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. Les mesures ont porté sur les régimes de 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. 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. Depending on the 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. The spring ice jamming of a river has been the 772 cause of many spring floods. Barnes [l, 2, 31 was perhaps the first person to systematically 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. 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, 773 etc. were also noted. Data at the bridge crossing were collected 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. A taut wire was tied cross the river at the first and second 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. This difficulty was overcome by quickly 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. 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. In Figures 1 to 8, the coordinate system at the three crossings 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. From the Peace River data, similar diagrams were prepared by the Calgary Office of Water Survey of Canada. The diagrams used in this study are shown in Figure 9. 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. As the ice cover began to form (see Figs. lb, 2a, and 2b), the resistance in the ice-covered 774 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.