Measurement of discharge under ice cover

P.W. Strilaeff Water Survey of Canada, Inland Waters Directorate Department of the Environment, Winnipeg, Manitoba

ABSTRACT: Many difficulties are involved in the measurement of dis- charge under ice cover, particularly in the operation of hydrologic data - gathering networks located in Arctic and Sub-Arctic regions. Difficulties result from extreme ice thickness, double thicknesses of ice, large depths of slush ice, coincidence of spring flows with ice breakup, freeze-up of gauging equipment in sub-zero temperatures, etc. Some of these problems are discussed and procedures used in the operation of networks in northern areas of Canada are outlined. A possible technique for estimation of river discharge using a single velocity in a cross-section is briefly described and a com- parison of accuracy is demonstrated between discharges derived using this technique and measured discharges from standard methods.

RESUME: On rencontre bien des difficultés pour mesurer le débit des écoulements sous les glaces surtout dans les réseaux de collationne- ment de données hydrologiques des régions arctiques et sub-arctiques. Ces difficultés ont leur origine dans l'extrême épaisseur des glaces, les bancs de glace en deux couches, la grande profondeur des couches d'eau et de glace mêlée, la coincidence de la débâcle avec les eaux de fonte au printemps, le gel des instruments de mesure en dessous de O'F, etc.. On discute de certains de ces problèmes et l'on esquisse des méthodes utilisées dans les réseaux de mesure du nord du Canada. On décrit brièvement une méthode d'évaluation du débit d'une rivière 2 l'aide d'un seul vecteur vitesse dans une section trans- versale et l'on compare les résultats, du point de vue de la précis- ion, üvec ceux qui sont donnés par les méthodes habituelles.

The collection of surface water data involves many problems be- lieved to be common to all countries. One of the greatest problems, however, is the collection of continuous records under winter con- ditions in northerly and remote areas. Most of the equipment and techniques used have been developed for data collection in southern areas of the country, and have had to be adapted or modified to sat- isfy the requirements of work in the far north. These adaptations and modifications have been only moderately successful. Collection of surface water data is also often frustrated by physical conditions such as severe low temperatures combined with high winds, "white out" conditions that make aircraft navigation dif- ficult, extreme ice thicknesses combined with slush or frazil ice, and the coincidence of spring flows with ice break-up. Some of the problems in northern areas of Canada, and the procedures used to overcome them, are discussed.

AIRCRAFT NAGIVATION Travel for the purpose of streamflow data collection in north- ern Canada is usually via single-engine aircraft. Common Arctic win- -79.7 ter conditions, such as snow cover, drifting snow, and ice-fog, con- ceal river channels, lakes, and other natural landmarks, making map- reading for navigational purposes extremely difficult. It takes a great deal of concentrated effort under prevailing lighting condi- tions to locate a gauging station. Aircraft travel also becomes ex- tremely frustrating when, for example, hours are spent getting ready to fly only to find a grey blanket of ice-fog settling in around the aircraft. These situations are frequent, and the successful arrival at a station is often a major accomplishment. To assist navigation under these conditions, gauging stations in the Arctic regions are normally equipped with marker pylons, which can be best described as steel tripods, approximately 3 metres in height, with large triangu- lar plates mounted at the apex and painted fluorescent orange (Fig. 1)

. ICE THICKNESS

In Arctic and Sub-Arctic regions, ice thicknesses of 3 m or more combined with large depths of slush are not uncommon. However, it is possible to mitigate or circumvent the complicating factors assoc- iated with difficult ice cover of this nature by the judicious sel- ection of winter measurement sites. It is a rather common occurrence, for example, to find reaches of open water immediately below the out- lets of large lakes, and the erection of a cableway at such a loca- tion reduces the problems of winter measurements. Likewise, a sat- isfactory winter measurement section frequently can be located at the lip of a rapids where thin ice occurs, thus alleviating ice-cutting problems. In addition, backwater effect can be substantially reduced or eliminated entirely by locating the gauging station above a rapids or waterfall (Fig. 2). At the Back River gauging station located at latitude 67'40' North, for example, backwater effect has not been known to exceed 35 cm even though ice thickness at the gauge site exceeds 3 m during most winters.

ICE CUTTING

Where it is not possible to avoid cross-sections with large depths of ice, ice cutting becomes a major obstacle to the proper conduct of winter discharge measurement programs. In Canada, a pow- ered ïce drill is now a common tool for this purpose (Fig. 3). Its power pack, weighing approximately 9 kilograms, consists of a three- horsepower, air-cooled, two-cycle engine; an appropriate set of re- duction gears; and a clutch assembly. Fastened to the power pack by means of a splined connector is a flighted auger shaft, of a length appropriate for penetrating the ice layer, and a toothed cutting head. In the District of Keewatin, where ice thickness averages 2.8 m in late winter, it is common practice to drill the necessary holes in two stages; a 1.5-m length of auger shaft is first used, followed by a second shaft of longer length. A multi-purpose tool is used to measure ice thickness where only a simple ice cover exists (Fig 4). The graduated shaft of this measuring device is fabricated out of wood or metal and has attached, at its lower end, two perforated semi-circular plates that fold up- wards from a normal horizontal position. When this tool is thrust downwards through the ice holes, the plates fold to allow easy pas- sage. As it is raised the plates return to a horizontal position, thus permitting the detection of the underside of the ice for the

798 purpose of determining ice thickness, When the tool is moved upward through the drilled hole, ice chips and fragments resulting from drilling are easily removed.

OBSERVATION OF WATER DEPTH AND VELOCITIES

Coincident with the increased use of the powered ice drill, it became necessary to re-design standard metering squipment in order that it might be accommodated in a drilled hole of approximately 20 cm in diameter. Two weight assemblies with a maximum diameter of 16 cm were developed. The "Calgary" type weighs 18 kilograms and em- ploys a teardrop shaped lead weight. The llWinnipeg'ltype weighs 8.2 kilograms and also employs a teardrop shaped weight with some modification for lower resistance to the current (Fig. 5). Both types have proven to be fairly stable in velocities of less than 2 m per second. Incorporated within the framework for the weight is a modi- fied pattern 622 Price-type current meter. Where the depth of water is greater than 3 or 4 m, a sounding reel or handline is used to suspend the weight assembly. The reel is mounted on a collapsible support set on runners (Fig. 6). Because of the extremely cold weather, the support is equipped with a chamber heated by catalytic heaters to prevent the meter from freezing while moving the equipment from one metering position to the next. Where the depth of water is less than 4 m, a graduated steel wading rod is used.

SLUSH ICE CONDITIONS

The problems presented by extreme ice thickness are aggravated further during freeze-up by the formation of thick slush ice layers, which adhere to the under-surface of the ice cover. Slush ice forms on the surface of moving water exposed to sub-zero air temperatures. The slush ice, in the form of ice panes, slivers, and crystals, is swept under newly formed ice blankets and becomes trapped. Heaviest slush ice formations occur in the early winter'and then gradually dissipate or form a part of the solid ice cover. Where the combined thickness of ice and slush does not exceed 6 m, a sectional flanged slush pole is used initially to loosen the slush formations suffic- iently to allow easy passage of the weight and meter assembly. Where the combined thicknesses are greater than 6 m, a specially designed, elliptical weight is used to penetrate the slush horizon (Pig. 7). This weight, which has enlarged tail fins,. is suspended at a point slightly off centre of gravity, permitting it to assume a vertical, nose-up position when suspended in the air. When the weight is low- ered through the slush ice in this position, the enlarged tail fins act as cutting and breaking edges. When lowesed below the interface between water and slush, a slight current is sufficient to force the weight into a horizontal position. Accurate determination of the depth to the interface is obtained by raising the metering assembly until the meter rotor stops.

RECORDING OF WATER LEVELS

Low temperatures, permafrost, and rocky terrain preclude the use of conventional, float-operated, water stage recorders. Two types of pressure water level recorders, both of which utilize nitrogen bubble systems to transfer river stage to the recording pen, are being used exclusively to record stages at isolated locations in northern Canada.

799 One type of recorder is the mercury manometer gauge, which balanccs a column of mercury against the pressure due to river stage (Fig. 8). The other type of recorder is the "Winnipeg" type pressure gauge in which the pressure due to river stage is transferred to the recording pen by means of mechanical linkage (Fig. 9). To operate at prevalent temperatures, it is necessary to provide a heated shelter for the mercury manometer gauge in order to maintain the temperature of mer- cury above its freezing point. The "Ninnipeg" type gauge does not require heat for its operation. However, the Stevens strip chart drive, which is common to both of these instruments, is unreliable at temperatures of about -40'F; consequently a variety of propane fuelled heaters are in use in northern Canada. Experimentation with electrical heat, provided by a wind-driven generator (Fig. lo), was carried out in the Northwest Territories, but fuelled heaters were found to be more reliable and economical. Experimentation with special low temperature lubricants such as "Moebi us Syntalube" in- dicates that the reliability of the Stevens strip chart recorder can be improved to temperatures of -45°F.

ALTERNATIVE STREAMFLOW MEASUREbIENT TCCINIOUE

Standard procedure in the making of a current meter discharge measurement requires that the total area of the cross-section at the place of measurement be divided into small or partial sections, and that the area and the mean velocity at each section be determined separately. The sum of the discharges from all the partial sections is the discharge of the stream. If correctly applied, this procedure is the most accurate of the known methods of measuring discharge. However, it is time consuming particularly when making measurements under ice cover, and a possible alternative is described. This alternative is based on the belief that a relationship exists between the discharge computed on the basis of a single vel- ocity in a cross-section and that obtained using the standard method. In pursuit of this possibiliry, an analysis was made in 1968 of a number of gauging stations located at points ranging from the U.S. Canada border to near the Arctic circle. A paper was presented on the subject at the International Hydrological Decade Symposium on Hydrometry at Koblenz, Germany, in September, 1970. This paper is a summary of a report by Messrs P.W. Strilaeff and W. Bilozor dated September, 1971, which was based on a more in- tensive and exhaustive analysis of the relationship. It explains the principles and some of the procedures involved when applying the single-velocity method in discharge measurement work.

1. UnderZying PrincipZe (1) Single-velocity method of measuring discharge requires three parameters: Single Velocity observation; Cross-Sectional Area for time of velocity observation; and Coefficient of Relationship between the product of the above two parameters and actual discharge. (2) Product of observed single velocity and cross-sectional area is related to actual discharge through the cross-section (Fig. 11). (3) The relationship is depicted as a straight line when plotted on rectangular paper except for near minimum flows. If the actual discharge is represented by the vertical ordinate, 800 the lower portion of the relationship generally deviates slightly in a concave direction from the straight line. This deviation is due to the higher percentage of near nil flow that naturally occurs at river banks at minimum flows. Similar deviation can be expected to occur for flood flow conditions when velocities of the large volumes of water outside the river banks are generally much lower than the average within the cross-section. However, the deviation shown from the straight line relationship in this case will be in a convex direction. This relationship is not subject to normal shift in control. It has been found that, although a stage-discharge relation- ship can have three or four rating curves for each year, the relationship of actual discharge to the product of single velocity and cross-sectional area remains constant. Any method for measuring discharge that does not directly record velocity distribution in the vertical and horizontal directions within the entire cross-section is subject tcl errors, due to irregularity of velocity distribution in nat- ural streams. Fluctuations in velocities also are frequent- ly evident and velocities observed at one point a few min- utes apart may vary by a large percentage. The inaccuracies in the single-velocity method due to fluc- tuation of velocities can be minimized by extending the length of period of observation beyond the normal minimum period of 40 seconds. It can be noted from Tables 1 and 2 that measurements within acceptable percent deviations from discharge measurements made by regular techniques can be expected through use of single-velocity method. Even greater reliability will pre- vail when velocity is observed for longer periods. In add- ition, it has been found that stage-discharge relationships developed from single velocity measurements are virtually coincident with stage-discharge relationships drawn using discharge measurements obtained by regular method. 2. SingZe VeZocity (1) For best accuracy velocity should be observed at or near the point where the velocity is at its maximum for the cross- section, and as far as possible from boulders, bridge piers, and other obstructions. This location can be determined from examination of notes for regular discharge measurements obtained to determine Coefficient of Relationship described below. (2) It is recommended that several separate observations of the velocity at a point be made and the average be used for computation of dishcarge. 3. Cross-Sectional Area (1) An accurate determination of the cross-sectional area of the stream is required for the time of single velocity ob- servation. (2) For convenience it is necessary to develop stage-area curve or table. This can be done by obtaining accurate soundings at low river discharge, or by extracting areas from regular discharge measurements obtained to determine Coefficient of Relationship. Om0 ClrtNwMdu) d\Ddu)NdmOrr~ ol ...... wU~OrlNrl0NOrrOrrrlrlrl~~ddW-I +I+ +++III+ 11+++1++11

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803 (3) Once stage-area curve or tables are developed, periodic check of the soundings are required; frequency of this check is dependent on stability of streambed, but a check after each high water period may be warranted. 4. Coefficient of ReZationship (1) Coefficient of relationship is obtained by dividing the actual or measured discharge by the product of single uel- ocity and cross-sectional area for time of velocity obser- vation. (2) In practice the coefficient is computed from the relation- ship given in Figure 11. 5. AppZicahiLity of SingLe-Velocity Method for Measurement of Dischurge under Ice Cover (1) Analysis made to date indicates that the method is applic- able for measurement of discharge under ice cover (Fig. 11). (2) Its application to winter work, however, is more involved as it is necessary to record ice thickness across the cross- section in order to compute the cross-sectional area applic- able to the time of velocity observation. Opportunity ex- ists, therefore, for development of an ice thickness recor- ding instrument. (3) Because the coefficient of relationship is not affected by normal backwater conditions, accuracy gained through stab- ility of the relationship can outweigh accuracy lost through insufficient ice thickness information. Further study is required to prove this statement. 6. Comments One of the objectives of research into the single-velocity method of measuring river discharge was to automate as much as possible the work of stream gauging. With an automated method a velocity meas- uring instrument would be anchored firmly in the streambed and the velocities would be recorded at constant depth, eg., 1 m above the streambed. Research to date involved mainly velocities at 0.2 m depth. It is felt, however, that since velocity distribution in the vertical is normally a parabola, the method described in this paper applies also when velocities are observed at constant depth. Greater care, however, is required in its application. Because of the par- abolic distribution of velocities in the vertical, the velocity pro- file near the streambed is relatively flat and can be subject to fluctuation due to roughness of the streambed. This problem can be minimized through judicious selection of the vertical at which the velocity recording instrument is to be located, and through placement of the instrument as far above the streambed as possible. Further research into this aspect of the method will. be done upon advent of a suitable recording velocity meter. In the meantime possibility exists for the use of the single-velocity method with conventional instruments to reduce manpower requirements in hydrometric surveys. For the reader's information I have added a list of "Suggestions for Additional Reading".

SUGGESTIONS FOR ADDITIONAL READING ANDERSON, w. (1968). Combination bearingless deflection vane and stilling well, U.S. Geological Survey, Water Resources Division

804 Bulletin Oct. - Dec. BEAUMONT, E.L. and WARREN, J.D. (1971). Application of the digitizer for streamflow computations, U.S. Geological Survey, Water Resources Division Bulletin, Apr. - Sept. BILOZOR, W. (1971). Application of single-velocity method of measuring discharge under backwater conditions, Department of the Environment, Inland Waters Branch, Water Survey of Canada, Winni- peg, Manitoba, Internal Circular. CARTER, R.W. (1965). Streamflow and water levels - Effects of new instruments and new techniques on network planning, IASH, Inter- national Symposium on Design of Hydrological Networks, Quebec. CORBETT, D.M. (1962). Stream-gauging procedure, U.S. Geological Survey Water Supply Paper 888.

DU", B. (1965). Three-section method for checking existing high end of a rating curve, U.S. Geological Survey, Water Resources Division Bulletin. ENGEL, P. (1972). Fixed point velocities for discharge measurements in open channel flow, Department of the Environment, Inland Waters Branch, Water Survey of Canada, Halifax, Nova Scotia, Internal Report.

LeFEVER, F.F. (1961). Index Method of Rating Gauging Stations and Computing Records, U.S. Geological Survey, Water Resources Div- ision Bulletin, May.

STRILAEFF, P.W. (1968). Preliminary investigation into relation- ship between discharge computed using single velocity in a cross- section and discharge measured using standard techniques, Depart- ment of Energy, Mines and Resources, Inland Waters Branch, Water Survey of Canada, Winnipeg, Manitoba, Internal Report.

STRILAEFF, P.W. and BILOZOR, W. (1971). Single-velocity method of measuring discharge, Department of the Environment, Inland Waters Branch, Water Survey of Canada, Winnipeg, Manitoba. STRILAEFF, P.W. and WEDEL, J.H. (1970). Measurement of discharge under ice cover, Proceedings International Symposium on I-Iydrom- etry, Koblenz, Germany, September.

Water Survey of Canada. (1972). The development of the Braincon depth-velocity meter, Department of the Environment, Inland Waters Branch, Water Survey of Canada, Halifax, Nova Scotia.

WELCH, C. (1969). Digitizing graphic deflection vane record for streamflow computation, U.S. Geological Survey, Water Resources Division Bulletin, .July - September.

805 .. . ..

Fig. 1. Marker pylon at gauging station

Fig. 2. A section of rapids which remains ice free year round on the Back River (Latitude 67" 40")

806 Fig. 3. Powered ice drill

Fig. 4. Multi-purpose tool for measuring ice thickness

807 Fig. 5. Weight assembly, Winnipeg type, incorporating a modified Price-type current me ter

Fig. 6. A heated chamber below the reel preve nt s the meter from freezing when not submerged 808 Fig. 7. Specially-designed elliptical weight in nose-up position

Fig. 8. Mercury manometer water level recorder

809 Fig. 9. "Winnipeg-type" pressure water level recorder

Fig. 10. Wind-driven electric generator

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811 DISCUSSION G. T’sang (Canada) - The technique described in the paper does give an estimate of the discharge. However, theoretically it is difficult for one to accept the technique, as a one-to-one corres- ponding relationship between the rate of discharge and the velocity at one point only exists when: (1) The velocity distribution and the flow area are self-preserving, eg., the velocity distribution contours and the river cross-section are always similar at different river stages, and (2) The point at which the velocity is measured is characteristic.

As the metering technique does not take these points into con- sideration, the technique will only work when the flow is in the rough turbulent regime, or to a less accurate extent, when the flow is in the smooth turbulent regime; in these instances, the velocity distribution throughout the flow area is uniform except near the boundary, so that the above two requirements are automatically sat- isfied. In a wide, deep river with a fast flow, the portion of the flow affected by the boundary is small, and the one point metering method given by the paper should give an acceptable measurement. The technique is certainly doubtful in measuring flow in an irregu- lar, narrow, or shallow river near the laminar flow regime. In fact the curve in Fig. 11 does show that in the low discharge range the straight line relationship begins to deviate. The rate of flow obtained by the technique is a simple multip- lication of the single point velocity by the flow area. The flow area is calculated from the cross-sectional contour surveyed pre- season and the thickness of the ice cover. The paper does not point out whether the thickness of the ice cover should include only the solid ice or the solid ice and the slush combined. The presence of frazil in a river can greatly change the velocity distribution. The accumulation of frazil to the underside of an ice cover is uneven so that its effect on velocity distribution modification is highly un- predictable. The accumulated frazil pack has also been found to be at times permeable to flow and at other times impermeable to flow. The ignoring of the frazil presence in a stream would certainly lead to large errors. For some rivers, such as the Peace River at Peace Point, frazil accumulation is always abundant throughout the season, and for all rivers great quantities of frazil are produced in the freeze-up period. It will be interesting to see whether the tech- nique works in such a situation. For streams with sandy and silt bottoms, it has been shown by field experiments that bed erosion takes place during the ice-covered period. Bed erosion will affect the flow area and introduces another source of error. Periodic sounding during the ice cover period will reduce this error. It is puzzling to see that the technique attempts to reduce the measurement error by time averaging, although completely overlooking space averaging. Backwater effect is mentioned in the paper. However, it is difficult to see there is any connection between backwater effect and river metering. To sum up, the technique lacks theoretical support. It should be viewed as an empirical procedure only applicable to some rivers under some conditions. A wider adaptation of the technique has yet 812 to be justified. The paper does contain some good information about winter hydro- logical instrumentation that is worth learning. P.W. StriZaeff (Canada) - Mr. Tsang you indicated that you do not feel that measurement of velocity at a single point in a stream would be representative of that over the cross-section. This was also our concern when we began research on this subject. However, we found that (as indicated in Figure 11) if care is taken in sel- ecting the vertical, a simple point measurement may be representative. Because every velocity vector within a given cross-section is inter- related to a certain extent, it is reasonable to expect the relation- ship that we found. With regard to ice thickness, it is necessary to measure the thickness at the time the single velocity measurement is made. This procedure must be followed in any winter discharge meas- urement and the volume of ice must be subtracted from the total cross- section area. M.C. Quick (Canada) - In view of the uncertainty of the cross- section area, do you know whether any work has been done in using’ salt solution or dye techniques to define this area? If so, were they successful, and if not, why? P.W. StriZaeff (Canada) - Yes .. these techniques have been tested but they are entirely different in concept to the approach presented in my paper. The method I described requires consistent evaluation of the cross-section area. The frequency of m,easurement depends on the stability of the section. The advantage of the single-velocity method is that one can makc an observation quickly and in time obtain the cross-section area.

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