Hydraulic Engineering

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Browsing Hydraulic Engineering by Subject "Aggradation"

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    Effect of Sediment Aggradation on the Hydraulic Performance of River Cross Drainage Structures (The case of Mekanisa Bridge on Mekanisa to Gofa Highway Road)
    (Addis Ababa, 2016-05) Hailu, Natnael; Seleshi, Yilma (PhD)
    Mekanisa Bridge is constructed on Little Akaki River in Addis Ababa city as cross drainage structure to connect the Highway Road from Mekanisa Round About to Gofa camp. It was constructed just before six years and currently the Bridge faces a serious problem of sediment aggradation both at the left and right side of the piers and the abutment. Due to sediment aggradation on the Bridge, reduction in the hydraulic performance was observed and flooding both upstream and downstream were also occurred frequently. HEC HMS and HEC RAS software were used to determine both the peak discharge, water surface profile level and sediment transport at the Bridge location based on the current river section profiles. Using the HEC HMS model the 50 year peak discharge of 405.5m3/sec was determined. The discharge which was used to determine the maximum water surface of the Bridge was 497.8m3/sec. From this one can observe no significant difference in peak discharge to cause a problem. But on HEC RAS model the water surface level using 50 year peak discharge was 2213.95m amsl before Bridge inserted into the model and 2214.6m amsl after Bridge inserted into the model. This is significant figure as compared to the maximum water surface level 2211.04m amsl which is used for the actual Bridge design. This clearly shows significant reduction in hydraulic performance of the Bridge and increase bed level at the Bridge location. In addition to this significant bed scour was modeled just upstream of the Bridge and insignificant deposition on the left and right side of the piers and abutments of the Bridge. This is mainly due to reduction in the opening and increase both velocity and sheer force of the water at the Bridge location. Pier layout, agricultural activity along the river reach and bank erosion are the major contributors of sediment at the Bridge crossing. River training work, dredging and retaining walls recommended to tackle sediment transport and deposition at the Bridge location. Key words:-Bridge, Sediment, HEC-HMS, HEC-RAS, Aggradation, Pier