The baseflow separation method based on a digital filter is a simple method for separating the baseflow from streamflow. Appropriate estimation of filter parameters is required to use the digital filter method for analysis. We carried out sensitivity analysis on four digital filter methods: Lyne–Hollick (LH), Chapman, Chapman and Maxwell (CM), and exponentially weighted moving average (EWMA). Furthermore, appropriate filter parameters were suggested for each method in this study. By applying them to 25 stage stations in the Nakdong River in the Republic of Korea, the four methods were evaluated. The results of the evaluation showed that the Chapman and CM methods had problems separating the baseflow during the dry seasons. The EWMA and LH methods were able to achieve reliable baseflow separation of the outcomes by selecting appropriate the filter parameters. Thus, the EWMA and LH methods can be used easily and reasonably among the digital filter methods that have one filter parameter.

Unlike general landslides, debris flow has a high water content, due to large floods and rainfall. On steep slopes, it behaves like a large specific-weight sediment–water mixture, rather than general fluid. Thus, its momentum rapidly increases, damaging human lives and properties. This study developed a numerical model of debris flow considering driftwood and entrainment erosion, and performed numerical simulations based on the 2011 observation data of Mt. Umyeon. To develop the debris-flow model, the Nays2DFlood model, which is a flooding model based on the shallow-water equation, was coupled with the advection and diffusion of the sediment–water mixture, debris-flow bottom shear-stress, and entrainment-erosion modules. The simulation better reproduced the depth, flow velocity, and debris-flow volume of Mt. Umyeon. In particular, the driftwood generation and motions, by debris flow, were well demonstrated in the numerical simulation. Moreover, the driftwood partially reduced the drag force, and the shielding effect of the forest caused a delay in the runoff duration-time. The results of this study are expected to help establish measures to reduce debris disasters that can respond to the current situation, wherein debris damage is increasing, owing to the increased rainfall because of climate change. This subsequently increases the possibility of debris flows and the consequent damage. In particular, the predictive methodology of the refined model expands the understanding of numerical treatment for debris flow and driftwood, by reflecting the detailed physical regime.

The wet-dry scheme for moving boundary treatment is implemented in the framework of discontinuous Galerkin shallow water equations. As a formulation of approximate Riemann solver, the HLL (Harten-Lax-van Leer) numerical fluxes are employed and the TVB (Total Variation Bounded) slope limiter is used for the removal of unnecessary oscillations. As benchmark test problems, the dam-break problems and the classical problem of periodic oscillation in the parabolic bowl are solved with linear triangular elements and second-order Runge-Kutta scheme. The results are compared with exact solutions and the numerical solutions of previous study. For a more practical application, the implicit Runge-Kutta scheme is employed for the bottom friction terms and the moving shoreline in a rectangular basin of varying slopes is simulated. In all case studies, good agreement is observed with exact solutions or other well-known numerical solutions.

Here, the results of Soil and Water Assessment Tool (SWAT) simulation are calibrated and validated using SWAT-CUP Premium, and the R² (coefficient of determination) values of simulated and observed flows are compared. According to the SWAT analysis of the Cheongsong Yongjeon stream basin, low accuracy of 0.40 was the R² value for simulated and observed flows, and the baseflow was calculated at an annual average of 46.659 m³/s. During the same period, after calibration and validation using SWAT-CUP Premium, the R² value of simulated and observed flows was improved to 0.71, and the evaluation indexes showed 0.51 for Nash–Sutcliffe efficiency (NS), 37.1 for percent bias (PBIAS), 0.73 for P-factor, and 0.33 for R-factor, indicating that the relationship between observed and simulated flows was improved. In addition, after calibration and validation, the baseflow was 56.951 m³/s, which was a more accurate value than the one before calibration and validation.

Baseflow separation method based on the digital filter is a technique for separating the baseflow from the total streamflow, and appropriate estimation of the filter parameter is required for analysis. In this study, we reviewed the analysis characteristics of the four baseflow separation methods (Lyne-Hollick, Chapman, Eckhardt, and EWMA (exponentially weighted moving average)) based on the digital filter according to the change in the filter parameter and evaluated their applicability. Each digital filter method was applied to Daeri and Cheonsong stage stations, which have measurement records for five consecutive years for tributaries located of the Nakdonggang River in the Republic of Korea. The Chapman method had an error in the baseflow separation during the dry season. The EWMA method was not appropriately analyzed within the parameter range used in previous studies. The Lyne-Hollick and Eckhardt methods provided appropriate results within the exploring range of the filter parameter. However, in the Eckhardt method, we had to be careful in determining the filter parameter because the variation in analysis results was relatively large according to the parameter change.

Climate change-induced heavy rainfall during summer months can further increase suspended solid loads in rivers, elevating turbidity. Such elevated turbidity can compromise fish gill tissue integrity and impair oxygen uptake, potentially leading to fatal impacts in aquatic ecosystems. Therefore, this study aims to examine fish migratory behaviors and physiological responses to varying turbidity levels through experimental trials to generate baseline data for assessing fish habitat suitability. The experimental design comprised two primary components: an investigation of turbidity avoidance behaviors and an analysis of habitat compatibility through extended exposure to turbid conditions. This study focused on dominant freshwater fish species native to South Korea, Zacco platypus, Pseudopungtungia nigra, and Zacco koreanus. Fish condition in response to turbidity was monitored over a 15-day period, during which locomotor activity and water quality parameters were recorded. In the control group tank with no turbidity, all species exhibited unrestricted swimming patterns without depth preference. However, in moderate and high turbidity treatments, all demonstrated preferential utilization of middle- and lower-depth strata. In addition, the highest number of fish mortality occurred in high-turbidity zones because of respiratory impediments from elevated suspended solid concentrations. These findings provide valuable insights into fish mobility and habitat utilization patterns in rivers experiencing sudden turbidity events, such as those associated with weir operations.