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Local scour around a bridge pier under ice-jammed flow condition – an experimental study

Local scour around a bridge pier under ice-jammed flow condition – an experimental study J. Hydrol. Hydromech., 69, 2021, 3, 275–287 ©2021. This is an open access article distributed DOI: 10.2478/johh-2021-0014 under the Creative Commons Attribution ISSN 1338-4333 NonCommercial-NoDerivatives 4.0 License Local scour around a bridge pier under ice-jammed flow condition – an experimental study 1 1 1 1 2* 3 Jun Wang , Zhixing Hou , Hongjian Sun , Bihe Fang , Jueyi Sui , Bryan Karney College of Civil and Hydraulic Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, Anhui, China. School of Engineering, University of Northern British Columbia, 3333 University Way, Prince George, BC, Canada. Department of Civil Engineering, University of Toronto, Canada. Corresponding author. E-mail: Jueyi.sui@unbc.ca Abstract: The appearance of an ice jam in a river crucially distorts local hydrodynamic conditions including water level, flow velocity, riverbed form and local scour processes. Laboratory experiments are used for the first time here to study ice-induced scour processes near a bridge pier. Results show that with an ice sheet cover the scour hole depth around a bridge is increased by about 10% compared to under equivalent open flow conditions. More dramatically, ice-jammed flows induce both greater scour depths and scour variability, with the maximum scour depth under an ice-jammed flow as much as 200% greater than under equivalent open flow conditions. Under an ice-jammed condition, both the maximum depth and length of scour holes around a bridge pier increase with the flow velocity while the maximum scour hole depth increases with ice-jam thickness. Also, quite naturally, the height of the resulting deposition dune downstream of a scour hole responds to flow velocity and ice jam thickness. Using the laboratory data under ice-jammed conditions, predictive relationships are derived between the flow’s Froude number and both the dimensionless maximum scour depth and the dimensionless maximum scour length. Keywords: Ice jam; Ice cover; Riverbed deformation; Local scour; Bridge pier. INTRODUCTION piers under the open flow conditions and various relations have been derived to predict the maximum scour depth around a Ice jams can represent key hydrologic elements in temperate single bridge pier (Alemi et al., 2019; Sonia Devi and Barbhui- and polar rivers, occurring repeatedly during some winters. The ya, 2017; Hosseini and Amini, 2015; Zaid et al., 2019). Recent appearance of an ice jam increases the flow’s wetted perimeter, work has considered local scour around multiple piers or pile thus increasing a reach’s resistance, decreasing local velocities groups (Amini and Mohammad, 2016; Khaple et al., 2017) and increasing water levels, possibly even to the point of caus- including the impact of grain size and flow structure on local ing ice flooding. Moreover, an ice jam alters the transport of scour (Abou-Seida et al., 2012; Schendel et al., 2018). However, sediment by temporarily impounding water, thus deforming the much less work has considered local scour under ice-covered riverbed compared to under open flow conditions. The riverbed conditions. scour under an ice-jammed flow condition may precipitate the Clearly, ice covers and ice jams inevitably add an extra solid failure of hydraulic structures such as bridges (Beltaos, 2012; boundary to the flow and increase its wetted perimeter. Under Buzin et al., 2015; Munck et al., 2017). Indeed, bridge piers ice-covered conditions, the location of the maximum flow affect the transportation of both sediments and river ice, induc- velocity migrates closer to the riverbed increasing the bed shear ing variations in local ice jam thickness and water level (Sui et stress around bridge piers. Thus, the riverbed scour depth al., 2010; Wang et al., 2017). Since velocity profiles under ice- around bridge piers under an ice-covered flow condition is covered conditions differ from those under open flow condi- expected to increase (Wang et al., 2008). The first research tions, local scour processes around bridge piers are often pro- work regarding local scour process under ice-covered flow foundly altered. Based on field observations at the Melvin condition was conducted by Bacuta and Dargahi (1986). Based Prince Ship-lock on the Mississippi River, Carr noted the for- on clear-water washout experiments, Bacuta and Dargahi inves- mation of a huge scour hole in front of the foundation of a ship tigated local scour process around a circular bridge pier under lock (Carr and Tuthill, 2012). Although this scour hole was an ice-covered flow condition, arguing that the resulting scour repaired/filled, it reappeared within a year. A laboratory model depth was much greater than under open flow conditions. indicated that even a 100-year flood under an open flow condi- Ackerman et al. (2002) also investigated the impact of ice cover tion could not create such a large scour hole. However, under on local scour around a circular bridge pier performing both an ice-covered flow conditions, a huge scour hole around the clear-water tests and live bed tests. Ackerman and Shen foundation of the Ship-lock was developed with a smaller dis- claimed that the maximum scour depth under an ice-covered charge. As the size of a scour hole is an important indicator of flow condition increased about 25 to 35%, comparing to under the turbulent intensity around a hydraulic structure, so the max- an open flow condition. Munteanu (2004) did experiments imum depth of a scour hole is a helpful index of the stability under different boundary conditions: free surface (FS), total and safety of such a structure (Link et al., 2020; Yang et al., ice-cover (TIC) and partially ice cover along flume walls 2020). (2PIC). Surprisingly, the local scour process around a pier To satisfy engineering requirements, considerable work has under the condition of partially ice-covered flow was found to explored the mechanisms of local scour in the vicinity of bridge be the strongest, and the maximum scour depth around a pier 275 Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney under the condition of partially ice-covered flow being about long to clear-water scouring. The sand bed was leveled before 55% more than that under the open flow condition. Hains and each experimental run and the slope was maintained at 0. An Zabilansky (2004), Zabilansky et al. (2006) investigated the ice hopper which was located between CS-4 and CS-5, was local scour around bridge piers, pointing out that the maximum used to add model ice particles into the flume. At the down- depth of scour hole under the smooth ice-covered flow condi- stream cross section CS-20 which is 2.24 m away from the tion is about 10% greater than that of under the open flow con- outlet of the flume, a Styrofoam panel with the length of 1 m dition. They also noted that the depth of scour holes around was placed on water surface to initiate the formation of an ice bridge piers increases with the increase in the roughness of jams. Due to limitation of laboratory conditions and budget, model ice cover. Sui et al. (2009) assessed the scour patterns researchers normally use other material to model ice. Up to caused by submerged jet under conditions of open flow, smooth date, the commonly used materials to model ice particles or ice ice-covered flow and rough ice-covered flow. Their results floe in laboratory experiments include paraffin, polystyrene, showed that the scour depth under rough ice-covered condition polyethylene, polypropylene (Beltaos, 1995; Healy and Hicks, exceeds that under the open flow condition and that decreasing 2007; Urroz and Ettema, 1992; Urroz et al., 1994; Wang et al., the flow depth increases the scour depth. Wu et al. (2014, 2015) 2015). In the present experimental study, model ice particles also studied local scour finding that an increase in the particle used are made of polyethylene with the mass density of 0.918 size of the armor-layer resulted in a reduction in the depth of g/cm , which is nearly the same as the mass density of natural scour holes. Under rough ice-covered flow condition, the scour ice of 0.917 g/cm . Model ice particles have a flat-ellipsoid depth was found to be the largest. With the increase in grain shape. The longest diameter of model ice particles is 3.5 mm size, the maximum scour depth decreases. However, the maxi- long. The model bridge piers which are cylindrical and installed mum scour depth will increase with the increase in the dimen- in the center of the flume at CS-16 have diameters of D = 2 cm, sionless shear stress. Namaee and Sui (2019a, 2019b, 2019c, D = 3 cm, D = 4 cm, respectively. Before each experimental 2020) assessed the impacts of ice cover on the scour process run, the flow condition in the flume was adjusted by changing around 2 side-by-side bridge piers, again showing that the local the flow rate and the tailgate. The rate of ice particles dis- scour around side-by-side bridge piers was greater than that charged from the ice hopper was adjusted and controlled. around a singular bridge pier. Their results also indicated that According to the standard requirements of the People's Re- more sediment deposited at the downstream side of those side- public of China (MTPRC, 2004), the bridge pier diameter by-side bridge piers comparing to that of singular bridge pier. should be less than 0.8 m when the bridge span is in the range Regardless of flow cover, the vertical turbulent intensity was of 5 – 20 m, namely, the pier-to-span radio (d/B) ranges from highest exactly over the channel bed and diminished towards 0.04 to 0.16. Also, a lot of researchers did numerical simula- the flow surface, implying that the shear stress is greatest on the tions and experimental studies on local scour around cylindrical channel bed causing sediment to be transported at a higher rate. piers under open flow conditions (Jiang et al., 1994; Ling et al., Additionally, under the same flow condition, the value of turbu- 2007; Wei et al., 2015; Zhu and Liu, 2011). Existing research lence kinetic energy increased with the pier size. works showed that the D/B ranges from 0.04 to 0.11. In the All previous studies regarding the impact of ice cover on the present study, the ratio of bridge pier diameter (D = 2 cm) to local scour processes around bridge piers/abutments have been the width of the laboratory flume (B = 40 cm) is 0.05, which is conducted in the laboratory using a sheet ice cover. However, reasonable. the extreme riverbed deformation occurs under the ice-jammed Cross section CS-5 was used as the control cross section. flow condition in natural rivers (Sui et al., 2000). Yet, flow The approaching water depth (H ) and the initial average flow conditions and local scour processes around bridge piers under velocity (V ) at CS-5 are used as the initial hydraulic condition. ice-jammed conditions have not yet been investigated. To ad- At each cross section between CS-5 and CS-20, piezometers dress this and to provide engineers with some knowledge re- were installed to monitor the variation in water level. During garding the mechanism of local scour process at piers, the cur- each experimental run, measurements of water level, ice jam rent conceptual study reports on laboratory experiments on the thickness and length of scour hole around the pier were impact of an ice jam on the pier scour processes, for the first recorded. The maximum scour depth was measured at the end time measuring and modeling the interaction between the chan- of each experimental run. Experiments showed that after about nel bed deformation and ice accumulation around a bridge pier. 6 hours, no significant changes in the scour depth and jam thickness, and the quasi-equilibrium depth of scour hole and EXPERIMENT SETUP thickness of ice jam achieved. However, all experiment runs lasted 24 hours to ensure that the local scour process and ice Experiments have been carried out in a laboratory flume at jam accumulation around the bridge pier reach equilibrium. Hefei University of Technology. As sketched in Figure 1, the Under such an equilibrium condition, the shape of the flume is 26.68 m long and 0.4 m wide. In total, 22 observation deposition dune doesn’t change since the scour process at the cross sections (CS) along the flume with an equal spacing dis- pier stops completely. Additionally, the ice jam thickness tance of 1.2 m have been setup. Between CS-2 and CS-22, a doesn’t change. In total, 45 experiments have been conducted sand bed with an initial thickness of 10 cm was formed. The under different flow conditions (flow depth, flow velocity) and median grain size (d ) of sand bed material is 0.71 mm and the cover conditions (open flow, sheet ice cover, and ice jam), as inhomogeneity coefficient η is 1.61. All experiment runs be- showed in Table 1. Fig. 1. Flume for experiment of local scour under ice jammed condition. 276 Local scour around a bridge pier under ice-jammed flow condition – an experimental study Table 1. Flow conditions for each experimental run. Average ap- Average ap- Grain Grain Approaching Pier Approaching Pier proaching flow proaching flow size of size of Number flow depth H diameter Number flow depth H diameter 0 0 velocity velocity sand bed sand bed (m) D (m) (m) D (m) –1 –1 V (m·s ) d (mm) V (m·s ) d (mm) 0 50 0 50 A1 0.17 0.20 0.02 0.714 B12 0.20 0.30 0.02 0.714 A2 0.18 0.20 0.02 0.714 C1 0.16 0.20 0.02 0.714 A3 0.19 0.20 0.02 0.714 C2 0.17 0.20 0.02 0.714 A4 0.20 0.20 0.02 0.714 C3 0.18 0.20 0.02 0.714 A5 0.17 0.25 0.02 0.714 C4 0.19 0.20 0.02 0.714 A6 0.18 0.25 0.02 0.714 C5 0.20 0.20 0.02 0.714 A7 0.19 0.25 0.02 0.714 C6 0.16 0.25 0.02 0.714 A8 0.20 0.25 0.02 0.714 C7 0.17 0.25 0.02 0.714 A9 0.17 0.30 0.02 0.714 C8 0.18 0.25 0.02 0.714 A10 0.18 0.30 0.02 0.714 C9 0.19 0.25 0.02 0.714 A11 0.19 0.30 0.02 0.714 C10 0.20 0.25 0.02 0.714 A12 0.20 0.30 0.02 0.714 C11 0.16 0.30 0.02 0.714 B1 0.17 0.20 0.02 0.714 C12 0.17 0.30 0.02 0.714 B2 0.18 0.20 0.02 0.714 C13 0.18 0.30 0.02 0.714 B3 0.19 0.20 0.02 0.714 C14 0.19 0.30 0.02 0.714 B4 0.20 0.20 0.02 0.714 C15 0.20 0.30 0.02 0.714 B5 0.17 0.25 0.02 0.714 C16 0.16 0.25 0.03 0.714 B6 0.18 0.25 0.02 0.714 C17 0.18 0.25 0.03 0.714 B7 0.19 0.25 0.02 0.714 C18 0.20 0.25 0.03 0.714 B8 0.20 0.25 0.02 0.714 C19 0.16 0.25 0.04 0.714 B9 0.17 0.30 0.02 0.714 C20 0.18 0.25 0.04 0.714 B10 0.18 0.30 0.02 0.714 C21 0.20 0.25 0.04 0.714 B11 0.19 0.30 0.02 0.714 OBSERVATIONS OF EXPERIMENTAL RUNS gresses upstream, water level increases simultaneously along the entire flume. In the first stage, experiments show that the The local scour processes around a bridge pier under the water level near the pier approaches to be constant value after open flow condition are clearly different from those occurring an increase of about 0.5 cm (comparing to the initial water level under the sheet ice-covered flow condition. But in the experi- under the open flow condition). Also, before the initial ice jam ment, it should be noted that the sheet ice cover normally floats approaches the bridge pier, due to the local scour process in on the water surface and remains stationary. By contrast, under front of the bridge pier, the water level increases gradually. the ice-jammed conditions, both the scour hole and ice jam With the development of an initial ice jam, both the depth and deform simultaneously. Since the thickness of ice jam has width of the scour hole increase gradually. The development of significant impact on the deformation of scour hole (Wang et a scour hole starts from the front face of the bridge pier, ex- al., 2016; Wang et al., 2018), the local scour around bridge tends on both the left and right sides, eventually reaching the piers under the ice-jammed flow condition is inevitably more rear side of the bridge pier. This scouring process develops complex than that occurring under the sheet ice-covered flow slowly. After about 30 minutes, the initial ice jam nearly ap- condition. Based on the scour hole around the bridge pier under proaches the bridge pier (say, before the bridge pier is sur- an ice-jammed flow condition, the development of the ice jam rounded by the initial ice jam). During this period, due to the can be divided into 3 stages. As observed that all experimental increase in water depth caused by the initial ice jam, the local runs showed the same characteristics in the scour process and scour process around the bridge pier gradually weakens. Name- ice accumulation around bridge piers. To clearly illustrate the ly, the rate of development of the scour hole around the bridge development of an ice jam and the local scour process around a pier is reduced. Results showed that, before the bridge pier is bridge pier, one experimental case is used to explain the scour surrounded by the initial ice jam, both the depth and width of process and ice accumulation around a bridge pier. This case the scour hole are less than those under an open flow condition has the initial approaching flow depth of H0 = 25 cm and the with the same initial flow condition (flow depth and velocity), initial average approaching flow velocity of V0 = 18 cm/s. as indicated in Table 2. During the initial development of an ice jam, the discharged As the ice jam arrives at the bridge pier, local scour around ice particles from the ice-hopper are transported downstream, the bridge pier intensifies. After the ice jam moves beyond the and an initial ice jam starts to form at the downstream cross bridge pier, the scour hole near the pier clearly develops more section CS-21. With the continuously incoming ice particles quickly than it did before the ice jam reached the pier. During from upstream, the initial ice jam progresses from the down- the development period of an initial ice jam, local scour process stream cross section CS-21 to upstream. Experiments showed around a bridge pier is similar to that under a sheet ice-covered that the initial ice jam has approximately the same thickness flow condition, since the thickness of the ice jam along the along the entire flume. Before the initial ice jam approaches the entire flume is approximately constant. After about 90 minutes bridge pier, all model ice particles that floating on water surface or so, the head of the initial ice jam has reached to CS-5. As pass around the bridge pier, and water flow in front of the ini- shown in Tables 2 and 3, one can see the differences in scour tial ice jam remains free surface. As the initial ice jam pro- depth and water level before and after the initial ice jam passes 277 Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney the pier, compared to that under an open flow condition. Re- (at the crest of an “ice-particle wave”) can reach to 14 cm. Note sults showed that, after the ice jam passes the bridge pier, the though that the thickness of an ice jam decreases along the depth of the scour hole is increased to a depth which is the flume from upstream to downstream. During the downstream same as the maximum scour depth under an open flow condi- migration process of an “ice-particle wave”, the jam thickness tion with the same initial flow condition (flow depth and veloci- also increases. The local scour process around the bridge pier ty). Also, the length of the scour hole under the initial also increase correspondingly. When the crest of an “ice- ice-jammed flow condition exceeds that under an open flow particle wave” approaches/passes the bridge pier, the local condition with the same initial flow condition. As indicated in scour process around the bridge pier is obviously intensified. Table 3, during the development of an initial ice jam, the water Many sediment particles are eroded from the scour hole and level increased significantly comparing to that under an open transported to the downstream side of the bridge pier. After flow condition with the same initial flow condition. Thus, the about 4 hours (240 minutes) in the experiment, quite a few ice- presence of an ice jam clearly induces increases in the scour particle waves had migrated downstream and passed the bridge depth, scour length and water level. pier. Both the depth and length of scour holes around the bridge After the development of an initial ice jam (which has ap- pier further increased to values much greater than those occur- proximately the same thickness along the entire flume) along ring under open flow conditions, as summarized in Table 4. the entire flume, then enters the thickening process of an ice As shown in Figure 2, an ice-particle wave migrates at cross jam accompanied by the ice-particle wave migration. The in- section CS-16. At the crest of the ice-particle wave, the jam coming ice particles from the ice hopper will be submerged and thickness is obviously greater than that before the ice wave entrained by the flowing water. These submerged ice particles migrates downstream. The flow depth at CS-16 (where the crest accumulate under the head of the initial ice jam. Thus, the head of the ice-particle wave is located) decreases, and thus the flow of the initial ice jam will become progressively thicker. During velocity increases. During the migration of an ice-particle this thickening process, the flow cross-sectional area naturally wave, an intensified local scour process around the bridge pier decreases and the velocity of flowing water correspondingly at CS-16 will result. Clearly, the propagation of ice-particle increases. Thus, more of the ice particles accumulated under the waves plays is a key factor in the evolution of the channel bed head of ice jam will be eroded and transported downstream. around the bridge pier. Over time, the initial ice jam will be thickened from upstream to To be more specific, a coordinate system was established at downstream. This will cause an increase in water level from CS-16. The origin of the coordinate system is located at the upstream to downstream. Interestingly, the underside surface of center of the bridge pier. Flow direction is opposite to the ab- ice jams has been gradually developed as an undulating or wavy scissa direction (X-axis). The Y-axis is perpendicular to flow surface, similar to the migrating sand or bed deposits during direction from the left flume wall to the right flume wall. The sediment transport. This undulating surface of the underside direction of the vertical axis (Z-axis) is upward. Figures 3a and surface of an ice jam is defined as the “ice-particle migrating 3b show the contour lines of scour holes around the bridge pier wave” (termed as the “ice particle wave” in this study). after the local scour process achieves an equilibrium condition Due to the presence of this ice particle wave, the thickness under both open flow and ice-jammed flow conditions, respec- of an ice jam consequently varies. Experiments show that this tively. Figures 4a and 4b are 3D illustrations of the scour holes “ice-particle wave” phenomenon affects the local scour around at equilibrium under both open flow and ice-jammed condi- the bridge pier. Results indicate that when the thickness of an tions, respectively. The dotted lines in the contour maps repre- ice jam around the pier increases (wave crest), local scour in- sent the scour hole whereas solid lines indicate the deposition tensifies; when the thickness of an ice jam around the bridge mound downstream of the pier. pier decreases (wave trough), the local scour is moderated. As the scour patterns around bridge piers in Figures 3a to 4b When the thickness of an initial ice jam is about 5 cm, due to show, all scour holes are approximately symmetrically distrib- the accumulation of ice particles, the equilibrium thickness of uted along the centerline of flume passing the center of the an ice jam at the same cross section often increases to about bridge pier. The maximum scour depths are located at the front double, or about 10 cm. However, the maximum jam thickness face of the bridge pier (facing flow). Under an ice-jammed flow Table 2. Comparison of the depth and length of scour hole and water level (30 minutes after experiment started, initial flow condition: H0 = 25 cm, V0 = 0.18 m/s). Scour depth (cm) Scour length (cm) Water level (cm) Initial ice-jam nearly approaches the pier 0.3 4.9 35.3 Open flow condition 1.2 6.5 25.2 Table 3. Comparison of the depth and length of scour hole and water level (90 minutes after experiment started, initial flow condition: H0 = 25 cm, V0 = 0.18 m/s). Scour depth (cm) Scour length (cm) Water level (cm) Initial ice-jam passes the pier 1.2 7.2 35.3 Open flow condition 1.2 6.5 25.2 Table 4. Comparison of the depth and length of scour hole and water level (240 minutes after experiment started, initial flow condition: H0 =25 cm, V0 = 0.18 m/s). Scour depth (cm) Scour length (cm) Water level (cm) Ice-jammed flow condition 2.2 10.5 35.5 Open flow condition 1.2 6.5 25.2 278 Local scour around a bridge pier under ice-jammed flow condition – an experimental study Fig. 2. Migration of an ice-particle wave around CS-16. Fig. 3a. Contour lines for scour hole and deposition mound around the bridge pier under an open flow condition (unit: cm). Fig. 3b. Contour lines for scour hole and deposition mound around the bridge pier under an ice-jammed flow condition (unit: cm). Fig. 4a. The 3D illustration for a scour hole and deposition mound Fig. 4b. The 3D illustration for a scour hole and deposition mound around the bridge pier under the open flow condition (unit: cm). around the bridge pier under an ice-jammed flow condition (unit: cm). 279 Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney condition, both the maximum depth and the maximum length of particles are rapidly eroded and transported downstream. scour holes exceed those under an open flow condition with the Whenever the trough of an ice-particle wave passes the cross same initial flow condition. Also, patterns of the downstream section where the bridge pier is located, the flow depth under deposition dune under an ice-jammed condition differ from that the ice jam is increased. Thus, flow velocity around the scour under an open flow condition. An intuitive comparison of scour hole decreases, and the local scour around the bridge pier is hole profiles under different flow conditions is given in Figure weakened. Overall, once initiated, the downstream propagation 5. Obviously, both the maximum depth and the maximum of the ice-particle waves continued throughout each experi- length of scour holes under an ice-jammed condition are great- mental run. The alternation from the crest of “ice-particle er. Interestingly, the slope of the deposition dune downstream wave” to the trough of “ice-particle wave” was continually of the bridge pier under an ice jammed flow condition is con- repeated. Correspondingly, the alternation of the local scour siderably gentler and extended further downstream than those process continues from the intensified scour process to the under an open flow condition. As indicated in Table 5, under an weakened scour process, until the local scour process reached ice-jammed flow condition, the maximum scour depth of 3.5 an equilibrium condition. cm is much more than that of 1.7 cm under an open flow condi- tion. Also, the maximum length of the deposition dune under an RESULTS ice-jammed flow condition is 17.45 cm which exceeds that under an open flow condition. The height of the deposition Under an open flow condition, without a deformation of dune under an ice jammed flow condition is 1.44 cm, which is channel bed, the flow field around the bridge pier is highly about twice of that under an open flow condition. complex. With the development of a scour hole around the Experiments showed that the development of scour holes bridge pier and a deposition dune downstream of the bridge under the initial ice-jammed condition differs from that under pier, the flow field becomes even more complicated. According the thickening process of an ice jam. Also, before an initial ice to Hafez (2016), the scour commences in the region of the jam approaches the bridge pier from downstream, the local highest velocity in the vicinity of the separating streamline. The scour processes around the bridge pier differs from that after horseshoe vortex which forms at the pier face shifts the maxi- the initial ice jam passes the bridge pier. Under conditions of mum downflow velocity closer to the pier in the scour hole. The downflow acts as a vertical jet to erode a groove in front of both open flow and the initial ice jam, the local scour process starts at the front face of the bridge pier. The scour hole gradu- the pier. The eroded sand particles are carried around the pier ally becomes deeper and wider. After an initial ice jam passes by the combined action of an accelerating flow and the spiral the bridge pier, the local scour process at the pier is similar to motion of the horseshoe vortex. Melville and Coleman (2000) that under a sheet ice-covered flow condition, since the thick- report a wake-vortex system which occurs behind the pier, acts ness of the initial ice jam doesn’t change much. During the ice like a vacuum hose sucking up bed material and transporting jam thickening (or ice-particle wave migration) process, the sand particles moved by the horseshoe vortex system and by the thickness of the ice jam varies. Whenever the crest of an “ice- downward flow to locations downstream of the pier. However, particle wave” passes the cross section where the bridge pier is wake-vortices are not normally as strong as the horseshoe vor- located, the cross-sectional area for flow decreases, and the tex and therefore, are not able to carry as great a sediment load flow velocity around the scour hole is clearly increased. Thus, as the horseshoe vortex. Therefore, sediment deposition is the local scour around the bridge pier is strengthened when the likely to occur downstream of the pier in the form of a sediment crest of an “ice-particle wave” passes the bridge pier. Sediment deposition dune. Table 5. The maximum depth and length of scour hole and height of deposition dune (Equilibrium ice jammed condition, initial flow con- dition: H = 25 cm, V = 0.18 m/s). 0 0 Scour depth (cm) Scour length (cm) Height of deposition dune (cm) Condition of equilibrium ice jam 3.5 17.45 1.44 Open flow condition 1.3 7.15 0.70 Open channel flow Ice-jammed flow Qi=0.026L/s, V0=0.18m/s, H0=25cm -30 -20 -10 0 10 20 30 -1 -2 -3 -4 Along flow direction (cm) Note: Flow direction to the left <---------------- Fig. 5. Profiles of scour hole the under open flow and ice-jammed conditions. Scour depth hs(cm) Local scour around a bridge pier under ice-jammed flow condition – an experimental study When an ice cover or ice jam is present, the velocity profile laboratory setups, all results indicate that an ice sheet intensifies is significantly modified. Quite naturally, the location of the the local scour at the pier. maximum velocity is now intermediate between the channel However, under an ice-jammed flow condition, the maxi- bed and ice cover, with a specific location that depends on the mum scour depth is much more (or over 200% more) than that ratio of the cover roughness to the bed roughness (Sui et al., under an open flow condition. Clearly, the impact of an ice jam 2010). As a result, the local scour process is modified as well. on the local scour around the bridge pier is much more than of a sheet ice-cover. Depth of scour hole When the initial approaching flow velocity is low, the max- imum scour depth under an ice-jammed flow condition is much The comparison of the maximum depths of scour holes un- greater (200 to 400%) than that under an open flow condition. der the open flow, sheet ice-covered flow and ice-jammed flow By contrast, when the initial approaching flow velocity is high, the maximum scour depth under an ice-jammed flow condition conditions are given in Figures 6a and 6b for different initial is about twice than that under an open flow condition, which is approaching flow depths of 25 cm and 30 cm, respectively. The itself much greater than that under both sheet ice-covered flow following summary observation can be drawn: With the same and open flow conditions. initial flow condition (same initial approaching flow depth and Regardless of flow cover conditions (open flow, sheet ice- flow velocity), the maximum scour depth under a sheet ice- covered flow and ice-jammed flow), the maximum scour depth covered flow condition is greater (about 13% more) than that increases with the flow velocity. Interestingly, under an ice- under a free surface condition. This finding is different from jammed flow condition, the maximum scour depth shows a results reported by other researchers as mentioned above gentler increase with velocity comparing to that under both (Ackermann et al., 2002; Munteanu, 2004). Although results open flow and sheet ice-covered flow conditions. obtained different researchers are different due to the different Ice-jammed flow Open channel flow Sheet ice-covered flow 4.00 3.00 d50=0.71mm Qi=0.026L/s H =25cm 2.00 1.00 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0)(m/s) Fig. 6a. The maximum scour depth vs. the initial approaching flow velocity (initial approaching flow depth: H = 25 cm). Ice-jammed flow Open channel flow Sheet ice-covered flow 5.00 4.00 3.00 d =0.71mm Qi=0.026L/s H =30cm 2.00 1.00 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0)(m/s) Fig. 6b. The maximum scour depth vs. the initial approaching flow velocity (initial approaching flow depth: H = 30 cm). Depth of scour hole (hs)(cm) Depth of scour hole (hs)(cm) Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney Results showed that, when the initial approaching flow depth maximum lengths of scour holes under open flow, sheet ice- is shallow, changes in the flow velocity will dramatically affect covered flow and ice-jammed flow conditions are given in the maximum scour depth under an ice-jammed flow condition. Figures 10a and 10b for different initial approaching flow Figure 7 gives the dependence of the dimensionless maxi- depths of 25 cm and 30 cm, respectively. The following state- mum scour depth (hs/H) on the flow Froude Number (Fr = ments summarize these observations: 0.5 V/(gH) , where, V = average flow velocity, g = gravitational With the same initial flow condition (same initial approaching acceleration, and H = water depth). Under an ice-jammed flow flow depth and flow velocity), the maximum scour length under a condition, the data sets are clearly above those under both open sheet ice-covered flow condition slightly exceeds (by about 5%) flow and sheet ice-covered flow conditions. that occurring under an open flow condition. However, under an Overall (regardless flow cover condition): ice-jammed flow condition, the maximum scour length tends to be much greater (even 110% more) than that under an open flow hV condition. Clearly, the impact of an ice jam on the local scour =− 2.439 0.227 with R² = 0.950 (1) process greatly exceeds that of a sheet ice-cover.Regardless of H gH flow cover conditions (open flow, sheet ice-covered and ice- jammed flow), the maximum scour length increases with the Under an ice-jammed flow condition: flow velocity. With the same initial flow condition, the maxi- mum scour range under an ice-jammed flow condition is much hV with R² = 0.951 (2) =− 2.014 0.146 larger than those under both open flow and sheet ice-covered H gH flow conditions. When the initial approaching flow depth is shallow, changes Clearly, regardless of flow cover conditions, the dimension- in flow velocity will dramatically affect the maximum scour less maximum scour depth increases with the flow Froude length under an ice-jammed flow condition, comparing to that Number, as showed in Equations (1) and (2). with a deep initial approaching flow depth. Figure 8 shows the relationship between the dimensionless Figure 11 gives the dependence of the dimensionless maxi- maximum scour depth (hs/H) and the dimensionless thickness mum scour length (Ls/H) on the flow Froude Number (Fr). of ice jam (Ti/H). One can see that the thicker an ice jam, the Clearly, regardless of flow cover conditions, the dimensionless deeper the maximum depth of a scour around the pier. Namely, maximum scour length increases with the flow Froude Number: a thicker ice jam can lead to a deeper scour hole around a Overall (regardless flow cover condition): bridge pier. This explains why the local scour process is inten- sified when the crest of an “ice-particle wave” passes the bridge LV =− 11.011 0.986 , with R² = 0.943 (3) pier, and why the local scour process is diminished when the gH trough of an “ice-particle wave” passes the bridge pier. The size of bridge pier is also an important factor affecting Under an ice-jammed flow condition: the scour depth. Results showed that the larger the pier diame- ter, the deeper the maximum depth of scour hole around the LV pier. Figure 9 shows the relationship between the maximum =− 11.67 1.074 , with R² = 0.912 (4) H gH scour depth (hs/H) and the pier size (D/H). Length of scour hole Under an ice-jammed flow condition, the observed data clearly lie above those under both open flow and sheet ice- The length of scour hole is an indicator of the range of a lo- covered flow conditions. cal scour hole around a bridge pier. The comparison of the Fig. 7. Dependence of the maximum scour depth on the flow Froude Number. 282 Local scour around a bridge pier under ice-jammed flow condition – an experimental study Fig. 8. Relationship between the maximum scour depth and the thickness of ice jam. Re lation be tween the maximum s cour de pth and bridge 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.05 0.075 0.1 0.125 0.15 0.175 The bridge pie r diame ter( D/H) Fig. 9. Relationship between the maximum scour depth and the bridge pier diameter. Deposition dune deposition dune is different from that under both sheet ice- covered flow and open flow conditions. One can see from Fig- Experiments show that, during a local scour process around ure 12, the height of a deposition dune under an ice-jammed the bridge pier, an associated deposition dune develops down- flow condition increases gently with the initial approaching stream of the pier. The features of a deposition dune (such as flow velocity. However, under both sheet ice-covered flow and length, height and slope) under an open flow condition are open flow conditions, the height of deposition dunes increases different from those under both sheet ice-covered flow and ice- rapidly with the initial approaching flow velocity. jammed flow conditions. The comparison of the maximum Under an ice-jammed flow condition, with the same initial ap- heights of deposition dunes under open flow, sheet ice-covered proaching flow depth, the thickness of an ice jam decreases with flow and ice-jammed flow conditions are given in Figure 12 for the increase in the flow velocity. As a consequence, with the an initial approaching flow depth of 20 cm. Following observa- increase in flow velocity, the wave height of the ice particle wave tions can be summarized: decreases. Thus, under a relatively high flow velocity, the impact With the same initial approaching flow depth, regardless of of the migration process of an “ice particle wave” on the height flow cover conditions, progressively more sediment is eroded of a deposition dune is diminished. Namely, the height of a depo- from the scour hole and delivered downstream, thus forming sition dune under an ice-jammed flow condition is less than that the deposition dune. The higher the initial approaching flow under both sheet ice-covered flow and open flow conditions. velocity, the larger (thicker) will be the deposition dune. Under an ice-jammed flow condition, much more sediment Under varied cover conditions, different characteristics of is eroded from the scour hole around the bridge pier and trans- the deposition dune will be evident. Under an ice-jammed flow ported to downstream, compared to that under both sheet ice- condition, since flow velocity is increased, the thickness of an covered flow and open flow conditions. However, the slope of a ice jam varies. This process affects the local scour process deposition dune under an ice-jammed flow condition is much around the bridge pier, and thus, the pattern of a deposition longer but milder than those under both sheet ice-covered flow dune. Namely, the impact of an ice jam on the pattern of a and open flow conditions. M aximum s cour de pth( hs/H) Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney Ice-jammed flow Open channel flow Sheet ice-covered flow 20.00 15.00 Qi=0.026L/s H =25cm 10.00 5.00 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0)(m/s) Fig. 10a. The maximum scour length vs. the initial approaching flow velocity (initial approaching flow depth: H = 25 cm). Ice-jammed flow Open channel flow Sheet ice-covered flow 20.00 15.00 Qi=0.026L/s H =30cm 10.00 5.00 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0)(m/s) Fig. 10b. The maximum scour length vs. the initial approaching flow velocity (initial approaching flow depth: H0 = 30 cm). Fig. 11. Dependence of the maximum scour length on the flow Froude Number. Results showed, regardless of flow cover conditions, the  TV maximum height of deposition dune (Ts/H) increases with the =+ 0.108 ln 0.271 with R² = 0.86 (5)  H gH flow Froude Number (Fr), as presented in Figure 13,  Length of scour hole (Ls)(cm) Length of scour hole (Ls)(cm) Local scour around a bridge pier under ice-jammed flow condition – an experimental study Ice-jammed flow Sheet ice-covered flow Open channel flow 2.00 1.60 1.20 0.80 d =0.71mm Qi=0.026L/s H =20cm 0.40 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0) Fig. 12. The maximum height of deposition dune vs. the initial approaching flow velocity (initial approaching flow depth: H = 20 cm). Fig. 13. Dependence of the maximum height of deposition dune (Ts/H) on the flow Froude Number (Fr). CONCLUSIONS process in the vicinity of a bridge pier under an ice-jammed flow condition. the following strong tendencies can be noted: The present experimental research is only a conceptual study 1). With the same initial flow condition, the maximum for providing engineering with some knowledge regarding the scour depth under a sheet ice-covered flow condition exceeds mechanism of local scour process at piers under an ice-jammed that under an open flow condition. However, under an ice- flow condition. The evolution process of ice jams in natural jammed flow condition, the maximum scour depth is much rivers is very complex. The coefficients of the proposed equa- greater than that under an open flow condition. tions derived from data gained in laboratory experiments need 2). When the initial approaching flow velocity is low, to be further modified based on data collected in natural rivers. both the maximum scour depth and maximum length of scour Based on both laboratory experiments and field investigations hole under an ice-jammed flow condition are much greater than of ice jams in natural river, Sui et al. (1994) claimed that the that under both sheet ice-covered flow and open flow evolution mechanism of model ice jam in laboratory is similar conditions. Also, the range of the deposition dune under an ice- to that of ice jams in the Hequ Reach of the Yellow River. The jammed flow condition is much larger, comparing to that under purpose of this research work is to explore the complex mecha- both sheet ice-covered flow and open flow conditions. nism of local scour around bridge piers under an ice-jammed 3). Both the maximum scour depth and scour length flow condition. Laboratory experiments clarify the local scour under an ice-jammed flow condition increases with the flow Height of deposition dune (Ts)( cm) Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney velocity. The dimensionless maximum scour depth increases LLC, Highlands Ranch, Colorado, U.S.A., pp.155–172. with the flow Froude Number. Results show that the thicker the Beltaos, S., 2012. Distributed function analysis of ice jam flood ice jam, the deeper the maximum depth of a scour hole around frequency.Cold Regions Science and Technology, 71, 2, the bridge pier. One can say that a thicker ice jam will likely 1–10. lead to a deeper scour hole around a bridge pier. When the crest Buzin, V.A.,Goroshkova, N.I.,Strizhenok A.V., 2015. Max- of an ice-particle wave passes the bridge pier, the local scour imum ice-jam water levels on the northern rivers of Russia process is intensified; but when the trough of this wave passes, under conditions of climate change and anthropogenic im- the local scour process diminishes. pact on the ice jamming process. Russian Meteorology & 4). Under an ice-jammed flow condition, the impact of an Hydrology, 39, 12, 823–827. ice jam on the pattern of a deposition dune is different from that Carr, M.L., Tuthill, M.A., 2012. Modeling of Scour-Inducing under both sheet ice-covered flow and open flow conditions. Ice Effects at Melvin Price Lock and Dam. Journal of Hy- With the same initial approaching flow depth, regardless of draulic Engineering, 138, 1, 85–92. flow cover conditions, the higher the initial approaching flow Günal, M., Gelmeran, T.A., Günal, A.Y., 2017. Local scour velocity, the higher the deposition dune. The height of a around group bridge pier with different shapes. Acta Physica deposition dune under an ice-jammed flow condition increases Polonica Series a, 132, 3, 632–633. gently with the initial approaching flow velocity. However, Hafez, Y.I., 2016. Mathematical modeling of local scour at under both sheet ice-covered flow and open flow conditions, slender and wide bridge piers. Journal of Fluids, Article ID: the height of deposition dune increases rapidly with the initial 4835253. http://dx.doi.org/10.1155/2016/4835253 approaching flow velocity. With the increase in flow velocity, Hains, D.B., Zabilansky, L.J, 2004. Laboratory test of scour the wave height of the ice particle wave decreases. Thus, under under ice: Data and preliminary results. U.S. Army Engineer a relatively high flow velocity, the impact of the migration Research and Development Center,Cold Regions Research process of an ice particle wave on the height of a deposition and Engineering Laboratory, Hanover, New Hampshire, dune is diminished. Also, the slope of the deposition dune Technical Report TR-04-9 under an ice-jammed flow condition is much longer but milder (http://www.crrel.usace.army.mil/techpub/CRREL_Reports/ than that under both sheet ice-covered flow and open flow reports/TRO4-9.pdf). conditions. Regardless of flow cover conditions, the Healy, D., Hicks, F.E., 2007. Experimental study of ice jam dimensionless maximum height of deposition dune increases thickening under dynamic flowconditions. Journal of Cold with the flow Froude Number. Regions Engineering, 21, 3, 72–91. 5). Most of the existing studies regarding local scour at Hosseini, R., Amini, A., 2015. Scour depth estimation methods piers are cylindrical piers. To our knowledge, the mechanisms around pile groups. KSCE Journal of Civil Engineering, 19, regarding local scour around piers under an ice-jammed 7, 2144–2156. condition has never explored. The present research is the first Jiang, H., 1994. Experimental study of local scour protection on one regarding this topic. In present study, only cylindrical piers bridge pier. Highway, 8, 1–8. with different pier diameters have been used. As reported by Khaple, S., Hanmaiahgari, P.R., Gaudio, R., Dey, S., 2017. researchers (Günal et al., 2017; Tyminski, 2010), under the Interference of an upstream pier on local scour at down- open flow conditions, the scour process around different pier stream piers. Acta Geophysica, 65, 1, 29–46. types is different. The future research work will be focused on Ling, J., Lin, X., Zhao, H., 2007. Analysis of three-dimensional the local scour process by using different pier types and flow field and local scour of riverbed around cylindrical arrangement under an ice jammed condition. pier. Nature Science, Journal of Tongji University, 35, 5, 582–586. Acknowledgement. This research is supported by the National Link, O., Garcia, M., Pizarro, A., Alcayaga, H., Palma, S., Natural Science Foundation of China (Grant Nos. 51879065). 2020. Local scour and sediment deposition at bridge piers The authors are grateful for the financial support. during floods. Journal of Hydraulic Engineering, 146, 3, Ar- ticle Number: 04020003. REFERENCES Melville, B.W., Coleman, S.E., 2000. Bridge Scour. Water Resources Publications. Abou-Seida, M.M., Elsaeed, G.H., Mostafa T.M., Elzahry, E.F., MTPRC, 2004. General Specifications for Design of Highway 2012. Local scour at bridge abutments in cohesive soil. Bridges and Culverts (JTG D60-2004). Ministry of Journal of Hydraulic Research, 50, 2, 171–180. 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A 864. review on the methods used to reduce the scouring effect of Urroz, G.E., Schaefer, J., Ettema, R., 1994. Bridge-pier location bridge pier. Energy Procedia, 160, 45–50. and ice conveyance in curved channels. Journal of Cold Re- Zhu, Z., Liu, Z., 2011. Three-dimensional numerical simulation gions Engineering, 8, 2, 66–72. for local scour around cylindric bridge pier. China Journal of Wang, J., Sui, J., Karney, B.W., 2008. Incipient motion of non- Highway and Transport, 24, 2, 42–48. cohesive sediment under ice cover – An experimental study. Journal of Hydrodynamics, 20, 1, 117–124. Received 17 January 2021 Wang, J., Chen P., Yang, Q., 2015. Impact of bridge piers on Accepted 2 June 2021 ice jam stage variation: An experimental study. Advances in Water Science, 26, 6, 867–873. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Hydrology and Hydromechanics de Gruyter

Local scour around a bridge pier under ice-jammed flow condition – an experimental study

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J. Hydrol. Hydromech., 69, 2021, 3, 275–287 ©2021. This is an open access article distributed DOI: 10.2478/johh-2021-0014 under the Creative Commons Attribution ISSN 1338-4333 NonCommercial-NoDerivatives 4.0 License Local scour around a bridge pier under ice-jammed flow condition – an experimental study 1 1 1 1 2* 3 Jun Wang , Zhixing Hou , Hongjian Sun , Bihe Fang , Jueyi Sui , Bryan Karney College of Civil and Hydraulic Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, Anhui, China. School of Engineering, University of Northern British Columbia, 3333 University Way, Prince George, BC, Canada. Department of Civil Engineering, University of Toronto, Canada. Corresponding author. E-mail: Jueyi.sui@unbc.ca Abstract: The appearance of an ice jam in a river crucially distorts local hydrodynamic conditions including water level, flow velocity, riverbed form and local scour processes. Laboratory experiments are used for the first time here to study ice-induced scour processes near a bridge pier. Results show that with an ice sheet cover the scour hole depth around a bridge is increased by about 10% compared to under equivalent open flow conditions. More dramatically, ice-jammed flows induce both greater scour depths and scour variability, with the maximum scour depth under an ice-jammed flow as much as 200% greater than under equivalent open flow conditions. Under an ice-jammed condition, both the maximum depth and length of scour holes around a bridge pier increase with the flow velocity while the maximum scour hole depth increases with ice-jam thickness. Also, quite naturally, the height of the resulting deposition dune downstream of a scour hole responds to flow velocity and ice jam thickness. Using the laboratory data under ice-jammed conditions, predictive relationships are derived between the flow’s Froude number and both the dimensionless maximum scour depth and the dimensionless maximum scour length. Keywords: Ice jam; Ice cover; Riverbed deformation; Local scour; Bridge pier. INTRODUCTION piers under the open flow conditions and various relations have been derived to predict the maximum scour depth around a Ice jams can represent key hydrologic elements in temperate single bridge pier (Alemi et al., 2019; Sonia Devi and Barbhui- and polar rivers, occurring repeatedly during some winters. The ya, 2017; Hosseini and Amini, 2015; Zaid et al., 2019). Recent appearance of an ice jam increases the flow’s wetted perimeter, work has considered local scour around multiple piers or pile thus increasing a reach’s resistance, decreasing local velocities groups (Amini and Mohammad, 2016; Khaple et al., 2017) and increasing water levels, possibly even to the point of caus- including the impact of grain size and flow structure on local ing ice flooding. Moreover, an ice jam alters the transport of scour (Abou-Seida et al., 2012; Schendel et al., 2018). However, sediment by temporarily impounding water, thus deforming the much less work has considered local scour under ice-covered riverbed compared to under open flow conditions. The riverbed conditions. scour under an ice-jammed flow condition may precipitate the Clearly, ice covers and ice jams inevitably add an extra solid failure of hydraulic structures such as bridges (Beltaos, 2012; boundary to the flow and increase its wetted perimeter. Under Buzin et al., 2015; Munck et al., 2017). Indeed, bridge piers ice-covered conditions, the location of the maximum flow affect the transportation of both sediments and river ice, induc- velocity migrates closer to the riverbed increasing the bed shear ing variations in local ice jam thickness and water level (Sui et stress around bridge piers. Thus, the riverbed scour depth al., 2010; Wang et al., 2017). Since velocity profiles under ice- around bridge piers under an ice-covered flow condition is covered conditions differ from those under open flow condi- expected to increase (Wang et al., 2008). The first research tions, local scour processes around bridge piers are often pro- work regarding local scour process under ice-covered flow foundly altered. Based on field observations at the Melvin condition was conducted by Bacuta and Dargahi (1986). Based Prince Ship-lock on the Mississippi River, Carr noted the for- on clear-water washout experiments, Bacuta and Dargahi inves- mation of a huge scour hole in front of the foundation of a ship tigated local scour process around a circular bridge pier under lock (Carr and Tuthill, 2012). Although this scour hole was an ice-covered flow condition, arguing that the resulting scour repaired/filled, it reappeared within a year. A laboratory model depth was much greater than under open flow conditions. indicated that even a 100-year flood under an open flow condi- Ackerman et al. (2002) also investigated the impact of ice cover tion could not create such a large scour hole. However, under on local scour around a circular bridge pier performing both an ice-covered flow conditions, a huge scour hole around the clear-water tests and live bed tests. Ackerman and Shen foundation of the Ship-lock was developed with a smaller dis- claimed that the maximum scour depth under an ice-covered charge. As the size of a scour hole is an important indicator of flow condition increased about 25 to 35%, comparing to under the turbulent intensity around a hydraulic structure, so the max- an open flow condition. Munteanu (2004) did experiments imum depth of a scour hole is a helpful index of the stability under different boundary conditions: free surface (FS), total and safety of such a structure (Link et al., 2020; Yang et al., ice-cover (TIC) and partially ice cover along flume walls 2020). (2PIC). Surprisingly, the local scour process around a pier To satisfy engineering requirements, considerable work has under the condition of partially ice-covered flow was found to explored the mechanisms of local scour in the vicinity of bridge be the strongest, and the maximum scour depth around a pier 275 Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney under the condition of partially ice-covered flow being about long to clear-water scouring. The sand bed was leveled before 55% more than that under the open flow condition. Hains and each experimental run and the slope was maintained at 0. An Zabilansky (2004), Zabilansky et al. (2006) investigated the ice hopper which was located between CS-4 and CS-5, was local scour around bridge piers, pointing out that the maximum used to add model ice particles into the flume. At the down- depth of scour hole under the smooth ice-covered flow condi- stream cross section CS-20 which is 2.24 m away from the tion is about 10% greater than that of under the open flow con- outlet of the flume, a Styrofoam panel with the length of 1 m dition. They also noted that the depth of scour holes around was placed on water surface to initiate the formation of an ice bridge piers increases with the increase in the roughness of jams. Due to limitation of laboratory conditions and budget, model ice cover. Sui et al. (2009) assessed the scour patterns researchers normally use other material to model ice. Up to caused by submerged jet under conditions of open flow, smooth date, the commonly used materials to model ice particles or ice ice-covered flow and rough ice-covered flow. Their results floe in laboratory experiments include paraffin, polystyrene, showed that the scour depth under rough ice-covered condition polyethylene, polypropylene (Beltaos, 1995; Healy and Hicks, exceeds that under the open flow condition and that decreasing 2007; Urroz and Ettema, 1992; Urroz et al., 1994; Wang et al., the flow depth increases the scour depth. Wu et al. (2014, 2015) 2015). In the present experimental study, model ice particles also studied local scour finding that an increase in the particle used are made of polyethylene with the mass density of 0.918 size of the armor-layer resulted in a reduction in the depth of g/cm , which is nearly the same as the mass density of natural scour holes. Under rough ice-covered flow condition, the scour ice of 0.917 g/cm . Model ice particles have a flat-ellipsoid depth was found to be the largest. With the increase in grain shape. The longest diameter of model ice particles is 3.5 mm size, the maximum scour depth decreases. However, the maxi- long. The model bridge piers which are cylindrical and installed mum scour depth will increase with the increase in the dimen- in the center of the flume at CS-16 have diameters of D = 2 cm, sionless shear stress. Namaee and Sui (2019a, 2019b, 2019c, D = 3 cm, D = 4 cm, respectively. Before each experimental 2020) assessed the impacts of ice cover on the scour process run, the flow condition in the flume was adjusted by changing around 2 side-by-side bridge piers, again showing that the local the flow rate and the tailgate. The rate of ice particles dis- scour around side-by-side bridge piers was greater than that charged from the ice hopper was adjusted and controlled. around a singular bridge pier. Their results also indicated that According to the standard requirements of the People's Re- more sediment deposited at the downstream side of those side- public of China (MTPRC, 2004), the bridge pier diameter by-side bridge piers comparing to that of singular bridge pier. should be less than 0.8 m when the bridge span is in the range Regardless of flow cover, the vertical turbulent intensity was of 5 – 20 m, namely, the pier-to-span radio (d/B) ranges from highest exactly over the channel bed and diminished towards 0.04 to 0.16. Also, a lot of researchers did numerical simula- the flow surface, implying that the shear stress is greatest on the tions and experimental studies on local scour around cylindrical channel bed causing sediment to be transported at a higher rate. piers under open flow conditions (Jiang et al., 1994; Ling et al., Additionally, under the same flow condition, the value of turbu- 2007; Wei et al., 2015; Zhu and Liu, 2011). Existing research lence kinetic energy increased with the pier size. works showed that the D/B ranges from 0.04 to 0.11. In the All previous studies regarding the impact of ice cover on the present study, the ratio of bridge pier diameter (D = 2 cm) to local scour processes around bridge piers/abutments have been the width of the laboratory flume (B = 40 cm) is 0.05, which is conducted in the laboratory using a sheet ice cover. However, reasonable. the extreme riverbed deformation occurs under the ice-jammed Cross section CS-5 was used as the control cross section. flow condition in natural rivers (Sui et al., 2000). Yet, flow The approaching water depth (H ) and the initial average flow conditions and local scour processes around bridge piers under velocity (V ) at CS-5 are used as the initial hydraulic condition. ice-jammed conditions have not yet been investigated. To ad- At each cross section between CS-5 and CS-20, piezometers dress this and to provide engineers with some knowledge re- were installed to monitor the variation in water level. During garding the mechanism of local scour process at piers, the cur- each experimental run, measurements of water level, ice jam rent conceptual study reports on laboratory experiments on the thickness and length of scour hole around the pier were impact of an ice jam on the pier scour processes, for the first recorded. The maximum scour depth was measured at the end time measuring and modeling the interaction between the chan- of each experimental run. Experiments showed that after about nel bed deformation and ice accumulation around a bridge pier. 6 hours, no significant changes in the scour depth and jam thickness, and the quasi-equilibrium depth of scour hole and EXPERIMENT SETUP thickness of ice jam achieved. However, all experiment runs lasted 24 hours to ensure that the local scour process and ice Experiments have been carried out in a laboratory flume at jam accumulation around the bridge pier reach equilibrium. Hefei University of Technology. As sketched in Figure 1, the Under such an equilibrium condition, the shape of the flume is 26.68 m long and 0.4 m wide. In total, 22 observation deposition dune doesn’t change since the scour process at the cross sections (CS) along the flume with an equal spacing dis- pier stops completely. Additionally, the ice jam thickness tance of 1.2 m have been setup. Between CS-2 and CS-22, a doesn’t change. In total, 45 experiments have been conducted sand bed with an initial thickness of 10 cm was formed. The under different flow conditions (flow depth, flow velocity) and median grain size (d ) of sand bed material is 0.71 mm and the cover conditions (open flow, sheet ice cover, and ice jam), as inhomogeneity coefficient η is 1.61. All experiment runs be- showed in Table 1. Fig. 1. Flume for experiment of local scour under ice jammed condition. 276 Local scour around a bridge pier under ice-jammed flow condition – an experimental study Table 1. Flow conditions for each experimental run. Average ap- Average ap- Grain Grain Approaching Pier Approaching Pier proaching flow proaching flow size of size of Number flow depth H diameter Number flow depth H diameter 0 0 velocity velocity sand bed sand bed (m) D (m) (m) D (m) –1 –1 V (m·s ) d (mm) V (m·s ) d (mm) 0 50 0 50 A1 0.17 0.20 0.02 0.714 B12 0.20 0.30 0.02 0.714 A2 0.18 0.20 0.02 0.714 C1 0.16 0.20 0.02 0.714 A3 0.19 0.20 0.02 0.714 C2 0.17 0.20 0.02 0.714 A4 0.20 0.20 0.02 0.714 C3 0.18 0.20 0.02 0.714 A5 0.17 0.25 0.02 0.714 C4 0.19 0.20 0.02 0.714 A6 0.18 0.25 0.02 0.714 C5 0.20 0.20 0.02 0.714 A7 0.19 0.25 0.02 0.714 C6 0.16 0.25 0.02 0.714 A8 0.20 0.25 0.02 0.714 C7 0.17 0.25 0.02 0.714 A9 0.17 0.30 0.02 0.714 C8 0.18 0.25 0.02 0.714 A10 0.18 0.30 0.02 0.714 C9 0.19 0.25 0.02 0.714 A11 0.19 0.30 0.02 0.714 C10 0.20 0.25 0.02 0.714 A12 0.20 0.30 0.02 0.714 C11 0.16 0.30 0.02 0.714 B1 0.17 0.20 0.02 0.714 C12 0.17 0.30 0.02 0.714 B2 0.18 0.20 0.02 0.714 C13 0.18 0.30 0.02 0.714 B3 0.19 0.20 0.02 0.714 C14 0.19 0.30 0.02 0.714 B4 0.20 0.20 0.02 0.714 C15 0.20 0.30 0.02 0.714 B5 0.17 0.25 0.02 0.714 C16 0.16 0.25 0.03 0.714 B6 0.18 0.25 0.02 0.714 C17 0.18 0.25 0.03 0.714 B7 0.19 0.25 0.02 0.714 C18 0.20 0.25 0.03 0.714 B8 0.20 0.25 0.02 0.714 C19 0.16 0.25 0.04 0.714 B9 0.17 0.30 0.02 0.714 C20 0.18 0.25 0.04 0.714 B10 0.18 0.30 0.02 0.714 C21 0.20 0.25 0.04 0.714 B11 0.19 0.30 0.02 0.714 OBSERVATIONS OF EXPERIMENTAL RUNS gresses upstream, water level increases simultaneously along the entire flume. In the first stage, experiments show that the The local scour processes around a bridge pier under the water level near the pier approaches to be constant value after open flow condition are clearly different from those occurring an increase of about 0.5 cm (comparing to the initial water level under the sheet ice-covered flow condition. But in the experi- under the open flow condition). Also, before the initial ice jam ment, it should be noted that the sheet ice cover normally floats approaches the bridge pier, due to the local scour process in on the water surface and remains stationary. By contrast, under front of the bridge pier, the water level increases gradually. the ice-jammed conditions, both the scour hole and ice jam With the development of an initial ice jam, both the depth and deform simultaneously. Since the thickness of ice jam has width of the scour hole increase gradually. The development of significant impact on the deformation of scour hole (Wang et a scour hole starts from the front face of the bridge pier, ex- al., 2016; Wang et al., 2018), the local scour around bridge tends on both the left and right sides, eventually reaching the piers under the ice-jammed flow condition is inevitably more rear side of the bridge pier. This scouring process develops complex than that occurring under the sheet ice-covered flow slowly. After about 30 minutes, the initial ice jam nearly ap- condition. Based on the scour hole around the bridge pier under proaches the bridge pier (say, before the bridge pier is sur- an ice-jammed flow condition, the development of the ice jam rounded by the initial ice jam). During this period, due to the can be divided into 3 stages. As observed that all experimental increase in water depth caused by the initial ice jam, the local runs showed the same characteristics in the scour process and scour process around the bridge pier gradually weakens. Name- ice accumulation around bridge piers. To clearly illustrate the ly, the rate of development of the scour hole around the bridge development of an ice jam and the local scour process around a pier is reduced. Results showed that, before the bridge pier is bridge pier, one experimental case is used to explain the scour surrounded by the initial ice jam, both the depth and width of process and ice accumulation around a bridge pier. This case the scour hole are less than those under an open flow condition has the initial approaching flow depth of H0 = 25 cm and the with the same initial flow condition (flow depth and velocity), initial average approaching flow velocity of V0 = 18 cm/s. as indicated in Table 2. During the initial development of an ice jam, the discharged As the ice jam arrives at the bridge pier, local scour around ice particles from the ice-hopper are transported downstream, the bridge pier intensifies. After the ice jam moves beyond the and an initial ice jam starts to form at the downstream cross bridge pier, the scour hole near the pier clearly develops more section CS-21. With the continuously incoming ice particles quickly than it did before the ice jam reached the pier. During from upstream, the initial ice jam progresses from the down- the development period of an initial ice jam, local scour process stream cross section CS-21 to upstream. Experiments showed around a bridge pier is similar to that under a sheet ice-covered that the initial ice jam has approximately the same thickness flow condition, since the thickness of the ice jam along the along the entire flume. Before the initial ice jam approaches the entire flume is approximately constant. After about 90 minutes bridge pier, all model ice particles that floating on water surface or so, the head of the initial ice jam has reached to CS-5. As pass around the bridge pier, and water flow in front of the ini- shown in Tables 2 and 3, one can see the differences in scour tial ice jam remains free surface. As the initial ice jam pro- depth and water level before and after the initial ice jam passes 277 Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney the pier, compared to that under an open flow condition. Re- (at the crest of an “ice-particle wave”) can reach to 14 cm. Note sults showed that, after the ice jam passes the bridge pier, the though that the thickness of an ice jam decreases along the depth of the scour hole is increased to a depth which is the flume from upstream to downstream. During the downstream same as the maximum scour depth under an open flow condi- migration process of an “ice-particle wave”, the jam thickness tion with the same initial flow condition (flow depth and veloci- also increases. The local scour process around the bridge pier ty). Also, the length of the scour hole under the initial also increase correspondingly. When the crest of an “ice- ice-jammed flow condition exceeds that under an open flow particle wave” approaches/passes the bridge pier, the local condition with the same initial flow condition. As indicated in scour process around the bridge pier is obviously intensified. Table 3, during the development of an initial ice jam, the water Many sediment particles are eroded from the scour hole and level increased significantly comparing to that under an open transported to the downstream side of the bridge pier. After flow condition with the same initial flow condition. Thus, the about 4 hours (240 minutes) in the experiment, quite a few ice- presence of an ice jam clearly induces increases in the scour particle waves had migrated downstream and passed the bridge depth, scour length and water level. pier. Both the depth and length of scour holes around the bridge After the development of an initial ice jam (which has ap- pier further increased to values much greater than those occur- proximately the same thickness along the entire flume) along ring under open flow conditions, as summarized in Table 4. the entire flume, then enters the thickening process of an ice As shown in Figure 2, an ice-particle wave migrates at cross jam accompanied by the ice-particle wave migration. The in- section CS-16. At the crest of the ice-particle wave, the jam coming ice particles from the ice hopper will be submerged and thickness is obviously greater than that before the ice wave entrained by the flowing water. These submerged ice particles migrates downstream. The flow depth at CS-16 (where the crest accumulate under the head of the initial ice jam. Thus, the head of the ice-particle wave is located) decreases, and thus the flow of the initial ice jam will become progressively thicker. During velocity increases. During the migration of an ice-particle this thickening process, the flow cross-sectional area naturally wave, an intensified local scour process around the bridge pier decreases and the velocity of flowing water correspondingly at CS-16 will result. Clearly, the propagation of ice-particle increases. Thus, more of the ice particles accumulated under the waves plays is a key factor in the evolution of the channel bed head of ice jam will be eroded and transported downstream. around the bridge pier. Over time, the initial ice jam will be thickened from upstream to To be more specific, a coordinate system was established at downstream. This will cause an increase in water level from CS-16. The origin of the coordinate system is located at the upstream to downstream. Interestingly, the underside surface of center of the bridge pier. Flow direction is opposite to the ab- ice jams has been gradually developed as an undulating or wavy scissa direction (X-axis). The Y-axis is perpendicular to flow surface, similar to the migrating sand or bed deposits during direction from the left flume wall to the right flume wall. The sediment transport. This undulating surface of the underside direction of the vertical axis (Z-axis) is upward. Figures 3a and surface of an ice jam is defined as the “ice-particle migrating 3b show the contour lines of scour holes around the bridge pier wave” (termed as the “ice particle wave” in this study). after the local scour process achieves an equilibrium condition Due to the presence of this ice particle wave, the thickness under both open flow and ice-jammed flow conditions, respec- of an ice jam consequently varies. Experiments show that this tively. Figures 4a and 4b are 3D illustrations of the scour holes “ice-particle wave” phenomenon affects the local scour around at equilibrium under both open flow and ice-jammed condi- the bridge pier. Results indicate that when the thickness of an tions, respectively. The dotted lines in the contour maps repre- ice jam around the pier increases (wave crest), local scour in- sent the scour hole whereas solid lines indicate the deposition tensifies; when the thickness of an ice jam around the bridge mound downstream of the pier. pier decreases (wave trough), the local scour is moderated. As the scour patterns around bridge piers in Figures 3a to 4b When the thickness of an initial ice jam is about 5 cm, due to show, all scour holes are approximately symmetrically distrib- the accumulation of ice particles, the equilibrium thickness of uted along the centerline of flume passing the center of the an ice jam at the same cross section often increases to about bridge pier. The maximum scour depths are located at the front double, or about 10 cm. However, the maximum jam thickness face of the bridge pier (facing flow). Under an ice-jammed flow Table 2. Comparison of the depth and length of scour hole and water level (30 minutes after experiment started, initial flow condition: H0 = 25 cm, V0 = 0.18 m/s). Scour depth (cm) Scour length (cm) Water level (cm) Initial ice-jam nearly approaches the pier 0.3 4.9 35.3 Open flow condition 1.2 6.5 25.2 Table 3. Comparison of the depth and length of scour hole and water level (90 minutes after experiment started, initial flow condition: H0 = 25 cm, V0 = 0.18 m/s). Scour depth (cm) Scour length (cm) Water level (cm) Initial ice-jam passes the pier 1.2 7.2 35.3 Open flow condition 1.2 6.5 25.2 Table 4. Comparison of the depth and length of scour hole and water level (240 minutes after experiment started, initial flow condition: H0 =25 cm, V0 = 0.18 m/s). Scour depth (cm) Scour length (cm) Water level (cm) Ice-jammed flow condition 2.2 10.5 35.5 Open flow condition 1.2 6.5 25.2 278 Local scour around a bridge pier under ice-jammed flow condition – an experimental study Fig. 2. Migration of an ice-particle wave around CS-16. Fig. 3a. Contour lines for scour hole and deposition mound around the bridge pier under an open flow condition (unit: cm). Fig. 3b. Contour lines for scour hole and deposition mound around the bridge pier under an ice-jammed flow condition (unit: cm). Fig. 4a. The 3D illustration for a scour hole and deposition mound Fig. 4b. The 3D illustration for a scour hole and deposition mound around the bridge pier under the open flow condition (unit: cm). around the bridge pier under an ice-jammed flow condition (unit: cm). 279 Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney condition, both the maximum depth and the maximum length of particles are rapidly eroded and transported downstream. scour holes exceed those under an open flow condition with the Whenever the trough of an ice-particle wave passes the cross same initial flow condition. Also, patterns of the downstream section where the bridge pier is located, the flow depth under deposition dune under an ice-jammed condition differ from that the ice jam is increased. Thus, flow velocity around the scour under an open flow condition. An intuitive comparison of scour hole decreases, and the local scour around the bridge pier is hole profiles under different flow conditions is given in Figure weakened. Overall, once initiated, the downstream propagation 5. Obviously, both the maximum depth and the maximum of the ice-particle waves continued throughout each experi- length of scour holes under an ice-jammed condition are great- mental run. The alternation from the crest of “ice-particle er. Interestingly, the slope of the deposition dune downstream wave” to the trough of “ice-particle wave” was continually of the bridge pier under an ice jammed flow condition is con- repeated. Correspondingly, the alternation of the local scour siderably gentler and extended further downstream than those process continues from the intensified scour process to the under an open flow condition. As indicated in Table 5, under an weakened scour process, until the local scour process reached ice-jammed flow condition, the maximum scour depth of 3.5 an equilibrium condition. cm is much more than that of 1.7 cm under an open flow condi- tion. Also, the maximum length of the deposition dune under an RESULTS ice-jammed flow condition is 17.45 cm which exceeds that under an open flow condition. The height of the deposition Under an open flow condition, without a deformation of dune under an ice jammed flow condition is 1.44 cm, which is channel bed, the flow field around the bridge pier is highly about twice of that under an open flow condition. complex. With the development of a scour hole around the Experiments showed that the development of scour holes bridge pier and a deposition dune downstream of the bridge under the initial ice-jammed condition differs from that under pier, the flow field becomes even more complicated. According the thickening process of an ice jam. Also, before an initial ice to Hafez (2016), the scour commences in the region of the jam approaches the bridge pier from downstream, the local highest velocity in the vicinity of the separating streamline. The scour processes around the bridge pier differs from that after horseshoe vortex which forms at the pier face shifts the maxi- the initial ice jam passes the bridge pier. Under conditions of mum downflow velocity closer to the pier in the scour hole. The downflow acts as a vertical jet to erode a groove in front of both open flow and the initial ice jam, the local scour process starts at the front face of the bridge pier. The scour hole gradu- the pier. The eroded sand particles are carried around the pier ally becomes deeper and wider. After an initial ice jam passes by the combined action of an accelerating flow and the spiral the bridge pier, the local scour process at the pier is similar to motion of the horseshoe vortex. Melville and Coleman (2000) that under a sheet ice-covered flow condition, since the thick- report a wake-vortex system which occurs behind the pier, acts ness of the initial ice jam doesn’t change much. During the ice like a vacuum hose sucking up bed material and transporting jam thickening (or ice-particle wave migration) process, the sand particles moved by the horseshoe vortex system and by the thickness of the ice jam varies. Whenever the crest of an “ice- downward flow to locations downstream of the pier. However, particle wave” passes the cross section where the bridge pier is wake-vortices are not normally as strong as the horseshoe vor- located, the cross-sectional area for flow decreases, and the tex and therefore, are not able to carry as great a sediment load flow velocity around the scour hole is clearly increased. Thus, as the horseshoe vortex. Therefore, sediment deposition is the local scour around the bridge pier is strengthened when the likely to occur downstream of the pier in the form of a sediment crest of an “ice-particle wave” passes the bridge pier. Sediment deposition dune. Table 5. The maximum depth and length of scour hole and height of deposition dune (Equilibrium ice jammed condition, initial flow con- dition: H = 25 cm, V = 0.18 m/s). 0 0 Scour depth (cm) Scour length (cm) Height of deposition dune (cm) Condition of equilibrium ice jam 3.5 17.45 1.44 Open flow condition 1.3 7.15 0.70 Open channel flow Ice-jammed flow Qi=0.026L/s, V0=0.18m/s, H0=25cm -30 -20 -10 0 10 20 30 -1 -2 -3 -4 Along flow direction (cm) Note: Flow direction to the left <---------------- Fig. 5. Profiles of scour hole the under open flow and ice-jammed conditions. Scour depth hs(cm) Local scour around a bridge pier under ice-jammed flow condition – an experimental study When an ice cover or ice jam is present, the velocity profile laboratory setups, all results indicate that an ice sheet intensifies is significantly modified. Quite naturally, the location of the the local scour at the pier. maximum velocity is now intermediate between the channel However, under an ice-jammed flow condition, the maxi- bed and ice cover, with a specific location that depends on the mum scour depth is much more (or over 200% more) than that ratio of the cover roughness to the bed roughness (Sui et al., under an open flow condition. Clearly, the impact of an ice jam 2010). As a result, the local scour process is modified as well. on the local scour around the bridge pier is much more than of a sheet ice-cover. Depth of scour hole When the initial approaching flow velocity is low, the max- imum scour depth under an ice-jammed flow condition is much The comparison of the maximum depths of scour holes un- greater (200 to 400%) than that under an open flow condition. der the open flow, sheet ice-covered flow and ice-jammed flow By contrast, when the initial approaching flow velocity is high, the maximum scour depth under an ice-jammed flow condition conditions are given in Figures 6a and 6b for different initial is about twice than that under an open flow condition, which is approaching flow depths of 25 cm and 30 cm, respectively. The itself much greater than that under both sheet ice-covered flow following summary observation can be drawn: With the same and open flow conditions. initial flow condition (same initial approaching flow depth and Regardless of flow cover conditions (open flow, sheet ice- flow velocity), the maximum scour depth under a sheet ice- covered flow and ice-jammed flow), the maximum scour depth covered flow condition is greater (about 13% more) than that increases with the flow velocity. Interestingly, under an ice- under a free surface condition. This finding is different from jammed flow condition, the maximum scour depth shows a results reported by other researchers as mentioned above gentler increase with velocity comparing to that under both (Ackermann et al., 2002; Munteanu, 2004). Although results open flow and sheet ice-covered flow conditions. obtained different researchers are different due to the different Ice-jammed flow Open channel flow Sheet ice-covered flow 4.00 3.00 d50=0.71mm Qi=0.026L/s H =25cm 2.00 1.00 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0)(m/s) Fig. 6a. The maximum scour depth vs. the initial approaching flow velocity (initial approaching flow depth: H = 25 cm). Ice-jammed flow Open channel flow Sheet ice-covered flow 5.00 4.00 3.00 d =0.71mm Qi=0.026L/s H =30cm 2.00 1.00 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0)(m/s) Fig. 6b. The maximum scour depth vs. the initial approaching flow velocity (initial approaching flow depth: H = 30 cm). Depth of scour hole (hs)(cm) Depth of scour hole (hs)(cm) Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney Results showed that, when the initial approaching flow depth maximum lengths of scour holes under open flow, sheet ice- is shallow, changes in the flow velocity will dramatically affect covered flow and ice-jammed flow conditions are given in the maximum scour depth under an ice-jammed flow condition. Figures 10a and 10b for different initial approaching flow Figure 7 gives the dependence of the dimensionless maxi- depths of 25 cm and 30 cm, respectively. The following state- mum scour depth (hs/H) on the flow Froude Number (Fr = ments summarize these observations: 0.5 V/(gH) , where, V = average flow velocity, g = gravitational With the same initial flow condition (same initial approaching acceleration, and H = water depth). Under an ice-jammed flow flow depth and flow velocity), the maximum scour length under a condition, the data sets are clearly above those under both open sheet ice-covered flow condition slightly exceeds (by about 5%) flow and sheet ice-covered flow conditions. that occurring under an open flow condition. However, under an Overall (regardless flow cover condition): ice-jammed flow condition, the maximum scour length tends to be much greater (even 110% more) than that under an open flow hV condition. Clearly, the impact of an ice jam on the local scour =− 2.439 0.227 with R² = 0.950 (1) process greatly exceeds that of a sheet ice-cover.Regardless of H gH flow cover conditions (open flow, sheet ice-covered and ice- jammed flow), the maximum scour length increases with the Under an ice-jammed flow condition: flow velocity. With the same initial flow condition, the maxi- mum scour range under an ice-jammed flow condition is much hV with R² = 0.951 (2) =− 2.014 0.146 larger than those under both open flow and sheet ice-covered H gH flow conditions. When the initial approaching flow depth is shallow, changes Clearly, regardless of flow cover conditions, the dimension- in flow velocity will dramatically affect the maximum scour less maximum scour depth increases with the flow Froude length under an ice-jammed flow condition, comparing to that Number, as showed in Equations (1) and (2). with a deep initial approaching flow depth. Figure 8 shows the relationship between the dimensionless Figure 11 gives the dependence of the dimensionless maxi- maximum scour depth (hs/H) and the dimensionless thickness mum scour length (Ls/H) on the flow Froude Number (Fr). of ice jam (Ti/H). One can see that the thicker an ice jam, the Clearly, regardless of flow cover conditions, the dimensionless deeper the maximum depth of a scour around the pier. Namely, maximum scour length increases with the flow Froude Number: a thicker ice jam can lead to a deeper scour hole around a Overall (regardless flow cover condition): bridge pier. This explains why the local scour process is inten- sified when the crest of an “ice-particle wave” passes the bridge LV =− 11.011 0.986 , with R² = 0.943 (3) pier, and why the local scour process is diminished when the gH trough of an “ice-particle wave” passes the bridge pier. The size of bridge pier is also an important factor affecting Under an ice-jammed flow condition: the scour depth. Results showed that the larger the pier diame- ter, the deeper the maximum depth of scour hole around the LV pier. Figure 9 shows the relationship between the maximum =− 11.67 1.074 , with R² = 0.912 (4) H gH scour depth (hs/H) and the pier size (D/H). Length of scour hole Under an ice-jammed flow condition, the observed data clearly lie above those under both open flow and sheet ice- The length of scour hole is an indicator of the range of a lo- covered flow conditions. cal scour hole around a bridge pier. The comparison of the Fig. 7. Dependence of the maximum scour depth on the flow Froude Number. 282 Local scour around a bridge pier under ice-jammed flow condition – an experimental study Fig. 8. Relationship between the maximum scour depth and the thickness of ice jam. Re lation be tween the maximum s cour de pth and bridge 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.05 0.075 0.1 0.125 0.15 0.175 The bridge pie r diame ter( D/H) Fig. 9. Relationship between the maximum scour depth and the bridge pier diameter. Deposition dune deposition dune is different from that under both sheet ice- covered flow and open flow conditions. One can see from Fig- Experiments show that, during a local scour process around ure 12, the height of a deposition dune under an ice-jammed the bridge pier, an associated deposition dune develops down- flow condition increases gently with the initial approaching stream of the pier. The features of a deposition dune (such as flow velocity. However, under both sheet ice-covered flow and length, height and slope) under an open flow condition are open flow conditions, the height of deposition dunes increases different from those under both sheet ice-covered flow and ice- rapidly with the initial approaching flow velocity. jammed flow conditions. The comparison of the maximum Under an ice-jammed flow condition, with the same initial ap- heights of deposition dunes under open flow, sheet ice-covered proaching flow depth, the thickness of an ice jam decreases with flow and ice-jammed flow conditions are given in Figure 12 for the increase in the flow velocity. As a consequence, with the an initial approaching flow depth of 20 cm. Following observa- increase in flow velocity, the wave height of the ice particle wave tions can be summarized: decreases. Thus, under a relatively high flow velocity, the impact With the same initial approaching flow depth, regardless of of the migration process of an “ice particle wave” on the height flow cover conditions, progressively more sediment is eroded of a deposition dune is diminished. Namely, the height of a depo- from the scour hole and delivered downstream, thus forming sition dune under an ice-jammed flow condition is less than that the deposition dune. The higher the initial approaching flow under both sheet ice-covered flow and open flow conditions. velocity, the larger (thicker) will be the deposition dune. Under an ice-jammed flow condition, much more sediment Under varied cover conditions, different characteristics of is eroded from the scour hole around the bridge pier and trans- the deposition dune will be evident. Under an ice-jammed flow ported to downstream, compared to that under both sheet ice- condition, since flow velocity is increased, the thickness of an covered flow and open flow conditions. However, the slope of a ice jam varies. This process affects the local scour process deposition dune under an ice-jammed flow condition is much around the bridge pier, and thus, the pattern of a deposition longer but milder than those under both sheet ice-covered flow dune. Namely, the impact of an ice jam on the pattern of a and open flow conditions. M aximum s cour de pth( hs/H) Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney Ice-jammed flow Open channel flow Sheet ice-covered flow 20.00 15.00 Qi=0.026L/s H =25cm 10.00 5.00 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0)(m/s) Fig. 10a. The maximum scour length vs. the initial approaching flow velocity (initial approaching flow depth: H = 25 cm). Ice-jammed flow Open channel flow Sheet ice-covered flow 20.00 15.00 Qi=0.026L/s H =30cm 10.00 5.00 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0)(m/s) Fig. 10b. The maximum scour length vs. the initial approaching flow velocity (initial approaching flow depth: H0 = 30 cm). Fig. 11. Dependence of the maximum scour length on the flow Froude Number. Results showed, regardless of flow cover conditions, the  TV maximum height of deposition dune (Ts/H) increases with the =+ 0.108 ln 0.271 with R² = 0.86 (5)  H gH flow Froude Number (Fr), as presented in Figure 13,  Length of scour hole (Ls)(cm) Length of scour hole (Ls)(cm) Local scour around a bridge pier under ice-jammed flow condition – an experimental study Ice-jammed flow Sheet ice-covered flow Open channel flow 2.00 1.60 1.20 0.80 d =0.71mm Qi=0.026L/s H =20cm 0.40 0.00 0.15 0.16 0.17 0.18 0.19 0.2 0.21 Initial approaching flow velocity (V0) Fig. 12. The maximum height of deposition dune vs. the initial approaching flow velocity (initial approaching flow depth: H = 20 cm). Fig. 13. Dependence of the maximum height of deposition dune (Ts/H) on the flow Froude Number (Fr). CONCLUSIONS process in the vicinity of a bridge pier under an ice-jammed flow condition. the following strong tendencies can be noted: The present experimental research is only a conceptual study 1). With the same initial flow condition, the maximum for providing engineering with some knowledge regarding the scour depth under a sheet ice-covered flow condition exceeds mechanism of local scour process at piers under an ice-jammed that under an open flow condition. However, under an ice- flow condition. The evolution process of ice jams in natural jammed flow condition, the maximum scour depth is much rivers is very complex. The coefficients of the proposed equa- greater than that under an open flow condition. tions derived from data gained in laboratory experiments need 2). When the initial approaching flow velocity is low, to be further modified based on data collected in natural rivers. both the maximum scour depth and maximum length of scour Based on both laboratory experiments and field investigations hole under an ice-jammed flow condition are much greater than of ice jams in natural river, Sui et al. (1994) claimed that the that under both sheet ice-covered flow and open flow evolution mechanism of model ice jam in laboratory is similar conditions. Also, the range of the deposition dune under an ice- to that of ice jams in the Hequ Reach of the Yellow River. The jammed flow condition is much larger, comparing to that under purpose of this research work is to explore the complex mecha- both sheet ice-covered flow and open flow conditions. nism of local scour around bridge piers under an ice-jammed 3). Both the maximum scour depth and scour length flow condition. Laboratory experiments clarify the local scour under an ice-jammed flow condition increases with the flow Height of deposition dune (Ts)( cm) Jun Wang, Zhixing Hou, Hongjian Sun, Bihe Fang, Jueyi Sui, Bryan Karney velocity. The dimensionless maximum scour depth increases LLC, Highlands Ranch, Colorado, U.S.A., pp.155–172. with the flow Froude Number. Results show that the thicker the Beltaos, S., 2012. Distributed function analysis of ice jam flood ice jam, the deeper the maximum depth of a scour hole around frequency.Cold Regions Science and Technology, 71, 2, the bridge pier. One can say that a thicker ice jam will likely 1–10. lead to a deeper scour hole around a bridge pier. When the crest Buzin, V.A.,Goroshkova, N.I.,Strizhenok A.V., 2015. Max- of an ice-particle wave passes the bridge pier, the local scour imum ice-jam water levels on the northern rivers of Russia process is intensified; but when the trough of this wave passes, under conditions of climate change and anthropogenic im- the local scour process diminishes. pact on the ice jamming process. Russian Meteorology & 4). Under an ice-jammed flow condition, the impact of an Hydrology, 39, 12, 823–827. ice jam on the pattern of a deposition dune is different from that Carr, M.L., Tuthill, M.A., 2012. Modeling of Scour-Inducing under both sheet ice-covered flow and open flow conditions. Ice Effects at Melvin Price Lock and Dam. Journal of Hy- With the same initial approaching flow depth, regardless of draulic Engineering, 138, 1, 85–92. flow cover conditions, the higher the initial approaching flow Günal, M., Gelmeran, T.A., Günal, A.Y., 2017. Local scour velocity, the higher the deposition dune. The height of a around group bridge pier with different shapes. Acta Physica deposition dune under an ice-jammed flow condition increases Polonica Series a, 132, 3, 632–633. gently with the initial approaching flow velocity. However, Hafez, Y.I., 2016. Mathematical modeling of local scour at under both sheet ice-covered flow and open flow conditions, slender and wide bridge piers. Journal of Fluids, Article ID: the height of deposition dune increases rapidly with the initial 4835253. http://dx.doi.org/10.1155/2016/4835253 approaching flow velocity. With the increase in flow velocity, Hains, D.B., Zabilansky, L.J, 2004. Laboratory test of scour the wave height of the ice particle wave decreases. Thus, under under ice: Data and preliminary results. U.S. Army Engineer a relatively high flow velocity, the impact of the migration Research and Development Center,Cold Regions Research process of an ice particle wave on the height of a deposition and Engineering Laboratory, Hanover, New Hampshire, dune is diminished. Also, the slope of the deposition dune Technical Report TR-04-9 under an ice-jammed flow condition is much longer but milder (http://www.crrel.usace.army.mil/techpub/CRREL_Reports/ than that under both sheet ice-covered flow and open flow reports/TRO4-9.pdf). conditions. Regardless of flow cover conditions, the Healy, D., Hicks, F.E., 2007. Experimental study of ice jam dimensionless maximum height of deposition dune increases thickening under dynamic flowconditions. Journal of Cold with the flow Froude Number. Regions Engineering, 21, 3, 72–91. 5). Most of the existing studies regarding local scour at Hosseini, R., Amini, A., 2015. Scour depth estimation methods piers are cylindrical piers. To our knowledge, the mechanisms around pile groups. KSCE Journal of Civil Engineering, 19, regarding local scour around piers under an ice-jammed 7, 2144–2156. condition has never explored. The present research is the first Jiang, H., 1994. Experimental study of local scour protection on one regarding this topic. In present study, only cylindrical piers bridge pier. Highway, 8, 1–8. with different pier diameters have been used. As reported by Khaple, S., Hanmaiahgari, P.R., Gaudio, R., Dey, S., 2017. researchers (Günal et al., 2017; Tyminski, 2010), under the Interference of an upstream pier on local scour at down- open flow conditions, the scour process around different pier stream piers. Acta Geophysica, 65, 1, 29–46. types is different. The future research work will be focused on Ling, J., Lin, X., Zhao, H., 2007. 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Journal

Journal of Hydrology and Hydromechanicsde Gruyter

Published: Sep 1, 2021

Keywords: Ice jam; Ice cover; Riverbed deformation; Local scour; Bridge pier

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