numerical analysis of bed change and bank erosion at the ...
Extended Abstract
11th ISE 2016, Melbourne, Australia
CASE STUDY: NUMERICAL ANALYSIS OF BED CHANGE AND BANK EROSION AT THE CONFLUENCES IN NAMHAN AND NAKDONG RIVERS, SOUTH KOREA EUN-KYUNG JANG Dept. of Civil and Environment Engineering, Myongji University, Yongin, 17058, Korea /Hydro Science and Engineering Research Institute, Korea Institute of Civil Engineering and Building Technology, Goyang-Si, 10223, Korea
UN JI Hydro Science and Engineering Research Institute, Korea Institute of Civil Engineering and Building Technology, Goyang-Si, 10223, Korea /Dept. of Construction Environment Engr., University of Science & Technology, Goyang-Si, 10223, Korea
1. INTRODUCTION River morphology is continuously changed by flow including bed change and bank erosion. In particular, river confluences produce complex flow patterns due to inflow variation from the main channel and tributary. The complex flow characteristics at the confluence section have a great effect on geomorphological changes in the channel. Geomorphological changes are generated by interactions between erosion and deposition of channel bed and bank materials by flow; quantitative analysis is required to examine the possible changes at the river confluence for different hydraulic conditions. The main purpose of this study is to perform numerical simulations for flow, bed change, and bank erosion characteristics using the CCHE2D (Jia et al. [1]; Wu. [2]) model for different study reaches in Korea where the 4 Major Rivers Restoration Project (4MRRP) (Ministry of Land, Infrastructure and Transport (MLIT) [3]) has been applied. The study reaches are the confluences of Namhan River and Geumdang Stream; and Nakdong River and Geumho River in Korea (Figure 1). The first site is where headcut erosion has been generated and the knickpoint has moved in the upstream direction towards the tributary (Figure 1(a)). Also, bed protection and bank failure have occurred after the 4MRRP. In the second section, headcut erosion is expected along the Geumho River due to a large amount of dredging in Nakdong River during the 4MRRP (Figure 1(b)). The inflow variation between Geumho and Nakdong Rivers is much greater than the first study site.
Figure 1. Study Sites of Confluence Sections. 2. MODELING CONDITIONS The inflow discharge from July 20 to July 29 in 2013 was the highest measured at the confluences of Namhan River and Geumdang Stream after the 4MRRP and was used for the modeling condition. The sediment discharge data measured at the Cheonmi station 9.3 km away is applied for bed change simulation. The total load transport mode and Wu et al. [4] equation for sediment transport capacity were applied in the model for three modeling cases with different ratios between inflow discharge of the main channel (Qm) and the tributary (Qt). In the
modeling for the confluence of Nakdong and Geumho Rivers, the flood event from Sept. 15 to Sept. 21 2012 has been applied. The measured data for sediment discharge at the Dongchon station in Geumho River and Waegwan station in Nakdong River have been used for boundary conditions. The total load transport mode and Wu et al. [4] equation for sediment transport capacity were also applied for the second site.
3. SIMULATION RESULTS 3.1 CONFLUENCE OF NAMHAN RIVER AND GEUMDANG STREAM In the simulation result of the confluence section of Namhan River and Geumdang Stream, the velocity downstream of the confluence was greater than the velocity upstream of the confluence in the main channel, irrespective of the magnitude of tributary inflow. However, as tributary discharge increased, the channel erosion was accelerated and a dry area was produced at the tributary. The deposition in the confluence area changed the flow direction and the main channel to the left side, and the localized flow then eroded the channel bed at the left side. Therefore, it is expected that bank failure due to continuous bed degradation is possible in this area (Figure 2).
Figure 2. Bed Changes Simulation Results for the Confluence of Namhan River and Geumdang Stream (Qt: Tributary Discharge) (Flow direction).
3.2 CONFLUENCE OF NAKDONG AND GEUMHO RIVER Figure 3 shows the result of bed changes after 4 days and 6 days for the second site of Nakdong and Geumho Rivers. Sediment accumulation caused bed deposition of about 0.1 m to 1 m thickness in some parts of the section. The simulation results indicated that bed erosion occurred in the meandering section, therefore the flow in the main channel and the bed degradation were concentrated in the right side. However, in the bed downstream of the meandering section, sediment was deposited to about 1 m thickness due to bed erosion at the upstream section.
Figure 3. Bed Change Simulation Results for the Confluence of Nakdong and Geumho River
4. CONCLUSION Based on the modeling results, eroded sediments from the tributary, Geumdang Stream, was deposited in the confluence area and the left side bed of the Namhan River was eroded due to flow concentration. In case of Nakdong and Geumho Rivers, in the meandering section downstream of the confluence area, flow was accelerated and bed erosion was increased. According to two case studies, results indicated that the complexity of geomorphological characteristics affected flow patterns and bed erosion and sedimentation, but with different trends in flow and bed configuration changes at the two sites.
5. ACKNOWLEDGEMENT This research was supported by a grant (11-TI-C06) from Advanced Water Management Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
REFERENCES [1] Jia, Y. and Wang. S, S.Y., “CCHE2D:Two-dimensional Hydrodynamic and Sediment Transport Model for Unsteady Open Channel Flows Over Loose Bed”, NCCHE-TR-2001-1, School of Engineering the University of Mississippi, USA, (2011). [2] Wu, W., “CCHE2D sediment transport model (version 2.1)”, NCCHE-TR-2001-3, School of Engineering the University of Mississippi, USA, (2001). [3] Ministry of Land, Infrastructure and Transport, “Master plan report of 4-river Restoration Project, Ministry of Land”, Infrastructure and Transport (MLIT), Korea, (2009). [4] Wu, W., Wang, S., S.Y. and Jia, Y, “Nonuniform sediment transport in alluvial river” Journal of Hydraulic Research, Vol. 38, No. 6, (2000), pp 427-434.
11th ISE 2016, Melbourne, Australia
CASE STUDY: NUMERICAL ANALYSIS OF BED CHANGE AND BANK EROSION AT THE CONFLUENCES IN NAMHAN AND NAKDONG RIVERS, SOUTH KOREA EUN-KYUNG JANG Dept. of Civil and Environment Engineering, Myongji University, Yongin, 17058, Korea /Hydro Science and Engineering Research Institute, Korea Institute of Civil Engineering and Building Technology, Goyang-Si, 10223, Korea
UN JI Hydro Science and Engineering Research Institute, Korea Institute of Civil Engineering and Building Technology, Goyang-Si, 10223, Korea /Dept. of Construction Environment Engr., University of Science & Technology, Goyang-Si, 10223, Korea
1. INTRODUCTION River morphology is continuously changed by flow including bed change and bank erosion. In particular, river confluences produce complex flow patterns due to inflow variation from the main channel and tributary. The complex flow characteristics at the confluence section have a great effect on geomorphological changes in the channel. Geomorphological changes are generated by interactions between erosion and deposition of channel bed and bank materials by flow; quantitative analysis is required to examine the possible changes at the river confluence for different hydraulic conditions. The main purpose of this study is to perform numerical simulations for flow, bed change, and bank erosion characteristics using the CCHE2D (Jia et al. [1]; Wu. [2]) model for different study reaches in Korea where the 4 Major Rivers Restoration Project (4MRRP) (Ministry of Land, Infrastructure and Transport (MLIT) [3]) has been applied. The study reaches are the confluences of Namhan River and Geumdang Stream; and Nakdong River and Geumho River in Korea (Figure 1). The first site is where headcut erosion has been generated and the knickpoint has moved in the upstream direction towards the tributary (Figure 1(a)). Also, bed protection and bank failure have occurred after the 4MRRP. In the second section, headcut erosion is expected along the Geumho River due to a large amount of dredging in Nakdong River during the 4MRRP (Figure 1(b)). The inflow variation between Geumho and Nakdong Rivers is much greater than the first study site.
Figure 1. Study Sites of Confluence Sections. 2. MODELING CONDITIONS The inflow discharge from July 20 to July 29 in 2013 was the highest measured at the confluences of Namhan River and Geumdang Stream after the 4MRRP and was used for the modeling condition. The sediment discharge data measured at the Cheonmi station 9.3 km away is applied for bed change simulation. The total load transport mode and Wu et al. [4] equation for sediment transport capacity were applied in the model for three modeling cases with different ratios between inflow discharge of the main channel (Qm) and the tributary (Qt). In the
modeling for the confluence of Nakdong and Geumho Rivers, the flood event from Sept. 15 to Sept. 21 2012 has been applied. The measured data for sediment discharge at the Dongchon station in Geumho River and Waegwan station in Nakdong River have been used for boundary conditions. The total load transport mode and Wu et al. [4] equation for sediment transport capacity were also applied for the second site.
3. SIMULATION RESULTS 3.1 CONFLUENCE OF NAMHAN RIVER AND GEUMDANG STREAM In the simulation result of the confluence section of Namhan River and Geumdang Stream, the velocity downstream of the confluence was greater than the velocity upstream of the confluence in the main channel, irrespective of the magnitude of tributary inflow. However, as tributary discharge increased, the channel erosion was accelerated and a dry area was produced at the tributary. The deposition in the confluence area changed the flow direction and the main channel to the left side, and the localized flow then eroded the channel bed at the left side. Therefore, it is expected that bank failure due to continuous bed degradation is possible in this area (Figure 2).
Figure 2. Bed Changes Simulation Results for the Confluence of Namhan River and Geumdang Stream (Qt: Tributary Discharge) (Flow direction).
3.2 CONFLUENCE OF NAKDONG AND GEUMHO RIVER Figure 3 shows the result of bed changes after 4 days and 6 days for the second site of Nakdong and Geumho Rivers. Sediment accumulation caused bed deposition of about 0.1 m to 1 m thickness in some parts of the section. The simulation results indicated that bed erosion occurred in the meandering section, therefore the flow in the main channel and the bed degradation were concentrated in the right side. However, in the bed downstream of the meandering section, sediment was deposited to about 1 m thickness due to bed erosion at the upstream section.
Figure 3. Bed Change Simulation Results for the Confluence of Nakdong and Geumho River
4. CONCLUSION Based on the modeling results, eroded sediments from the tributary, Geumdang Stream, was deposited in the confluence area and the left side bed of the Namhan River was eroded due to flow concentration. In case of Nakdong and Geumho Rivers, in the meandering section downstream of the confluence area, flow was accelerated and bed erosion was increased. According to two case studies, results indicated that the complexity of geomorphological characteristics affected flow patterns and bed erosion and sedimentation, but with different trends in flow and bed configuration changes at the two sites.
5. ACKNOWLEDGEMENT This research was supported by a grant (11-TI-C06) from Advanced Water Management Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
REFERENCES [1] Jia, Y. and Wang. S, S.Y., “CCHE2D:Two-dimensional Hydrodynamic and Sediment Transport Model for Unsteady Open Channel Flows Over Loose Bed”, NCCHE-TR-2001-1, School of Engineering the University of Mississippi, USA, (2011). [2] Wu, W., “CCHE2D sediment transport model (version 2.1)”, NCCHE-TR-2001-3, School of Engineering the University of Mississippi, USA, (2001). [3] Ministry of Land, Infrastructure and Transport, “Master plan report of 4-river Restoration Project, Ministry of Land”, Infrastructure and Transport (MLIT), Korea, (2009). [4] Wu, W., Wang, S., S.Y. and Jia, Y, “Nonuniform sediment transport in alluvial river” Journal of Hydraulic Research, Vol. 38, No. 6, (2000), pp 427-434.
