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In 2009, South Korea started a large multi-purpose water project on its four major rivers. The main purposes of the Four Major Rivers Restoration Project (FMRRP), which are directly related to water itself, were: (1) securing water resources to combat water scarcity; (2) reducing flood risks by riverbed dredging and reinforcing levees; and (3) restoring the river's environmental functions. Despite socio-environmental opposition, the project was completed in 2011. This article reviews the sedimentation issues raised during and after the FMRRP regarding technological developments and the lessons learned from the project. A total of five sedimentation issues raised directly from the project are reviewed in this article: (1) riverbed dredging and sediment redeposition; (2) tributary headcut; (3) riverbed scour downstream of the movable weir structures; (4) sedimentation management schemes; and (5) a near-prototype river experiment facility. Each issue is addressed by identifying and analyzing field data, developing new numerical models, and testing at a large-scale experiment facility. From this project, we have found that a comprehensive numerical technique is required to predict the sediment issues as mentioned above in a watershed-scale riverwork. [ABSTRACT FROM AUTHOR]

A comprehensive review of the existing river flow and sediment measurement data of South Korea shows specific sediment yield values ranging from 5 to 1,500 tons/km2·year. The watershed characteristics decisively affect the shape of mass curves for water and sediment, as well as the flow duration and sediment rating curves. In terms of flow regime, small watersheds have flashy hydrographs and high sediment concentrations at a given flow discharge. The sediment rating curve coefficients decrease from 1 to 0.02 as the watershed area increases from 100 to 20,000 km2, while the exponents of these curves remain relatively constant between 1.5 and 2.0. The sediment transport in small watersheds depends on large floods, and the half‐yield discharge decreases with watershed area and typically ranges from 5 to 40 times the mean discharge. This study defines equations to calculate the mean annual flow discharge, flow duration curves, sediment yield, and cumulative water and sediment distribution curves validated with field measurements in South Korea. Additionally, the sediment regime is also presented in a continental perspective. [ABSTRACT FROM AUTHOR]

Examining the physical mechanisms through which large-scale climate indicators, e.g., El Niño–Southern Oscillation and Indian Ocean Dipole, affect hydroclimatic variables in the tropics and extratropics is a forefront scientific challenge. We examined climatic teleconnections between large-scale climate indices and temperature variability over South Korea. To do this, we calculated not only leading patterns of observed monthly mean and extreme temperature through an empirical orthogonal teleconnection (EOT) decomposition technique but also statistical correlations on a monthly basis using cross-correlation and lag regression analyses for the leading modes and global atmospheric circulation dataset. As a result, the spatial pattern of the leading EOT modes for mean (extreme high) temperature represents an eastern (southern) coastal mode for boreal summer and a northern (middle) inland mode in boreal winter, while extreme low EOTs exhibit a northern inland mode in summer and a western coastal mode in winter. The temporal evolution of the leading EOT modes exhibits a mostly increasing trend and an interdecadal oscillation. The leading EOT modes of mean temperature explain more variance than those of extreme temperature during warm and cold seasons. The findings from this study illustrate that tropical ENSO forcing has a coherent association with August and December temperature patterns, while the Indian Ocean Dipole is identified as a driver for temperature variability during fall season. The monsoon circulation over the western North Pacific also exhibits a significant negative correlation with the December temperature EOTs. The leading EOTs for October temperature exhibit the positive correlation with the tropical cyclone variability, while the leading EOTs for mean and extreme high temperature exhibit significant negative correlations with the snow depth over northeastern Eurasia in November. The leading patterns of the August and December mean temperature time series are predictable at up to 5-month lead time from the tropical Pacific sea surface temperatures (SSTs), while a predictable response from Indian Ocean SSTs was detected at up to 4-month lead time. [ABSTRACT FROM AUTHOR]

Summary The climatic relationship of the ENSO phenomena (El Niño/La Niña) and monthly precipitation patterns over South Korea is examined based on the composite and harmonic analysis. Three core regions, namely the Upper Region (UR), the Middle Region (MR), and the Lower Region (LR), were identified with a high level of the spatial coherence and temporal consistency rate, which represent the geographical extent and magnitude of the response of the ENSO forcing to the precipitation patterns. During the El Niño events, the February (+) to May (+), November (0) to April (+), and November (0) to May (+) wet period for UR, MR, and LR respectively, are the signal seasons having a high level of coherence and consistency. The spatial coherence rates of each region are 0.94, 0.98, and 0.98, and the temporal consistency rates are 0.80, 0.90, and 0.80 respectively. On the other hand, in case of the La Niña events, the October (0) to January (+), November (0) to May (+) dry period for UR and MR respectively, are the signal seasons showing a strong and consistent teleconnection. The spatial coherence rates of each region are 0.98, 0.96 and the temporal consistency rates are both 0.78. According to the comparative analyses for both extreme episodes, in three core regions, the El Niño/La Niña–precipitation relationships show the opposite sign, positive and negative precipitation anomalies respectively. Based on the results of annual cycle analysis, Mann–Whitney U hypothesis test and cross-correlation analysis, the wet anomalies during the warm event years, which prevail over the whole region, are more remarkable and significant than the dry departures during the cold event years. In conclusion, the climatic teleconnection between the extreme phase of SO and mid-latitude precipitation is identified over South Korea. [ABSTRACT FROM AUTHOR]

The concept of vegetation freeboard equivalence (VFE) is presented from the comparison between the rise in stage with/without vegetation and the freeboard height under design discharge conditions. In South Korea, the freeboard height of large, medium and small rivers is defined as a function of river discharge. Two models are used for this analysis of flood stage with and without vegetation: the 1-D model HEC-RAS and the 2-D model RMA-2. Both models are applied to three river study sites of the Geum River in South Korea as representative sites for a large, a medium and a small river. The analysis shows that without vegetation, both models provide comparable results and the calculated results are in very good agreement with the design configuration. The vegetation effects on the medium river are less significant, and the freeboard is adequate to contain the rise in stage from the added floodplain vegetation in large rivers. The concept of vegetation freeboard equivalence is therefore useful for the analysis of flood river stages after the restoration of channels with increased floodplain vegetation. [ABSTRACT FROM AUTHOR]

The changes in channel geometry downstream of Hapcheon Dam, South Korea, are closely examined. Daily pulses of water from peak hydropower generation and from sudden sluice gate operations affect the 45-km reach of the Hwang River between the Hapcheon Reregulation Dam and the Nakdong River. From 1983 to 2003, the median bed-material size, d50, increased from 1.0 to 5.7 mm, and the bed slope of the reach decreased from 94 to 85 cm/km. The vertical riverbed degradation averaged 2.6 m for a distance of 20 km below the reregulation dam. A simple analytical model is developed to predict the increase in sediment transport and the river bed adjustments from flow pulses in comparison with steady flow discharges. Numerical model simulations confirm the theoretical prediction that sediment transport rates from daily pulses are 21% higher than for steady flow discharges. Unsteady sediment transport simulations indicate that the channel bed degradation should extend mostly 20-25 km below the reregulation dam and should not change much after 2013. [ABSTRACT FROM AUTHOR]

This paper presents a new multiobjective approach to solve sedimentation problems behind weirs and low-head dams. The multicriteria decision analysis (MCDA) framework is used to improve reservoir operation rules for Sangju Weir in South Korea. A series of stage and discharge constraints can be developed to include consideration for reservoir sedimentation, hydropower generation, flood control, water supply, irrigation and drainage, and environment. Seasonally changing operation rules can help mitigate reservoir sedimentation while improving hydropower production, water supply, water quality, and environmental issues. Based on a 22-year daily reservoir operation simulation, improved operation rules to mitigate reservoir sedimentation include: (1) a nonflood season stage kept high (EL 47.0 m); (2) a flood season stage between EL 47.0 m and EL 44.5 m depending on the magnitude of the upstream flow discharge; and (3) gates should be opened during floods (Q>600 m3/s). [ABSTRACT FROM AUTHOR]

A three-tiered approach was used to assess erosion risks within the Nakdong River Basin in South Korea and included: (1) a screening based on topography and land use; (2) a lumped parameter analysis using RUSLE; and (3) a detailed analysis using TREX, a fully distributed watershed model. These tiers span a range of spatial and temporal scales, with each tier providing increasing detail and resolution. The first two tiers were applied to the entire Nakdong River Basin and the Naesung Stream watershed was identified as having the highest soil erosion risk and potential for sedimentation problems. For the third tier, the TREX watershed model simulated runoff, channel flow, soil erosion, and stream sediment transport in the Naesung Stream watershed at very high resolution. TREX was calibrated for surface flows and sediment transport, and was used to simulate conditions for a large design storm. Highly erosive areas were identified along ridgelines in several headwater areas, with the northeast area of Songriwon having a particularly high erosion potential. Design storm simulations also indicated that sediment deposition of up to 55 cm could occur. [Copyright &y& Elsevier]

Highlights • Teleconnection between the global and regional scale climate indices and precipitation over Korean peninsula is examined. • Two significant modes were detected based on empirical orthogonal teleconnection method. • The CI-precipitation relationships show significant positive and negative correlations. • Precipitation predictability by SST were demonstrated through the regression of the SST data on to EOT modes with varying lead times. Abstract Spatiotemporal variability, teleconnection, and predictability of the Korean precipitation related to large scale climate indices were examined based on leading patterns of observed monthly Rx5day and total precipitation through an empirical orthogonal teleconnection (EOT). Cross-correlation and lag regression analyses for the leading modes and global atmospheric circulation dataset were employed on a monthly basis. The spatial pattern of the leading EOT modes for Rx5day and total precipitation represents a northern inland mode for boreal summer and a southern coastal mode in boreal winter. The temporal evolution of the leading EOT modes exhibits increasing trends during summer season and decadal variability for winter season. The leading EOT patterns of Rx5day precipitation show more widespread coherent patterns than those of total precipitation during warm and cold seasons, while the former explains less variance in precipitation variability than the latter. The tropical ENSO forcing has a coherent teleconnection with September and November-December precipitation patterns, while the Indian Ocean dipole is identified as a driver for precipitation variability in September and November. The monsoon circulation over the western North Pacific also exhibits a significant negative correlation with winter precipitation EOTs, while tropical cyclone indices are positively correlated with the fall precipitation EOTs. The leading patterns of the September and December Rx5day precipitation time series are predictable at up to six month lead time from the tropical Pacific sea surface temperatures (SSTs), while a somewhat weak predictable response from Indian Ocean SSTs was only detected at longer lead times. In addition, predictability from the Pacific SSTs for above normal precipitation is greater than that for below normal precipitation. [ABSTRACT FROM AUTHOR]

• Annual sediment yields using SEMEP are typically 50–300 ton/km2·yr. • RMSE of model with SEMEP is 53 ton/km2·yr less than that of existing models. • Sediment transport into alluvial rivers is affected by river wetlands and water. • Watershed morphometric characteristics can represent sediment transport. • Geospatial analysis shows urbanization's erosion features, yielding rich sediment. South Korea experiences numerous local sedimentation problems, such as landslides, upland erosion, aggradation and degradation, and flood plain sediment deposition. This has necessitated the development of a reliable and consistent approach for modeling sediment processes in the country. In this study, samples obtained from 35 gauging stations at five alluvial river basins in South Korea were used together with the modified Einstein procedure and series expansion of the modified Einstein procedure to determine the total sediment load at the sampling locations. Using two different methods, the total sediment load of majority of the 35 considered rivers were found to be typically 50–300 ton/km2·yr. A model tree data mining technique was used to develop a model for estimating the specific degradation based on certain meaningful parameters, namely, the 1) elevation at the middle relative area of the hypsometric curve [m], 2) percentage of wetland and water, 3) percentage of urban land, 4) mean annual precipitation [mm], 5) main stream length [km], and 6) watershed form factor [km2/km2]. The root mean square error of the predictions of the proposed model was found to be 55 ton/km2·yr less than those of existing statistical models. Erosion loss maps obtained by the revised universal soil loss equation (RUSLE), satellite images, and aerial photographs were also used to represent the geospatial features affecting erosion and sedimentation. The results of the geospatial analysis indicated that the transport of sediment into the alluvial rivers was affected by the wetlands located near the rivers, and also enabled clear delineation of the unique erosion features of construction sites in the urban areas. In addition, the watershed morphometric characteristics could be used to accurately represent the sediment transport. The proposed data mining methodology promises to facilitate the solution of various erosion and sedimentation problems in South Korea. The geospatial analysis procedure would also enable the understanding of spatially varied erosion and sedimentation processes under different conditions. [ABSTRACT FROM AUTHOR]