Flow Structure and Channel Stability at the Site of a Deep Scour Hole, Mackenzie Delta, Canada

Unusually deep scour holes in distributary channels of the Mackenzie Delta are of concern for oil and gas resource development, particularly with respect to buried pipeline crossings. Surveys of one such hole, carried out in 1985 and 1992, indicated vertical stability and slight lateral movement. The present study examines how the hole may have changed by the mid-2000s and documents the complex local velocity field and related bed material properties. Small discrepancies between isobaths of different years suggest a dynamic stability that involves short-term fluctuations in erosion and deposition. This suggestion was corroborated by detailed measurements of the highly three-dimensional velocity field, which revealed major eddy structures and flow reversals that help maintain sizeable velocity magnitudes despite low mean velocities. The composition of the bed material suggests cohesive behaviour, but the literature indicates a range of critical shear stresses that spans two orders of magnitude. The more probable lower end of this range is consistent with the observed dynamic stability of the scour hole.Des fosses d’affouillement exceptionnellement profondes se trouvant dans les effluents du delta du Mackenzie risquent de poser des problèmes en matière de mise en valeur des ressources pétrolières et gazières, surtout en ce qui a trait aux pipelines enfouis. Selon les levés d’une de ces fosses effectués en 1985 et en 1992, cette fosse serait verticalement stable et aurait un léger mouvement latéral. La présente étude se penche sur la façon dont la fosse pourrait avoir changé une fois rendu vers le milieu des années 2000 et documente le champ de vitesse local complexe de même que les propriétés des matériaux de fond connexes. De petits écarts entre les isobathes de différentes années laissent entrevoir une stabilité dynamique assortie de fluctuations à court terme sur le plan de l’érosion et de la déposition. Cette suggestion a été corroborée à l’aide de mesures détaillées du champ de vitesse hautement tridimensionnel, qui a révélé d’importantes structures à grands tourbillons et des écoulements de retour qui favorisent le maintien d’intensités de vitesse appréciables malgré des vélocités moyennes faibles. Bien que la composition des matériaux de fond suggère un comportement cohésif, la documentation indique une gamme de contraintes de cisaillement critique qui s’étend sur deux ordres de grandeur. Le bas de cette gamme plus probable est conforme à la stabilité dynamique observée à l’égard de la fosse d’affouillement

Estimating river discharge during ice breakup from near-simultaneous satellite imagery

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 40 km 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 25 m and an accuracy of ~ 0.07 m/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 ~ 27,000 m3/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

Reply to Discussions by Timoney et al (2018) and Hall et al (2018) on “Frequency of ice-jam flooding of Peace-Athabasca Delta” (Beltaos 2018; CJCE 45(1): 71-75)

The writer systematically refutes all of the discussers’ criticisms and supplies more details on the rigour of his “slope-based” frequency analysis. He also identifies key flaws of alternative and conflicting statistical interpretations proposed by the two discussion teams; and provides detailed documentation of the large uncertainty associated with paleolimnological assessments of ice-jam flood frequency.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Numerical prediction of ice-jam profiles in lower Athabasca River

A recent study of dynamic ice breakup processes and their erosional potential in the Lower Athabasca River concluded that breakup can result in very large sediment loads, which cannot be predicted at present. As a first step towards building suitable modelling capability, a user-friendly, public-domain, ice jam model is calibrated and validated using 2013 and 2014 water level measurements as well as historical data sets by others. The calibrated model is shown to reliably compute the profiles of different ice jams occurring in a 60 km reach that extends both above and below Fort McMurray. The model also enabled development of an ice jam stage-flow relationship for the city of Fort McMurray, which can help assess present and future, climate-modified, flood risk.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Assessing the Frequency of Floods in Ice-Covered Rivers under a Changing Climate: Review of Methodology

Ice-influenced hydrologic and hydrodynamic processes often cause floods in cold regions of the globe. These floods are typically associated with ice jams and can have negative socio-economic impacts, while their impacts on riverine ecosystems can be both detrimental and beneficial. Several methods have been proposed for constructing frequency distributions of ice-influenced annual peak stages where historical data are scarce, or for estimating future frequencies under different climate change scenarios. Such methods rely on historical discharge data, which are generally easier to obtain than peak stages. Future discharges can be simulated via hydrological models, driven by climate-model output. Binary sequences of historical flood/no-flood occurrences have been studied using logistic regression on physics-based explanatory variables or exclusively weather-controlled proxies, bypassing the hydrological modelling step in climate change projections. Herein, background material on relevant river ice processes is presented first, followed by descriptions of various proposed methods to quantify flood risk and assess their advantages and disadvantages. Discharge-based methods are more rigorous; however, projections of future flood risk can benefit from improved hydrological simulations of winter and spring discharges. The more convenient proxy-based regressions may not adequately reflect the controlling physics-based variables, while extrapolation of regression results to altered climatic conditions entails further uncertainty

Celerity of Ice Breakup Front in the Regulated Peace River, Canada, and Implications for the Recharge of the Peace–Athabasca Delta

Timely release of flow from upstream hydropower generation facilities on the Peace River can enhance potential ice-jam flooding near the drying Peace–Athabasca Delta (PAD), a Ramsar wetland of international importance and homeland to Indigenous Peoples. An important consideration in deciding whether and when to commence a release is the celerity of the breakup front as it advances along the Peace River. Relevant historical data for a key stretch of the river are analyzed to determine average celerities, which can vary by an order of magnitude from year to year. Seven breakup events are identified that might have been candidates for a release, and the predictability of associated celerities is explored in terms of antecedent hydroclimatic variables, including cumulative winter snowfall, snow water equivalent on 1 April, ice cover thickness, coldness of the winter, and freezeup level. It is shown that celerity can be predicted to within a factor of two or less, with the freezeup level giving the best results. Three of the seven “promising” events culminated in PAD floods and were associated with the three highest celerities. The empirical findings are shown to generally align with physical understanding of breakup driving and resisting factors