Evaluation of Existing Storm Water Drainage System of Aba-Geda Sub-City in Adama City, Ethiopia

Abstract

Improper management of urban runoff poses a significant threat to cities worldwide. It stems from a combination of various factors, including hydrological elements, urbanization, climate change, and inadequate infrastructure. This issue can lead to devastating consequences, such as property damage, the destruction of critical infrastructure, and even loss of life. Many cities around the world have been facing with this disastrous trait, which arises due to the complexity and uncertainty surrounding these interconnected factors. It is crucial for cities to address and tackle this issue promptly to mitigate its detrimental effects on society and the environment. During the rainy season in Adama city of Aba-Geda Sub City often experience frequent flooding. The abundance of rainfall, combined with the absence of proper drainage systems and inadequate solid and liquid waste management practices, contribute to this issue.This paper focuses on the challenges of storm water drainage management in Adama city of Abageda Sub-city, Ethiopia. The study highlights the need for an integrated early warning system for flood risk management and ongoing improvement in analysis, mitigation, and management of flood damages. The objective of this study is to assess the existing storm water drainage system in Adama, specifically for Aba-Geda Sub-City drainage line. The data collection process involved gathering both primary and secondary data were subsequently analyzed and presented. Landsat imagery from 2018 and 2022 was utilized for land use and land cover classification. Geographic information system (GIS) software was employed to classify the land use and land cover map based on ground truth observation. Additionally, an accuracy assessment was conducted to verify the consistency of the classified map with the ground truth data. This involved comparing the classified map with actual on-site observations to ensure the reliability and precision of the classification results. The frequency analysis of rainfall was computed using the Gumbel distribution, which is commonly used to analyze extreme value data, such as rainfall. The intensity-duration frequency (IDF) curve was developed to understand the relationship between rainfall intensity, rainfall duration, and return period. The peak discharge flowing in the main catchment up to the final outlet point was calculated using the rational method, which is a widely used method for estimating peak discharges in small watersheds. These methods and analyses provided valuable insights into the characteristics of rainfall and its impact on the drainage system. The comparison of the model results with the existing hydraulic capacity of the channels revealed that, the channels from CH-8 to CH-14, known as Mebrat Hayil to Posta Bet, as well as the channels from CH-15 to CH-16, surrounding Bekele Mola to Posta Bet, and the channels from CH-17 to CH-26, from Sar Tera to Franco, are all experiencing a lack of hydraulic capacity to handle the runoff water they receive. Similarly, the channels from CH-27 to CH-30, from Franco to Pan-Africa, are also inadequate in carrying the contributed runoff water. These channels are currently ill equipped to manage the runoff generated by a 25-year return period design discharge. The primary issue in this area stems from the insufficient capacity of the channels and the incorrect sizing of the inlet culverts, which were not appropriately designed considering the contributing catchment area of 220ha.Therefor, BMP’s will help in solving the problem.