A novel methodology to quantify hydraulic conveyance through an ice-impacted lake-outlet system

Abstract

River ice can pose considerable challenges to hydropower operations in cold regions. Relative to open-water conditions, the presence of ice in various forms contributes to lower hydraulic efficiency. At the outlet of Lake Winnipeg, maintaining winter conveyance through the channels of the Outlet Lakes Area (OLA) is of paramount importance for downstream energy production on the Nelson River. The complex hydraulic conditions of the OLA are attributed to various factors, including the influence of hydraulic system operation, flow routing through lakes and rivers with non-uniform attributes, and the presence of dynamic ice processes throughout winter. In this study, a methodology was designed to quantify the impacts of ice on the OLA hydraulics using a combination of numerical modelling and statistical techniques. This approach includes adaptation and configuration of a two-dimensional river ice model, selection of suitable performance metrics, and derivation of hydraulic datasets for model simulations using historical observations and statistical relationships. Numerical modelling simulations feature treatments for predominant ice processes in the OLA, including freeze-up jamming, thermal growth and decay of a stable ice cover, and under cover transport and deposition of brash ice. The methodology was evaluated using historical winters 1996–1997 to 2020–2021, with most results falling within an acceptable performance range, even in response to varying hydro-meteorological conditions. Instances of error exceeding accepted thresholds were attributed mainly to factors outside of the study, including uncertainty in discharge estimation and the effects of mid-winter forebay operation. Influence of Lake Winnipeg water levels on winter hydraulics was identified as significant, as higher upstream Lake Winnipeg water levels can mitigate ice impacts by providing more hydraulic head for conveyance. As expected, ice restriction severity also varies in accordance with heat fluxes. Targeted areas for improvement of the methodology and future work are discussed, including enhancing the simulation of the thawing (‘pre-breakup’) period and field verification of flow split quantities during winter. The methodology presented provides an estimate of ice-impacted hydraulic conveyance which serves to benefit hydropower operators, regional water resources studies, and further investigations of river ice impacts that can be attributed to future climate change.