Un Ji

An Experimental Study to Evaluate the Subsidence Stability of Riprap Protection without Filters

Many countermeasures for local scour at bridge piers constructed on the river and sea have been developed and researched to protect piers against local scour. The most commonly employed method is riprap protection, which is sometimes required the filter installation between riprap and base layers. However, the construction of stone filters are really hard to perform in the field, require the high cost, or sometimes are impossible. The experimental modeling is conducted to analyze the riprap failure modes and the stability of riprap protection without filters based on the different approach velocity and riprap layer thickness conditions. Also, the stability index to evaluate the performance of riprap protection is developed using the experimental results. The cover and thickness of the riprap layer play a very important role in the stability and thicker riprap layers can prevent a total disintegration of the riprap layer effectively, especially due to winnowing failure. keywords : pier scour, riprap protection, winnowing failure, riprap subsidence ..............................................................................................................................................................................................

Experimental Study on Mechanism Analysis of Headcut Erosion in the Noncohesive Sediment Bed

The headcut erosion at the confluence section of a mainstream and tributary can migrate up the tributary streams, and rapid degradation can threaten the stability of hydraulic structures installed in the channel. Therefore, quantitative analysis for the development and mechanism of headcut erosion is needed to prevent damage due to the headcut. In this study, hydraulic experiments for headcut erosion in the channel with noncohesive materials were performed and the knickpoint movement and final bed slope change were analyzed based on the different hydraulic conditions. As a result, the knickpoint movement was 1.5 times faster when the difference in velocity between the upstream and downstream sections was 2.5 times greater and the central part of the cross-section was eroded and collapsed earlier than the left and right sides. The movement length of headcut erosion was longer and the final bed slope was milder as the velocity difference between the upstream and downstream sections was increased. This study showed that a correlation between the knickpoint movement and bed slope change by headcut erosion and the water level difference of upstream and downstream sections was not constant compared to the velocity difference.

Stable Channel Design for the Gravel-bed River Considering Design Constraints

Abstract Stable channel design is to determine the width, depth and slope for satisfying the condition that the upstream incoming sediment rate is equal to the sediment transport rate at the design channel. Therefore, the most sensitive variable when designing a stable channel is the selection of a sediment transport equation applied for the channel design. Especially if in the case of gravel beds the designer uses the equation developed by using the dataof sand rivers, the calculation result of the stable channel section has large errors. In this study, the stable channeldesign has been applied to the gravel bed river using the previous stable channel design program with newly added the sediment transport equation for gravel beds; and the stable channel section considering design constraints has beenproduced by using the analytical method.As results, in the case of the application with the fixed width, the depth predicted by Ackers and White’s equationwas the shallowest and Meyer-Peter and Muller’s equation was 0.8 m deeper than the current section of 2.4 m. Inthe case of the application with the fixed depth, the width predicted by Engelund and Hansen’s equation was twice wider than the current section and by Meyer-Peter and Muller’s equation was 20 m wider than the current sectionof 44 m.

Long-term Bed Change Analysis and Equilibrium Bed Elevation Prediction after Weir Construction in Nakdong River

Bed changes in the Nakdong River were analyzed with long-term monitoring data for analyzing riverbed change patterns after Four Major Rivers Restoration Project (FMRRP). Also, possible long-term bed changes were predicted using one-dimensional numerical model for the section where the largest change was observed after FMRRP. The sensitive analysis was performed with different incoming sediment discharge conditions and sediment transport equations. The numerical model was calibrated by comparing short-term monitoring data and simulated results, and was applied for predicting bed change after 10 years. As a result of monitoring data analysis, the largest change in bed elevation occurred at the section between the Changnyeong-Haman and Hapcheon-Changnyeong weirs. The result of one-dimensional numerical modeling for 10 years indicated that maximum depositions of 2.07 m and 3.26 m were produced in this section.

Numerical Analysis of Flow and Bed Changes for Selecting Optimized Section of Buried Water Pipeline Crossing the River

Abstract A water pipeline buried under the riverbed could be exposed by bed erosion, therefore safe crossing sections should be analyzed for preventing damages due to the exposure of pipelines. In this study, flow and bed changes have been simulated using a two-dimensional numerical model for selecting the optimized section of pipeline crossing in the Geum River. As a result of simulation with the 20-year recurrence flood, sediment deposition has been distributed overall in the channel and bed erosion over 2 m has occurred near bridge piers. For the extreme flood simulation, the channel bed near the bridge piers has been eroded down to the buried depth. Therefore, within 140 m upstream of the bridge piers, bed erosion affects a buried pipeline in safety due to bridge pier effects and the crossing section over 150 m upstream of bridge piers is selected as a safe zone of a water pipeline.Key Words : Bed Change, CCHE2D, Numerical Analysis, Pipeline Crossing, River Crossing Structure x90W?A@ A: E£%CW? x1aDÐW?AE ]£ (12 i x8bx90 ´ C02) LÐs x87{AEG ¯pz .