Multifunctional weirs can be used to maintain water supply during dry seasons and to improve downstream water quality during drought conditions through discharge based on retained flux. Sixteen multifunctional weirs were recently constructed in four river systems as part of the Four Rivers Restoration Project. In this study, three multifunctional weirs in the Geum River Basin were investigated to analyze the environmental effects of multifunctional weir operation on downstream flow. To determine seasonal vulnerability to drought, the basin was evaluated using the Palmer Drought Severity Index (PDSI). Furthermore, the downstream flow regime and the effect on water quality improvement of a coordinated dam-multifunctional weir operation controlled by: (a) a rainfall-runoff model; (b) a reservoir optimization model; and (c) a water quality model, were examined. A runoff estimate at each major location in the Geum River Basin was performed using the water quality model, and examined variation in downstream water quality depending on the operational scenario of each irrigation facility such as dams and weirs. Although the water quality was improved by the coordinated operation of the dams and weirs, when the discharged water quality is poor, the downstream water quality is not improved. Therefore, it is necessary to first improve the discharged water quality on the lower Geum River. Improvement of the water quality of main stream in the Geum River is important, but water quality from tributaries should also be improved. By applying the estimated runoff data to the reservoir optimization model, these scenarios will be utilized as basic parameters for assessing the optimal operation of the river.
In this study, we tried to determine the vertical variation of sediment structure and geochemical characteristics, core sediment was collected in the Nakdong River midstream on August, 2014. Core sediment mainly composed of sand (51.48%) and silt (46.21%) and coarsing upward changed from sM to mS facies. IL and TOC were decreased from lower to upper layer. C/N ratio was lower than 10 so the organic matters were originated from underwater creatures and C/S ratio was decreased from lower to upper layer. Heavy metal (Al, Fe, Zn, Cr, Pb, Ni, Cu, Cd) content were decreased from lower to upper layer and seriously polluted condition is not. These results are thought to be due to the effect of natural and anthropogenic in the fluctuation of flow.
In order to determine the best operating rules for the Nakdong River, three cases were applied to analyze the simulated results of water supply capacity by HEC-ResSim model. This study discussed to present the best operating rules for conjunctive operating of existing the dams and new constructed the weirs through system network. The system network was constructed considering the water supply, the river environment and the operating facility. The water supply capacities are separately evaluated for each case applying the best rules. It is case1 that the dams are operated individually, case2 that the dams are operated in conjunction with the others dams, and case3 that dams and weirs are operated in conjunction with the others dams-weirs. Comparing the cases, case 3 has shown the best water supply capacity of the Nakdong River.
The multi-functional weirs constructed as part of the Four Major River Restoration Project in Korea are operated for water level management and may have a backwater effect in estuaries. If the main channel of the Nakdong River flows backward and affects the estuary water, the water quality in the estuaries may not be representative of the tributary water quality. In this study, we confirmed the representativeness of the existing water quality monitoring networks using spatiotemporally disperse electrical conductivity observations, self-organizing maps (SOMs) for monthly pattern analysis, and the LOcally WEighted Scatter plot Smoother (LOWESS) technique for trend analysis. The results show that the Namgang 4-1 site, which is located in the Nam River estuary, is not affected by the Nakdong River, while the Baekcheon (Sunwongyo) site in the Baekcheon estuary is always affected by the Nakdong River. Therefore, it is necessary to relocate the existing monitoring network or establish a new monitoring network for locations affected by mainstream backflow, as is seen in Baekcheon (Sunwongyo). The methods proposed in this study, including spatiotemporally diverse electrical conductivity measurement, dimensionless fluctuation values, SOMs, and LOWESS, can be used to verify the representativeness of water quality measurement networks in other regions.
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