Assessment of the impact of selective withdrawal on downstream fish habitats using a coupled hydrodynamic and habitat modeling

The Soyang-gang Dam is one of large dams in Korea. Highly turbid water has sometimes occurred in the reservoir after heavy rainfall, lasting for long periods. This turbid water necessitated the building of a selective withdrawal structure in 2017. The original purpose of this structure was to supply clean water from the dam to the downstream area. The present study investigated the possibility of the other purpose to improve the quality of downstream fish habitats using selective withdrawal operations. For this, a coupled hydrodynamic and habitat modeling approach was used. The study area was an 11 km reach located downstream of the Soyang-gang Dam. To compute the change of the water temperature in the Soyang Reservoir, the CE-QUAL-W2 model was used. Using the results from the CE-QAUL-W2 model, the temperature of the water discharged from the dam was obtained for three selective withdrawal operations, namely surface-layer withdrawal, middle-layer withdrawal, and bottom-layer withdrawal. Hydrodynamic simulations were performed using the HEC-RAS model, and the distributions of the flow depth, velocity, and water temperature were obtained over the study area. Physical habitat simulations (PHSs) were carried out using habitat suitability curves (HSCs) for both adult and spawning Zacco platypus. Results from the PHSs indicated that fish habitats in the regulated streams can be seriously degraded due to cold water releases from the dam, but its impact can be reduced significantly using selective withdrawal operations.