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Modelling Under-ice Movement of Cohesive Sediments : Hay River , Northwest Territories , Canada
| Content Provider | Semantic Scholar |
|---|---|
| Author | Milburn, Duncan Krishnappan, Bommanna G. Krishnappan B. G. |
| Copyright Year | 2002 |
| Abstract | An intensive field program was conducted just before river-ice breakup at the Hay River, Northwest Territories, Canada in April, 2000, followed by controlled laboratory experiments on Hay River water and sediments in a rotating circular flume at Burlington, Ontario, Canada to better understand the nature of cohesive sediment transport in the Hay River. Results from these earlier studies have shown that the deposition of fine sediment is possible in the shallower portion of the river along the river banks, where the bed shear stress is lower than the critical shear stress for deposition of the Hay River sediment during the winter months. The remobilization and the transverse dispersion of the sediment across the width of the river are attributed as possible causes for the formation of sediment plume just prior to breakup when the bed shear stress exceed the critical shear stress for the erosion. This hypothesis will be tested using a new modelling strategy proposed in this paper. INTRODUCTION – THE NATURE OF COHESIVE SEDIMENTS Cohesive sediments are characterized as a mixture of predominantly clay and silt-sized fractions of clay-type minerals but may also contain a range of organic compounds (Raudkivi, 1998). Much uncertainty exists about the hydraulic forces that erode, resuspend and transport cohesive sediments, particularly the difference in the critical condition for initiation and cessation of sediment movement in a flowing medium. For coarse-grained material, erosion and deposition can occur simultaneously under a constant bed shear stress. For cohesive material, however, simultaneous erosion and deposition is not possible because different critical conditions occur one for erosion and a different one for deposition. Cohesive sediments tend to be transported not as single constituent grains, but rather in a flocculated/aggregated form (Droppo, 2001; Lick, 1982; Mehta and Partheniades, 1 Water Resources Division, Department of Indian Affairs and Northern Development, Yellowknife, Northwest Territories, Canada 2 Aquatic Ecosystem Impacts Research Branch, National Water Research Institute, Burlington, Ontario, Canada 1975; Partheniades and Kennedy, 1966). These “flocs” comprise a complex matrix of microbial communities, organic and inorganic particles and substantial interfloc spaces that allow retention or passage of water. Furthermore, because of the electrochemical forces in combination with biological factors, flocs will settle at rates completely different from those of their primary constituent particles (Lau and Krishnappan, 1992). As Droppo et al. (1997) explain these flocs are like individual microecosystems with autonomous and interactive physical, chemical and biological behaviours. Their reactivity and relatively large pore spaces form an important medium for removal of contaminants from a water column. Low flow conditions favour deposition of these flocs only to be resuspended during more dynamic conditions such as river-ice breakup resulting in an important vector for contaminant transport. Some of the main differences between the cohesionless and cohesive sediments are listed in Table 1. Table 1: Comparison of Cohesionless and Cohesive Sediments Cohesionless Sediments Cohesive Sediments • tend to move as single particles or grains • size ranges from silts (2 to 62 μm) to sands (>62 μm) • erosion and deposition is a function of size and shear stress • can be eroded and deposited simultaneously • can be transported as bed load • form as agglomerates or “flocs” • very fine clay (<2 μm) and fine silt (< 16 μm) particles • erosion and deposition a function of size, shear stress, mineralology and chemicalbiological properties • cannot be eroded and deposited simultaneously • rarely transported as bed load remain in suspension The transport relationships developed for cohesionless sediment are not adequate to predict the transport behaviour of cohesive sediment (Raudkivi, 1998; Shen and Julien, 1992). At the present state of knowledge, the fine sediment transport characteristics can only be obtained by direct measurements in special flumes such as rotating circular flumes (Krishnappan, 1993). Previous work by Milburn and Prowse (2002) shows that the bed sediments deposited as winter accumulations at Hay River, Northwest Territories, Canada are cohesive in nature. It has also been shown (Milburn and Prowse, 1996; 1998) that the protracted period of winter-ice cover in northern rivers favours the accumulation of fine-grained (<62 microns), contaminant-bearing sediments. These sediment accumulations can be mobilized immediately prior to breakup when discharges and bed shear stress first start to rise from the winter low flow condition (Milburn and Prowse, 1996; 2002). The environmental implications of this phenomenon are largely unknown and poorly understood, but the spring breakup could represent a significant, episodic release of sediment-bound contaminants. RESEARCH OBJECTIVES In view of the above, the objective of this research was to extend the work of Milburn and Prowse (2002) and Milburn and Krishnappan (2002) by taking further detailed field measurements just before breakup that can be used for modelling cohesive sediment transport. Results from these earlier studies have shown that the deposition of fine sediment is possible along the shallower portion of the river along the river banks, where the bed shear stress is lower than the critical shear stress for deposition of the Hay River sediment during the winter months. The remobilization and the transverse dispersion of the sediment across the width of the river are attributed as possible causes for the formation of sediment plume just prior to breakup when the bed shear stress exceed the critical shear stress for the erosion. This hypothesis will be tested using a new modelling strategy proposed in this paper. The strategy consists of applying an advection-dispersion equation expressed in stream-tube coordinate system to model the sediment deposition in the river during winter months. The stream tubes will be constructed using measured flow depths and predicted velocity distributions in the lateral direction in a number of cross-sections. The water surface slopes that are required for predicting the velocity profiles will be obtained by applying the MOBED model, which solves the St. Venant equations and a sediment mass balance equation. With a better understanding of the cohesive forces governing erosion and deposition of these materials, a new modelling strategy to predict the transport of the sediment in the Hay River under ice-covered conditions is formulated. The details of the modelling strategy are described below. A MODELLING STRATEGY TO PREDICT TRANSPORT OF FINE SEDIMENT A fine sediment dispersion and deposition model developed by Krishnappan (1991) is adapted for modelling sediment transport in the Hay River. The model solves an advection-diffusion equation expressed in curvilinear coordinate system (see Figure 1). Some of the salient features of the model are discussed here for the sake of completeness. The governing equation solved in the model is shown below: 2 1 2 2 x z x x m Uh E m m C C C x Q U Uh λ ∂ ∂ ∂ λ ∂ ∂η ∂η Ê ˆ = + + Á ̃ Ë ̄ (1) where C is the depth averaged volumetric concentration of the fine sediment, x is the distance measured along the longitudinal coordinate axis, U is the depth-averaged velocity component in the x direction, h is the flow depth, Q is the volumetric flow rate, Ez is the transverse dispersion coefficient, mx is the metric coefficient of the coordinate system, λ1 is the rate coefficient for reactions, λ2 is the rate of source or sink and η is the normalized cumulative discharge defined as: |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://web2.clarkson.edu/projects/iahrice/IAHR%202002/Volume%201/201.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
