Ice-covered Flow Conditions Research Articles

Rough ice cover modified hyporheic exchange: A numerical study on the mechanical effect propagating from the ice-water interface to the sediment–water interface

Many rivers are subject to ice-covered flow conditions in cold climate regions or during the winter months. The presence of ice cover can change stream flow hydrodynamics and thus affect the hyporheic exchange involved in many biogeochemical processes in aquatic environments. Among the various features of ice cover, its underside roughness is a main factor modifying the surface–subsurface flow hydrodynamics. To date, there has been little research on how ice cover affects the hyporheic exchange, even less on the effect of the ice cover roughness. This leaves open questions on the mechanical changes initiated at the ice-water interface and transferred to the water–sediment interface. In this study, a coupled surface–subsurface computational approach was used to model conjunctively the channel flow over dune bed and the underlying porous flow under different ice cover conditions. The results show that ice cover can enhance the hyporheic flux and reduce the hyporheic zone depth when compared to open channel flow at equivalent discharge. As the river flow depth increases to 4 times the height of the dune, the effect of the ice cover on the hyporheic flow becomes negligible. Within this effective range, the hyporheic flux via the channel water depth follows a power-law function. Ice cover roughness greatly augments the exchange rate while compresses the exchange space, therefore reduces flow residence time in the hyporheic zone. Based on the modeling results, prediction models were proposed for evaluating the impacts of ice cover roughness on the hyporheic exchange flux and depth. Our study indicates that hyporheic flow exchange is significantly impacted by the roughness of the ice cover which in turn the hyporheic exchange is likely to affect the river ice processes as well as the water quality and ecological functions throughout the river corridors.

Deformation of vegetated channel bed under ice-covered flow conditions

A common feature of cold regions is the presence of ice cover on water surface. In the presence of vegetation in the channel bed which is normally leafless in winter, interaction between river ice and vegetated channel bed becomes very complex. In the present study, the impacts of ice cover and submerged leafless vegetation on channel bed deformation and flow resistance have been investigated based on 144 experiments conducted in a large-scale outdoor flume. The independent variables associated with the maximum scour depth around vegetation elements have been assessed and equations have been developed. Results indicate that the most important variable affecting the maximum scour depth around vegetation under ice-covered flow conditions is the ratio of the ice cover roughness to the bed roughness (ni/nb). However, under open channel flow conditions, the flow Froude number is the most important variable influencing the maximum scour depth. The methods based on the law of the wall provide reliable Manning's coefficient for both smooth and rough ice covers in the presence of submerged vegetation in the channel bed. As the vegetation density increases, the size of scour holes becomes smaller. With the increase in vegetation density, the median grain size of the armour layer in scour holes decreases correspondingly. When vegetation elements are placed in a staggered configuration, the median grain size of the armour layer in scour holes is less than that of squared configuration of vegetation elements. Regardless of the roughness of ice cover on water surface and the grain size of sediment in the bed, the maximum depth of scour holes always occurs at the upstream front face of each vegetation element. Under the rough ice-covered flow condition, the maximum depth of scour holes is more than that under the smooth ice-covered and open flow conditions.

Characteristics of turbulent flow around bridge abutments in the presence of vegetation in channel bed under ice-covered flow conditions

During winter, ice cover frequently forms on the water surface of rivers with vegetated channel beds in cold regions. The investigation of the impacts of both ice cover and channel bed vegetation on flow structures around bridge abutments is essential for engineers to gain a comprehensive understanding of the complex interactions occurring in such a situation. In the present study, the flow structure around a rectangular bridge abutment in the presence of vegetation under ice-covered conditions has been studied. Considering different vegetation densities by arranging vegetation elements in square and staggered configurations, this study incorporates the influence of ice covers with different roughness, namely smooth and rough ice cover. Key turbulence parameters, including turbulence intensity, Reynolds shear stress (RSS), and turbulent kinetic energy (TKE), are also examined based on laboratory experiments. Results show that the shape of velocity profiles for flow over a vegetated channel bed changes from an S-shaped curve under an open flow condition to a convex shape under ice-covered conditions. The effects of an ice cover and vegetation on the flow around the bridge abutment create unpredictable turbulence intensity patterns. Under a rough-covered flow condition, there appears a larger area with negative Reynolds shear stress (RSS) downstream of the abutment. Turbulent kinetic energy (TKE) under ice-covered conditions has substantially lower magnitudes than in open flow conditions.

Sediment Transport Beneath a Simulated Partial Ice Cover: Effects of Asymmetric Border Ice

With the onset of winter in cold regions, border ice begins to form, impacting sediment transport rate and distribution. Understanding the effect of ice cover is crucial in regions with prolonged sub-freezing temperatures, as water bodies remain frozen for a considerable part of the year. While the existing literature includes many studies on sediment transport in open channel flow and several studies on completely ice-covered flow, there is limited research on sediment transport in partially ice-covered channels. Addressing this gap, the current study conducted laboratory experiments in a rectangular flume at the Hydraulics Research and Testing Facility, University of Manitoba, Canada. The investigation focused on examining the influence of asymmetric border ice, varying coverage ratios in asymmetric partially ice-covered flow, and changing flow strengths on bedload transport rate and distribution. Additionally, a comparison was made with symmetric partially ice-covered flow conditions. The findings indicate that the presence of asymmetric border ice indeed affected the bedload transport distribution within the channel, causing non-uniform bedload transport distribution across the channel width, with peak values concentrated in the center of the open flow section. Increased asymmetry in border ice leads to greater asymmetry in bedload transport rate distribution. Despite these pronounced differences in bedload transport rate across the channel width, the cross-section-averaged bedload transport rate could still be estimated utilizing the conventional equations, used for open channel flow, fully ice-covered flow, and symmetric partially ice-covered flow, with the effect of ice cover accounted for by incorporating an additional boundary in the calculation of the wetted perimeter, leading to adjustments in the flow strength.

Local scour around submerged angled spur dikes under ice cover

Local scour is a phenomenon leading to the localized lowering of the channel bed due to the imbalance of sediment transport. As spur dikes protrude into the natural channels, local scour could be triggered. Accurate estimation of local scour around spur dikes is crucial for the effectiveness of erosion control and prevention and habitat enhancement measures. In the current study, the correlations between the maximum scour depth and the overtopping ratio, spur dike dimensions, ice cover roughness, and grain size of the bed material are investigated. Under both open channel and ice-covered flow conditions, a variety of experiments were done in a large-scale outdoor flume with different experimental setups. The results revealed that the scour depths around submerged spur dikes increased with increases in the densimetric Froude number and the decreases in the overtopping ratio and alignment angle. The maximum scour depth around a submerged angled vertical wall spur dike is significantly affected by the presence of an ice cover on the water surface, namely, the rougher the cover, the deeper the scour hole. Based on data collected from the laboratory experiments, an existing maximum scour depth estimation equation has been modified to consider the influence of the cover condition and the submergence level. The calculated results showed high accuracy in estimation of the measured data.

Channel Bed Deformation around Double Piers in Tandem Arrangement in an S-Shaped Channel under Ice Cover

Flow structure and channel bed deformation caused by double piers in a tandem arrangement under ice-covered flow conditions in a bent channel is more complicated than those around a single pier in a straight channel. Based on experiments in an S-shaped flume, the scouring phenomenon at double piers in a tandem arrangement under an ice cover has been conducted by varying pier spacing distance, bend apex cross section (BACS), and hydraulic parameters. Results show that, under identical hydraulic conditions, the variation trend of the scour depth in the vicinity of double piers in a tandem arrangement in a bent channel is similar to that in a straight channel. The deepest depth of scour holes at the upstream BACS is more than that at piers at the downstream BACS. At each BACS, the effect resulting from the interaction of double piers gradually decreases with the pier spacing distance. Different from the characteristics of local scour at double piers in a tandem arrangement in the straight flume, when the ratio of pier spacing distance to pier diameter (L/D) is more than 15, the horseshoe vortex generated by the front pier has negligible impact on the rear pier, and the maximum depth of scour hole at the rear pier scour hole is about 90% that of the front pier. Also, when L/D is higher than 15, the influence of the rear pier on the front one is negligible, and the scour hole depth at the front pier remains the same. However, this phenomenon occurs when the straight flume’s L/D is greater than 17.

Local Scour around Side-by-Side Double Piers in Channel Bends under Ice-Covered Conditions—An Experimental Study

The pier scour process is normally intensified in the presence of an ice cover, which poses risks to the longevity and safety of bridges. In the present study, the impact of the densimetric Froude number, locations, and pier spacing of side-by-side piers on the local scour depth under ice-covered flow conditions were investigated based on clear water scour experiments in an S-shaped laboratory flume. The results demonstrated that the local scour at piers along the convex bank was more substantial than that along the concave bank when other factors stayed identical. The densimetric Froude number clearly has more impact on local scour at piers along the convex bank than that along the concave bank. Different from the mechanism of the pier scour in a straight channel, the scour depth around a pier along the convex bank in the S-shaped flume increases as the distance between two piers (or pier spacing) increases, while it decreases around the piers along the concave bank. Similar scour patterns were observed when the side-by-side piers were installed at different bend apex cross-sections. The maximum local scour depths at piers along the convex bank measured at different bend apex cross-sections were relatively unchanged when other influencing factors were held constant. However, the maximum scour depth around piers along the concave bank decreased as the bends increased toward downstream.

Channel Bed Deformation and Ice Jam Evolution around Bridge Piers

The interaction between the evolution of an ice jam and the local scour at bridge piers becomes much more complicated due to the evolution of both the channel bed and ice jam. Thus, research work regarding this topic has been hardly conducted. In the present study, experiments under different flow conditions with three different pier shapes were carried out. Through laboratory experiments, the development of scour holes around bridge piers under open flow, ice-covered, and ice-jammed flow conditions was compared. The results show that under the same hydraulic condition and with the same ice discharge rate (Qi/Q), the development of an initial ice jam with a local scour around bridge piers along the entire flume takes a relatively short time. However, it takes a longer time for an ice jam to achieve an equilibrium state. With the presence of a local scour at bridge piers, after an ice jam reaches an equilibrium state, the ice jam thickness, water level, and water depth for flow are relatively larger compared to that without a local scour at the pier. The equilibrium ice jam thickness around the pier is negatively correlated with the initial flow Froude number. When the development of an initial ice jam is dominated by a mechanical thickening process, the rate of the development of a scour hole around a pier is faster. On the other hand, when the development of an initial ice jam is dominated by a hydraulic thickening process, the development of a scour hole around a pier can be treated as a scour process under an ice-covered flow condition. An equation was developed to determine the scour depth around a pier under an ice-jammed flow condition by considering related factors such as the flow Froude number, ice jam thickness, and ice discharge rate. The results of this research can provide a reference for bridge design and safety protection, as well as the interaction mechanism of local scour and ice jam evolution.

Local Scour around Tandem Double Piers under an Ice Cover

Compared to the scour around a single pier, the local scour process around tandem double piers is much more complicated. Based on laboratory experiments in a flume, we conducted the scour process around tandem double piers under an ice-covered flow condition. The results showed that when the pier spacing ratio L/D = 2 (where L = the pier spacing distance, and D = the pier diameter), the rear pier (the downstream one) will intensify the horseshoe vortex process behind the front pier, and the scour depth around the front pier will increase by about 10%. As the pier spacing ratio L/D increases, the scour depth around the front pier will gradually decrease. When the pier spacing ratio L/D = 5, sediment scoured around the front pier begins to deposit between these two piers. To initiate a deposition dune between piers, the pier spacing distance under an ice-covered condition is about 20% more than that under an open flow condition. The results also showed that the existence of the rear pier will lead to an increase in the length of the scour hole but a decrease in the depth of the scour hole around the front pier. The local scour around the front pier interacts with the local scour of the rear pier. The maximum scour depth of the scour hole around the rear pier increases first, then decreases and increases again afterward. When the pier spacing ratio L/D = 9, the scour depth around the rear pier is the least. With an increase in the pier spacing ratio, the influence of the local scour around the front pier on the local scour around the rear pier gradually decreases. When the pier spacing ratio L/D is more than 17, the scour around the front pier has hardly any influence on that around the rear pier. The scour depth around the rear pier is about 90% of that around the front pier.

A revisit of the local scour around bridge piers under an ice-covered flow condition—An experimental study

A revisit of the local scour around bridge piers under an ice-covered flow condition—An experimental study

Velocity Field and Turbulence Structure around Spur Dikes with Different Angles of Orientation under Ice Covered Flow Conditions

Spur dikes are well-known structures that are widely used in rivers and coastal regions. Depending on their types, sizes, and orientation angles, spur dikes can substantially change flow characteristics. Results of previous studies indicate that the presence of an ice cover in rivers can cause complicated flow structures. The present experimental study investigates velocity fields and turbulence structures in the vicinity of spur dikes under ice cover with different roughness coefficients. The spur dikes were set up at the following three angles of orientation, 90°, 60°, and 45°. Our results show that the strongest velocity fluctuation occurs immediately above the scour hole surface and very close to the dike tip. The increase in the dike angle toward upstream, the velocity component values increase, leads to a larger scour hole. Results show that an increase in dike angle of each 10° (from 45° to 90°) increases the scour depth between 5% and 10%, depending on flow conditions. Furthermore, the increase in the cover roughness coefficient and the blockage ratio of a spur dike leads to a further increase in turbulence kinetic energy and 3D velocity components values. The findings of this study imply that the appearance of an ice cover can increase turbulence intensities up to nearly 30%.

Macro-Turbulent Flow and Its Impacts on Sediment Transport Potential of a Subarctic River during Ice-Covered and Open-Channel Conditions

Macro-turbulent flows (i.e., coherent flow structures reaching through the whole water column), have not been studied widely in northern seasonally frozen rivers during both open-channel and ice-covered flow conditions. Thus, we aim: (1) to detect and compare the macro-turbulent flow, both at open-channel and ice-covered flow conditions; (2) to explore spatial variation of macro-turbulent flow characteristics within a meander bend; and (3) to detect the effects of near-bed layer velocity fluctuation on bedload transport during differing overall flow conditions. The analyses are based on 5–10 min-long acoustic Doppler current profiler (ADCP) measurements from a subarctic river. The ice-covered low flow, and open-channel higher and lower flow conditions were measured over the period of 2016 to 2020. This study found that macro-turbulent flow existed at all measurement locations under both open-channel and ice-covered flow conditions. Macro-turbulent flow was most consistent and obvious in the streamwise velocity component, and in particular at the inlet and outlet of the investigated meander bend. During all seasons, the near-bed velocities consistently exceeded the sufficient amount for sediment transport. At inlet and outlet areas, the greatest near-bed velocity fluctuation across the critical threshold for sediment transport coincided with the measurement times having frequent macro-turbulent flow.

Velocity profiles and turbulence intensities around side-by-side bridge piers under ice-covered flow condition

Abstract Recent studies have shown that the presence of ice cover leads to an intensified local scour pattern in the vicinity of bridge piers. To investigate the local scour pattern in the vicinity of bridge pier under ice-covered flow condition comparing to that under open channel flow condition, it is essential to examine flow field around bridge piers under different flow conditions. In order to do so, after creation of smooth and rough ice covers, three-dimensional timeaveraged velocity components around four pairs of bridge piers were measured using an Acoustic Doppler velocimetry (ADV). The ADV measured velocity profiles describe the difference between the velocity distributions in the vicinity of bridge piers under different covered conditions. Experimental results show that the vertical velocity distribution which represents the strength of downfall velocity is the greatest under rough covered condition which leads to a greater scour depth. Besides, results show that the turbulent intensity increases with pier size regardless of flow cover, which implies that larger scour depth occurs around piers with larger diameter.

Impact of armour layer on the depth of scour hole around side-by-side bridge piers under ice-covered flow condition

Abstract In the present study, experiments were conducted in a large-scale flume to investigate the issue of local scour around side-by-side bridge piers under both ice-covered and open flow conditions. Three non-uniform sediments were used in this experimental study. Analysis of armour layer in the scour holes around bridge piers was performed to inspect the grain size distribution curves and to study the impact of armour layer on scour depth. Assessments of grain size of deposition ridges at the downstream side of bridge piers have been conducted. Based on data collected in 108 experiments, the independent variables associated with maximum scour depth were assessed. Results indicate that the densi-metric Froude number was the most influential parameter on the maximum scour depth. With the increase in grain size of the armour layer, ice cover roughness and the densimetric Froude number, the maximum scour depth around bridge piers increases correspondingly. Equations have been developed to determine the maximum scour depth around bridge piers under both open flow and ice covered conditions.

Effects of ice cover on the incipient motion of bed material and shear stress around side-by-side bridge piers

In the current study, a series of experiments were carried out to investigate the scouring process around side-by-side bridge piers under both open flow and ice-covered flow conditions. Three non-uniform natural sediments and four pairs of side-by-side bridge piers were used in this experimental study. The mechanism of incipient motion of the sediments under ice-covered conditions has been investigated. The role of the boundary shear Reynolds number, critical dimensionless shear stress, particle densimetric Froude number, grain size and roughness of ice cover on the incipient motion of bed material have been studied. Results show that the critical dimensionless shear stress increases with an increase in the particle densimetric Froude number. In addition, with an increase in the boundary shear Reynolds number, the critical dimensionless shear stress increases correspondingly. For the same critical dimensionless shear stress, the finer sediment needs a lower boundary shear Reynolds number for the incipient motion of sediment particles. For the same grain size of bed material, a lower critical dimensionless shear stress is needed for the incipient motion of sediment particles under rough ice-covered flow condition. Results of this study present valuable outcome to identify the feature of sediment motion near the scour hole under ice-covered flow conditions.

Local scour around two side-by-side cylindrical bridge piers under ice-covered conditions

In the current study, 108 flume experiments with non-uniform, cohesionless sediments have been done to investigate the local scour process around four pairs of side-by-side bridge piers under both open channel and ice-covered flow conditions. Similar to local scour around bridge piers under open channel conditions and a single bridge pier, it was observed in the experiments that the maximum scour depth always occurred at the upstream face of the pier under ice-covered conditions. Further, the smaller the pier size and the greater the spacing distance between the bridge piers, the weaker the horseshoe vortices around the bridge piers, and, thus, the shallower the scour holes around them. Finally, empirical equations were developed to estimate the maximum scour depth around two side-by-side bridge piers under both open channel and ice-covered flow conditions.

Comparison of Three Commonly Used Equations for Calculating Local Scour Depth around Bridge Pier under Ice Covered Flow Condition

A precise prediction of maximum scour depth around bridge foundations under ice covered condition is crucial for their safe design because underestimation may result in bridge failure and over-estimation will lead to unnecessary construction costs. Compared to pier scour depth predictions within an open channel, few studies have attempted to predict the extent of pier scour depth under ice-covered condition. The present work examines scour under ice by using a series of clear-water flume experiments employing two adjacent circular bridge piers in a uniform bed were exposed to open channel and both rough and smooth ice covered channels. The measured scour depths were compared to three commonly used bridge scour equations including Gao’s simplified equation, the HEC-18/Jones equation, and the Froehlich Design Equation. The present study has several advantages as it adds to the understanding of the physics of bridge pier scour under ice cover flow condition, it checks the validity and reliability of commonly used bridge pier equations, and it reveals whether they are valid for the case of scour under ice-covered flow conditions. In addition, it explains how accurately an equation developed for scour under open channel flow can predict scour around bridge piers under ice-covered flow condition.

Local Scour around Bridge Piers under Ice-Covered Conditions

AbstractAn experimental study evaluated the local scour caused by flow past bridge piers in a channel covered by ice. Results show that ice cover affects the scour characteristics, especially at lower flow depths. In the range of test conditions used in the study, with the increase in relative bed coarseness, dimensionless scour depth decreases correspondingly. Including scour radius as an indicator brings forward a new perspective for scour estimation. Furthermore, empirical equations for both scour depth and scour radius are developed under both open channel and ice-covered flow conditions. Present equations are also supported by previous findings related to the influence of relative bed coarseness, flow shallowness, and pier Froude number.

3D modelling of ice-covered flows in the vicinity of a deep hole in the East Channel of the Mackenzie Delta, N.W.T.

The discovery of vast quantities of hydrocarbons during exploration of the Mackenzie Delta has led to the detection of numerous anomalous deep holes in various delta channels in the past four decades. The proposed pipeline crossing from the Mackenzie Delta to northern Alberta has renewed interest in studying the stability of these deep holes. The main goal of this research project was to study ice-covered flow velocity and bed shear stress distributions using a previously calibrated computational fluid dynamics (CFD) model for a 30 m deep hole in the East Channel of the Mackenzie Delta to provide an initial assessment of its stability. In this study, a previously developed 3D CFD model using the FLUENT code with the renormalization group theory (RNG) k-ε turbulence closure model with nonequilibrium wall functions was adapted to represent ice-covered flow conditions for the study reach. The numerical model was applied to simulate four flow conditions (500, 720, and 1000 m3/s along with a moving ice cover during a flow rate of 720 m3/s). Results indicated that a single vortex was formed near the inner bend above the hole and deposition may be a key process occurring during ice-covered flow conditions.

Mixing coefficient for ice-covered and free-surface flows

Dispersion experiments were conducted in four river reaches under both open-water and ice-covered flow conditions. The data were used to obtain the transverse mixing coefficient and to investigate which dimensionless mixing coefficient should be used for ice-covered flows. The results also demonstrate that the sinuosity of a river reach affects the value of the mixing coefficient and can thus be used to guide the selection of mixing coefficient for natural streams.