When domestic sewage and rainwater runoff are discharged into a single sewer pipe, it is called a “combined sewer system.” The sewage design standards in Korea specify the flow velocity based only on the volume of rainfall; therefore, sedimentation occurs on non-rainy days owing to the reduced flow rate and velocity. This sedimentation reduces the discharge capacity, causes unpleasant odors, and exacerbates the problem of combined sewer overflow concentration. To address this problem, the amount of sewage on non-rainy days, not just the volume of rainfall, should also be considered. There are various theories on sedimentation in sewer movement. This study introduces a self-cleansing velocity based on tractive force theory. By applying a self-cleansing velocity equivalent to the critical shear stress of a sand particle, sedimentation can be reduced on non-rainy days. The amount of sewage changes according to the water use pattern of citizens. The design hourly maximum wastewater flow was considered as a representative value, and the velocity of this flow should be more than the self-cleansing velocity. This design method requires a steeper gradient than existing design criteria. Therefore, the existing sewer pipelines need to be improved and repaired accordingly. In this study, five types of improvement and repair methods that can maximize the use of existing pipelines and minimize the depth of excavation are proposed. The key technologies utilized are trenchless sewer rehabilitation and complex cross-section pipes. Trenchless sewer rehabilitation is a popular sewage repair method. However, it is complex because the cross-section pipes do not have a universal design and require continuous research and development. In an old metropolis with a combined sewer system, it is difficult to carry out excavation work; hence, the methods presented in this study may be useful in the future.

ABSTRACT
1. 서 론
2. 연구방법
    2.1 소류력과 자가세정유속
    2.2 연구지역
3. 결과 및 토의
    3.1 하수도설계기준 반영안
    3.2 자가세정유속의 최소경사
    3.3 자가세정유속을 고려한 개보수 방안
4. 결 론
References

• 지현욱(한국건설기술연구원 국토보전연구본부) | Hyon Wook Ji (Department of Land, Water and Environment Research, Korea Institute of Civil Engineering and Building Technology)
• 유성수(한국건설기술연구원 국토보전연구본부) | Sung Soo Yoo (Department of Land, Water and Environment Research, Korea Institute of Civil Engineering and Building Technology)
• 송호면(한국건설기술연구원 국토보전연구본부) | Homyeon Song (Department of Land, Water and Environment Research, Korea Institute of Civil Engineering and Building Technology)
• 강정희(한국건설기술연구원 국토보전연구본부) | Jeong-Hee Kang (Department of Land, Water and Environment Research, Korea Institute of Civil Engineering and Building Technology) Corresponding author