Changes in hydraulic geometry of the Hwang River below the Hapcheon Re-regulation Dam, South Korea

The changes in hydraulic geometry of the Hwang River downstream of Hapcheon Dam have been investigated since 1982. The study reach is 45 km long from the Hapcheon Re-regulation Dam to the confluence with the Nakdong River, in South Korea. The geographic information system (GIS) analysis of a time series of aerial photographs taken in 1982, 1993 and 2004 showed that the non-vegetated active channel width decreased an average of 152 m (47% width reduction since 1982). The average median bed material size increased from 1.07 mm in 1983 to 5.72 mm in 2003, and the bed slope of the reach decreased from 94 to 85 cm/km from 1983 to 2003. The analysis of aerial photographs and field surveys shows that the 2004 channel width was asymptotically approaching the estimated equilibrium width from the Julien and Wargadalam (Julien, P.Y. and Wargadalam, J., 1995. Alluvial channel geometry: theory and applications. Journal of Hydraulic Engineering, 121 (4), 312–325) hydraulic geometry equations. An exponential model is proposed and is in good agreement with field measurements. The sediment transport model GSTAR-1D shows that the thalweg elevation will reach equilibrium around 2013–2015. The thalweg elevation simulated for 2013 is expected to remain almost identical to the predictions for 2023. The bed elevation changes should affect primarily the 20 km reach immediately downstream of the re-regulation dam.