Numerical analysis of morphological changes by opening gates of Sejong Weir

Chang-Lae Jang; Tae Hyo Baek; Taeun Kang; Giyoung Ock

doi:10.3741/JKWRA.2021.54.8.629

All Issue

2021 Vol.54, Issue 8 Preview Page

Research Article

Numerical analysis of morphological changes by opening gates of Sejong Weir 보 개방에 의한 하도의 지형변화 과정 수치모의 분석(세종보를 중심으로)

31 August 2021. pp. 629-641

PDF XML

Abstract

In this study, a two-dimensional numerical model (Nays2DH) was applied to analyze the process of morphological changes in the river channel bed depending on the changes in the amount of flooding after fully opening the Sejong weir, which was constructed upstream of the Geum River. For this, numerical simulations were performed by assuming the flow conditions, such as a non-uniform flow (NF), unsteady flows (single flood event, SF), and a continuous flood event (CF). Here, in the cases of the SF and CF, the normalized hydrograph was calculated from real flood events, and then the hydrograph was reconfigured by the peak flow discharge according to the scenario, and then it was employed as the flow discharge at the upstream boundary condition. In this study, to quantitatively evaluate the morphological changes, we analyzed the time changes in the bed deformation the bed relief index (BRI), and we compared the aerial photographs of the study area and the numerical simulation results. As simulation results of the NF, when the steady flow discharge increases, the ratio of lower width to depth decreases and the speed of bar migration increases. The BRI initially increases, but the amount of change decreased with time. In addition, when the steady flow discharge increases, the BRI increased. In the case of SF, the speed of bar migration decreased with the change of the flow discharge. In terms of the morphological response to the peak flood discharge, the time lag also indicated. In other words, in the SF, the change of channel bed indicates a phase lag with respect to the hydraulic condition. In the result of numerical simulation of CF, the speed of bar migration depending on the peak flood discharges decreased exponentially despite the repeated flood occurrences. In addition, as in the result of SF, the phase lag indicated, and the speed of bar migration decreased exponentially. The BRI increased with time changes, but the rate of increase in the BRI was modest despite the continuous peak flooding. Through this study, the morphological changes based on the hydrological characteristics of the river were analyzed numerically, and the methodology suggested that a quantitative prediction for the river bed change according to the flow characteristic can be applied to the field.

Keywords

Numerical simulation

Hydrograph normalizing

Phase lag

Bed deformation

본 연구에서는 2차원 수치모형(Nays2DH)을 적용하여 금강에 건설된 세종보를 중심으로 보 개방 후에 홍수량 변화에 의한 하도의 지형변화 과정을 분석하였다. 이를 위해 부등류, 부정류(단일사상), 연속홍수사상을 수문특성을 반영한 흐름조건으로 수치모의를 수행하였다. 부등류 수치모의 조건은 유황에 따른 정상유량으로 상류단 경계조건을 가정하였다. 부정류와 연속홍수사상의 경우에는 실제 홍수사상들로부터 정규 수문곡선 (normalized hydrograph)을 산정 한 후, 시나리오에 따른 첨두유량으로 정규 수문곡선을 재구성하여 상류단 유량으로 가정하였다. 본 연구에서는 지형학적 변화를 정량적으로 평가하기 위해 하상기복지수(BRI)를 산정하여 시간에 따른 하상변동을 분석하였으며, 연구지역의 항공사진과 수치모의 결과를 정성적으로 비교하였다. 부등류 수치모의 결과, 각 유황별 유량이 증가하면 하폭 대 수심의 비는 감소하고, 사주의 이동속도는 증가하였다. 하상기복지수는 초기에는 증가하지만, 시간이 증가함에 따라 변화량이 작아졌다. 또한 유량이 증가하면 하상기복지수가 증가했다. 부정류 수치모의 결과 사주의 이동속도는 유량의 변화에 따라 감소했다. 또한 첨두홍수에 대한 지형적 반응에서 시간지체(time lag)가 발생했다. 즉, 부정류에서는 하상고의 변화는 수리학적 조건에 대하여 위상지연(Phase lag) 이 나타났다. 연속홍수사상 발생에 의한 부정류 수치모의 결과에서는 각각 첨두홍수 발생에 따른 사주의 이동속도는 홍수발생이 반복됨에도 불구하고 급격하게 감소했다. 또한 부정류 수치모의 결과와 마찬가지로 위상지연이 나타났으며 사주의 이동속도는 지수적으로 감소하는 특성을 보였다. 하상기복지수는 시간경과에 따라 증가하였으나, 첨두홍수가 연속으로 발생하였음에도 불구하고 하상기복지수의 증가율은 완만하였다. 본 연구를 통해 하천의 수문특성을 반영한 지형변화 과정을 수치모의를 수행하여 분석하였으며, 이를 통해 흐름특성에 따른 하상변동의 정량적인 예측모의를 현장에 적용할 수 있는 방법을 제시할 수 있을 것으로 판단된다.

키워드

수치모의

수문곡선 일반화

위상지연

하상변동

References

Ashida, K., and Michiue, M. (1972). "Study on hydraulic resistance and bedload transport raten alluvial streams." Transactions, Japan Society of Civil Engineering, Vol. 206, pp. 59-69. (in Japanese) 10.2208/jscej1969.1972.206_59

Chai, T., and Draxler RR. (2014). "Root mean square error (RMSE) or mean absolute error (MAE)? Arguments against avoiding RMSE in the literature." Geoscientific Model Development, Vol. 7, No . 3, pp. 1247-1250. 10.5194/gmd-7-1247-2014

Colombini, M., Seminara, G., and Tubino, M. (1987). "Finite-amplitude alternate bars." Journal of Fluid Mechanics, Vol. 181, pp. 213-232. doi: 10.1017/S0022112087002064 10.1017/S0022112087002064

Crosato, A., and Mosseman, E. (2009). "Simple physics-based predictor for the number of river bars and the transition between meandering and braiding." Water Resources Research, Vol. 45, W03424. doi:10.1029/2008WR007242 10.1029/2008WR007242

East, A.E., Pess, G.R., Bountry, J.A., Magirl, C.S., Ritchie, A.C., Logan, J.B., Randle, T.J., Mastin, M.C., Minear, J.T., Duda, J.J., and Liermann, M.C.(2015). "Large-scale dam removal on the Elwha River." Geomorphology, Vol. 228, pp. 765-786. 10.1016/j.geomorph.2014.08.028

Engelund, F. (1974). "Flow and bed topography in channel beds." Journal of the Hydraulics Division, ASCE, Vol. 100, No. 11, pp. 1631-1648. 10.1061/JYCEAJ.0004109

Garcia, M., and Nino, Y. (1993). "Dynamique des bancs de sediments dans des chenaux droits et a méandres: Essais sur le phénomène de resonance." Journal of Hydraulic Research, Vol. 31, No. 6, pp. 739-761. 10.1080/00221689309498815

Hoey, T.B., and Sutherland, A.J. (1991). "Channel morphology and bedload pulses in braided rivers: A laboratory study." Earth Surface Processes and Landforms, Vol. 16, No. 5, pp. 447-462. 10.1002/esp.3290160506

Im, D., Kang, H., Kim, K.-H., and Choi., S.-U. (2011). "Changes of river morphology and physical fish habitat following weir removal." Ecological Engineering, Vol. 37, pp. 883-892. 10.1016/j.ecoleng.2011.01.005

Jang, C.-L.(2013). "Dynamic characteristics of multiple bars in the channels with erodible banks." Journal of Korea Water Resources Association, Vol. 46, No. 1, pp. 25-34. doi: 10.3741/JKWRA.2013.46.1.25. (in Korean) 10.3741/JKWRA.2013.46.1.25

Jang, C.-L.(2014). "Numerical experiments of the behavior of bars in the channels with periodic variable width." Journal of Korea Water Resources Association, Vol. 47, No. 1, pp. 37-47. doi: 10.3741/JKWRA.2014.47.1.37 10.3741/JKWRA.2014.47.1.37

Jang, C-L., and Lee, K.S. (2020). "Experimental analysis on the channel adjustment processes by weir removal." Journal of Korea Water Resources Association, Vol. 53, No. 11, pp. 951-960. doi: 10.3741/JKWRA.2020.53.11.951. (in Korean)

Jang, C.-L., and Shimizu, Y. (2005). "Numerical simulations of the behavior of alternate bars with different bank strength." Journal of Hydraulic Research, Vol. 43, No. 6, pp. 595-611. 10.1080/00221680509500380

Jang, C.-L., and Shimizu, Y. (2007). "Vegetation effects on the morphological behavior of alluvial channels." Journal of Hydraulic Research, Vol. 45 No. 6, pp. 763-772. 10.1080/00221686.2007.9521814

Kang, K.-H., Jang, C.-L., Lee, G. H.., and Jung, K. (2016). "Numerical analysis of the morphological changes by sediment supply at the downstream channel of Youngju dam." Journal of Korea Water Resources Association, Vol. 49, No. 8, pp. 693-705. doi: 10.3741/JKWRA.2016.49.8.693 (in Korean) 10.3741/JKWRA.2016.49.8.693

Korea Institute of Civil Engineering and Building Technology (KICT) (2008). Guideline for small dam removal. (in Korean)

Ministry of Environment (ME) (2019). Monitoring report of weir operation in the 16 weirs of the 4 major rivers. (in Korean)

Ock, G., Choi, M., Kim, J.-C., Park, H.-G., and Han, J.-H. (2020). "Evaluation of habitat diversity changes by weir operation of the Sejongbo Weir in Geum River using high-resolution aerial photographs." Ecology and Resilient Infrastructure, Vol. 7, No. 4, pp. 366-373. doi: 10.17820/eri.2020.7.4.366. (in Korean)

Ritchie, A.C., Warrick, J,A., East, A.E., Magirl, C.S., Stevens, A.W., Bountry, J.A., Randle, T.J., Curran, C.A., Hilldale, R.C., Duda, J.J., Gelfenbaum, G.R., Miller, J.M., Pess, G.R., Foley, M,M., McCoy, R., and Ogston, A.S. (2018). "Morphodynamic evolution following sediment release from the world's largest dam removal." Scientific Reports, Vol. 8, No. 1. doi: 10.1038/s41598-018-30817-8 10.1038/s41598-018-30817-830185796PMC6125403

Seminara, G., and Tubino, M. (1989). "Alternate bars and meandering: Free, forced and mixed interactions." River Meandering, Edited by Ikeda, S., and Parker, G., AGU Water Resources Monograph 12, AGU, Washington, D.C., U.S., pp. 267-320. 10.1029/WM012p0267

Shimizu, Y., Nelson, J., Kattia, A.F., Asahi, K., Giri, S., Inoue, T., Iwasaki, T., Jang, C., Kang, T., Kimura, I., Kyuka, T., Mishra, J., Nabi, M., Patsinghasanee, S., Yamaguchi, S. (2020). "Advances in computational morphodynamics using the International River Interface Cooperative (iRIC) Software." Earth Surface Processes and Landforms, Vol. 45, No. 1, pp. 11-37. doi: 10.1002/esp.4653 10.1002/esp.4653

Tubino, M., (1991). "Growth of alternate bars in unsteady flow." Water Resources Research, Vol. 27, No. 1, pp. 37-52. 10.1029/90WR01699

Watanabe, A., Fukuoka, S., Yasutake, Y., and Kawaguchi, H. (2001). "Method for arranging vegetation grions at bends for control of bed variation." Collection of Papers on River Engineering, Vol. 7, pp. 285-290.

Wilcox, A.C., O'Connor, J.E., and Major, J.J. (2014). "Rapid reservoir erosion, hyperconcentrated flow, and downstream deposition triggered by breaching of 38-m-tall Condit Dam, White Salmon River, Washington." Journal of Geophysical Research: Earth Surface, Vol. 119, pp. 1376-1394. doi: 10.1002/2013JF003073 10.1002/2013JF003073

World Commission on Dams (WCD) (2000). Dams and development, a new framework for decision-making. The report of the World Commission on Dams, Earthscan, London, UK and Stering, VA, U.S.

Information

Publisher :KOREA WATER RESOURECES ASSOCIATION
Publisher(Ko) :한국수자원학회
Journal Title :Journal of Korea Water Resources Association
Journal Title(Ko) :한국수자원학회 논문집
Volume : 54
No :8
Pages :629-641
Received Date : 2021-04-26
Revised Date : 2021-06-15
Accepted Date : 2021-06-15
DOI :https://doi.org/10.3741/JKWRA.2021.54.8.629

Journal of Korea Water Resources AssociationISSN:2799-8746(Print) 2799-8754(Online)한국수자원학회

All Issue