Journal of Korea Water Resources Association (한국수자원학회논문집)

Korea Water Resources Association (한국수자원학회)
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Numerical analysis of flow and bed change at a confluence of the Namhan River and the Seom River using a two-dimensional model

2차원 수치모형을 이용한 남한강과 섬강 합류부 구간의 흐름 및 하상변동 해석

  • Park, Moonhyung (Department of Land, Water and Environment Research, Korea Institute of Civil Engineering and Building Technology) ;
  • Kim, Hyung Suk (Department of Land, Water and Environment Research, Korea Institute of Civil Engineering and Building Technology) ;
  • Baek, Chang Hyun (Water Resources Information Center, Han River Flood Control Office)
  • 박문형 (한국건설기술연구원 국토보전연구본부) ;
  • 김형석 (한국건설기술연구원 국토보전연구본부) ;
  • 백창현 (한강홍수통제소 수자원정보센터)
  • Received : 2018.11.02
  • Accepted : 2018.11.28
  • Published : 2018.12.31
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Abstract

The flow and bed change were analyzed using the CCHE2D model, which is a two-dimensional numerical model, at a confluence of the Namhan River and Seom River where deposition occurs predominantly after the "Four Major Rivers Restoration Project." The characteristic of the junction is that the tributary of Seom River joined into the curved channel of the main reach of the Namhan River. The CCHE2D model analyzes the non-equilibrium sediment transport, and the adaptation lengths for the bed load and suspended load are important variables in the model. At the target area, the adaptation length for the bed load showed the greatest influence on the river bed change. Numerical simulation results demonstrated that the discharge ratio ($Q_r$) change affected the flow and bed change in the Namhan River and Seom river junction. When $Q_r{\leq}2.5$, the flow velocity of the main reach increased before confluence, thereby reducing the flow separation zone and decreasing the deposition inside the junction. When $Q_r$>2.5, there was a high possibility that deposition would be increased, thereby forming sand bar. Numerical simulation showed that a fixed sand bar has been formed at the junction due to the change of discharge ratio, which occurred in 2013.

본 연구에서는 4대강 살리기 사업 후 퇴적현상이 지배적으로 발생하는 남한강과 섬강 합류부 구간을 대상으로 2차원 수치모형인 CCHE2D 모형을 이용하여 하천의 흐름 및 하상변동에 대한 해석을 수행하였다. 대상지점 합류부는 남한강 본류의 만곡부에 지류 섬강이 유입되는 특성을 갖는다. CCHE2D 모형은 비평형 유사이송을 해석하며 소류사와 부유사 조정거리가 중요한 입력변수로 대상지점에서는 소류사 조정거리가 하상변동에 가장 큰 영향을 주는 것으로 나타났다. 수치모의 결과 유량비($Q_r$) 변화가 남한강과 섬강 합류부 지점에서 흐름 및 하상변동에 영향을 미쳤으며, $Q_r{\leq}2.5$인 경우에는 합류전 본류의 유속이 증가하여 흐름박리구역을 감소시켰으며 이로 인해 합류부 내측의 퇴적이 감소하였다. $Q_r$>2.5이면 합류부 구간에 퇴적이 증가하여 사주가 형성될 가능성이 높은 것으로 나타났다. 수치모의를 통해 2013년에 발생한 유량비 변화에 의해 합류부에 고정사주가 형성된 것을 알 수 있었다.

Keywords

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Fig. 1. Study reach of river confluence in the Namhan River and Seom River (Daum map)

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Fig. 2. Initial bed elevation of the Namhan River and the Seom River

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Fig. 3. Discharge hydrograph and water surface level hydrograph. Hydrological data was obtained from the H an R iver Flood Control Office (HRFCO)

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Fig. 4. Aerial photographs (2011, 2012, 2015) of a field site (Daum map)

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Fig. 5. Channel cross-section profiles at No. 153. Bed elevation data was obtained from the Korea Institute of Hydrological Survey (KIHS)

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Fig. 6. Bed elevation change at the confluence of the Namhan River and the Seom River; (a) Run-2, (b) Run-4, (c) Run-5, (d) Run-7

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Fig. 7. Comparison of the bed elevation between calculated and observed data at No. 153

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Fig. 9. Flow velocity distributions of (a) Case 2, (b) Case 6, (c) Case 10 and (d) Case 14

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Fig. 10. Lateral profiles of the velocity magnitude at (a) No. 152, (b) No. 153, (c) No.154 and (d) 200 m upstream from the Seom River confluence

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Fig. 11. Changes in bed topography. (a) Case 2; (b) Case 4; (c) Case 6; (d) Case 9; (e) Case 12; (f) Case 14

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Fig. 12. Lateral profiles of the bed change at (a) No. 152 and (b) No. 153

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Fig. 13. Deposition volume and area versus the discharge ratio (Qr)

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Fig. 8. (a) Initial bed elevation and (b) mesh generation for CCHE2D modeling

Table 1. Review of sensitivity analysis for numerical parameters

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Table 2. Simulation conditions and results

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