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Abstract Major ice jams forming in the lower reaches of Peace River during the spring breakup of the ice cover are the main agents of flooding and replenishment of the Peace–Athabasca Delta ecosystem. Their paucity following construction of the Bennett Dam (1968) and the potential impacts of climate change have raised concerns regarding habitat degradation. The most recent ice-jam flood, which occurred in 2014, was the fifth in the post-regulation period. Relative to previous occasions, the 2014 event was monitored in considerable detail. Unusually complete water level recordings at Delta hydrometric stations, high-water-mark elevations, and aerial observations furnish adequate quantitative evidence for application of numerical models. Ice-jam modelling capability is essential to quantification of flow magnitudes that help replenish the Delta and can facilitate assessment of climate-change impacts on flood frequency and development of adaptation strategies. Following a brief chronology of the 2014 ice breakup in the lower Peace River and presentation of key hydrometric data, the user-friendly, public-domain model HEC-RAS is briefly described and its setup for the lower Peace River outlined. It is then calibrated and validated on two ice jam configurations that occurred on distinct dates. Application of the calibrated model to different ice-jam scenarios indicated that jams of moderate, rather than extreme, length are more effective in flooding of the Delta. Highlights • Peace River ice jams cause ecologically vital flooding of Peace-Athabasca Delta • Ice-jam flood of 2014 only fifth since regulation in 1968 and first since 1997 • Relatively well documented event; adequate data for numerical modelling • HEC-RAS model calibrated and validated on jam configurations of May 3 and 7 • Model used to determine optimal jam configuration for widespread flooding [ABSTRACT FROM AUTHOR]

The breakup of the ice cover in cold-region rivers is a brief but seminal period of their hydrologic regime, with important ecological and socio-economic implications. The main driver of ice breakup processes is the flow discharge hydrograph. It is generated by runoff from snowmelt and rainfall but can be modified by rapid release of water from storage as the ice cover recedes by ablation and mechanical breaking up. Despite its potential importance, there is very limited quantitative information concerning the hydrodynamic processes that control storage release during ice breakup in rivers, while the issue of climate change underscores the need for improved understanding of the relevant mechanisms. Quantitative analysis for assumed prismatic channels shows that ablation and sustained ice dislodgment and breaking can cause significant flow enhancement via storage release. The latter process is far more dynamic than ablation and, under certain conditions, may lead to formation of a self-sustaining wave (SSW). Analytical results are applied to natural stream conditions, using the Lower Peace River as a case study. Observed rates of ice recession typically indicate ice melt as the dominant process. A rare occurrence of rapid ice breaking over hundreds of kilometres (2014) indicated partial agreement with the SSW concept and revealed that discrepancies may arise from the characteristic irregularity of rivers. Impacts on storage release by climate-driven changes to river ice regimes are examined, along with their implications to ice-jam formation and associated flooding. [ABSTRACT FROM AUTHOR]

River concentrations of suspended sediment and particulate contaminants, such as trace metals, increase sharply during ice breakup, with potentially detrimental ecological impacts that may be complicated by changing climatic conditions. To enhance the very limited knowledge on this issue, comprehensive data have been collected on the Saint John River (SJR). During breakup, the suspended sediment concentration (SSC) was found to rise gradually, crest, and decline, roughly in step with the runoff, but occasionally spiked to extremely high peaks. The latter ranged from 4.2 to 6.5 times the runoff-generated peak concentrations (RPCs), which ranged from 35 to 150 mg/L. Peak RPCs and individual-event sediment loads generally increased with flow discharge. The sediment spikes were invariably associated with waves resulting from releases of upstream ice jams and with the ensuing ice runs. Concentration–discharge graphs exhibited pronounced clockwise hysteresis, indicative of sediment supply constraints. This feature is more prominent in high-runoff events and typically associated with a lag of 1–3 days between peak concentration and peak discharge, which arrives later. Prediction of SSC via sediment-rating curves is hopeless, but such curves can be helpful in computing loads associated with individual events using a modified approach that terminates load computation 3 days after the arrival of peak discharge. The bulk of the sediment load is delivered on the rising limb of the hydrograph and is likely to be missed in routine sediment monitoring programs. Practical steps to capture this information are suggested. Increases in SJR spring flows during recent decades are projected to continue under a warming climate, resulting in considerable increases of SSCs and loads by the end of this century. [ABSTRACT FROM AUTHOR]

The Peace-Athabasca Delta (PAD) in northern Alberta is one of the world's largest inland freshwater deltas, home to large populations of waterfowl, muskrat, beaver, and free-ranging wood bison. Following construction of the Bennett Dam in the late 1960s, a paucity of ice-jam flooding in the lower Peace River has resulted in prolonged dry periods and considerable reduction in the area covered by lakes and ponds that provide habitat for aquatic life in the PAD region. Past studies have shown that both regulation and climate have contributed to the drying trend, via increased freezeup levels and reduced breakup flows, respectively. However, it has not so far been possible to assess the relative impacts of these two factors. This question is addressed herein by developing quantitative relationships based on the concept of conditional probability. The new methodology is first tested against observed frequencies of ice-jam floods both before and after regulation; it is subsequently applied to quantify the effects of climate and of regulation on the post-regulation reduction in ice-jam flood frequency. The results indicate that both factors have contributed significantly to the drying of the PAD, with regulation having had the more pronounced effect. Complications that may arise from secondary effects of regulation on breakup flows or of climate on freezeup levels are discussed and quantified. It is shown that such effects do not alter the overall conclusions of the paper. [ABSTRACT FROM AUTHOR]

Abstract: River ice jams can cause extreme flood events with major consequences to infrastructure, riverside communities, and aquatic life. The emerging issue of climate change and the growing appreciation of related ecological linkages underscore the need for process-based predictive capability, such as theoretical advances and numerical modelling. A key element of such capability is the internal strength of the rubble comprising an ice jam, which is quantified by a few empirical constants, most notably the angle of internal friction, φ. Though this angle is considered a material property, there is considerable variation in reported values, which derive from applications of established theoretical concepts to actual case studies. The source of this discrepancy is identified by re-examining the theoretical formulations of ice jam stability and noting certain restrictive assumptions that were made in early literature. A less restrictive analysis takes into account the three-dimensional state of stress within an ice jam and leads to a more correct formulation. The resulting φ-values are consistent with experimental data obtained with specially-designed experimental setups. Relevant information can also be gleaned from more recent developments, intended to describe the dynamic evolution of ice accumulations into ice jams, via adaptations of constitutive laws that were originally developed for sea and lake ice. Analysis of the constitutive equations indicates that it is only when full lateral confinement occurs and the static (ice jam) condition is approached that φ can be considered a material constant. For this late phase of ice jam formation, the constitutive law appears to under-predict the value of φ while over-predicting the ratio of lateral-to-streamwise stresses. [Copyright &y& Elsevier]

Abstract: Since the late 1960s, a paucity of ice-jam flooding in the lower Peace River has resulted in prolonged dry periods and considerable reduction in the area covered by lakes and ponds that provide habitat for aquatic life in the Peace–Athabasca Delta region. To help identify the causes of this trend and assess future conditions under a changing climate, a field observation program was conducted during 1999–2003. Limited data were also obtained during a significant event in 2007. This program has furnished the first detailed documentation of ice processes in the lower Peace. Though focusing on the breakup period, relevant antecedent conditions were also documented. Freezeup often involves formation of thickened ice covers, which lead to high freezeup stages and potentially to enhanced growth of the solid-ice layer during the winter. Ice decay prior to breakup can be moderate to severe, and is manifested in saturation and candling. The type of observed breakup ranged from essential melt-out in 2000, to occasional jamming upstream of the delta reach in 2003, and to a moderate delta flood in 2007. The field observations and measurements, including the only ice jam profile that has been measured in the lower Peace, are shown to be consistent with current physical understanding of breakup and jamming processes. The main factors conducive to ice-jam flooding are identified and their relative importance is assessed using physically-based breakup criteria. Breakup patterns are shown to be strongly influenced by the very low slope of the river, which amplifies the importance of waves generated by ice jam releases. [Copyright &y& Elsevier]

Abstract: River ice jams can cause extreme flood events with major consequences to infrastructure, riverside communities, and aquatic life. Yet, it is only in the last few decades that concerted efforts have been made to understand and predict ice-jam occurrence and severity. Building on a 1990s state of the art review, new physical knowledge, prediction capabilities, and management methods are discussed herein. The thickness and hydraulic roughness of ice jams have been elucidated, and flow through ice-jam voids quantified. Under-ice transport of frazil ice, which can lead to enormous freezeup accumulations, has been placed on a more rigorous footing, while heat exchange with the water flowing under a jam can be predicted with some confidence. Systematic field measurements have produced new understanding of the waves that are generated by ice-jam releases. Increased understanding of the physical processes has enhanced confidence in older numerical models and motivated development of more sophisticated ones, leading to two-dimensional dynamic algorithms based on continuum as well as discrete element approaches. Ice-jam management and control continue to be difficult tasks, but new structural and non-structural techniques offer promising avenues, at least in the case of relatively small rivers. The emerging issue of climate change and the growing appreciation of related ecological linkages have led to important, but still few, insights on how ice jamming regimes can be modified by altered climatic conditions and what the repercussions might be on river ecology. Despite the progress, there are still major unknowns, particularly related to the conditions of formation and release of ice jams. [Copyright &y& Elsevier]

Abstract: The breakup of the ice cover in northern rivers is a brief but crucial event in the life cycle of many aquatic species and can trigger extreme ice jam events with major socio-economic impacts and significant climate change implications. An important, but vaguely understood, breakup process is the fracture of the winter ice cover by low-amplitude water waves. Previous work on this subject has been based on the assumption of an infinitely long wave propagating under an infinitely long and “edgeless” ice cover. This configuration does not account for structural constraints imposed by the proximity of an ice edge or a transverse crack. Consequently, it only furnishes approximate values of bending stresses, and tells little about the spacing of cracks that may be generated by an advancing wave, which is the only visual evidence that can identify the relevant fracture mechanism in the field. Herein, edge proximity is taken into account by making plausible simplifications to the ice response equation, and using wave forms of limited extent. It is shown that such conditions generally produce higher bending stresses than does the infinite wave/edgeless cover configuration. The distance of the peak bending stress from the edge, which defines the spacing of cracks, varies with wavelength and is less than 100 ice thicknesses or so. This is comparable to that of high-amplitude, single waves (or surges) that result from ice jam releases, but much less than fractures generated by bending on horizontal planes, caused by the meandering river plan form. Comparison of the present results with the limited available evidence indicates that wave-generated fractures occur during the passage of ice jam release surges. [Copyright &y& Elsevier]

Major ice jams forming in lower reaches of the Peace River during spring breakup have been identified as the main agents of flooding and replenishment of perched basins of the Peace–Athabasca Delta, one of the world's largest inland freshwater deltas that supports a dynamic and valuable ecosystem. The paucity of ice jamming in the lower Peace River following construction of the Bennett Dam and the potential impacts of climate change have raised concerns regarding habitat degradation. One of the factors controlling ice-jam formation is the thickness of the winter ice cover, which may decrease in the future as a result of climatic warming, known to be most pronounced in northern parts of the globe and during the winter season. Using downscaled daily output from six Global Climate Models under two Representative Concentration Pathway scenarios, ice thickness is calculated for the years 1950 to 2100. Comparisons of monthly model-generated air temperatures and winter snowfall to historical data at nearby Fort Chipewyan indicate satisfactory agreement in terms of both magnitude and variability. Future projections show that under the highest emission scenario, average thickness decreases by about 0.2 m between the 1980s and the 2050s; and by an additional 0.1 m by the 2080s. Projected probability distributions of ice thickness point to increased incidence of thermal breakup, potentially reducing the already-low chances of ice-jam flooding. At the same time, a need for enhanced ice thickness monitoring is identified, aiming to develop a database of annual thickness values at selected sites within the lower reach of Peace River. • Peace-Athabasca Delta is partly sustained by Peace River ice-jam flooding. • Ice cover thickness can be a limiting factor to ice-jam occurrence. • Downscaled daily temperature and precipitation from six CMIP5 GCMs examined. • Thinner ice covers projected for the 2050s and 2080s under two RCP scenarios. • Likely to result in reduced ice-jam flood frequency. [ABSTRACT FROM AUTHOR]