During the winter months, production of frazil ice upstream of Jenpeg Generating Station in northern Manitoba has been known to cause operational problems, including the blockage of powerhouse intakes. To solve this problem, owner Manitoba Hydro deployed a new ice boom, replacing a previously existing ice boom that was first installed in 1988. The new ice boom is designed to form and retain an ice cover upstream of Jenpeg Generating Station in order to minimize the amount of frazil ice buildup at the intakes.
Understanding the problem
The 168-MW Jenpeg Generating Station is the most upstream component of a system of dams along the Nelson River in the Canadian province of Manitoba. Jenpeg’s powerhouse and spillway structures are used to control and regulate the outflow waters of Lake Winnipeg whenever the lake level is between 216.7 m (711 feet) and 217.9 m (715 feet). When Lake Winnipeg water levels are outside of this range, operation of the Jenpeg Generating Station is dictated by the operating license conditions.
|The new ice boom for the 168-MW Jenpeg Generating Station was designed in three distinct sections and was moved upstream 300 m from the existing ice boom.|
To solve the frazil ice problem at the Jenpeg Generating Station, the only feasible option was to redesign the existing ice boom. The option of deploying concrete piles was briefly contemplated, however such a structure may have caused significant headlosses and would have been much more expensive and difficult to construct in comparison to an ice boom.
The existing ice boom at the Jenpeg Generating Station was installed in 1988. This ice boom successfully retained the ice during each winter, however it was not placed in the best possible location. During the spring and summer months, the currents at this location were so high that they were wearing out the components of the boom. The old boom required annual repair and maintenance because the components would fail frequently due to fatigue wear from excessive water movement.
It is for this reason that the new ice boom was relocated 300 m upstream of the old boom to a location with less current. Remarkably, relocating the boom allowed for the span cable thickness of the new boom to be reduced to 1.75 in diameter from 2.75 in.
In 2011, to avoid annual maintenance costs, Manitoba Hydro decided to install the new boom before the 2011-2012 winter season. The company hired Geniglace Inc. to design the new boom.
Designing the new ice boom
The new ice boom is designed to form an upstream ice cover during the freeze-up period, to retain the ice in winter and to resist the mobile ice forces during the breakup period. It is also designed to minimize the wear and tear on the components from high currents that occur during the times of the year when no ice is present.
Below are the design criteria for the new ice boom:
– It must be permanently installed;
– It should retain the majority of ice driven by wind, currents and waves;
– It should resist ice forces during the freeze-up, winter and breakup periods;
– It must be able to operate when water levels are between 213 m (689.9 ft) and 218 m (715.2 ft);
– The boom should be placed in the area of the reservoir with the least amount of current; and
– The boom should be placed as close as possible to the dam, to minimize frazil ice generation in open water areas, without sacrificing its efficiency.
The new boom is 311 m wide and divided into three sections, consisting of varying numbers of pontoons, with different spacing for each section due to the different currents across the channel. The layout of the boom was selected based on bathymetry and velocity profile data collected and modeled by Manitoba Hydro. Dividing the boom into three sections instead of one section allows for more flexibility under ice loading. The pontoons were made from steel pipe with elliptical-shaped end caps. The pontoons are individually connected to a steel cable with a diameter of 1.75 in that is suspended 1.3 m below the water level. The new boom is fixed to the channel by four anchor points, one on each shore of the river and two in the middle of the river.
There was concern that moving the boom upstream by 300 m might be problematic because it would initiate more frazil ice to be generated downstream of the boom. However, it was decided that the impact of this short distance for frazil ice production should be negligible.
As requested by Manitoba Hydro personnel, the boom was designed to resist debris but not to collect and retain it, as Jenpeg Generating Station already has a debris removal system at the dam where debris is removed regularly. Therefore, the gaps between the pontoons were made to be large enough to allow some of the debris to pass.
Installation of the new ice boom
The boom spans were assembled from the shoreline using a lift truck with an extending fork. Each pontoon was lifted and the chains and shackles were then attached to the span cables. The anchors were drilled using a barge equipped with a 50-ton drill. Full construction was completed in less than two weeks.
Installation of the new ice boom was completed in the fall of 2011, and it formed a stable ice cover in the reservoir over the winter of 2011-2012. The new ice boom has operated flawlessly and without maintenance since its installation three years ago and has resisted the ice formation and breakup periods of three winters thus far. Most notably, the new ice boom withstood the winter of 2013-2014, which was the worst winter on record in more than 20 years in northern Manitoba.
– By Elie Abdelnour, P.Eng., senior engineer, Geniglace Inc.; Razek Abdelnour, Ing., president, Geniglace Inc.; and Jarrod Malenchak, PhD, P.Eng., section head sediment and ice studies, water resources engineering, Manitoba Hydro