57 FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF WATER RESOURCE fe \sibii in sum and detail design of R\I FGVDl I \ IRRIGATION PROJEC T V OK ME 4 ANNEX 3 HYDRO-GEOLOGICAL STUDY FEASIBILITY FINAL REPORT MAY 2010 in association with WjUU Woilt 41 Ml Supci v UKMJ i iMcipiIm: IWWIM) I ntcfcont menial Consultants and Technocrats PVT LTD (ICT)Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Studer FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF WATER RESOURCES FEASIBILITY STUDY AND DETAIL DESIGN OF BALE GADULA IRRIGATION PROJECT FINAL FEASIBILTY REPORT VOLUME 4-ANNEX 3 HYDRO-GEOLOGICAL STUDY MAY 2010 WWDSE in Association with ICT Final Feasibility Study ReportFederal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study List of Volumes in the Final Feasibility Report VOL 1 Executive Summary VOL 2 Main Report VOL 3 Annex 1 Meteorological and Hydrological Study VOL 4 Annex 2 Geological and Geotechnical Investigations VOL 4 Annex 3 Hydro-geological Study VOL 5 Annex 4 Soil Survey VOL 5 Annex 5 Land Evaluation VOL 6 Annex 6 Socio Economic Study VOL 6 Annex 7 Settlement Study VOL 7 Annex 8 Irrigation Agronomy VOL 7 Annex 9 Farm Mechanisation VOL 7 Annex 10 Agricultural Marketing VOL 8 Annex 11 Livestock Study VOL 9 Annex 12 Environmental Impact Assessment VOL 9 Annex 13 Watershed Management — VOL 10 Annex 14 Institutional Dev. VOL 11 Annex 15 Financial & Economic Analysis WWDSr in Association with ICT Final Feasibility Study ReportFodoral Democratic Republic of Ethlopla-Mlniatry of Water Reaourcen Feasibility Study and Detail Design of Balo Gadula Irrigation Project VOL 4- Annex 3 Hydro geological Study TABLE OF CONTENTS 11ST Or TABUS.................................................................................................................................................................... H LIST or FIGURfS................................................................................................................................................................... II 1. INTRODUCTION.............................................................................................................................................................. 1 1 1 Ba< "GROUND TO THE PROJECT.................................................................................................................................................................................... 1 1 2ORJECTIVESOF THE HYDROGEOLOGICAL INVESTIGATION....................................................................................................................................... 1 2. METHODOLOGY.............................................................................................................................................................3 2 1 FlElD SURVEY, APPROACH AND DATA ACQUISITIONS............................................................................................................................................... 3 2 2 Previous works........................................................................................................................................................ 5 2.2.1. Summary from Scientific Literature................................................................................................................ 5 2.2.2. Summary from previous projects.................................................................................................................... 6 2.3 Hydrogeological data.............................................................................................................................................7 3. GEOLOGY, CLIMATE, HYDROGRAPHY AND GEOMORPHOLOGY........................................................................10 3.1 Regional Geology................................................................................................................................................ 10 3 2 Local Geology....................................................................................................................................................... 11 3.3 Topography and geomorphology......................................................................................................................... 13 3.4 Climate................................................................................................................................................................. 16 3.5 Hydrography........................................................................................................................................................18 4. HYDROGEOLOGY...................................................................................................................................................... 19 4.1 Regional Hydrogeology...................................................................................................................................... 19 4.2 Groundwater Potential of the Bale Gadula Plain and Surrounding Plateau..................................................... 22 4.3. Water supply for settlement areas once irrigation starts.................................................................................29 4.4 Predicting the groundwater level rise under irrigation.................................................................................... 31 4.5 Water logging and water level rise management option.................................................................................... 34 5. CONCLUSIONS AND RECOMMENDATIONS...........................................................................................................36 5.1 Specific Hydrogeological Recommendations......................................................................................................36 5.2 Recommendation on the way foreword.............................................................................................................. 36 REFERENCES................................................................................................................................................................ 37 WWDSE in Association with ICT Final Feasibility Study ReportFederal Democratic Republic of Ethiopia-Ministry of Water Resources VOL 4- Annex 3 Feasibility Study and Detail Design of Bale Gadula Irrigation ProjectHydro-geological Study LIST OF TABLES Table 2.1 Water points within the Bale Gadula Project............................................................................. ........ 7 Table 2. 2 Physico chemical characteristics of the Weyib River water.................................................. ........ 9 Table 3. 1a Mean monthly rainfall data............................................................................................................17 Table 3.1 b Mean monthly, mean minimum and mean maximum temperatures at the Goro Metrological Station..........................................................................................................................27 Table 4.1: Basis for classification of aquifers in the Genale Dawa basin................................................... 20 Table 4. 2: Estimation of potential evapo-transpiration, actual evapo-transpiration and MONTHLY AND ANNUAL GROUNDWATER RECHARGE RATES IN THE BALE GADULA IRRIGATION COMMAND AREA.................................................................................................................................... 27 Table 5.1 Geographic coordinates of proposed pieozmeters.......................................................................... 36 LIST OF FIGURES Figure 2.1 Location map of the Bale Gaduux Irrigation Project.......................................................................4 Figure 2.2 Water points in the project area........................................................................................................... 8 Figure 3.1 Geological map of the Bale Gardula project site and surrounding area................................ 12 Figure 3.2 Topographic features of the Bale Gadula Command area and its bounds............................... 1 5 Figure 3.3 Typical North South profile’ of across the Bale Gadula plain.................................................. 16 Figure 3.4 Mean monthly rainfall distribution of the Goro Station............................................................. 17 Figure 4.1 Groundwater potential zonation of the Genale Dawa Basin.......................................................21 Figure 4.2 Hydrogeological map of the Bale Gadula Plain and surrounding areas................................. 23 Figure 4.3 Conceptual groundwater flow and recharge model for the Bale Gadula Plain.................. 25 Figure 4.4 Assumed sites of discharge for ground waters of the Bale Gadula Irrigation Command Area..................................................................................................................................... 33 Figure 4. 5 Potential site of water logging..........................................................................................................35 VWVDSE in Association with ICT Final Feasibility Study ReportFederal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study 1. INTRODUCTION 1.1 BACKGROUND TO THE PROJECT The WWDSE has entered to an agreement with MoWR to render consultant services for Yadot Feasibility Irrigation Project. One of among the study component is the investigation of hydrogeology, which assist the design of Irrigated agricultural development of the area. The feasibility irrigation project is planned to be conducted in Mena-Angetu woreda, Bale zone, Oromia Regional State following the course of Yadot River, which stretches from west to east in the zone. The Bale Gadula irrigation project covers and areas of 12,000 ha.. 1.2 OBJECTIVES OF THE HYDROGEOLOGICAL INVESTIGATION The purpose of this investigation principally it to address the objectives set in the TOR of the Bale Gadula Irrigation Project as regard to hydrogeology. The principal objectives of the hydrogeological study are • To assess the groundwater potential and its quality for the water supply system of the irrigation project o Assess the extent of the available groundwater resources with possible average water table yield and quality, o Prepare hydro geological map to the scale of 1:250,000 o Select appropriate location of well fields and estimate its yield to be used for water supply to settlement areas o Design borehole to be drilled for water supply to the settlement areas • To predict the groundwater level rise and change of water quality on the irrigable land under the impact of irrigation, and o Work out long term forecast of probable changes in quality and quantity o Assess the effect of abstraction on other elements of the hydrologic cycles and adjacent aquifers o Evaluate time of impact of irrigation on the groundwater level and quality WWDSE in Association with ICT Final Feasibility Study Report 1Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study • To determine the requirement of subsurface drainage and to predict time for the construction of the subsurface drainage facilities to avoid water logging during irrigation development. o Recommend shallow piezometers as required for future water level and water quality monitoring purposes o Analyze the possibility of advanced groundwater management schemes such as conjunctive use with surface water as a supplementary use in irrigation o Predict groundwater level rise and the time of construction of drainage systems In order to address these objectives a number of activities have been carried out. The activities include collection, review and analysis of available reports, data and maps. Collection of relevant information related to borehole, hand dug well, springs yield, location, and geology. Field survey has also been carried out for the purpose of sampling water points, mapping the terrain and landscape. Physico chemical analyses has been conducted on selected water points for the purpose of determining water quality, water quality impacts, and water quality determination for different water uses. WWDSE in Association with ICT Final Feasibility Study Report 2Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study 2. METHODOLOGY 2.1 FIELD SURVEY, APPROACH AND DATA ACQUISITIONS Accessibility The Bale Gudula Irrigation project (Figure 2.1) is bounded by of 7°00’ and 7°12’ North and 40°18 and 40°38 E coordinates. The project site can be accessed by driving through Robe- Ginnir road and turning left after reaching Gorro town on the way to Alem Kerem town. The project site is at a distance of about 20km from the main road. The main project site (irrigation command area) is situated on the left bank of the Weyib River, bordered by the river from the southwest and, Kubsah village from the west and the Asendabo dry stream from North and north east. The altitude of the proposed irrigation area is between 1800m and 1900m. Goro town is located at elevation of 2000 masl. WWDSE in Association with ICT Final Feasibility Study Report 3I I I I I I Federal Democratic Republic of Ethiopia-Ministry of Water Resources VOL 4- Annex 3 Feasibility Study and Detail Design^f Bale^GadulaJrrigation ProjectHydro-geological Study Field Work, Data Acquisition and Methodology A one week field survey has been conducted during the month of July 2009 to survey the geology, groundwater occurrence and map field characteristics of the lithologies underlying the Bale Gadula Plain Existing data has been acquired from the previous works and from the current study. The fact that the groundwater table is deep in the area did not allow extensive drilling Only limited data exist on well and spring discharge data, well logs or geophysical surveys. The most important data source is the water resources inventory report of the Genale Dawa master clan study. Hydrometrological data from the nearest station (Goro) has been acquired from the Ethiopian Metrological Agency. Geologic mapping and hydrogeologic maps has been produced based on the field acquired observations, oreviously acquired regional data and from combination on indirect evidences such as topographic and hydroggrphic informations. Digital elevation model of I 'esolution 30 m by 30 meter has been used to help in the mapping process. The DEM has been used in mapping tectonic features and regional lineaments. Water I samples have been collected in polyethelyn plastic bottles from existing water sources and have been analyzed for their physic-chemical characteristics in the I laboratory of Water Works Design and Supervision Enterprise. Prior to going to the field the consultant has revisited existing information from the prevous works. The most prominent previous work used in this study is the Genale Dawa Master Plan Study. Published works on the Sof Omr Cave and the aquifer properties of basalts of Ethiopian highlands has also been used for the purpose of attaining the objectives of this work. 2.2 PREVIOUS WORKS 2.2.1. Summary from Scientific Literature The Bale Gaduai Irrigation Project Site makes a small part of the Bale Highlands. The Bale Highlands are known for their massive volcanic chains, intervening plateau and depressions. The basaltic highlands of southern Eastern Ethiopia are scarcely investigated compared to other volcanic terrains of Ethiopia. However a few geological studies exist on the Mesozoic sediments bordering the bale highlands from East and South. The volcanic products are mostly basaltic VWVDSE in Association with ICT Final Feasibility Study Report' 5Federal Democratic Republic of Ethiopia-Ministry of Water Resources VOL 4- Annex 3 Feasibility Study a nd Detail Design of Bale Gadula Irrigation Project Hydro-geolo gical Study ryholitic and trachyte. The volcanic missives are do minated by tracytic with typical characteristics dominant large phenocrysts of feldspar s. Some intervening depression and intermountain valleys are filled sites of alluvio lacustrine sediment accumulation. Glaciations of the Bale Mountains during the last glacial maximum have also led to formation of glacial deposits, moraines and has left glacial landscape in the highest places of the Bale Mountains. The details of the finding from previeous studies has been presented in sections 3 and 4. 2.2.2. Summary from previous projects Existing hydrological, hydrgeological, climatological, geological data has been gathered. These include: • BALE GUDULA IRRIGATION PROJECT- PHASE I- MAIN REPORT, By Water Resource Development authority, Addis Ababa, 1992 • GENALE -DAWA RIVER BASIN Integrated resources development master plan study report, Ministry of Water Resources, 2007 • EGS, 1996. Geological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia • EGS, 1993. Hydrogeological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia • EGS, 1972. Geological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia In these reports the geological and hydrogeological results were depicted at the scale of 1:2000000. There reports provide general information as to the lithologies, age relationships, and groundwater occurrences. However there are not at the scale required in this particular study. Furthermore this is a significant mismatch between the maps geological or hydrogeological maps produced by these investigations. This necessitates a detailed field investigation in the project site to as to attain the objectives. It should be noted that the Ganale Dawa River Basin integrated master plan study report provide the most accurate geological map of the region. WWDSE in Association with ICT Final Feasibility Study Report 6Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula irrigation Project VOL 4- Annex 3 Hydro-geological Study 2.3 HYDROGEOLOGICAL DATA Previous hydrogeological investigation in the Bale Gadula area and its surroundings is scarse It cannot be denied that this has affected the detailed hydrogeological knowledge of the Bale Gadula plain. Hydrogeoloical data (well logs, spring yields, well depth, yield, SWL, etc) are scarce. For a site of thousands of sq km the number of identified water points does not exceed 10 Table 1 shows the water points and associated hydrogeological data. The data has been compiled from previous works and current field investigation. Figure 2.2 show locations of water points on which hydrogeological data has been retrieved. It is noted that out of the drilled wells all turn out to be not functional. This probably is related to deeper water table than the drilled depth. Table 2 shows the physic chemical characteristics of the Weyib River analyzed under this particular field investigation and project. The typical characteristics of the Weyib River water is its very low total dissolved solids (less than 100 mg/L) and low salt content in general. This is make the river water the most favorable for irrigation practices and reduce significantly the salinity risk. Table 2. 1 Water points within the Bale Gadula Project UTM N UTM E Water Point Name Depth Jm) SWL Yield (US) Number of Users Remark 667575 784572 Bale Dureni Spring 2.5 Population of 1 Kebele 664178 780630 Bale Alem Kerem No 1 93 NF 663729 780088 Bale Alem Kerem No 2 NF 658012 782490 Waltai Negaya 92 No 3 NF 657319 782613 Waltai Negaya No 2 657086 782985 Waltai Negaya No 1 650924 787142 Sinana Dinsho 650580 787352 Sinana Dinsho 89 NF 89 93 85 NF NF NF WWDSE in Association with ICT Final Feasibility Study ReportFederal Democratic Republic of Ethlopla-Mlnhitry of Water Resources Feasibility Study and Dotall Doslgn of Balo Gadulajrrlgatlon Project VOL 4- Annex 3 Hydro-geological Stud^ It should be noted that all 7 drilled wells in the Bale Gadula command area are non functional because they turn out to be dry during drilling. Table 2. 2 Weyib at Gadula chemical characteris ics of the ib River water pH EC TDS Na Mg Ca K HCO3 Cl SO4 NO3 P04 6.92 97 66 7.9 3.6 8.4 2.5 61.49 3.09 0.38 0 05 (Samples collected at Weyib bridge on July 2009) WWDSE in Association with ICT Final Feasibility Study Repo~ 9Federal Democratic Republic of Ethiopia-Ministry of Water Resources VOL 4- Annex 3 Feasibility Study and Detail Design of Bale Gadula Irrigation ProjectHydro-geological Study- 3. GEOLOGY, CLIMATE, HYDROGRAPHY AND GEOMORPHOLOGY 3.1 REGIONAL GEOLOGY The Genale Dawa basin in which the Gadula irrigation areas belongs is underlain by Proterozoic crystalline basement complexes, covered by marine Jurassic to early fluviatile Caenozoic and successions and Tertiary volcanic rocks. The basement complexes (gneissic terrane and narrow low grade belts) are designated as parts of the Mozambique belt and the Arabian-Nubian Shield, respectively. These rocks are then intruded by syn-and post-tectonic organic Dlutons. The crystalline basement rocks are suggested to be affected by the late D roterozoic deformation, metamorphism, and magmatism and are contemporaneously intruded by syn and post tectonic basic to acid intrusive rocks. The late Triassic is a time of regional subsidence during which rifting begins. During this period the progression in the Karroo rifting allowed the deposition of a thick clastic rocks of continental origin thicker and ticker toward the central part of the rift. After a long period of subsidence, in the Callovian early Oxfordian the sea floor spreading began (separation of east Gondwana from west Gondwana). The floor spreading ended in the early Hauterivian (121-120Ma). According to Dainelli, the Jurassic transgression came from the southeast, reaching its maximum limit in western Ethiopia and Eritrea during the Kimmeridgian. This transgression deposited a sandy formation (Adigrat Sandstone), followed by neritic sediments composed mainly by thick limestones. In cenozoic crustal uplifting lead to consecutive formation of the Upper Sandstone due to a forced regression of the sea. Tertiary uplift of the Arabian-Ethiopian swell was accompanied by laterization processes and followed by eruption of trap volcanics In some localities quaternary volcanics has continued to erupt. In literature, the Mesozoic sedimentary rocks have been divided into two different successions on the base of their presumable age and limits. The lower carbonate succession is referred to be Jurassic (Kimmeridgian) while the upper carbonate WWDSE in Association with ICI Final Feasibility Study Report 10Federal Democratic Republic of Ethiopia-Ministry of Water Resources VOL 4- Annex 3 Feasibility Study and Detail Design of Bale Gadula Irrigation Project Hydro-geological_SLjdy__ succession is considered to be of Jurassic to Cretaceous age (Kimmeridgian to Cenomanian). Between the two, a very slight unconformity v/as referred to exist by Kazmin. Although this, no field evidences were noted and being the limit iota y transitional and arbitrary, the two successions will be treated as a sing e geological unit of Mesozoic age in general. The Tertiary basaltic flows of Oligocene to Miocene age outcrop at the Northern part of the Genale Dawa basin including the areas surrounding the Bale Gadula plain and upper Weyb river basin. The basaltic flow are mainly constituted by olivine basalt very dark grey in colour and reddish brown when weathered. The volcanic rocks belonging to the trap basalts are known for rarity of paleosol layers as the whole succession is emplaced in very short (1 million year) span of time (Kiffer et al.. 2004). 3.2 LOCAL GEOLOGY Three principal lithologies underlie the Gadula plain and its sholders (figure 3.1). These are Mesozoic sediments at the outlet of the Wayibe River from the Gaduala plain, basalts belonging to the trap series, and younger basalts belonging to the quaternary. Aerially less extensive alluvio - colluvial materials mantling the topography particularly along the foot hills of the escarpments bounding the Gadula plain are also notable. VWVDSE in Association with ICT Final Feasibility Study Reoc.t 11Federal Democratic Republic of Ethiopia-Ministry of Water Resources VOL 4- Annex 3 Feasibility Study and Detail Design of Bale Gadula Irrigation ProjectHydro-geological Study succession is considered to be of Jurassic to Cretaceous age (Kimmeridgian to Cenomanian). Between the two, a very slight unconformity was referred to exist by Kazmin. Although this, no field evidences were noted and being the limit totally’ transitional and arbitrary, the two successions will be treated as a single geological unit of Mesozoic age in general. The Tertiary basaltic flows of Oligocene to Miocene age outcrop at the Northern part of the Genale Dawa basin including the areas surrounding the Bale Gadula plain and upper Weyb river basin. The basaltic flow are mainly constituted by olivine basalt very dark grey in colour and reddish brown when weathered. The volcanic rocks belonging to the trap basalts are known for rarity of paleosol layers as the whole succession is emplaced in very short (1 million year) span of time (Kiffer et al., 2004). 3.2 LOCAL GEOLOGY Three principal lithologies underlie the Gadula plain and its sholders (figure 3.1). These are Mesozoic sediments at the outlet of the Wayibe River from the Gaduala plain, basalts belonging to the trap series, and younger basalts belonging to the quaternary. Aerially less extensive alluvio - colluvial materials mantling the topography particularly along the foot hills of the escarpments bounding the Gadula plain are also notable. WWDSE in Association with ICT Final Feasibility Study ReportFigure 3.1 Geological Map of Bale Gadula Project Site and Its Surroundings —i— 672000 Quaternary Volcanics Geologic StructuresFederal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study' The Gadula plain and the irrigation command area is entirely underlain by Quaternary basalt lithology. Overlaying the quaternary basalts rests layers of colluvial deposits, black cotton soils. The Weyib River flows along a fault line, which has a prevalent direction N100o W or 80°E. At the field outcrop both basalt units are s dense black with vesicular textures. In general, Weyib River flows along a fault line, which has a prevalent direction N100°W or 80°E. Most of the catchment area is covered with dense forest, which account for the low sediments in the river. The project area is surrounded by hard formations of the Trap series of the basaltic rock, outcropping on both sides of the valley and the river stream. In the valley there is thick layer of colluvial deposits of black cotton soil and slopes gently decrease toward the river stream. The colluvial material thickness is low and therefore is not mappable at the scale in the geologic map. The basaltic rock which is dense black with vesicular texture is outcropping on both sides of the valley. This basalt was formed in tertiary period. The Gadula plain itself is underlain by the quaternary basalts. Regardless of the age difference between the Quaternary and the Cenozoic trap series basalts, the appearance in the field (degree of fracturing, flow bed thickness of joint settings) of the two lithologies is similar. South wards the Weyib River leaves the Bale gadula plain and enters into the Mesozoic sediments which underlie the basalt formation (see geologic map). The Mesozoic sediment particulary the Antalo formation at the outlet of the Weyib River from the Baslatic highlands is known for extensive karst geomorphic development. The Sof Omar Cave maze forms part of this karst. At 15.1 kilometres long, Sof Omar Cave is the longest cave in Ethiopia and ranks as the 2nd longest in the World. The Weyib River flows through the cave system. It sinks at the Ayiew Maco entrance and reappears at the Holuca resurgence 1 kilo meter away. 3.3 TOPOGRAPHY AND GEOMORPHOLOGY Topography, climate, hydrogaphy, and land use are important elements that determine the groundwater availability, groundwater circulation, recharge and discharge processes. These factors obviously are determinants of impacts of WWDSE in Association wiih ICT Final Feasibility Study Report 13’ ■*' I\mcc» KcccbUv cl 222 lL-21^Iv<'\ 1 ,x>‘* '1 Mlci*h\ cl \VM»m tttmmuw* n*1’ »' I't'-j. ' < \OI l Acc»«% ' vylc '\bUhMI Oil \|IO|||ld\\alOI \\aloi lonUlllU » OlldlllOllh mI|II||\ blllldllp Ulld OV»‘l.ll| \\,Ur ivmouu o*» a\ ailabilll\ Uh' Halo Gadiila pio|ml nilo in a llal banallh plain noiludad bolwoon llal loppod platoaun liom iumIIi and noiilli (lluum I r) Ilin nonHimi boundaiy n! Iln» pio|m I aira m boundchi by Iho Waibo Uivoi whli'h i ill an nun li an lull innli’i bolow Him Radula Plain A olowni look al Ilia plain llhull nlinwh llial Ilin plain I* Imnind by combination ol nionional pioiunnun aldnil by Hiiillinu and lui'liiniih Ilin Walin* l\iv('i follow* a mpinnal laiill llial 11 in NW Hl //// • z •r* fFederal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study Note : That the dissected gored running in the middle of the figure is the Waibe River and the command area is located the plain bound in the central depression. VVWDSE in Association with ICT Final Feasibility Study ReportII Uli 44 • I Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study Regardless of the fact that the Gadula plain is bounded by highlands from north and south, the plain itself is an isolated plateau secluded between the highlands (figure 3.3). This should make the plain site of recharge rather than site for groundwater discharge. Any regional groundwater flow emerging from the north should discharge at the foot of highland (eg. Bale Dureni spring is manifestation of such a process), while in the southern part of the plain any groundwater emerging from the south should discharge to the Waibe River before reaching the Gadula plain. The fact that the northern and eastern boundary of the Gadula plain is bounded by the Asendabo dry stream is an indication of minimal groundwater inflow to the Gadula plain from the-North. ‘rom Pos. 668272.399. 77B077.163 To Pos: 666137.496. 789414.234 2.5 km 5.0 km 7.5 km 10.0 km 11.61 km Figure 3. 3 ‘Typical North South profile’ of across the Bale Gadula plain. The Gadula plain is the flat laying area in the middle of the profile. 3.4 CLIMATE Table 3.1a and figure 3.4 show the metrological data for the Goro town, the nearest metrological station to the project site. As recorded in the station mean monthly annual temperature of the region is 17.3 °C (table 3.1b). Air temperature at Gorro is uniform throughout the year. On the basis of the three years of available data, mean monthly temperature vary between 17°C (June) and 20 °C in January. Mean monthly maximum temperature vary between 24°C (June) and 28°C (February). WWDSE in Association with ICT Final Feasibility Study Report 16Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibjhty Study and Detail Design of Bale Gadula Irrigation Project Table 3.1a Mean monthly rainfall data VOL 4- Annex 3 Hydro-geological Study Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual 15 21 111 137 135 46 12 32 112 77 21 5 724 Table 3.1b Mean monthly, mean minimum and mean maximum temperatures at the Goro Metrological Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Min 12 11 10 10 10 10 10 10 9 7 10 10 10 Mean 20 19 19 18 18 17 18 18 18 17 19 19 18 Max 28 28 27 26 26 24 25 27 27 27 27 28 27 Rainfall in mm Figure 3. 4 Mean monthly rainfall distribution of the Goro Station Between 1973 and 2002 the maximum rainfall recorded was 1383 mm/yr while the minimum record shows rainfall amount as low as 732.9 mm. In an average year, more than 50% of the total annual rain falls during Beige (March, April), which is the principal cropping season. Meher rainfall (July, August) falls account for more than 25% of the annual total. The other seven months are relatively dry having only 25% of the total annual rain fall. WWDSE in Association with ICT Final Feasibility Study Rcpoit 17Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and DetaiHDesign of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study The mean monthly pan evaporation varies from 128mm/month (Sep) to 180mm (march) with annual mean of 1732mm. The mean daily wind velocity varies between 1.5m/s and 4m/s with the high wind speeds occurring from June till August. 3.5 HYDROGRAPHY The hydrography of the regions straddling the Gadula plain are characterized by dendritic, parallel and rectangular pattern which are manifestations of tectonics, geology processes in the region (figure 3.2). The Waibe River, the River south of the Waibe River, the tributaries of Waibe form a set of parallel pattern which is manifestation of control of the drainable pattern by regional parallel faults. The Gadula Plain has two separate hydrographic patterns whereby in the North Western Sector the plain is drained southward while n the South Eastern Sector, the plain is drained north east ward. WWDSE in Association with ICT —— Final Feasibility Study Report 18n u «„a i>< .w Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study* 4. HYDROGEOLOGY As depicted in the TOR the hydrogeological investigation has three principal objectives. These are • Assessing groundwater potential • Prediction of water level rise and water logging effects related to irrigation activities, • Proposing approaches of irrigation water management so as to manage or reduce effects of irrigation activities. 4.1 REGIONAL HYDROGEOLOGY Generally, the principal aquifers the Genale Dawa basin including the Weyib River basins are basalts belonging to the trap series. The basement lighologies are known for their poor potential. The Mezozoic sediments are known for their good potential but the fact that they are dissected by Major River decrease their importance unless otherwise favorable geologic structural conditions allow. In the highlands particularly close to the Weyib river basin the limestones belonging to the Mesozoic sediments show extensive features of karstification and cave formation. This make the Antalo limestone sequence in the upper part of the Genale Dawa basin attractive sites for groundwater development. Regardless of the enhanced porosity by karstification the groundwater has short residence time and fast flow velocities in the karst features. This makes the karstified iimestone aquifers succeptible to variation in climate and landuse. Generally the hydraulic conductivity and yield of wells increases from the highlands surrounding the Bale massif to the lowlands bordering Kenya from South. The region surrounding the foothills of the Bale massive are known for their high groundwater potential. The Genale Dawa Master Plan Study classify the aquifers of the basin into four major groups (figure 4.1) depending on the type of permeability and the extent of the aquifer as set in tables 4.1. These are: WWDSt in Association with ICT Final Feasibility Study Report 15Democratic Republic of Ethiopia Ministry of Water Resources and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study • Extensive aquifers with intergranular permeability(unconsolidateci sediments, alluvim. eluvim.colluviums, lacustrine sediments and poorly cemented sand stons • Extensive aquifers with fracture and/or karastic Dermpability(consolidated sediments, limestone, sand stone, shale marl, evaporate . marble) • Extensive aquifers with fracture permeability (volcanic rocks,basalts, rhyolites, trachytes and ignimbrites) • Localized aquifers with fractured and intergranular permeability ( noncarbonated metamorphic rocks . granitic intrusive, dolerites) General permeability and productivity classification are made for regional surveys the basis of field observations and borehole data. ’able 4 1: Basis for classification of aquifers in the Genale Dawa basin v/yyyi i I hi Abi.u‘-idliuii mlh IL I I null I easibility Study Repoit 204°<H» Figure 4.1 Ground Water Potential Zonation map of Genale Dawa Basin 38'00 E 39;0'0"E 40-00" E 41-0 0' E 42-0'0'E 43=0-0 'E Legend Goro Gale Gadula Command Area Aquifer productivity map Productivity | High to Moderate .ov, Aquifuge Moderate Moderate to _o ;. .o.ii.SicctEii mmuiatvi Federal Democratic Republic of Ethiopln-Mlnlstry of Water Resources f ojsibihty Study and Detail Design of Balo Gadula Irrigation Projoct VOL 4- Annex 3 Hydro-geological Study In the Genale Dawa basin recharge generally, decrease from the highlands to the lowlands. Baseflow separation method (Cherent. 1993) show recharge is in the order of exceeding 150 mm in the highlands and less than 50 mm in the lowlands bordering Kenya In the Bale Gadula plain and surrounding regions recharge has been estimated to be around 100 mm/year. 4.2 GROUNDWATER POTENTIAL OF THE BALE GADULA PLAIN AND SURROUNDING PLATEAU Groundwater potential is measured by a) rate and sustainability of recharge, b) storage and tramsimisivity characteristics of the aquifers and c) the suitability of the groundwater for different uses. From existing literature recharge rate in the Bale plateau and upper Weyib catchement is estimated at 100 mm/yr. Regardless of the high recharge rate a number of wells drilled into the quaternary basaltic volcanics and in the trap series basalts turn out to be dry (table 2.1). Generally the topography around the Gadula plain favors formation of springs and presence of wetlands in the depression. However the number of springs emerging in the project area does not exceed 5 in number and discharge rate of the only prominent springs are low (2.5 l/s). It should be noted that the low groundwater potential of the Bale Gadula plain is an exception in a region which is otherwise known as high groundwater potential zone. In the regional hydrogeological classification the region is classified as high potential zone. The low groundwater potential of the Bale Gadula region is related to the presence of hydrogeologic shadow whereby groundwaters emerging from the highlands surrounding the plain discharge into the Weyib River (in south) or into the Asendabo Stream (in north and north east). Therefore the Bale Gadula plain is a local plateu secluded between two river gorges- this put. WWDSE in Association with ICT Final Feasibility Study Report 22I Hydrogeological Map of Bale Gadula Command Area and Its Surroundings 4.2 a-rannn | Legend * Water Points and their yield Major rivers Major rivers Geological Structures Infered groundwater flow direction I Extensive aquifer in dual porosity medium 'associated with fractured and karstrfied limestone The Sof Omor Cave is assoicated with this lithlology Medium to high productivity aquifer with fracture porosi Lrthologies are dominantly basalts groundwaters are contained in bedding plains and joints. Low productivity aquifers composed of tryacytes and ryholites. Groundwater mainly discharge to springs at the contact betweem the Tryachytes and the underlying trap series basalts. High productivity and extensive aquifers composed of qutemary basalts. Groundwater occures both in fractures and is co re a beds associated with the basalt flows 10 Kilometers JFederal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study the gadula palin as a 'local groundwater shadow’. Figure 4.2 given the detailed hydrogeological map and aquifer classification of the region. In the classification criteria set in table 4.1 has been used. Groundwater flow map inferred from the general topographic feature show flow towards the south west and discharge of groundwater takes place to the Weyib and Asendabo Rivers. Measurement of EC value of the Waibe River during the dry season show a value less than 500 micro simens/cm. Measurement of EC value of the Yadot River during the dry season show a value less than 200 micro simens/cm. Because of scarcity of water points intensive measurement of EC and water quality was not attained. However the combination of geology, topography and drainage should result in low TDS of the groundwater and surface waters. This is confirmed by the regional observation under the Genale Dawa Master Plan study which reveals that groundwaters of the volcanic highlands has low TDS in the order of less than 500 mg/L. The few measurement sin the irrigation command areas and their surroundings confirm this low TDS of surface and groundwaters. Table 2.2 show the river water is suitable for irrigation and domestic water supply. Combination of geological information, recharge, and water table depth and water quality has been used to draw a conceptual model for groundwater occurrence and flow in the Bale Gadula plain and surrounding high grounds (figure 4.3). As shown in the figure the Mesozoic sediments are assumed to exist beneth the basaltic cap. However the exact depth to the Mesozoic sediments is unknown because of absence of exposure of such a lithology in the Bale Gadula Plain. WWDSE in Association with ICT Final Feasibility Study ReportFederal Democratic Republic of Ethiopia-Ministry of Water Resource*! Feasibility Study and Detail Design of Bale Gadulfi Irrigation rroject VOI t Annex .1 Hydro qculoqiGal Study Z-onprlly OCCU' in frac'.rP'. .vsa-sandcorn?. • be- jer- a a fl? -.s grounr'we emeroin; 4r.: - rhe highlands ir the north a xl set d schtrge in the '■■•epl: R ve- before reaching theGadula iNam Gmundweter flow IM th is gene' ? . shod ano residence trie c'g rod" ct.-xue isalsoshort lr the genet, a ouifer classification o' the Gena C av* basin th $ sector s ciass •>. as high potential agu for - Depth to wsfor tabe >s m onler of excelling Ilin peters. Recharge ta*«e« place from ■ imfai is estimated at 1 AO romp Groun f.-fitei xurs m fi actureel quaternary basalts. Loca ly cJluvrum and al u\ ial materials of small latered and vertical aquifers hold and transmit water, Since regional groundwaters get discharged into the alleys bounding the plain from south north and north east, the Gadula plain is considered as groundwater shadow zone. Total dissolved solids is less than 400 mg-L. GroutKl'.'jflter ienedly occur n fractured ha salts and contact ler.’^n lava II0/.5. gr'Tinrlw'f emerging from the highlands in te north and south discharge in rhe Weyib River before reaching the Gadula plain. Groundwater flow path is general/ short and residence time of groundwater is also short. In the general aquifer classification of theGenal Dawe basin this sector is classified as high potential aquifer 5 K~t Figure 4.3 Conceptual groundwater flow and recharge model for the Bale Gadula Plain ////DSE in Association with ICT Final Feasibility Study Report 25Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadulalrrigatior^Project Recharge estimation in the Bale Gadua Plain VOL 4- Annex Hydro-geological btuay In order to estimate groundwater recharge in the Gadula Plain first estimate has been made on the potential evapotranspiration and actual eveportranspiration using the Thornthwaite Matter Soil Water Balance Method. The method is based on the consideration of monthly soil water balance accounting where by the Potential evapo-transpiration is estimated from the classical relation of Thornthwaite and the Actual evapotranspiration is estimated from soil water holding capacity and rooting depth of the soils, monthly soil water deficit and accounting for monthly potential accumulated water losses. Thornwaite formula for calculating PET is based on temperature with an adjustment being made for the number of daylight hours. The equation for calculating PET on monthly base is given by:- / = 16 Nm Where wis the month 1, 2, 3... 12, and 10 Tn. mm is the monthly adjustment factor related to hours of daylight, Tm is the monthly mean temperature OC ), a = 6.7xl O’7 Z3 - 7.7x10"5 Z2 +1.8x1 O'2 Z + 0.49 Z is the heat index for the year, given by: m = l, 2,3................ 12 The result of the estimation of the potential evapotranspiration is given in table 4.2. The actual evapo-transpiration is then calculated from the Thornwaite- Mattar soil water balance approach as depicted in table 4.2. Recharge to groundwater is then estimated by considering that half of the monthly surplus goes to recharging groundwater and the remaining half leaves the terrain as runoff. The recharge water of any given month leaves the aquifer in the next month. The bulk annual recharge is therefore the sum of the monthly recharge of each month. By doing so the recharge for the Gadula plain irrigation area is estimated at half of 106 mm of surplus water which is equal to 53 mm/yr of recharge. It should be noted that the VVWDSE in Association with ICT Final Feasjbjlity 26Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation ProjectHydro-geological Study VOL 4- Annex 3 results obtained from the soil water balance method is lower than previous estimates from the base flow separation method taking large rivers only. The discrepancy lies in the fact that the regional recharge estimates by Tesfaye Chemet (1994) does not take into consideration the local soil water balancing and is based on regional assessment of river discharges. Given that the deep water table and the general groundwater scarcity in the Bale Gadula plain, the lower "echarge rate estimated by the current study should be the most valid. Table 4. 2: Estimation of potential evapo-transpiration, actual evapo transpiration and monthly and annual groundwater recharge rates in the Bale Gadula Irrigation command area. •' ater Sglsnce Esti—aticn for 3 silty clay soil with water capacity cf 350 Panterer PIT ? Jan Feb Mar Apr May Jun Jul Aus Sep Oct Nov Dec Annual 15.00 21.00 111.00 13".00 105.00 16.00 12.00 32.00 112.00 ”00 21.00 5.0) ”2 » 100.00 55 00 -s5.ee 105.00 100.00 103 00 S5 00 ’2 00 *3 00 "00 50 00 53 00 53 00 2.00 S’.OO 32.00 -35.00 -50.00 -100 35 00 -13 00 -*2 00 ss 00 1091 .-.cnaiuiated Potential Water _o -25S.OO -333 00 -35 00 •55.00 -1-0 00 • 13.00 -Si 00 -1*3 00 Sri moisture Sm’ It*-* 135.1* 13* I" r-.r 206.1* 313.05 253." 23-1.51r 2:5 51 33* .25 2*: 53 213 50 15- Actual S'aposanspeanon Sod Mtxsture Deficit Suplus l.’.; -32.30 15 00 21.02 S5 00 *? 00 0.00 0 00 0.00 3*00 32 00 106 53 -5.32 -25 16 35 00 5* 63 -62 *0 •61 03 135 00 C' 12.00 32 02 " JO " 00 21 00 500 650 0 00 0.00 35.00 60.00 -1.00 13 00 *200 SS 00 2.00 3*.00 32.00 0.00 0.50 0 00 35.00 0.00 0.00 0 00 106 "otal A."adahle Water for P.unof 2.15 0 00 ’-05 0 00 2 00 3S.00 51.00 ■'*' 11.25 5 5-r 35 00 r.5o S.*5 -3S Detesnon •—> 0.00 15.00 25 50 1125 5.6- 2 SO r 50 S *5 - 3$ 215 106 1.00 15.00 25 50 11.25 5.65 2.S2 1-50 S'5 - 3S 2 15 ■ P - Average monthly rain fall (obtained from rainfall data from Goro station) • PET- Potential Evapotranspiration (obtained from Thornwaite formula) ■ P-PET - difference between rainfall and potential evapotranspiration, positive values indicate wet season and negative values indicate dry season Positive values are representing additions of moisture to the soil while the negative values are showing the monthly demand of moisture by the vegetation which is not satisfied by the monthly rainfall. ■ Lam - accumulated potential water loss, is obtained by cumulating the negative values of the differences between monthly precipitation and evapotranspiration for dry season only; and the summation begins with the first month of dry season; for our catchment. October and November for lowlands and highlands respectively. . sm- Soil moisture. Accumulated potential water loss is used to obtain the soil moisture during the dry months using the following formula: WWDSE in Association with ICT “ Final Feasibility Study Report T7Federal Democratic Republic of Ethiopia-Ministry of Water Resources easi ility Study and Detajl JJesign of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study (Water balance calculation of the Thornthwaite type, 1997) Where, Sm - Soil moisture during the month M (mm), Lam - Accumulated potential water loss at month M (mm), ana W- available water capacity of the root zone (mm) Soil moisture values for each wet month are obtained by adding the excess of rain of the current month to the soil moisture of the month before. However, this sum may not exceed the water capacity and excess is booked as moisture surplus. • AET - Actual evapotranspiration. Monthly actual evapotranspiration (AETm), is found as: AET = PET if Pm > PETm Otherwise, AETm = Pm + Sm -1 - Sm, , m stands for month. Where, Sm.i and Sm are soil moisture during the month m-1 and m respectively. ■ SMD - Soil moisture deficit. Monthly SMD is the difference between PETm & AETm ■ S - Soil moisture surplus, is the excess of soil moisture values(S ). m • TARO - total available for runoff. The value is determined, starting from the first month of the water surplus period. For our catchment, May in highlands and June in lowlands. The first months surplus is the TARO for that month, and from the surplus, 50% value is detained (D) and carried over to the next month TARO, and 50% is river discharged (RO). Therefore, the TARO of preceding month is given by the surplus of that month (S ) plus the detention of M last month (D _ j). m Therefore the total recharge for the 12000 ha of the irrigation command area is estimated at 6 4 x 10 m3/yr of water. 6 WWDSE in Association with ICT Final Feasibility Study Report 28Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological_Study__ 4.3. WATER SUPPLY FOR SETTLEMENT AREAS ONCE IRRIGATION STARTS The hydrogeological investigation envisages to propose best alternative water supply source in line with the settlement or resettlement options that is proposed by the Socio economic survey. The socio economic survey indicates the need for resettling or relocating the population is minimal or insignificant. See excerpt below: One of the expected adverse effects of the Bale- Gadula irrigation project is the displacement of families and population from their homes. Nonetheless, during the fieldwork it was observed that, the proposed irrigation command area is very sparsely populated. In addition almost all of the residential houses are concentrated around the Kebele administration offices forming well patterned rural village town, where social services like school, health post, grain mills, farmers' training center, mosques and other infrastructures are relatively available. Five Kebeles fall in the irrigation command area. The total population living in five Kebeles falling in the command area was estimated at 16,472 people in 2008 of which 8068 are male and 8404 are female. Weltai Nagaya Kebele is located along the bank of Web River, which is technically not feasible for irrigation. The other Kebeles (Bale Anole and Bale- Gadula) are located along the gravel road that connects Robe town, the capital of Bale Zone with Ginir Woreda traversing Goro town. Moreover, in all of the villages found in the proposed irrigation command area, there is at least one health post, one primary school, two mosques, three grain mills, WWDSE in Association with ICT Final Feasibility Study ReportFederal Democratic Republic of Ethiopia-Ministry of Water Resources -L££^j^^y_Study_and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study one framers' training center and Kebele administration office. Thus, the relocation of these villages to other places could not be technically, economically as well as socially feasible so as to realize the implementation of the proposed irrigation project. Thus, the project engineering design should be carefully undertaken so as not to disturb the existing villages. However, 230 households and 1,610 populations who are haphazardly settled in the proposed irrigation command area may be relocated during project implementation. In this scenario, the number of households to be displaced from their residential area will be insignificant. As agreed with engineering design team of the project, these villages will be excluded from command area. They should not be relocated so as to minimize the adverse impact of the project on local community. Only those households which are haphazardly settled will be relocated during the implementation of the project. Under this condition of no major resettlement need water supply for domestic water use for the community need to consider the following options: • Under the present condition depth to water table exceeds 90 meter. But once the irrigation starts option may exist in the command area to drill shallow wells and hand dug wells and tap infiltrating irrigation water. The exact location of this wells should be selected based on post irrigation practice site investigation • Use of community based surface water purification system- this can be achieved by installing disilting containers accompanied by some treatment for chemical quality by chlorination techniques • Option to strike the underlying Mesozoic sediments. It is known that the Mesozoic sediments particularly the Antalo limestone which is found WVVDSE in Association with ICT Final Feasibility Study Report 30ii vn w. »• n « .1 • ••> Federal Democratic Republic of Ethiopia-Ministry of Water Resources casibinty^Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study downstream of the Weyb River Valley is karstified because of the high rainfall and vegetation cover of the area. Deeper drilling in the order of 20D meters may penetrate the high potential limestone aquifers (as shown in figure 4.2 and 4.3). However as to the judgment of the consultant, deep drilling may appear costly for the purpose of merely community water supply. Prior to drilling appropriate geophysical survey has to be conducted to delineate the depth to the Mesozoic sediments underneath the basalt overburden. Therefore the irrigation practice itself would favor easy availability of water resources within the command area its surroundings. 4.4 PREDICTING THE GROUNDWATER LEVEL RISE UNDER IRRIGATION Data requirement for prediction of groundwater level rise include transmisivity of aquifers, hydraulic gradient of groundwaters, and boundary conditions around the impacted aquifers. However obtaining the stated data under the current project may be costly or may not be necessary. This is because groundwater table is very deep, and the topography favors easy discharge of the infiltrating irrigation water to adjacent gorges. Using a simple equation which relate water level rise to specific capacity of the aquifers, and Darcy discharge a preliminary prediction to water level rise is predicted as follows. Assuming the porosity/specific capacity of aquifer 0.21 (this is appropriate for the basaltic aquifers underlying the area) assuming annul irrigation water application of 0.360 m/year, water level rise is estimated at 1.8 meters/year. Assuming unsaturated thickness of 100 meters (estimated from the elevation difference between the Gadula Plain and the bottom or the Wybe River valley), it takes more than five decades for the groundwater to rise to the surface. However, since part of this irrigation recharge water is discharged out of the irrigation area because of sub surface flows the water logging should take longer time that stated. 1 in absence of clear aquifer property data this value is widely accepted Figure for porosity of basaltic aquifers. It should be noted this value is also from experience of the consultant rather than from literature sources. WVVDSE in Association with ICI Final Feasibility Study Report 31Federal Democratic Republic of I thlopla Ministry of Water Roaourca* Feasibility Study and Detail Design of Halo Gadula Irrhjwtlon Project VOI 4 Annex I I lydro geological SIihF However since all the infiltiatmg recharge walor doos not go Io rise the walor l;ihl<• alone and part of it is discharged out ol the Irrigation areas as groundwater out flow the predicted watoi level rise should bn lower than estimated A slrnph* calculation involving Dairy's law has been used Io calculate the subsurface* discharge from the irrigation command area Becauso of lack ol hydraulic gradient data and transmisivity of aquifer preliminary estimates has been used Hydraulic, gradient was obtained from the slope of the irrigation area, cross section along with discharge takes place is delineated from topographic map, and rock permeability from literature has been used I ho conceptual discharge takes pl;ir.<? is depicted in figure 4 4 Discharge Q = kdh/dx , where k is permeability of the soils and dh/dx is hydraulic gradient Assuming a k value of 0 1 m/day (this is high value) for the basaltic outcrops of the area and hydraulic gradient of 0 1 for the Gadula plain, discharge cross section of 36000 meters (figure 10 ) and aquifer thickness of 100 motors annual discharge from the plain is estimated at 36000 m3/day It should bo noted Ilia! total annual recharge from irrigation and rainfall (0 36 rn/yr) is far lower than the total amount of subsurface discharge from the command area suggesting no major water logging or water level rise problems in the Gadula plain Other contributing factors to the water level rise could bo channel loss during rogation exercises However considering the depth of the unsaturated /one. and the high infiltration the impact from such activity should bo minimum 7//7OSI 1(1 With if I Hull I I'HMllilily ,f •’!/Feden c Rcpub c c’ E’K cc <> V ' e’ * .»te- Resounrc* Feas > S\ ?\ .- c ?cts Des c” c* Ba c va<k a f>•’« c ' r’C c4 VOL 4 Annrw ? Hydro ucv ouivdl S\uG» 11 -1 J ---------- 0 2 4 & Kilometers I 1 1 1 I L_ 1 1 I Figure 4. 4 Assumed sites of discharge for ground waters of the Bale Gadula Irrigation Command Area (The dark bold line shows this area) WWDSI with IC > i ’.v > eas t' t\ Stua\ KeivtV .11 < MJ t.W 5U W. lit. W JL '■ .1 1 ill. Ill—V II S-JUMJJI *11 11 « 11 W U*K- Federal Democratic Republic of Ethiopia-Ministry of Water Resources ^^ikililX^M^y^Ilj-PgtailjJesign of Bale Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Studjr 4.5 WATER LOGGING AND WATER LEVEL RISE MANAGEMENT OPTION Management option for water logging and salinity buildup is depends whether such problem is going to be encountered or not. From the simple Darcy calculation in section 4.4 it is predicted that neither permanent water logging nor salinity buildup would be encountered in the Bale Gadula irrigation project site. The possibility of encountering temporary depression storage and lower grounds and formation of short lived water logging may not be avoided. This kind of short lived surface accumulation of irrigation water is entirely related to non groundwater related processes. Presence of local depression, presence of clay soils, or other low permeability soils may increase the risk of temporary detention storage. Based on topographic evidence the north eastern sector (figure 4.5) of the Gadula plain is identified as the potential site of temporary water logging. WWDSE in Association with ICT Final Feasibility Study Report 34w-n-vrtt-tlllllll llllf Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project VOL 4- Annex 3 hydro-geological Study Figure 4. 5 The northern Potential site of water logging. sector of the study area is identified as high likelihood zone of temporary water logging site WWDSE in Association with ICT Final Feasibility Study ReportFederal Democratic Republic of Ethiopla-Ministry of Water Resources Feasibility Study and Detail Design of Balo Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological Study 5. CONCLUSIONS AND RECOMMENDATIONS 5.1 SPECIFIC HYDROGEOLOGICAL RECOMMENDATIONS The Bale Gadula Irrigation project site because of its favorable topography, geology and hydrograpy should not encounter a major water logging problem or salinity buildup once irrigation started. Even under condition whereby subsurface discharge is not taking place the plain, more than 10 decades has to pass before the deep groundwater rises to the surface. Community based surface water purification systems are the best option for domestic water supply in Irrigable area, as groundwater level is deeper and experience show drilled wells in the region fail to provide water. Another option for the community water supply may be to drilled shallow wells and hand dug wells once the irrigation starts so as to tap infiltrating water. 5.2 RECOMMENDATION ON THE WAY FOREWORD Site visits, hydrogeological investigation, water quality data all lead to the conclusion that groundwater level rise and salinity are not major challenges if irrigation starts in the Bale Gadula plain. Furthermore the deep water table is indicative of limited option for use conjunctive surface water groundwater irrigation. The author recommends that peizometirc wells be installed and drainage channels designed and constructed although the treat water logging and salinization is minimal. Table 5.1 the recommended sites for small diameter pizemeter well installation. Each piezometer should have a depth in the order of 10 to 20 meters. Table 5.1: Geographic coordinates of proposed pieozmeters E N Depth in diameter 667388 783819 10 to 20 670701 784334 10 to 20 665622 785218 10 to 20 666873 782420 10 662751 781684 10 658849 782935 10 672983 781684 10 to 20 yyWDSE in Association with ICT Final Feasibility Study Report 36federal Democratic Republic of Ethiopia-Ministry of Water Resources feasibility Study and Detail Design of Balo Gadula Irrigation Project VOL 4- Annex 3 Hydro-geological JJtudy REFERENCES 1. Genale Dawa Basin Master Plan Study, Ministry of Water Resources, 1998, Addis Ababa Ethiopia 2. BALE GUDULA IRRIGATION PROJECT- PHASE I- MAIN REPORT, By Water Resource Development authority, Addis Ababa, 1992 3. GENALE -DAWA RIVER BASIN Integrated resources development master plan study report, Ministry of Water Resources, 2007 4. EGS, 1996. Geological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia 5. Tesfya Chernet (EGS, 1993). Hydrogeological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia 6. EGS, 1972. Geological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia 7. Kieffer, B., Arndt, N., Lapierre, H., Bastien, F., Bosch, D., Pecher, A., Yirgu, G., Ayalew, D..Weiss, D., Jerram, D.A., Keller, F., Meugniot, C., 2004. Flood and shield basalts from Ethiopia: Magmas from the African superswell. J. Petrol. 45, 793-834. WWDSE in Association with ICT Final Feasibility Study Report 371
🔊 Read Aloud
⏹ Stop
Summery
<
×
Summery