Browsing by Author "Yilma, Sileshi (PhD)"

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    Flood Analysis and Hydraulic Competence Assessment of Railway Drainage Structures a Case of Meganagna-Ayat AALRT Line
    (Addis Ababa University, 2019-07) Kajubi, Enock; Yilma, Sileshi (PhD)
    The flooding problem along the magenagna-Ayat Addis Ababa Light Rail Train line has persisted since its construction. Therefore, flood analysis has been carried out as well as the hydraulic competence of the existing drainage structures along the line. Flood frequency analysis was carried out using the Gumbel distribution after best fit tests and the design storms that correspond to the 25, 50, 100 and 200-year return periods were computed. Using a high resolution Digital Elevation Model (DEM) of 12.5m, a watershed delineation was carried out and the sub-basin parameters calculated accordingly. A total of seven (7) watersheds fourteen (14) sub watersheds were obtained in project area. The drainage basins ranged between 1.02sq. Km for catchment six to 10.27 sq. Km for catchment five. Using the watershed parameters obtained, time of concentration, lag time and the storage attenuation coefficients were computed for each sub-basin to be used in the hydraulic computations. A rainfall-runoff transformation was carried to assess the hydraulic competence for the existing drainage structures for 25-year and 50-year return period for all culverts and 50-year and 100-year return periods for the bridges. One culvert was found over designed, three under designed, two inaccessible on site and one competent as well as the bridge. Poor interoperability between the railroad grade and the highway grade at the level crossing was identified as one of the major causes for flooding at level crossings thus must be redesigned. Transverse aggregate drains as well as silt traps in the longitudinal railroad side drains have been proposed, jointing of the railroad drainage system with the road system, public sensitization towards municipal solid waste management and establishment of a railway drainage asset management policy have been recommended to mitigate the and improve the infrastructure drainage to be safe, sustainable and resilient.
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    Flood Analysis and Hydraulic Competence of Drainage Structures along Addis Ababa Light Rail Transit (A Case Study on Mesualekia-Nefas Silk2 Stretch)
    (Addis Ababa University, 2019-07) Kiwanuka, Moses; Yilma, Sileshi (PhD)
    This study is on flood analysis and hydraulic competence of the existing drainage structures on some roads of Addis Ababa City after an integration of Roads and Addis Ababa Light Rail Transit Drainage Systems. Flooding in any circumstance causes major stresses on the economic, social and environmental regimes of the affected area. Addis Ababa has a pronounced rainfall peak which is common during the summer time months of July to August. Flooding is a major problem for Addis Ababa city roads. Therefore, this study was carried out in-order to assess the flood analysis and hydraulic competence of drainage structures along Mesualekia - Nefas silk2 stretch for the different rainfall intensities corresponding to different return periods. The existing side drains and cross drainage structure located within the study area were inspected and assessed in order to ascertain different aspects relating to their performance. The hydrological analysis was assessed in-order to determine the different watershed contributing to the flows. This was done by use of Geo spatial tools like ARCGIS, ARCSWAT, and HEC GEOHMS. The flows were obtained by using the rational method for areas less than 0.5km2 and HEC HMS for areas greater than 0.5km2. Rational method was used to obtain different design flows for watershed 1 to 4, including flow from the carriageway runoff and the following were obtained as 2.66, 0.08, 3.23, 6.20, 2.12, 0.08, 0.08, 0.08 m3/s corresponding to a 10-year ARI. HEC HMS was used to obtain a design flow of 29.52 m3/s corresponding to watershed 5. A design flow of 46.3m3/s was used for assessing the cross culvert which was the summation of all the flows from side drains and carriageway runoff. The hydraulic analysis was carried out using HY-8 for the culvert and Hydraulic tool box for existing side drains. From field survey and measurements, it was discovered that the existing side drain of 900mm circular pipe couldn’t convey a flow of 6.2 m3/s. The existing box culvert comprised of one barrel of 4000mm x 3300mm, which was sufficient in conveying the designed flow because its headwater elevation was lower than that of the roadway elevation. However, observations like presence of accumulated silt, debris within most side drains and an undersized side drain reduced its hydraulic competence. Based on the findings, recommendations were made including regular desilting, screens at inlets of side drains and at some distance before the entrance of the culverts, periodic maintenance of the existing drainage structures and widening of the pipe conduits for the side drains to about 1200mm for easy maintenance. There is also need to monitor performance of existing side drains and culverts, that, basing on field data assessed, might have resulted in under- or overdesigns. Regarding the on-track drainage network, this research provided for aggregate drains of size 400mm and the existing longitudinal drains were efficient to convey the flow.
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    Hydraulic Performance of River Crossing Structure Against Sediment Aggradation and Trash Accumulation (The Case of Arsema Bridge from German round about to Lebu Mebrate Haile)
    (Addis Ababa University, 2019-05) Wendmagegnehu, Taye; Yilma, Sileshi (PhD)
    Arsema Bridge was constructed on Jemo River, Jemo River is tributary of Little Akaki River which is the sub catchment of Akaki basin and study area has an inclusive area of 73.3 Square Kilometer, Arsema Bridge built as River crossing structure to connect the ring road from German round about to Lebu Mebrate Haile under Addis Ababa city Administration. It had been built just before 15 years ago and currently the Bridge faces a serious problem of sediment aggradations both at the left and right side of the abutment, and trash accumulation on the entrance of the Bridge. Due to this; overtopping of the Bridge and flooding of residential house were observed frequently. HEC- HMS modeling was used to determine peak discharge of 2, 5, 10, 25, 50 and 100 year return periods and then 67, 97.6, 117, 136.1, 154.5 and 174.9 m /sec results were found, subsequence HECRAS Modeling was used to determine water surface profile level, performance of existing bridge when difference return period peak discharge pass through it and sediment transport at the bridge location based on the current river sectional profiles however the bridge accommodate only up to 10 year return flow. Then Q50 was used to analysis before and after bridge constructed, then after the water surface elevation is 2214.36m and 2214.57m amsl respectively however existing bridge high chord is 2213.684m amsl i.e. 0.676m and 0.886m difference before and after the bridge constructed, this show us the existing bridge is submerged as per previously estimated flood. After detail analysis of the result and discussion I recommend the appropriate remedial measure for sedimentation aggradations and trash accumulation as well as flooding of the residential house, HECRAS was used with combination of HEC- RAS Mapping: to redesign of Bridge with Trash Trap Screen and Flood Hazard map, the result show us road surface finishing level has to increase at least 0.766 m high at the bridge and refill both left and right side of the bridge up to 150 m length as per road profile; to accommodate the redesigned bridge high chord of 2214.45m amsl, residential house upstream to bridge has to consider flood protection or guide wall up to 2.0m high depending on their OGL against the flood hazard map, relocate of all utilities line which are pass through bridge and period clearing of trash trap screen is highly recommended as main mitigation measure.
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    Investigating the Effect of Scouring on Hydraulic Performance Of River Cross Drainage Structure: (Case study of Ginchi Awash Bridge)
    (Addis Ababa University, 2018-06) Hawi, Adicho; Yilma, Sileshi (PhD)
    Bridges that cross the rivers and valleys are vital components of the road network that contributes significantly to the national development and public daily life. Any damage or collapse of bridge can risk the lives of road users as well as create serious influence to the entire country. Ginchi Awash Bridge is found at 79.5km west of Addis Ababa Ethiopia, which is bounded in geographical coordinate of 38°35'E to 38°7'E longitude and 9°8'N to 9°1'N latitude. Ginchi Awash Bridge is the one which this thesis work is conducted on, in case the bridge is in risk due to the scour at the abutment and the river bed of the bridge structure. Hence this research provides the investigation of the effect of scouring on the hydraulic performance of the Bridge from the hydrological and hydraulic analysis. From the hydrological aspect the design flood at different return periods is estimated by Flood Frequency Analysis method and the peak discharge for 100 year return period is 291.04m v 3 /s. Based on the result from HEC-RAS model the significant bed scour was modeled and abutment local scour depth for the estimated design flood of 100 year return period is 4.12m at left abutment and 4.84m at right abutment and the contraction scour or bed scour is 1.66m. The total scour depth is the sum of local abutment scour depth and contraction scour depth which is 5.78m at left abutment and 6.50m at right abutment. This is mainly caused by constriction of the main channel and about 70.5 percent of river bed and bank material is sand and gravel which are rapidly eroded under water action. In addition to this the steady flow water surface profile is checked and the result shows that the water surface elevation of Ginchi Awash River at the Bridge is lower than the low chord elevation of the Bridge, so the flood could not over top even for design flood of 100 year return period. Finally the Scour mitigation methods are studied in HEC-RAS software. Taking into account the possible geotechnical failure repair and construction costs, constructing riprap at the bridge abutment is recommended and its detail design calculation is discussed.
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    Investigation on the causes of failure of Tana Beles Weir
    (Addis Ababa University, 2019-07) Seife, Tadesse; Yilma, Sileshi (PhD)
    An investigation on Tana Beles diversion head-work is carried out for this research work and attempt is made to understand the failure problems encountered. This serve for different purposes such as for construction of new diversion structure (either as replacement for existing structures, or as an entirely new structure), rehabilitation of existing structures (from minor repairs to complete re-engineering either to maintain existing function, or to meet new requirements). Hydrology and hydraulic analysis, structural analysis and evaluation of the foundation condition of the weir has been made to investigate the causes for the failure of the weir. The hydrology analyses is made by Hec-hms and the result shows that the peak flood used for the design of the weir is smaller than the result obtained in this investigation, but the water way of the weir built can accommodate the estimated peak flood. Models used for the hydrology analysis include Global Mapper, Arc-GIS, Hec-Geohms and Hec-hms. On the other side, hydraulic design is evaluated and the evaluation exposes the absence of concrete floor, absence of energy dissipater and protection works on the hydraulic design of the weir and clogging of under sluice as a cause for the failure of the weir. The structural analysis involves checking stability analysis of the weir against overturning, sliding and shear. Surveying data upstream and downstream of the weir is collected and foundation condition were investigated by comparing the collected surveying data with the design bed level.in addition to this the applied foundation treatment is evaluated by comparing it with standards and found that the main structure was built on weak foundation which activates the weir failure. This investigation shows that the main cause of Tana Beles weir failure is due to the weak foundation condition of the weir and absence of the necessary concrete floor and energy dissipater on its design. Finally, the remedial design for the weir is recommended.
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    Small Scale Irrigation Project Structural Failures and Their Management Problems Related to Water Loss in Ethiopia
    (Addis Ababa University, 2017-06) Tirfessa, Tolera; Yilma, Sileshi (PhD)
    The challenges in reducing water losses of the conveyance structures such as earthen and lined canals were assessed considering the influence of participating parties (farmers, contractor and funding agencies) and contributing factors (hydrologic and hydraulic factors). The effect of conveyance efficiency, structural failure/performance, irrigation system management problems and water losses in Jemjem SSIP (which was constructed in 2004- 2006E.C), were the major problems for this study. The main objective of this study is to assess the reason why more than 50% of the water diverted is lost and to discover the solutions that can minimize the problems. To solve or estimate existing problems scientifically, the method proceeded are ‘measuring flow volume of the main canal at different points using v-notch weir to estimate loss in each interval and their conveyance efficiency, calculating Manning’s roughness of the main canal to compare with designed value of ‘n’, Estimating evaporation loss using energy balance method and estimating seepage loss of the main canal using Kostiakov A.N empirical formula and using some questionery results to estimate the influence of water management problems. The designed value was taken from the design report and the existing conveyance efficiency is obtained by measuring discharge and depth of flow of the canal at different locations. The conveyance efficiency given at the design stage was 90% for the scheme which was directly taken from FAO recommendations. However, the values obtained during the study were 34.9%. But, the conveyance efficiency selected should be based on construction materials, methods and qualities, managerial activities, technological and other factors affecting conveyance efficiency. The evaporation and seepage losses together constitute 3.63lit/sec while leakage and other operational losses account 23.07lit/sec/ (which is 56.27%) of the total main canal inflow. Such a decrease in conveyance efficiency or increase in water loss is found to be due to inefficiency of management issue and due to inefficiency of contributing factors such as construction material and (hydrologic and hydraulic factors). Thus, management problems, structural under performance and a designed conveyance efficiency taken directly from standard books had effect on the overall performance of Jemjem irrigation project. Irrigation structures should be maintained from time to time and Gates should be operated by trained operator to create equity of water distribution and also to make water management easy. Water shed management should be well implemented in the area so that quantity of silt deposited will be reduced. Water committee should be changed to WUA and sufficient follow up should be made to overcome water management problems existing in the project and similar projects in Ethiopia.
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    Water Delivery Performance Evaluation of Koga Irrigation Scheme
    (Addis Ababa University, 2018-11) Yeshi, Andualem; Yilma, Sileshi (PhD)
    Water delivery performance is the degree to which whether the system delivers water at the required rate at the right place and time so, water delivery performance evaluation of irrigation system plays a dynamic role in identifying the gaps and applying mitigation measures. Koga irrigation scheme was designed to irrigate around 7000ha but, it does not meet the proposed objective, hence the objective of the study was evaluating the water delivery performance of Koga irrigation scheme. This study was carried out for three month on a single irrigation season from February to April; 2018. Water delivery performance of the scheme was evaluated by comparing irrigation water demand with actual irrigation water supply by using 7 indicators classified under water delivery and water use or water supply indicators. As per the result, deficit of supply has been progressive in March and April; because actual irrigation water supply in March and April was less than the crop demand. However, supply was exceeding the demand during February. The average, irrigation water supply was observed in shortage of the requirement at the head middle and tail reach of the main canal. The overall relative water supply, relative irrigation supply and deficiency of the scheme were 0.65, 0.75 and 0.28 respectively. As shown from the result water delivery performance indicators; adequacy, dependability, equity and efficiency were found to be 0.68, 0.29, 0.08 and 0.95 respectively. According to these values, performance of water delivery of the main canal was rated as poor in terms of adequacy and dependability; however it was good in relation to efficiency and equity. These values indicate that, actual water level as well as command area was reduced from initially designed. The results from this study are very clear to that of, non-proportionality of irrigation water demand with supply, poor temporal water delivery system, absence of frequent management aspect have been strongly affected the water delivery performance of the schemes. Therefore, it is suggested that, effective water management, supplying irrigation water based on crop demand , optimal crop plan, and apply a strong water delivery plan are required to mitigate these problems.