The behavior of merging buoyant jets discharged from uni-directional diffuser has been examined considering the interaction of individual axi-symmetric jets. Some series of experiments have been performed to define the range of merging transition based on the variations of local flow properties. The properties for the large eddies on the boundaries; passing frequency, vortex spacing, and Strouhal number, are also measured to investigate the evolution of individual jets to a merged flow. The results of this study show that the local flow properties change from those of axi-symmetric to merged flow over a merging transition where b/p _s = 0.3 to 1.2. The flapping motion is observed as the individual jets get merged with neighboring ones. After the merging transition (b/p _s = 0.3 to 1.2), the flows show a typical behavior of plane flows with a constant value of Strouhal number.

The Korean Ministry of Environment has established management policies for four major rivers; these policies include plans to restore ecosystems and manage hazardous materials and non-point source pollution in the watersheds. The purpose of this study was to select priority tributaries in the Nakdong River basin for immediate management under these plans by using water quality index (WQI), principal component analysis (PCA), and non-point source pollution modeling data. First, comprehensive water pollution was assessed for 22 sub-basins in the Nakdong River basin by using a WQI. Seasonal pollution was then categorized as either point or non-point source by using the PCA results based on observed water quality data, and lastly, a distributed model was developed to analyze non-point source pollutant loads. This allowed for an assessment of the degree of potential non-point source pollution from several land cover types and the selection of basins where such pollution should be managed on a priority basis. Four sub-basins (Nos. 12, 13, 19, and 21) were found to have high pollution levels, and sub-basin No. 12 had the highest non-point loading. As a result of this comprehensive basin assessment, the Geumho River was selected as the first priority tributary for management. The approach applied in this study should be useful for future efforts to develop effective strategies to improve water resources in river basins.

Harmful algal blooms (HABs) have been recognized as a serious problem for aquatic ecosystems and a threat to drinking water systems. The proposed method aimed to develop a practical and rapid countermeasure, enabling preemptive responses to massive algal blooms, through which prior to the algal bloom season we can identify HAB-prone regions based on estimations of where harmful algae initiates and develops significantly. The HAB-prone regions were derived from temperature, depth, flow velocity, and sediment concentration data based only on acoustic Doppler current profilers (ADCPs) without relying further on supplementary data collection, such as the water quality. For HAB-prone regions, we employed hot-spot analysis using K-means clustering and the Getis-Ord G*, in conjunction with the spatial autocorrelation of Moran’s I and the local index of spatial association (LISA). The validation of the derived HAB-prone regions was conducted for ADCP measurements located at the downstream of Nam and Nakdong River confluence, South Korea, which preceded three months of algal bloom season monitored by unmanned aerial vehicles (UAVs). The visual inspection demonstrated that the comparison resulted in an acceptable range of agreement and consistency between the predicted HAB-prone regions and actual UAV-based observations of actual algal blooms.

Many environmental problems have arisen due to algae since the large‐scale river projects were implemented in Korea. Therefore, it is imperative to monitor algal distribution status in order to find a solution to these problems. Sporadic sampling and population estimation by microscopy are traditional algae monitoring methods. However, these techniques yield spatiotemporally limited information. In order to overcome these weaknesses, the present study aimed to investigate the applicability of a new monitoring technique using an in situ optical particle sizer (LISST‐100X) for algal size and distribution estimation. The survey site was the mountain range where the Nakdong River and Nam River converge. The number of algal populations and dominant species were identified, and suspended particle concentrations and distribution were surveyed based on hydraulic characteristics, water quality, and water sampling. Field monitoring data were also collected using a unmanned aerial vehicle (UAV) in order to assess the applicability of the algal distribution estimation method using an in situ optical particle sizer for natural streams. At the time of the algae outbreak, relatively large‐sized suspended particles are distributed throughout the surface layer, which is distinguished from the similar distribution in the general water bodies. Because of these characteristics, it was possible to investigate vertical and spatial algae distribution. Additionally, the mixing pattern was analyzed according to pulsed flow in order to estimate the applicability of monitoring algal bloom removal according to discharge.

In this study, we modified and applied an open source CFD software package, the TELEMAC, to simulate sediment transport and bed morphology around Gangjeong Weir on Nakdong River in Korea. The simulations have been carried out with the real river bathymetry and the weir geometry, and for different operation scenarios of the weir. The hydrodynamic parameters and bed evolutions obtained from the numerical results have been validated against field observation data. Particularly, the scour depth has been verified against the results obtained from a well-known empirical formula. Agreement between numerical computations, observed data and empirical formulas is judged to be satisfactory on all major comparisons. This study is not only to point out the locations where deposition and erosion taken place due to the weir gate operation, but also to analyze the mechanism of formation and evolution of scour holes after the weir gates.

This study discerns optimal rules for coordinated operation of existing dams and eight newly constructed weirs in the Nakdong River through use of a system network while considering the water supply and operating facilities. To determine the operation rules, capacity was simulated by reservoir operations modeling and evaluated separately under the following three cases: (1) when dams were operated individually; (2) when they were operated in conjunction with other dams; and (3) when dams and weirs were operated in conjunction with each other. The water supply capacity of the Nakdong was highest in Case 3. When considering operational losses (9.1 and 14.8% in Cases 2 and 3, respectively), water supply decreased. Findings show that when water levels drop below the target, hydrological facilities should flexibly adjust water storage and flow rates at each dam in the system. Further studies are needed to investigate the influence of agricultural reservoirs and river channel discharged flows and to identify regions vulnerable to water shortages driven by future fluctuations in water resources due to climate change.