Four series of sediments flushing experiment were carried out in a trunk sewer. The objectives of the experimental study is to survey the physical and chemical characteristics of sediments and estimate the effect of flushing. The flow rate of flushing were in the range of 0.40-0.44 m3/s. Tractive force at the flat portion of trunk sewer was calculated 0.3 N/m2 thus the critical particle size was predicted as 0.36 mm. However the decrease of sediments height was only 1 mm, and no difference of particle size distribution between before and after flushing. Sediments consisted of inorganic and organic substances. For organic substances the specific gravity was 2.50, water content was 73%, ignition loss was 39%. For inorganic substances, specific gravity was 2.60, water content was 20%, ignition loss was 1%. Sulfide concentration suggested no serious problem about odor and corrosion. The flushed load were in the range of 57-401 kg for BOD and in the range of 108-2,960 kg for suspended solids. The concentration of flushing water showed in the range of 100-640 mg/s as suspended solids. Estimation predicted 1-30% of total deposited load in the trunk sewer can be removed by one flushing event. The ratio of reduced load out of total discharged load is estimated as 4%.

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