discharge. solely the effect of the ice cover. ity and dune movement in the break-up period may also be noted from Figure 10. In this period, as the discharge was increasing, the dune movement therefore also showed a greater amplitude.

It was thought that the presence of an add-

The lateral sediment activity may be seen from examining the

The sediment activity thus was Similar lateral sediment activ-

During its ice-cover period, the river experienced a slow and

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gradual erosion (note that the curves in Figure 10b between Feb. 18 and March 11 do not indicate a decrease in erosion, but the lack of data points). The greatest erosion happened at the second crossing where the depth of the river was the least and consequently the shear of the flow was felt the most. The erosion of the river bed at the second crossing in mid-winter was j.n the order of 20 ft3/sec. width of the river at the second crossing was in the order of 100 ft. This gave a depth in the order of 1/5 ft by erosj-on. The average depth of flow at the second crossing in mid-winter is in the order of 1.5 ft. Erosion there caused an increase in depth of close to 15%. If winter flow metering is calculated from the river contour obtained in the summer, an error of 15% will be conceived. For a shallower river, greater error may be expected. Although the river was eroded in winter months, the greatest erosion occurred during the spring flood, which came after the break-up flood, as may be seen from Figure 8c.

On Feb. 24 ice clusters were noted suspended in the water. Fine sed- iment particles, mostly clay, were picked up by the clusters. In the drilled holes, the dirty ice clusters quickly accumulated. It seemed that the ice clusters resulted from the erosion of the underside of the ice cover rather than from the agglOmeratiOn of frazil crystals; this was because on this day there was no large open-water surface upstream for frazil ice production. The ice clusters, when brought down to the bottom by turbulence, picked up fine sediment particles; this constituted as a mode of sediment transport. This mode of sed- iment transport, to the knowledge of the authors, had not been re- ported elsewhere.

The

A new mode of sediment transport in winter flow was observed.

CONCLUS ION

The present study showed the factors affecting the flow of water, the formation, growth, and break-up of an ice cover, the behavior of frazil ice under an ice cover, and sediment activities in natural rivers in winter. as it is believed that there is not enough knowledge of winter hydrau- lics. Too hasty a theoretical approach therefore may lead to premat- ure modelling. known, further research may be done by isolating each factor. experimental findings reported in this paper are true only for rivers of similar characteristics. Rivers of different characteristics may behave differently .

No theoretical modelling is attempted at this time

After the factors influencing winter hydraulics are The

AC KNOW LEDGMENTS

Part of the work presented in this paper was done under the financial support .of the Dept. of Energy, Mines and Resources, Canada, when the authors were with the School of Engineering, University of Guelph. The rest of the work was done at the Canada Centre for In- land Waters. The authors wish to thank Mr. Paul Graham for his help in the initial development of the program, Mr. Peter Poortinga for his help in engineering installation, and Mr. K.L. Ho for helping in data collection. The authors also wish to show their gratitude to Mr. Hugh McClean, who kindly permitted the use of his property for field experiments.

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REFERENCES

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BARNES, H.T. (1906). Ice formation with special reference to anchor ice and frazil. New York, Wiley.

BARNES, H.T. (1926). Ice formation in the St. Lawrence River. Canadian Eng. J., V.50, No. 2, pp 207-208.

BARNES, H.T. (1928). Ice Engineering. Montreal, Renouf Publ. Co.

PARISET, E. and HAUSER, R. (1961). Formation of ice cover on rivers. Trans. of the Institute of Canada, V. 5, No. 1.

CAREY, K.L. (1966). Observed configuration and computed roughness of the underside of river ice. U.S. Geological Survey Research, professional paper No. 550-B, pp 192-198.

CAREY, K.L. (1967). The underside of river ice. St. Croix River, Wisconsin. U.S. Geological Survey Research, professional paper No. 575-C, pp 195-199.

MICHEL, B. (1963). Theory of formation and deposit of frazil ice. Proceedings of 20th Annual Meeting, Eastern Snow Conference, pp 130-148.

CARSTENS, T. (1968). Hydraulics of river ice. La Houille Blanche-Revue internationale de l’eau, No. 4, pp

TSANG, G. (1970). Change of velocity distribution in a cross- section of a freezing river and the effect of frazil ice load- ing on velocity distribution. Proc. of I.A.H.R. Symposium on ice and its action on hydraulic structures. Iceland. pp 3.2.1.- 11, September.

MOORE, J.H. and WATSON, C.H. (1971). Field tests of ice jam prevention techniques, A.S.C.E., Hydraulics Div.,V. 97, pp

271-284.

777-789.

Review of current ice technology and evaluation of research priorities. (1970). A report by H.G. Acres Ltd., Niagara Falls, Canada.

BENNETT, R.M. (1968). The stage-discharge relationship under ice cover for the Peace and Slave Rivers. M.Sc. thesis, Utah State University.

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100 200 300 400 500 600 700 M I I I '1

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Area - 12,700 It? Discharge - 22,300 cfs Mean Velocity - 1.76 fps

600 1100 1600 2100 M.

a: DECEMBER 1 2 , 1966

100 200 300 400 500 600 700 M. I l l I I I I I

Area - 12,000 ít? Discharge - 19,100 Cfs Mean Velocily-1.59 fps I I I

600 1100 1600 2100 M.

, Fig. 9. Accumulation of frazil ice in the Peace River

79 4

b : TOTAL EROSION AT THE THREE CROSSINGS

Fig. 10. Sediment activities at the Nottawasaga Research Site

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DISCUSSION

L.R. Mayo (U.S.A.) - Two questions: (1) How did you measure the water velocity in frazil slush? (2) How large was the hole you used to measure this velocity with the Ott current meter?

G. Tsung (Canada) - (1) This can be done by making a large hole in the ice. There will be flow through the holes. (2) We had an Ott-meter which was about 12 inches long and therefore had to dig a hole about 15 inches in diameter.

C. ObZed (France) - Y a-t-il apparition d'une convection nat- urelle dans la rivière, provoquée par example par la différence de temperatures entre le fond, les rives et la surface, et dans ce cas quel est son ordre de grandeur par rapport ?i la vitesse du courant principal? (Cette remarque n'est inspirée par votre figure no. 7 et 8.) Avez-vous aussi fait des mesures de temperature sur le fond et dans les rives?

G. Tsung (Canada) - Natural convection is only important when the temperature gradient is large and the turbulence produced by boundary roughness is low. These situations never exist in natural rivers. This natural convection would be rather insignificant in river flow. In ponds and reservoirs, the natural convection may play a larger role. No measurements of either the temperature gradient of the river (and the river bed), or the convective velocity were made. My best estimate would be that the temperature of the flowing water was roughly at its freezing point.

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