Assessment of Flow/Discharge Variability Effects on Bridges and Culverts along the Holeta to Ambo Road, West Shewa Zone, Oromia Region
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Date
2024-04
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Addis Ababa University
Abstract
This study assessed the effect of variable flow on bridges and culverts along Holeta Ambo Road, West Shewa Zone, Oromia Regional State. In this study, the HEC-HMS hydrological model is used for analysing the flow variability. The simulation uses meteorological gauge stations over a period of thirty-year (1991-2020) for the catchment, which is sourced from Ethiopian metrology agency. The model was calibrated and validated using observed discharge data from three-gauge stations—Berga Nr. Addis Alem, Awash Bello (031020), and Debis Nr. Guder—that was collected by the Ministry of Water and Energy, Addis Ababa.
This study uses a calibrated HEC-HMS model to simulate daily flow in 56 drainage structures (bridges and culverts), over a period of 30-years (1991–2020). A coefficient of variance more than 40% is used as the threshold for variable flow. As a result, more than 71% of the structures exhibit a coefficient of variance of more than 40%.
The study also develops design rainfall and rainfall intensity–duration frequency using Gen. Extreme Value (GEV). The design rainfall is used as an input in flood frequency modelling in HEC-HMS to estimate the peak discharge. The estimated peak flow is compared with the discharge capacity calculated using Manning’s formula, and the result showed that nearly 38% of drainage structure is prone to flooding.
The effect of sedimentation on culvert structure is assessed through comparing the estimated peak flow and discharge capacity before and after sedimentation, and around 13% of the culverts are affected by sedimentation. The scouring effect of structure is computed by comparing the permissible velocity with the velocity calculated using Manning's formula. Consequently, 25% of the structures, particularly the bridges, are affected by scouring.
This study reveals the impact of variable flow on drainage structures along the Holeta to Ambo Road, which is prone to flooding, sedimentation, and scouring. It is essential to replace the existing structures with those designed to manage variable flow, improve routine culvert maintenance and prioritizing the replacement of aging structures. These measures ensure the long-term safety, durability and functionality of the road network and structures. Additionally, this study provides the opening size and structure type of the selected drainage structures are used for other study.
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Keywords
Flow variability effect, Drainage structures, flooding, scouring, sedimentation