Search

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]

As part of the Oil Sands Joint Monitoring Program, the present study aims to understand and quantify the erosional and sediment transport capacity of the Lower Athabasca River during the ice breakup period. Of special interest is the very steep reach above Fort McMurray, which is a potential natural source of polycyclic aromatic hydrocarbons. A two-year field program (2013–2014) has furnished unique data, reported in a companion paper, on the spatio-temporal water level variation during the breakup event. Herein, methodology is developed and applied for using the water level data sets to extract values of key hydrodynamic variables, which cannot be measured during breakup and ensuing ice runs. The results of the analysis indicated that flow velocity, discharge, and bed shear stress can exceed 4 m/s, 10,000 m 3 /s, and 100 Pa during the passage of sharp waves caused by releases of ice jams. According to past reports, the 2013 and 2014 breakup events, though dynamic, were not extreme. Yet, their severity exceeded by far the highest open-water flow values in the same reach over the period of record (2.5 m/s, 4440 m 3 /s and 35 Pa). Suspended sediment concentrations and loads during breakup were estimated via flow-based rating relationships developed from WSC sediment data. It was shown that concentrations could easily exceed 10 g/L, while a breakup event could supply a significant fraction of the total sediment load for the year and transport naturally occurring PAHs into the industrial oil sands reach. [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]

Highly dynamic flow conditions that often occur during the river ice breakup can cause greatly increased in-stream sediment concentrations and loads, with significant impacts on aquatic ecology and the environment. Sharp waves accompanying the releases of ice jams cause transient but very high flow velocities and shear stresses. However, very limited quantitative information is available on how such ice jam-generated waves (or “javes” for short) influence the transport of suspended sediment even though their effects on it are visually apparent. Suspended sediment concentrations (SSCs), obtained during a 5-year study of ice and sediment processes in the Saint John River (NB), reveal occasional spikes in concentration, consisting of relatively brief upward excursions to nearly order-of-magnitude higher values. Such spikes are concurrent with javes and ice runs. Documented cases of sediment pulses during river ice breakup events in the Saint John River form a unique data set demonstrating temporal and spatial characteristics. Peak SSC values approach 1 g/L and approximately coincide with peak jave stages and surface ice concentrations. Pulse durations amount to several hours, and are shorter than those of the concomitant javes. Despite their brevity, pulses can deliver sediment loads that are significant fractions of the total load for the entire breakup event. Measurement of breakup sediment pulses is not feasible under existing monitoring programs. Instead, focused short-term studies could lead to development of predictive capability, linking pulse properties to hydro-climatic variables that drive the breakup event. A strong influence of river discharge has been detected, leading to projected increases in pulse sediment amounts under a changing climate. [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: Ice breakup is a controlling factor in the hydrology of arctic Deltas, including the Mackenzie River Delta, which is characterized by numerous channels and lakes. Ice jams that form during dynamic spring breakup events often result in flooding, thus replenishing Delta lakes with essential water, sediment and nutrients. The present study, carried out under the auspices of the International Polar Year, aims to gather and analyze essential quantitative information for improving understanding of Delta ice processes and associated prediction capability. Recent work has shown that dynamic breakup in the flat Delta channels can only be triggered by javes, the sharp waves generated by ice jam releases, which amplify the hydrodynamic forces applied on, and can dislodge, the winter ice cover. First-ever measurements of javes in Delta channels are presented and their hydrodynamic properties quantified via the Rising Limb Analysis Method (RLAM), which enables calculation of the hydrodynamic properties of the measured waveforms. Typical leading edge and crest celerities were 10 and 5m/s respectively, while a record value of 16.5m/s was obtained on one occasion and linked to large local depths in Middle Channel. The hydrodynamic forces applied on the riverbed and the ice cover, resulting from various javes, are subdued in Delta channels relative to those in the lower Mackenzie River and in other, previously studied, rivers. This is primarily due to low water surface slope and points to a northward-decrease in the incidence of dynamic breakup and ice jams in Delta channels. [Copyright &y& Elsevier]

Abstract: River ice breakup has many socio-economic and ecological implications that primarily result from the formation and release of major ice jams. A key driver of breakup processes is the river discharge, but the presence of fractured ice sheets or moving ice rubble render its measurement or estimation very difficult. Here, we exploit the fact that the acquisitions of individual images of a satellite stereo scene are separated by about one minute. Between three stereo images taken from the PRISM instrument onboard the Japanese ALOS satellite on May 20, 2010, we track the displacements of river ice debris using automatic image correlation along a 40km long reach of the Mackenzie River, just above the entrance to its delta. This results in an almost complete ice velocity field over the river area studied with a spatial resolution of 25m and an accuracy of ~0.07m/s for the speeds. Channel bathymetry, slope, and hydraulic resistance along the study reach, known from previous studies, are then utilized to compute discharge at 15 cross sections. Calculated values vary from ~20,000 to ~27,000m3/s and are comparable to 23,800, the value estimated by Water Survey of Canada. Variability is much smaller, and close to that of conventional flow measurements, within the relatively straight sub-reach immediately upstream of the delta, where the effects of boundary constraints caused by a sharp bend farther upstream are minimal. This study proves for the first time that it is feasible to estimate river discharge during ice breakup at a reasonable accuracy using near-simultaneous satellite images. While this work had to rely on data that were coincidentally contained in satellite archives, special satellite or aircraft acquisitions could greatly enhance the probability of obtaining suitable data and thus the applicability of the method. [Copyright &y& Elsevier]

Abstract: Ice breakup is a controlling factor in the hydrology of arctic deltas, including the Mackenzie River Delta, which is characterized by numerous channels and lakes. Ice jams that may form during the spring breakup often result in flooding, thus replenishing delta lakes with essential water, sediment and nutrients. These processes are primarily driven by the flow of the lower Mackenzie River. The present study, carried out under the auspices of the International Polar Year, examines how the ice cover of the lower Mackenzie River can break up while still retaining a significant portion of its mechanical strength. This is the so-called mechanical breakup, a necessary condition for occurrence of ice jams farther downstream. In most rivers, mechanical breakup can simply result from the rising freshet flow, but this is doubtful for the Lower Mackenzie, owing to the low water surface slope and thick winter ice cover. Analysis of extensive measurements obtained during the 2008 breakup event indicates that mechanical breakup can primarily result from javes, the sharp waves generated upon ice-jam release, which are known to amplify hydrodynamic forces. Further confirmation is provided by archived hydrometric station data for the period 2000–2011. The present results on the conditions of breakup initiation are consistent with past findings while indicating a lower base temperature for the accumulation of “thawing” degree-days. This difference is shown to be linked to water–ice heat exchanges and radiative fluxes. Recommendations for future research include upstream extension of the study reach to include the range of influence of releasing ice jams, and physics-based modeling of the decreasing ice competence during the pre-breakup period. [Copyright &y& Elsevier]

Abstract: Ice-jam release waves, or javes for short, are highly dynamic events that occur in ice-forming rivers. They are known for serious ecological and socio-economic impacts, which are usually detrimental but on occasion beneficial, particularly with respect to ecosystem maintenance in the large freshwater deltas of northern Canada. Detailed water level data have been obtained during the passage of javes in the past 10years or so, but their study is hampered by lack of velocity and discharge measurements, owing to the presence of moving ice in the flow. A previously developed method to analytically compute such parameters, which is based on measured waveforms, has given encouraging results in several applications. Herein, this method is first subjected to comprehensive validation using numerical model output for a hypothetical jave in a prismatic channel. The analytical method is shown to perform adequately with respect to all hydrodynamic jave characteristics, such as celerity, velocity, discharge, and bed shear stress. Jave data from various rivers are then compiled in tabular form and it is shown via dimensional analysis that bulk predictions can be made in terms of jave height and rate of rise of the water level. Implications of the present findings to riverbed scour and to ecologically vital ice-jam flooding of flat rivers and deltas are explored and research challenges identified. [Copyright &y& Elsevier]

Abstract: Ice breakup is a controlling factor in the hydrology of arctic deltas, including the Mackenzie River Delta, which is characterized by a flat front and numerous channels and lakes. Ice-jam flooding replenishes delta lakes with essential water, sediment and nutrients. The present study, carried out under the auspices of the International Polar Year, aims to gather essential quantitative information for improving understanding of delta ice processes and associated prediction capability. Comprehensive observations and measurements, which were carried out during the 2007 and 2008 breakups, are described along with relevant analysis and data interpretation. The spacing of pre-breakup transverse cracks in delta channels is similar to that of rivers, typically ranging from 2 to 5 ice cover widths. Ice jamming in the delta is driven by major ice runs originating in the Mackenzie River and stalling in the upper reaches of Middle Channel. The data collected during the mechanical breakup event of 2008 permitted crude, but first-ever, quantification of flow re-distribution resulting from the formation of a Middle Channel jam. This jam also generated jams in East Channel and smaller distributaries. Examination of threshold velocity for ice block submergence indicated that, with notable exceptions, the expected kind of jam in delta channels is a surface accumulation of ice blocks and plates. This prediction is in accord with observations and hydrometric data at the Inuvik and Aklavik gauges. The use of portable loggers to record water level variations at selected sites proved to be a robust and effective technique for obtaining key hydraulic data. [Copyright &y& Elsevier]

Abstract: River ice jams can cause extreme flood events with major socio-economic and ecological impacts. A major practical question is how to quantify and assess ice-jam flood risk. Ideally, this question can be answered by means of historical ice-influenced water level peaks. More commonly, however, the available historical information is scarce and determination of ice-jam flood frequencies or probabilities must rely on a synthetic method. After noting that empirical evidence does not support the assumption of discrete stage outcomes, which is central to the existing methodology, a new synthetic method is developed. It hinges on the fact that the peak stage can take on any value between discharge-dependent envelopes and is thence called the distributed-function method, or DFM, and successfully tested by means of four case studies. It is shown further that the DFM produces more realistic results relative to those of the existing approach. Practical limitations are common to both methods and arise primarily from the need to use the peak runoff discharge, rather than the unknown value that prevails at the time of the peak stage. Resulting errors in probability estimates are evaluated in an extreme case and shown to be both conservative and tolerable. Analysis of additional case studies is recommended as a means of enhancing the utility of the DFM. [Copyright &y& Elsevier]

Abstract: Dynamic processes that occur during the breakup of river ice covers have important socio-economic and ecological impacts, but development of predictive capability is hampered by the brevity of the event and the ever-changing flow and ice conditions. The spatial and temporal variation of river water levels reflects the evolution of breakup processes and may be used to quantify their characteristics. However, various practical difficulties in measuring such variations have contributed to a scarcity of relevant data. A recently developed technique for remote recording of river levels involves pre-breakup deployment of portable pressure loggers, which are retrieved weeks or months later. Logger memory can accommodate high sampling frequencies, sufficient to capture very rapid water level changes. Application of this technique to document a highly dynamic breakup in the Saint John River, New Brunswick, resulted in an extensive data set. The processed logger output is described in detail and shown to furnish important insights on the chronology of breakup events. Analysis of the water level records that were obtained at different locations resulted in quantification of the hydrodynamic properties of waves generated by ice jam releases, also known as “javes”. Jave-induced amplification of flow and shear stress decreased with traveled distance, being between 2.5 and 3.0 at a distance of 11km. Extensive mobilization of the riverbed during the passage of javes in the study reach is likely. Application of a numerical model to the measured WL profile of a major ice jam resulted in default-range calibration coefficients, further corroborating current modeling capability. [Copyright &y& Elsevier]

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: During the 1994 and 1995 spring breakup events, ice jams were formed on the lower Matapedia River and remained in place long enough to permit collection of detailed field data on jam thickness as well as on water surface elevations. Together with bathymetric data that were obtained in the following summers, the spring measurements form one of the most complete ice-jam data sets that have been obtained to date. This allowed examination of how well current understanding of the hydromechanics of ice jams, incorporated in a numerical model, can describe the results of the measurements. The availability of jam thickness data was particularly advantageous because it removed the ambiguity associated with model calibrations that are solely based on water level measurements. With suitable choice of the model parameters, the model reproduced the measured water level profiles of the jam very closely, while providing reasonable simulations of jam thickness. The selected values of the model parameters were consistent with values established by previous applications on other rivers, and almost identical between the two Matapedia jams, despite contrasts in jam severity and river flow. Model sensitivity was explored using the more complete 1995 data set and found to be moderate. The detail of the available data also made it possible to scrutinize hydraulic resistance characteristics of flows underneath ice jams, and confirm earlier formulations specifically designed to address extreme-roughness conditions. [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 water temperature decay under ice jams that form during the breakup of the river ice cover is evaluated using different data sets from several Canadian rivers. In all cases, the water temperature varies with downstream distance in negative exponential fashion, as indicated by a simplified, one-dimensional, formulation of heat conservation in a river. The corresponding heat transfer coefficients and derivative dimensionless quantities are very high relative to the values that would be expected by extrapolation of existing formulae for turbulent convection in rough pipes. It is suggested that these high heat transfer rates are due to two factors, both characteristic of breakup ice jams: the extreme roughness of the jam underside, and the flow through the voids of the accumulation of ice blocks that comprise the jam. [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]

Extreme ice-jam flood events in rivers occur during a type of breakup that is partly governed by the mechanical properties of the ice cover, and known as “mechanical”. By contrast, thermal breakups are preceded by advanced thermal decay of the ice and can only produce insignificant, if any, jamming. Identification and quantification of threshold conditions that demarcate the two types of breakup events will improve current capabilities in a variety of issues, including river ice–aquatic ecosystem interactions, socioeconomic impacts of river ice, and climate-change impacts via modified river ice regimes. Extending recent advances in the prediction of the onset of mechanical breakups, a simple threshold criterion is postulated and its quantitative consequences are worked out. It is shown that there is a site-specific rise in water level above the freeze-up elevation, which delineates mechanical from thermal events. The threshold value is approximately proportional to the thickness of the ice cover, and also depends on local river morphology and hydraulics. These predictions are tested against three extensive case studies and found to perform satisfactorily. Environmental implications of the present results are discussed. [Copyright &y& Elsevier]

Various criteria have been advanced to describe and predict the conditions under which a load will break through a floating ice cover. As a rule, the experimental basis for each of these criteria has been limited, and a comprehensive performance evaluation has yet to be carried out. Criteria based on stress, strain, deflection and strain energy are discussed first, in conjunction with the viscoelastic nature of ice and the mode of failure of an ice sheet. Five data sets are then described and utilized to test and compare the performance of failure criteria that have been proposed in the literature. Stress-based criteria, though simple to use, are only useful for very brief loading because they do not account for loading time and history. Strain- and deflection-based criteria are also subject to time and history effects that similarly limit their applicability to short-term loads. On the other hand, strain-energy criteria are found to apply equally well to short- and long-term loads of different histories. This feature is attributed to implicit consideration of time and load-history effects through integration of the deflection–load variation. Safety implications and future research avenues are discussed. [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]

The Peace-Athabasca Delta (PAD) in northern Alberta is one of the world's largest inland freshwater deltas, home to many species of fish, mammals, and birds. Coincident with regulation that commenced in 1968, the PAD has experienced prolonged dry periods in-between floods, accompanied by reduction in the area covered by lakes and ponds that provide habitat for aquatic life. This likely resulted from reduced frequency of formation of major spring ice jams, which are the only mechanism for replenishing the higher-elevation, or "perched", basins of the PAD. The inundation effectiveness of ice jams depends on the magnitude of the prevailing river flow, but it has not been possible so far to assess whether regulation has had a positive or negative effect on breakup flows. Using a comprehensive BC Hydro data set, containing river inflows up to the W.A.C. Bennet Dam, "naturalized" breakup flows are computed; these are flows that would have occurred under natural flow conditions and are tabulated for each year of the study period (1972–2016). The rigour and utility of the adopted "lagged-flow" methodology are demonstrated via basic hydrodynamic principles and comparisons with independently generated numerical-modelling data for an early portion of the regulation period. Though there can be significant differences between naturalized and regulated flows in individual years, average flow during the first week of May, the typical time when major ice jams form, has not been affected by regulation. During the last week of April and the second week of May, regulation has respectively had a positive and negative effect on average flow. Naturalized flows exhibit practically no temporal trend in the study period (1972–2016), while regulated flows exhibit decreasing, but not statistically significant (P -value > 0.05), trends. It is shown further that the time when the regulated spring outflow begins to be curtailed can be a key factor in Peace River flow conditions near the PAD. • Peace River ice jams generate vital flooding of Peace-Athabasca Delta • Regulation effect on spring breakup flow, a relevant control, is unclear • Simple and rigorous methodology is developed to compute naturalized flows • Differences from regulated flows vary in sign and magnitude in different years • Naturalized breakup flows exhibit no temporal trend during 1972–2016 [ABSTRACT FROM AUTHOR]

Major ice jams forming in the lower reaches of Peace River during the spring breakup of the ice cover have been identified as the main agents of flooding and replenishment of the perched basins of Peace–Athabasca Delta, one of the world's largest inland freshwater deltas. The paucity of ice jamming in the lower Peace River 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 only the fifth in the post-regulation period and the first since 1997. Despite the sizeable number of years in the regulation period, conventional estimates of flood frequency are shown to be uncertain, but become robust when based on the slope of the cumulative number of floods. The evolution of ice breakup in 2014 was in full accord with previously postulated mechanisms for the flat reach of Peace River, which extends for several hundred kilometres above its mouth. An updated examination of historical hydrometric data at the WSC Peace Point gauge took into account the years 2003–2018, extending the record of the regulation period by 50%. The analysis further confirmed and fine-tuned physically based predictions of the effects of the freezeup level, the breakup flow, the thickness of the winter ice cover, and the accumulated degree-days of thaw on the incidence of ice-jam flooding. Commercially motivated reduction of freezeup flows in recent years likely contributed to the occurrence of the 2014 flood. Total winter snowfall at an index station appears to have reversed a pre-2000 decreasing trend, but its benefit to breakup flow may be moderated by increased incidence of mid-winter thaws. • Peace River ice jams generate vital flooding of Peace-Athabasca Delta. • Drying trend and paucity of ice jams experienced since regulation in 1968. • Ice-jam flood of 2014 first since 1997, fifth since 1968, well documented. • Observed ice breakup evolution confirms postulated mechanism. • Updated time series of hydroclimatic controls reveal new insights. [ABSTRACT FROM AUTHOR]