Design | Out in the cold
Mark Baker, Principal of DamCrest Consulting located in the US, gives an insight into cold weather impacts on dams
Above – Figure 1: Cold Regions of the World. Mountainous areas with snow are also considered cold regions (Mountain Warfare and Cold Weather Operations, US Army, 2016) Key
Moderately Cold Severely Cold
Below – Figure 2: Bystanders view the thick ice rubble that clogged the spillway at Vaux Dam, Montana in 1951. The embankment dam overtopped, eroded, and failed. [MonDak Heritage Center]
DAMS IN COLD REGIONS [see figure 1] have the potential to experience ice runs, spillway clogging, gate inoperability/damage, frost heave, inlet blockage, concrete freeze/thaw damage, and hazards to operation and maintenance staff. Cold weather effects on dams may be currently under-incorporated in dam engineering work. Efforts to understand and address these vulnerabilities will make dams in cold regions safer, more reliable, and safer for dam on-site personnel. This article is to raise awareness of cold weather impacts on dams and to suggest the application of existing research and guidelines for dam siting, design, construction, risk analysis, and O&M. [1]
Ice runs, jams, and clogging Seasonally freezing/thawing rivers drain over one
third of the world’s land surface. Ice runs can occur at river dams when winter ice breaks up and moves downstream typically during the spring. The ice rubble varies in sizes and can weigh many tons. During an ice run, rubble can form ice jams at any place but particularly at river bends, constrictions, and bridges. During an ice jam river water impounds upstream, rises, and eventually breaks through the jam releasing a surge of water. This surge of water and ice rubble can impact dams downstream by damaging or failing a dam’s gates. Spillway clogging can also occur, leading to dam overtopping and failure.
Ice runs can be episodic with minor thicknesses of ice passing through a dam each year and major thicknesses of ice runs occurring only after many years or decades. The US has an ice jam historical database (see
https://icejam.sec.usace.army.mil/ ords/f?p=1001:7) with 18,000 ice run events. During siting of new dams or evaluating the vulnerability of existing dams to ice runs, professionals can research a river’s history of ice jams. Dams just downstream of river bends or constructions could be more susceptible to the effects of ice runs. For off-stream dams with river intakes, the use of stout (concrete) intake structures on a side of the river with deep entrances can lessen the damage if ice runs occur.
Mitigation of ice run vulnerability to dams can include the use of: Wide, ungated overflow structures to lessen clogging. Submersible roller gates within broad crested spillways to keep gates out of the path of ice runs.
Left – Figure 3: In March 2019, an upstream ice jam broke and sent a flood with 0.5m thick ice rubble downstream to Spencer Dam in the US. The flood of ice rubble destroyed the powerplant, failed gates, and failed its earthen dike by overtopping. One person died in the event. [Nebraska Department of Natural Resources]
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River jetties and ice booms to direct ice toward less vulnerable stations of the dam Upstream reinforced concrete icebreakers to break flowing river ice into smaller pieces. Designing gate piers and the gates themselves for not only static/hydraulic loading but also for dynamic impact ice loads.[1]
To minimise the impacts of ice jams, dam operations can include monitoring upstream river ice conditions, breaking of upstream ice jams before a large impoundment develops (explosives have been used early in jam development), and deployment of an ice breaking ship. Lowering the reservoir during periods when ice jams may occur may lesson ice jam impacts – however this may not be effective for small reservoirs.
Reservoir ice sheet loading Dams with frozen reservoirs can be subject to
loadings from reservoir ice sheet thermal expansion or sheet movement due to river flow or wind. These loadings can damage gates. Ensuring that all gates are functional can be critically important – several methods can be used to reduce freeze up and accumulation of ice on gates including bubblers, water movers (stirrers), and heating systems. For embankment dams, vertical intake towers
need to be designed for lateral ice loads and the dam upstream slope riprap should be adequately sized to minimize displacement and bedding disturbance due to ice movement.
Concrete freeze/thaw If concrete has cracks, water can enter, freeze, and
expand cyclically which can cause spalling. If not repaired, water can reach steel rebar causing corrosion, and popouts. To avoid freeze/thaw damage, the concrete mix and construction need to be high quality. Any cracks that exist should be promptly sealed and repaired.
Frost
If embankment dam construction has a winter shut down, susceptible materials such as silt may absorb water, freeze, and expand to create a loose layer in the dam which may be more susceptible to internal erosion. To address this concern, a dry sacrificial layer can be placed before winter and removed in the spring.
Expansion of freezing water around structures can cause movement cracks, gaps or gate binding. Frost impacts can be addressed by burying vulnerable areas below frost depth, battering structures away from vertical to reduce the potential for gaps at embankment-structural interfaces, including expansion joints/waterstops, and adding drainage. Internal erosion through zones affected by frost heaving likely contributed to the failure of Hadlock Pond Dam in 2005 [3]
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