Investigation of storm water drainage system. The case of shashemene to hawassa road

Resumen o Introducción

Road side drainages and their design are essential for sustainable performance of the road. Due to urbanization, deforestation and lack of good management, Shashmene to Hawassa road was facing frequent flooding and siltation. Therefore, the aim of this study was to investigate storm water drainage condition of the study area. Primary and secondary data were collected, analyzed and presented in compatible form. A number of Statistical Tools and Hyfran-plus hydrological analysis softwares were employed to conduct statistical tests and to perform analysis of hydro-meteorological data.

Missing rainfall data infilling was conducted by apt technique, frequency analysis was performed and Log Pearson type III probability distribution function was applied after identifying the best fit distribution using GOFT and easy fit 5.6 professional software finally to developing IDF curve. Watershed catchment delineation performed using Arc GIS software and 65.80 m3/s design runoff was generated using SCS-CN method. The computed rainfall versus runoff accuracy for different return periods were checked by using coefficient of determination (R2=0.982) which means input rainfall better explains output runoff and correlation coefficient (R=0.99) which means strong positive relationship exists between rainfall and computed runoff. From 1km chainage of road section evaluated, only 20 % of cross slope pass the ERA Paved Flat Terrain Typical Cross Section Standard. This was the reason behind flooding and siltation on the way along the road.

The sediment estimation on gully channel side slope indicated that average 200.08m3 or (530.2kg/year) sediment annually eroded from the area. Therefore, source of sediment accumulation was found to be from the stream banks on the gully side slope. In conclusion, based on the output of the analysis, remedy actions such as; providing proper road geometry and slope; practicing different erosion protection mechanism of channel and agricultural land; fill gravel or hard core on that sag section of road in order to increase the bed level of asphalt road; providing efficient drainage channel and gabion wall with integration of selected fill and sand fill bag for stream bank gully slope erosion protection were recommended in the study area.