Physical habitat simulations using unsteady flow model

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Sung-Uk Choi, Byungwoong Choi, Seung Ki Kim

Tuesday 30 june 2015

15:05 - 15:20h at Amazon (level 1)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 6G. Environment - Ecosystem

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Dam construction affects the discharge downstream, which, in turn, alters the stream morphology and ecology. Hydropeaking increases unsteadiness of the flow downstream. Interruption of the sediment supply downstream of the dam induces degradation and armouring of the bed, which affects the suitability of aquatic habitats. Thus, it is necessary to evaluate the impact of the dam before construction in order to establish the countermeasures. Most previous studies examined the impact of the dam construction on the aquatic habitats using PHABSIM (Gore et al., 1989; Valentin et al., 1996; Booker and Dunbar, 2004). Such conventional approaches are based the IFIM concept, assuming the quasi-steady flow, and they are not capable of reflecting unsteadiness of the flow due to hydropeaking. This study investigated the impact of hydropeaking on the stream morphology and habitat suitability of fish. The 1D unsteady flow model and the habitat suitability index model were used for hydraulic and habitat simulations, respectively. Wu et al.’s (2000) formula was used to estimate the total sediment load for mixtures. The Exner’s equation was solved to predict the morphology change due to sediment transport. A 10 km long reach downstream of the Youngjoo Dam in the Naeseong Stream, Korea was chosen for the study area. The construction of the Youngjoo Dam will be completed and the dam will start the impoundment at the end of 2014. The Naeseong Stream, a tributary of the Nakdong River, has a watershed area of 1,806.7 km2. The stream is 109 km long, and the average slope is 0.011. Bed sediment consists of sand and gravel, the median size of which is 1 mm, indicating a sand-bed stream. Physical habitat simulations were carried out, and the results indicated that the hydropeaking decreases the physical habitat suitability of the stream. The unsteadiness of the flow due to hydropeaking was also investigated. It was found that the unsteadiness propagates downstream like water wave, and its amount decreases with distance from the dam. The flow depth computed by the unsteady model was smaller than that by the quasi-steady model, increasing the velocity and bed shear stress. This suggested that the quasi-steady flow computation can under-predict significantly the erosion capability of the pursing flow resulting from hydropeaking.