E-proceedings of the 36th IAHR World Congress 28 June – 3 July, 2015, The Hague, the Netherlands
NUMERICAL MODELLING OF SEDIMENT TRANSPORT IN AN ICE COVERED RIVER (1)
(2)
(3)
AHMAD SHAIBAEINIA , SHALINI KASHYAP , YONAS B. DIBIKE , TERRY D. PROWSE
(4)
& IAN G. DROPPO
(1)
Water & Climate Impact Research Centre, Environment Canada, Victoria, Canada,
[email protected]
(2)
Water & Climate Impact Research Centre, Environment Canada, Victoria, Canada,
[email protected]
(3)
Water & Climate Impact Research Centre, Environment Canada, Victoria, Canada,
[email protected]
(4)
Water & Climate Impact Research Centre, Environment Canada, Victoria, Canada,
[email protected] (5)
(5)
Canada Centre for Inland Waters, Environment Canada, Burlington, Canada,
[email protected]
ABSTRACT The transport of sediment and the associated chemical constituents originating from potential anthropogenic and natural sources are becoming an issue of increasing importance in the lower reaches of the Athabasca River ecosystem in northern Alberta, Canada. As a cold region river, the annual cycle of ice cover formation and breakup play a key role in sediment transport in this river. This numerical modelling study investigates the transport of fine cohesive sediments in the lower Athabasca River during both ice-covered and open-water periods. A one-dimensional (1D) river ice model is used to predict the ice coverage and its effect on the flow characteristics. The results of the ice model are used to investigate the effect of ice-cover on the sediment erosion, deposition and transport pattern within a ~200 km reach of the lower Athabasca River. A two-dimensional (2D) hydrodynamic/sediment transport model is also applied for more detailed simulations of lateral variations in specific regions. Results are validated and evaluated using available field measurements. Model output is discussed in relation to their utility for understanding and assessing the implication of the sediment dynamics on chemical contaminant fate and its effect on ecological indicators such as the benthic community. Furthermore, results help to obtain a better understanding of the important effects of ice-cover on sediment transport in cold region rivers. Keywords: Lower Athabasca River; Fine sediment transport; Ice covered and open water conditions. 1.
INTRODUCTION
The Lower Athabasca River (LAR), in northern Alberta, Canada, begins north of Fort McMurry and flows to the Athabasca Delta and Lake Athabasca. Throughout its course, the river cuts through natural bitumen deposits, (Conly et al., 2002) and runs adjacent to oilsand developments. Fine cohesive sediments play an important role in river ecosystem as many contaminants such as metals and polycyclic aromatic hydrocarbons (PAHs) can be associated and transported with the fine sediment fraction (Ghosh et al., 2000). The Canada-Alberta Joint Oil Sands Monitoring Program (2012) identified a need for a more systematic and comprehensive quantification and modeling of the sources, transport, flux, and fate of materials and contaminants entering the Lower Athabasca watersheds. To achieve this objective, there was a requirement to develop integrated hydrodynamic, sediment transport and water quality models of the LAR. Numerical models can be effective tools to predict the erosion, transport and fate of sediments and associated contaminants within river environments. However, modelling of flow and sediments in the lower Athabasca River can be challenging due to its complex morphology and highly seasonal flow, reflecting the climatic condition (as an unregulated river). Being located in a cold region, the ice formation, breakup and the relatively long ice-cover period can further complicate the modelling, as Ice processes can significantly change flow, and geomorphic processes (Prowse, 2001; Ettema and Daly, 2004). The objective of this paper is to numerically investigate the transport of suspended fine cohesive sediments in LAR for both open-water and ice-covered periods. The focus of this study is on ice coverage, and the other ice processes (such as ice breakup) are not taken into account. A one-dimensional (1D) model is used for large-scale long-term simulation of sediment transport patterns. For short term detailed simulations (e.g., to predict high-resolution longitudinal and lateral variations), a two-dimensional (2D) model is used. Furthermore, a 1D ice model is used to predict the winter ice coverage and its effect on the river flow characteristics. The results of this ice model are used to modify the sediment transport and hydrodynamic models to account for the effect of winter ice-cover. 2.
NUMERICAL MODELS
The 1D and 2D numerical models of choice for this study are MIKE 11 (DHI, 2012) and Environmental Fluid Dynamics Code (EFDC) (Hamrick 1992), respectively. Both models solve shallow flow equations (area-averaged, and depth1
E-proceedings of the 36th IAHR World Congress, 28 June – 3 July, 2015, The Hague, the Netherlands
averaged, respectively) using a finite difference method. The transport of suspended fine sediments is described by the advection-dispersion (AD) (i.e., conservation of mass of suspended particles and flocs):
c c c ui D S t xi xi xi
[1]
where c is the concentration, D is dispersion coefficient, ui is velocity component in xi direction, and S represent sink/source terms. Sediment deposition (Sd) and erosion (Se) are the two main source/sink terms in AD equations. Deposition of suspended particles and flocs occurs when the bed shear stress, τb, falls below a critical shear stress for deposition, τcd, causing particles/ flocs to fall to the bed and remain there (without becoming re-suspended immediately). The river bed will be eroded when the bed shear stress, τb, exceeds a critical shear stress for erosion, τce. The deposition and erosion rates (Sd and Se respectively) can be expressed by:
S d Ws c cd b cd
for b cd
Se E0 ce b ce
for b ce
n
[2]
where Ws is settling velocity of sediments, and E0 and n are the erosion coefficient and exponent, respectively. In this study the erosion rate and critical shear stress are taken from a set of experiments on sampled bed materials conducted in a circular channel (Droppo et al., 2014). MIKE 11 can include up to three sediment layers at the bed. Both hindered settling and consolidation are considered in the process of transition from layer 1 to layer 2 and from layer 2 to layer3. Note that flocculation process is not explicitly accounted in the numerical models used for this study. Nevertheless, experimental and numerical results of Droppo and Krishnappan (2014) have shown that, considering the flow and sediment characteristics, flocculation process are not likely to influence sediment transport characteristics in the LAR and tributaries. Note that the sediment/floc size is implicitly considered to the model by its effect on parameters such as settling velocity, critical shear stress, and erosion rate. Ice processes (ice formation, coverage and breakup) can affect flow and sediment transport in cold region rivers (Prowse 2001). Ice-cover eliminates the surficial wind shear stress and adds an under-ice shear stress, resulting in an increase in flow resistance and water level and decrease in bulk velocity and bed shear stress (Ettema and Daley, 2004). Such changes can reduce the rate of bed sediment transport. MIKE11 and EFDC sediment transport models don’t have internally coupled ice-process models. Therefore, an external coupling approach is used in this study, where the simulated results from an ice process model (i.e. CRISSP1D, Shen 2006) is used to modify some flow parameters in the hydrodynamic and sediment transport models. Change in flow resistance due to ice friction can be taken into account by applying the ice shear stress τice and limiting the other surficial shear stresses τs (e.g., wind stress). The modified surface shear stress τ’s is described as:
s ice (1 ) s
where
ice fi U U 8
[3]
where (x, t)[0, 1] is the ice surficial coverage, fi is ice friction coefficient and U is average velocity magnitude. 3.
STUDY AREA AND DATA
The study reach of the LAR extends from below the confluence of the Athabasca and Clearwater Rivers (near the city of Fort McMurray) to Old Fort (a 200 km distance) (Figure 1). Major tributaries contributing to this reach are the Steepbank, Ells, MacKay, Muskeg, and Firebag Rivers. Bathymetry data was obtained from five different sources, including (in order of priority in case of overlap): (1) high resolution (0.5m) Geoswath bathymetry collected by Environment Canada for about ~40km; (2) 54 detailed surveyed cross sections collected by Faye Hicks (2011); (3) six high resolution surveyed reaches collected by CEMA (2012); (4) DEM synthetic sections for further downstream (Pietroniro et al., 2011); and (5) high resolution (5m) LIDAR and the digital elevation model (DEM) data (Geobase) were used to produce the topography of the flood plain and islands. Figure 1 shows an example of the combined bathymetry. To be used in the 1D model, the bathymetry data were interpolated on 200 cross-sections at an average interval of ~1km. Depending on the availability of data, the hydrometric data (flow and water level), to be used as boundary conditions, were obtained from Water Survey Canada (WSC), Regional Aquatics Monitoring Program (RAMP), and flow data simulated using a hydrologic model (developed by Environment Canada). Climate data, to be used in the ice model, (e.g., temperature) where obtained from Environment Canada climate database. Suspended sediment concentration data are taken from WSC and RAMP sediment data. Since the available sediment data are very scattered, these data initially were used to develop the sediment rating curves, then the required continues time series of sediment inflow are produced from the rating curves. 4.
RESULTS AND DISCUSSION
Figure 2 compares the measured and simulated time series (with CRISPP1D with and without ice processes) of water depth at the WSC station 07DA001 (near the upstream boundary - M3 on JOSMP data portal). It also shows a snapshot of the simulated ice covered profile of CRISSP1D (on January 30, 2002). The winter ice coverage causes an increase in water depth which has correctly been predicted by the ice model. A key factor in sediment transport is the bed shear stress. Since we are not able to internally couple the ice model with the MIKE 11 sediment transport model, we have modified the MIKE 11 model parameters to reproduce the effect of winter ice coverage in terms of both water level and 2
E-proceedings of the 36th IAHR World Congress 28 June – 3 July, 2015, The Hague, the Netherlands
bed shear stress. Figure 3 compares the time series of the bed shear stress (near the upstream boundary) obtained from the ice model, MIKE 11 with and without ice effect. The figure shows that, considering the ice effect we have been able to reproduce the significant drop in the winter bed shear stress, predicted by the ice model.
Figure 1. Study area (left) and example of combined river bathymetry (right)
Figure 2. Time series simulated water depth and temperature near the upstream boundary (left) and a snapshot of simulated ice covered river profile (right).
Figure 3. Time series simulated bed shear stress (near upstream) from the ice model and MIKE 11 (with and without ice effect)
Figure 4a compares the time series of the simulated and measured (RAMP) suspended sediment concentration for the Athabasca River at a location downstream of the oilsand developments (~85 km from upstream boundary). The concentrations are plotted in a logarithmic scale to be able to differentiate the lower concentration values. As the figure shows, the simulated and measured sediment concentrations are in a good agreement. The maximum concentrations occur in the warm season in June and July. The ice effect leads to a lower sediment concentration value, although the effect is quite small. This is due to the winter low flow condition causing a low bed shear stress, regardless of the presence or absence of ice-cover. Figure 4b ( sediment concentration rating curve) Illustrates that the simulated and measured results are quite compatible and the effect of ice-cover with respect to winter suspended sediment concentrations is relatively small. Note that the simulations are based on effect of the ice coverage on flow resistance and water depth and have not consider ice formation, breakup and some other ice processes that will have additional significant effect on sediment transport. Figure 5 shows an example of a 2D simulation result for suspended and deposited fine sediments corresponding to a relatively high flow condition. The result shows that fine sediment deposition occurs mostly in the flood plain and on the islands, which is expected as these are areas exposed to wetting and drying with low flow velocities and even stagnant water. Total cohesive suspended sediment concentration is also predicted to be higher in the main channel compared to the surrounding floodplain and islands due to the higher flow. 5.
CONCLUSIONS
1D and 2D hydrodynamic and sediment transport models, externally coupled with a 1D ice model, were used to predict the erosion, deposition and transport of fine sediments within a ~200 km reach of the LAR for both ice-covered and open-water condition. The simulated results for suspended sediment concentration showed a good agreement with the available measurements. The results showed that although the winter ice coverage decreases the bed shear stress, it has relatively minor effect on the magnitude of suspended fine sediment concentration in LAR. 2D simulation results showed a lower concentration of the suspended sediments when flood plains are inundated due to deposition processes. 3
E-proceedings of the 36th IAHR World Congress, 28 June – 3 July, 2015, The Hague, the Netherlands
(a)
(b)
Figure 4. (a) Time series and (b) sediment rating curve, for measured and simulated (with and without ice effect) suspended sediment transport (~85 km from upstream boundary)
Figure 5. Simulated suspended and deposited fine sediment based on the flow condition of July 31, 2011.
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