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Numerical Modeling of Flow and Sediment Transport within the Lower Reaches of the Athabasca River: a Case Study

  • Environment and Climate Change Canada

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This study investigates flow and sediment transport patterns within the lower reaches of the Athabasca River (~200 km) in Alberta, Canada. These reaches are characterized by complex bathymetry, regions of high tortuosity, and variable discharges and bed slopes. Sediment within this reach is primarily sand and gravel, but there is also a high percentage (>10%) of cohesive sediment with unique settling properties. A regional Environmental Fluids Dynamics Code (EFDC) 2D numerical model was setup to predict hydrodynamics of the flow and suspended sediment transport. Bathymetry measurements were obtained from a combination of high resolution 3D Geoswath and ADCP surveys, and detailed 2D cross-section measurements. A local high resolution 2D numerical simulation was also completed for a reach near Steepbank River (<20 km) to better understand the effects of a coarser grid resolution on the regional model predictions. Model results were validated using field measurements including water surface elevations collected with Global Positioning System (GPS), water velocities collected using a Gurley current meter, and suspended sediment measurements obtained from the Regional Aquatics Monitoring Program. The results showed that the regional model was capable of making reasonable predictions of water surface elevations, flow velocities, and suspended sediment concentrations. Simulation results with a rigid bed, estimated sediment inputs and assumed parameters, have also shown that a large proportion of incoming sediments get deposited along the lower reaches of the Athabasca River, and the model was able to identify those major depositional areas.
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A large data set of grain size and lithology covering 7 600 km of river length is presented for 12 river Albertan rivers. The downstream change in lithological content largely reflects the relative resistance to wear of quartzite and limestone and the introduction of granite erratics. Diminution coefficients for river and alluvial fans are presented to show the dominant influence of differential transport in the aggrading fan environment. Comparison of diminution coefficients for various gravel rivers with abrasion coefficients established in controlled experiments, reveals that abrasion coefficients consistently underestimate diminution coefficients. Analysis, which uses diminution and abrasion coefficients for different lithologies, reveals that the abrasion coefficient for rivers can be subdivided into two components--"abrasion during transport" and abrasion at rest". The analysis indicates that the condition of "abrasion at rest" is dominant in Alberta rivers. Grain-size distributions for alluvial gravel rivers are commonly bimodal. The gravels of this study show a deficiency in the range from coarse sand to granules. This deficiency relates to the style of sediment transport, whether suspended or bedload.
The Athabasca River drains an area of 160 000 km2 in northern Alberta, Canada, with much of the lower basin underlain by oil-sand deposits. The oil sands occur primarily in the McMurray Formation of the Cretaceous Period, with outcrops evident along the banks of the Athabasca River, as well as the lower portions of several tributaries. Since the oil sands represent a natural diffuse source of hydrocarbons to the aquatic environment, understanding the nature and extent of sediment-bound hydrocarbon contaminants in the context of the sediment regime of the Athabasca River is important. Described are fluvial geomorphic characteristics of the lower Athabasca River, which provide a basis for assessing sediment-bound hydrocarbon contaminants. Suspended sediment derived from main stem and tributary sources between Fort McMurray and Embarras account for 1.2 Mt, or 18%, of the mean annual load of the Athabasca River. Of this load, approximately 53% of the sediment input originated from tributaries, the remainder from main-stem sources. The majority of sediment contributed along the main stem occurs in the vicinity of Embarras, well downstream of oil-sand sources. Natural oil-sand sediment contributions are likely much less than 3% of the annual load downstream of Fort McMurray. Key words: sediment, fluvial geomorphology, oil sands, hydrocarbons, polycyclic aromatic hydrocarbon (PAH), contaminants, environmental monitoring, Athabasca River.
PurposeThis research aims to investigate: (1) the evolutional sequence of erosion of cohesive sediments entering the Athabasca River, (2) the influence of consolidation/biostabilization time on bed sediment stability, and (3) the implication of these results on contaminant transport within the Athabasca River. Materials and methodsA 5-m annular flume was used to generate bed shear to assess cohesive sediment dynamics for eroded beds with consolidation/biostabilization periods (CBs) of 1, 3, and 7days. Additional laser particle sizing, image analysis, densitometry, and microbial analysis were employed to further the analysis with respect to bed erosion and eroded floc characteristics. Results and discussionThe critical bed shear stress for erosion increased from 0.16 (1-day CB) to 0.26Pa (7-day CB) with an inverse relationship observed for both suspended sediment concentration and erosion rate with respect to CBs. The 7-day CB yielded the largest eroded flocs that initially have high organic content but were quickly broken up with increasing shear. The strongest eroded floc population occurred for the 3-day CB. Eroded flocs were found to be of an open matrix with high water content and low density. Flocs contained a mixture of clay and silt particles, microbes, algae, diatoms, and secreted extracellular polymeric substances (EPS). Counts of bacteria were observed to decrease with CBs while an increase in the algal community is suggested with time. ConclusionsConsolidation was believed to have limited effect on erosion while biostabilization was the main controlling factor. Increasing biostabilization with time resulted in a more stable surficial layer with a reduced erosion rate relative to less biostabilized beds. The highly biostabilized bed (7-day CB), however, yielded the largest flocs which broke up easily compared to those eroded from 1- and 3-day CBs. It is believed that the EPS produced by the sediment biological community is the main component of the bed and flocs that is responsible for the observed stability results. KeywordsAthabasca River–Cohesive sediments–EPS–Erosion–Floc strength–Microbes–Rotating annular flume–SFGL
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