Erosion characteristics and floc strength of Athabasca River cohesive sediments: towards managing sediment-related issues

Purpose This 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 methods A 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 7 days. 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 discussion The critical bed shear stress for erosion increased from 0.16 (1-day CB) to 0.26 Pa (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. Conclusions Consolidation 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.