The effect of coarse gravel on cohesive sediment entrapment in an annular flume

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.

Sediment‐phosphorus dynamics can shift aquatic ecology and cause downstream legacy effects after wildfire in large river systems

“…As sediment is the primary vector of P transport in aquatic systems (Baker & Richards, 2002), associated increases in P export to downstream environments (Bayley et al, 1992;Hauer & Spencer, 1998;Burke et al, 2005;Lane et al, 2008;Silins et al, 2014) that are coupled with increased sediment erosion rates after wildfire are not surprising and have been commonly reported (Kunze & Stednick, 2006;Moody et al, 2013), including in the present study watersheds (Silins et al, 2009a). Here, it should be underscored that both the Crowsnest and Castle Rivers, as well as their tributaries, have gravel beds that can entrap postfire, P-enriched fine sediment originating from hill slope runoff and bank erosion (Krishnappan & Engel, 2004;Glasbergen et al, 2014). Collectively, the interaction of these components governs the longevity of watershed scale effects on P dynamics and aquatic ecology, as well as their disturbance-response trajectories.…”

“…Such changes are ecosystem specific because of spatial and temporal variability in hydro-climatic conditions, geology, soils, topography, vegetation, river and bed morphology, and intensity and severity of disturbances such as wildfire, as well as proportion of watershed area impacted. Here, it should be underscored that both the Crowsnest and Castle Rivers, as well as their tributaries, have gravel beds that can entrap postfire, P-enriched fine sediment originating from hill slope runoff and bank erosion (Krishnappan & Engel, 2004;Glasbergen et al, 2014). Accordingly, the release of SRP from fine interstitial sediment stored in these gravel beds is the mechanism by which bioavailable P is released to the water column both in headwaters and downstream reaches, across increasingly larger river basin scales where the importance of bed sediment storage will be more pronounced with increasing channel bed area.…”

Quantifying bed‐related suspended load in gravel bed rivers through an analysis of the bedload‐suspended load relationship

“…This hypothesis is consistent with observations made in several flume and field studies considering sand, silt or clay particle infiltration in porous beds [Frostick et al, 1984;Glasbergen, 2014;Hamm et al, 2009;Harvey et al, 2012;Krishnappan and Engel, 2006;Mooneyham and Strom, 2018]. This infiltration process was found to occur even for very low Rouse numbers (~0.01), especially when pore spaces were empty [Khullar, 2007;Mooneyham and Strom, 2018].…”

“…Similar observations were made at the reach scale in the Isabena catchment where changes in fine sediment stocks were correlated with the maximum flood discharge [Buendia et al, 2016]. The strong interactions observed in these field studies between suspended load and the river bed were also demonstrated in several flume experiments [Glasbergen, 2014;Hamm et al, 2009;Krishnappan and Engel, 2006;Mooneyham and Strom, 2018]. Navratil et al (2010) estimated these stocks in a typical mountainous braided river by considering stocks in dried braided channels, bars and vegetated bars in addition to sediments stored in the wetted channel.…”

“…It is important to note that, even though it is often considered that only coarser materials interact with the bed, all of these field measurements used the Lambert and Walling or the Navratil technique and thus mainly concern the finest fraction (<63 μm) of fine sediments [Lambert and Walling, 1988;Navratil et al, 2010]. The strong interactions observed in these field studies between suspended load and the river bed were also demonstrated in several flume experiments [Glasbergen, 2014;Hamm et al, 2009;Krishnappan and Engel, 2006;Mooneyham and Strom, 2018].…”