Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd–6th December 2002 International Association of Hydraulic Engineering and Research DESIGN AND MONITORING OF A PROTOTYPE ICE BOOM FOR

USE IN AN AREA OF HIGH CURRENT VELOCITY

Andrew Liddiard1, Razek Abdelnour1, Raymond-Marie Tremblay2, Tung Thanh Quach2 and Brian Morse3

ABSTRACT A prototype ice boom has been deployed in the Rivière des Prairies in Montréal, Québec, Canada. The section of river generates a significant amount of frazil ice and has a high current velocity such that a conventional boom would be ineffective much of the time. The new design consists of deploying floating nets downstream of round steel pontoons to assist in the formation of an ice cover. The winter of 2001/02 was the warmest on record since 1949 and had a higher discharge than average. Despite this, a partial ice cover was formed on the boom and nets. Following an analysis of historical data for years when an ice cover formed naturally, and looking at the data collected over the course of the 2001/02 winter, a new boom configuration is being developed. There is good reason to believe that the improved boom design will allow a stable ice cover to form across the width of the river at this particular section of the river (Figure 1). INTRODUCTION This paper is complementary to those presented by Quach et al, 2002 and Morse et al, 2002 at this conference. Whereas those two papers discuss the problem and recommendations for improved ice control in Rivière des Prairies (Montréal, Québec, Canada), this paper presents the design of the selected option and subsequent performance monitoring. The problems experienced by Hydro-Québec (HQ) at the Rivière des Prairies hydroelectric generating station are well documented in the other two papers and will only be summarized herein. The hydroelectric plant has been in operation by HQ since its construction in 1926. In 1971 two ice booms were installed

1 BMT Fleet Technology Limited, Kanata, Ontario, Canada 2 Hydro-Québec, Montréal, Québec, Canada 3 Université Laval, Sainte-Foy, Québec, Canada

Figure 1: The Bordeaux boom site showingthe boom in Rivière des Prairies partlyfilled with ice.

along the river upstream of the plant to control problems with frazil ice generation. A 3.5 km reach of the river between the closest boom and the power plant remained open but was not plagued by frazil ice due to warm water inputs (sewer discharges) from the Cities of Montréal and Laval, which are on the south and north shores of the river, respectively. In 1992 these warm water discharges ceased and the power plant began to experience problems with frazil ice. To help combat the problem of ice damming, HQ employed the use of a mini-icebreaker to create a short cut path to direct the frazil through the sheet ice that formed immediately upstream of the plant and over the spillway. This is a less than ideal remedy as it is not only costly to operate but also results in a loss of power to the plant and is not a guaranteed means to prevent flooding. The section of the river that generates the most frazil has a high current velocity such that a conventional ice boom would prove ineffective when the discharge exceeds 1000 m3/s, which is the mean discharge of the river during the start of the winter. This average discharge is about 850 m3/s for the months of January and February. The selected option was to design and install a prototype boom that combines round steel pontoons with floating nets, deployed downstream of the boom. This boom was developed and deployed for the winter of 2001/02. CONCEPT DESIGN AND DEPLOYMENT For more than half a century, booms have been used by hydropower utilities to prevent blockage of their water intakes by debris, ice and frazil. A boom accelerates the formation of a stable ice cover upstream of the power plant thus minimising new ice growth and ensuring the water intake trashracks are free of ice. During the past decade, significant research was carried out to improve the design of a number of booms deployed in rivers and reservoirs across North America. Numerical models to evaluate the forces on these booms generated by both the wind and the current drag forces, with and without ice or debris behind them, were developed. The instrumentation of a number of ice booms provided data to calibrate these models and improve their accuracy. In addition to the force, a better understanding was achieved of the behaviour of the ice during its interaction with the boom for a variety of loading scenarios and a range of environmental conditions experienced during the past few winters. The design of the pontoons used for the ice boom in this study is based on this previous research and development in ice booms going back to 1993 with the New York Power Authority’s Lake Erie ice boom study (Abdelnour et al., 1994) and the Canadian Coast Guard’s Lavaltrie ice boom in the Saint Lawrence River (Abdelnour et al., 1993). The addition of netting behind the boom came partly from a site visit to Umea, Sweden where a net boom has been in use in a canal upstream of a water intake for a number of years (Sahlberg, 1990). This net is attached to a cable that allows it to be easily lowered in and raised out of the canal. The net works well in forming a stable ice cover as there is quite a low current velocity in the canal, however the net is torn and damaged each year during spring break-up, and sometimes during the formation of an ice cover. Such net booms are not uncommon and are generally referred to as tension ice booms. The use of a tension ice boom was one of suggested options in Morse et al. (2002). A tension ice boom is essentially floating netting held in tension by cables that are

anchored to the river bottom. The idea is for the ice to form on the net itself as opposed to a conventional boom that traps the ice as it flows downstream. Morse et al. (2002) rejected this option due to cost and the difficulty of sizing the cables to carry the load of the ice. The prototype boom design overcomes the limitation of a net boom by incorporating benefits of a conventional boom that can support the ice loads on the nets. The boom was designed for the design flow rate of the power plant, which is 800 m3/s. The RMA2 two-dimensional hydrodynamic module of the SMS package was used to develop a numerical model of the river using bathymetry and validation data obtained from an acoustic Doppler current profiler (ADCP). The model was used to visualize the velocities in the river for the design flow rate of 800 m3/s, and a validation discharge of 1200 m3/s. The mean design velocities obtained from the model for the area selected for the placement of the boom were 0.9 m/s and 1.27 m/s for the two discharges, respectively. The validation data indicated that the model was somewhat conservative in that the modeled velocities were, on average, approximately 0.11 m/s higher than the measured velocities. In the area of the boom, and in particular along the north shore, the model gave results that were approximately 0.25 m/s or 17 % lower than the meas-ured values (local measured velocity of 1.45 m/s). However the model results remained conservative along the south shore at the boom location. The intended configuration of the booms and nets is shown in Figure 2. The pontoons were sized at a dia-meter of 0.61 m and a length of 10 m. Each pontoon was attached to the span cable by chains so that the cables were held approximately 1 m below the water surface. The span cables were attached to buoys at each end, which were in turn attached to the anchor cables. The buoys were sized at a diameter of 1.12 m and 1.27 m long. The anchors consisted of 7-m long Dywidag bolts installed in concrete filled holes that were drilled into the bedrock. The anchors are rated for a load of 150 tonnes for the single anchors and 300 tonnes for the double anchors (anchors 2&3, 4&5 and 6&7 in Figure 3). Two types of netting materials and mesh spacing were used in the project: nylon nets with 5 cm spacing and polypropylene with 10 cm spacing. Both types of nets used a square mesh and were fabricated by Barry Cordage Limited of Montréal. Personnel from Barry Cordage also conducted the installation of the nets, assisted with the field observations and carried out the aerial

Figure 3: Configuration of booms andnets as deployed.

Figure 2: Design layout ofbooms and nets.

photography. Each section of the net was 15 m wide by 20 m long and the nets were to be joined together to form six 100-m long sections, three on each of the north and south booms. The two additional downstream booms were included both to catch the nets should they tear or come loose and to assist in the formation of ice around the downstream end of the nets. The boom and netting were deployed during the month of November 2001, while the discharge was approximately 1500 m3/s and the current velocity was 1.5 m/s. The configuration of the booms and nets as they were deployed is shown in Figure 3 (the numbers refer to the anchors and letters refer to the net assembly sections). This image was recreated from aerial photographs taken shortly after installation. Only one net section reached the full 100-m design length; all of the other sections would have reached the downstream booms had the full number of nets been attached. As can be seen, the boom positions were greatly affected by the strong current velocity and the nets did not expand to their full width. The tendency was for the nets to curl and collapse in the current so that at the end of a section the width was less than 5 m instead of 15 m.

Aluminium spreader bars were installed in several of the net assemblies to help maintain the desired coverage. Small floats were attached to the corners and intersections of the nets to provide additional buoyancy. Sections D, E and F were also tied to the downstream boom to provide an additional amount of spread and to keep the nets from getting tangled. Figure 4 shows a close-up photograph of the nets on the southern-most boom and Figure 5 shows a photograph of the booms following deployment.

MONITORING In general, the discharge of the river was higher than normal over the course of the 2001/02 winter, which in turn caused current velocities that were above the critical velocity of deposition of ice from upstream (normally taken to be 0.7 m/s). This was also an extraordinarily warm winter, which is likely the root cause of the higher than normal discharge rate and the delayed ice formation upstream of the boom. The first ice formation event occurred at the end of December 2001 when a strong Southwest wind caused ice to detach from the banks of Lac des Deux Montagnes and

Figure 4: Close–up of net section dfollowing installation

Figure 5: Boom layout following deployment, lookingupstream from the Bordeaux train bridge.

travel down Rivière des Prairies. Much of the ice passed the three ice booms on the river and was deposited a few kilometres downstream of the Bordeaux boom causing a blockage of the river and thus raised the water level. The power plant operators managed to raise the water level between the blockage and the plant by closing the spillway, thereby allowing the water to pass under the ice again. The Bordeaux boom retained some ice, but it was only about 5 cm thick and since the air temperature remained close to freezing throughout January, very little ice formed on the boom. Figure 6 shows how the netting on the north side of the river and the 100 m long section on the south boom are attracting snow and ice, although not necessarily forming a complete ice cover. February was colder than January and an ice cover formed on the Montréal (south) side of the river. A lesser cover was also formed on the Laval (north) side of the river. This difference is attributed to local current velocities, which are much higher on the Laval side. On February 13 the ice around the net was examined and was found to be on the order of 40 cm in thickness. The ice formed on either side of the netting as shown in Figure 7. The upper part was formed by the entrapment and freezing of fallen snow. The lower part was made up of naturally forming ice and the accumulation of ice that passed under the pontoons and re-floated and deposited under the net. Unfortunately, aerial photos could not be taken after January 15 owing to unfavourable atmospheric conditions and the difficulty in obtaining a permit from Montréal airport authorities since the site is close to the airport. Figure 8 shows an estimate of the ice extent around the boom at the end of February based on photos taken from the shore. Two problems that were not anticipated were that (1) the pontoons regularly banged into one another causing a tremendous noise, exceeding the local by-law limit of 50 dB,

Figure 7: Ice formed on net.

Figure 8: Estimated extent of ice cover at the Bordeaux boom at the end of February.

Figure 6: Ice formation on the Bordeauxboom as of January 16, 2002.

and (2) the pontoons which were perpendicular to the flow would submerge and emerge at a regular frequency proportional to the discharge. This second phenomenon is the more interesting of the two since the first can be overcome by either lashing the problem pontoons together for a temporary solution (as was done in this case) or placing the pontoons further apart. The oscillating effect of the pontoons is an indication that the pontoon has insufficient buoyancy for the observed current velocity. The force of the on-coming current creates low pressure underneath the pontoon as the water flows underneath and also an increase in the head immediately upstream of the pontoon. These combine to force the pontoon to submerge, but only to the point where its buoyancy will allow it. Once the buoyant force becomes dominant, the pontoon emerges from the water. Assisting in the emergence of the pontoons is the fact that once the water begins to flow over the pontoon it creates a lift force that works with the buoyancy of the pontoon. This scenario repeats continually resulting in the observed oscillations. An analogous phenomenon was reported by Dean and Dalrymple (1991) for pipelines resting on the seafloor. The obvious solution to this problem is to increase the pontoon diameter to provide additional buoyancy. A third problem that was anticipated, although it was hoped would not occur, was that the nets presented a navigational obstruction to boats. Before the nets and booms could be removed from the river in the spring, a boat motored through the netting, ensnaring its prop and motor and destroying the net. ANALYSIS One of the first tasks that was carried out towards improving the boom performance was to conduct another bathymetric and current profile survey to concentrate on the area around the boom. This was completed on July 18, 2002 and covered a 700-m long section of the river (Figure 9). This data was used in conjunction with the data that was obtained in 2001 to create a new numerical model of the river for just this area. The model was run for flow rates of 600, 793 (validation), 1200, 1500, 1800, 2139 (1 in 10 year winter flow) and 2910 m3/s (1 in 100 year winter flow). The probabilistic determinations of the flows were obtained from fitting the historical winter flow rates (December to March) to a Gumbel distribution. The model validation was similar to that obtained for the earlier model despite the higher concentration of data, although the new model results were closer to the measured values along the north shore. Simulations were carried out for open water conditions, a partial ice cover (similar coverage to that shown in Figure 7) and for a full ice cover. In order to simulate an ice cover with the free-surface model, equivalent Manning’s roughness values for the ice covered portions of the river were computed based on the reduced conveyance that would be seen in an ice-covered river (USACE, 1998). From the simulations with a partial ice cover the velocity in the centre channel increases significantly such that at the

Figure 9: Current distribution obtainedfrom a survey done on July 18, 2002.

higher flow rates the water would likely erode any partial ice cover along the shores. Thus in redesigning the boom it will be important to form an ice cover across the entire width of the river. In addition to running more simulations, the environmental data for the area was also analysed. The historical discharges for the river and the mean daily air temperatures for St-Hubert, Québec (22 km to the southeast) were obtained for the period from 1949 to 1994. The mean daily air temperatures for this past winter were also obtained. The data from winter 2001/02 were compared to the winter 1967/68 where a complete ice cover was known to have formed naturally in the area (Figure 10 shows the ice cover in 1968) and also how the hydraulics and environ-mental conditions compared to the those of the winter 2000/01 including river discharge, water elevation and air temperature. It was found that the air temperature during the winter of 1967/68 was below average where the freezing degree-days value was 808 compared to an average value of 880. What did become obvious is that the winter 2001/02 was the warmest winter for the period between 1949 and 1994 as shown in Figure 11. The freezing degree-days was 434, which is about 100 freezing degree-days below the record low. The average flow for the winter 1967/68 was 992 m3/s, compared to a mean flow between 1949 and 1994 of 947 m3/s (data obtained from Cartierville station, about 3.5 km upstream from the Bordeaux Boom). This is shown in Figure 12. The flow for 2001/02 was comparable at about 1000 m3/s. Figure 12 also shows, the water level increased above 3 m during the winter of 1968 while the average level for the same period was below 2.5 m. The presence of an ice cover resulted in a 0.5 m headloss, and the water elevation remained well below the 4 m, the level that can be reached during the spring runoff (data from 1970 to 1997,

Figure 11: The air temperatures in 2001/02in comparison to historical records.

Figure 12: Comparison of the discharge andthe water level for the data between 1949/94and for 1968.

Figure 10: An ice cover formed in 1968at the same location where the BordeauxBoom was placed in 2001/02.

with no ice). This observation shows that it is possible to form an ice cover in this location and the effect of the ice should not increase the headloss to result in any flooding in the area. It should be noted however that in 1968, no ice booms were placed upstream, so abundant supply of ice to make an ice cover was available. BOOM REDESIGN From the observations made over winter 2001/02 and from the numerical model results, it is more difficult to form an ice cover on the north shore of the river than on the south. Not only is the north shore the outside of a mild bend, but there is also a slight constriction at that point so the water velocity is a little higher than on the south shore. Due to the difficulty in deploying and recovering the nets, not to mention the navigational hazard, HQ has decided that nets will not be used in the future. The new design of the pontoon configuration was on-going at the time of writing this paper, but the most promising concept at the point of writing was to add an additional boom of smaller diameter pontoons downstream of the existing primary boom. The idea behind this is to create a cascade effect that will result in a localized slowing down of the water velocity. It is not believed to be necessary to add these additional pontoons to the boom on the south shore since the velocities are considerably lower and a fair ice cover formed there this past winter. The additional booms will mostly likely be sized with 0.46 m diameter pontoons with 0.61 m diameter pontoons in the centre to resist the submergence forces. The pontoons in the centre of the existing booms will be replaced with larger diameter pontoons; 0.76 m diameter pontoons should provide a freeboard of 0.29 m even at a flow rate of 2910 m3/s. The length of the anchor cables will also be lengthened and shortened accordingly in order to provide an orientation that is closer to the one that was originally envisioned (compare Figures 2 and 3). Additionally, it is planned to assist the formation of an ice cover upstream of the boom by breaking up the shore ice formed upstream and allow it to drift and collect on the Bordeaux boom. CONCLUSIONS A prototype ice boom was designed and deployed in an area of high current velocity on Rivière des Prairies in 2001. The boom performance was monitored over the course of the first winter and environmental data have been analysed to provide a means with which to improve the design for future seasons. HQ recognizes that this is a difficult area to work in and that an ice cover will not form every year. However, given that a partial ice cover was formed in 2001/02 during the warmest winter on record (since 1949), and with higher than average discharge, it appears promising that with a well-designed boom for this site, an ice cover can be consistently formed across the river. The presence of an ice cover in the river will introduce a headloss, that will result in an increase of the water level by about 0.5 m, an increase that is well below the water level recorded during the spring runoff. ACKNOWLEDGEMENT The authors gratefully acknowledge the assistance of the Des Prairies generating station staff for their cooperation throughout this project. The authors also thank Peter Barry and Jean-François Robitaille of Barry Cordage, Montréal, Québec for their assistance with the deployment of the nets and the ice observations.

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