Hydraulic Engineering

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ANALYSIS AND CHARACTERIZATION OF HYDROLOGICAL DROUGHT UNDER CLIMATE CHANGE IN HAMASSA WATERSHED, RIFT VALLEY BASIN
(Hawassa University, 2024-07-20) REDIAT LEGESE SIME
Hydrological drought occurs when there is an extended period of significantly reduced water availability, leading to depleted water sources and severe impacts on ecosystem and communities. Water scarcity caused by prolonged periods of reduced rainfall due to climate change can lead to the natural disaster of drought. However, little has been done so far on hydrological drought under climate change in Hamassa watershed. This study aimed to analyze and characterize hydrological drought under climate change in the Hamassa watershed, Rift Valley Basin, Ethiopia. Hydrological data (1992-2015), meteorological data (1992-2022), future climate data 2030-2090), spatial data, DEM, land use land cover, and soil were collected. CMhyd software package was used for bias correction of the climate data. The hydrological model soil and water assessment tool (SWAT) was used for hydrological analysis. The simulation result was calibrated and validated using the SWAT calibration uncertainty procedure (SWAT-CUP). Standard precipitation index (SPI) and stream flow drought index (SDI) are used to decide drought conditions in a watershed and to identify drought-prone areas in the watershed. Temperature projections for both the near and long term indicate an increase compared to the current period under both RCP2.6 and RCP8.5 scenarios. Meanwhile, precipitation projections suggest a decrease for the periods 2040-2060 and 2061-2072 under both RCP2.6 and RCP8.5 scenarios. The standard precipitation index (SPI) and stream flow drought index (SDI) results showed that the watershed experiences mild (-0.5- -0.999), moderate (-1- -1.49), severe (-1.5- -1.99), and extreme (≤ - 2) drought events. Droughts are projected to occur in the periods 2040-2060 and 2061-2072 under both RCP2.6 and RCP8.5 scenarios. Sub-watersheds 7, 8, 9, 10, and 11 showed high vulnerability to severe and extreme drought. Drought-mitigating structures are needed to mitigate drought in the watershed
ANALYSIS AND PRIDICTION OF METEOROLOGICAL DROUGHT USING STANDARDIZED PRECIPITATION INDEX: CASE STUDY OF EASTERN OROMIA
(Hawassa University, 2019-02-14)
Ethiopia is one of the most drought affected country in Africa and suffered from acute drought many time in recent years. The objective of this study is to understand the rainfall variation and conduct an in-depth analysis and prediction of drought for to Eastern Oromia. Standardized precipitation index (SPI) was selected to characterize drought condition in the region. Drought characteristics at a time scale of 2-month, 3-month, 6month and 12-month were computed separately for 22 selected rain gauge station. The most maximum drought characteristics at 2-month, 3-month, 6-month and 12-month time scales have been found in Hararegie over last 36 years (1979-2015). The spacial extent of drought and rainfall variation were determined from SPI and CV values computed separately for each rainfall have been interpolated between station in Arc view GIS environment. The rainfall variation shows that the rainfall over the study area is highly variable and increases from Arsi to East Harargie and in the recent last decade (2005-2015) the map of SIP-3 (Belg) shows that a spacial extent drought over study area was irregular and there were no period in recent decades (2005-2015) without drought at least mild drought and this shows that the area is vulnerable to drought. The maximum intensity of (5.33) was recorded at Fedis Station for SPI-3 (kiremt season) in a year 1996, maximum intensity of (4.04) was recorded at Kobo Station in a year 1985 for SPI-3 (Belg season), maximum intensity of (4.77 and 4.5) were recorded at Ticho Station for SPI-6 (both seasons) and SPI-12 (annual) respectively. The drought prediction was made until 2043, using the Representative Concentration Path way (RCP) 8.5 Scenario.on the basis of the drought index measured by SPI. From the analysis of the drought index through the RCP 8.5 Scenario, extreme drought intensity will be more likely to occur. The long-term forecast (12-month period unit) showed that extreme drought would occur severely at station Diksis Sude, Hirna and Adelle from 2035 to 2040 and need more attention for this area
ANALYSIS OF DAM BREACH PARAMETERS AND INUNDATION MAPPING (THE CASE OF KALID-DIJO EMBANCKMENT DAM)
(Hawassa University, 2023-07-08) Lemeneh Mersha
Embankment dam risk assessment is a method used to evaluate the catastrophic impact of dam break flooding. This study was conducted to analyze dam breach parameters of Kalid Dijo dam and inundation mapping. The dam breaching outflow hydrographs, inundation mapping and downstream flood propagations were simulated by applying HEC-RAS 5.0.6 model to study the possible relationships among the peak flows, dam breach parameters, and map the inundation area. The initial parameters are taken from literature based on the dam type and its silent features. HEC-RAS model 2D reservoir and river routing were performed and resulted in multiple breaching outflow hydrographs for specified conditions at the dam site and at specified downstream reach stations. The maximum breach discharge resulted from the model was 852.18 m3 /sec which results in overtopping the dam by 0.6m. Four Time to Failure Hour and four Breach Side Slope values are used for the sensitivity analysis at the dam site. Accordingly, the change of Time to Failure Hour resulted 403% change of the maximum discharge at the dam site which is high compared to the change in Breach Side Slope which produced 100% reduction in maximum discharge. This implied that the peak discharge was highly sensitive to changes in Time to Failure Hour more than five times than Breach Side Slope; and Time to Failure Hour is a controlling parameter at the dam site. At two randomly selected downstream observation stations 01 and 02, an increase in TFH and increase in Manning coefficient resulted in decrease in peak flow at reach station-01 and 02. The total inundated area is 454.19 sq.km. It covers 1100ha of command area and some parts of Dalocha town.
ASSESSMENT OF CLIMATE CHANGE IMPACT ON STREAM FLOW OF GIDABO SUBBASIN, RIFT VALLEY BASIN, ETHIOPI
(Hawassa University, 2022-08-10) YIDIDYA TSEGAYE ALEMU
Climate changes alter regional hydrologic conditions and result in a variety of impacts on water resource systems. Such hydrologic changes will affect almost every aspect of human well-being. The goal of this thesis is to assess the impact of climate change on the hydrology of Gidabo subbasin located in the Rift Valley basin of Ethiopia. The RCP scenarios of types 2.6, 4.5, and 8.5 were used for the climate projection from the CORDEX Africa domain from CMIP5. The RCM of RCA4 was used to generate future possible local meteorological variables in the study area. These data were used as input to the Soil and Water Assessment Tool (SWAT) model to simulate the corresponding future streamflow Variability in the Gidabo subbasin. SWAT-CUP, a program for calibration and uncertainty was utilized for uncertainty analysis. The three projected time periods for this study were the 2040s, 2060s, and 2090s. The time series generated by RCM of RCA4 driven by MIROC5 indicate a significant increasing trend in maximum and minimum temperature values and a decreasing trend in precipitation for all RCP emission scenarios in Measso station for all time periods. The hydrologic impact analysis made with the downscaled temperature and precipitation time series indicates variation in an increasing and decreasing trend for the three RCP scenarios at different periods respectively. The model output shows that there may be a mean annual, seasonal, and mean monthly decrease in stream flow volume for all RCP scenarios in the Projected time periods in the future. It also shows most of the projections are within the uncertainty bandwidth of 95PPU
Assessment of Climate Change Impact on Surface Water Availability: Gerhu-Sirnay Catchment, Mereb Sub- Basin,
(Hawassa University, 2024-07-12) ZEGEYE TAMIRU HAMESSO
Investigating surface water availability under climate change impact is vital to ensure water resource sustainability. The general objective of this thesis was to assess the impact of climate change over surface water availability of Gerhu-Sirnay catchment using CORDEX-RCA4 with soil and water assessment tool (SWAT). To achieve this, quality of observed data was accepted for outlier, adequacy and consistency tests, and CORDEX-RCA4 datasets were passed the Power transformation and variance scaling bias correction and square root normalization. The baseline monthly stream flow (1990-2003) was modeled using SWAT, calibrated (1992-1999) and validated (2000-2003) in SWAT-CUP under SUFI2 tool. The CORDEX-RCA4 models were compared their performance at the baseline period (1990 to 2001) using volumetric metrics and Taylor diagram to predict future precipitation and stream flow variability by the best fit RCA4 model under RCP4.5 and 8.5 emission scenarios for 2050s and 2080s periods. The results showed that SWAT was very good at modeling baseline stream flow indicated by R 2 , NSE, PBIAS and RSR as 0.93, 0.94, 6.3% and 0.13 for calibration, and 0.82, 0.89, 10.4% and 0.07 for validation respectively. ICHEC-EC-EARTH-RCA4 was best fitted by scoring 0.9838, 0.0000, 0.9838 and 0.0162 for VHI, VFAR, VCSI and VMI respectively for volumetric, and 0.749, little less than 75 and little less than 100 for CC, NRMSE and δN respectively and better at annual scale at Taylor diagram. The baseline variability of seasonal rainfall indicated that an increments on the winter and autumn and decrease on the spring season. In the 2050s and 2080s of both emission scenarios significant increase and decrease was projected than the baseline periods at seasonal and annual scales. The mean annual rainfall was decreased by; 7.58% and 9.82% at 2050s and 2080s, and 4.92% and 9.28% during 2050s and 2080s under RCP4.5 and RCP8.5 respectively. The total change of rainfall was; 9.89% for 2050s and 13.52% for 2080s, and 8.79% at 2050s and 13.31% at 2080s for RCP4.5 and RCP8.5 scenarios respectively. Future annual stream flow will be decreased by; 8.84% in 2050s and 10.59% in 2080s, and 6.32% in 2050s and 9.88% at 2080s under RCP4.5 and RCP8.5 respectively. The total annual stream flow change will be; 9.88% during 2050s and 13.67% at 2080s, and 9.96% at 2050s and 13.86% at 2080s for the RCP4.5 and RCP8.5 scenarios respectively. Findings of this study indicated that climate change has significant impact over surface water availability of Gerhu-Sirnay catchment. To conduct policy oriented climate change impact over surface water availability, future researchers should consider multiple; climate variables, dynamic drivers and uncertainty analysis, and improve CORDEX inputs
ASSESSMENT OF CLIMATE CHANGE IMPACT ON SURFACE WATER RESOURCE IN WOSHA WATERSHED, RIFT VALLEY LAKES BASIN, ETHIOPIA
(Hawassa University, 2024-10-16) ELSAE WARE GIDESA
Climate change (CC) and Land Use/Land Cover (LULC) changes present significant threats to global water resources and socio-economic systems. This study aimed to evaluate the impact of climate change and LULC changes on the availability of surface water resources in the Wosha watershed, located in the Rift Valley Lakes Basin (RVLB) of Ethiopia. The available water resources were assessed using the Soil and Water Assessment Tool (SWAT), a semi-distributed, physically-based hydrological model. Calibration and validation of computed stream flow were conducted using SWAT-CUP with the SUFI-2 algorithm. Bias-corrected data from three climate models output from Coupled Model Intercomparison Project Phase 6 (CMIP6) models such as CANESM5, MIROC6, and NESM3 were used to assess baseline (1985-2014), mid-term (2041- 2070), and long-term (2071-2100) periods under the Shared Socioeconomic Pathways (SSP2 4.5 and SSP5 8.5) climate scenarios. The SWAT model's performance was robust, achieving R² values of 0.88 and NSE values of 0.75 during calibration, and R² values of 0.83 and NSE values of 0.72 during validation for monthly simulations. The projections indicate that both rainfall and temperature will increase under SSP2 4.5 and SSP5 8.5 scenarios in the mid-term period, with a temperature rise of 1.2°C expected under SSP5 8.5. Precipitation is also expected to increase by 1% to 10% in the long term for both scenarios. Therefore, the primary objective of this study was to assess the impact of climate and LULC changes on the availability of surface water resources in the Wosha watershed. The results underscore the importance of understanding surface water availability and mitigating the impact of climate change to ensure future water resources for the region.
ASSESSMENT OF DESIGN PRACTICES AND PERFORMANCE EVALUATION OF RIVER DIVERSION STRUCTURES FOR IRRIGATION: A CASE STUDY OF RASSA AND WODESA SMALL SCALE IRRIGATION SCHEMES IN WONDO GENET WOREDA, SIDAMA REGION, ETHIOPIA.
(Hawassa University, 2024-12-16) HENOK NIGUSSIE ALEMU
Diversion headworks play a vital role in diverting water from rivers or channels for irrigation. However, many such structures face hydrological, hydraulic, structural, operational, and management challenges, leading to reduced functionality and high maintenance costs. Some recently constructed diversion structures have been reported to work properly while others have what could be termed as partial failures; parts of the structures get damaged as time passes. This study focuses on assessing the design practices and performance evaluation of the Rasa and Wodesa diversion headwork structures, which were constructed for irrigation purposes. The research evaluates the current conditions of these structures, analyzes the design practices, and identifies key factors contributing to their underperformance. Primary data, including field observations and measurements, were collected, along with secondary data from sources such as the National Meteorology Agency, the Ministry of Water and Energy, and the Sidama Regional State Irrigation Development Agency. The tools used include GPS, GIS 10.3.1, Excel, DEM, Bentley Flow Master, and HEC-HMS. The hydrological analysis, conducted using the HEC-HMS model, was calibrated and validated with observed streamflow data from the Tukur Wuha River for the periods 1991–2000 and 2001–2007, respectively. The model achieved good performance, with Nash-Sutcliffe Efficiency (NSE) values of 0.671 and 0.672, R² values of 0.79 and 0.8, Percent Bias (PBIAS) of 7.26% and 7.41%, and RMSE (Stdev) of 0.6 for both periods. The optimized parameters from the calibrated model were transferred to the ungauged Rasa and Wodesa watersheds using the regionalization technique. Design storm analysis of the model presented the extreme flows of 59.2m³/s for Rasa and 98.2m³/s for Wodesa which are beyond the weir design capacity of 44.2m³/s and 84m³/s respectively which results in flooding. Poor flood passage capacity and deposition of sediment were also recorded, which hindered efficiency. More specifically, structural evaluations indicated that floor thicknesses of concrete were low, cutoffs inadequate, and weir dimensions too small to provide adequate protection against uplift pressures and piping. Despite these problems, the stability for both weirs in terms of sliding, overturning, overstressing, and contact pressures were determined to be safe. The retaining walls were also safe from a structural perspective although contact pressures were higher than the recommended safe limits and therefore required further assessment and strengthening. This paper points out key aspects that would enable a small-scale diversion weir to function with greater durability and efficiency in irrigation systems
ASSESSMENT OF GROUNDWATER POTENTIAL ZONES: THE CASE OF WOGAYI WATERSHED IN THE OMO-GIBE RIVER BASIN, ETHIOPIA
(Hawassa University, 2023-08-10) WUBISHET SISAY WOLDEYES
This study was aimed at assessing the groundwater potential of Wegayi watershed in the Omo Gibe river basin. Secondary data: climate, topography, well log information and different thematic layers were used. The thematic layers considered in this study are geology, geomorphology, drainage density, lineament density, rainfall, soil types, slope and land use/cover. Arc GIS and Surfer 20 softwares were used to produce spatial maps of the different thematic layers and water table contours respectively. The geological formation of the catchment was evaluated using existing well logs data of the boreholes. The groundwater flow direction of the catchment was identified by using ground water level maps. The Saaty analytical hierarchy process (AHP) method was implemented to identify the groundwater potential zones based on their relative assigned weights. Eventually, the weights were normalized so as to indicating on their importance in groundwater occurrence. The major lithological units of the area are weathered and fractured basalts. These types of geological formation are recognized to be good groundwater formations. According to the groundwater flow lines and the ground water contour maps produced in the catchment, the groundwater flows from southern and eastern parts of the highlands of the catchment towards the plain areas of the northwestern parts of the catchment. The hydraulic characteristics are variable throughout the catchment. According to the result of groundwater potential map, the major recharge of groundwater occurs in the Easter and southwestern highlands of the catchment, which is said to be recharging zone. However, the area situated in the southeastern, northwestern and northeastern parts of the catchment is discharge zones. The results of AHP indicate that 58.12% moderate, 20.82% high, 21.03% low and 0.032% very low groundwater potential zones have been identified. Finally, it is concluded that the integrated GIS and remote sensing techniques are very efficient and useful for the identification of groundwater potential zones
ASSESSMENT OF LAND USE / LAND COVER CHANGE IMPACT ON STREAM FLOW USING SWAT MODEL, THE CASE OF KULFO RIVER CATCHMENT, RIFT VALLEY BASIN, ETHIOPIA
(Hawassa University, 2024-01-06) OBSE FUFA ARFASA
In order to conserve land and water resources and ensure its long-term sustainability, land use land cover change dynamics and its impact on the water resources of watersheds have emerged as key issues in hydrology. The main objective of this study was to investigate impact of land use land cover change on stream flow of Kulfo River. ERDAS IMAGINE-2015 using the maximum likelihood algorithm of supervised classification was used to evaluate changes in land use and land cover of study area. Land sat-5 images of year 2000 and 2010 as well as land sat-8 images of year 2022 obtained from USGS with spatial resolution of 30m were used for land use land cover classification. Accuracy of model to classify land features accurately and precisely was evaluated by making comparison matrix between classified classes with those collected by GPS and from Google earth. Kappa coefficient from comparison matrix has been used to evaluate performance of model and it has been determined over 80% for all classification periods verifying the accuracy of classification. From comparison observed from each periods of land use land cover maps, there are some significant changes on land use and land cover features of study area. Over the past study years, the watershed has undergone significant changes, including the expansion of agricultural land and towns at the expense of the reduction of grassland, forest, and shrub area. Those LULC maps of different periods were varied as SWAT inputs while maintaining the remaining inputs the same in order to evaluate its impact on stream flow using SWAT model. The sensitivity analysis, calibration and validation was conducted using Sequential Uncertainty Fitting (SUFI-2) within SWAT-CUP. The calibration results showed very good match of simulation with observation with Nash-Sutcliff efficiency (NSE) of 0.81, coefficient of determination (R2 ) of 0.83, root mean square error (RMSE) of 0.12 and percent of bias (PBIAS) of -0.1.Perspective objective functions during validation have been determined as 0.84,0.86,0.01 and 0.03 respectively. Well calibrated stream flow for different periods has been compared to evaluate impact of LULC changes on stream flow. It has been pointed out that some increases in agricultural land and urbanization promoted runoff formation by reducing water infiltration into deep soil, result in an increase in surface runoff and a decrease in ground water flow. It is also revealed by the study that increased surface due to land use land cover change is more significant during wet seasons than that of dry seasons. The study's methodology and findings will be pertinent to environmental policy makers and other relevant bodies which will involve in the development, occupancy, and management of resources related to water and land
ASSESSMENTS OF SUITABILITY OF CONSTRUCTION MATERIAL WITH SPECIAL EMPHASIS ON FILTER: - THE CASE OF AGUAT WUHA EARTH DAM AT
(Hawassa University, 2020-07-10) HENOK DESSALEGNTILAHUN
Aguat-wuha dam is an earth fill dam, which is one of the Irrigation Projects undertaken by AWDSE found in South Wello Zone, Sayint Ajibar Woreda. As it was observed, during the construction process the construction material, the filter material in particular, was not properly prepared while placement. This could result in malfunctioning of the filter zone as a because of material poor gradation and contamination. Hence, this work was aimed to evaluate the suitability of construction material especially on filter material assessments. During this research work materials for analysis were taken from the dam which is on construction and filter material samples stored in stoke. The geometrical criteria have been mainly checked using the formulas and recommendations proposed by scholar and standard. In addition to this, the final gradation or grain size distribution of a suitable filter material has been prepared based on the gradation of the base material following the design procedures prepared by USSCS and Sherard recommendations. It was found that the filter material taken from the dam and analyzed for grain size distribution doesn’t satisfy most of the filter criteria. On the other hand the treated sample of this same filter material showed better in satisfying most of the filter criterion and also the stocked filter material was contaminated due to improper placement and poor prevention. The analysis result (especially USSCS method) indicated that the treated (washed) filter material almost satisfies the geometrical criteria and it had a little bit fine (1.7-2.2 %.), which was still in the limit of the filter band. On the other side, the constructed sample was not satisfied the gradation curve. Because it was over fined, that 5.5-5.8% passes the fine limit of filter which can result clogging, poor permeability, weak draining capacity and crack for coming serviceability. Additionally the material contamination result shows, mostly on the underneath of stoke especially from ground to 80 cm because the material was near to unexpected runoff entry. Therefore the field manager or site engineer should apply efficiency checking and giving remedial measure carried out not only at quarry site but also during construction. Additionally design of quality control and quality assurance should properly apply
DAM BREACH ANALYSIS AND DOWNSTREAM FLOOD MAPPING USING HEC-RAS AND HEC-FIA (A CASE STUDY OF GERHU-SIRNAY DAM)
(Hawassa University, 2020-07-03) KEHASE NEWAY GEBRETSADKAN
Dam breach analysis is a science that quantifies the hazard sourced from dam failures. On downstream of Gerhu-sirnay dam there are developments including water treatment plant and multi owner seasonal agricultural areas. The objective of this study was to estimate breach outflow hydrograph to the downstream reach for overtopping and piping scenarios henceforward to assess flood damage for the worst scenario. The quality of rainfall data was tested for outlier, adequacy, homogeneity using Pettitt (1979) test and consistency tests. The probable maximum precipitation calculated by Hershfield (1965) method was 129.7 mm. The inflow hydrograph was obtained by using soil conservation service (SCS) method and its probable maximum flood (PMF) reaches 329.1 m 3 /s. The breach parameters for both scenarios were early calculated by the four regression equations integrated with Hydrologic Engineering Center River Analysis System (HEC-RAS) breach calculator tab. Due to comparison in both scenarios, the final breach parameters were taken from Von Thun and Gillette (1990). For overtopping the resulting parameters were 54 m breach bottom width, 85.75 m breach top width, 0.5 side slope (H: V) and 0.73 hrs breach formation time whereas in case of piping these parameters were 32 m, 57.75 m, 0.5 and 0.53 hrs respectively. Then HEC-RAS version 5.0.3 has used to model the dam breach analysis with two dimensional unsteady flow conditions of inflow hydrograph as upstream boundary condition and normal depth as downstream boundary condition. The peak breach outflow for both scenarios were 2,239.7 and 1,282.68 m 3 /s respectively as overtopping was the worst scenario. From the prepared inundation maps by GIS for about 521.1 ha area was inundated under the worst scenario with a maximum depth 24.07 m and duration of 1.0 hrs. By using Hydrologic Engineering Center Flood Impact Analysis (HEC-FIA) version 2.2, the flood damage was estimated in terms of direct economic damage and life loss. The total ex-ant direct economic damage result shows 63,650,493.3 Birr. From the report of LIFE-Sim dynamic model integrated with HEC-FIA, about four persons could be lost their life. To minimize the quantified damage, well operation of emergency action plan would be an important tool. The water treatment plant of Gerhu-sirnay town must be displaced from its current place to at least 0.073 km apart from both sides of the river banks until to do this the stakeholders of the plant must be under the age of 65 years to have an efficient warning mobilization
DAM BREACH ANALYSIS OF BISANDIMA DAM, SIDAMA REGINAL STATE, ETHIOPIA
(Hawassa University, 2023-07-23) TAFESE FONA DEBESA
Due to the construction of Bisandima dams, the downstream ecosystem is highly changed in that a huge area is covered with irrigation farms, new settlements, and residence areas of inhabitants living on the farms, and fishery communities were formed due to this dam. All these investments and newly settled inhabitants are highly exposed to flooding and they are at risk from the possible failure of this dam. The objective of this study was to model the Bisandima dam breach phenomena and to develop flood inundation maps. In this study dam breach, flood routing was carried out by using unsteady flow routing in HEC-RAS 2D model using geometric data to determine flood-susceptible areas downstream of the dam for the study area. The key inputs required in the dam breach flood routing processes include Precipitation data to determine PMF, digital Elevation model, and land use data were used as input for the HEC-RAS model. Some parametric methods were also used to predict dam breach parameters for use in the model. The breach discharge resulted from the HEC-RAS model for all methods such as Thun & Gillete (1990), Froehlich (2008), Froehlich (1995), MacDonald and Langridge –Monopolis (1984), Xu & Zhang (2009) were 2079.01, 1040.21, 1332.49, 531.91, 592.22 m3 /s and 1796.52, 620.37, 572.02, 530.53, 498.46 m3 /s for overtopping failure and piping failure respectively and also breach outflow at the downstream cross-section were 969.32, 625.48, 699.46, 538.96, 592.38 m 3 /s for overtopping failure respectively. The hazard mapping due to the combined effect of flood depth and velocity showed that an area of 28ha, 9.5ha, 11ha, 3ha, and 0.5ha are under low, medium, high, very high, and extreme hazard respectively for overtopping failure and an area of 20ha, 8ha, 10ha, 2.2ha, and 0.8ha are under low, medium, high, very high and extreme hazard respectively for piping failure. As noticed from the flood inundation map almost all critical areas downstream of the dam are in immediate danger. Based on the finding of the study outcomes the following recommendations are drawn: flood early warring, watershed treatment, provide dike at both sides of the downstream
DEVELOPING OPTIMAL RESERVOIR OPERATION (CASE OF KOKA MULTIPURPOSE RESERVOIR, ETHIOPIA
(Hawassa University, 2023-07-18) GEMECHU SHIFERAW BELACHEW
Reservoir operation is the most challenging task in the management of water resources systems. Hence, water resources models are needed to minimize these challenges, as much as possible if they are supplemented with sound engineering judgments. In this study, the rainfall runoff simulation HEC-Hydrologic Model System (HEC-HMS) and reservoir simulation model (HEC-ResSim) were applied to the koka multipurpose reservoir to optimize the power fluctuation observed and water scarcity due to reduced water release from the reservoir. Inflow was generated by the HEC-HMS model by using twenty years of daily meteorological data collected from NMA. The generated inflow was calibrated and validated with 16 years of observed flow data from Melka Kunture, Hombole, and Mojo gauging stations. The performance of the model was evaluated by NSE, R2, RMSE, and PBIAS performance indices criteria reviewed in different literature. For example, the NSE value of Melka Kunture, Hombole, Mojo, and Koka Inlet is 0.75, 0.78, 0.63, and 0.8 for calibration, and 0.7, 0.72, 0.53, and 0.75 for validation, respectively. The value of R2 for Melka Kunture, Hombole, Mojo and Koka Inlet is also 0.74, 0.77, 0.64 and 0.77 for calibration and 0.74, 0.75, 0.52 and 0.76 for validation respectively. Although the model slightly underestimates the flow, for both calibration and validation, the model shows acceptable performance to generate an inflow of the upper awash watershed. To simulate the reservoir the inflow generated by the HEC-HMS model, reservoir physical, and operational data were collected from governmental organizations and provided to the model. Since the HEC-ResSim model cannot optimize the constraints directly, trial and error have been applied through a prioritization rule between three main demands; i.e., Hydropower, Irrigation, and Domestic, Municipal and Industrial demands. The best alternative was selected based on the power target, release target, and pool elevation target. Accordingly, from the three alternatives applied in the simulation, ALT-1 gives maximum power and maximum release that supports the downstream water needs. When power demand was given the highest rule priority, the reservoir generate an average energy of 504.76MWh per day or 55GWh per year which is greater than the power generated in ALT- 2 and ALT-3 by 32% and 64%, respectively. The reservoir reaches its minimum elevation of 103.93m in June (except in the drought year of 2003 and 2016) and its maximum elevation of 110.39m in August. Generally, the reservoir can support the downstream water needs safely if the operation will be conducted by the power demand priority rule.
EFFECT OF BAFFLE BLOCK PARAMETERS ON HYDRAULIC JUMP CHARACTERISTICS
(Hawassa University, 2023-08-07) MEKONNEN BEKELE WORKU
The experimental determination of baffle blocks installed on stilling basins has been also utilized to stabilize the formation of the jump and increase the turbulence, there by assisting in the dissipation of energy. The main objective of this study is to assess the effects of baffle block parameters on hydraulic jump characteristics. Investigate the influence of baffle blocks on hydraulic jump properties of flow over a weir, to evaluate the effects of different shapes and arrangements of baffle blocks on hydraulic jump characteristics and identify the best shape and arrangement of baffle blocks increasing efficiency of energy loss. The experimental work was performed in a rectangular flume with a dimension of (14 m* 0.3 m* 0.45 m) long, wide, and deep, respectively with Steel, different shapes of baffle blocks (Trapezoidal, Rectangular, and Triangular) with the dimension of (0.05 m*0.0375 m*0.06 m) height, width and length bed material. The experiment was being tested with various flow rates for different shapes and arrangements to analyze the effect on the sequent depth ratio, energy loss, Froud number, and efficiency. Each shape and arrangement baffle have five discharges that were used to maintain the upstream depth the relationship between the Froude number and the discharge for different baffle depth and different velocity; where the average value of the depth was varied (10mm - 50mm). Froude number decreases (1.92-1.54) with the decreases in the average value of loss of energy (0.20–0.14), as the sequent of depth ratio decreases (17.85*10-3 –13.41*10-3 ). The relation between Shape of Baffle Block, Energy Dissipation, and efficiency Trapezoidal (32.19, 70.86%), Rectangular (37.35, 83.98%), Triangular (32.03, 67.23%), and Mixed (34.41, 74.08%) respectively. From above clearly shows that rectangular baffle blocks have higher energy dispassion and efficiency which indicate that the energy dispassion and efficiency in row three and four arrangement 4*3*2 and 4*3*2*1 obtained best are energy dispassion (32.77%) and efficiency (83.98%) receptively under rectangular shapes, which is highest from baffle block shapes relatively better than the other baffle block shapes this is due to may be part of energy dissipate increases efficiency. From this observation rectangular baffle blocks are better for energy dissipation since higher efficiency
Estimation of Groundwater Recharge Using GIS Based Wetspass-M Model: The Case of Dedaba Watershed, Rift Valley Lakes Basin, Ethiopia
(Hawassa University, 2024-10-18) JIBRIL WAKEYO WARIO
The Dedaba watershed, located within the Rift Valley Lakes Basin in Oromia, Ethiopia, is experiencing significant changes driven by agricultural expansion, land use and land cover (LULC) changes, and a growing population. These dynamics, combined with insufficient watershed management, have resulted in water resource depletion, pollution, and environmental degradation. The escalating demand for groundwater, driven by the population increase, present a considerable challenge in this region. This study utilized the WetSpass-M (Water and Energy Transfer between Soil, Plants, and Atmosphere under quasi Steady State – Monthly) model, a spatially-distributed water balance model, to assess seasonal and annual groundwater recharge, actual evapotranspiration, and surface runoff in the Dedaba watershed. The model integrates spatially distributed data on precipitation, potential evapotranspiration, temperature, wind speed, soil types, LULC, and topography. These datasets, processed using GIS techniques, allowed for the generation of detailed spatial water balance components. Calibration and validation of the model were conducted using observed groundwater levels and streamflow data, ensuring accurate simulations. The calibrated WetSpass-M model revealed groundwater recharge estimates ranging from 0.46 to 65.4 mm/year, with an average of 37.47 mm/year, representing 3.4% of the total recharge. To understand the impacts of LULC changes on groundwater recharge, the model was applied using LULC data from 1990 and 2020. Results indicated a continuous decline in recharge rates over this period, underscoring the significant influence of LULC on groundwater resources. Specifically, the model estimated recharge at 3.29 mm in January 1990, peaking at 6.03 mm in September, and dropping to 0.13 mm in December. By 2005, these values had decreased, with January at 2.84 mm, September at 5.2 mm, and December at 0.12 mm. The downward trend persisted into 2020, with recharge starting at 2.61 mm in January, peaking at 4.52 mm in September, and reaching 0.12 mm in December. The study highlights the critical need to consider temporal variability and long-term trends in groundwater recharge for sustainable water management in the Dedaba watershed. The analysis of LULC changes shows a rapid urban expansion, reduction of forests and grasslands, and consequent threats to groundwater recharge. Mitigating these risks requires collaborative efforts, including promoting afforestation, water-conserving urban farming, sustainable agricultural practices, and artificial recharge techniques. Future research should incorporate climate change projections to enhance groundwater recharge predictions and improve water resource management strategies.
ESTIMATION OF RUNOFF AND SEDIMENT YIELD USING SWAT MODEL: THE CASE OF KATAR WATERSHED, RIFT VALLEYLAKE BASIN OF ETHIOPIA
(Hawassa University, 2019-07-27) Dulo Husen
Estimating of runoff and sediment yield at watershed level is important for better understanding of hydrologic processes and identifying appropriate measures to combat erosion. In this study, Soil and Water Assessment Tool (SWAT) was used to calibrate and validate a hydrologic component on Katar river discharges at Habura gauging station and predict the stream flow of Katar watershed. The objective of the study was estimating the runoff and sediment yield for the Katar watershed using SWAT model. Sensitivity analysis, model calibration and validation were also performed to assess the model performance. From the result of Global sensitivity analysis, twelve(12) highly sensitive parameters were identified, and coefficient of determination (R2 ), Nash-Sutcliffe (ENS) and percent bias (PBIAS) were used as objective function to evaluate model calibration and validation on the monthly basis, and it could simulate runoff to a good level of accuracy. The results obtained were satisfactory for the gauging station (R2 = 0.80, ENS = 0.6 and PBIAS=0) for calibration and (R2 = 0.6, ENS = 0.55 and PBIAS=1.2) validation period. The simulated runoff and sediment yield of Katar watershed was quantified and also the utmost erodible part of the watershed was identified and prioritized. Among all sub-watersheds, nine (9) sub watersheds were more vulnerable to soil loss and potentially prone to erosion risk, which was out of range of tolerable soil loss rate (18 tha-1 yr-1 ). Large area of watershed covered by Haplic Luvisols(high clay content) and agriculture is the dominant activities in area. The simulated mean of sediment yield and runoff loss from watershed for 26 years were 11 tha-1 yr-1 and 12.3 m 3 s -1 respectively. The result of the study could help stakeholders to plan and implement appropriate watershed management strategies based prioritizations of severity of erosion. In conclusion, the SWAT model could be effectively used to predict runoff and sediment yield and result of the study could help different stakeholders to plan and implement appropriate interventions strategies in the Katar watershed.
ESTIMATION OF SOIL EROSION AND SEDIMENT YIELD OF GEFFERSA RESERVOIR WATERSHED
(Hawassa University, 2021-07-10) KANAOL MERERA ABDISA
Sedimentation is an important parameter to measure the life of a reservoir. It depends on sediment yield and sediment yield depends on soil erosion. This study has been conducted in Geffersa reservoir watershed in the upper Awash River basin of Ethiopia. The objective of the study is to estimate potential soil erosion, sediment yield from the watershed and identify hotspot areas for proper planning using Arc GIS and RUSLE adopted to Ethiopian condition. The revised universal soil loss equation (RUSLE) was integrated with Geographic Information System (GIS) to model the spatial patterns in soil erosion in the watershed. All the parameters of the model (erosivity, erodibility, steepness, land use land cover, and supportive practice factors) were used in ArcGIS to create a soil erosion map. The annual soil loss of the watershed range from 0 to 728.48 ton/ha/year. The average annual soil loss value was found to be 23.6 ton/ha/year and has been classified into five erosion severities classes as very slight, slight, moderate, severe, and very severe to identify erosion hotspot area. Based on those results, 772.73 ha & 606.34 ha of the watershed were felt under severe and very severely vulnerable to soil erosion. The estimated Sediment yield delivered to the outlet was found ranges 0 to 78 ton/ha/year. The average annual sediment yield from the entire watershed is 5.3 ton/ha/year. If the same rate of sedimentation continues, the total storage volume will be filled up in 394 years. Out of the available reservoir sedimentation management strategies, watershed management is the best technique to minimize the sediment yield and its flow into the reservoir.
EVALUATING THE PERFORMANCE OF SWAT AND HEC- HMS MODELS ON RAINFALL-RUNOFF ESTIMATION AT THE GREAT AKAKI RIVERWATERSHED, ETHIOPIA.
(Hawassa Unversity, 2023-10-17) HENOK GEZAHEGN MULUNEH
The present study was conducted to examine the accuracy and applicability of the hydrological models Soil and Water Assessment Tool (SWAT) and Hydrologic Engineering Center (HEC)- Hydrologic Modeling System (HMS) to simulate stream flow. Models combined with the ArcGIS interface have been used for hydrological study in the Great Akaki River watershed. The critical focus of the stream flow analysis was to determine the efficiency of the models when the models were calibrated and optimized using observed flows in the simulation of stream flow. Daily weather gauge stations data were used as inputs for the models from the 1995-2017 periods. Other data inputs required to run the models included land use/land cover (LU/LC) classes resulting from Map agency and related offices, soil map and digital elevation model (DEM). For evaluating the model performance and calibration, daily stream discharge from the catchment outlet data was used. For the SWAT model calibration, ALPHA_BF (Base flow alpha factor (days), curve number (CN) and GW_DELAY(Groundwater delay(day)) are identified as the sensitive parameters. SCS.lag (MI), Curve number(AMC_II) and Initial abstraction (Ia) are the significant parameters identified for the HEC-HMS model calibration. The models were subsequently adjusted by auto calibration for 1995-2010 to minimize the variations in simulated and observed stream flow values at the catchment outlet (Aba Samuel). The hydrological models were validated for the 2011-2017 period by using the calibrated models. For evaluating the simulating daily stream flow during calibration and validation phases, performances of the models were conducted by using the Nash-Sutcliffe model efficiency (NSE) and coefficient of determination (R2). The SWAT model yielded high R2 and NSE values of 0.85 and 0.82 for daily stream flow comparisons for the catchment outlet at the calibration and validation time, suggesting that the SWAT model showed relatively good results compared to the HEC-HMS model. Also, under modified LU/LC and ungaugedstream flow conditions, the calibrated models can be later used to simulate stream flow for future predictions. Overall, the SWAT model seems to have done well in stream flow analysis for hydrological studies.
EVALUATING THE STABILITY OF EMBANKMENT DAM BY INTRODUCING CONCRETE FACE ROCK FILL DAM (CASE OF LEGEMERA DAM SOUTH WOLLO, ETHIOPIA)
(Hawassa University, 2018-03-13) JEMAL KASSIM MOHAMMED
Design of an embankment dam is a vital issue from the standpoint of safety, economy, controlling seepage, and speed of construction. Concrete face rock fill dams (CFRDs) virtually impervious, resistant to erosion and aging, workable, shorter period of construction and small volume of construction material. It is very well suited when there is no ample amount of impervious material within economically feasible hauling distance and severe weather conditions during construction. This study was aimed at addressing biggest challenge of hauling distance of the impervious clay core material by changing earth fill dam with central clay core to concrete face rock fill dam in case of Legemera micro earth dam, found in South Wollo. Universally recommended standard values beside to primary and secondary data essential for the work has been used. GeoStudio 2012 software was used for stability, seepage and deformation analysis of the dams. Consequently, by using this software good result of safety factor has been found for concrete face rock fill dam with regard to stability of the dam during maximum pool level, steady state condition and end of construction. During steady state condition, the upstream and downstream factor of safety computed for the dam is 1.954 and 1.544 respectively. Likewise, at the end construction, the upstream and downstream safety factor is 1.824 and 1.59 respectively. The seepage is also reduced to values of 2.42 x10-6 m 3 /sec by implementation of concrete face rock fill dam. Economic analysis result similarly shows that, concrete face rock fill dam is more economical by saving 22,441,387.75 ETB due to fast rate construction of the dam independent of weather condition. Generally, application of CFRD can fulfill the basic requirement and minimum factor of safety under all loading condition, and dam of such types is seepage free
EVALUATION OF IMPACT OF INTERNAL EROSION ON THE STABILITY OF DAM
(Hawassa Inversity, 2019-07-10) KETEMA TESFAYE ADUGNA
Embankment dams encounter several problems in terms of dam safety. One of those problems is called internal erosion. This phenomenon is brought by the movement of fine particles within the dam due to seepage forces. Internal erosion represents a complex combination of several mechanisms related to the failure or near failure of dams and levees. If the dam is not able to self heal, the eroded zones will increase which will eventually cause the dam to fail. Specially if the discontinuity such us concrete conduit is there in the dam embankment the probability of concentrated flow occurrence through the embankment body is increased. The dam selected for this study Arjo Dhidhessa rock fill dam is currently under construction by MoWIE, Due to regied structure embedded in the body of the embankment dam it is related to the problem of internal erosion within the core. The impact of this internal erosion is analyzed in this thesis with the use of Finite Element Method/Analysis (FEM/A). FEA models simulate the in-situ stresses in the dam and calculate the strength. It also enables the analysis of changing hydraulic conductivity and its effect on the overall effective strength due to changing pore pressure and seepage forces. The analysis using numerical methods was performed in the program PLAXIS2D and SEEP/W while limit equilibrium analysis was done in SLOPE/W. The calculation in PLAXIS2D was performed by using the Mohr-Coulomb constitutive model. The in-situ stresses are initially calculated using gravity loading since this is the preferred method on an uneven terrain instead of a K0-calculation. Then, through a set of phases in the program, zones where erosion is assumed to have occurred are changed. These zones have a higher permeability and will thus affect the pore pressures in the dam following Darcy’s law with permeability through a set medium. The increased permeability is set to follow an increased void ratio due to loss of fine material in the core. How this increase of void ratio affects the permeability is investigated through using Ren et al. (2016) proposed equation for calculating permeability with a set void ratio. Their equation, apart from the usually used Kozeny-Carman equation, considers both effective and ineffective void ratio where the ineffective void ratios refers to the volume of pores that is immobile when flow is considered. Conduits through embankment dams are prone to seepage and internal erosion around the surrounding soil. The increased flow in the eroded zones of the core did not seem to impact the strength of the dam in much regard. The phreatic surface and thus the pore pressure did not change enough to influence the overall effective strength of the dam. It raises the question if the stability of an earth-rock fill dam will be affected due to increased pore pressure at all due to its draining properties and if it would rather fail due to increased seepage forces. Throughout design and construction of rockfill dam as much as possible fixing the conduit out of the embankment dam is recommendable to avoid the probability of concentrated flow which potential cause internal piping