Abstract
Abstract. While cohesive sediment generally represents a small fraction (<0.5%) of the total sediment mass stored in gravel-bed rivers, it can strongly influence physical and biogeochemical processes in the hyporheic zone and alter aquatic habitat. This research was conducted to examine mechanisms governing the interaction of cohesive sediments with gravel beds in the Elbow River, Alberta, Canada. A series of erosion and deposition experiments with and without a gravel bed were conducted in a 5-m diameter annular flume. The critical shear stress for deposition and erosion of cohesive sediment without gravel was 0.115 Pa and 0.212 Pa, respectively. In experiments with a gravel bed, cohesive sediment moved from the water column into the gravel bed via the coupling of surface and pore water flow. Once in the gravel bed, cohesive sediments were not mobilized under the maximum applied shear stresses (1.11 Pa) used in the experiment. The gravel bed had an entrapment coefficient (ratio between the entrapment flux and the settling flux) of 0.2. Accordingly, when flow conditions are sufficient to produce a shear stress that will mobilize the armour layer of the gravel bed (>16 Pa), cohesive materials trapped within the gravel bed will be entrained and transported into the Glenmore Reservoir, where sediment-associated nutrients may pose treatment challenges to the drinking water supply.
Highlights
Cohesive sediment is environmentally significant in aquatic systems because it can have a deleterious impact on biota (Ankers et al, 2003), habitat (Cobb et al, 1992) and influence the transport and fate of contaminants (Horowitz & Elrick, 1987; Owens et al, 2005)
An increasing number of laboratory and field-scale studies have advanced knowledge of cohesive sediment transport and storage mechanisms in aquatic systems (Packman et al, 2000; Rehg et al, 2005; Krishnappan & Engel, 2006; Collins & Walling, 2007; Krishnappan, 2007). These and other studies show that entrapment of cohesive sediment is dependent on the concentration of suspended sediment, that entrapment continues until a clogging layer is formed (Diplas, 1947), and that fine sediments remain in the bed until a critical shear stress mobilizes the gravel bed (Einstein, 1968; Rehg et al, 2005)
Steady state concentrations can be expressed as a fraction of the initial sediment concentration and the critical shear stress for deposition can be extrapolated using a fitted power law relationship between the fraction deposited and bed shear stress (Milburn & Krishnappan, 2003)
Summary
Cohesive sediment is environmentally significant in aquatic systems because it can have a deleterious impact on biota (Ankers et al, 2003), habitat (Cobb et al, 1992) and influence the transport and fate of contaminants (Horowitz & Elrick, 1987; Owens et al, 2005). An increasing number of laboratory and field-scale studies have advanced knowledge of cohesive sediment transport and storage mechanisms in aquatic systems (Packman et al, 2000; Rehg et al, 2005; Krishnappan & Engel, 2006; Collins & Walling, 2007; Krishnappan, 2007) These and other studies show that entrapment of cohesive sediment is dependent on the concentration of suspended sediment, that entrapment continues until a clogging layer is formed (Diplas, 1947), and that fine sediments remain in the bed until a critical shear stress mobilizes the gravel bed (Einstein, 1968; Rehg et al, 2005). This study examines mechanisms governing the interaction of cohesive sediments with gravel beds in the Elbow River, Alberta, Canada, and builds upon earlier entrapment studies (Krishnappan & Engel, 2006) to advance the entrapment ratio concept for modelling fine sediment transport
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