doi:10.5194/piahs-367-157-2015
Sediment Dynamics from the Summit to the Sea
157
(Proceedings of a symposium held in New Orleans, Louisiana, USA, 11–14 December 2014) (IAHS Publ. 367, 2014).
The effect of coarse gravel on cohesive sediment entrapment in
an annular flume
KEN GLASBERGEN1, MIKE STONE2, BOMMANNA KRISHNAPPAN3,
JAMIE DIXON4 & ULDIS SILINS5
1 GeoProcess Research Associates Inc, Burlington, Ontario, Canada
2 Geography and Environmental Management, University of Waterloo, Waterloo, Ontario, Canada
mstone@uwaterloo.ca
3 Environment Canada, Burlington, Ontario, Canada
4 City of Calgary, Calgary, Alberta, Canada
5 Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada
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.
Key words erosion; cohesive sediment; entrapment; sedimentation; gravel bed
INTRODUCTION
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). Bed form and bed mobility also influence the entrapment of cohesive sediments
(Schalchli, 1992; Rehg et al., 2005; Krishnappan & Engel, 2006). However, some uncertainty
exists regarding the magnitude of conveyance losses, the environmental significance of
remobilization from temporary storage as well as the duration and magnitude of long-term storage
of cohesive sediments within river channel systems (Lambert & Walling, 1998).
In a study of water quality in the Elbow River and its potential impact on water supply for the
City of Calgary, Sosiak & Dixon (2004) reported that many of the water quality problems in the
Glenmore Reservoir are directly related to land-use change and its effect on the source, quality,
transport and fate of cohesive sediment in the Elbow River. Accordingly, to understand and better
manage the long-term impacts of land-use change on water quality and drinking water supply,
there is a need to rigorously quantify processes that influence the in-stream source, transport and
fate of fine sediment in this predominantly gravel-bed river. Currently, little is known about the
mass of cohesive sediment stored in coarse gravel beds of the Elbow River or in-stream processes
that govern entrapment of cohesive sediment. The objectives of this study are: (1) to quantify the
transport and depositional properties (critical shear stress for erosion and deposition, density,
Copyright 2014 IAHS Press
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K. Glasbergen et al.
settling velocity) of Elbow River cohesive sediments experimentally in an annular flume with and
without gravel; (2) to evaluate the processes governing entrapment of cohesive sediment in gravel
beds; and (3) to quantify the entrapment ratio of gravel from the Elbow River. 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.
METHODS
Experimental approach
Two sets of flume experiments (with and without gravel beds) were conducted to quantify the
transport and depositional properties of cohesive sediments in the Elbow River. This approach
enables the entrapment of cohesive sediment by Elbow River gravel to be rigorously quantified.
The experimental conditions for erosion and deposition experiments conducted in this study are
listed in Table 1.
Table 1 Description of erosion and deposition experiments in the annular flume.
Experiment
Deposition
Deposition
Deposition
Erosion
Erosion
Deposition
Erosion
(no gravel)
(no gravel)
(no gravel )
(no gravel)
(no gravel)
(gravel bed)
(gravel bed)
Shear stress
(Pa)
0.123
0.123
0.212
variable
variable
0.48
variable
Initial concentration
(ppm)
289
614
593
614
614
181
1.7
Consolidation period
(h)
0
0
0
113
39
0
39
Sediment collection
Water, gravel and cohesive sediments were collected from partially submerged gravel bars of the
Elbow River during low flow conditions in autumn 2011 near the confluence of the river with the
Glenmore Reservoir in the City of Calgary, Alberta. Cohesive sediment and river water were
collected using an inverted cone sampler (Milburn & Krishnappan, 2003). Approximately 750kg
of gravel was collected and this material was sufficient to create an 8-cm thick gravel bed in the
rotating flume. All materials were shipped to the Canadian Centre for Inland Waters (CCIW) in
Burlington, Ontario. Prior to the flume experiment, the gravel was pre-washed and sieved five
times using a SWECO Vibro-Energy Separator®. Materials >10 mm were used to create a gravel
bed in the flume.
Flume experiments
The series of deposition and erosion experiments listed in Table 1 were conducted in a 5-m (ring
diameter) rotating annular flume located at CCIW. The flume and experimental methods are
described in detail (Krishnappan, 1993; Krishnappan & Engel, 1994, 2004). The deposition
characteristics of cohesive sediment without gravel were studied in two separate experiments by
adding filtered river water and a known mass of sediment to produce fully mixed concentrations of
289 mg/L and 614 mg/L in the flume. The flume was operated at high speed for 20 minutes then
the speed was lowered to a constant bed shear stress of 0.123 Pa. The flume was operated at this
level for about 5 hours. During this experiment, suspended sediment concentrations were
monitored at regular time intervals. Deposition experiments were repeated under the same
conditions with gravel beds. The flow characteristics and bed shear stresses in the flume during
these experiments were calculated using a three dimensional hydrodynamic flow model called
PHOENICS (Rosten & Spalding, 1984).
The effect of coarse gravel on cohesive sediment entrapment in an annular flume
159
The erosion characteristics of cohesive sediment without gravel were determined in the first
erosion experiment. A cohesive sediment layer was created by mixing river water and sediment at
high speed then bringing the flume to rest. The sediment–water mixture was left undisturbed for
113 hours to allow the sediment to settle and age on the flume bed. The experiment began by
increasing the speed of the flume and top cover in steps, thereby incrementally applying shear
stress as a stair-case function. Suspended sediment concentration in the flume of the eroded
sediment was measured as a function of time. In the second erosion experiment with a gravel bed,
a cohesive sediment mixture was added while the flume ran at high speed and then sediment
settled in the flume as the speed decreased slowly until it stopped. After 113 hours, the speed of
the flume and top cover were increased as described above and sediment concentrations in the
flume were measured as a function of time. An entrapment coefficient (defined as the ratio
between the entrapment flux and the settling flux) was calculated using a fine sediment transport
model developed by Krishnappan & Engel (2006). Settling velocity of cohesive sediment was
calculated using the method described by Krishnappan (2007). The mass of fine sediments stored
within the gravel bed of the flume was calculated using the method of Lambert & Walling (1988).
RESULTS AND DISCUSSION
Deposition and erosion characteristics of cohesive sediment
The depositional characteristics of cohesive sediment without gravel were studied by using the
same initial suspended sediment concentrations (614 mg/L) for two different shear stress
conditions (0.212 and 0.123 Pa). 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). The deposition experiment data are presented in Fig. 1. The fractions of
cohesive sediment deposited were 0.57 and 0.94 for an applied shear stress of 0.123 and 0.212 Pa,
respectively. The critical shear stress for deposition for the Elbow River cohesive sediments was
determined as 0.115 Pa.
The critical shear stress for erosion of cohesive sediment without gravel was investigated
using two consolidation periods (39 and 113 h). Results of the experiments are presented in Fig. 2.
The critical shear stress for bed erosion was 0.212 Pa for both the 39 and 133 h consolidation
periods. The critical shear stress for erosion of the Elbow River sediment was approximately two
times higher than the critical shear stress for deposition (0.115 Pa), which is consistent with results
observed in other studies (Stone & Krishnappan, 1997; Milburn & Krishnappan, 2003).
Fig. 1 Deposition of cohesive sediment for two bed shear stress conditions (0.123 Pa and 0.212 Pa).
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Fig. 2 Cohesive sediment bed erosion for 39 h and 113 h consolidation periods.
Effect of gravel on deposition and erosion of cohesive sediment
The results of deposition experiments in the flume with and without gravel under the same
conditions of shear stress are presented in Fig. 3. During the initial 15 minutes of the experiment,
suspended sediment concentrations declined rapidly to 11.9 mg/L at steady state. Figure 3 shows
that 93% of the fine sediment was deposited either on or in the gravel bed compared to 56%
deposited under similar flow conditions without gravel. The results are comparable to Krishnappan
(2007) and demonstrate the significant effect of gravel on the deposition of cohesive sediment.
Fig. 3 Sediment deposition with and without gravel at the same flume speed.
The effect of gravel on cohesive sediment erosion was also evaluated experimentally in the
annular flume. A very small fraction of the deposited sediment was remobilized under the
maximum shear stresses applied in the erosion experiment with gravel (Fig. 4). Some relatively
constant sediment pulsing from the gravel bed was observed was over time. The pulsing appears to
have resulted from the erosion of fine sediments deposited on the top surface of the gravel bed.
The effect of coarse gravel on cohesive sediment entrapment in an annular flume
161
Fig. 4 Erosion experiment with coarse gravel bed.
The stochastic nature of the sediment remobilization into the water column is most likely related to
the turbulent nature of flow at the boundary layer (Kirkbride & Ferguson, 1995). It should be
noted here that the concentration of the eroded sediment was very small (<2 mg/L) and is only
slightly higher than the measurement accuracy of 1 mg/L.
The entrapment coefficient for the Elbow River gravel was estimated to be 0.2. Krishnappan
& Engel (2006) found that the entrapment coefficient for sands varied as a function of applied
shear stress. The observed difference in entrapment for the same sized sand was attributed to the
stability of the bed which at low shear was stable but at a higher shear stress was mobilized.
Krishnappan & Engel (2006) observed that a mobile sand bed used in their experiments prevented
a clogging layer from forming, thus resulting in a higher entrapment value than for the Elbow
River gravel. The present study shows that gravel reduced the steady state sediment concentration
of suspended sediment by 83% compared similar flow conditions with no gravel bed.
Implications for water quality of the Glenmore reservoir
Knowledge of cohesive sediment transport characteristics and storage/mobilization within gravel
deposits of the Elbow River bed is critical to understanding its effect on water quality of the
Glenmore Reservoir. The cohesive sediment fraction stored in gravel of the Elbow River was
approx. 0.5% of the total sediment mass in the upper 8 cm of the river bed. Particle density of
cohesive sediment approached that of water for size fractions >100 µm. The maximum settling
velocity of 50-µm flocs was 0.37 mm/s which decreased to 0.07 mm/s for flocs of 128 µm
diameter (Fig. 5). Such information is relevant to the City of Calgary in the context of drinking
water supply because cohesive sediment is the primary vector for phosphorus transport which can
influence the trophic status of the Glenmore Reservoir (Sosiak & Dixon, 2004). The most
bioavailable particulate phosphorus forms are associated with sediment size fractions <20µm
(Stone & English, 1993) and the phosphate desorption potential from cohesive sediment into the
water column is most pronounced in the smallest size fraction (Stone & Mudroch, 1989).
Accordingly, when the armour layer of the gravel bed is mobilized during high flow events,
cohesive sediment stored in the gravel will be re-entrained and transported into the reservoir.
However, given the low settling velocities of the fine sediment in the water column, phosphorus
desorption from the sediment into the water column will be maximized thereby increasing the
potential for algal blooms.
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Fig. 5 Density and settling velocity of cohesive sediment.
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