Browsing by Author "Samuel, Tadesse"

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    Application of Dispersive Soils as a Sub Grade and Embankment Materials (Case Study on Adama – Awash Railway Section)
    (Addis Ababa University, 2016-04-27) Yohannes, Woldechirkos; Samuel, Tadesse
    Dispersive soils are prevalent in many areas of Ethiopia (especially in the rift valley). The use of dispersive soils in embankment structure can lead to serious engineering problems if the soil is not accurately identified before use and appropriate mitigation measures are not taken. The current tests used for identification include pinhole, double hydrometer, crumb and chemical tests. The main objective of this research is to use dispersive soil as an alternative embankment construction material. The light greyish sub grade soil of the pilot – study area (Adama – Welenchity section) have been tested with all identification tests of dispersive soil to check the extent of dispresivity of the soil. According to the result, the sub grade soil is dispersive with Pinhole test D2 (dispersive soil category 2), double hydrometer percent dispersion value TP1=87.17% and TP2=83.24%, crumb test result of grade 3 and Chemical analysis exchangeable sodium percentage of 31.93%. Treatments to dispersive clay using gypsum was carried out in this study. The optimum amount of gypsum required to treat a dispersive clay sample was 5%. The identification test results obtained by stabilizing with 5% recycled gypsum are: - double hydrometer percent dispersion value 24.72%, pinhole test ND1 (non-dispersive soil 1) and chemical analysis exchangeable sodium percentage of 6.54%. The engineering index tests of the dispersive soil have also been checked and found that the dispersive soil qualifies the specification requirement of embankment construction materials. After being treated with 5% recycled gypsum the dispersive soil can be used for embankment construction.
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    Estimation of Sediment Yield under the Impact of Climate Change (Case of Mille River Watershed, Lower Awash Sub-Basin, Ethiopia)
    (Addis Ababa University, 2022-03) Samuel, Tadesse; Bayou, Chane (PhD)
    Climate change impact and its variability on stream flows and sediment yield are now becoming one of the significant challenges. Hence evaluation of the impact and the trends of the climate change on hydrological process is very important at watershed level. The aim of this study was to assess the impact of climate change on the sediment yield of Mille river watershed at the lower Awash sub-basin using both climate and SWAT hydrological model. The climate model variables (precipitation and temperature) were obtained from Coordinated Regional Downscaling Experiment (CORDEX) of RACMO22 RCM climate model under HadGEM2-ES GCM and CCLM4 regional Climate model under ICHEC-ES global climate model with representative concentration pathway (RCP4.5) and (RCP8.5) scenarios. Bias correction was applied to climate variables and average value of the two RCM models arranged before future climate data’s transferred to hydrological model (SWAT) to simulate discharge and sediment for future time period. For model performance evaluation; Coefficient of determination (R2) and Nash Sutcliffe Efficiency (NSE) were used during calibration (1989-1997) and validated (1998-2002) period. The result of calibration and validation flow (R2 and NSE) are 0.68, 0.54 and 0.7, 0.6 respectively. Calibration and validation of sediment also provided a result of 0.81, 0.81 during calibration and 0.8, 0.79 during validation for R2, NSE respectively. Stream flow was simulated for two consecutive thirty years periods of (2031–2060) and (2061-2090) for both scenarios and compared with the base period (1986–2015) to investigate impact of climate change on hydrological process. The result obtained shows that overall mean annual precipitation projected to increase by 3.86% in 2050s and 7.82% during 2070s under RCP 4.5. For RCP 8.5 climate scenario precipitation increase by 4.61% during 2050s and 10.57% during 2070s. The hydrological response of a catchment also show an increment for the increasing precipitation amount such as sediment yield increase by 5.85% and 10.11% during 2050s and 2070s respectively under RCP 4.5 and 8.99% and 14.25% of increment occur in 2050s and 2070s respectively for RCP 8.5 climate scenario. Temperature maximum also shows an increment with a magnitude of 0.11oc and 0.09oc under RCP 4.5 of the 2050s and 2070s on the other hand 0.2oc and 0.15oc increment was observed with in the same period of RCP 8.5. Temperature minimum will be increase for the future over the Mille watershed by 0.09oc and 0.15oc Under RCP 4.5 and by 0.13oc and 0.18oc under RCP 8.5 for the same time horizons of 2050s and 2070s.Overall there will be a variations of precipitation, temperature and sediment yield month to month and season to season. Therefore, it is important to consider this variation of sediment yield and climatic variable to draw appropriate guidelines for planning and management of different water resources project.