Water leakage investigation of micro-dam reservoirs in Mesozoic sedimentary sequences in Northern Ethiopia

摘要

Millions of people throughout the world depend on dam reservoirs for domestic water supply, irrigation, electricity and flood protection. In the last two decades, 54 micro-dam reservoirs have been constructed in Northern Ethiopia to fight the recurrent drought and improve agricultural productivity through irrigation. However, about 60% of these micro-dam reservoirs are suffering from excessive leakage. Comprehensive studies have been carried out on two micro-dams to assess and pinpoint the causes of leakage. Arato and Hashenge micro-dams located in Northern Ethiopia have 20 m and 19 m height, and 2.59 Mm~3 and 2.23 Mm~3 reservoir capacities respectively. Observational geological description, shallow hand dug test pits, vertical electrical sounding and drilling of geotechnical holes were used to understand the overall geological, engineering geological and geo-hydrological set-up of the area. The different methods applied, such as discontinuity analysis, geophysical surveys, drilling and packer tests, delivered results that were found to be in close agreement and led to the identification of the leakage zone. The geological units found in both sites are limestone-shale-marl intercalation, dolerite and recent soil deposits. The research results revealed that the limestone-shale-marl intercalation unit is heterogeneous and shows alternating sequences. Analysis of the different data shows that the limestone-shale-marl intercalation is a pervious unit (hydraulic conductivity in the range of 10~(-4)-10~(+2) cm/s) and was found to be responsible for the excessive leakage of the micro-dams. It is hoped that the observations, data and insights gathered from these case studies will enable to plan technically and economically viable anti-leakage measures for these schemes and help for future new site selection and design activities in the region and other regions with a similar geological environment.