Kyong Oh Baek

Analysis of transverse mixing in natural streams under slug tests

To investigate the effects of the stream geometry on the transverse mixing of the pollutant, tracer tests were conducted at seven different sites in tributaries of the Han River in Korea. The routing procedure combined with the stream-tube concept was developed to analyze the dispersion data collected in a natural stream with irregular geometry under slug tests. The routing procedure was used to calculate the transverse dispersion coefficient from the observed concentration data, and the result was compared with the results by the moment methods. The field data showed that, in most tests on the curved reaches of the streams, the high tracer concentration as well as the high velocity occurred following the thalweg line. However, the local irregularities inside the stream cross-section significantly affected the transverse distributions of stream velocity and tracer concentration as much as the channel meander did. The observed transverse dispersion coefficients by the stream-tube routing procedure were in good agreement with those by the stream-tube moment method. The analysis of the relations between the transverse dispersion coefficients and the basic hydraulic parameters showed that as the ratio of width to depth (W/H) increased, the transverse dispersion coefficients tended to increase. The transverse dispersion coefficients were also proportional to the sinuosity of the stream. The transverse dispersion coefficients also tended to increase as the ratio of mean velocity to shear velocity (U/U*), which is the friction term, increased.

Transverse Dispersion Caused by Secondary Flow in Curved Channels

A new theoretical equation is proposed to describe the streamwise variations of the transverse velocity along a curved channel with a constant curvature. Furthermore, based on this theoretical equation for the transverse velocity, a new equation for the transverse dispersion coefficient is developed to incorporate the effect of the secondary flow on the transverse dispersion in curved channels. The new equations for the transverse velocity and dispersion coefficient are verified with experimental data sets that were obtained from laboratory experiments conducted in two different channels. The results show that the proposed velocity equation properly describes the streamwise variations of the secondary flow developed in the curved channels. The reach-averaged values of the transverse dispersion coefficient calculated by the new equation are in relatively good agreement with the observed values from the laboratory channels. Sensitivity analysis reveals that both the secondary flow and the transverse dispers...

Empirical equation for transverse dispersion coefficient based on theoretical background in river bends

There are different approaches to estimating the transverse dispersion coefficient in river mixing. Theoretical approaches have derived the dispersion coefficient from the concept of shear flow, which has dominant effects on the transverse mixing. Empirical approaches have developed an equation using the hydraulic and geometric data of rivers through dimensional analysis and regression techniques. These two equations interact closely with each other. For example, the complicated theoretical equation can be simplified by empirical approaches, and the functional relationships of the empirical equation can be derived from theoretical bases. In this study, a new empirical equation for the transverse dispersion coefficient has been developed based on the theoretical background in river bends. As a regression method, the least-square iterative method was used because the equation was a nonlinear model. The estimated dispersion coefficients derived by the new equation were compared with observed transverse dispersion coefficients acquired from natural rivers and coefficients calculated by the other existing empirical equations. From a comparison of the existing transverse dispersion equations and the proposed equation, it appears that the behavior of the existing formula in a relative sense is very much dependent on the flow condition and the river geometry. Moreover, the proposed equation does not vary widely according to variation of flow conditions. Also, it was revealed that the equation proposed in this study becomes an asymptotic curve as the curvature effect increases.

Assessment of Influx Efficiency at By-Pass Fishway Using Two-Dimensional Physical Habitat Simulation Model -Focused on Zacco Platypus-

In this study, the efficiency of the by-pass fishway installed at Kangjung-Goryong Weir in Nakdong River was assessed by using River2D which is a two-dimensional physical habitat simulation model. The model was calibrated and validated through the measured water elevation. The assessment was performed according to flow condition such as flood, normal, and low flow. Especially the low flow condition was focused on because the target fish, Zacco Platypus, have moved frequently up and downstream at the spawning season from April to June. From simulation results, it can be deduced that the influx efficiency and the passage efficiency of the fishway in the low flow is higher than that in the flood and normal flow due to occurrence of proper velocity at fishway entrance.

Equation for Streamwise Variation of Secondary Flow in Sinuous Channels

The secondary flow in a bend takes the helical motion by which the water in the upper part of the river is driven outward and the water near the bottom is driven inward. This phenomenon is commonly explained as the result of the interplay between the centrifugal force, the pressure gradient due to the decline of the water surface, and the bottom friction. The behavior of secondary flow in alternating bends is more complicated than that in a bend. In this study, a theoretical equation was derived based on Odgaard (1986) and Chang (1988) to reveal the stream-wise variation of the secondary flow in sinuous channels. The derived equation describes the transverse component of the secondary flow as a function of stream-wise and vertical coordinates. To validate the proposed equation, hydraulic experiments were conducted in laboratory meandering channels having different sinuosity. Comparison of experimental results with the proposed equation and an existing equation revealed that the new equation was in good agreement with the experimental data, whereas the existing equation overestimated. Sensitivity analysis of the new equation with respect to hydraulic parameters showed that the size of helical motion of secondary flow tended to increase as the sinuosity, the roughness, and the aspect ratio (ratio of width to depth) became larger. Also, the behavior of the secondary flow along the channel was sensitive to variations of the roughness and the aspect ratio.

Assessment of Attraction Efficiency of By-pass Fishway at Dalseong Weir According to Operating Attraction Waterway

In this study, the attraction efficiency of the by-pass fishway installed at Dalseong Weir in Nakdong River was assessed according to operation of the attraction waterway by using River2D which is a two-dimensional physical habitat simulation model. The model was calibrated and validated through the measured water elevation. The attraction efficiency of the fishway was evaluated at the low flow condition because the target fish, Zacco platypus (Z. platypus), have moved frequently up and downstream at the spawning season from April to June. From simulation results, it can be deduced that the attraction efficiency at situation of open attraction waterway is superior to that of close attraction waterway. Also it is shown that velocity field at inner region of the fishway is suitable for migration of the fish.

Comparative Study on Evaluating Low-Flow in Ungauged Watershed

In this study, the methodologies for evaluating the low-flow at the ungauged watershed are reviewed and assessed. The ungauged watershed can be classified into different situations such as the partially recorded watershed and the completely ungauged watershed. The extension method and the percentile method are used to evaluated the low-flow at the partially recorded watershed. The drainage-area ratio method and the regional regression method are used at the completely ungauged watershed. These four methods are applied and validated based on the hydrological and geometric data acquired from unit watersheds in Han River basin for TMDLs. In case of partially recorded watershed, the values of low-flow evaluated by the extension method are in better agreement with measured flow-rate rather than those by the percentile method. In case of completely ungauged watershed, the drainage-area method is broadly used to estimate the low-flow. It must be paid attention to consider the treated sewage discharge produced at watersheds when applying the method.