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Riprap Design Criteria related to Ice Movements

Author(s): Isabelle Theriault; Tristan Aubel

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Keywords: River Ice; Ice-Structure Interaction

Abstract: Hydro-Québec is reassessing dikes safety on both sides of the Beauharnois canal as required by the Dam Safety Act. Initially assessment is only performed considering wave loading on the riprap. However, the February 2020 ice jam in the canal showed that ice loading must be accounted for in the riprap design. The combination of an exceptionally high discharge from Lake Ontario with a 24-hour persisting wind of 40 km/h is the most probable cause of the lac Saint-François ice cover break-up. The ice then drifted through the Beauharnois canal and jammed upstream of the powerhouse and along the dikes of the canal. The hydraulic conditions having led to that event were reproduced using a 2D numerical model. The ice cover stability is assessed by computing the resulting force acting on the ice cover. When that force is higher than one of the breakup modes resisting forces, breakup occurs. To identify the most critical case, sensitivity simulations were done by varying ice thickness, compressive strength and wind speed. The resulting ice thickness was selected to design a stable riprap in response to ice loading using the relation between riprap size, slope, ice thickness and compressive strength proposed in Carter’s Practical Guide to Calculation of Ice Forces [Carter, 2003] and established through his interpretation of 1996 Cold Regions Research and Engineering Laboratory scale model results of damage assessment on banks covered by riprap submitted to moving ice [CREEL, 1996]. Since Hydro-Quebec has few sites where interaction between moving ice and riprap has been observed, the intention is to pursue the investigation by collecting data of either the successes or failures experienced by other dam owners or embankments protected by riprap. This will make possible the construction of a database to refine and validate the computation method.

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Year: 2022

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