HYDROGEOLOGICAL, HYDROCHEMICAL AND ENGINEERING GEOLOGY MAPS OF ADDIS ABABA NC 37-10 EXPLANATORY NOTES

Bereket Fentaw and Leta Alemayehu (Chief Compilers) Jiri Sima (Editor) The Main Project Partners

The Czech Development Agency (CzDA) cooperates with the Ministry of Foreign Affairs on the establishment of an institutional framework of Czech development cooperation and actively participates in the creation and financing of development cooperation programs between the Czech Republic and partner countries. www.czda.cz

The Geological Survey of Ethiopia (GSE) which is accountable to the Ministry of Mines and Energy, collects and assesses geology, geological engineering and hydrogeology data for publication. The project beneficiary. www.geology.gov.et (www.mome.gov.et)

AQUATEST a.s. a Czech consulting and engineering company in water management and environmental protection. The main aquatest contractor. www.aquatest.cz

The Czech Geological Service collects data and information on geology and processes it for political, economical and environmental management. The main subcontractor. www.geology.cz

Copyright © 2011 AQUATEST a.s., Geologicka 4, 152 00 Prague 5, Czech Republic First edition ISBN 978-80-260-0330-4 AcknowledgmentAcknowledgment

Field work and primary compilation of the map and explanatory notes was done by a team from the Geological Survey of Ethiopia (GSE) consisting of staff from the Groundwater Resources Assessment Department; the Czech experts from AQUATEST a.s. and the Czech Geological Survey in the framework of the Czech Official Development Assistance Program. The team is greatly indebted to administrative offices of zones and Weredas of of Oromia regional state and Amhara regional state as well as Addis Ababa city administration for their limitless cooperation. The team is grateful to the management of the Geological Survey of Ethiopia, particularly to Director General (GSE) Mr. Masresha G/Selassie and Mr. Yohannes Belete, Head of Groundwater Resources Assessment Department (GSE) and Mr. Getnet Mewa, Head of Geohazard Investigation Department (GSE). Special thanks go to the NGOs and private water drilling and consultant companies for providing data from private databases. Finally, the team acknowledges the untiring support of the people living in the mapped area who assisted the team by all means possible and facilitated the data collection and those who helped us in different ways.

Contents

Acknowledgment ...... 3 Extended Summary...... 13 Introduction ...... 17 1. Basic Characteristics of the Area ...... 19 1.1 Location and Accessibility ...... 19 1.2 Population, Settlements and Health Status ...... 20 1.3 Land Use ...... 27 2. Selected Physical and Geographical Settings ...... 29 2.1 Geomorphology ...... 29 2.2 Soil and Vegetation Cover ...... 30 2.3 Climatic Characteristics ...... 32 2.3.1 Climatic Zones and Measurements...... 32 2.3.2 Precipitation ...... 38 2.4 Hydrography and Hydrology of the Area ...... 45 2.4.1 Surface Water Network Development ...... 47 2.4.2 Surface Water Regime ...... 47 2.4.3 Baseflow ...... 52 2.5 Water Balance ...... 58 2.6 Drought and Climate Change ...... 59 3. Geological Settings ...... 63 3.1 Previous Work ...... 63 3.2 Stratigraphy ...... 63 3.3 Lithology ...... 64 3.3.1 Precambrian Formation (Basement) ...... 64 3.3.2 Mesozoic Sedimentary Rocks ...... 66 3.3.3 Tertiary Volcanic Rocks ...... 68 3.3.4 Quaternary Volcanic and Sedimentary Rocks ...... 69 3.4 Structure ...... 70 3.5 Geological History ...... 70 4. Engineering Geologocal Settings ...... 73 4.1 General ...... 73 4.2 Methodology ...... 74 4.2.1 Data Acquisition ...... 74 4.3 Information Layers ...... 75 4.4 Definition of Engineering Geological Zones ...... 78 4.5 Basic Characteristics of Important Engineering Geological Regional Units ...... 85 4.5.1 Rock mass ...... 86 5. Hydrogeology ...... 89 5.1 Water Point Inventory ...... 89 5.2 Hydrogeological Classification/Characterization ...... 92 5.3 Elements of the Hydrogeological System of the Area (Aquifers, Aquicludes and Aquitards) ...... 93 5.3.1 Extensive and Moderately Productive Porous Aquifers ...... 94 5.3.2 Extensive and Highly Productive Fissured and Karstic Aquifers...... 95 5.3.3 Extensive and Moderately Productive Fissured and Mixed Aquifers...... 97 5.3.4 Extensive and Low Productive Fissured Aquifers ...... 100 5.3.5 Minor Aquifer with Local and Limited Groundwater Resources (Aquitard) ...... 101 5.3.6 Formation with Essentially no Groundwater Resources (Aquitard) ...... 101 5.4 Hydrogeological Conceptual Model ...... 102 5.5 Annual Recharge in the Area ...... 103 6. Hydrogeochemistry ...... 105 6.1 Sampling and Analysis ...... 105 6.2 Classification of Natural Waters ...... 106 6.2.1 Rain Water ...... 109 6.2.2 Surface Water ...... 110 6.2.3 Groundwater in Volcanic Rocks ...... 110 6.2.4 Groundwater in Mesozoic and Paleozoic Sediments ...... 110 6.2.5 Groundwater in Quaternary Sediments ...... 111 6.3 Water Quality ...... 111 5.3.1 Domestic Use ...... 112 5.3.2 Irrigation Use ...... 113 5.3.3 Industrial Use ...... 114 6.4 Mineral and Thermal Water ...... 116 7. Natural Resources of the Area ...... 129 7.1 Economic Geology ...... 129 7.2 Water Resources...... 133 7.2.1 Surface Water Resources and Development ...... 134 7.2.2 Groundwater Resources and Development ...... 137 7.3 Human and Land Use Resources and Development ...... 140 7.4 Wind and Solar Energy Development ...... 140 7.5 Environmental Problems and their Control / Management ...... 140 7.6 Touristic Potential of the Area ...... 146 Conclusions ...... 149 References...... 151 Annex 1 – Field Inventory Data...... 155 Annex 2 – Water Chemistry...... 183 Annex 3 – Well Logs ...... 189 List of Figures

Fig. 1.1 Location map ...... 19 Fig. 1.2 The main roads and settlements...... 20 Fig. 1.3 Administrative zones ...... 21 Fig. 1.4 Malaria risk in Ethiopia ...... 24 Fig. 1.5 Land use ...... 28 Fig. 2.1 Generalized physiographic units ...... 29 Fig. 2.2 Distribution of soil types ...... 31 Fig. 2.3 Climatic zones ...... 34 Fig. 2.4 Temperature at Addis Ababa and Fiche meteo-stations ...... 36 Fig. 2.5 Mean monthly relative humidity [%] and wind speed [km/d] at Addis Ababa meteo-station ...... 37 Fig. 2.6 Mean monthly number of sunshine hours and radiation [MJ/m2/d] at Addis Ababa meteo-station ...37 Fig. 2.7 Mean monthly Piche evaporation (Ev) [mm] and potential evapotranspiration (ETo) [mm] at Addis Ababa–Bole meteo-station ...... 38 Fig. 2.8 Seasonal classification and precipitation regimes of Ethiopia (source: NMSA, 1996) ...... 40 Fig. 2.9 The Addis Ababa meteo-station precipitation pattern ...... 41 Fig. 2.10 Long-term fluctuation and average of precipitation for the Addis Ababa meteo-station ...... 42 Fig. 2.11 The Fiche meteo-station precipitation pattern...... 42 Fig. 2.12 Long-term fluctuation and average of precipitation for the Fiche meteo-station ...... 44 Fig. 2.13 The Kachise meteo-station precipitation pattern ...... 44 Fig. 2.14 Long-term fluctuation and average of precipitation for the Kachise meteo-station...... 45 Fig. 2.15 The principal river basins of the area ...... 46 Fig. 2.16 River flow in the Muger, Jemma and Abay in standard variables ...... 46 Fig. 2.17 The Abay river discharge at Kessie river gauge station ...... 48 Fig. 2.18 Annual variability of the mean annual flow of Abay River at Kessie river gauge ...... 49 Fig 2.19 The Muger river discharge at Chancho river gauge station ...... 49 Fig. 2.20 Annual variability of the mean annual flow of Muger River at Chancho river gauge ...... 50 Fig. 2.21 The Aleltu river discharge at Muka Ture river gauge station ...... 50 Fig. 2.22 Annual variability of the mean annual flow of Aleltu River at Muka Ture river gauge ...... 51 Fig. 2.23 Method of Kille baseflow assessment ...... 53 Fig. 2.24 Kille baseflow separation ...... 53 Fig. 2.25 Method of baseflow separation ...... 55 Fig. 2.26 Hydrograph baseflow separation ...... 56 Fig. 2.27 The most drought prone areas of Ethiopia (source: RRC, 1985) ...... 60 Fig. 3.1 Biotite hornblende gneiss ...... 65 Fig. 3.2 Contact between tertiary volcanic and Debre Libanos sandstone at Ziga Wedem river gorge ...... 67 Fig. 3.3 Mesozoic propagation of the Karoo rift to the southeastern part of Ethiopia (modified after Gani et al., 2008) ...... 71 Fig. 4.1 Overall statistics of individual stratigraphic rock groups according areas of their outcropping ...... 73 Fig. 4.2 Example of hill shading visualization of DEM ...... 75 Fig. 4.3 Example of ranging of slope angles ...... 76 Fig. 4.4 Example of visualization of geology ...... 76 Fig. 4.5 Conceptual model for relief categorization in accordance with relief energy as a driving force of destruction, transport and sediment processes acting on the landscape surface ...... 77 Fig. 4.6 Position of plateau and valley provinces within the mapped area ...... 79 Fig. 4.7 Proxy of plateau and valley provinces within the mapped area ...... 79 Fig. 4.8 Proxy of geo-risk susceptibility for the whole mapped area ...... 85 Fig. 4.9 Proxy of geo-risk susceptibility for the plateau province ...... 85 Fig. 5.1 Field collection of data from drilling near Ginchi ...... 91 Fig. 5.2 Distribution of porous aquifers ...... 95 Fig. 5.3 Frequency of yield of springs in fissured and karstic aquifer developed in limestone ...... 96 Fig. 5.4 Distribution of fissured and karst aquifer developed in limestone ...... 96 Fig. 5.5 Frequency of yield of springs in fissured aquifers developed in volcanic rocks ...... 98 Fig. 5.6 Frequency of yield of springs in fissured aquifers developed in sandstones...... 99 Fig. 5.7 Distribution of moderately productive fissured and mixed aquifers ...... 100 Fig. 5.8 The extent and location of aquitards and low productive fissured aquifers ...... 101 Fig. 5.9 Conceptual hydrogeological model of the Addis Ababa area...... 103 Fig. 6.1 Level of cation-anion balance ...... 106 Fig. 6.2 Piper diagram for classification of natural waters ...... 108 Fig. 6.3 Content of nitrate in analysis of water in the Addis Ababa area ...... 113 Fig. 6.4 Swimming pool with thermal water pool in Hilton hotel ...... 118 Fig. 6.5 Swimming pool with thermal water pool in Ghion hotel ...... 121 Fig. 6.6 Piper diagram for cold and thermal waters ...... 127 Fig. 7.1 Existing ignimbrite Quarry ...... 130 Fig. 7.2 Basalt quarry for crushed aggregate production ...... 130 Fig. 7.3 Scars of rockfalls with volumes in the order of 103-105 m3 at the TV4/TV5 contact above Debre Libanos village ...... 143 Fig. 7.4 Huge blocks from rockfalls accumulated on the slopes just above Debre Libanos and inside the village, are testament to the very high risk of rockfall in such a crowded place ...... 144 Fig. 7.5 Scar of large, fossil rockfall from a geomorphologically over-steepened rock wall in the Upper sandstone of the Jemma river valley NE from Fiche ...... 145 Fig. 7.6 Crown and transportation zone of a medium sized landslide in Upper Sandstones and colluvial soils which blocked the road from Fiche to Jemma river bridge ...... 145 Fig. 7.7 Portuguese bridge from the 16th century ...... 147 List of Tables List of authors and professionals participating in the project ...... 18 Tab. 1.1 Population in the study area ...... 22 Tab. 1.2 Mortal diseases in Ethiopia (WHO, 2006) ...... 23 Tab. 1.3 Rural water facilities by Zones and Weredas ...... 25 Tab. 1.4a Leading causes of hospital and health center morbidity 2008/2009 in Oromia Region ...... 26 Tab. 1.4b Leading causes of hospital and health center morbidity 2008/2009 in Amhara Region ...... 27 Tab. 2.1 Ethiopian climate classification ...... 33 Tab. 2.2 Climatic station of the Addis Ababa and Fiche area ...... 35 Tab. 2.3 Temperature variation in Addis Ababa and Fiche meteo-stations ...... 36 Tab. 2.3 Monthly total Piche evaporation [mm] at Addis Ababa–Bole meteo-station ...... 37 Tab. 2.4 Characterization of the precipitation pattern in Ethiopia ...... 39 Tab. 2.5 Monthly long-term average precipitation at Addis Ababa, Fiche meteo-stations [mm] ...... 39 Tab. 2.6 Long-term monthly rainfall at Addis Ababa [mm] (fully recorded years only) ...... 41 Tab. 2.7 Long-term monthly rainfall at Fiche [mm] (fully recorded years only) ...... 43 Tab. 2.8 Long-term monthly rainfall at Kachise [mm] (fully recorded years only) ...... 44 Tab. 2.9 Data river gauging stations ...... 47 Tab. 2.10 Runoff data ...... 51 Tab. 2.11 Baseflow data for the Addis Ababa sheet area ...... 57 Tab. 2.12 Water balance of Mugher basin ...... 59 Tab. 2.13 Water balance of Aleltu basin...... 59 Tab. 3.1 Stratigraphy of the area ...... 63 Tab. 4.1a Overall occurrence of geological slope class units ...... 80 Tab. 4.1b An overview of classified regional engineering geological units with their size within the mapped area; part two ...... 80 Tab. 4.1c An overview of classified regional engineering geological units with their size within the mapped area; part three ...... 81 Tab. 4.1d An overview of classified regional engineering geological units with their size within the mapped area; part four ...... 81 Tab. 4.1e An overview of classified regional engineering geological units with their size within the mapped area; part five ...... 82 Tab. 4.2 Documented risky geodynamic processes ...... 82 Tab. 4.3 Basic land use and building activity restrictions assigned to basic geo-risks classes ...... 84 Tab. 4.4 Summary of the laboratory measured physical properties of samples belonging to high rock mass strength units...... 86 Tab. 4.5 Summary of the laboratory measured physical properties of samples belonging to moderate rock mass strength units ...... 87 Tab. 4.6 Summary of the laboratory measured physical properties of samples belonging to rock units of moderate to low mass strength ...... 87 Tab. 5.1 Summary of field inventory ...... 91 Tab. 5.2 Basic data about selected inventoried wells ...... 97 Tab. 5.3 Recharge in the Abay plateau and adjacent areas ...... 104 Tab. 6.1 Level of balance ...... 106 Tab. 6.2 Summary of hydrochemical types ...... 107 Tab. 6.3 Groundwater and surface water descriptive statistics of TDS, EC and Cl values ...... 109 Tab. 6.4 Chemical composition of rain water ...... 109 Tab. 6.5 Groundwater chemistry compared to drinking water standards and guidelines ...... 112 Tab. 6.6 Suitability of water for irrigation ...... 114 Tab. 6.7 Suitability of water for use in industry ...... 114 Tab. 6.8 Concentration limits for incrustation ...... 115 Tab. 6.9 Concentration limits for corrosion ...... 115 Tab. 6.10 The basic characteristics of thermal water observed in different water points by Kondo (1975)...... 117 Tab. 6.11 The basic chemistry of thermal water [mg/l] observed in different water points by Kondo (1975) ...118 Tab. 6.12 Chemical analysis from Hilton hotel well ...... 119 Tab. 6.13 Chemical composition of well BH-1 ...... 122 Tab. 6.14 Chemical composition of thermal water (Berhanu Gizaw, 2002) ...... 123 Tab. 6.15 Chemical data (Tilahun Azagegn Tafere, 2008) ...... 125 Tab. 6.16 Tritium data (Tilahun Azagegn Tafere, 2008) ...... 126 Tab. 6.17 Stable isotope data (Tilahun Azagegn Tafere, 2008)...... 126 Tab. 6.18 Well data (sine, 2008) ...... 128 Tab. 7.1 Selected sites for quarries ...... 131 Tab. 7.2 Physical characteristics of construction material...... 133 Tab. 7.3 Aquifers of the area ...... 133 Tab. 7.4 Assessment of water resources of the Addis Ababa sheet ...... 134 Tab. 7.5a Selected SHP sites within the area...... 136 Tab. 7.5b Selected dam sites within the area during field work ...... 136 Tab. 7.6 Suspended sediments transported by the Mugher River ...... 142

Under Separate Cover (see attached CD) Annexes: Annex 1 Field Inventory Data Annex 2 Water Chemistry Annex 3 Well Logs

Maps: Hydrogeological Map of Addis Ababa NC 37–10 - full size and A3 size Hydrochemical Map of Addis Ababa NC 37–10 - full size and A3 size Engineering Geology Map of Addis Ababa NC 37–10 - full size and A3 size Sheet 1: Basic Units Sheet 2: Geo-Risk Susceptibility Extended Summary The Addis Ababa area is located in Central Ethiopia on the Addis Ababa map sheet (NC 37-10) at the scale of 1:250,000, covering an area of 18,204 km2. The area is a part of the Amhara and Oromia regional states and Addis Ababa city administration and is inhabited by 4.3 million people and about 60 % of the area is intensively cultivated.

The western and northeastern parts of the Addis Ababa area are below 1,500 m above sea level (a.s.l.) and represent the low reaches of the Abay Guder, Mugher and Jemma rivers. These areas rise to the southeast to the Central Plateau with an altitude of 1,500–3,200 m a.s.l. with peaks above 3,500 m a.s.l. The area is a part of the Abay river basin, except the southeastern corner which is a part of the Awash basin. The rainy season is bimodal from March to May (Belg rainy season) and from June to September (Kiremt rainy season); the annual mean rainfall was adopted as being 1,250 mm/year for the Addis Ababa area. There are several permanent rivers (Abay Guder, Mugher and Jemma) and intermittent rivers particularly in the northwestern part of the area. Specific surface runoff was adopted as being a value of 15 l/s.km2. The adopted value of specific baseflow is 2.0 l/s. km2 representing 62.5 mm/year and 5 % of precipitation. The Addis Ababa area faced severe Kiremt drought in period from 1970 to 1973 and 1984. The years that drought was most serious in the Addis Ababa area were 1984 and 2002. The area shows high Kiremt drought probability.

The aquifer system has been defined based on the hydrogeological characteristics of lithological units described by the geological maps and data from the field inventory and desk study. The characterization of the area shows the following aquifer/aquitard systems: 1. Local and moderately productive porous aquifers with spring and well yield Q = 0.51–5 l/s developed in Quaternary unconsolidated deposits. 2. Extensive and highly productive fissured and karst aquifer with spring and well yield Q = 5.1– 25 l/s developed in limestone. 3. Extensive and moderately productive fissured and karst aquifer with spring and well yield Q = 0.51–5 l/s developed in volcanic rocks (not forming plugs) and sandstones. 4. Extensive and low productive fissured aquifers with spring and well yield Q = 0.051–0.5 l/s developed in basement rocks. 5. Extensive formation consisting minor fissured aquifers with local and limited groundwater resources – Aquitard with spring and well yield Q = 0.05 l/s developed in plug forming rhyolites along the Abay/Awash basins surface water divide. 6. Formation with essentially no groundwater resources – Aquiclude consisting of gypsum and mudstone.

The water balance, hydrograph separation and Kille methods show that the infiltration coefficient (recharge) is about 5 % of the total precipitation. Part of the groundwater infiltrates

13 directly from precipitation and groundwater flows laterally to local and/or regional drainage base levels represented by rivers in deep valleys where it emerges as springs or flows vertically recharging deeper aquifers. This type of front recharge is dominant on the Central Plateau, but the aquifers outcropping in the lower reaches of the main rivers limited surplus of water for infiltration have limited direct recharge. Recharge from the Central Plateau with higher precipitation to aquifers developed in limestone and sandstone is inferred. The flood episodes on intermittent and perennial rivers contribute significantly to the recharge of aquifers along river banks. Bank recharge provides a relatively large amount of good quality groundwater with low TDS in the alluvial aquifers. Some part of the deep regional flow appears as hot springs in the town of Addis Ababa in the Filwoha area.

Chemistry of groundwater in the Addis Ababa area is relatively uniform reflecting a hydrogeological system with relatively fast groundwater circulation despite the variability in geology and hydrogeology. The dominant hydrochemical type of groundwater is bicarbonate type.

The basic Ca–HCO3 type dominates and transitional types (Ca–Mg)–HCO3 and (Ca–Na)–HCO3

occur in the central part of the map sheet. There is an occurrence of basic Na–HCO3 type along surface water divide between the Muger and Jemma river basins and this type of chemistry is also typical for hot springs of the Filwoha thermal area. This transitional bicarbonate type verges into

transitional sulphate type (Ca–SO4) which is characteristic for aquifer developed in limestone. In general, the TDS increase from the infiltration areas along main water divide receiving the highest proportion of precipitation to the northwest where groundwater is drained by the main rivers. Groundwater TDS varies from 46 mg/l to 1,560 mg/l for cold water and is about 3,455 mg/l for hot springs of the Filwoha area. The majority of groundwater is good for drinking. The use of groundwater can be limited by pollution particularly of human and animal origin and some samples show increasing concentrations of nitrates additional to high TDS.

The total amount of water resources of the area has been assessed to be 8,667 Mm3/year. The use of surface water for irrigation is the most important development factor and 80 % of available surface water resources will be used for irrigation. This portion represents 5,333 Mm3/ year. Considering that about 10,000 m3 of water is needed to irrigate 1 ha of land, the calculated irrigation resources represent an irrigation potential of 533,300 ha (5,333 km2) which is about 29 % of the whole area and 50 % of the arable land (moderately to intensively cultivated plateau).

Additional to existing dams there are 13 places recommended for construction of irrigation and/ or hydropower dams.

The total volume of renewable groundwater resources of active aquifers in the area has been assessed to be 1,082 Mm3/year. Considering the total number of people living within the area is 4.3 million the need for water supply can be nearly 32 Mm3/year (22.5 l/c.d). The figure shows that recent demand represents about 3 % of renewable groundwater resources of active aquifers, meaning that aquifers can provide adequate drinking water even in the future considering the trends in population growth and can be also used for supply of areas adjacent to the map sheet.

Most of the people within the area live in small towns and villages additional to Addis Ababa. There is a good practice to develop big springs which form regional drainage of aquifers developed in volcanic rocks for drinking water supply of towns. Developed and well protected springs at Entoto, Debre Tsigie, Fital and Ejeri having yield 10–30 l/s are indicators for such development. Additional to development of big springs the water supply based on drilled wells represents the most secure water and should be applied for small towns and concentrated village settlements. Technically, it is recommended to drill wells as follows: a) In aquifers developed in volcanic rocks, it is recommended to drill wells with a depth of about 300 m. Each of the wells can yield about 7 l/s (recent average).

14 b) In aquifers developed in basement rocks, it is recommended to drill wells with a depth of about 30–70 m. Each of the wells can yield about 1 l/s (recent average) and can provide 86,400 l/day which is enough to supply a small town or group of villages with about 4,320 inhabitants considering a daily consumption of 20 l/c.d. c) In aquifers developed in the volcanic rocks of the plateau the building of a well field is recommended. The well field consisting of about 20 wells can provide about 150 l/s to be used locally or transferred by pipe to the end user. In total, 14 well fields were located on the map with a potential capacity of 1,650 l/s. The total yield of well fields is 66 Mm3/ year and represents 6 % of the existing renewable resources of the area. Well fields were located in two lines perpendicular to the regional groundwater flow direction. The first line is located between Daleti nad Muka Ture. The second line is located between Bicho, Gola, Shino, Inchini and Chancho.

The minimum required distance of water supply wells and potential pollution sources should be maintained during the development of groundwater resources for towns and villages. In addition to priority in development of groundwater for safe drinking water supply it should be possible to select the most fertile soil to be developed by small scale irrigation and livestock watering based on groundwater to increase the stability of food supply in prolonged periods of drought in the Addis Ababa area.

Soil erosion and protection is one of the limiting factors of sustainable development of agriculture within the area and should be addressed in all development projects, but data about soil erosion are scarce in the area.

Additional to assessment of engineering geology characteristics of various rocks of the area, natural hazard and protection against the consequences of earthquakes, landslides, rock falls and other hazards were discussed because these are important for the preservation of human lives, property and arable land. Susceptibility to exogenous risks differs both in quantity and quality between engineering geological provinces defined for the valley and plateau areas. The high susceptibility class occupies 56 % of the area.

The hot-spots of natural hazard have been identified as follows: • repeated rockslides of all sizes, including large deep-seated ones (activating volumes in the order of millions of m3), and small to medium sized rockfalls (up to hundreds of m3) occurring along sides of the deep erosion valleys of the main rivers and their main tributaries, • repeated catastrophic rock falls with volumes of hundreds to hundreds of thousands of m3 are bound to the contact which is exposed in kilometer long passages along the upper rims of the deeply cut valley sides and • river flood plains because of the possibility of floods where changes in the quantity or quality of river sediments could be also used as a long-time reaching indicator of environmental changes.

The work which is summarized in the presented explanatory notes shows the excellent water, agricultural, industrial, human as well touristic potential of the Addis Ababa area.

15 16 IntroductionIntroduction

Background Ethiopia is a country affected by environmental degradation including recurrent droughts which lead to food insecurity, and drought stricken areas have been constantly degraded over the past several decades by improper utilization of natural resources. The central part of the dissected highlands of Ethiopia is no exception to the above mentioned fact. The rugged topography and high gradient coupled with increasing population and intense deforestation and town development aggravates the problem. It is therefore important to compile a map of water resources, engineering geology and geological risk to be able to propose and implement appropriate protection measures during development efforts. It is also vital in identifying and tackling existing problems and proposing their solution. In this context the project for hydrogeological investigation of the “Addis Ababa Sheet Engineering Geology and Hydrogeological Investigation” was performed in the Addis Ababa sheet, in 2009 and 2010 by the Geological Survey of Ethiopia. The publication of the project results was conducted in the framework of bilateral cooperation between the Czech and Ethiopian governments, where the participation of the Czech experts was financed by the Czech Development Agency in the framework of the Czech Republic Development Assistance Program and the project entitled “Capacity Building in the Field of Hydrogeology and Engineering Geology”. Participation of the Ethiopian professionals was financed by the Ethiopian government. This report deals with the assessment of hydrogeological and hydrochemical characteristics and other environmental parameters acquired during the desk and field work and discussion between stakeholders and the joint Czech-Ethiopian team of professionals.

Objective and Scope Water is a finite resource and must be managed in a sustainable way. For sustainable development, water resource investigation can play an important role in the efficient and optimal utilization of the water resources available to a country. The main objectives of the study for hydrogeological mapping were to identify water-bearing lithological units and their basic characteristics, to indentify recharge and discharge areas as well as groundwater flow direction, to categorize water quality within water bearing formations, to indicate the suitability of groundwater for different purposes, and to compile hydrogeological and hydrochemical maps with accompanying explanatory notes of the study area based on the information and analysis made. The work covers the interpretation of aerial photos and satellite images, meteorological and hydrological data analysis, quantification of inventoried water points, collection of representative water samples and data for hydrochemical studies, and evaluation of water resource management of the area. The hydrogeological and engineering geological investigation of the Addis Ababa map sheet is part of the project entitled “Addis Ababa Sheet Engineering Geology and Hydrogeological Investigation” that was conducted between 2009 and 2011 to alleviate water shortage in the area.

17 The main objective of the engineering geology mapping was to compile a map for the following purposes: (i) to test the suitability of the methods chosen for the job and ways of practically fulfilling the tasks defined by the methodology chosen in the field as well as in the subsequent desktop data processing and assessment, (ii) to identify drawbacks and gaps in the methods and in data gained during the mapping season to reflect the actual engineering geology in the chosen area, (iii) to synthesize lessons learned and to transform them into a proposal for improvements. The findings have been compiled into maps in the scale of 1:250,000 to provide basic information for the development of potential natural resources within the area of the Addis Ababa sheet.

The desk and field work was carried out by a group of Ethiopian hydrogeologists. Final assessment and publication of the map was carried out by a joint Czech-Ethiopian team of professionals. The names of participating experts are shown in the following list.

List of professionals participating in the project Name Institution Participation field Jiri Sima AQUATEST a.s. Editor

Bereket Fentaw Geological Survey of Ethiopia Chief compiler - hydrogeology

Leta Alemayehu Geological Survey of Ethiopia Chief compiler - engineering geology

Jiri Zvelebil AQUATEST a.s. Engineering geology expert

Ondrej Nol AQUATEST a.s. Hydrogeological expert

Antonin Orgon AQUATEST a.s. GIS expert

Romana Suranova AQUATEST a.s. Printing expert

Craig Hampson AQUATEST a.s. Language revision

Tenebit Zecariyas Geological Survey of Ethiopia Data acquisition and evaluation - hydrogeology

Getachew Zewdie Geological Survey of Ethiopia Data acquisition and evaluation - hydrogeology

Tsehay Amare Geological Survey of Ethiopia Data acquisition and evaluation - hydrogeology

Data acquisition and evaluation - engineering Zulfa Abdurahman Geological Survey of Ethiopia geology Data acquisition and evaluation - engineering Bruk Abel Geological Survey of Ethiopia geology Data acquisition and evaluation - engineering Melkamu Tegegn Geological Survey of Ethiopia geology

Tamrat Fantaye Geological Survey of Ethiopia Geophysical study and field data interpretation

Yielak Alemu Geological Survey of Ethiopia Geophysical study and field data interpretation

Asres Haile Geological Survey of Ethiopia Geophysical study and field data interpretation

AEGOS project expert - coordination, technical Dana Capova Czech Geological Survey architecture, interoperability AEGOS project expert data conversion and pro- Vladimir Ambrozek Czech Geological Survey cessing

Petr Coupek Czech Geological Survey AEGOS project expert - data on-line provision

Shiferaw Ayele Geological Survey of Ethiopia AEGOS project country representative 1. Basic Characteristics 1. of the Area

1.1 Location and Accessibility The study area is located in Central Ethiopia, in part of the Central Ethiopian highlands (plateau). It belongs to the Amhara and Oromia regional states and Addis Ababa city administration. Geographically, the study area is bounded from north to south by latitudes 8°00‘ N and 9°00‘ N, and from west to east by longitudes 37°30‘ E and 39°00‘ E. The area covers approximately 18,204 km2 of the topographic map sheet at a scale of 1:250,000 of Addis Ababa (NC 37-10). The location of the map is illustrated in Fig. 1.1. The sheet is bounded by Debre Markos sheet in the north, Nekemte sheet in the west, Debre Birhan sheet in the east and Akaki sheet in the south.

The area can be accessed by a number of all weather road networks. One of the main asphalt roads connects Addis Ababa – Debre Tsige – Fiche which is the main road to the north. In addition Location and Accessibility Basic Characteristics of the Area

Fig. 1.1 Location map

Basic Characteristics of the Area 19 Location and Accessibility Basic Characteristics of the Area

Fig. 1.2 The main roads and settlements

there is another main asphalt road from Addis Ababa to Sendafa – Debre Birhan which is important for accessing the southeastern part of the area. There are a number of all weather gravel and dry weather soil roads which are interconnected with each other and with the asphalt roads. These are Fiche – Fital – Muka Ture, Degem – Ejeri, Mukturi – Lemi, Mukuturi – Kobtebe, Chancho – Segno Gebeya – Debra and Sululta – Legedade which play a vital role in accessing the area. There is also a third asphalt road from Addis Ababa to Menagesha and Holeta which is important for accessing the southwestern part of the area. The main accessible roads and settlements are shown in Fig. 1.2.

1.2 Population, Settlements and Health Status The population distribution of the study area is not uniform. A relatively high population is observed in towns than rural areas. In the plateau area permanent peasant villages are located along roads. There are also few people living in the lowlands (inside the deep gorges of the Jemma, Zegawedem and Mugher).

Settlements are highly dependent on the availability of water, farmlands, proximity to the roads and market places. Settlements are denser in the highlands than the lowlands (river gorges), with the exception of Addis Ababa city. The study area is dominantly inhabited by two ethnic groups; Oromo and Amhara. There are also some Gurage and Tigray people live in towns like Fiche, Muka Ture, Chancho, Sululta and Menagesha. But in Addis Ababa city there is a very dense population with diverse and variable ethnic groups. Oromifa and Amharic languages are widely spoken. The people in general subsist their living from agriculture and animal husbandry. Teff is widely cultivated in the area as well as wheat, barely and sorghum. The common domestic animals raised

Population, Settlements and Health Status Population, include goats, sheep and horses. Manufacturing, services and trade are developed in Addis Ababa and other smaller towns.

There are 7 Zones divided into 38 Weredas within the mapped area (see Fig. 1.3), however none of them are located entirely within the boundary of the map sheets. To calculate the total number of people living within the mapped area the number of people living in Weredas was assessed from the total Wereda population and by the percentage of the Wereda area located within the map sheets.

20 Basic Characteristics of the Area Population, Settlements and Health Status Settlements and Health Status Population, Basic Characteristics of the Area

Fig. 1.3 Administrative zones

Tab. 1.1 shows the population in the different Zones /Weredas within the mapped area.

The total population is assumed by the Central Statistics Authority to be 4,304,735 however this figure could in reality be several thousand higher. The urban population comprises more than 50 % (mainly Addis Ababa) and the remaining population live in rural areas.

Considering the trends in population growth, access to water will become worse by 2015 in urban areas and 2025 in rural areas, respectively. People in the area could face a water scarcity i.e. less than 1,000 m3/year, and/or even water stress i.e. availability less than 500 m3/year (Tesfay Tafese, 2001).

The life expectancy at birth is 49 years for males and 51 years for females (WHO, 2006). As in most developing countries, Ethiopia‘s main health problems are communicable diseases caused by poor sanitation and malnutrition. Mortality and morbidity data are based primarily on health facility records which show that the leading causes of hospital deaths were dysentery and gastroenteritis, tuberculosis, pneumonia, malnutrition and anemia, and liver diseases including hepatitis, tetanus, and malaria. The situation is complicated by the fact that Ethiopia s population mainly lives in rural areas (84 %) where access to healthcare is more complicated than in urban areas.

The country faces chronic problems with malaria (Fig. 1.4) which is endemic over 70 % of the country, and was once a scourge in areas below 1,500 m a.s.l. which represent only small part of the Addis Ababa sheet, particularly the deep valleys and gorges along the Abay and its main tributaries. The threat of malaria had declined considerably as a result of government efforts supported by the WHO and AID, but occasional seasonal outbreaks are common. UNICEF estimated that the number of malaria cases

Basic Characteristics of the Area 21 Tab. 1.1 Population in the study area (Part 1) Wereda area in Assessed Total population Region Zone Wereda mapped area population in mapped [km2] [%] area Addis Addis Ababa City Admin. 427 81.2 2,738,248 2,223,431 Ababa

Amahra East Gojam Gozamn 6 0.4 131,273 467

Amahra East Gojam Awabel 41 3.6 109,752 4,003

Amahra East Gojam Baso Liben 424 52.7 131,031 69,000

Amahra East Gojam Aneded 99 30.2 89,418 26,961

Amahra North Shewa Merhabete 188 19.1 113,361 21,628 Population, Settlements and Health Status Population,

Basic Characteristics of the Area Amahra North Shewa Moretina Jiru 35 5.0 83,889 4,173

Amahra North Shewa Saya Debir Wayu 6 0.9 55,522 508

Amahra North Shewa Ensaro 375 93.9 55,984 52,584

Oromia North Shewa Were Jaso 749 67.3 135,768 91,333

Oromia North Shewa Hidabu Abote 169 49.3 77,229 38,088

Horo Gudru Oromia Gudru 422 34.5 92,495 31,902 Wellega

Horo Gudru Hababo Oromia 606 57.6 42,495 24,470 Wellega Gudru

Oromia North Shewa Degem 525 97.4 92,144 89,735

Oromia North Shewa Girar Jarso 411 99.4 67,298 66,913

Oromia North Shewa Kuyu (12) 1,175 100.0 103,065 103,053

Oromia North Shewa Yaya Gulete 431 100.0 52,355 52,341

Oromia North Shewa Wuchale 712 81.0 91,050 73,784

Oromia North Shewa Sululta 896 100.0 116,870 116,868

Oromia North Shewa Bereh 424 58.2 81,205 47,221

Oromia North Shewa Jido 71 21.4 53,769 11,530

Oromia North Shewa Mulo 353 100.0 32,835 32,829

Oromia North Shewa Debere Libanos 319 100.0 36,216 36,213

Oromia West Shewa Ginde Beret 1,046 100.0 88,894 88,891

Oromia West Shewa Meta Robi 817 100.0 136,561 136,546

Oromia West Shewa Jeldu 1,640 100.0 188,278 188,204

Oromia West Shewa Ambo Zuria 754 76.5 109,468 83,722

22 Basic Characteristics of the Area Tab. 1.1 Population in the study area (Part 2) Wereda area in Assessed Total population Region Zone Wereda mapped area population in mapped 2 [km ] [%] area Oromia West Shewa Dendi 653 66.2 143,435 94,910

Oromia West Shewa Elfata 293 99.9 55,468 55,409

Oromia West Shewa Midakegn 768 74.0 76,912 56,902

Oromia West Shewa Toke Kutayu 172 25.7 104,003 26,758

Abuna Oromia West Shewa 902 99.9 106,844 106,790 Gendeberet

Oromia East Shewa Akaki 44 6.9 71,160 4,945

South West Population, Settlements and Health Status Settlements and Health Status Population,

Oromia Sebeta Hawas 87 6.9 127,173 8,829 Basic Characteristics of the Area Shewa South West Oromia Dawo 2 0.3 82,925 285 Shewa

Total 18,202 4,304,735 Source: Central Statistics Authority Statistical Abstract (2007) per year is about 9 million and the number of extra cases in an epidemic year is about 6 million. The occurrence of outbreaks is largely a result of heavy rain, unusually high temperatures, and the settling of peasants in new lowland locations. An example of the different diseases in Ethiopia is shown in Tab. 1.2.

Access to safe drinking water is limited and some statistics suggest that only 15 % of rural inhabitants have access to safe drinking water. The WHO (2006) statistics show that 31 % of the rural population has sustainable access to improved drinking water sources (96 % of the urban population). This low number is alarming because 70 % of contagious diseases are caused by contaminated water. This is a serious problem for Ethiopia in the effort to establish a strong agricultural community that will be able

Tab. 1.2 Mortal diseases in Ethiopia (WHO, 2006) Type of disease Total inhabitants [%] Children under 5

Respiratory 12 22

HIV/AIDS 12 4

Prenatal / neonatal 830

Diarrheal 617

Tuberculosis 4

Measles 44

Cardio-vascular 3

Ischemic heart diseases 3

Malaria / injuries 32

Syphilis / others 214

Basic Characteristics of the Area 23 Population, Settlements and Health Status Settlements and Health Status Population, Basic Characteristics of the Area

Fig. 1.4 Malaria risk in Ethiopia

to safeguard the supply of food for the whole country. One of the priorities of government policy is therefore to provide safe drinking water to rural communities.

The supply of safe water is not equal in all of the Zones of the region. The total number of facilities and the number of inhabitants (excluding area under Addis Ababa City Administration) for a single facility are shown in Tab. 1.3. Preliminarily results of the population and housing census of 2007 show that a particularly ponds serving for water supply can provide an adequate volume of water, but do not follow the requirements for safe water supply to inhabitants.

The leading causes of hospital and health center morbidity in 2008/2009 in the Oromia region are shown in Tab. 1.4a and Amhara region in Tab. 1.4b.

Conclusions of a review made by the Ethiopian Health Sector Development Program (HSDP, 2008) show that despite the significant rise in access to water and improved sanitation, there is no

24 Basic Characteristics of the Area Tab. 1.3 Rural water facilities by Zones and Weredas (Part 1) Zone / Wereda Number of facilities Number of inhabitants per facility East Gojam / Gozamn 29 bono, 4 ponds, 6 tankers 3,366

Awabel 32 bono 3,429

Baso Liben 20 bono, 5 ponds, 1 tanker 5,040

Aneded 26 bono 2,991

North Shewa / Merhabete 1 bono, 7 springs 6,998

Moretina Jiru 7 bono, 1 tanker, 18 springs 3,227

Saya Debir Wayu 6 bono 6,018

Ensaro 1 bono, 7 springs 6,996

Were Jaso 81 bono, 5 tankers, 4 ponds 1,509 Population, Settlements and Health Status Settlements and Health Status Population, Basic Characteristics of the Area Hidabu Abote 15 bono, 3 ponds, 8 tankers 1,980

Horo Gudru Wellega / Gudru 1 bono 92,495

Hababo Gudru 3 bono, 5 tankers 8,578

North Shewa / Degem 11 bono, 25 ponds 2,490

Girar Jarso 24 bono, 4 sprins 2,404

Kuyu 2 bono, 1 pond, 1 tanker 25,766

Yaya Gulete 7 bono 7,479

Wuchale 8 tankers 11,381

Sululta 13 bono, 5 ponds, 1 tanker 6,151

Bereh 7 bono 11,601

Jido 6 bono 8,962

Mulo 2 bono 16,418

Debere Libanos 7 bono 5,174

West Shewa / Ginde Beret 5 ponds 17,779

Meta Robi 35 bono, 2 tankers 3,691

Ada Berga 4 bono, 3 ponds 14,835

Jeldu 7 bono, 1 tanker 23,536

Ambo Zuria 2 bono, 2 tankers 27,367

Wolmera 9 bono 8,937

Ejere 1 bono 78,795

Dendi 44 bono, 1 tanker 3,187

Elfata No facility 55,468

Midakegn 2 tankers 38,456

Basic Characteristics of the Area 25 Tab. 1.3 Rural water facilities by Zones and Weredas (Part 2) Zone / Wereda Number of facilities Number of inhabitants per facility

Toke Kutayu 14 bono, 4 tankers 5,778

Abuna Gendeberet 3 bono, 1 tanker, 2 ponds 17,807

East Shewa / Akaki 3 tankers, 72 pomds 949

South West Shewa / Dawo 14 bono, 1 well, 2 ponds 4,878

3 bono, 75 ponds, 2 tankers, Sebeta Hawas 1,413 10 springs

data on rates of usage of these services. Ethiopia still suffers from a heavy disease burden that is directly related to poor hygiene practices and sanitation services. Each year, the average Ethiopian child has five to twelve diarrhea episodes and diarrheal illnesses kill between 50,000 to 112,000 Population, Settlements and Health Status Population,

Basic Characteristics of the Area children each year. Women and girls are most affected by inadequate sanitation services as they are forced to spend more time fetching water and caring for the sick than participating in income- generating activities or attending school.

During the last few years, there has been an increased level of political commitment to hygiene and environmental health services in Ethiopia leading to the Ministry of Health defining a Hygiene and Environmental Health Program (www.moh.gov.et). The program is based on key policies such as the National Sanitation Strategy and Protocol and the Millennium Sanitation Movement has established a framework that serves to motivate and align relevant actors to speed up sanitation coverage and hygiene behavioral change. In addition, three key ministries – Health, Water

Tab. 1.4a Leading causes of hospital and health center morbidity 2008/2009 in Oromia Region Rank Diagnosis No. of all cases % of all cases

1 Acute upper respiratory tract infection 103,154 5.93

2 Other helminthes 106,620 4.81

3 Other unspecified malaria 103,154 4.65

4 Gastritis and duodenitis 102,252 4.61

Homicide and injury purposely 5 100,161 4.52 inflicted by another person (not in war)

6 All other diseases of gento – urinal system 82,493 3.72

7 Bronchopneumonia 78,189 3.53

8 Infection of skin and subcutaneous tissue 72,881 3.29

Muscular rheumatism and rheumatism 9 72,868 3.29 unspecified

10 Pyrexia of unknown origin 70,226 3.17

Total of leading diseases 920,411 41.50

Total of other diseases 1,297,453 58.50

Total of all diseases 2,217,864 100.00 Source: Oromia Regional Health Bureau

26 Basic Characteristics of the Area Tab. 1.4b Leading causes of hospital and health center morbidity 2008/2009 in Amhara Region Rank Diagnosis No. of all cases % of all cases

1 All forms of malaria 317,213 19.08

2 Intestinal parasites 171,584 10.32

3 Diarrhea 151,166 9.09

4 All other respiratory diseases 120,016 7.22

5 Gastritis 108,643 6.53

6 Rheumatism 83,912 5.05

7 Skin lesions 83,224 5.00

8 Upper respiratory tract infection 79,752 4.80 Population, Settlements and Health Status Population, Basic Characteristics of the Area 9 Eye diseases 77,857 4.68

10 Fever of unknown origin 59,263 3.56

Total of leading diseases 1,252,630 75.32

Total of other diseases 410,340 24.68

Total of all diseases 1,662,970 100.00 Source: Amhara Regional Health Bureau

Resources and Education – have joined to launch the National WASH program, which provides a strategic framework for achieving a national vision for universal access to hygiene sanitation.

The Ministry of Health has defined the following objectives of the program: • Increase sanitation measures including latrine coverage and ensure facilities are properly handled, sustained and utilized. • Promote communal solid waste disposal sites, including improvement of medical and other waste management systems in public and private health institutions. • Increase drinking water quality monitoring; and monitor food safety and food processing industries.

Health Extension Workers (HWEs) play a significant role in carrying out the key activities of the program throughout communities. HEWs promote personal and environmental hygiene and provide support to the community; increase community awareness and involvement in safe water supply and prevention of water contamination; promote behavioral change to improve food safety and control vector born diseases; build a Healthy House Model and work with the relevant institutions to ensure irrigation development projects and water conservation schemes.

Improving safe water supply to people living in the mapped area basin contributes to an improvement in their health which is one of the fundamental problems for the creation of strong pastoral and farm communities capable of full time engagement in agricultural activity.

1.3 Land Use

Poor land use practices, improper management systems and lack of appropriate soil conservation Land Use measures have played a major role in causing land degradation problems in the country. Because

Basic Characteristics of the Area 27 Land Use Basic Characteristics of the Area

Fig. 1.5 Land use

of the rugged terrain, the rates of soil erosion and land degradation in Ethiopia are high. Setegn (2010) mentions the soil depth of more than 34 % of the land area is already less than 35 cm, indicating that Ethiopia loses a large volume of fertile soil every year and the degradation of land through soil erosion is increasing at a high rate. The highlands are now so seriously eroded that they will no longer be economically productive in the foreseeable future.

The land and water resources are in danger due to the rapid growth of the population, deforestation and overgrazing, soil erosion, sediment deposition, storage capacity reduction, drainage and water logging, flooding, and pollutant transport. In recent years, there has been an increased concern

over climate change caused by increasing concentrations of CO2 and other trace gases in the atmosphere. A major effect of climate change is alterations in the hydrologic cycles and changes in water availability. Increased evaporation combined with changes in precipitation characteristics has the potential to affect runoff, frequency and intensity of floods and droughts, soil moisture, and water supplies for irrigation and generation of hydroelectric power.

About 60 % of the Addis Ababa sheet area is made up of intensively and moderately cultivated land (Fig. 1.5).

28 Basic Characteristics of the Area 2. Selected Physical and 2. Geographical Settings

The area is predominantly composed of plain and hill domes with summits greater than 3,000 m a.s.l. but also deep gorges with altitudes of about 700 m a.s.l (Fig. 2.1). The physical- geographical variation also reflects variation in soil, vegetation, climate, hydrology, hydrogeology, engineering geology characteristics as well as in population and settlement development.

2.1 Geomorphology

The geomorphology of the area is related to tectonic events followed by erosion. The northern part Geomorphology of the area is located on the eastern margin of the Abay (Blue Nile) basin and is typically characterized by deep cut gorges with sharp escarpments, flat top hills, gently sloping and undulating plains; whereas the southern part is associated with the development of the Main Ethiopian Rift system and recent erosion and sedimentation processes related to river development. There is a high variation in Selected Physical and Geographical Settings

Fig. 2.1 Generalized physiographic units

Selected Physical and Geographical Settings 29 altitude within the study area ranging from 1,000– 3,462 m a.s.l. The area can be divided into three geomorphic zones based on variation in elevation, morphological setting and topographic setting: • Deep gorges of rivers with steep slopes and cliffs • Central plateau with gentle slopes • Separated mountain hills and ridges

Blue Nile–Jemma–Mugher–Guder river gorges This physiographic zone covers the northeastern and northwestern and western parts of the map

Geomorphology area. It is characterized by the development of deep gorges with steep slopes and escarpments of the Abay, Jemma, Mugher, Guder and Zega Wedem (Robi Gemero) rivers. The elevation ranges from 1,000–2,500 m a.s.l. The lowest altitude of this zone is recorded inside the Abay river gorge in the northwestern part of the area which is less than 1,000 m a.s.l. This zone is composed of a Precambrian basement, Paleozoic and Mesozoic sediments and Tertiary volcanic rocks in which especially basalt with hexagonal columnar joints form steep escarpments and cliffs. In the Muger and Guder river gorges, Mesozoic limestone and sandstone also form steep escarpment and cliffs often with waterfalls. The drainage system is dendritic with a number of perennial and intermittent rivers.

The Central Plateau and Gentle Slope This physiographic zone covers wide area of the map. It is characterized by a flat plain with gently slopes and undulating terrain build by Tertiary volcanic rocks (basalt and ignimbrite) and covered by eluvia. The altitude ranges from 2,500–2,800 m a.s.l. The region is desiccated by

Selected Physical and Geographical Settings Selected Physical and Geographical Settings various big rivers and small streams such as Awash, Holeta, Duber, Aleltiu, Legedadi, Sokoro. In this zone there are numerous numbers of lineaments and most of them have NE-SW alignment. The drainage pattern shows parallel to dendritic pattern. The most of the rivers in this zone flow to the Abay basin and the rest, in the south, flow to the Awash basin.

Entoto–Chelelka–Gura–Guda–Wechecha–Chilmo mountains This zone is localized in the southeastern, southwestern and northern parts of the study area at Entoto, Chelelka, Gura Guda (near Fiche), Wechecha and Chilmo (north of Ginchi) areas. This region forms E-W trending ridges and mountains peaks of tertiary volcanic rocks (basalt, trachyte, trachybasalt, rhyolite and ignimbrite). The altitude of this zone reaches up to 3,400 m a.s.l. on Cheleleka and Entoto ridges. The maximum altitude is 3,571 m a.s.l at Mt. Cheleleka. This zone forms the Entoto and Chilmo mountain ridges, which in turn form the surface water divide between the Blue Nile and Awash river basins.

2.2 Soil and Vegetation Cover Soil and vegetation cover reflects the basic climatic condition of the area as well as the regional and site specific geological, geomorphological and erosion characteristics.

Soil The development of soils is mainly dependent on the type of rock from which they are derived and the condition of the depositions directed by climate and geomorphological position. The highlands are dominated by shallow black to gray silty soil derived from the volcanic rock. The river valleys and their main tributaries are covered by silty to sandy soil and alluvial depositions.

Soil and Vegetation Cover Soil and Vegetation According to the soil map provided by the Ministry of Agriculture, the study area is mainly covered by three main soil types (Fig. 2.2) black cotton soil (vertisols), brown soil (rendzinas), and lateritic soil (cambisols). The Addis Ababa map sheet is covered by 8 soil types in total.

Cambisols have limited agricultural value as they occur dominantly on slopes, are often shallow or have many stones or rock outcrops. Where cambisols are deep and not stony they are good for agriculture but available P content can be low. Cambisols have a strong brown or red color.

30 Selected Physical and Geographical Settings Soil and Vegetation Cover Soil and Vegetation Selected Physical and Geographical Settings

Fig. 2.2 Distribution of soil types

Rendzinas This soil type is dark, grayish-brown and humus rich. It is one of the soils most closely associated with the bedrock type and an example of the initial stages of soil development. The soil of this type contains a significant amount of gravel and stones. It is usually developed beneath grassland formed by the weathering of soft rock types: usually carbonate rocks (dolomite, limestone, marl, chalk) but occasionally sulfate rocks (gypsum).

Phaeozems Phaeozems are characterized by a humus-rich surface layer covered in the natural state with abundant grass or forest vegetation. They are highly arable soils and are used for growing wheat, tef, and pasture for cattle, as well as for wood production. Phaeozems have a high content of available calcium ions bound to soil particles, resulting in a very permeable, well-aggregated structure. These soils occur in association with Chernozems but under more humid climatic conditions (more than 550 mm of rainfall per year), which results in the absence of calcium carbonate or salt accumulation in subsurface layers. They may exhibit a layer of clay accumulation, however.

Lithosols Lithosols are mineral soils less than 10 cm thick, developed over hard rock. These soils have no agricultural value. They are often referred to as ”skeletal soils“ because of their extreme shallowness and steepness and consequently their high erosion hazard.

Selected Physical and Geographical Settings 31 Luvisols Luvisols have a distinct clay accumulation horizon. Most Luvisols are well-drained but Luvisols in depression areas with shallow groundwater may develop gleyic soil properties in and below the argic horizon. Stagnant properties are found where a dense illuvial horizon obstructs downward percolation and the surface soil becomes saturated with water for extended periods of time.

Nitosols Nitosols are the most inherently fertile of the tropical soils because of their high nutrient content and deep permeable structure. They can be exploited for plantation agriculture.

Arenosols are soils formed from coarse textured unconsolidated material. Arenosols are

Soil and Vegetation Cover Soil and Vegetation excessively drained, moderately deep, coarse textured sandy clay loam to sandy loam soil derived either from alluvial/colluvial or sandstone parent material. The Cambic Arenosols usually cover mountain areas. These soils are very permeable with no natural fertility.

Vertisols are heavy clay soils in flat areas that have a pronounced dry season during which they shrink and have large deep cracks in a polygonal pattern. During the wet season the clay swells and causes pressure in the subsoil. Vertisols have a fairly good but limited agricultural potential because the land is rather difficult to prepare. Dry soils are hard and wet soils are sticky. There is only a short period when moisture conditions of the surface layer are favorable to prepare land. Another difficulty is that the drainage of the subsoil is very low, because of the swelling clay. Very Selected Physical and Geographical Settings Selected Physical and Geographical Settings often the soils are flooded or have stagnant water during the wet season. The organic matter content in vertisols is often not more than 1 %. The soil has high water retention, but a relatively small amount of water is available for plant growth. Rooting might be restricted because of the swelling and shrinking properties of the soil.

Vegetation The main vegetation types of the area include eucalyptus trees, junipers, acacia, olive trees, bisana, shrubs and many undifferentiated ever green plants. Dense vegetation cover is observed at Mt. Entoto, Ankorcha, Chelelka, Menagesha forest, Chilmo and river gorges of Abay, Mugher, Zega Wedem and Duber.

The forest of hills and mountains are dominated by eucalyptus trees and some no-indigenous juniper trees. In Zega Wedem gorge around Debre Libanose and Duber river valleys and Chilmo Mountain there are naturally growing indigenous trees and plants. The rest of the plateau area is characterized by poor vegetation cover except with sparsely spaced shrubs and eucalyptus trees. The deforestation of the area increased from year to year.

2.3 Climatic Characteristics The area is mainly characterized by a wet climate in which the rainy season passes from July to September. The mean annual rainfall is between 800 mm in the lowlands and 2,000 mm in the highlands. The mean monthly maximum temperature is 32 °C and the mean monthly minimum temperature is 5.4 °C based on temperature – elevation relationship for the area.

2.3.1 Climatic Zones and Measurements The climatic conditions of Ethiopia are mostly dominated by altitude. According to Daniel Gamatchu (1977) there are wide varieties in climatic zones. Climatic zones defined by Javier

Climatic Characteristics Gozálbez and Dulce Cebrián (2006) and Tesfaye Chernet (1993) are shown in Tab. 2.1.

A climatic zoning map (Fig. 2.3) has been compiled based on the climatic region classification given in Tab. 2.1 and the elevation of the study area. Only a negligible area of 1 % lies in the Wurch

32 Selected Physical and Geographical Settings Tab. 2.1 Ethiopian climate classification Name / Altitude Precipitation Precipitation Precipitation / Mean annual between 900 and below 900 mm above 1,400 mm temperature 1,400 mm

Afro-alpine High Wurch (Kur) meadows of grazing land above 3,700 m and steppes, no farming below 5 oC Helichrysum, Lobelia

Wurch (Kur) Sub-afroalpine Sub-afroalpine

3,700–3,200 m barley barley Climatic Characteristics 5–10 oC Erica, Hypericum Erica, Hypericum

Afro-mountain (tem- Afro-mountain (temperate) perate) Dega bamboo forest forest – woodland 3,200–2,300 m barley, wheat, nug, pulses barley, wheat, pulses 10–15 oC Juniperus, Hagenia, Podo- Juniperus, Hagenia, carpu, bamboo Podocarpus

Shrub-savannah Wooded savannah Savannah

Weina Dega (sub-tropical) (sub-tropical) Selected Physical and Geographical Settings (sub-tropical) 2,300–1,500 m corn, sorghum, teff, en- corn, teff, nug, enset, barley wheat, teff, some corn 15–20 oC set, nug, wheat, barley Acacia, Cordia, Ficus, acacia savannah Acacia, Cordia, Ficus bamboo

Tropical Wet tropical Kolla Tropical sorghum, teff, nug, pe- mango, sugar cane, corn, 1,500–500 m sorghum and teff anuts coffee, oranges above 30 oC acacia bushes Acacia, Cordia, Ficus Cyathea, Albizia

Semi-desert and desert Bereha crops only with irrigation below 500 m thorny acacias, Commi- above 40 oC phora

Remark: after Javier Gozáblez and Dulce Cebrián (2006), Tesfaye Chernet (1993)

(sub-afroalpine) zone and covers the summit of the Cheleleka and Mute Kerensa Mountains, 50 % of the area lies in the Dega (temperate) region which covers a large part of the eastern and central part of the area, 49 % in the Weina Dega and Kolla (sub-tropical and tropical) region and covers a large part of the northern and northwestern part of the area along valleys of the main rivers of the sheet.

The outstanding modern quantitative climatic classification of Koeppen (1989) defines the climatic types according to the values of temperature and precipitation regardless of the geographic location of the region. Criteria for classification of the principal climatic types in a modified Koppen system are based on the mean annual and mean monthly precipitation and temperature values. The actual application of the Koeppen system to climatological statistics shows that the Ethiopian climate is grouped into three main categories, each divided into three or more types making a total of 11 principal climatic types.

The highlands of the Central plateau belong to the Cwb zone – characterized by a warm temperate rainy climate with dry winter. The mean temperature of the coldest month is below 18 °C and for

Selected Physical and Geographical Settings 33 Climatic Characteristics Selected Physical and Geographical Settings Selected Physical and Geographical Settings

Fig. 2.3 Climatic zones

more than four months it is above 10 °C. Precipitation during the driest winter months is less than one tenth of the wettest summer months. The volume and distribution of precipitation varies considerably from one area to the next such that the lowest is about 600 mm and the highest is about 2,000 mm. Forests are predominant in areas of heavy precipitation, while savannah grass is present in areas with moderate precipitation, which prevails at elevations from 1,750 to 3,200 m a.s.l.

The rest of the area represented by lowlands along the Abay and the lower reaches of its main tributaries belongs to Aw zone – characterized tropical climate with distinct dry winter where the mean temperature of the coldest month is above 18 °C and the mean annual rainfall is 680– 1,200 mm. This type of climate prevails up to and elevation of 1,750 m a.s.l. The length of dry and wet periods varies considerably from the western part to the northern and eastern parts of the country. This climate is characterized by intermingled tall grass and trees.

There are 17 meteorological stations operated by the Meteorological Institute within the mapped area (Tab. 2.2). The stations located in Fiche and Addis Ababa provided basic meteorological characteristics of the area.

The temperature is variable throughout the entire area. Even within the same area it shows seasonal variation. The maximum temperature is expected to be between March and May and the minimum between October and January. The mean annual temperature recorded at Addis Ababa and Fiche stations ranges from a maximum of 18.7 °C to a minimum of 13.0 °C. The variation is expected to be higher than this (Tab. 2.3 and Fig. 2.4). In the mountains the temperatures are lower whereas in the north lowland part of the area temperatures are relatively high.

34 Selected Physical and Geographical Settings Tab. 2.2 Climatic station of the Addis Ababa and Fiche area ID Altitude Assessed Acquired Station name Class X UTM Y UTM map [m a.s.l.] period parameters

MS1 Addis Alem 3 432246.9 1000109.0 2,400 1999–2008 P, T

Addis Ababa – MS2 1 479118.7 995316.5 2,354 1999–2009 P, T, H, E Bole Addis Ababa – MS3 3 474772.4 997517.1 2,408 1999–2009 P, T Observation st.

MS9 Gojo 4 398842.9 1024279.0 2,884 T Climatic Characteristics

MS4 Fiche 1 471192.6 1082582.0 2,793 1954–2005 P, T

MS5 Debre Tsige 4 480916.0 1064744.0 2,700 1989–2005 P, T

MS6 Sululta 1 473558.0 1014759.0 2,599 1975–2005 P, T

MS7 Lemi 3 489910.3 1082770.0 2,500 1973–2005 P, T, H, E

MS8 Chancho 3 472358.0 1028435.0 2,587 1954–2005 P,T

MS10 Derba 4 461772.1 1042712.0 2,350 1999–2009 P Selected Physical and Geographical Settings

MS11 Inchini 4 431466.7 1029676.0 2,598 1999–2009 P

MS14 Muger 4 454570.0 1048968.0 1,513 1999–2009 P

Ms15 Kachisi 4 372484.6 1061259.0 2,565 1999–2008 P

MS16 Ginchi 3 405728.0 998431.3 2,205 1999–2009 P, T

MS17 Fital 4 460909.3 1063113.0 2,600 1999–2009 P

MS18 Ejere 3 447375.7 1095741.0 2,100 1999–2009 P, T

MS19 Muka Ture 4 486031.8 1056448.0 1,979 1970–2005 P

Due to the high altitude particularly in the northwestern part of the area temperatures are not high during the Belg season. On the other hand, most of the country´s extreme minimum temperature values were recorded during the Bega dry season. Climate records show the temperature values below freezing point can occur in December and January.

Relative Humidity The relative humidity of the area determines the rate of evaporation. The mean relative humidity of the area recorded at the Addis Ababa (Akaki) station is 58.5 % and reaches its peak in the rainy season from July to September, while the minimum relative humidity is registered in March and December (Fig. 2.5). The annual range of relative humidity is about 40 %.

Wind Speed One of the factors of evaporation is wind speed. Any decrease in wind speed, resulting in the non removal of saturated vapor, affects the evaporation rate. The wind speed at the Addis Ababa meteo-station reaches its peak in October and the wind is relatively calm in the month of July and August (see Fig. 2.5).

Selected Physical and Geographical Settings 35 Tab. 2.3 Temperature variation in Addis Ababa and Fiche meteo-stations Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

A.A. Bole-max (1999-2009) 24.1 25.5 25.9 25.8 25.4 23.3 21.4 21.0 22.0 23.5 23.2 23.2

A.A. Bole-min (1999-2009) 8.2 8.8 10.6 11.6 11.6 11.0 11.4 11.4 10.6 8.9 7.1 7.2

A.A. Bole-av (1999-2009) 16.1 17.1 18.2 18.7 18.5 17.1 16.4 16.2 16.3 16.2 15.1 15.2

Fiche-max (1973-2005) 20.2 21.2 21.6 22.0 22.4 21.5 18.8 18.0 18.8 19.2 19.6 19.9

Fiche-min (1973-2005) 6.8 7.8 8.7 8.9 9.2 8.8 8.4 8.3 8.3 6.3 5.6 6.1 Climatic Characteristics

Fiche-av (1973-2005) 13.5 14.5 15.1 15.4 15.8 15.1 13.6 13.1 13.5 12.7 12.6 13.0

30

25 AA-min

Selected Physical and Geographical Settings Selected Physical and Geographical Settings 20 AA-max C] o 15

t [ AA-av 10 Fitche-min Fitche-max 5 Fitche-av 0 123456789101112 month

Fig. 2.4 Temperature at Addis Ababa and Fiche meteo-stations

Sunshine Hours and Radiation The total daily evapotranspiration rate is dependent on the total daily sunshine hours. It can be noted from Fig. 2.6 that the mean maximum number of sunshine hours is recorded in November and December whilst the mean minimum is registered in July and August.

Evapotranspiration The mean evapotranspiration rate in the month of February is relatively high compared to the other months (Fig. 2.7). It can be noted from Fig. 2.5 that the wind speed is also high in this month illustrating the fact that wind is an important factor for the evapotranspiration rate. The mean minimum evapotranspiration rate in July and August coincides with the minimum mean wind speed and sunshine hours during this period. Annual evapotranspiration at Addis Abba meteo- station is 1,358 mm and at Fiche meteo-station is 1,438 mm.

The Piche evaporimeter is an atmometer and it measures the amount of water lost to the atmosphere usually over a period of one day. Results of Piche evaporation measurements are shown in Tab. 2.3 and Fig. 2.7.

36 Selected Physical and Geographical Settings 160 140 120

100 R_humidity 80 Wind

ve humidity [%] 60 Ɵ Climatic Characteristics wind speed [km/d] speed wind

rela 40 20 0 123456789101112 month

Fig. 2.5 Mean monthly relative humidity [%] and wind speed [km/d] at Addis Ababa meteo-station Selected Physical and Geographical Settings 25

20 /d] 2 15 Sunshine

on [MJ/m RadiaƟon

Ɵ 10 sunshine [h] sunshine radia 5

0 123456789101112 month

Fig. 2.6 Mean monthly number of sunshine hours and radiation [MJ/m2/d] at Addis Ababa meteo-station

Tab. 2.3 Monthly total Piche evaporation [mm] at Addis Ababa–Bole meteo-station (Part 1) Year/ Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

1999 195.2 270.1 166.5 223.4 209.8 128.9 71.0 80.4 176.7 83.1 163.7

2000 209.0 223.4 247.1 202.4 165.9 108.9 64.7 66.5 70.4 86.8 135.8 172.3

2001 116.1 93.1 90.9 139.2 93.6 64.1 57.2 55.5 74.7 117.6 147.4

2002 108.5 145.6 141.9 185.8 181.3 108.9 59.2 53.9 88.2 187.7 176.5 126.8

Selected Physical and Geographical Settings 37 Tab. 2.3 Monthly total Piche evaporation [mm] at Addis Ababa Bole meteo-station (Part 2) Year/ Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

2003 121.4 98.6 137.0 233.7 124.2 61.2 68.5 83.0 235.1 223.5 162.4

2004 147.6 200.5 221.9 133.2 226.1 106.3 77.3 64.8 85.2 145.6 166.6 143.5

2005 138.7 199.4 170.3 164.7 116.3 94.6 58.8 66.3 74.6 180.4 289.7

Aver. 130.7 179.1 162.5 169.4 175.2 105.1 64.2 65.1 93.3 148.0 121.4 151.2 Climatic Characteristics

Evapotranspiration is the combination of soil evaporation and vegetation transpiration and has a considerable impact on the water balance of the study area. The Piche evaporation (Ev) is shown together with the potential evapotranspiration (ETo) in Fig. 2.7.

200 180 Ev 160 ETo

Selected Physical and Geographical Settings Selected Physical and Geographical Settings 140 120 100 Ev [mm] Ev

ETo [mm] ETo 80 60 40 20 0 123456789101112 month

Fig. 2.7 Mean monthly Piche evaporation (Ev) [mm] and potential evapotranspiration (ETo) [mm] at Addis Ababa – Bole meteo-station

2.3.2 Precipitation The Ethiopian territory is divided into four zones marked as A, B, C, and D, each of them with different precipitation patterns. The seasonal classification and precipitation regimes of Ethiopia (after NMSA, 1996) are characterized in Tab. 2.4 and shown on Fig. 2.8.

The mapped area belongs mainly to region A which is characterized by three distinct seasons, and by bimodal precipitation patterns with small peaks in April and the main rainy season during mid June to mid September with peaks in July. Only a small part on the western border of the sheet belongs to region B (b2). This region is characterized by two distinct seasons, one being wet and the other dry, with the wet season during April/May to October/November. In general, the annual rainfall depends on the regional altitude variation of the area and precipitation increases from east to west. The more elevated areas of the Addis Ababa sheet at the east have precipitation about 1,100 mm/year and about 2,000 mm/year in areas on the western part of the sheet.

38 Selected Physical and Geographical Settings Tab. 2.4 Characterization of the precipitation pattern in Ethiopia Zone Precipitation pattern

This region mainly covers the central and central eastern part of the country. It is characterized by three distinct seasons, and by bimodal precipitation patterns with A small peaks in April and the main rainy season during mid June to mid September with peaks in July.

This region covers the western part of the country. It is characterized by a single pre- cipitation peak. Two distinct seasons, one being wet and the other dry, are encoun- tered in this region. The analysis of mean monthly precipitation patterns shows that B this zone can be split into southwestern (b1) with the wet season during February/ Climatic Characteristics March to October/November, western (b2) with the wet season during April/May to October/November, and northwestern (b3) with the wet season during June to September parts.

This region mainly covers the southern and southeastern parts of the country. It has C two distinct precipitation peaks with a dry season between. The first wet season is from March to May and the second is from September to November.

The Red Sea region in the extreme northeastern part of the country receives diffused D precipitation with no distinct pattern; however precipitation occurs mainly during the winter. Selected Physical and Geographical Settings

Tab. 2.5 Monthly long-term average precipitation at Addis Ababa, Fiche meteo-stations [mm] Stat/ Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Addis 15.40 14.00 55.50 79.10 72.90 121.90 240.90 248.50 119.20 33.40 6.80 6.60 Ababa

Fiche 17.80 33.30 63.40 71.90 47.80 78.20 329.90 315.50 110.40 17.70 5.50 6.50

Kachise 12.95 10.23 74.01 134.56 98.61 331.50 420.00 432.00 245.00 93.76 22.06 19.40

The basic characteristics of the stations are show in Tab. 2.2. Data from Fiche and Addis Ababa meteo-stations were used to demonstrate precipitation character of the area.

Mean monthly precipitation data from Addis Ababa meteo-station (Tab. 2.5) represents the typical precipitation pattern of the region A and a graphical presentation of precipitation pattern is shown in Fig. 2.9.

Years with a full set of data were extracted from the review of data from the period from 1999 to 2008. Long-term precipitation data is given in Tab. 2.6 and shown in Fig. 2.10. The long-term average annual precipitation at Addis Ababa (Bole) meteo-station is 1,018 mm/year for the assessed 10 years.

The graph in Fig. 2.10 shows relatively small fluctuations in precipitation at Addis Ababa meteo- station where differences in precipitation are about 20 % in some years.

Mean monthly precipitation data from Fiche meteo-station (Tab. 2.5) represents the typical precipitation pattern of the region A and a graphical presentation of precipitation pattern is shown in Fig. 2.11.

Selected Physical and Geographical Settings 39 Climatic Characteristics Selected Physical and Geographical Settings Selected Physical and Geographical Settings

Fig. 2.8 Seasonal classification and precipitation regimes of Ethiopia (source: NMSA, 1996)

40 Selected Physical and Geographical Settings 300

250

200

150 Climatic Characteristics 100

50 average precipitation [mm/month]

0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

month

Fig. 2.9 The Addis Ababa meteo-station precipitation pattern Selected Physical and Geographical Settings

Tab. 2.6 Long-term monthly rainfall at Addis Ababa [mm] (fully recorded years only) Year/ Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Month

1999 4.4 0.0 35.0 17.8 30.5 104.6 294.0 271.0 62.8 127.1 0.0 0.0 946.7

2000 0.0 0.0 17.6 175.6 95.2 102.1 193.0 222.0 158.0 19.6 7.5 0.0 989.9

2001 0.0 10.3 174.3 14.8 116.7 166.0 289.0 207.0 113.0 10.6 0.0 0.0 1,102.7

2002 30.6 25.9 79.4 36.6 49.6 115.5 214.0 234.0 72.6 0.5 0.0 32.8 891.0

2003 0.0 34.1 48.9 111.5 18.0 111.0 204.0 238.0 130.0 4.6 0.0 33.3 934.3

2004 26.1 11.7 32.4 104.2 7.0 114.5 241.0 230.0 122.0 50.0 0.6 0.0 939.3

2005 55.4 14.1 41.8 116.2 164.6 159.1 174.0 248.0 77.6 25.8 7.2 0.0 1,084.1

2006 2.0 36.6 107.8 93.9 37.8 115.1 313.0 331.0 133.0 35.9 0.0 0.0 1,205.9

2007 9.9 21.3 61.1 86.8 134.0 157.6 191.0 245.0 131.0 37.2 0.0 0.0 1,075.5

2008 0.0 0.0 0.0 34.0 75.3 73.1 295.0 259.0 193.0 22.2 53.1 0.0 1,004.6

Average 1,017.4

Years with a full set of data were extracted from the review of data from the period from 1984 to 2005. Long-term precipitation data is given in Tab. 2.7 and shown in Fig. 2.12. The long-term average annual precipitation from Fiche meteo-station is 1,102 mm/year for the assessed 20 years and its precipitation pattern is shown in Fig. 2.12. The year 1984 was very dry, however values presented in official records are so small that they also represent errors in the records.

Selected Physical and Geographical Settings 41 1400

1200

1000

800

600 Climatic Characteristics

400 precipitation [mm/year] precipitation 200

0 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 Avg year

Selected Physical and Geographical Settings Selected Physical and Geographical Settings Fig. 2.10 Long-term fluctuation and average of precipitation for the Addis Ababa meteo-station

The graph in Fig. 2.12 shows relatively small fluctuations in precipitation. Differences in precipitation can reach 30 % in some years.

Mean monthly precipitation data from Kachise meteo-station (Tab. 2.5) represents the typical precipitation pattern of the region A, however the station is located at the border between region A and b2. The graphical presentation of precipitation pattern is shown in Fig. 2.13.

350

300

250

200

150

100

average precipitation [mm/month] 50

0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec month

Fig. 2.11 The Fiche meteo-station precipitation pattern

42 Selected Physical and Geographical Settings Tab. 2.7 Long-term monthly rainfall at Fiche [mm] (fully recorded years only)

Year/ Jan Feb Mar Apr May Jun July Aug Sep Oct Nov Dec Total Month

1984 0.0 0.1 0.3 0.1 2.9 4.4 8.9 4.1 3.4 0.0 0.0 0.0 24.2

1986 12.2 75.6 19.5 109.3 113.2 57.5 234.1 367.8 72.0 24.0 20.0 3.0 1,108.2

1987 0.0 25.6 269.0 107.1 151.4 36 107.1 270.7 39.9 3.8 0.0 7.6 1,018.2

1989 19.4 45.2 66.8 117.7 35.4 45.6 260.0 386.0 93.9 33.6 2.0 30.4 1,136.0 Climatic Characteristics 1990 0.0 106.4 56.5 70.0 11.2 12.9 398.0 281.9 216.8 11.8 0.0 0.6 1,166.1

1991 21.6 53.8 80.5 1.9 32.1 94.7 256.7 365.7 143.4 8.5 0.0 1.4 1,060.3

1992 48.1 100.9 63.4 35.9 40.8 70.4 251.0 336.4 99.3 49.0 16.5 18.3 1,130.0

1993 21.4 57.1 20.8 133.0 94.6 78.2 377.5 285.9 123.0 14.4 0.0 0.0 1,205.9

1994 1.6 2.1 70.2 55.1 18.8 98.4 307.1 324.4 128.0 0.1 10.6 0.0 1,016.4

1995 0.0 52.3 45.2 123.2 53.0 52.6 329.2 331.7 130.1 0.0 0.0 35.5 1,152.8 Selected Physical and Geographical Settings

1996 29.5 11.5 102.3 79.8 80.1 209.6 393.0 397.9 198.9 7.2 18.1 1.8 1,529.7

1997 43.3 0.1 72.0 45.1 29.0 149.8 347.7 276.8 51.1 62.6 20.1 1.8 1,099.4

1998 11.2 19.0 49.1 58.3 45.8 87.8 266.9 383.8 181.1 40.5 0.2 0.0 1,143.7

1999 42.6 0.0 3.6 12.3 21.8 64.2 422.4 496.0 57.1 79.9 6.6 0.9 1,207.4

2000 0.0 0.0 24.0 88.3 35.5 52.5 442.7 381.8 91.7 10.9 8.3 4.3 1,140.0

2001 1.8 10.4 99.2 22.9 66.1 135.9 406.8 214.9 55.9 1.9 0.0 15.4 1,031.2

2002 41.5 16.8 138.8 41.9 23.6 70.3 230.8 313.0 141.2 0.0 0.0 0.0 1,017.9

2003 23.4 51.1 50.8 112.6 2.1 121.9 360.0 311.0 124.1 2.8 1.8 15.0 1,176.6

2004 0.9 11.7 38.4 123.7 27.2 151.6 296.6 299.2 129.5 14.9 3.7 0.0 1,097.4

2005 48.7 1.0 73.0 69.0 81.6 76.4 885.5 207.4 119.5 10.7 4.4 0.0 1,577.2

Average 1,101.9

Years with a full set of data were extracted from the review of data from the period from 1999 to 2008. Long-term precipitation data is given in Tab. 2.8 and shown in Fig. 2.14. The long-term average annual precipitation from Kachise meteo-station is 1,893.4 mm/yeara for the assessed 10 years and its precipitation pattern is shown in Fig. 2.14.

The graph in Fig. 2.14 shows relatively small fluctuations in precipitation. Differences in precipitation can reach 40 % in some years.

The adopted average precipitation for the Addis Ababa area is 1,250 mm.

Selected Physical and Geographical Settings 43 1800

1600

1400

1200

1000

800 Climatic Characteristics 600 precipitation [mm/year] 400

200

0

4 8 2 4 6 8 4 8 86 9 9 9 9 00 0 9 9 990 9 9 0 002 0 1 1 198 1 19 1 19 1 2 2 2 Avg year

Selected Physical and Geographical Settings Selected Physical and Geographical Settings Fig. 2.12 Long-term fluctuation and average of precipitation for the Fiche meteo-station

500 450 400 350 300 250 200 150 100 50 average precipitation [mm/month] precipitation average 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec month Fig. 2.13 The Kachise meteo-station precipitation pattern

Tab. 2.8 Long-term monthly rainfall at Kachise [mm] (fully recorded years only) (Part 1) Year/ Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Month

1999 11.1 0.0 0.9 38.5 112.0 333.9 390.0 556.0 224.0 271.0 8.4 6.8 1,952.7

2000 0.0 0.0 18.5 109.1 114.0 335.9 365.0 499.0 353.0 143.1 64.3 40.7 2,043.5

2001 0.0 7.0 92.7 61.7 127.4 342.1 430.0 396.0 147.0 92.7 9.6 17.5 1,723.3

44 Selected Physical and Geographical Settings Tab. 2.8 Long-term monthly rainfall at Kachise [mm] (fully recorded years only) (Part 2) Year/ Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Month

2002 63.4 6.6 93.6 52.3 34.3 387.5 214.0 333.0 156.0 0.9 0.0 74.9 1,416.6

2003 7.8 17.2 110.4 94.8 8.4 294.8 515.0 380.0 194.0 40.1 4.1 4.7 1,670.7

2004 9.5 1.1 135.3 108.0 39.0 247.8 396.0 496.0 218.0 87.1 8.7 12.0 1,758.1

2005 18.8 3.9 159.0 70.0 127.3 315.0 486.0 367.0 330.0 106.4 6.7 0.0 1,989.6 Climatic Characteristics 2006 7.2 27.8 100.7 685.9 121.3 305.7 485.0 423.0 242.0 74.9 43.2 22.7 2,539.0

2007 3.4 37.9 19.3 93.3 133.5 309.8 379.0 397.0 329.0 52.1 0.8 0.0 1,755.0

2008 8.3 0.8 9.7 32.0 168.9 442.3 536.0 473.0 255.0 69.3 74.8 14.7 2,085.1

Average 1,893.4

3000 Selected Physical and Geographical Settings

2500

2000

1500

1000 precipitation [mm/year] 500

0 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 Avg year

Fig. 2.14 Long-term fluctuation and average of precipitation for the Kachise meteo-station

2.4 Hydrography and Hydrology of the Area The Addis Ababa sheet is located mainly within the Abay river basin and a small southeastern part is located in the Awash river basin. The Jemma, Muger and Guder rivers are the principal left Hydrography and Hydrology of the Area tributaries of the Abay. Small rivers like the upper reaches of the Akaki, Kokebe, Holeta, Berga and Jemjem rivers form the most upper part of the Awash basin. The principal river basins of the area are shown in Fig. 2.15.

Records from all river gauging stations reflect the fact that the river discharge is directly proportional to the intensity of rainfall within the basin. There is a high discharge fluctuation between wet and dry seasons of the year. The highest flow period is from June to October and peak flow for all rivers is usually recorded in August (Kiremt rainy season). Discharge graphs also

Selected Physical and Geographical Settings 45 Hydrography and Hydrology of the Area Selected Physical and Geographical Settings Selected Physical and Geographical Settings

Fig. 2.15 The principal river basins of the area

show small increase of flow during small rainy season (Belg) in period from March to May. Fig. 2.16 shows a comparison of surface flow graphs of the Jemma, Muger and Abay from the Kessie stations. The river discharge is expressed as standardized variable values that enable a comparison of the flow trends of rivers with the different flow volumes.

3,5

3 SV Abay Kessie

2,5 SV Mugher

2 SV Jemma Nr. Lemi

1,5

1

0,5 standardised variable

0

-0,5

-1

month

Fig. 2.16 River flow in the Muger, Jemma and Abay in standard variables

46 Selected Physical and Geographical Settings 2.4.1 Surface Water Network Development The drainage pattern of the area is symmetric and in some parts dendritic with a moderate to high drainage density. Small rivers rise on the slopes or foothills along the watershed between the Awash basin and the Abay basin. Rivers in the Abay basin flow along the western plateau to the northwest. Rivers flow at the beginning in shallow valleys but then they start to erode deep valleys and dissect the plateau into deep river canyons. There are many waterfalls in the area.

Rivers in the Awash basin flow along the slopes of the Entoto to the south; they cross Addis Ababa and join the Awash River in the rift valley.

2.4.2 Surface Water Regime There are lots of river gauging stations within the Abay and Awash river basins. Some of them are operational but many stations have no data and/or data are fragmental. In the Addis Ababa sheet there are 14 river gauging stations with available data. Data on the river gauges representing the basic hydrological characteristics in Addis Ababa are summarized in Tab. 2.9. The station on the Hydrography and Hydrology of the Area Abay at Kessie (village) near the confluence of the Abay with the Jemma is used for comparison.

It is necessary to point out the fact that the flow measurements are mainly performed on small tributaries of the main rivers at gauging stations located on the plateau, while measurements of the flow of the main rivers Jemma, Mugher, Guder in deep parts of their valleys are very sporadic or not existing at all. The measured tributaries provide enough data on surface flow and Selected Physical and Geographical Settings baseflow on the plateau and can be used for accurate characterization of outcropping aquifers,

Tab. 2.9 Data river gauging stations (Part 1) Atitude Catchment No. River / Location X UTM Y UTM Starting date [m a.s.l.] area [km2]

Abay Basin (Abay River)

112001 Abbay / Kessie 410800 1223363 1,100 1/7/1953 65,784.0

Jemma Basin

112027 Aleltu / Muka Ture 494514 1066620 2,650 18/7/1983 447.0

112029 Robi Gumer / Lemi 500000 1077675 2,650 22/7/1983 887.0

112034 Jemma / Lemi 489037 1096102 1,330 28/2/1996 5,412.0

Jemma at new 442850 11110 4 2 1,16 0 N ot measured N.D. road * bridge

Muger Basin

112002 Mugher 411000 1077000 1,400 1/1/1959 489.0

11204 4 Gorfo / Gorfo 481701 1038986 2,600 10/6/1998 49.2

112012 Aleltu / Chancho 473800 1031800 2,650 1/1/1982 29.0

112013 Deneba / Chancho 471000 1028000 2,650 1/1/1993 86.0

112014 Sibilu / Chancho 472539 1020567 2,650 1/1/1981 375.0

112015 Roba / Chancho 474000 1022500 2,650 1/1/1981 (15.0)

Selected Physical and Geographical Settings 47 Tab. 2.9 Data river gauging stations (Part 2) Atitude Catchment No. River / Location X UTM Y UTM Starting date [m a.s.l.] area [km2]

Awash / Rift Valley

031033 Awash / Ginchi 404740 996718 2,195 1/5/1993 75.6

Berga / 031001 428556 996668 2,150 15/1/1975 248.0 Addis Alem

031002 Holeta / Holeta 446886 1004010 2,383 16/1/1975 119.0

Jemjem / 031031 419100 994843 2,150 1/6/1998 147.9 Wolonkomi

031021 T. Akake / AA 465908 999964 2,350 1/1/1981 131.0

Hydrography and Hydrology of the Area Remark: * Road connecting Gebre Guracha with Gundo Meskel

but characterization of deeper aquifers outcropping in deep valleys is more difficult. This fact is the main obstacle for a more precise assessment of groundwater resource on the Addis Ababa sheet.

Selected Physical and Geographical Settings Selected Physical and Geographical Settings Measured discharge of the Abay River at the Kessie river gauge between 1973 and 2004 is shown in Fig. 2.17. Fig. 2.18 shows that flow is highly variable and reflects years with high and low precipitation. The calculated mean annual flow of 531.09 m3/s represents flow generated mainly in the northern highlands where the Abay River rises (originates) and where precipitation is also highly variable. The lowest annual recorded mean occurred in 1984 and 1987 and corresponds with a period of drought. The highest mean annual flow of 1,052.22 m3/s was recorded in 2001. The Abay River represents the total regional drainage of the northern and central part of the western Ethiopian highlands.

Measured discharge of the Muger River at the Chancho river gauge between 1959 and 2004 is shown in Fig. 2.19. Fig. 2.20 shows that flow is highly variable and reflects years with high and low precipitation. The calculated mean annual flow of 7.89 m3/s represents flow generated mainly

16000

14000

12000 /s]

3 10000

8000

6000 discharge [m 4000

2000

0 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006 year

Fig. 2.17 The Abay river discharge at Kessie river gauge station

48 Selected Physical and Geographical Settings 1200

1000 /s] 3 800

600

400 average discharge [m

200 Hydrography and Hydrology of the Area

0

2 76 82 1972 1974 19 1978 1980 19 1984 1986 1988 1990 199 1994 1996 1998 2000 2002 Avg

year Selected Physical and Geographical Settings Fig. 2.18 Annual variability of the mean annual flow of Abay River at Kessie river gauge in the central part of highlands where the Muger River rises (originates) and where precipitation is also highly variable. The lowest annual recorded mean annual flow of 1.81 m3/s occurred in 1965 and corresponds with a period of drought. The highest mean annual flow of 12.1 m3/s was recorded in 1993. The Muger River is one of the main the left tributaries of the Abay River and represents the deep regional drainage of the central part of the western Ethiopian highlands.

Measured discharge of the Aleltu River at the Muka Ture river gauge between 1983 and 2004 is shown in Fig. 2.21. Fig. 2.22 shows that flow is highly variable and reflects years with high and

300

250

200 /s] 3

150

100 discharge [m

50

0 1956 1959 1962 1965 1968 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 year

Fig 2.19 The Muger river discharge at Chancho river gauge station

Selected Physical and Geographical Settings 49 14

12 /s]

3 10

8

6

4 average discharge [m Hydrography and Hydrology of the Area 2

0

4 68 82 02 990 992 Avg

Selected Physical and Geographical Settings Selected Physical and Geographical Settings 1964 1966 19 1970 1972 1974 1976 1978 1980 19 1984 1986 1988 1 1 199 1996 1998 2000 20 year

Fig. 2.20 Annual variability of the mean annual flow of Muger River at Chancho river gauge

low precipitation. The calculated mean annual flow of 4.38 m3/s represents flow generated mainly in the central part of highlands where the Aleltu River rises (originates) and where precipitation is also highly variable. The lowest annual recorded mean annual flow of 2.15 m3/s occurred in 1973 and corresponds with a period of drought. The highest mean annual flow of 7.13 m3/s was recorded in 1977. The Aleltu at Muka Ture is one of the left tributaries of the Jemma River and represents the local drainage of the central part of the western Ethiopian highlands. The other rivers in the upper reaches of the Jemma, Muger and Guder are intermittent and were not flowing during dry periods of the observed years.

180

160

140

120 /s] 3 100

80

60 discharge [m 40

20

0 1983 1986 1989 1992 1995 1998 2001 2004 2007

Fig. 2.21 The Aleltu river discharge at Muka Ture river gauge station

50 Selected Physical and Geographical Settings 9

8

7 /s] 3 6

5

4

3

average discharge [m 2 Hydrography and Hydrology of the Area

1

0

3 987 1983 1985 1 1989 1991 199 1995 1997 1999 2001 2003 Avg Selected Physical and Geographical Settings year

Fig. 2.22 Annual variability of the mean annual flow of Aleltu River at Muka Ture river gauge

Tab. 2.10 Runoff data (Part 1) Mean Annual Specific Area No. River / Location flow flow runoff Aquifer [km2] [m3/s] [mm] [l/s.km2] Abay Basin (Abay River) Volcanic + 112001 Abbay / Kessie 531.09 254.7 65,784.0 7.97 sedimentary

Jemma Basin

112027 Aleltu / Muka Ture 3.38 238.6 447.0 9.82 Volcanic

112029 Robi Gumer / Lemi 8.82 313.8 887.0 9.90 Volcanic

Volcanic + 112034 Jemma / Lemi 80.43 469.0 5,412.0 14.78 sedimentary

Jemma / confluen- Volcanic + 103.00 216.7 15,000.0 6.87 ce – model sedimentary Muger Basin Volcanic + 112002 Mugher 7.75 500.1 489.0 15.87 sedimentary

11204 4 Gorfo / Gorfo 1.00 644.0 49.2 20.35 Volcanic

112012 Aleltu / Chancho 0.47 511.5 29.0 16.20 Volcanic

112013 Deneba / Chancho 2.99 1,097.2 86.0 34.74 Volcanic

Selected Physical and Geographical Settings 51 Tab. 2.10 Runoff data (Part 2) Mean Annual Specific Area No. River / Location flow flow runoff Aquifer [km2] [m3/s] [mm] [l/s.km2]

112014 Sibilu / Chancho 5.50 462.8 375.0 14.66 Volcanic

112015 Roba / Chancho 0.28 883.6 (10.0) 16.67 Volcanic

Awash / Rift Valley

031033 Awash / Ginchi 1.18 492.6 75.6 15.61 Volcanic

Berga / 031001 2.83 360.1 248.0 11.41 Volcanic Addis Alem

031002 Holeta / Holeta 2.41 513.9 147.9 16.28 Volcanic Hydrography and Hydrology of the Area Jemjem / 031031 1.78 472.0 119.0 14.96 Volcanic Wolonkomi

031021 T. Akake / AA 3.58 862.4 131.0 27.33 Volcanic Selected Physical and Geographical Settings Selected Physical and Geographical Settings There are no big differences in mean flow (runoff) within the main river catchments located at different parts of the map sheet. The highest specific runoff is from the Muger river basin which is located in the center and its catchment represents the highest part of the sheet receiving the highest portion of precipitation. The adopted specific runoff for the Addis Ababa sheet is 15.0 l/s. km2 based on data from flow measurements and calculated specific runoff in gauging station shown in Tab. 2.10. This specific runoff has been used for further calculations. The main difficulty is the nonexistence of flow measurements in the lower reaches of the main rivers.

2.4.3 Baseflow The same gauging stations were used for calculation of baseflow, because these stations have provided flow data for several years.

Baseflow represents one of the most important types of information on groundwater resources in the basin. The methods were analyzed by Bogena et al. (2005) and it was found by means of a correlation analysis that the appropriate baseflow values can be determined on the basis of daily river discharge data. The baseflow can be identified from a series of observed monthly low- water runoff values (MoLR) as the simplest assessment method. It has been shown that a long- term average of MoLR of a 20-year period is a good approximation for groundwater recharge in unconsolidated rock areas. However, in consolidated rock areas the MoLR values are often affected by interflow leading to a significant overestimation of groundwater recharge. Hence, a more sophisticated hydrograph separation method based on the Kille method is recommended in these areas.

The Kille method (see Fig. 2.23) for calculation of baseflow was used in the study together with separation of hydrographs where baseflow data is deduced from the discharge record of a stream by separating the baseflow component from the total discharge.

The application of the method can be summarized as follows: 1. For each month in a year the minimum daily discharge rate (Q in m3/s) was selected. In total, the number of Q values is n = 12 × length of the record set in years.

52 Selected Physical and Geographical Settings 0,6 MoLR = 0.00060 n/2 + 0.02296 0,5 interflow 0,4 /s]

3 linear zone of the distribution curve 0,3

MoLR [m MoLR 0,2 baseflow n 0,1 n/2

y0 0 0 100 200 300 400 500 600 i Fig. 2.23 Method of Kille baseflow assessment Hydrography and Hydrology of the Area

2. Sort the n rates into ascending order and plot them against the corresponding orders (i). In general, a subset of points of low discharge in the scatter plot fits on a straight line. 3. The linear zone of the distribution curve represents the baseflow. The MoLR is calculated by means of the gradient m, the number of values n and the axis intercept y0: MoLR = m × n / 2 + y0. If the hydrographic basin is closed (i.e. there is no water flowing in/out from/to an Selected Physical and Geographical Settings adjacent basin) and the aquifer is in steady state with respect to storage on an annual basis, then the average groundwater recharge rate R = MoLR. 4. Convert R into a value in mm/y, i.e. multiply the value in m3/s by 60 × 60 × 24 × 365 × × 1,000 and subsequently divide the result by the drainage area of the basin in m2.

Data on baseflow assessed by the Kille method is shown in Fig. 2.24 and in Tab. 2.11 together with baseflow data assessed by the hydrograph separation method.

Assessment of baseflow using the Kille method revealed comparable results for all rivers within the area. Results of the Kille method should be considered as granted because there is enough data to eliminate short-term climatic variations, excluding results for the Jemma River. Assessment of baseflow for the Jemma is based on data from two years only.

0,25 0,75 0 0,5 Berga - Addis Alem 3 Awash - Ginchi 0,25 0.18 m /s -0,25 0.076 m3/s 0 -0,5 -0,25

-0,75 -0,5

Q -0,75 -1 og

l -1

-1,25 Q log -1,25

-1,5 -1,5

-1,75 -1,75 -2 -2 -2,25

-2,25 -2,5

-2,5 -2,75

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Fig. 2.24 Kille baseflow separation (Part 1)

Selected Physical and Geographical Settings 53

1 1 0,5 0,5 0 0 -0,5 -0,5 -1 -1 -1,5 -1,5 Holeta - Holeta (Genet) -2 -2 3 -2,5 -2,5 0.25 m /s -3 -3 Little (Tinishu) Akaki -3,5 -3,5 0.22 m3/s -4 -4 Q -4,5 -4,5 og l log Q -5 -5 -5,5 -5,5 -6 -6 -6,5 -6,5 -7 -7 -7,5 -7,5 -8 -8 -8,5 -8,5 -9 -9 -9,5

Hydrography and Hydrology of the Area -9,5

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 20 40 60 80 100 120 140 160 180 200 220 240 260

1,5 1 3 0,5 2 0

Selected Physical and Geographical Settings Selected Physical and Geographical Settings -0,5 1 -1 -1,5 0 Abay - Kessi 117.79 m3/s -2 Muger - Chancho -1 -2,5 0.24 m3/s -3 -2 -3,5 -4 -3 log Q log -4,5 log Q log -4 -5 -5,5 -5 -6 -6,5 -6 -7 -7 -7,5 -8 -8 -8,5 -9 -9 -9,5

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 50 100 150 200 250 300 350 400 450 500

1,25 1 Aleltu - Muka Ture 3 2,25 0,75 0.99 m /s Jemma - Lemi 37.37 m3/s 0,5 0,25 2 0 -0,25

-0,5

1,75 -0,75 log Q log Q -1 -1,25

-1,5 1,5 -1,75

-2

-2,25 1,25 -2,5

-2,75

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 20 40 60 80 100 120 140 160 180 200 220 240 Fig. 2.24 Kille baseflow separation (Part 2)

54 Selected Physical and Geographical Settings Separation of the hydrograph (see Fig. 2.25) is another method that was used for assessment of baseflow. Baseflow separation techniques use the time-series record of stream flow to derive the baseflow signature. The common separation methods are either graphical which tend to focus on defining the points where baseflow intersects the rising and falling limbs of the quickflow response, or involve filtering where data processing of the entire stream hydrograph derives a baseflow hydrograph.

The daily flow data were used to plot the baseflow component of a flood hydrograph event, including the point where the baseflow intersects the falling limb. Stream flow subsequent to this point was assumed to be entirely baseflow, until the start of the hydrographic response to the next significant rainfall event. These graphical approaches (Fig. 2.25) to partitioning baseflow vary in complexity and include (Linsley, 1958): a) the constant discharge method (green line on the chart) assuming that baseflow is constant during the storm hydrograph; the minimum streamflow immediately prior to the rising limb is used as the constant value; Hydrography and Hydrology of the Area b) the constant slope method (blue line on the chart) connecting the start of the rising limb with the inflection point on the receding limb; this assumes an instant response in baseflow to the rainfall event; c) the concave method (violet line on the chart) attempting to represent the assumed initial decrease in baseflow during the climbing limb by projecting the declining hydrographic trend evident prior to the rainfall event to directly under the crest of the flood hydrograph; this Selected Physical and Geographical Settings minimum is then connected to the inflection point on the receding limb of storm hydrograph to model the delayed increase in baseflow.

The constant slope method was used for assessment of baseflow. The selection of years for baseflow separation was done based on the average mean flow of the river. Results of the baseflow separation are shown in Fig 2.26.

flow crest

inflexion point b c a

time

Fig. 2.25 Method of baseflow separation

Selected Physical and Geographical Settings 55 3 600 3 400 Abay - Kessi (1992) 3 3 200 196.84 m /s 3 000 2 800 2 600 2 400 2 200 2 000 Q 1 800 1 600 1 400

1 200 1 000 800 600 400 200 Hydrography and Hydrology of the Area 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 day

1 150 1 100 1 050 Jemma - Lemi (1997) 1 000 39.74 m3/s 950 Selected Physical and Geographical Settings Selected Physical and Geographical Settings 900 850 800 750 700 650 600 Q 550 500 450 400 350 300 250 200 150 100 50 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 day

200 190 180 Muger - Chancho (1999) 3 170 1.52 m /s 160 150 140 130 120 110

Q 100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 day Fig. 2.26 Hydrograph baseflow separation (Part 1)

56 Selected Physical and Geographical Settings 70

65 Aleltu - Muka Ture (1986) 60 1.32 m3/s 55

50

45

40

Q 35

30

25

20

15

10

5 Hydrography and Hydrology of the Area 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 day Fig. 2.26 Hydrograph baseflow separation (Part 2)

Tab. 2.11 Baseflow data for the Addis Ababa sheet area (Part 1) Selected Physical and Geographical Settings Specific Kille Hydrograph Specific Area Map ID River runoff method separation baseflow Aquifer [km2] [l/s.km2] [m3/s] [m3/s] [l/s.km2] [mm]

Abbay / Volcanic + 112001 65,784.0 7.97 117.790 196.84 1.79/2.99 76.8 Kessie sedimentary

Aleltu / 112027 447.0 9.82 0.990 1.32 2.21/2.95 70.6 Volcanic Muka Ture

Robi Gu- 112029 887.0 9.90 0.140 0.26 0.16/0.29 7.1 Volcanic mer / Lemi

Jemma / Volcanic + 112034 5,412.0 14.78 37.370 39.74 6.91/7.43 221.6 Lemi sedimentary

Volcanic + 112002 Mugher 489.0 15.87 0.240 1.52 0.49/3.11 64.5 sedimentary

Gorfo / 11204 4 49.2 20.35 0.020 0.18 0.41/3.65 64.4 Volcanic Gorfo

Aleltu / 112012 29.0 16.20 0.014 0.019 0.48/6.40 108.8 Volcanic Chancho

Deneba / 112013 86.0 34.74 0.110 0.91 1.28/10.58 183.5 Volcanic Chancho

Sibilu / 112014 375.0 14.66 0.190 1.01 0.51/2.70 50.5 Volcanic Chancho

Awash / 031033 75.6 15.61 0.076 No data 1.01 41.7 Volcanic Ginchi

Berga / 031001 248.0 11.41 0.180 No data 0.73 25.4 Volcanic Addis Alem

Selected Physical and Geographical Settings 57 Tab. 2.11 Baseflow data for the Addis Ababa sheet area (Part 2)

Specific Kille Hydrograph Specific Area Map ID River runoff method separation baseflow Aquifer [km2] [l/s.km2] [m3/s] [m3/s] [l/s.km2] [mm]

Holeta / 031002 147.9 16.28 0.250 No data 1.69 42.6 Volcanic Holeta

T. Akake / 031021 131.0 27.33 0.220 No data 1.68 48.2 Volcanic AA

Comparison of the assessment of baseflow using the Kille method and hydrograph separation is shown in Tab. 2.11. Results show differences between the assessment of baseflow using the Kille method and hydrograph separation. Differences are particularly visible in small intermittent Hydrography and Hydrology of the Area rivers but differences are not significant for bigger rivers for the majority of the year. Values of specific baseflow calculated by both methods were averaged and adopted as a specific baseflow for aquifers of the Addis Ababa area.

An adopted average baseflow value of 2.0 l/s.km2 represents a depth of about 62.5 mm and compared to the adopted average depth of precipitation of 1,250 mm the calculated Selected Physical and Geographical Settings Selected Physical and Geographical Settings infiltration (recharge) can be assessed as being 5 %.

2.5 Water Balance Precipitation is partly evaporated, partly transpired and part of the water flows to nearby rivers as runoff (surface runoff and shallow local baseflow). The rest of the water infiltrates into deeper parts of volcanic aquifers and forms deep local or deep regional groundwater flow. The balance was assessed for Fiche and Muger meteo-stations, which are located on the volcanic plateau and river gauging stations on the surrounding rivers. The upper part of the aquifer developed

Water Balance Water in volcanic rocks is drained as shallow local baseflow on the plateau which is represented by the Mugher river gauging stations and the Aleltu river gauging station near Muka Ture. The aquifer developed in volcanic rocks is totally drained by deeper local drainage level occurring downstream parts of the main rivers, but discharge measurements are rare and its reliability is under question.

The water balance assessment is based on the following considerations: • The average monthly precipitation from Muger and Fiche meteo-stations (Tab. 2.12 and Tab. 2.13) represents the input recharge for the whole plateau. • The average monthly evapotranspiration in Akaki meteo-station is shown in Tab. 2.12 and Tab. 2.13. • The deficits in the water balance of basins of Mugher River and Aleltu River represent infiltration into deeper aquifers and its value is manifested as deep local and/or deep regional baseflow. Infiltration into deeper aquifers can be expressed by the equation

Ideeper = Pprecipitation – Etevapotranspiration – TRtotal runoff.

The highest monthly precipitation occurs in June, July and August. During these months not all of the water volume is consumed either by evapotranspiration or by runoff and the rest of the water can infiltrate into the aquifer developed in volcanic rocks. After full saturation of the aquifer in June or August total runoff is increased. Assessment of the total volume of infiltration (deep local and regional flow) for the Mugher basin is 314 for mm/year and 214 mm/year for the Aleltu basin.

58 Selected Physical and Geographical Settings Tab. 2.12 Water balance of Mugher basin Month/ Units Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec parameter

Precipitation mm/month Muger 12 20 61 80 98 225 399 307 164 15 26 22

Evapotranspi- mm/month Akaki 111 111 135 125 131 102 90 93 100 127 117 115 ration

Total runoff mm/month Muger 1111241142441151752 Balance Water

Deep local and mm/month 119 19 5 regional flow

Tab. 2.13 Water balance of Aleltu basin Month/ Units Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec parameter

mm/ Precipitation Fiche 18 33 63 72 48 78 330 316 110 18 6 6 month

Evapotranspi- mm/ Akaki 111 111 135 125 131 102 90 93 100 127 117 115 ration month Selected Physical and Geographical Settings

mm/ Aleltu at Total runoff 0011126418449311 month Muka Ture

Deep local and mm/ 176 38 regional flow month

The presented water balance is calculated based on available data and demonstrates a system approach to assessment of hydrological and hydrogeological data and is in conformity with the conceptual hydrogeological model presented in Chapter 4.

2.6 Drought and Climate Changes The whole Ethiopian territory is often affected by reoccurring droughts causing famine. The impact of drought is severe in both the arid lowlands as well as the highlands of Ethiopia. The existence of drought and desertification is well known from geological and archeological evidence as well as from historical documents and on-going measurements. It is matter of fact that the centre of the Ethiopian civilization was shifted about 1,000 km from Axum in the dry north to Addis Ababa located in the more humid centre of the current (modern) Ethiopia over the last 2,000 years. The northern and eastern parts of the country appeared to be highly vulnerable to reoccurring drought and famine. The most drought-prone regions of Ethiopia are shown in Fig. 2.27.

There are many causes of drought, starting with a local deficit of vapor and condensation nuclei

and changes in land use causing changes in soil reflectivity etc., to global changes related to Drought and Climate Changes the greenhouse effect with the warming of the surface water of tropical seas. Climate change is dangerous because it can accelerate irregularities in the behavior of synoptic weather systems over the country which is one of the main reasons for the failure of the seasonal rains. Geological and historical evidence was described in detail by Brooks (draft, 2005) and Sima et al. (2009).

The study of NMSA (1996) considers an occurrence of meteorological drought when seasonal rainfall over a region is less than 19 % of its mean. In addition, a drought is classified as moderate

Selected Physical and Geographical Settings 59 Drought and Climate Changes Selected Physical and Geographical Settings Selected Physical and Geographical Settings

Fig. 2.27 The most drought prone areas of Ethiopia (source: RRC, 1985)

60 Selected Physical and Geographical Settings and severe if seasonal rainfall deficiency is between 21–25 % and more than 25 %, respectively. A year is considered to be a drought year for the country as a whole in the case the area affected by one of the above criteria for drought, either individually or collectively, is more than 20 % of the total area of the country. The study of drought incidence, intensity and frequency within the whole Ethiopian territory takes into consideration data from the period 1969 to 1987 resulting in the following: 1. Occurrence of drought in the Belg season was serious in mapped area in (severe drought in bold italics) 1973, 1975, 1977, 1978, 1980, and 1984 affected more than half the regions. The year 1975 was the most serious, including in the mapped area. The impact is considered to be catastrophic if drought occurs continuously for three or more years. 2. Occurrence of drought in the Kiremt season has more of an effect because 95 % of crop production relies on these rains. Drought occurred in 1972, and 1987 of which the latter

affected about 70 % of the country, including a part of the mapped area region. Drought and Climate Changes 3. Occurrence of drought in both the Belg and Kiremt seasons (drought year) in 1973 and 1984 with failure of rain in 6 out of 14 regions.

The conclusions of the study revealed that the period 1970 to 1973 was the worst time of the country and 1984 was the worst year where most areas, including central part of the country were badly affected by drought in both seasons. For Belg, Kiremt and both seasons together, 1973, 1987 and 1984 are years when the most regions were affected. The map area was part of the country area receiving insufficient water on an annual basis in 1972, 1973, 1980 and 1984. Selected Physical and Geographical Settings

The rainfall deficit assessment of the mapped area revealed that the most serious deficit in Belg rainy season occurred in 1984, 1988, 1991, 1994, and also during the last decade starting from 1998.

The rainfall deficit during Kiremt season was devastating in 1982, 1984, 1987 but in the last decade it was relatively hospitable.

Considering the critical situation resulting from a rainfall deficit in both the Belg and the Kiremt seasons, the assessment shows that 1984 and 2002 were the most difficult years.

Dealing with classification of rainfall, 1978, 1984–85, 1990–91, 1995 and 2002 can be identified as years with the highest deficit in rainfall.

Climate Change Current climate change poses a significant challenge to Ethiopia by affecting food security, water and energy supply, poverty reduction and sustainable development efforts, as well as by causing natural resource degradation and natural disasters. For example the impacts of past droughts such as those of 1972/73, 1984 and 2002/03 are still fresh in the memories of many Ethiopians. Floods in 2006 caused substantial loss to human life and property in many parts of the country. In this context, planning and implementing climate change adaptation polices, measures and strategies in Ethiopia will be necessary.

The agricultural sector is the most vulnerable to climate variability and change. In terms of livelihoods, small scale rain-fed subsistence farmers and pastoralists are the most vulnerable.

The annual minimum temperature is expressed in terms of temperature differences from the mean and averaged for 40 stations. There has been a warming trend in the annual minimum temperature over the past 55 years. It has increasing by about 0.37 °C every ten years. The trend analysis of annual rainfall shows that precipitation remained more or less constant when averaged over the whole country.

Selected Physical and Geographical Settings 61 For the IPCC mid-range (A1B) emission scenario, the mean annual temperature will increase in the range of 0.9–1.1 °C by 2030, in the range of 1.7–2.1 °C by 2050 and in the range of 2.7–3.4 °C by 2080 over Ethiopia compared to the 1961–1990 normal. A small increase in annual precipitation is also expected over the country.

The other climate related hazard that affects Ethiopia from time to time is flooding. Major floods occurred in different parts of the country in 1988, 1993, 1994, 1995, 1996 and 2006. All of them caused loss of life and property.

In recent years the environment has become a key issue in Ethiopia. The main environmental problems in the country include land degradation, soil erosion, and deforestation, loss of biodiversity, desertification, recurrent drought, flood and water and air pollution. Drought and Climate Changes A large part of the country is dry sub-humid, semi-arid and arid, which is prone to desertification and drought. The country has also fragile highland ecosystems that are currently under stress due to population pressure and associated socio-economic practices. Ethiopia s history is associated –more often than not – with major natural and manmade hazards that affect the population from time to time. Drought and famine, flood, malaria, land degradation, livestock disease, insect pests and earthquakes have been the main sources of risk and vulnerability in most parts of the country. Especially, recurrent drought, famine and recently floods are the main problems that affect millions of the country s population almost every year. While the causes of most disasters are climate Selected Physical and Geographical Settings Selected Physical and Geographical Settings related, the deterioration of the natural environment due to unchecked human activities and poverty has further exacerbated the situation.

The major adverse impacts of climate variability in Ethiopia include: • Food insecurity arising from the occurrence of droughts and floods. • Outbreaks of diseases such as malaria, dengue fever, water borne diseases (such as cholera, dysentery) associated with floods and respiratory diseases associated with droughts. • Heavy rainfalls which tend to accelerate land degradation. • Damage to communication, road and other infrastructure by floods.

For example in 2006 flooding in the main rainy season (June–September) caused the following disasters (NMA, 2006): • More than 250 fatalities and about 250 people unaccounted for in Dire Dawa flood. • More than 10,000 people in Dire Dawa became homeless. • More than 364 fatalities in Southern Omo and more than 6,000 (updated to 8,350 after August 15) people were displaced over Southern Omo, where around 14 villages were flooded. • More than 16,000 people over West Shewa were been displaced. • Similar situations also occurred over Afar, Western Tigray, Gambella Zuria and over the low lying areas of Lake Tana.

In terms of loss in property and livestock • The DPPA estimate is about 199,000 critically affected people due to the flood in the country. • More than 900 livestock drowned over South Omo. In addition, 2,700 heads of cattle and 760 traditional silos were washed away (WFP). • About 10,000 livestock encircled by river floods in Afar. • Over Dire Dawa, the loss in property is estimated in the order of millions of dollars.

62 Selected Physical and Geographical Settings 3. 3. Geological Settings

The geology of the area includes complete succession starting from Precambrian basement, Paleozoic and Mesozoic sediments, Tertiary and Quaternary volcanic and sedimentary rocks.

In general, the basic litho-stratigraphical units from the oldest to the youngest include meta gabbro, biotite gneiss, granite, biotite-hornblende gneiss, Paleozoic sandstone, Lower sandstone (Adigrat sandstone), gypsum (Gohatsion formation), Antalo limestone, Muger mudstone formation, Upper sandstone (Debere Libanos sandstone), Aiba basalt, Tarmaber basalt, Entoto mixed volcanic rocks, Wechecha-Yere-Furi ignimbrite/trachyte and trachybasalt, Quaternary plateau basalts of Previous Work Previous Central Ethiopia and Quaternary super facial deposits.

3.1 Previous Work Geological Settings The regional characteristics were described by Kazmin et al. (1972) on the Geological map of Ethiopia at a scale of 1:2,000,000. The petroleum potential of Debre Libanos was studied by Alua (1997). The study revealed that there was no available data to diagnose the petroleum potential of the area and advised for further detailed study.

Detailed geological maps of Addis Ababa sheet in scale 1:50,000 were prepared by The regional department of the GSE and were used for compilation of geological background for hydrogeological and engineering geology maps at a scale of 1:250,000.

3.2 Stratigraphy Stratigraphy is represented by rocks of various ages and lithology starting from the youngest most recent Quaternary superficial deposits and volcanic rocks, Tertiary rhyolites, ignimbrites, basalts and inter-bedding sediments, two Mesozoic sequences of sandstone, mudstone, gypsum and limestone, Paleozoic Abay sandstone to the Precambrian basement as the oldest rock. Tab. 3.1 Stratigraphy Tab. 3.1 Stratigraphy of the area (Part 1) Relative age Major group Mapped unit

Quaternary Plateau covering sediments and volcanic rocks

Alluvial sediments Fluvio-coluvial deposits Elluvial – colluvial soil/sediment

Volcanic rocks including ash layers

Geological Settings 63 Tab. 3.1 Stratigraphy of the area (Part 2) Relative age Major group Mapped unit

Tertiary Plateau forming volcanic rocks

Central type of volcanism as an Ignimbrites, rhyolites and ignimbrites equivalent of Balchi rhyolite

Stratigraphy Termaber basalt and phonolite Stratiod basalts and trachytes

Fissured type of basalt as an Ignimbrites and basalts intercalated with equivalent of Alaji rhyolite lake type of sediments Geological Settings Geological Settings Gorges forming volcanic rocks

Upper basalt/Intera upper basalt/Upper Le- mi-Debre Libanos-Fitche basalt Aiba basalts Basalt accompanied with or alternating with relatively thick layers of pyroclastic

Lower basalt representing the first fissured (Ashangi??, Blue Nile??) type of volcanism

Mesozoic Gorges forming sedimentary and basemen rocks

Debre Libanos sandstone/Upper sandstone

Mugher mudstone

Antalo limestone

Gypsum

Adigrad sandstone/Lower sandstone

Paleozoic

Abay sandstone with some shale at the top

Precambrian

Metamorphic and igneous rocks

refers to the stratigraphy of the area identified by the various authors preparing the geology of the area.

3.3 Lithology The description of the lithological units is mainly taken from various sheets of geological maps in at a scale of 1:50,000 prepared by GSE.

Lithology 3.3.1 Precambrian Metamorphic Rocks Basement rocks are exposed in the western part of the sheet and consist of metamorphosed and igneous rocks.

64 Geological Settings 3.3.1.1. Biotite Gneiss (Pbgn) They crop out mainly in the northern and western parts of the study area. The rocks are exposed in the gorges of Abay and Guder rivers, and also on the banks and in the stream beds of their big tributaries. Gneiss forms gently undulating surfaces below the Mesozoic sedimentary rocks.

The biotite gneiss is dark grey, medium to coarse grained and shows gneissosity, which is defined by the alignment of biotite, feldspar and quartz. The strike of foliation is NNW and NNE and it dips Lithology gently or moderately towards the east or west (Ilfious, 2008). It is highly granitized, pegmatized and at places injected by quartz veins. It was found that intensity of granitiztion, pegmatization and migmatization decreases from north to south. In some places the quartz is highly recrystallized and shows intergranular cracking. In most places it is massive with a slight degree of weathering. The weathered surfaces is covered by greenish and brownish material originated by weathering. Geological Settings

3.3.1.2 Biotite Hornblende Gneiss (Pbhgn) This unit is exposed in the northwestern and western parts of the study area (Fig.3.1). It forms north-south aligned hills and is observed to intrude the biotite gneiss.

There is weakly developed foliation and migmatization. The gneiss has a medium to coarse grained texture. This rock develops a slight degree of weathering and facture, while outcrops along trails show a high degree of weathering. Whenever the biotite gneiss is exposed, the unit is observed to outcrop sporadically as lenses within it. It is mainly composed of hornblende, biotite, plagioclase and quartz.

Fig. 3.1 Biotite hornblende gneiss

Geological Settings 65 3.3.1.3 Meta Granite (Pgrt) Granite is underlying the sandstone. Its color varies from pink to light grey. Mostly, it is fresh, weakly to moderately foliated with visible biotite, k-feldspar and quartz grains alignment (Ilfious, 2008; Assegid, 2006). It is observed to intrude the biotite gneiss. When it is intensively weathered it is altered to reddish brown soil. Jointing and fracturing perpendicular to the foliation are also commonly observed phenomena. Generally, rugged topography is formed by this unit and it Lithology usually forms discontinuous outcrops.

3.3.1.4 Meta Gabbro (Pmg) Gabbro outcrops in the western part of the area. On fresh surfaces it is dark grey, medium

Geological Settings Geological Settings to coarse grained. Generally, it shows weak to moderate foliation and is injected by pegmatite and quartz material in form of veins. Often it forms domes and discontinuous ridges. Jointing is a commonly observed structural feature. Gabbro is slightly to moderately weathered.

3.3.2 Paleozoic and Mesozoic Sedimentary Formations 3.3.2.1 Paleozoic Sandstone (Plst) Sandstone (Abay sandstone) is exposed in the northwestern and western parts of the study area in the Abay and Guder river gorges. Sandstone overlies non-conformably the foliated granite and the biotite gneiss and is overlaid by the Mesozoic sandstone unit (Ilfious, 2008). Lithologically, it is represented by varying proportions of sandstone, siltstone, mud stone, shale and some paleosoil. The sandstone dominates at the top, while shale is abundant in the lowermost parts. Siltstone usually occupies the middle part and mud stone occurs as intercalation within all of the units. There is a development of paleosoil with a thickness of 2–3 m between the Mesozoic sandstone and the Paleozoic sandstone. The thickness of the sandstone unit reaches up to 200 m in some outcrops (Assiged Getahun, 2006; Ilfious, 2008).

The color of sandstone varies from light grey and reddish brown to yellowish grey. The sandstone is highly to moderately weathered with a characteristic of light grey color when fresh but reddish brown color when weathered and has a medium to coarse grained texture with some conglomeratic texture. Sandstone is highly compacted and shows jointing in places. There are also intercalations of highly fractured shale and massive and compacted mudstone and siltstone.

3.3.2.2 Mesozoic Sandstone (Msst) This sandstone (Lower sandstone) unit is exposed in the northwestern, western, southwestern and central parts of the area. It forms the cliffs of the Abay, Guder and Mugher river gorges below the volcanic rocks but mainly below the Mesozoic limestone and above the Paleozoic sandstone and basement rocks. It has a maximum thickness of 1,131 m (Ilfious, 2008). The succession mostly consists of sandstone with very thin intercalations of siltstone, mudstone and some paleosoil. In the top part it is conglomeratic and fine to medium grained, reddish brown to light gray color. In most cases it develops structures like lamination and cross bedding. The degree of weathering and fracturing is high in the top part.

3.3.2.3 Gypsum (Mgb) Gypsum (Gohatsion formation) formation is mainly exposed in the northern, central and northeastern part of the study area following the Abay, Jemma and Mugher river gorges forming steep cliffs. Assiged Getahun (2006) mentioned that the nature of the contact with the overlying limestone is sharp while with the underlying sandstone the contact is gradational.

This formation is composed mainly of gypsum and mudstone with variegated color i.e. gray at the top, pink at the middle and white at the bottom. There are intercalations of yellow limestone at

66 Geological Settings the base and shale towards the top (Assiged Getahun, 2006). It is slightly to moderately weathered and compact.

3.3.2.4 Limestone (Lst) Limestone (Antalo limestone) is exposed in the northern, northeastern, central and western parts of the area. It is mainly outcrops in the Abay, Jemma and Mugher river valleys. Most of the time the limestone forms cliffs, however in some places it develops a gently sloping ridge. Lithology

The contact with the underlying mudstone formation is gradational which is marked by a siltstone layer followed by calcareous siltstone, silty limestone and gradually to limestone. However, the contact with the underlying gypsum unit is sharp (Assiged Getahun, 2006). This formation is characterized by alternating beds of marl. There are also shale intercalations which Geological Settings are frequent towards the bottom. Most of the time the limestone appears as light gray and yellow; when weathered its color changes to dark, white and sometimes to deep yellow. In places, a higher degree of weathering is observed, the precipitation of the secondary minerals such as calcite and silica are observed along fractures and weak zones. Structures such as karst openings, chert nodules and stylelites are observed at the bottom of the limestone. Assiged Getahun (2006) mentioned that the petrographic study indicated that this limestone has a range of textures from mudstone to wackstone and packstone.

3.3.2.5 Muger Mudstone – Siltstone Formation (Mmst) This formation is exposed in the northern eastern part of the area at the Jemma and Mugher river valleys. It forms a relatively gentle slope, with lower topography. It is overlain conformably by a sandstone unit and underlain by Antalo limestone having gradational contact in both cases.

Fig. 3.2 Contact between tertiary volcanic and Debre Libanos sandstone at Ziga Wedem river gorge

Geological Settings 67 The dominant types of rocks in this formation are mudstone, siltstone and shale. However, there are multiple beds of different intercalations. This formation has variegated colors from red and multicolored mudstone to light green and gray shale and yellow to white siltstone. It exhibits a high degree of weathering. The main structures are laminations, cross laminations, ripple marks and bedding.

Lithology 3.3.2.6 Debre Libanos Sandstone (Musst) The sandstone (Upper sandstone) is exposed in the northern eastern and central parts of the map area, within the Jemma, Zega Wedem and Mugger river gorges. It outcrops in the form of cliffs. The maximum measured thickness is about 328 m. The thickness generally declines from east to west (Assiged Getahun, 2006). It has a sharp and unconformable contact with the overlying basalt Geological Settings Geological Settings (see Fig. 3.2) whilst the contact with the underlying unit is gradational.

The sandstone unit exhibits a wide range of compositional variation ranging from yellow color, well sorted, medium grained at the top to red color, conglomeratic cross bedded sandstone in the middle. The bottom part is dominated by jointed, fine grained white sandstone. This unit is slightly weathered at the top and highly weathered at the bottom. In general it exhibits a coarsening upward sequence.

3.3.3 Tertiary Volcanic Rocks Tertiary volcanic rocks are the main constituent of the Ethiopian plateau. They are found in a wide variety from liquid basalt rocks to highly viscose acidic rhyolite representing a fissured and central type of volcanism. Various volcanic episodes and lava flows were followed by short or longer periods of river, lake sediment and soil development that can be found within volcanic sequences.

3.3.3.1 Aiba Basalt (Tv1) This unit is exposed in northern and central parts of the map area in river valleys and canyons. It mainly forms steep slopes and cliffs as well as gentle slopes in some areas. The contact with the underlying sandstone is characterized by an abrupt nature. There is a red limonite layer of 3 m thick on contact with the overlying pyroclastic layer (Assiged Getahun, 2006).

This basalt has a dark grey color on fresh outcrops. Upon weathering, it has a dark-brown, gray and reddish brown color. In this unit there is vertical compositional variation. The top part is composed of vesicular basalt. In the middle, coarse grained basalt is noticed. The bottom of this unit is made up of columnar jointed, cliff forming and relatively fresh aphanitic basalt. These columnar joints are characterized by well developed hexagonal faces. The maximum thickness measured is about 356 m around Shenhea-Cheka (Assiged Getahun, 2006).

3.3.3.2 Olivine – Plagioclase Porphyric Basalt (Tv2) This basalt (Tarmaber-Megzeze basalt) is exposed in central and southern parts of the map area. It mainly forms a gentle slope plateau and its origin results from fissure eruption. In some localities extensive ridges and mountains (such as the Cheleka and Chilmo Mountains) are also observed. The basalt overlies non-conformably over the upper sandstone and conformably over the lower ignimbrite. It exhibits stratified layers with varied compositions and structures. The layers are fine to very coarse grained, aphanitic to porphyritic and sometimes very coarse grained porphyritic basalt and vesicular basalt. Intense fracturing, columnar jointing and spheroidal weathering are very common features. Matebie Meten et al. (2006) classify this unit into porphyritic aphanitic and very coarse grained porphyritic basalt based on compositional and structural variations. The rock shows a high degree of weathering and fracturing.

68 Geological Settings 3.3.3.3 Lower Ignimbrite (Tv3) This unit is exposed in southern eastern part of the study area. It consists of interlayers of ignimbrite, ash and tuff. It is grey and black in color and shows columnar jointing and is medium to fine grained. It generally consists of two layers the coarse ignimbrite at the top and fine ignimbrite at the lower layer. This unit is highly affected by joints which are vertical and horizontal plunging in N340° E and N150 °E direction (Assiged Getahun, 2007). It is intensively weathered fractured. Lithology 3.3.3.4 Inter Layers of Ignimbrite, Welded Tuff and Ash (Tv4) This unit is found outcropping in the southern and eastern part of the study area. It is characterized by grey color, containing fragments of ignimbrite, tuff, and pumice and is associated with some

rhyolites. It is fine to medium grained. In most places, it is overlain by aphanitic to medium grained Geological Settings vesicular trachyte (Wechecha-Yerer-Furi trachyte and trachy basalt) and it overlies the lower ignimbrite, Entoto mixed rock and Tarmaber basalt. It is slightly to moderately weathered. The unit is with massive surface and is rarely fractured.

3.3.3.5 Entoto Rhyolite and Trachyte Rocks (Tv5) The Entoto mixed rocks are found in the southeastern part of the map area. This unit constitutes of rhyolite, trachyte, ignimbrite pyroclastic rocks and sediments. All the rocks are highly weathered and jointed with few layers of agglomerate in some places. There is a red baked soil developed at the contact with Tarmaber basalt. It shows a variegated color of weathering, mainly pink, yellow, white and grey and sometimes light green and reddish brown. This lithological unit is highly affected by joints trending E-W and N29°. It forms a high mountain chain called Entoto running in an E-W direction (Assiged Getahun, 2007).

3.3.3.6 Aphanitic to Medium Graind Vesicular Trachyte (Tv6) This formation (Wechecha-Furi-Yerer trachyte and trachy basalt) is exposed in southwestern and southeastern parts of the map area. It is found overlying Tarmaber basalt marked by a thick paleosoil at the contact of these two units. Trachyte forms large mountains such as Furi, Wechecha and Yerer. It has an aphanitic to medium grained texture with vesicular varieties mostly in its lower part. The characteristic color would be light grey to dark grey, often to greenish grey. Mostly trachyte and the trachy basalts are found alternatively layered with the trachyte being dominant. In its lower part it shows columnar jointing and is affected by two sets of joints (Assiged Getahun, 2007).

3.3.3.7 Aphanitic to Phorphyritic Trachyte (Tv7) The Entoto trachyte is exposed in the southeastern part of the map area. This unit is generally coarse grained porphyritic and highly weathered. Weathered material has a light pink to white color. Slight fracturing is developed in the unit which also has EW and SE-NE trending joints. The joints are filled with dark brown clay. This unit is covered by patches of olivine Quaternary basalt.

3.3.4. Quaternary Volcanic and Sedimentary Rocks 3.3.4.1 Quaternary Plateau Basalts of Central Ethiopia (Qb) Plateau basalts are exposed in the southeastern and northeastern parts of the map. The southern part is dominantly olivine basalt with a characteristic grey color on fresh outcrops whilst it becomes reddish brown when weathered. In most cases it outcrops in boulder form and vesicles are filled by secondary minerals. It mainly forms ridges with the maximum thickness of about 50 m (Assiged Getahun, 2007).

The northern part is dominantly trachyte and trachy basalt with dark green color having an aphanitic and porphyritic texture. It forms topographically high domes and stains at Gara Guda,

Geological Settings 69 Degem and Yabeno areas. In this unit, a sheeted and layered flow structure is observed at the top of the dome and it is oriented in an EW direction with a shallow dip angle (22°/35°) (Assiged Getahun, 2006).

3.3.4.2 Quaternary Superficial Deposits (QS) This unit comprises mainly the eluvial soil and very small position of alluvial soil. The eluvial soil Lithology is deposited in eastern, southeastern, and central part of the area. It is dark grey dark brown and black color, its thickness ranges from 1–5 m, (Matebie Meten et al., 2007). The alluvial sediments are deposited in northern, northeastern and western parts of the study area along Jemma, Zega Wedem and Muger river valleys. Its texture varies from sand to silt in size. The basalt, limestone and quartz grain fragment association indicate the probable parent rocks from which this alluvial Geological Settings Geological Settings is derived (Assiged Getahun, 2006).

3.4 Structure The northern part of the area is structurally simple and only occasionally tectonized whereas, on the contrary, the southern part is highly tectonized and has a complex structure due to its vicinity to the Main Ethiopia Rift margin. The main structures encountered in the area are lineaments,

Structure faults, joints and dykes.

Lineament – The major lineaments in the study area trend in NE-SW, NW-SE, N-S and E-W directions. The NE-SW lineaments are dominant in the area and are parallel to the structures of the rift or rift margin. Most of the lineaments are observed in all of the units. In addition, there are some NWW and NEE trending lineaments which are more concentrated in sedimentary rocks. Most of the lineaments follow liner trends of ridges, mountains and mainly river valleys and streams. The length of the lineaments varies from few meters to about 12 km. (Assiged Getahun, 2006; Matebie Meten et al., 2007).

Faults – There are few normal faults in the area. They are mainly found in the NE part of the area. They are mainly found in the NW and dips towards the NE. They cut most of the lithological units.

Joints – Joints are widely observed in tertiary basalts and upper sandstone. The EW and NW trending joints are more common in the upper sandstone. Most of these joints are filled by secondary material such as calcite, iron oxide, silica and feldspar.

Dykes – These features are more observable in the northern part of the area cutting the Alaji rhyolite and trachyte. They are parallel and oriented in a NE direction with a maximum width of about 2 m (Assiged Getahun, 2006). Its composition varies from pyroxene porphyric to aphanitic and vesicular basalt.

3.5 Geological History The geological history of the map area may have started in the Late Paleozoic, when Gondwana began to brake up again and Africa, which occupied almost the central part of the supercontinent, exercised extensional deformation throughout the Phanerozoic. Hence, sedimentary basin development in Africa is commonly related with the polyphase break-up of Gondwana. The geodynamic setting of Gondwana (and of Africa) during the Paleozoic is mainly related to drifting through which Africa migrated northwards across the South Pole. This process provided glacial Geological History sediments and transgression-regression records in connection with the formation and melting of ice in North Africa. During the Late Paleozoic to Cenozoic, the geological processes primarily worked to disintegrate the super continent and the geologic history revolves mainly around rifting. Thus, rifting associated with the break-up of Gondwana was dominant from the Middle Paleozoic

70 Geological Settings (Late Carboniferous) onwards. During the first stage of rifting (Late Carboniferous-Early Jurassic) the process of disintegration was, in its infancy, marked by the beginning of the Karoo basin and the opening of Neotethys. It affects the Central Atlantic Province, the north and east margins of Africa and Arabia with typical development in the Karoo area. In North Africa, it is noted by widespread deposition of sandstone. In the map area this stage is represented by locally developed Graben filling sediments and Lower Glacial sediments of the Ordovician age and by Upper Glacial sediments (Upper clastic) probably of the Silurian age which are overlain non-conformably by widely developed Adigrat sandstone.

The second stage of rifting (Middle Jurassic to early Cretaceous) was connected with regression Geological History of the Indian Ocean towards Southeastern Ethiopia which deposited the Mesozoic mudstone and sandstone. The sandstone is formed by a braided river. It forms the cyclic reputation of fining Geological Settings upward sequences. This is a clue indicating that the deposition was continuous for a certain period of time. Its cross-bedding indicates the intensity of water supply during deposition.

Following the Late Mesozoic-Early Tertiary transgression-regression cycles, there was a strong epireogenic uplift of the whole of East Africa together with Arabia (Kazmin, 1972). As a result, there was an eruption of fissural volcanism that covers a wide area on the plateau. According to Zanettine et al. (1974) these volcanisms erupted in a cyclic manner rather than one prolonged eruption. In the map area the first fissural eruption was marked by the Lower basalt. This was followed by uplifting and deposition of conglomerate or friable sandstone. After some time there was another fissural eruption. Silicic and basaltic fissural magmas erupted from the end of the

Fig. 3.3 Mesozoic propagation of the Karoo rift to the southeastern part of Ethiopia (modified after Gani et al., 2008)

Geological Settings 71 Oligocene until the Upper-middle Miocene (25-15 Ma) (Zanettine et al., 1978). The rest of the basalts are the result of this silicic and basaltic magma. So after the eruption of the Middle basalt there was uplifting and local tectonics in the upper part of the study area. This was also followed by the formation of a local basin and deposition of Tertiary sediment. After the eruption of the Upper basalt there may also have been another uplifting with accompanying tectonics which formed a local basin in the central eastern part of the study area. This activity was followed by the deposition of Tertiary sediments. The deposition of the later Tertiary sediment was followed by another cyclical fissural eruption which began and ended with basaltic and silicic magmas, respectively. This eruption produced the ignimbrites and basalts. After this stage the fissural Geological History volcanism may have been radically changed to a central type and gave the basalts whose center could be Mount Megezez. Another small center is found around Maset. Geological Settings Geological Settings After a certain time the young Quaternary ignimbrites erupted from centers probably located around Addis Ababa, and were underlain non-conformably by younger basalts.

72 Geological Settings 4.Engineering Geological 4. Settings

4.1 General Results of two seasons of engineering geology mapping 2010–11 over a total area of 18,204 km2 carried by GSE are represent by an engineering geological map in scale 1:250,000 (Addis Ababa sheet NC 37-10), and by the following text. General The study area falls in the southern part of the Abay river basin. The mapped area is part of the uplifted Ethiopian Dome, the structure belonging to the broad regional structure of the northern part of the East African Rift. This rift-related development is mirrored by the high altitude of the central plateau of Ethiopia 1,000–3,500 m a.s.l. and the specific geological and geomorphologic features.

The highest altitude of the plateau is about 3,520 m a.s.l. on the “Gara Guda” mountain peak in the NE near Fiche and Degem. The southeastern and the northwestern part of the study area are lower – at about 2,000 m a.s.l. It belongs to the Abay River, Guder and Muger and Akaki river Engineering Geological Settings 60

50

40

30

proxy of area [%] area of proxy 20

10

0 Quaternary Teritary Mesozoic Paleozoic Proterozoic age

Fig. 4.1 Overall statistics of individual stratigraphic rock groups according areas of their outcropping

Engineering Geological Settings 73 catchment areas. Canyon-like valleys of the rivers listed above deeply dissected the plateau, so the lowest part is the gorge of the Abay River.

Long-term denudation and erosion proceeded in accordance with regional and partial uplifts. The regional uplift proceeded in several stages. The older in the southwestern part culminating uplift had caused stripping of most of the tertiary flood basalts to the older tuffs and basalts of TV2 flow in the northeastern and central parts. On the western border of the mapped area, even the underlying Mesozoic sediments, their Paleozoic basement and even Proterozoic crystalline rocks were erosionally exposed. The younger uplift phase had been most active in the northeastern part, Geological History where the present day highest parts of the area occur.

Proxy of areas occupied by individual stratigraphic groups can be seen on Fig. 4.1.

4.2. Methodology An engineering geological map represents a synthesis of all data relevant for civil engineering, Engineering Geological Settings geological risks zoning and land use. A database needed to compile such a map encompasses complex information from a broad field of geo-science branches.

The maps show basic information on the region in question: a) by setting regional ranges for

Methodology variability of engineering geology conditions to specify the main engineering geology characteristics of the region in question; b) by drawing attention to the main regionally characteristic engineering geology problems - mainly natural/geological risks, to be tackled; c) by providing initial guidance and thematic proposals for more detailed, site-oriented engineering geology mapping or specialized construction site surveys.

4.2.1 Data Acquisition The desk study concentrated on a review of archive data and excerption from topographic maps, thematic maps and existing reports, meteorological data, remote sensing imageries and field work planning.

During the field work the primary geological, geomechanical and geomorphological information was acquired.

The methodology of mapping and layout of the field maps was prepared in accordance with the methodology proposed in the early stages of the project and the quantitative descriptions, mainly of the physical-mechanical properties of rocks respect the standard Geological Survey of Ethiopia Standards given by the Technical Standards for Engineering Geology Mapping (Belete, 2006). • Technical data acquisition formats were prepared and used for collecting and recording field data (in-situ soil and rock tests, inventory of land cover/use, geo-hazard etc). The geological map at a scale of 1:50,000 and topographic maps at various scales have been used for planning field traverses and in-situ testing and a site inventory of geo-hazards and land cover/use. • The area was mapped as soil when the soil thickness exceeds 2 m; if it is less it is mapped as rock. • In-situ field testing (L-type Schmidt Hammer and point load) and a discontinuity survey on rock units were conducted to classify the rocks into various rock mass strength units. • A detailed discontinuity survey (orientation, condition, separation) was conducted on the representative rock units. Different discontinuity sets were measured using a geological compass and visual assessment and descriptions were made following the standard or rock exposure characteristic formats. • Soil samples were collected from representative soil units for laboratory index tests (grain size, Atterberg limits, and free swell). Test pits were manually dug to the required depth in order to collect soil samples for laboratory tests and to inspect the in-situ soil three dimensionally.

74 Engineering Geological Settings • Soils were classified genetically as residual, colluvial and alluvial. Because of frequent difficulties in distinguishing residual and short-way transported colluvial soils, common class residual- colluvial soils have been used on the map. All soils were further classified into various strength classes based on the relative compactness for cohesionless soil and relative consistency for cohesive soil. In-situ pocket penetrometer tests and field manual tests (dilatancy, toughness and dry strength) were used to classify cohesive soil according to their consistency. • Rock samples were collected from representative outcropping geological units for laboratory

unconfined compressive strength tests, porosity and water absorption tests. Methodology • Surface water and construction material potential of the area are assessed and special attention was given to the establishment of suitable construction sites for small dams.

4.3 Information Layers The engineering geology map has been prepared by an overlay integration of information layers and other relevant information. • Layer – Morphology (DEM) Engineering Geological Settings • Layer – Ranges of slope angle • Layer – Geology • Layer – Documentation points – test pits, rock sampling sites Information Layers Information Layer 1 - Hill Shading and Drainage Network The hill shading layer was processed from DGM using ENVI software. It replaces the contour lines for a synoptic expression of the terrain morphology. An example of hill shading is shown in Fig. 4.2.

Information Layer 2 - Ranges of Slope Angle This map has been derived from DEM using a standard algorithm based on a calculation of the angle between the dip and vertical line. The continuum of slope angle was divided into ranges and

Fig. 4.2 Example of hill shading visualization of DEM

Engineering Geological Settings 75 Information Layers Engineering Geological Settings

Fig. 4.3 Example of ranging of slope angles

a single value was assigned to each of them. Border values of the ranges were determined in the regular manner so that each range spans 5 degrees.

Fig. 4.4 Example of visualization of geology

76 Engineering Geological Settings Information Layer 3 - Geology The layer of bedrock and its soil cover is the same as for the compilation of the engineering geology as well as hydrogeological maps. Soils were mapped when their thickness was greater than 2 m. An example of the visualization of geology is shown in Fig. 4.4.

Information Layer 4 - Documentation Points - Test Pits, Rock Sampling Sites The layer contains geographical positions of all documentation points, while MS Excel spread sheets give all qualitative and quantitative characteristics of these points.

It is one of the background GIS layers included in the Engineering Geology Map Data Information Layers Catalogue (EGM-DC) showing 136 documentation sites and 52 exploratory pits, where the in-situ characteristics of rocks and soils were described, and rocks were tested in-situ as well as sampled for laboratory analysis. The sources of basic physical-mechanical, quantitative data gathered during the field work have a density of 0.01 point/km2.

Compilation of Engineering Geological Map - Engineering Geological Zoning Engineering Geological Settings The density of 0.01 documentation points per km2 is clearly insufficient to follow the standard procedure of engineering geological map compilation. The latter is in particular based on rock and soil ranking according physical-mechanical properties. Regarding that, a process oriented approach has been adopted.

Process Oriented Approach At the present state of knowledge, the process oriented approach is the only way to proceed. Moreover, it brings notable advantages, because it establishes a general framework for future categorization and interpretation of all documentation point data.

Relief Energy Zoning and its Backing by a Conceptual Model Process oriented IG zoning is a two-level hierarchical. At the first level it mirrors the main morphostructural and morphomerical features of the mapped area. Regarding them, the First

Fig. 4.5 Conceptual model for relief categorization in accordance with relief energy as a driving force of destruction, transport and sediment processes acting on the landscape surface

Engineering Geological Settings 77 Order Zones have been fixed. At the second level - more fine division is introduced according to the local geology and local level of relief energy. Relief energy is expressed via average inclination within the given area (pixel 90 x 90 m) as depicted in the map sketch 3 - Ranges of slope angle. The inclination is considered as an indirect indicator of activity of relief modeling exogenic processes such as physical and chemical weathering, erosion and carrying capacity of superficial water flow by precipitation. The driving of exogenic processes by other forms of climatic influence was considered to be the same throughout the whole area. A conceptual model for energy relief categorization is depicted in Fig. 4.5.

Information Layers Destruction zone/high energy zone consists of all areas with inclination above 27°. The angle of 27° represents the equilibrium slope angle of a freely scattered sand pile. It means, in that zone there is no possibility to accumulate any eluvial regolith, so a complete, highly active stripping from any medium to fine regolith takes place.

Only bare rocks exposed to weathering dominated by physical weathering occur in areas with inclination above 35° – i.e. the threshold angle of rock fill pile. In areas with inclination in the Engineering Geological Settings interval from 27° to 35°, occurrence of rock blocks scree accumulations are possible eluvial- deluvial mantle, but all finer material is transported down slope.

Transport/medium energy zone encloses areas with inclinations from 5° to 27°. The angle of 5° is an important geomorphological threshold known as the piedmont angle. Within this zone, the maximum amount of regolith transport takes place in time of rains. Therefore, only a thin layer of regolith can accumulate there in the meantime. This regolith forms deluvial deposits, and is dominated by medium-grained size fraction. Due to the presence of regolith mantle, it is possible to maintain some water content, so chemical weathering can take place on the contact of bare rock and moist deluvial-fluvial regolith. When the water content is increased it results in under- skin water flow, which removes all fine grained particles. When there is maximal water input due to heavy rains, nearly the entire regolith mantle is taken away and the underlying bare rock is again exposed to physical weathering. Therefore, the medium energy zone is also the zone of mixed physical and chemical weathering.

Accumulation/low energy zone can be found only in areas with inclination lower than 5°. As shown by its name, this zone is characterized by a mantle of very fine grained fluvial-deluvial or even alluvial and lacustrine regolith with medium or high thickness. Due to the thick, fine grained mantle, longer preservation of its water content is possible. So, there are better conditions for chemical weathering operation as at the bare rock/regolith contact, as well as within the regolith itself – i.e. secondary changes of mineral content (e.g. production of clay particles) can also change the original physical-mechanical properties of regolith. Therefore the low energy zone is also the zone of dominancy of chemical weathering.

4.4 Definition of Engineering Geological Zones I. Basic Zones of the First Hierarchic Level – Plateau and Valley Provinces Definition of Engineering Geological Zones Definition of Engineering Geological Zones I. Low relief-energy zone consisting of morphostructural and structural-denudation forms: only slightly undulating plains and shallow, wide basins. II. Medium relief-energy areas – areas adjacent to the rims of individual plains and near the edge of the high energy zone – i.e. as the edge of deep river valleys. III. High-relief-energy zone – deep river valleys which intervene with both Zone I and II in finger- like patterns, and have slope angles above 27°.

The low and medium energy zones form a plateau morphologic province, the high energy zone mainly occurs in the valley morphologic province.

78 Engineering Geological Settings Definition of Engineering Geological Zones Definition of Engineering Geological Zones Engineering Geological Settings

Fig. 4.6 Position of plateau and valley provinces within the mapped area

II. Zones of the Second Hierarchic Level – Detailed Regional Zoning Detailed regional zoning at the second hierarchic level has been supported using the ENVI SW pack. At first, map scheme 3 – Ranges of slope angle was reclassified to obtain only three of the above-defined classes of slope range/relief energy zones of 0–5°; 5–27°, and more than 27°. The new map obtained was merged with map scheme 1–Geology. The outcome yielded 56 classes of engineering geological regional units (see Tab. 4.1).

24%

plateau

valley

76% Fig. 4.7 Proxy of plateau and valley provinces within the mapped area

Engineering Geological Settings 79 Basic Engineering Geological Units An engineering geological map is a graphical presentation of spatial variability of occurrence of those units. The sequel according areas occupied by those individual classes within the mapped area is given in Tab. 4.1.

Tab. 4.1a Overall occurrence of geological slope class units IG Code Area [km2] Area [%]

ALLU_high 2.91 0.0151

ALLU_medium 33.33 0.1726

ALLU_low 49.06 0.2541

RES_high 1.01 0.0052

RES_medium 161.52 0.8365 Definition of Engineering Geological Zones Definition of Engineering Geological Zones Engineering Geological Settings RES_low 1,004.02 5.1997

Sub-total 1,251.85 6.4832

Tab. 4.1b An overview of classified regional engineering geological units with their size within the mapped area; part two (Part 1) IG Code Area [km2] Area [%]

TV7_high 770.00 0.0399

TV7_medium 540.19 2.7976

TV7_low 321.10 1.6629

TV6_high 1.34 0.0069

TV6_medium 130.98 0.6783

TV6_low 98.75 0.5114

TV5_high 1.24 0.0064

TV5_medium 117.47 0.6084

TV5_low 9.98 0.0517

TV4_high 0.05 0.0003

TV4_medium 15.17 0.0786

TV4_low 22.04 0.1141

TV3_medium 37.04 0.1918

TV3_low 164.03 0.8495

TV2_high 323.99 1.6779

TV2_medium 4,872.99 25.2366

80 Engineering Geological Settings Tab. 4.1b An overview of classified regional engineering geological units with their size within the mapped area; part two (Part 2) IG Code Area [km2] Area [%]

TV2_low 2,448.11 12.6785

TV1_high 88.24 0.4570

TV1_medium 385.47 1.9963

TV1_low 17.69 0.0916

Sub-total 9,603.57 49.7357

Tab. 4.1c An overview of classified regional engineering geological units with their size within the mapped area; part three IG Code Area [km2] Area [%] Definition of Engineering Geological Zones Definition of Engineering Geological Zones Engineering Geological Settings MEUSa_high 36.97 0.1915

MEUSa_medium 181.59 0.9404

MEUSa_low 2.65 0.0137

MEMu-Si_high 12.69 0.0657

MEMu-Si_medium 217.99 1.1289

MEMu-Si_low 65.88 0.3412

MELi_high 140.30 0.7266

MELi_medium 1,802.74 9.3362

MELi_low 92.43 0.4787

MEGy_high 15.51 0.0803

MEGy_medium 636.70 3.2974

MEGy_low 70.61 0.3657

MELSa_high 362.86 1.8792

MELSa_medium 2,142.96 11.0981

MELSa_low 421.13 2.1810

Sub-total 6,203.01 32.1246

Tab. 4.1d An overview of classified regional engineering geological units with their size within the mapped area; part four (Part 1) IG Code Area [km2] Area [%]

PASa_high 27.22 0.1410

PASa_medium 287.24 1.4876

Engineering Geological Settings 81 Tab. 4.1d An overview of classified regional engineering geological units with their size within the mapped area; part four (Part 2) IG Code Area [km2] Area [%]

PASa_low 26.08 0.1351

Sub-total 340.54 1.7636

Tab. 4.1e An overview of classified regional engineering geological units with their size within the mapped area; part five IG Code Area [km2] Area [%]

PRGn_high 5.08 0.0263

PRGn_medium 7.15 0.0370 Definition of Engineering Geological Zones Definition of Engineering Geological Zones Engineering Geological Settings PRGn_low 283.28 1.4671

PRGr_high 31.92 0.1653

PRGr_medium 0.07 0.0004

PRGr_low 18.10 0.0937

PRGa_high 6.62 0.0343

PRGa_medium 46.30 0.2398

PRGa_low 317.52 1.6444

PRHo_high 48.81 0.2528

PRHo_medium 1,145.40 6.9319

PRHo_low

Sub-total 1,910.25 9.8929

Geo-risk susceptibility Zoning of geo-risk susceptibility to four classes was done by reclassifying the total scores obtained by scoring the above-mentioned basic engineering geology.

The classified stratigraphy-terrain energy sequel was further classified by scoring individual units according rock strength, degree of weathering, and geomorphological character. Another

Tab. 4.2 Documented risky geodynamic processes (Part 1) X UTM Y UTM Lithology Type of hazard

435516 1078879 Basalt rock fall

419046 1080171 Basalt rock fall

433858 1084017 Basalt rock fall

82 Engineering Geological Settings Tab. 4.2 Documented risky geodynamic processes (Part 2) X UTM Y UTM Lithology Type of hazard

499903 1079384 Basalt rock fall

495399 1081691 Basalt rock fall

490161 1087491 Sandstone rock fall

417033 1077640 Basalt rock fall

412928 1083232 Basalt rock fall

412163 1082980 Basalt rock fall

341513 1073001 Basalt rock fall Definition of Engineering Geological Zones Definition of Engineering Geological Zones 453442 997993 Ignimbrite/tuff rock fall Engineering Geological Settings

435788 1078282 Basalt slope instability

469796 1084423 Basalt slope instability

496781 1079176 Tuff slope instability

490326 1083740 Basalt slope instability

493463 1015134 Soil surface erosion

461223 1089599 Soil surface erosion

435593 1082298 Soil surface erosion

449643 1082402 Soil surface erosion

469363 1082276 Soil surface erosion

423363 1081088 soil surface erosion

418759 1079470 Basalt surface erosion

434858 1084241 Soil surface erosion

498309 1079045 Soil surface erosion

412915 1083126 Soil surface erosion

336575 1040764 Soil surface erosion

333640 1053086 Soil surface erosion

339670 1007671 Soil surface erosion

335976 1013363 Soil surface erosion

449620 1002987 Soil surface erosion

453559 999670 Soil surface erosion

Engineering Geological Settings 83 Tab. 4.2 Documtented risky geodynamic processes (Part 3) X UTM Y UTM Lithology Type of hazard

455608 999239 Soil surface erosion

455735 999178 Soil surface erosion

459398 1003697 Soil surface erosion

446530 1009299 Soil surface erosion

359187 1076996 Basalt surface erosion

391574 1066790 Soil surface erosion

391515 1066767 Soil surface erosion

466603 1000365 Soil surface erosion Definition of Engineering Geological Zones Definition of Engineering Geological Zones Engineering Geological Settings

475647 1007043 Soil surface erosion

452403 1085890 Soil surface erosion

scoring criterion was occurrence of dangerous geodynamic processes documented within given rock type (Tab. 4.2). Finally, fault and joint structure were scored with regard to their possible connection with recent tectonic and seismic activity.

Final zoning of geo-risk susceptibility was done by scoring of basic engineering geological units and classifying the results of that scoring into four classes (Tab. 4.3).

The geo-risk sequel has been used twice: • To state their ranking for basic characterization with aspiration to preliminary fix their engineering geological hotspots.

Tab. 4.3 Basic land use and building activity restrictions assigned to basic geo-risks classes Color Geo-risk susceptibility

Low: no restricted land use, simple engineering geology and geotechnical conditions; no restriction of simple and medium structurally demanding engineering structures (up to two storeys, low and medium statically demanding with shallow foundations), simplified engineering geology survey and structural analysis, use of building code tabular values possible

Medium: moderately restricted land use, medium complicated engineering geology con- ditions – farming respecting geo-risk prevention; no restriction to simple, low demanding, statically determinate constructions up to two storeys with shallow foundations, in other cases full engineering geology site survey highly recommended

High: restricted land use, complicated engineering geology conditions - farming respecti- ng geo-risk prevention preferred; permanent habitation not recommended without engi- neering geology survey focusing on geodynamical hazards

Very high: strongly restricted land use, very complicated engineering geology conditions – to be left in natural state, restricted from any intervention without rigorous engineering geology survey focussing on prevention of geohazards and their remedial measures

84 Engineering Geological Settings 2% 23% Geological Settings 53%

22% Definition of Engineering Geological Zones

Fig. 4.8 Proxy of geo-risk susceptibility for the whole mapped area

0.6 % 1.4 % 5.2 %

92.8 %

Fig. 4.9 Proxy of geo-risk susceptibility for the plateau province

• To target future, more detailed works. Particularly the effort to get their fair characterization including a satisfactorily dense net of field documentation points within them.

4.5 Basic Characteristics of Important Engineering Geological Regional Units Quaternary Alluvium Soils (Allu) Quaternary alluvium is deposited in the eastern, western, and southern parts of the map area, Basic Characteristics of Important Units Engineering Geological Regional along Jemma, Zegawedem, and Shenkora river valleys. Its texture varies from sand to silt size.

Engineering Geological Settings 85 Various fragments of basalt, limestone, and rounded quartz grains are associated with it. These fragments are most probably parent rocks from which this alluvium is derived. The alluvium in general has reddish brown, dark brown and gray color. Its thickness varies from 1 to 3 m. At some places black cotton soil variety is also observed. The reddish brown clay has good plasticity. The black cotton soil is dry and fractured.

Quaternary Eluvium-Deluvium Soils (Dell) Eluvium soil is developed in-situ from the underlying parent rock by mechanical and chemical decomposition. This group of soil is found in the western and southeastern part of the project area on flat plateaus to gently sloping and low relief areas of basalt mainly found in Kewo, Dedu, Jemolefo and Hamus Gebeya sub-sheets.

It is reddish brown to brown in color. Clay and clayey silty soil types are found in this class.

The clay soil type is black and dark grey color, it cracks concoidally when broken, and it is wet. Its thickness varies from 1.8 to 3 m. The black soil has good plasticity and overlies basalt rock. Light Definition of Engineering Geological Zones Definition of Engineering Geological Zones Engineering Geological Settings grey soil is also observed. It is wet and shows plasticity and the underlying bedrock is ignimbrite. Clay silty soil is red and reddish brown in color, its thickness varies from 2 to 4 m. The bedrock under this soil is basalt. It has moderate plasticity and slight wetness.

The delluvium group of soil is found near mountain and hill foots such as Mt. Menagesha and Intoto. It has reddish brown color. The soil is clayey and silty clayey in texture and has a thickness of 1.7 to 3.3 m. In some place it breaks and forms small rectangular blocks.

4.5.1 Rock Mass 4.5.1.1 Rock with High Strength (Rhi) The basalt and trachytic lithological units have a 1 to 8 MPa index point load strength value

that is normalized to IS50. This is comparable to the estimated unconfined compressive strength (UCS) of 16 to 88 MPa. From the values, the basalt and the trachyte are classified as rock with high mass strength. The degree of weathering is slight to moderate. Tab. 4.4 summarizes the water absorption, porosity and bulk density of samples belonging to the high rock mass

Tab. 4.4 Summary of the laboratory measured physical properties of samples belonging to high rock mass strength units Water absorption [%] Porosity [%] Bulk density [g/cm3]

mean 3 7 2.67

range 30 77 3.11

standart deviation 7 14 0.56

count 43 43 43.00

strength. Although the average water absorption is only 3 %, extreme variations of rocks falling are expected in the engineering geological unit. This could be attributed to the presence of abundant inhomogeneous discontinuity at a smaller scale. The similar extreme variation in porosity on the other hand suggests the effect of differential weathering rather than the presence of fractures. In general, the bulk density is higher than the rocks in the other rock mass strength units.

86 Engineering Geological Settings 4.5.1.2 Rock with Medium Rock Mass Strength (Rme) Precambrian and Paleozoic sandstones and the pyroclastic deposits (mainly comprised of ignibrites) fall in the medium rock mass strength category. Ignimbrite rocks outcrop in the southeastern part of the mapped area. This unit has a 0.4 to 6 MPa index point load strength value that is normalized to IS50. The corresponding UCS value ranges from 3.75 to 45 MPa and hence falls in the medium rock mass strength category.

The water absorption of ignimbrites in the area is high making them very limited for use as aggregates for concrete and asphalt. The ignimbrites have a moderate to high degree of weathering. Relatively higher water absorption is noted in the samples of moderate strong rock units than the high strength rock mass units (Tab. 4.5). Slightly lower bulk density is observed Geological Settings relative to the stronger rock mass units.

Tab. 4.5 Summary of the laboratory measured physical properties of samples belonging to moderate rock mass strength units

3

Water absorption [%] Porosity [%] Bulk density [g/cm ] Definition of Engineering Geological Zones

mean 10 19 2.07

range 23 36 2.07

standart deviation 9 15 0.41

count 7 7 7.00

4.5.1.3 Rock with Medium to Low Rock Mass Strength (RMe to RLo) The Mesozoic sandstone and limestone units are categorized in this class. The Mesozoic sandstone outcrops in the northeastern and western part of the mapped area. The point load index strength ranges from 0.2 to 1.1 MPa which is comparable to UCS values of 4.57 to 26 MPa. The lithological units hence are categorized as having low to medium rock mass strength. An overall moderate degree of rock mass weathering is notable in the lithological units. In this rock mass unit water absorption and porosity are very low while a slight increase in the bulk density is observed (Tab. 4.6). Filling of discontinuities and pore spaces by a higher degree of weathering could be the cause of such behavior.

4.5.1.4 Rock with Low Rock Mass Strength (RLo) The Muger Mudstone unit and the Gypsum unit are categorized into this class. The Muger Mudstone unit outcrops in the northeastern and northern parts of the mapped area. The gypsum

Tab. 4.6 Summary of the laboratory measured physical properties of samples belonging to rock units of moderate to low mass strength Water absorption [%] Porosity [%] Bulk density [g/cm3]

mean 1 3 2.65

range 2 4 0.34

standart deviation 12 0.13 Basic Characteristics of Important Units Engineering Geological Regional count 5 5 5.00

Engineering Geological Settings 87 unit outcrops in the central part of the mapped area. No significant tests could be made in this unit due to their low overall strength. Hence, these units have been classified as rock with low rock mass strength. Geological Settings Geological Settings Basic Characteristics of Important Units Engineering Geological Regional

88 Engineering Geological Settings 5.5. Hydrogeology

Hydrogeology of the Addis Ababa sheet is based on the assessment of a large amount of data collected from existing reports and maps and during field work. The acquired data were processed in the form of maps with the final aim to assess the groundwater resources of the area.

5.1 Water Point Inventory The field water point inventory was based on a desk study, during which the relevant materials like geological reports and maps and aerial photographs were collected from the regional geology

department of GSE. Important climatic and gauging station data and topographic maps were Hydrogeology collected and purchased from various offices. The desk study also included preliminary data Inventory Point Water interpretation and preparation of field maps using satellite images, aerial photographs and a digital elevation model (DEM) of terrain with geology as a background.

The hydrogeological map of Ethiopia at a scale of 1:2,000,000 was published by Tesfaye Chernet (1993). He classified the geological units of Ethiopia into four major groups depending on the type of permeability and the extent of the aquifer. This hydrogeological map was the basic document for preparation of the field work. Tesfaye (1993) identified the following units: • Mesozoic limestone and the Upper sandstone with fissured and/or karst permeability were classified as highly productive aquifers; the specific yield of wells was estimated to be in interval 0.2–7.6 l/s.m and total yield of wells with 20 m of drawdown varies in interval 1.8–68.4 l/s in highly productive aquifers. • Other sedimentary (the Mesozoic Lower sandstone and Paleozoic sandstone) and volcanic rocks with fissured porosity were classified as moderately productive aquifers; the specific yield of wells was estimated to be in interval 0.05–1.1 l/s.m and total yield of wells with 20 m of drawdown varies in interval 0.45–9.9 l/s in moderately productive aquifers. • Gypsum formation was classified as low productive aquifer the specific yield of wells was estimated to be in interval 0.006–0.5 l/s.m and total yield of wells with 20 m of drawdown varies in interval 0.05–4.5 l/s in low productive aquifers. • Outcrops of basement rocks in the Guder valley are described as localized aquifers with fracture and intergranular porosity and are characterized as a regional low productivity aquiclude. • Recharge characteristics of the highlands were derived for 150–250 mm/year and discharge is characterized as moderate to high. • The highlands (in the northwest) were classified as an area with major water resources. These were assessed to be widespread and moderate to large in quantity. Groundwater and surface water are of good chemical quality with TDS less than 500 mg/l in the highlands and less than 1,500 mg/l in deep valleys. Most of the streams are perennial; there are many cold springs, and the groundwater level is between 0 and 100 m and can be exploited in low relief areas (valleys).

• Groundwater chemistry is characterized as being bicarbonate (HCO3) in the highlands.

Hydrogeology 89 Jiri Sima et al. (2009) in Water Resources Management and Environmental Protection Studies of the Jemma Basin for Improved Food Security described the plateau covered with various volcanic rocks intercalated by sediments in places as an area with extensive and moderately productive or locally developed and highly productive fissured aquifers. The aquifers are in parts classified as being mixed with porous sediments contributing to well yield. This area receives adequate precipitation and has moderate runoff resulting in good infiltration and the possibility to feed deeper fissured aquifers developed in older volcanic and sedimentary rocks which were classified as extensive fissured aquifers of low productivity. Muger mudstone forms the first base (aquitard) for groundwater that flows horizontally and emerges in deep valleys as springs from the upper Hydrogeology Hydrogeology sandstone. The upper sandstone forms extensive and moderately productive fissured aquifers. Water Point Inventory Point Water The mudstone layer is not continuous within the whole basin and this fact allows water to infiltrate to greater depths and feed underlying aquifers developed in limestone. Limestone represents an extensive and highly productive fissured and karst aquifer and an underlying thick layer of gypsum forms the second base (aquitard) for groundwater that also flows and emerges as large springs from the limestone. The mechanism for recharge and drainage of the lower sandstone underlying the gypsum is not known in the Jemma basin.

Berhanu Melaku (1982) in Hydrogeology of Upper Awash Basin (EIGS) discussed the hydrogeological setup of the upper Awash basin. He classified aquifers of the area as very good, good, moderate and poor based on surface geological hydrogeological studies and inventory of ground water points.

A study on hydrochemical and environmental investigation of Addis Ababa region was done by Berhanu Gizaw (2002). The study revealed that the regional ground water flow is north-south and

the dominant water type is Ca–Mg–HCO 3.

Recently the geology of different subsets of Addis Ababa sheet was studied by Matebie and Assiged et al. between 2006 and 2007. This study shows the area is dominantly covered by Mesozoic sediments to Tertiary volcanic rocks and Quaternary basalt and sediment.

WWDSE (2006) worked on an evaluation of water resources of the basin of the Ada’a and Becho plains for irrigation development. This is a comprehensive study and supported by geophysics and drilled mapping wells. The work classified the main aquifers of the area into upper and lower basalt aquifers and alluvial and lacustrine aquifers. The upper basalt aquifers include Weliso-Ambo basalt, Akaki basalt and Addis Ababa basalt, whereas the lower aquifers include Tarmaber basalt and Aiba basalt.

Tilahun (2008) in his MSc thesis presented a hydrogechemical characterization of aquifer systems in the Upper Awash and adjacent plateau using geochemical modeling and isotope hydrology. The research work addressed schematic conceptual models for spatial geochemical variations, evaluation and recharge areas for shallow and deep aquifer system along four ground water flow path starting from the plateau, through the transitional erosion escarpment into the rift valley.

Thermal water references and a description of previous work are presented in Chapter 6.4.

Topographic and geological maps of 1:50,000 scale were used during the field work as a base map in addition to 1:60,000 aerial photographs. A compass and a GPS were used for navigation and locating the water points. The water points were characterized by location, lithology, topography and field measurements of pH, temperature and EC were taken. Pictures and video sequences were captured for documentation and interpretation. Discharge of springs and rivers was measured by the floating, volumetric method and by visual assessment. The static water levels of boreholes with piezometers and open hand dug wells were measured using an electrical

90 Hydrogeology Tab. 5.1 Summary of field inventory Water point type Number of inventory Sampled

Borehole (BH) 87 57

Cold spring (CS) 128 93

Dug well (DW) 23 17

River water (RV) 44 Hydrogeology Water Point Inventory Point Water Rain water (RW) 22

Total 244 173 Source: Inventory of the study area 2009 –2010 sounding deeper. A summary of the field inventory is shown in Tab. 5.1 and an extract from the water point inventory database is shown in Annex 1.

Existing reports about borehole data were collected from regional water bureaus, Addis Ababa Municipality, NGOs and private drilling companies as well as from direct contact with drillers and geologists in the field (Fig. 5.1).

Data Assessment was mainly dedicated to data organization, processing, and interpretation in the form of maps and the text of the presented explanatory notes. Aquifers are classified in this report according to their productivity based on the yield measured in the field and hydraulic properties like transmissivity obtained from pumping test data together with topographic settings and recharge conditions. The geographic information system (GIS) ArcGis was used for compilation of maps.

Fig. 5.1 Field collection of data from drilling near Ginchi

Hydrogeology 91 Groundwater from water points representing important parts of the map sheet s hydrogeological system was sampled for chemical analysis (see Chapter 6).

5.2 Hydrogeological Classification/Characterization The qualitative division of lithological units is based on the hydrogeological characteristics of various rock types using field water point inventory data, including data from pumping tests of various boreholes. The lithological units were divided into groups with dominant porous and fissured permeability and impermeable rocks. This division served for definition of the map Hydrogeology Hydrogeology sheet s aquifer/aquitard system. Since quantitative data such as permeability, aquifer thickness and yield are not adequate or evenly distributed enough to make a detailed quantitative potential classification; analogy was used for characterization of rocks without the adequate number of water points. Hence, the hydrogeological characterization of the study area reveals the following aquifer/aquitard systems:

Units with porous permeability, where groundwater is accumulated in and is flowing through

Hydrogeological Classification/Characterization Hydrogeological pores of an unconsolidated or semi-consolidated material. Porous materials of Quaternary and Tertiary age (mixed aquifers) are represented by fluvial and colluvial sediments developed in depressions (sediments of lakes) and/or along valleys of former and existing rivers. The pyroclastic rocks between lava flows are generally porous but usually less permeable due to poor sorting. Layers of paleosoil of various thicknesses in between lava flows are also less permeable and consist usually of clay material.

Units with fissured and karst permeability, where groundwater is accumulating in and flowing through the weathered rock but mainly through fissures and joints developed in volcanic and sedimentary rocks. The porosity of lava flows may be high but the permeability is largely a function of a combination of the primary and secondary structures (joints and fissures) within the rock. In addition, the permeability of lava flows tends to decrease with geological time. Pyroclastic rocks associated with lava flows as well as paleosoils in between lava flows are generally porous but not very permeable because of poor sorting and the abundance of fine materials. Hence, extensive volcanic ash beds may form semi-horizontal barriers to water movement (infiltration) resulting in lower productivity of basaltic units located at greater depth. These rocks sometimes show cracks and fissures that can transmit and store groundwater between various lava flows. Mesozoic sediments represented by sandstone and limestone form aquifers with good fissured porosity which can be enhanced by dissolution and karstification of limestone in some places. The units with fissured and karst permeability forming aquifers are expressed on the hydrogeological map in green.

Basement rocks represent fissured aquifers of low potential. The groundwater in the hard rock is practically all stored in the fractured zones and the weathered mantle called overburden or regolith. The depth of fractured aquifer zones is generally no more than 50–70 m below the surface. The fractures will tend to close at depth. The faults and joints in igneous rocks are nearly vertical, except for narrow fractures, which are more or less parallel to the rock surface, sheeting and exfoliation. The greatest permeability is found in the sub-soil zone within the partly decomposed rock. Wells tapping this zone have yields roughly an order of magnitude greater than in the fresh rock. The aquifers are expressed in the model of the hydrogeological map in brown/red.

Impermeable units like fresh rhyolite, particularly when forming mountains, as well as mudstone and gypsum which are compact and massive with relatively few fissures having a low effective porosity represent impermeable layers and/or layers with limited groundwater resources within the studied area. The impermeable units form aquitards and aquicludes and are expressed on the hydrogeological map in brown.

92 Hydrogeology 5.3 Elements of the Hydrogeological System of the Area (Aquifers, Aquicludes and Aquitards) Geological description and qualitative division of various geological units together with their topographical position within the map sheet lead to a definition of elements of the hydrogeological system and its conceptual hydrogeological model. The system consists of the following elements: • Porous aquifer developed in alluvial and colluvial sediments of Quaternary age and in Tertiary semi-consolidated sediments and pyroclastics, where their outcrops can be shown on the map. • Fissured locally developed aquifers (minor aquifers) in rhyolites forming watershed mountains, particularly along the watershed of the Abay – Awash rivers. Hydrogeology • Fissured aquifer developed in basalts and other volcanic rocks on the plateau. • Mixed aquifer developed in fissured ignimbrite, rhyolite, trachyte and basalt and porous tertiary sediments intercalating volcanic rocks on the plateau. • Fissured aquifer developed in basalts and other volcanic rocks outcropping in deep valleys. • Fissured and karstic aquifers developed in Mesozoic and older sandstone and limestone in deep valleys. • Aquifer developed in fractured zones and the weathered mantle of basement rocks. • Aquicludes represented by Mesozoic mudstone and gypsum. Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological The hydrogeological map shows aquifers and aquitards defined based on the character of the groundwater flow (pores, fissures), the yield of springs and the hydraulic characteristics of boreholes. The following aquifers and aquitards were defined: 1. Extensive (larger than 100 km2 –1,252 km2) and moderately productive or locally developed and highly productive porous aquifers (T = 1.1 –10 m2/d, q = 0.011– 0.1 l/s.m, with yield of spring and well Q = 0.51–5 l/s). The aquifers are shown in light blue. Aquifers consist of Quaternary unconsolidated. 2. Extensive (larger than 100 km2–2,035 km2) and highly productive fissured / karst aquifer (T = 10.1–100 m2/d, q = 0.11–1 l/s.m, with spring and well yield Q = 5.1–25 l/s). The aquifer is shown in dark green. The aquifer consists of Mesozoic limestone. 3. Extensive (larger than 100 km2–13,071 km2) and moderately productive or locally developed and highly productive fissured aquifers (T = 1.1 –10 m2/d, q = 0.011– 0.1 l/s.m, with spring and well yield Q = 0.51–5 l/s). The aquifers are in light green. Aquifers consist of basalts and other volcanic rocks, including mixed units of various Tertiary rocks such as ignimbrites, basalts, rhyolite, trachyte and pyroclastic intercalated with sediments and Mesozoic and Paleozoic sandstones. 4. Extensive (larger than 100 km2–805 km2) low productive fissured aquifers (T = 0.11–1 m2/d, q = 0.0011–0.01 l/s.m, with spring and well yield Q = 0.051–0.5 l/s). The aquifers are shown in brown/red. The aquifers consist of basement rocks (no data on the sheet but analogy from other areas). 5. Minor fissured aquifers with local and limited groundwater resources (21 km2) – aquitard (T < 0.1m2/d, q < 0.001 l/s.m, with spring and well yield (Q) less than 0.05 l/s). The aquitard is shown in light brown. The aquitard consists of Tertiary rhyolite of a limited extend with outcrops forming watershed mountains along the Abay – Awash surface water divide. 6. Formation with essentially no groundwater resources (1,019 km2) – aquiclude. The aquicludes are shown in dark brown. The aquicludes consist of mudstone and gypsum.

The following detailed hydrogeological characteristics of the aquifers – aquitards and hydrogeological characteristics of the individual lithological units are described based on field observation in which 244 water points consisting of boreholes, springs and dug wells were inventoried during the Addis Ababa sheet field seasons of 2009 to 2010.

Hydrogeology 93 5.3.1 Extensive and Moderately Productive Porous Aquifers The porous aquifers altogether make up 1,252 km2, and consist of alluvial, colluvial and elluvial sediments of Quaternary age and Tertiary sediments. These aquifers cover 7 % of the map sheet and are shown in light blue.

Quaternary Deposits The general distribution of the deposits with porous permeability in the study area is localized to a gentle undulating plateau and along rivers. The area covered by deposits with porous permeability

Hydrogeology Hydrogeology is relatively large. The thickness of these deposits is variable from place to place. The alluvium along major rivers like the Abay, Jemma, Muger and Guder is reworked and seems less compacted and more porous. The alluvial sediments are represented mainly by porous-sandy-gravels to silty sand which can be exploited by shallow dug wells. Elluvial sediments of the plateau consist of silty to clayey soil. Aquifers in alluvial and elluvial sediments are recharged directly by infiltration of rain. Alluvial sediments can also be recharged by rivers during high discharge and by groundwater from aquifers which are cut by the river. Groundwater of aquifers in alluvial and elluvial sediments is drained by small springs and rivers. Plateau Quaternary deposits also recharge the underlying aquifers. Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological Dug wells sunk into the plateau´s Quaternary deposits supply the local community with groundwater of low TDS but in some situations where the elluvial cover is thin the wells fail during the dry season after pumping for only a few hours. The groundwater level was about 1–2 m below the surface during the water point inventory. In most of the visited wells a groundwater fluctuation of several meters is common. Groundwater is under water table conditions.

Perennial springs with variable yield depending on sediment thickness, degree of consolidation and extension of elluvial and/or alluvial sediments were seen on some of the inter-mountain plains. The safe yield of wells sunk into Quaternary deposits is estimated to be about 0.5 l/s. The thicker cover of Quaternary sediments can be located using simple geophysical measurements, e.g. VES.

Tertiary Sediments Tertiary sediments separate the lower and the upper part of the basalts and are represented by oilshale and tuffaceous sediments, and this is considered to be one of the major disconformities among the flows. The inter-volcanic sediments usually do not exceed 10 m as observed in the Tsid Mariam section, but can be about 50 m thick as observed in Hose and Gebre Guracha. The sediments consist of various permeable (sand, gravel and limestone) as well as impermeable (marl, clay and coal seams) rocks and represent lake sediments and/or fluvial and colluvial sediments of wide valleys and shallow depressions of a former plateau. The sediments can also contain some anhydrite-gypsum layers. No visible spring was found during the field work because the outcrops of Tertiary sediments are located in the upper parts of deeply dissected valleys and the contact of sediment with underlying basalt is covered with slope sediments. These intercalated sediments do not act as an independent aquifer but they form a mixed fissured and porous multilayered aquifer together with the volcanic rocks. Tertiary sediments can significantly contribute to the safe yield of wells when they are developed together with volcanic rocks. The permeable porous sediments in between lava flows form a body that can accumulate large volume of groundwater by drainage of surrounding fissured aquifers and contribute to the yields of wells developing groundwater from this mixed aquifer, which is more productive then fresh basalt, ignimbrite, trachyte and rhyolite that are normally considered as rocks with very low permeability. Tertiary sediments are recharged indirectly by groundwater from the overlying aquifers developed in volcanic rocks. No specific water point draining the Tertiary sediments was inventoried so groundwater probably drains into deep river valleys and also recharges the underlying volcanic aquifers. Groundwater is probably under water table or semi-confined conditions. The distribution of porous aquifers is shown in Fig. 5.2.

94 Hydrogeology Hydrogeology Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological

Fig. 5.2 Distribution of porous aquifers

5.3.2 Extensive and Highly Productive Fissured and Karstic Aquifer The limestone represents an extensive (2,035 km2) and highly productive fissured and possibly karst aquifer in the area. The aquifer covers 11 % of the sheet area and is shown in dark green.

The drainage of the aquifer is documented by a limited number of water points because aquifer outcrops can only be found along the deepest parts of the Abay, Jemma, Guder and mainly Muger rivers and some of their tributaries. These valleys are difficult to access and are sparsely inhabited. It seems that a large volume of groundwater of the aquifer is drained directly to rivers hidden (covered) by well-developed alluvial sediments of the main rivers. This idea is confirmed both by the chemistry of the lower reach of the Jemma River tending towards sulphate types and by the relatively high river flow during the dry season reflecting maintenance of surface flow by drainage of groundwater from medium to highly productive aquifers.

The discharge of 8 inventoried springs draining groundwater of the aquifer developed in limestone was measured within the Addis Ababa sheet. Data from the Addis Ababa sheet were combined with data from the whole Jemma basin (7 points) and plotted in Fig. 5.3.

The springs have an average discharge of 17 l/s and median discharge of 5.1 l/s. The lowest discharge was measured to be 0.05 l/s the highest discharge was measured to be 100 l/s. Large differences in the discharge of springs are given by the karstic character of the aquifer. Distribution of fissured and karst aquifer developed in limestone is shown in Fig. 5.4.

The limestone aquifer is enclosed between aquitards formed by the overlying mudstone and underlying gypsum. Both of the mudstone and the gypsum units are large; however, they do not represent regional units like lower sandstone and limestone. Therefore groundwater from the

Hydrogeology 95 10

8

6 Hydrogeology Hydrogeology

4 frequency

2

0

Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological <0.005 0.005 - 0.05 0.05 - 0.5 0.5 - 5 >5 classes of yield [l/s]

Fig. 5.3 Frequency of yield of springs in fissured and karstic aquifer developed in limestone

upper aquifers (upper sandstone and volcanic rocks) can infiltrate into the limestone and possibly to the lower sandstone as part of the deep regional groundwater circulation. Mudstone serves as a confining layer for the limestone aquifer in places where the layer overlies the limestone. The

Fig. 5.4 Distribution of fissured and karst aquifer developed in limestone

96 Hydrogeology high sulphate content in some springs can be explained by the occurrence of gypsum in the matrix of limestone and/or groundwater flowing along the boundary between the limestone and gypsum before it emerges from the spring.

5.3.3 Extensive and Moderately Productive Fissured and Mixed Aquifers Volcanic and sedimentary rocks of the map sheet represent an extensive (13,071 km2) and moderately productive fissured and mixed aquifers which are the principal aquifers in the area. Aquifers with fissured and mixed fissured and porous permeability cover 72 % of the map sheet

and are shown on the hydrogeological map in light green. Hydrogeology

Volcanic Rocks The rocks consist of basalts, ingnimbrites, trachytes rhyolites and tuffs and ash. Volcanic rocks are intercalated by various sediments and often form mixed units. Groundwater is mostly under water table conditions, however, semi-confined conditions can be found in a large number of the wells. These semi-confined conditions are a result of vertical as well as lateral non-homogeneity of volcanic rocks partly intercalated by various sediments. Basaltic lavas tend to be fluid and form relatively thin flows that have considerable pore space at the tops and bottoms of the flows.

Tab. 5.2 Basic data about selected inventoried wells of the Area System Elements of the Hydrogeological SWL Yield Drawdown Specific T Site ID Site name Aquifer [m b.g.l.] [l/s] [m] yield [l/s.m] [m2/d]

BH-6 Muka Ture 20.00 6.0 37.00 0.16 Basalt

BH-26 Legeta 8.14 50.0 48.98 1.02 294.00 Ignimbrite

BH-27 Welmera artesian 16.0 202.00 Basalt

Basalt + BH-33 Kebele 16 15.60 25.0 65.90 0.38 467.00 ignimbrite Basalt + BH-34 Wondira 26.37 27.0 5.60 4.82 657.00 ignimbrite

BH-35 Hamele 39.84 17.0 57.74 0.29 1,243.00 Basalt

Miki BH-36 artesian 20.0 18.18 1.10 248.00 Basalt Lyland

BH-50 Hambiso 32.94 7.0 380.00 Basalt

Garba BH-51 64.11 2.5 2.65 Basalt Sirea

BH-52 Bedhada 6.54 2.29 33.70 Basalt

BH-53 Bedhada 6.54 10.6 129.00 Basalt

BH-54 Selle 28.15 7.3 197.00 Basalt

Abysinia Basalt + BH-55 15.3 524.00 Flower ignimbrite Garad BH-79 94.00 5.0 14.68 0.34 Volcanics Flower

HBH-87 Filwoha artesian 4.5 15.50 0.29 33.12 Volcanics

Hydrogeology 97 30

25

20

15 Hydrogeology Hydrogeology frequency 10

5

0

<0.005 0.005 - 0.05 0.05 - 0.5 0.5 - 5< 5 Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological classes of yield [l/s]

Fig. 5.5 Frequency of yield of springs in fissured aquifers developed in volcanic rocks

Numerous basalt flows commonly overlap and the flows are separated by soil zones or alluvial materials that form permeable zones. Columnar joints that develop in the central parts of basalt flows create passages that allow water to move vertically through the basalt. Basaltic rocks are the most productive aquifers in volcanic rocks. Ignimbrite and trachyte form a non-homogeneous hydrogeological environment, with the amount of groundwater in these rocks varying from place to place depending on the frequency, intensity and distribution of the fracturing system. Silicic lavas tend to be extruded as thick, dense flows, and they have low permeability except where they are fractured. Aquifers with fissured and mixed fissured and porous permeability cover 72 % of the map sheet and are shown on the hydrogeological map in light green.

The major lineaments and faults in volcanic rocks units trend NE-SW, NW-SE, N-S and E-W directions. The NE-SW lineaments being dominant in the area and they are parallel to the structures of the rift or rift margin. Most of the lineaments are observed in all of the units and rivers follow these structures. The basalt formation in places has a large proportion of scoraceous lava flows which are highly porous, favoring groundwater storage and movement. Most of the springs of this unit are fracture controlled usually emerging at the intersection points of fractures or faults with a topographical depression. Discharge of 50 springs and wells was measured during the field inventory. The springs and wells have an average discharge of 6.51 l/s and a median discharge of 2.25 l/s. The lowest discharge was measured to be 0.01 l/s and the highest discharge was measured to be 50 l/s. Dry period discharge of springs varies from 0.01 to 35 l/s. Basic available data about inventoried wells is shown in Tab. 5.2.

Springs with high discharge (Csp-111 Guduru at Werka Gara: 35 l/s; Csp-15 Duber 01 at Sululta: 10 l/s; Csp-39 Laga Nora at Degem: 34 l/s; Csp-93 Keta at Gojo: 30 l/s) represent a regional drainage of volcanic rocks of the plateau reflecting the high potential of these aquifers. Yield of wells exceeds 10 l/s in 50 % of the inventoried wells. Frequency of yield of springs and wells discharging groundwater from aquifers developed in volcanic rocks is shown in Fig. 5.5.

Due to the relatively good yield and constant discharge rate of the springs emerging from volcanic rocks, they are widely used for small scale irrigation purposes in addition to being a sustainable

98 Hydrogeology 10

8

6 Hydrogeology 4 frequency

2

0 <0.005 0.005 - 0.05 0.05 - 0.5 0.5 - 5 >5 Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological classes of yield [l/s]

Fig. 5.6 Frequency of yield of springs in fissured aquifers developed in sandstones community water supply including parts of Addis Ababa. This aquifer is recharged directly by infiltration of precipitation and/or by infiltration from a porous aquifer developed in Quaternary sediments covering the plateau area and by rivers. The plateau sediments retain rainwater for a longer time and create favorable conditions for infiltration through a highly weathered, jointed and permeable upper layer. Groundwater is drained by shallow rivers and also recharges the underlying aquifers. The groundwater is probably under water table conditions.

The static water level measurements of boreholes in the volcanic rocks are in the range from 0 m – artesian condition to 94 m below the surface for BH-79 at Garod flower plc at Welmera (Annex 1).

Sandstone The Upper and Lower Mesozoic sandstone and Paleozoic sandstone outcrop on both sides of the Abay, Jemma, Muger and mainly Guder rivers and their major tributaries. The drainage of aquifers is documented by a limited number of water points because aquifer outcrops can only be found along the deepest parts of valleys of rivers and some of their tributaries. These valleys are difficult to access and are sparsely inhabited. It seems that a large volume of groundwater in the aquifer is drained directly to rivers hidden (covered) by the well developed alluvial sediments of the main rivers. The discharge of 7 inventoried springs draining groundwater of the aquifers developed in sandstones was measured within the Addis Ababa sheet. Data from the Addis Ababa sheet were combined with data from whole Jemma basin (10 points) and plotted in Fig. 5.6.

The yield of the springs varies from 0.07 to 20.2 l/s with an average of 3.22 l/s and a median of 1.75 l/s. From the information provided by local inhabitants, the yield of these springs remains virtually constant throughout the year. Most of the springs emerge near the contact with the underlying Mugher mudstone. Groundwater is mostly under water table conditions; however, semi-confined conditions can be found in some parts of aquifers where they are covered by impermeable layers.

Recharge to the aquifer is mainly through overlying aquifers developed in Tertiary volcanic rocks. Infiltrating rain water penetrates to greater and greater depth and saturates the aquifer in the

Hydrogeology 99 Hydrogeology Hydrogeology Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological

Fig. 5.7 Distribution of moderately productive fissured and mixed aquifers

sandstones. Groundwater flows along their impermeable bottom consisting of Mugher mudstone and/or basement and emerges from river gorges as local springs. Both the Muger mudstone and the gypsum units are large; however, they do not represent regional units like sandstones and limestone. Therefore groundwater from the upper aquifers (upper sandstone and volcanic rocks) can infiltrate into the limestone and to the lower Mesozoic and Paleozoic sandstone as part of the deep regional groundwater circulation. This hypothesis is confirmed by Csp-124 nearby the 7th camp of the Fincha sugar factory at Horro Gudur which emerges at the contact between sandstone and the underlying basement and has a yield of 2 l/s. Distribution of moderately productive fissured and mixed aquifers is shown in Fig. 5.7.

5.3.4. Extensive and Low Productive Fissured Aquifers The basement rocks are classified as an extensive (805 km2) and low productive fissured aquifer the area and consist of various crystalline (metamorphosed and igneous) rocks of Precambrian age. Aquifers with fissured permeability developed in basement rocks cover 4 % of map sheet and are shown on the hydrogeological map in brown/red color.

The basement rocks occupy a limited area at the bottom of deep valleys along the Abay and Guder rivers in the north and west. There is no water point inventoried within the Addis Ababa sheet and aquifer characteristics are described based on analogy from other mapped areas. Generally, in the Precambrian basement rocks the groundwater is accumulated or stored in the fracture openings, weathered parts, fractures and joints of the hard rock which may be closed at depth hindering deeper groundwater circulation and storage. Therefore, because of limited fracturing and jointing, small extent and impermeable nature of the rock, they have very little prospect of having sufficient water storage to be a good regional aquifers. In the study area the

100 Hydrogeology Hydrogeology Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological

Fig. 5.8 The extent and location of aquitards and low productive fissured aquifers

Precambrian basement rocks are mostly found at a depth overlain by sandstones. The extent and location of aquifers are shown together with aquitards and aquicludes in Fig. 5.8.

It is clear that the basement rocks have heterogeneous hydrogeological characteristics and each site needs to be treated separately for water well siting. The deeper weathering and fracturing along tectonic zones can be located using simple geophysical measurements, e.g. VES.

Recharge to the aquifer is mainly through overlying aquifers. Infiltrating rain water penetrates to greater and greater depths and saturates the basement aquifer. Limited recharge is also supposed directly through the outcrops of the aquifer.

5.3.5 Minor Aquifer with Local and Limited Groundwater Resources (Aquitard) Rhyolite The rhyolite unit covers a relatively small area (21 km2) and no springs emerge from this unit. As quantitative data are not available for this unit, hydrogeological classification is based on the geologic information. The unit forms small ridge along Abay – Awash surface water divide and hydro-structures such as joints and fissures are poorly developed. The recharge, permeability and storage of water are believed to be minimal therefore this unit is considered as an aquitard.

5.3.6 Formation with Essentially no Groundwater Resources (Aquitard) Mudstone and Gypsum The mudstone and gypsum units of the area are found in the lower valley of the Abay, Jemma and Muger rivers covering 1,019 km2 – 6 % of the map sheet. No water points emerge directly from

Hydrogeology 101 these units but since this unit acts as an impervious layer springs emerge at the contact zones with the overlying aquifers.

The extent and location of aquitards and aquicludes are shown together with low productive fissured aquifers in Fig. 5.8.

5.4 Hydrogeological Conceptual Model Minor aquifers with local and limited groundwater resources are found in mountains of a limited Hydrogeology Hydrogeology extent forming the peak of the plateau and ridges along the Abay and Awash river basin divide. Runoff is relatively fast and infiltration is limited. The rest of the plateau area is covered with various volcanic rocks forming a gently undulating plain that receives adequate rainfall and has moderate runoff resulting in good infiltration and formation of extensive and moderately productive or locally developed and highly productive fissured aquifers. Infiltration is particularly good in areas where the plateau is covered by thick elluvial sediments. Aquifers outcropping in the plateau area also feed deeper fissured aquifers developed in underlying volcanic and sedimentary rocks.

The groundwater flow direction in the whole area coincides with the topography following the Elements of the Hydrogeological System of the Area System Elements of the Hydrogeological surface water flow direction because small intermittent and particularly perennial rivers form shallow local drainage levels for shallow aquifers. The flow is partly controlled by the structure and partly by the geomorphology of the area. Most of the springs emerging from tertiary volcanic rocks are topographically controlled and others emerge along structures indicating that the groundwater flow is controlled by both factors. Local groundwater flow directions vary from place to place according to the local topography. Groundwater flow direction is not only oriented to the drainage system of the Abay basin formed by the Abay, Jemma, Muger and Guder rivers and their tributaries but also to the direction of the Awash river basin (Rift Valley). The groundwater divide between the Abay and Awash basins does not generally conform to the surface water divide but it is slightly shifted into the Abay basin because this part of the basin forms a plateau area while the

Hydrogeological Conceptual Model Awash basin is formed by relatively steep erosional escarpment in the Addis Ababa area. Deeper local flow emerges along the edges of the plateau in both the Abay and Awash rivers in the form of springs with high discharge. The yield of these springs discharged from aquifers developed in volcanic rocks varies from 7 to 50 l/s.

Outcrops of deeper aquifers can be found only in deep valleys and therefore the possibility for direct recharge by precipitation is limited. These aquifers are recharged from the upper overlying layers. Deep regional groundwater flow direction is mainly vertical and only limited groundwater emerges as springs in deep valleys. Utilizing the potential of the extensive fissured aquifers is difficult due to their thick cover. The prevailing vertical groundwater flow results in a lack of springs in the valleys, particularly from aquifers developed in volcanic rocks.

The Mugher mudstone forms the first base (aquitard) for groundwater that changes its vertical flow to horizontal flow and emerges in deep valleys as springs from the upper sandstone. The upper sandstone forms an extensive and moderately productive fissured aquifer. The mudstone represents the lower facie of the sandstone and is not continuous throughout the area. This allows water (deep regional flow) to infiltrate to greater depths and feed the underlying aquifer developed in the limestone.

Limestone represents an extensive and highly productive fissured / karst aquifer and the thick underlying layer of gypsum forms the second base (aquitard) that again flows along this base and emerges as large springs from limestone. The gypsum formation forms an important aquitard of the Addis Ababa sheet; however, it represents the upper facie of the limestone and is not continuous throughout the area. This allows water (deep regional flow) to infiltrate to greater depths and feed

102 Hydrogeology Surface water Groundwaterdivide divide

Spring Tv6-7

Spring Stream Fault Groundwater Landslide Awash level Stream () Muger Shallow local gw. flow river HS river Tv4-5 Hydrogeology Tv3 Tv2 Shallow local gw. flow Tv1 Abay Deep regional groundwater flow Usst river Conceptual Model Hydrogeological Mdst Fault Lsst Spring Gyps Lsst Deep regional groundwater flow Bas

Fig. 5.9 Conceptual hydrogeological model of the Addis Ababa area the underlying aquifer developed in the sandstones of various ages. Drainage of these sandstones is directly into the rivers at the bottom of their deep canyons and/or groundwater emerges as springs at contact between sandstones and underlying basement rocks.

A shallow local groundwater flow is developed in the basement rock composed of various crystalline rocks. The depth of groundwater circulation in the weathered and fissured part of crystalline rock is usually limited to 20–50 m. Deeper circulation up to 70 m occurs only exceptionally when basement rocks are disturbed by relatively thick fissured zones.

The specific character of the deep regional flow of groundwater is known for groundwater infiltrated on the Entoto Mountain ridge and flowing to the south to Addis Ababa. This groundwater infiltrates to great depths and is up flowing along a fault (group of faults) located in the central part of Addis Ababa and emerges as a hot spring in the Filwoha area. The former Menelik spring is substituted by a deep well developing thermal water of the Filwoha thermal area.

The hydrogeological conceptual model is shown in Fig. 5.9. The model was also used for a mathematical cross-section of groundwater flow within the Jemma basin. A cross section 2D and 3D model of the whole the Jemma basin was created to verify the assessment of recharge data, regionalization of the hydraulic characteristics of aquifers and confirm the groundwater resources assessment (Sima et al., 2009).

5.5 Annual Recharge in the Area

The regional mechanism of recharge of aquifers in the Addis Ababa area was described above. Annual Recharge in the Area There is also a seasonal, but significant, amount of recharge to localized aquifers from most of the permanent as well as intermittent streams after the Kiremt rains when the level of rivers is above the groundwater level. Aquifers along the rivers are recharged by the surface water of streams as the flow of many streams is controlled by structures.

Assessment of regional recharge in the area is difficult and various data sources provide different results. The recharge was estimated using the Kille analytical method, Bilan mathematical balance

Hydrogeology 103 model (Sima et al., 2009), soil water and chloride balance. A summary of the assessment of annual recharge using the various methods is shown in Tab. 5.3.

Tab. 5.3 Recharge in the Abay plateau and adjacent areas Annual Area Method Remarks recharge [mm]

Southern Plateau of the Kille / hydrograph rivers – Alleltu, Robi Gumero 40 Jemma basin separation and Jida

Hydrogeology Hydrogeology Kille / hydrograph rivers – Mugher, Aleltu, Sibilu, Mugher basin 110 separation Gorfo, Daneba

Annual Recharge in the Area Annual Recharge Muger River 314 Water balance see Chapter 2.5

Aleltu River 214 Water balance see Chapter 2.5

Kille / hydrograph Abay in Kessi 77 separation

Rift valley 30 Kille Jeweha

Awash, Berga, Holeta, Jem- Upper Awash 50 Kille jem, T. Akake

Mean annual precipitation 1,200 mm Ada´a and Becho plains* 250 Chloride balance Median Cl content in aquifer = 2.4 mg/l

Beresa river basin** 122 Soil water balance Source: * Evaluation of water resources of the Ada' a and Becho plains groundwater basin for irrigation development project, April 2006 ** Nigussie Kebede (2005)

104 Hydrogeology 6.6. Hydrogeochemistry

One of the important tasks of the water point inventory and data collection was to survey the groundwater chemistry and to assess the groundwater quality for its use within the mapped area. Therefore, a study of the groundwater quality was carried on the different aquifers (geological formations) of the area as well as various parts of the circulation system. The results of the hydrochemical study can help to understand the groundwater circulation within the aquifers in addition to comparing the water quality with various standards.

Tesfaye Chernet (1993) identified the hydrochemical characteristics of the natural waters which were collected from different sources and the recharge/discharge conditions of the groundwater. According to Tesfaye Chernet: Sampling and Analysis • the water resources in the highlands are classified as being water with good chemical quality with TDS less than 500 mg/l, Hydrogeochemistry • the water resources in the valleys of Abay, Muger and Guder are classified as being water with variable chemical quality with TDS 500–1,500 mg/l,

• the groundwater chemistry is characterized as being bicarbonate (HCO3) in the highlands.

Sima et al. (2009) characterized water type in the Jemma river basin, including the northeastern corner of the Addis Ababa sheet. Results of chemical analyses were interpreted graphically and were shown on the hydrochemical map of the Jemma basin.

6.1 Sampling and Analysis A total of one hundred and seventy three (173) water samples were collected from boreholes, dug wells, springs, water holes, river waters, and rain water in the study area. All of the water samples collected for laboratory analysis were submitted to the central laboratory of GSE and analyzed for chemical composition. The set of analytical data was extended for analysis obtained from various drilling companies and reports for assessment of thermal water chemistry in Filwoha area. The chemistry of the groundwater obtained from the samples taken during the field work is shown in Annex 2. Chemical analysis of the major constituents (Mg, Ca, Na, HCO3, SO4, Cl) and secondary constituents (K, NO3, F, HBO2, CO2, SiO2), and measurements of electrical conductivity (EC) and pH at room temperature were performed in the laboratory. Field measurements of pH, temperature and electrical conductivity were made at the time of sampling. The analytical results were presented graphically on a hydrochemical map to facilitate visualization of the water type and their relationships. Suitability of groundwater for drinking, industrial and agricultural purposes is assessed based on the pertinent quality standards.

Reliability of the analyses was assessed using the cation-anion balance. The assessment showed that only a limited number (16 out of 173 or 10 %) of the samples significantly exceeded the

Hydrogeochemistry 105 reliability level of 10 % of the cation-anion balance. The frequency of the level of balance is shown in Fig. 6.1 and Tab. 6.1.

Tab. 6.1 Level of balance Level of balance [%] Frequency Cumulative frequency [%]

5 116 67.1

10 41 90.8

15 7 94.8 Sampling and Analysis

Hydrogeochemistry 20 3 96.5

25 297.7

30 and more 4 100.0

140

120

100

80

60 frequency

40

20

0 5 10 15 20 25 30 and more balance level [%]

Fig. 6.1 Level of cation-anion balance

6.2 Classification of Natural Waters Classification of natural water was used to express the groundwater chemistry on the

Classification of Natural Waters hydrochemical map. Hydrochemical types are classified based on the Meq% representation of the main cations and anions by implementing the following scheme: • Basic hydrochemical type, where the content of the main cation and anion is higher than 50 Meq%. This chemical type is expressed on the hydrochemical map by a solid color. • Transitional hydrochemical type, where the content of the main cation and anion ranges between 35 and 50 Meq%, or exceeds 50 % for one ion only. A dominant ion combination is expressed on the hydrogeological map by the relevant colored horizontal hatching. The secondary ion within the type is expressed by an index (e.g. Mg2+).

106 Hydrogeochemistry • Mixed hydrochemical type, where the content of cations and anions is not above 50 Meq% and only one ion has a concentration over 35 Meq%. This type is expressed on the hydrogeological map by the relevant colored vertical hatching.

The hydrochemistry of groundwater of the area is expressed on the hydrochemical map by the relevant solid colors (for basic types) or colored hatching (for transitional and mixed types).

A general overview of the hydrochemistry of the natural water of the study area is shown in Tab. 6.2.

Tab. 6.2 Summary of hydrochemical types Hydrochemistry Type Number of cases Percentage Hydrogeochemistry Classification of Natural Waters Classification of Natural Ca–HCO3 Basic 124 71.7

Ca–Mg–HCO3 Basic 5 2.9

Mg–HCO3 Basic 1 0.6

Na–HCO3 Basic 7 4.0

Ca–HCO3 Trans 14 8.1

Ca–Mg–HCO3 Trans 5 2.9

Ca–Na–HCO3 Trans 3 1.7

Na–HCO3 Trans 2 1.2

Ca–HCO3 Mixed 1 0.6

Ca–HCO3–SO4 Basic 3 1.7

Na–Ca–HCO3 Trans 4 2.3

Na–Ca–HCO3 Basic 2 1.2

Ca–Cl Trans 1 0.6

Ca–SO4 Mixed 1 0.6

The dominant hydrochemical type of groundwater in aquifers developed in volcanic rocks of the plateau of the Addis Ababa sheet basin is bicarbonate type. The pure Ca –HCO3 type along with a few transitional types dominates within the area. In the central part of the map sheet, transitional types (Ca–Mg)– HCO3 and (Ca– Na) –HCO3 occur. There is an occurrence of the basic

Na–HCO3 type along surface water divide between the Mugher and Jemma river basins as shown on the hydrochemical map. The low TDS and bicarbonate groundwater type indicate the fast hydrogeological regime of the plateau receiving a relatively high volume of precipitation where groundwater flows in relatively lithologicaly homogeneous fissured aquifers developed in various volcanic rocks and associated (intercalated) Tertiary sediments.

A different situation is documented by the hydrochemistry of water points in the deep valleys of the Abay, Jemma, Muger and Guder rivers and the deep gorges of their tributaries. Transitional types are characteristic for the lower reaches of these rivers, where sedimentary rocks, including gypsum, contribute to groundwater discharge in springs. This type of groundwater is characterized by SO4

Hydrogeochemistry 107 aniont in addition to HCO3. Higher TDS and sulphate content in groundwater generally indicates its circulation in sedimentary rocks with higher solubility and its contact with gypsum which is a part of the sedimentary sequence or gypsum material present inside the rock matrix of other sedimentary rocks (sandstone, mudstone and limestone). The chemistry of groundwater particularly from

limestone varies from Ca– HCO3 to Ca–SO4 types based on contact of the circulating groundwater with gypsum layers inside the limestone sequence and/or with the underlying gypsum strata.

Deep groundwater circulation and heat flow in the central part of Addis Ababa leads to the

formation of thermal water with Na–HCO3 and high TDS.

To facilitate visualization of the classification of water types, the percentage of major cations and

Hydrogeochemistry anions of the analyzed samples is plotted on the Piper diagram as shown in Fig. 6.2. Classification of Natural Waters Classification of Natural

Basalt Basalt-v + Basalt-vi Limestone 80 80 Sandstone RIT 60 60 Soil River water Precipitation 40 40

20 20

Mg SO4

80 80

60 60

40 40

20 20

80 60 40 20 20 40 60 80

Ca Na HCO3 Cl Fig. 6.2 Piper diagram for classification of natural waters

The basic statistical data for values of electric conductivity (EC), total dissolved solids (TDS) and concentration of chloride (Cl) are shown in Tab. 6.3.

The content of TDS in springs is between 45 to 300 mg/l for the moderately yielding aquifer developed in the volcanic rocks forming the plateau, while most of the basalts from the deep gorges have a higher value of TDS mostly in the range of 300–400 mg/l. The content of TDS is generally above 400 mg/l for springs draining fissured aquifers developed in sedimentary rocks located in the deep valleys of the rivers. The basin topography is not uniform and the highest peaks where local

108 Hydrogeochemistry Tab. 6.3 Groundwater and surface water descriptive statistics of TDS, EC and Cl values TDS [mg/l] EC [μS/cm] Cl [mg/l]

Average 341.0 366 11.90

Median 292.0 318 4.00

Minimum 45.8 36 0.99

Maximum 3,445.0 3,350 411.00

Count* 171.0 171 171.00 Hydrogeochemistry

Remark: * Samples of rain water are not considered in the statistics Waters Classification of Natural groundwater circulation is very fast have groundwater with low TDS, in general less than 100 mg/l. TDS increases from the infiltration area along the watershed on the plateau to the drainage area formed by the valleys of the Abay, Jemma, Muger and Guder rivers. This trend is shown by idealized isosalinity lines on the hydrochemical map. The increase in TDS is in conformity with the groundwater flow direction on the plateau and also with the idea about the deeper circulation of groundwater emerging from aquifers developed in volcanic and sedimentary rocks at the bottom of the deep valleys.

6.2.1 Rain Water Hydrochemistry of rain water of the area is not known in detail; however, chemical composition of two samples taken from Holeta and Jewi Bone, respectively is shown in Tab. 6.4. A difference in the ion (cation and anion) balance of 5 % shows the analysis to be reliable. The water chemistry can be classified as basic Ca–HCO3 type.

Tab. 6.4 Chemical composition of rain water Cation [mg/l] Anion [mg/l]

Holeta

Na 0.6 HCO3 2

K 0.4 Cl 0.99

Ca 1.5 SO4 2

Mg 0.2 NO3 5

TDS in mg/l 14.19

Jewi Bone

Na 8 HCO3 106

K 3 Cl 5

Ca 23 SO4 4

Mg 5 NO3 7

TDS in mg/l 168.29

Hydrogeochemistry 109 The hydrochemistry of rain water is shown on the hydrochemical map by a pie chart.

6.2.2 Surface Water Four surface water samples were taken upstream and downstream of Jemma (Rw1), Holeta (Rw2), Lega Robi (Rw3), and Awash (Rw4).

Surface water is of basic Ca–HCO3 type for the Lega Robi and Awash rivers, basic Mg–HCO3 for the Holeta River and variable transitional Ca–Cl for the Jemma River. TDS of surface water varies from 250 mg/l to 777 mg/l and is not systematically distributed. It reflects more or less the local situation in hydrochemistry of the local aquifers which are drained by that particular segment of the river. There is an anomalous concentration of chloride in the Jemma River and together with Hydrogeochemistry increased content of sulphate it indicates solution of evaporite from the Ghoa Tsion formation. Classification of Natural Waters Classification of Natural

The hydrochemistry of surface water is shown on the hydrochemical map by a pie chart.

6.2.3 Groundwater in Volcanic Rocks Rain water infiltrates in outcrops of volcanic rocks and flows within fissured aquifers from recharge areas into discharge areas in shallow and deeper valleys. It can infiltrate also in great depths and appears as hot springs in the Filwoha thermal area in the centre of Addis Ababa. The bicarbonate type is typical for groundwater circulating in various volcanic rocks. There are no differences in groundwater types related to different types of basalt or to different volcanic formations. Groundwater dissolves volcanic rock minerals along its flow and is enriched by various

chemical compounds. The dominant water type of these volcanic rocks is the basic Ca–HCO3 type.

The total content of dissolved solids varies between 45 mg/l and 700 mg/l for cold water and is about 3,000 mg/l for the thermal water of the Filwoha thermal area.

The volcanic rocks forming aquifers in the Addis Ababa sheet basin are represented mainly by various types of basalts and ignimbrites with some rhyolites, tuffs, ashes and Tertiary sediments. The difference between the chemistry of basalt and ignimbrites was studied by Sima et al. (2009) within the Jemma basin on the plateau in the vicinity of Debre Birhan. It was found that groundwater in ignimbrite has a slightly higher content of dissolved solids and higher content of alkali earths. The higher content of dissolved solids is related to the higher porosity of some parts of the ignimbrite and higher vitreous material which is more soluble. The difference in alkali earth and particularly in fluoride content is important for deciding on the future development of aquifers with a majority of ignimbrite. The higher content of fluoride is also known from the thermal water of the Filwoha thermal area and other areas of Ethiopia, e.g. part of the Rift Valley, where groundwater circulates in ignimbrites and their ash derivates.

The hydrochemistry of groundwater discharged from volcanic rocks is expressed on hydrochemical map by the relevant solid colors (for basic types) or colored hatching (for transitional types). Chemistry of thermal water of the Filwoha thermal area is described and documented in Chapter 6.4.

6.2.4 Groundwater in Mesozoic and Paleozoic Sediments Springs of groundwater emerging from sedimentary rocks located in the canyons of the Abay, Jemma, Muger and Guder rivers and their tributaries represent the total drainage of the Addis Ababa sheet.

The groundwater from sandstone is of basic and transitional Ca–HCO3 types but Na, Mg and SO4 ions are also present as important constituents of the groundwater chemistry. The content of TDS

110 Hydrogeochemistry varies from 47 to 1,002 mg/l (average 477 mg/l) and the sulphate content varies from 1 to 76 mg/l. The higher content of sulphates indicates contact of circulating groundwater with the gypsum layer which is part of the sedimentary sequence or the presence of gypsum material inside the matrix of sandstone. The content of nitrates varies from 1 to 43 mg/l showing that local infiltration near the springs also contributes to the groundwater. Values of nitrates exceeding 10 mg/l in more than 50 % of samples is rather surprising and indicates that the aquifer is highly vulnerable to pollution.

The chemistry of groundwater from limestone is of basic Ca–HCO3 type but the other two springs have a transitional type Ca– (HCO3 –SO4) with varying levels of sulphate ions. The TDS varies from 400 to 800 mg/l (with an average of 600 mg/l) and sulphate content from 4 to 202 mg/l (with an average of 77 mg/l) indicating contact with the gypsum layer which is part of the sedimentary sequence or the presence of gypsum material inside the matrix of limestone. The Hydrogeochemistry content of nitrates varies from 0 to 29 mg/l. The results of the chemical analysis show variability Waters Classification of Natural of the chemistry of the groundwater limestone. High solubility of gypsum can easily contribute to the very high content of sulphate in the groundwater.

Sima et al. (2009) stated that ionic genetic ratios of groundwater from aquifers developed in sedimentary rocks (e.g. Na/SiO2, Na/Cl) are similar to the groundwater of volcanic rocks, indicating that groundwater from aquifers in volcanic rocks penetrates to the depth of the sedimentary rocks and recharges the aquifers.

The hydrochemistry of groundwater discharged from Mesozioc and Paleozoic sediments is expressed on the hydrochemical map by the relevant solid colors (for basic types) or colored hatching (for transitional types).

There were no samples taken from groundwater in mudstone and gypsum as well as from basement rocks because there is no water point inventoried there.

6.2.5 Groundwater in Quaternary Sediments Rain water infiltrates in Quaternary sediments and flows within porous aquifers from recharge areas into discharge areas in shallow valleys. It can infiltrate also from floods when the level of surface water in the river is higher than the groundwater level in the alluvial aquifer which is located along the river. The bicarbonate type is typical for the chemistry of groundwater in Quaternary sediments in the Addis Ababa sheet. Basic and transient Ca–HCO3 types are characterized for 28 from 31 water samples. The remaining three samples are of Na–HCO3 type. There are no differences in groundwater types related to different types of Quaternary sediments (alluvium, eluvium).

Groundwater dissolves minerals of sediments along its flow and is enriched by various chemical compounds. The Quaternary sediments forming aquifers in the sheet area are represented by various types of alluvial and elluvial sediments. The dominant water type of these sediments is basic Ca–HCO3 type. The mean total content of dissolved solids is about 300 mg/l, the lowest value of TDS is 49 mg/l and the highest value of TDS is 683 mg/l.

The hydrochemistry of groundwater discharged from volcanic rocks is expressed on the hydrochemical map by the relevant solid colors (for basic types) or colored hatching (for transitional types).

6.3 Water Quality Water Quality Water Water quality of the mapped area was assessed from the point of view of drinking, agriculture and industrial use.

Hydrogeochemistry 111 6.3.1 Domestic Use To assess the suitability of water for drinking purposes, the results of the chemical analyses were compared with the Ethiopian standards for drinking water published in the Negarit Gazeta No. 12/1990 and The Guidelines of Ministry of Water Resources (MoWR, 2002) (see Tab. 6.5.). The table shows that groundwater of the mapped area is convenient for drinking. This conclusion is valid particularly for groundwater of volcanic and limestone aquifers, but it is not unambiguous for groundwater in the shallow aquifers developed in fissured aquifers developed in volcanic rocks Water Quality Water and porous Quaternary sediments and when the groundwater dissolute gypsum material within sedimentary formations.

Tab. 6.5 Groundwater chemistry compared to drinking water standards and guidelines

Hydrogeochemistry Ethiopian standards (1) and Number of exceeding values Range MoWR Guidelines (2) [mg/l] Property (min–max) Highest Maximum Highest Maximum [mg/l] desirable permissible desirable permissible level level level level Na (2) 2–919 358 2

Ca (1) 1.5–134 75 200 16 0

Cl (1) 0.99–411 200 600 1 0

Cl (2) 0.99–411 533 0

HBO2 0.26–4.31 0.3 26

(free) 0.05 0.1 ammonia

Fe (1) 0.1 1

Fe (2) 0.4

Mg (1) 0.2–66 50 150 1 0

Mn (1) 0.05 0.5

Mn (2) 0.5

SO4 (1) 0.99–202 200 400 1 0

SO4 (2) 0.99–202 483 0

TDS (1) 14.19–3,446 500 1,500 26 2

pH (1) 5.37–8.68 7.0–8.5 6.5–9.2 0 172

pH (2) 5.37–8.68 6.5–8.5 172

NO3 (1) 0.04–98 10 45 61 3

NO3 (2) 0.04–98 50 2

F (1) 0.02–28.6 1 1.5 10 7

F (2) 0.02–28.6 3 4

112 Hydrogeochemistry 120

100 BH-59

80 Csp-93 Water Quality Water

60 BH-6 nitrates [mg/l] 40 Hydrogeochemistry

20

0 0 20 40 60 80 100 120 140 160 180 water points

Fig. 6.3 Content of nitrate in analysis of water in the Addis Ababa area

The content of calcium, sulphate and nitrate as well as TDS in some analyses exceeds the highest desirable level and reflects the main threats to the groundwater quality. Deterioration of groundwater quality by a high content of calcium and sulphate is caused by the natural character of the aquifers and results from dissolution of gypsum material through which the groundwater is circulating. The high content of nitrates is caused by human factors (pollution) that add allochthonous material to the groundwater in the aquifer (human and animal waste). The total content of dissolved solids can exceed the desirable and even maximum permissible level due to natural as well as human factors and usually by a combination of both.

Particular interest was paid to the content of nitrates in the groundwater. The content of nitrates is not related to the rock composition (type) but it reflects pollution of groundwater by human and/or animal waste. The background content of nitrates in groundwater is about 5 to 10 mg/l depending on the relevant land cover. In forest areas it can be even higher because of decomposition of various plants and other organic material. A relative high number of water analyses (70 out of 173 or 40 %) with a nitrate content of above 10 mg/l shows that the first (shallow) aquifers are polluted by human activity. The value of 10 mg/l is considered as the natural content of nitrates in the groundwater. There are positive findings that the concentration of nitrates in three samples exceeds the maximum permissible level of the standards for drinking water 45 mg/l. This fact also shows that the groundwater level is in the relative depth and the quality of groundwater is protected by a thick layer of overburden. This is an important factor particularly in highly vulnerable karst aquifers. This fact also has to be considered when planning for the future development and protection of groundwater resources in the area. Proper location of water points and suitable protective measures should be applied to springs and wells used for human water supply. Fig. 6.3 shows the content of nitrates in the analysis of water in the study area.

6.3.2 Irrigation Use Agricultural standards for the quality of groundwater used for irrigation purposes are determined based on the Sodium Adsorption Ratio (SAR), total dissolved solids and United States Salinity Criteria (USSC). The Sodium Adsorption Ratio (SAR) is used to study the suitability of groundwater

Hydrogeochemistry 113 for irrigation purposes. It is defined by SAR = Na/[(Ca+Mg)/2] where all concentrations are expressed in mg/l.

Most of the water samples (see Tab. 6.6) from the study area are found to be suitable for irrigation since they show the SAR value within the water quality class of excellent for agricultural purposes. Groundwater classified as poor quality water for irrigation corresponds with water points yielding

Na–HCO3 type of water. Water Quality Water Tab. 6.6 Suitability of water for irrigation Value of SAR Water class Number of samples in the range

<10 Excellent 164 Hydrogeochemistry

10–18 Good 5

18–26 Fair 0

>26 Poor 4

6.3.3 Industrial Use Industrial water criteria establish the requirements of water quality to be used for different industrial processes that vary widely. Thus, the composition water for high pressure boilers must meet extremely strict criteria whereas water of low quality can be used for cooling of condensers.

Tab. 6.7 Suitability of water for use in industry (Part 1) Number of Solids (TDS) Chlorides Sulfates as Industry or use pH samples in [mg/l] as Cl [mg/l] SO [mg/l] 4 the range

Brewing 500–1,500 6.5–7.0 60–100 0

Carbonated beverages <850 <250 <250 169

Confectionary 50–100 >7.0 2

Dairy <500 <30 <60 147

Food canning and freezing <850 >7.0 146

Food equipment washing <850 <250 169

Food processing general <850 169

Ice manufacture 170–1,300 133

Laundering 6.0–6.5 7

Paper and pulp fine <200 47

Paper groundwood <500 <75 147

Paper bleached cardboard <300 <200 90

Paper unbleached <500 <200 147 cardboard

114 Hydrogeochemistry Tab. 6.7 Suitability of water for use in industry (Part 2) Number of Solids (TDS) Chlorides Sulfates as Industry or use pH samples in [mg/l] as Cl [mg/l] SO [mg/l] 4 the range

Paper soda and sulfate <250 <75 67 pulps

Rayon and acetate fiber Quality Water <100 17 pulp production

Rayon manufacture 7.8–8.3 0 Hydrogeochemistry Sugar <100 <20 <20 17

Tanning 6.0–8.0 157

Textile <100 <100 169 Remark: Sugar requirements for TDS are in general low

The suitability of water for use in industry is shown in Tab. 6.7.

Tab. 6.8 Concentration limits for incrustation Component Concentration [mg/l] Number of samples in the range

– Bicarbonates (HCO 3) >400 490

– Sulfates (SO 4) >100 496

Silicon (Si) >40 448

Iron (total) >2 Not analyzed

Manganese (total) >1 Not analyzed

Hydrogen sulfide (H2S) >1 Not analyzed

Total hardness (TH as CaCO3) >200 Not calculated

Tab. 6.9 Concentration limits for corrosion Component Concentration and/or value Number of samples in the range

pH < 7 149

EC >1,500 μS/cm 171

Chloride (Cl–) >500 mg/l 173

Hydrogen sulfide (H2S) >1 mg/l Not analyzed

CO2 >50 mg/l Not analyzed

Dissolved oxygen (O2) >2 mg/l Not analyzed

Total hardness (TH as CaCO3) <100 mg/l Not analyzed

Hydrogeochemistry 115 Of almost equal importance for industry as quality of used water is the relative time constancy in concentration of various components. As a result, an adequate groundwater quality often becomes a primary consideration in selecting a new industrial plant location. Groundwater from the mapped area can be used for industry, but some specific technologies require water treatment before the water is used in the technology.

Incrustation hazard is important for the design of various pipes as well as technological

Water Quality Water processes. Incrustation occurs if concentrations exceed the limits shown in Tab. 6.8. Corrosion hazard occurs if concentrations exceed the limits shown in Tab. 6.9.

There is threat of incrustation for about 40 % of the samples because most of the groundwater

Hydrogeochemistry is circulating in carbonate aquifers or corrosion when groundwater of the area is used in pipes for public water supply or for delivery of water for industry or agriculture.

6.4 Mineral and Thermal Water Additional to the lake (Rift Valley) and the Afar zone of thermal springs there are several occurrences of thermal springs in the Ethiopian highlands. One of the prominent occurrences is found at the Filwoha (boiling water in Amharic) hot springs in Addis Ababa. The first hydrogeological description of Filwoha was presented by Chuzo Kondo (1958, 1975 and 1967). He recommended (in 1957) drilling wells instead of using shallow dug wells and at the same time he recommended drilling a well at the Ghion Hotel.

Filwoha hot springs are located in a small alluvial plain of the Bantiketu stream (Ginfile, Kechene) and were often subjected to flooding in the past.

Mineral and Thermal Water The general geology of the area consists of Quaternary sediments on the plain and volcanic rocks on the hills. Kondo (1975) defined the rock sequence as follows: • Quaternary formation • Youngest trachyte • Tuffs, felsitic and vesicular trachyte • Xenolitic, perlitic and flow structured trachyandesite • Olivine basalt intercalated with shale and sands • Volcanic breccias and oldest trachyte

Younger trachyte and hard basalts and compact trachyte were found in an old borehole (No. 1, 130.5 m deep), where 75 m alteration shows intrusion of trachyte into basalt.

The Quaternary formation formed mainly by layers of silty clay and gravel with some sand reaches up to 55 m. Kondo (1958) described three erosion levels +40 m, +20 and +5 to 20 m above the recent surface which are difficult to observe nowadays. These levels reflect block movement, volcanic activity or related earthquake with movement of the area as a whole.

The Filwoha fault running 55° N – 70° E and dipping southeastward (measured by Kondo (1975) divides the eastern hills into two parts.

Based on Kondo (1975) geophysical measurements the Filwoha fault in the northern part forms a relatively uplifted horst but depressed blocks in the southern part. The hot water is related to the southern side of the fault and has an elliptical shape elongated along the Filwoha fault. Accordingly Kondo (1958) conclude that Filwoha hot springs emerge along a NE-SW trending tectonic zone where it is crossed by a NNW-SSE tectonic zone. Extension of the Filwoha fault is indicated by bending of the Buhe and Little Akaki streams and the relatively high temperature of groundwater in

116 Hydrogeochemistry St. George Brewery wells west of Mexico Square (21.5 °C and 23.3 °C). The occurrence of hot water is related to trachyandesite. The well at the Hilton hotel is considered to be the eastern extremity of the thermal field. There were 5 wells drilled for Filwoha in 1957 and 4 new wells were drilled in the period from 1958 to 1971. Additional wells were drilled for Jubilee Palace and the Nigst Zewditu Memorial Hospital. A rough estimation of Filwoha spa of 11 l/s was made by Kondo in 1971.

Filwoha No. 1 is 245.1 m deep and was drilled in 1967 by WATENCO (Greek water engineering company). Three wells were drilled at the Ghion hotel i.e.“big well” 360 m deep (drilled in late 60s) with water temperature of 75 –78 °C and two well drilled before 1957 with a depth of 77.7 m (61.5 °C) and 56.4 m (35.8 °C), respectively. The Hilton hotel well is 480 m deep and has a water of temperature of 45.8 °C. Mineral and Thermal Water Mineral Hydrogeochemistry The well log of Filwoha well (No. 1, 245 m deep, drilled in 1967) shows basalts below 193 m with artesian water of 78 °C at a depth of 220.8 m, 80 °C at a depth of 215.5 m and 98 °C at the bottom. The highest temperature of 112 °C was measured at a depth of 112.3 m. Cold water inflows into well from a 4.5 m thick layer of sand at a depth of 115.5 m which causes the temperature to drop from 112 °C to 47 °C in this location. The results show the occurrence of hot water in two separate layers, the upper layer in trachybasalt and lower layer in basalt separated by relatively cold water from the sand layer in the interval from 115.5 to 120 m.

The basic characteristics of the thermal water observed in different water points by Kondo (1975) are shown in Tab. 6.10.

Tab. 6.10 The basic characteristics of thermal water observed in different water points by Kondo (1975) Water point Temp [°C] pH Conductivity [1/.cm] Depth [m]

Old reservoir (Menelik sp) 67.0 7.9 165

New reservoir 67.0 8.2 195

New reservoir (colder water) 30.2 8.8 160

Old well No. 1 72.0 7.8 170

Old well No. 4 57.5 7.7 240

New well No. 1 76.5 7.7 260 245.1

New Well No. 2 34.0 7.6 400 60.0

Ghion No. 2 35.8 8.2 255 56.4

Ghion pool side 35.5 8.2 450

Ghion “big well” 46.0 7.9 220

Hilton Hotel 45.8 7.2 260 480.0

The basic chemistry of the thermal water observed in different water points by Kondo (1975) are shown in Tab. 6.11.

Radioactivity measured by Institute Pasteur in 1951 shows a value of Mach 0.458.

Rao (1981) carried out a thermal water supply study of Filwoha. The Filwoha spa received its thermal water from six dug wells in Menelik spring (DW-2 and DW-5 were in use) and two

Hydrogeochemistry 117 Tab. 6.11 The basic chemistry of thermal water [mg/l] observed in different water points by Kondo (1975)

Year Na K Ca Cl SO4 HCO3 Source 1951 715.7 3.1 24.1 16.3 60.3 1,873.3 1

1952 641.4 2.0 18.5 4.4 46.9 1,743.7 13

1962 945.0 20.0 5.7 37.0 71.0 2,320.0 11

1962 955.0 18.0 5.6 39.0 73.1 2,327.0 11

1967 1,182.0 60.0 4.0 71.0 54.0 2,440.0 14 Mineral and Thermal Water Mineral Hydrogeochemistry

boreholes BW 1 (The American Well) and BW 2 (the new well approx. 1978, located between the laundry and the tennis court) at that time. Drillers reported that groundwater was struck at depth of 6 m under watertable conditions and again at a depth of 81 m under artesian conditions with pressure of about 4 m above the surface and having a discharge of about 8 l/s. (Well log and design of BW 2 are shown in Annex 3.)

Tesfaye Chernet (1982) performed a groundwater investigation for water supply of the Hilton Hotel Fig. 6.4. He studied the possibility of augmenting the water supply for the hotel which was at that time supplied by thermal water (47 °C) from a 400 m deep well drilled by WATENCO in 1967 (well log and design is shown in Annex 3). In the WATENCO well report it is mentioned that the hot water production zones are developed in fractured basalt and coarse grained parts of

Fig. 6.4 Swimming pool with thermal water pool in Hilton hotel

118 Hydrogeochemistry interflow sediments. The basalt is mainly encountered at depths below 75 m. The active part of the well filter is from 25.7 to 368 m. The static water level is reported to be 9.75, yield is 3.15 l/s and specific yield is 0.04 l/s.m of drawdown. Transmissivity (steady flow) calculated for 24 hours with drawdown of 85.34 m and yield of 189.24 l/min. is 4.25 m2/d. Chemical analyses are given in Tab. 6.12.

Tab. 6.12 Chemical analysis from Hilton hotel well Compound Units Tesfay (1982) WATECO (1967)

Ca mg/l 516

Mg mg/l 1 3.9 Mineral and Thermal Water Mineral Hydrogeochemistry Na mg/l 800 864

K mg/l 18 64.8

CO3 mg/l 144

HCO3 mg/l 1,070 2,000.8

Cl mg/l 48 0.053

SO4 mg/l 48 156

NO3 mg/l 20

F mg/l 25

SiO2 mg/l 100 49

HBO4 mg/l 401

TDS mg/l 3,021 3,298

pH 8.3

Berhanu Gizaw (2002) also estimates the potential of geothermal energy of the Filwoha area. He evaluated the thermal water upflow zone to be at depth of 80 m consisting of weathered and welded tuffs based on drilling logs of wells from the National Palace and the Sheraton Hotel drilled by Hydro in 1991. The tuff layer continues to a depth of 355 m. The highest temperatures appear close to the Filwoha fault (NW-SE trending zone) which is exposed inside the National Palace and the Ghion Hotel. The exact extension of the Filwoha thermal area is not known but estimates show an extent of about 3 km2. Thus a conservative estimate of the potential volume is about 1 km3. Assuming porosity of aquifer developed in fissures of volcanic rocks of 7 %, the energy potential of the area is estimated to be between 2,000 MW thermal year/km3 or 40 MW thermal/km3 for over 50 years. The estimation was based on a detailed study of the thermal points in the area.

Berhanu Gizaw (2002) described the thermal well at the Sheraton Hotel which is located in the SE corner of the compound about 7 m north of the main Kasanchis –Ambassador asphalt road. The well is the deepest in the compound (350 m deep). This well is planned to be utilized after the thermal pump is reinstalled. He also visited the Saint Joseph School Well (Mesekel Square well) in 1999. This is a warm thermal well located on Mesekel Square, 25 m from the NW corner of the Saint Joseph School compound. The school manages the well but there is no documentation

Hydrogeochemistry 119 about it. Reportedly it has been in service for more than 30 years. Information further suggests that the inside part of the well is wider but it narrows towards the surface (1.15 m diameter). A pipe is used for pumping out water for cleaning and flushing toilets but it was found to be unsuitable for gardening and domestic purposes. The well was sampled from a pipe about 80 m to the south of the wellhead before it enters a tank. The Gandi Memorial Hospital well was not functional during the survey due to pump failure as a result of drawdown. Information obtained from the technician ironically suggested that the water is cold despite its proximity to the Filwoha area and was used for washing although its depth is not known. The Greek Community well was visited in 2000. This well, which was found to be thermal, is owned by the Greek Community School and is located about half way between the ECA and Olympia on the Bole road. The water is pumped and used for washing and flushing toilets only because of its salinity. A sample was collected at the surface Mineral and Thermal Water Mineral Hydrogeochemistry from a pipe close to the wellhead near the entrance to the school compound.

The Finfine Hotel owns 5 wells which are located some 100 to 200 m to the south of the hotel in the old Filwoha compound and are all used for bathing and washing. In July and August the number of visitors is roughly 60,000 people/month. Water from the wells is piped through surface boosters to a nearby 4-compartmet tank in the middle of the old Filwoha area, where it is then pumped uphill to 2 big tanks and directed to the service rooms by gravity. The common features of the wells are the extensive deposition of calcite wherever there is a leakage or jet of water followed by algae growth on their channel (green, yellow) and steam separation. Four out of the five wells were sampled (Filwoha Wells No. 1–4) and are described below.

Filwoha Well No. 1 is the most NW drilled well in the compound. Initially the water comes up from a 4“ pipe but it is directed to the nearby tank through a 3“ pipe and is finally reduced to a 2“ pipe before it enters the tank. Since it is artesian the flow is uninterrupted (24 hours) but variable. The sample was collected from the 2“ pipe before it entered the tank (first compartment). Filwoha Well No. 2 was sampled from a pipe just before it enters the tank. Filwoha Well No. 3 drilled to about 90 m, is the closest to the Ghion Hotel compound, and is surrounded with a bubbling ground area of 4 x 4 m. The water is clear and ejects in a jet a few cm above the ground by the well side. During the second campaign, a sample was collected about 30 m north of the outlet from the small booster pumping house which in turn is located in between the source and the nearby tank. Filwoha Well No. 4 was sampled from a pipe just before it enters the tank.

Hilton Well No. 2 is located some 20 m to the east of the SE corner of the swimming pool and about 80 m to the south of the hotel building. Drilled in 1998, this well is the most recent one in the compound and currently supplies about 7 l/s water 24 hours a day, although it has a greater potential. The water was sampled from a 4“ pipe about 3 m away from the wellhead (chlorine addition house). The sample was collected from a pipe already under huge pressure and hence fractionation was a possibility. Hilton Well No. 1 is the oldest one drilled in the compound. Unfortunately, there is no documentation whatsoever on the geological formation encountered and its output. However, a senior technician of estimated that is has been in service over 25 years and when the installed pump is at full capacity it produces 5 l/s. It is located in the cellar of the Garden Wing, Apartment C, about 100 m SW of well No. 2. The well still functions 24 hours a day, although it recently encountered a change in color to green. A 6“ pipe is located directly into the well but a 4“ pipe redirects the water. Some white and yellow looking deposits were observed around the wellhead pipe. Sample was taken from a ½“ pipe at the wellhead.

There are three wells in the Ghion Hotel grounds, bordered by the Finfine Hotel compound to the SW, which are fitted with submersible pumps. Thermal water is used in the Ghion Hotel for showers and baths in the swimming pool area and in the hotel rooms Fig. 6.5. All of the three wells were sampled and are described below. The most powerful (temperature, discharge) of the wells in the compound, Ghion Hotel Well No. 3, is actually only separated from Finfine well No. 3 by

120 Hydrogeochemistry a fence. It is also an artesian well delivering variable discharge from an 8“ pipe. No submersible pump is used but a pump is fitted at the surface to boost the supply to the tank whenever the level in the tank drops. Records on the wells are unfortunately scarce and even less data are available when it comes to thermal wells. According to the information from the technicians, the well has been in operation for more than 25 years and the depth may reach 200 m. This well is sufficient to cover the demands of the showers and baths in the various hotel blocks. The other two wells are located on the extreme western edge of the big swimming pool which is close to both the National Palace and the Finfine wells but not as close as Ghion Hotel Well No. 3. A senior technician of the hotel estimated that wells Ghion Hotel Wells No. 1 and No. 2 were drilled to depths not deeper than 60–80 m some 30 years ago. Ghion Hotel Well No. 1 is located at the NW edge of the big swimming pool and is the closest to the National Palace compound and is currently used Mineral and Thermal Water Mineral as a reserve (the lowest output amongst the group). Ghion Hotel Well No. 2 was sampled from Hydrogeochemistry aerated sample (shower) some 15 m to the east of the wellhead, which is about 30 m west of the SW corner of the big swimming pool. Currently, it supplies thermal water for the showers of the swimming area (6 am to 6 pm). Ghion is currently the largest hotel in Ethiopia, however, taking into consideration its international size swimming pool and the shallow nature of wells No. 1 and No. 2 and hence their limited output it will be difficult to heat up the pool continuously without further development.

New wells were drilled in the Filwoha area in 2004 by WWDSE and WWDE for Spa Service Enterprise (well logs and design are shown in Annex 3).

BH-1 drilled by WWDSE is 300 m deep and encountered basalt, ignimbrite and layer of basalt at the bottom. Temperature increases from 35 °C at a depth of 283 m to 71.5 °C at a depth of 300 m. A safe yield of 2 l/s was recommended to the client. The chemical composition of well is given in Tab. 6.13.

Fig. 6.5 Swimming pool with thermal water pool in Ghion hotel

Hydrogeochemistry 121 BH-3 drilled by WWDE is 300 m deep and encountered basalt and ignimbrite. Thermal water with a temperature of 67 °C was struck at a depth of 284 m under artesian conditions. A safe yield of 0.6 l/s was recommended to the client. Transmissivity of the well is 14.05 m2/d.

BH-4 drilled by WWDE is 90 m deep and encountered basalt and ignimbrite. A safe yield of 0.6 l/s was recommended to the client. Transmissivity of the well is 1.9 m2/d.

Tab. 6.13 Chemical composition of well BH-1 Compound Units BH-1

TDS mg/l 2,216.5 Mineral and Thermal Water Mineral Hydrogeochemistry EC μS/cm 3,410

pH 7.51

NH4 mg/l 0.437

Na mg/l 910

K mg/l 19

TH CaCO3 mg/l 16.8

Ca mg/l 5.04

Mg mg/l 1.02

Fe mg/l 0.016

Mn mg/l 0.1

F mg/l 19.6

Cl mg/l 38.7

NO2 mg/l

NO3 mg/l 5.28

Alkalinity CaCO3 mg/l 1,855.2

CO3 mg/l Trace

HCO3 mg/l 2,263.6

SO4 mg/l 2.1

PO4 mg/l 0.384

Filwoha fault where the exposure (ignimbrite) starts inside the National Palace and extends SW until it diminishes inside the Ghion Hotel compound. The displacement is about 10 m. Due to its soft nature some concrete support has been put in place. The fault in the compound of the National Palace is located just to the southwest of the building and about 100 m to the east of the swimming pool. The fault has a 25° NE-SW direction. It is almost vertical ( 90° C) having a similar fracture direction. Both vertical and horizontal joints exist and are generally not uniform.

122 Hydrogeochemistry The average vertical joint spacing may be considered to be 50 cm. The fault is poorly consolidated and is predominantly ignimbrite with minor white patches (carbonates, retained presumably from fossil hot springs) in a fine ash matrix having only sporadic rock fragments (crystal poor).

Deep groundwater circulation and heat flow in central part of Addis Ababa leads to formation of thermal water with Na–HCO 3 and high TDS which is different from cold waters occurring in surrounding of the Filwoha thermal area. Results of chemical analysis of sampled thermal water are shown in Tab. 6.14. A difference in chemistry of cold and thermal waters in the area is shown in the Piper diagram in Fig 6.6

Tab. 6.14 Chemical composition of thermal water (Berhanu Gizaw, 2002) (Part 1) Mineral and Thermal Water Mineral Altitude T Cond Dis O Hydrogeochemistry No Point Date X UTM Y UTM pH 2 [m a.s.l.] [°C] [μS/cm] [mg/l]

29 Filwoha W1 25.05.99 473739 996201 2,330 78.0 6.60 3,640 1.1

30 Filwoha W3 25.05.99 473747 996190 2,330 58.0 6.94 3,710 1.3

25.05.99 473762 996176 2,340 78.0 7.10 3,800 1.2 31 Filwoha W4 08.12.00 76.0 7.38 3,420 OFL

32 Filwoha 2 25.05.99 473735 996188 2,330 58.0 6.84 3,520 1.5

33 Hilton W2 26.05.99 474539 996372 2,380 41.0 7.58 3,200 1.7

34 Hilton W1 26.05.99 474345 996472 2,350 50.6 6.96 3,200 1.6

35 Ghion W2 26.05.99 473417 996301 2,325 40.4 6.98 2,490 1.3

36 Ghion W1 26.05.99 473384 996279 2,370 38.0 7.02 2,700 1.5

37 Ghion W3 26.05.99 473429 996367 2,340 77.0 7.36 3,590 1.1

52 Joseph W 03.06.99 473476 995836 2,310 33.5 8.00 2,970 1.2

53 Ras Hotel 03.06.99 472675 996208 2,340 22.0 6.52 653 1.2

54 Africa Hotel 04.06.99 471942 996209 2,370 27.4 6.92 785 1.3

61 N P W1 10.06.99 473226 996424 2,350 67.1 7.12 3,850 1.0

62 N P W2 10.06.99 473467 996420 2,350 75.0 7.12 3,850 1.5

Tab. 6.14 Chemical composition of thermal water (Berhanu Gizaw, 2002) (Part 2) Dis O F. Coli B SiO HCO NO NO SO PO No 2 2 3 2 3 4 4 [% sat.] [C./100 ml] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l]

29 26 1 15.69 39 2,407 <0.01 0.68 82.39 <0.1

30 30 0 11.23 26 2,397 <0.01 <0.1 81.87 <0.1

21 1 14.50 50 2,385 <0.01 <0.1 78.15 <0.1 31 OFL 15.10 48 2,391 <1.0 69.46

Hydrogeochemistry 123 Tab. 6.14 Chemical composition of thermal water (Berhanu Gizaw, 2002) (Part 3) Dis O F. Coli B SiO HCO NO NO SO PO No 2 2 3 2 3 4 4 [% sat.] [C./100 ml] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] 32 30 13.34 29 2,385 <0.01 0.59 79.30 <0.1

33 32 5 9.65 42 2,083 <0.01 4.79 58.67 <0.1

34 Nil 10.23 62 2,031 <0.01 1.08 50.56

35 24 Nil 12.36 57 1,488 0.14 6.95 74.09 <0.1

36 24 7.77 70 1,806 <0.01 1.65 60.70 <0.1 Mineral and Thermal Water Mineral Hydrogeochemistry 37 25 Nil 8.65 48 2,318 <0.01 0.86 72.53

52 20 23 4.00 38 230 <0.01 0.17 34.43 <0.1

53 17 1 0.06 53 35 <0.01 55.30 33.68 <0.1

54 19 Nil 7.89 61 37 <0.01 89.90 20.08 <0.1

61 19 Nil 12.30 26 2,342 <0.01 0.73 68.06 <0.1

62 34 Nil 9.67 27 2,520 <0.01 <0.04 76.38 <0.1

Tab. 6.14 Chemical composition of thermal water (Berhanu Gizaw, 2002) (Part 4) F Cl Br Li Na K Rb Cs No. Point [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] 29 Filwoha W1 24.42 41.26 <0.10 0.41 992.0 15.90 3.25 1.05

30 Filwoha W3 24.61 41.90 <0.10 0.41 999.0 20.06 3.78 0.96

24.99 41.13 <0.10 0.40 930.0 14.90 3.88 1.12 31 Filwoha W4 < 1.00 39.68 0.15 0.36 937.0 17.30

32 Filwoha 2 24.63 41.90 21.32 0.39 906.0 16.92 2.98 0.89

33 Hilton W2 25.86 41.29 4.81 0.44 912.0 17.23 3.10 1.15

34 Hilton W1 26.53 30.49 <0.04 839.0 16.63 3.15 1.21

35 Ghion W2 19.90 39.34 0.85 0.26 656.0 21.14 4.01 1.30

36 Ghion W1 17.71 36.49 1.89 0.30 750.0 23.39 3.20 1.70

37 Ghion W3 36.17 39.12 <0.04 981.0 23.50 3.65 1.10

52 Joseph W 7.21 25.91 <0.10 0.19 795.0 11.22 1.45 0.02

53 Ras Hotel 0.34 41.80 0.15 <0.04 16.7 1.682 0.09 0.02

54 Africa Hotel 0.18 59.40 0.20 <0.04 19.5 2.227 0.16 0.01

61 N P W1 34.00 36.99 0.68 <0.04 965.0 21.16 4.02 1.36

62 N P W2 22.59 38.36 0.85 0.41 986.0 22.00 4.35 1.20

124 Hydrogeochemistry Tab. 6.14 Chemical composition of thermal water (Berhanu Gizaw, 2002) (Part 5) Mg Ca Mn Fe Al Cr Co Ni Cu Cd Pb TDS No. [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [μg/l] [μg/l] [μg/l] [μg/l] [μg/l] [μg/l] [mg/l]

29 5.820 1.62 <0.05 <0.10 <1.0 <1.0 <1.0 <1.0 <1.0 <0.25 <2.0 2,696.7

30 0.563 1.60 <0.05 <0.10 <1.0 <1.0 <1.0 <1.0 <1.0 <0.25 <2.0 2,680.2

0.511 1.39 <0.05 <0.10 <1.0 <1.0 <1.0 <1.0 <1.0 <0.25 <2.0 2,623.0 31 0.600 3.58 < 0.05 0.16 <1.0 2,566.0 Mineral and Thermal Water Mineral 32 1.960 1.96 <0.05 <0.10 <1.0 <1.0 <1.0 <1.0 <1.0 <0.25 <2.0 2,604.2 Hydrogeochemistry

33 1.405 1.53 0.08 0.20 <1.0 <1.0 <1.0 <1.0 12.270 <0.25 13.480 2,432.8

34 1.184 1.14 <0.05 <0.10 <1.0 <1.0 <1.0 <1.0 6.168 <0.25 <2.0 2,268.0

35 0.515 0.60 0.09 <0.10 <1.0 <1.0 <1.0 <1.0 2.270 <0.25 4.227 1,828.7

36 1.578 2.02 0.08 <0.10 <1.0 <1.0 <1.0 <1.0 <1.0 <0.25 <2.0 2,098.1

37 0.425 1.29 <0.05 <0.10 <1.0 <1.0 <1.0 <1.0 <1.0 <0.25 <2.0 2,644.1

52 5.980 0.46 <0.05 0.25 <1.0 <1.0 <1.0 1.965 6.903 <0.25 <2.0 1,123.2

53 25.600 85.50 <0.05 <0.10 <1.0 <1.0 <1.0 1.311 3.167 <0.25 <2.0 392.4

54 41.000 82.22 <0.05 <0.10 <1.0 <1.0 <1.0 <1.0 1.528 <0.25 <2.0 461.5

61 0.686 51.00 <0.05 0.12 <1.0 <1.0 <1.0 1.015 2.131 <0.25 <2.0 2,691.5

62 0.945 45.00 <0.05 <0.10 <1.0 <1.0 <1.0 <1.0 1.610 <0.25 <2.0 2,810.4

Tilahun Azagegn Tafere (2008) stated that the permeable formation favors mixing of deep high TDS thermal waters with fresh meteoric water. The high TDS thermal waters are found in areas like the Addis Ababa Filwoha area, where the deep thermal aquifer is overlain by a poorly permeable formation (massive ignimbrite, tuff, and trachyte) which prevents meteoric water mixing. This is also the reason why the aquifer system in the Filwoha area has confined artesian conditions but the shallow cold water aquifer system in the same area has a water level between 20 and 30 m below ground level.

He also presented data about the chemical and isotopic composition of the thermal water from the Filwoha field.

Tab. 6.15 Chemical data (Tilahun Azagegn Tafere, 2008) (Part 1) T No T ID Locality X UTM Y UTM Altitude [m a.s.l.] Depth [m] EC [μS/cm] TDS [mg/l] pH Na [mg/l] K [mg/l] Ca [mg/l] Mg [mg/l]

106 TCK9 AA-Filwoha 473276 996535 2,350 504.0 3,380.0 2,240.0 9.0 930.0 16.0 2.8 1.1

AA-Hilton 107 TCK13 474175 996550 2,381 120 3,359.0 2,049.0 7.5 840.0 15.0 6.0 2.0 Hotel

Hydrogeochemistry 125 Tab. 6.15 Chemical data (Tilahun Azagegn Tafere, 2008) (Part 2) 3 [mg/l] 3 3 4 F [mg/l] HCO T No T ID Locality Cl [mg/l] NO [mg/l] CO [mg/l] SO [mg/l] Water type Source

106 TCK9 AA-Filwoha 7.7 1.9 27.6 1,874.3 100.8 92.0 Na–HCO3 ABGREP

107 TCK13 AA-Hilton Hotel 43.0 0.0 21.1 2,198.0 0.0 55.0 Na–HCO3 ABGREP

Tab. 6.16 Tritium data (Tilahun Azagegn Tafere, 2008) Mineral and Thermal Water Mineral Hydrogeochemistry Data 3H value T ID Locality X UTM Y UTM Source source [T.U.]

TTR17 AAWSA Filwoha Well 473735 996188 borehole 0.30

TTR18 AAWSA Filwoha Well 473762 996176 borehole 0.60

TTR19 AAWSA Filwoha Well 473747 996190 borehole 0.00

TTR20 AAWSA Filwoha Well 1 473739 996201 borehole 0.00

TTR25 AAWSA Ghion Hotel Well 3 473429 996367 borehole 0.00

TTR26 AAWSA Ghion Hotel Well 473417 996301 borehole 0.30

TTR27 AAWSA Ghion Hotel Well 1 473384 996279 borehole 2.00

TTR29 AAWSA Hilton Well 1 474345 996472 borehole 0.00

TTR30 AAWSA Hilton Well 2 474539 996372 borehole 0.10

National Palace Ther- TTR46 AAWSA 473226 996424 borehole 0.00 mal Water No. 1

National Palace Ther- TTR47 AAWSA 473467 996420 borehole 0.70 mal Water No. 2

Tab. 6.17 Stable isotope data (Tilahun Azagegn Tafere, 2008) (Part 1) Data  18O  D T ID S No Locality X UTM Y UTM Source Temp source [‰] [‰]

TA70 29 AAWSA Filwoha Well 1 473739 996201 borehole -5.28 -25.45 thermal

TA71 30 AAWSA Filwoha Well 473747 996190 borehole -5.30 -23.45 thermal

TA72 31 AAWSA Filwoha Well 473762 996176 borehole -4.97 -22.30 thermal

TA73 32 AAWSA Filwoha Well 473735 996188 borehole -5.23 -23.35 thermal

TA74 33 AAWSA Hilton Well 2 474539 996372 borehole -5.11 -22.20 thermal

TA75 34 AAWSA Hilton Well 1 474345 996472 borehole -5.14 -22.60 thermal

TA76 35 AAWSA Ghion Hotel Well 473417 996301 borehole -2.06 -9.80 cold

126 Hydrogeochemistry Tab. 6.17 Stable isotope data (Tilahun Azagegn Tafere, 2008) (Part 2) Data  18O  D T ID S No Locality X UTM Y UTM Source Temp source [‰] [‰]

TA77 36 AAWSA Ghion Hotel Well 1 473384 996279 borehole -4.26 -16.75 thermal

TA78 37 AAWSA Ghion Hotel Well 3 473429 996367 borehole -5.27 -22.50 thermal

Saint Joseph TA93 52 AAWSA School Well (Me- 473476 995836 borehole -4.77 -20.85 thermal sekel Square) National Palace Mineral and Thermal Water Mineral TA102 61 AAWSA Thermal Water 473226 996424 borehole -5.36 -25.60 thermal Hydrogeochemistry No. 1 National Palace TA103 62 AAWSA Thermal Water 473467 996420 borehole -5.02 -22.90 thermal No. 2 Greek Communi- TA08 8 AAU 474665 995717 borehole -5.00 -23.00 thermal ty School

TA19 19 AAU Hilton Hotel 474188 996842 borehole -5.00 -24.00 thermal

Contrary to the general expectation that isotopic composition of thermal waters would plot graphically below the Local Meteoric Water Line (LMWL) following a horizontal line, the Filwoha geothermal waters plot close to the LMWL. Apart from the thermal springs in southern Afar, all of the thermal waters in the Upper Awash basin plot close to the LMWL. The thermal springs in the Filwoha graben in central Addis Ababa plot above the LMWL. Compared to the 18O depleted

Cold water 80 80 Hot water

60 60

40 40

20 20

Mg SO4

80 80

60 60

40 40

20 20

80 60 40 20 20 40 60 80

Ca Na HCO3 Cl Fig. 6.6 Piper diagram for cold and thermal waters

Hydrogeochemistry 127 Tab. 6.18 Well data (sine, 2008) /d] 2 Water point Water BH Local name X UTM Y UTM Altitude [m a.s.l.] depth Well [m] [m] SWL Discharge [l/s] down Draw [m] Specific capa- city [l/s.m] Transmissivi- ty [m

548 National Palace-1 473400 996400 2,350 200.0 0.0 5.00

549 National Palace-2 473400 996300 2,352 249.0 0.0 5.00 51.6 0.10 10.05

550 Filwoha Hotel-1 473100 996400 2,340 249.0 0.0 7.30 Mineral and Thermal Water Mineral Hydrogeochemistry

551 Filwoha Hotel-2 473200 996300 2,340 57.0

552 Filwoha Hotel-3 473200 996400 2,340 100.0 0.0

553 Ghion Hotel-1 473300 996100 2,342 77.7 0.0 4.16 47.0 0.09 9.18

554 Ghion Hotel -2 473300 996200 2,344 56.4 7.6 4.16 47.0 0.09 9.18

555 Ghion Hotel-3 473300 996300 2,342 60.0 59.0 0.83

556 St.Joseph School 473400 995800 2,338 50.0 6.0 0.93

groundwater (i.e. Mojo, Dukem, Nazareth, Wolenchiti) of the same transect, the Filwoha thermal waters contain higher d-excess. This is related to the negative oxygen shift (displacement of the 18 waters towards left of the LMWL in a  O - D plot). The Filwoha thermal waters are rich in CO2 18 which comes via the Filwoha fault. Exchange between the O of the waters and that of the CO2

can give rise to the negative oxygen shift, a well-known phenomena in CO2 rich thermal waters (D´Amore and Panichi, 1977). The Sodere, Wonji and Southern Afar thermal waters plot close to the slightly below the LMWL showing signs of 18O and D enrichment. Since these thermal waters emerge near the Awash River, the heavy 18O and D contents is related to mixing of the thermal water with Awash River water. The groundwater from Entoto Mountain contains relatively enriched 18O and D suggesting the groundwater inflow from the mountain is not the principal recharge water for the Filwoha graben thermal water. This further suggests that the Filwoha thermal waters must have their groundwater inflow from high altitude sources or that they are older. The latter is supported by a 14C date on a deep well (BH 201) in the graben which shows and age of 14,500 years. A deep well (BH 231) at the foot of Entoto Mountain also shows a 14C age of 19,750 years suggesting the presence of paleo-waters circulating in deeper levels of the region.

The HCO3 content is higher in the thermal waters, the highest being in the thermal waters of the

Filwoha graben. The high HCO3 in the thermal groundwater is due to hydrolysis reaction aided by 13 CO2 input from deeper sources, consistent with the  C enrichment.

The study carried out by the Geological Survey of Ethiopia (Zenaw Tessema, 1997) “Preliminarily Hydrogeological Study of Filwoha Area, Addis Ababa” was based on the requirement of the Sheraton Hotel to use 10 l/s from the Filwoha source. The Sheraton Hotel drilled 4 wells, only one borehole could strike hot water. Information on these wells has not been provided.

The Filwoha thermal area is a big heritage of Addis Ababa. It is necessary for the area to be protected either by the Addis Ababa Municipality or by the Spa Service Enterprise. Recent protective measures are not adequate particularly when construction activities are common in the vicinity of the thermal wells.

128 Hydrogeochemistry Natural Resources 7. of the Area

Natural resources of the Addis Ababa sheet consist of resources of various origin related to geological composition, soil condition, water, wind and solar radiation, however, human resources are the most important prerequisite (potential) for the future development in the area.

7.1 Economic Geology The petroleum potential of the area has been investigated by Serawit et al. (1996), who indicated the abundance of reservoir rocks but did not find the source rocks or the cap rocks. The occurrence of Antalo limestone and Gohatsion formations has been suggested to play the role of Economic Geology source rocks for the potential presence of petroleum in the area. Alula (1997) studied the Debre Libanos area from the point of view of petroleum reserves. Despite the lack of available data to diagnose the petroleum potential of the area, he found a succession of a limestone unit of 100 m thick, a sandstone unit of 400 m thick and capping rocks of tertiary basalts thicker than 600 m. The succession implies the presence of favorable conditions for petroleum, but the limited data

on the structural condition in the subsurface has not been addressed. Nevertheless, seepage of of the Area Natural Resources petroleum in the area has been known for generations.

Coal occurrence has been known in the form of intercalations of sediments between layers of Tertiary basalts in the area.

Limestone, gypsum and mudstone provide potential resources for development of cement and lime as well as for the chemical industry (paint production, plaster of Paris, dimension stones, etc.) which has long been developed by cement factories in the Muger Valley and there are a lot of reports describing limestone resources in the neighborhood. A detailed description of the limestone and gypsum resources was made by Wondafrash (1993). The limestone deposits were also studied from the point of view of the utilization of dimension stones. Considering joint spacing, the maximum block size in the Jemma Valley is measured to be 1 m3 and 0.5 m3, respectively. Samples of blocks have been cut and polished as slabs and cubes by the Ethiopian marble industry. Most of the slabs have a smooth bright surface without scratches. The gypsum beds form lenticular stratified lenses within the Antalo limestone. They are nearly horizontal, and are morphologically characterized by steep slopes on the banks of the Jemma and Bersina near their confluence. Two beds of gypsum were mapped having an average thickness of 2.16 m and 7.5 m, extending laterally for 800 m and 840 m, respectively. Wondafrash (1993) calculated that in the C2 category the total resources of gypsum that can be exploited from the Yegof deposit are 1,014,418 tons.

The upper sandstone which is normally used as a dimension stone for building of houses in many areas of the country is exposed here only in inaccessible areas of deep river valleys.

Natural Resources of the Area 129 Economic Geology Natural Resources of the Area Natural Resources

Fig. 7.1 Existing ignimbrite Quarry

Fig. 7.2 Basalt quarry for crushed aggregate production

130 Natural Resources of the Area Tab. 7.1 Selected sites for quarries (Part 1) Estimated Potential usage Quarry No. Type of material Location volume [m3] of material

Q1 75,000 Basalt Aggregate Gebre Guracha

Q2 500,000 Scoria Aggregate Mida

Q3 50,000 Trachyte and basalt Aggregate Sadamo

Q4 Basalt Abandoned Menagesha Economic Geology

Q5 Ignimbrite Abandoned Desta Mender

Q6 200,000 Soil For making bricks Kobi

Q7 50,000 Trachy basalt Masonry Burayou Mariam Natural Resources of the Area Natural Resources Q8 Basalt Abandoned Gefersa

Q9 12,500 Basalt Aggregate Markos

Q10 300,000 Basalt Aggregate Markos

Q11 125,000 Trachyte Masonry Burayou

Q12 500,000 Ignimbrite Masonry Asko

Q13 1,250,000 Ignimbrite Masonry Burayou

Q14 50,000 Ignimbrite Masonry Keta

Q15 800,000 Ignimbrite Masonry Fredoro

Q16 800,000 Ignimbrite Masonry Ashewa Meda

Q17 1,000,000 Ignimbrite Masonry Sululta

Q18 Basalt Abandoned Dire

Q19 140,000 Scoraceous basalt Sub base Dire

Q20 8,000 Basalt Aggregate Abado

Q21 Basalt Aggregate Sululta

Q22 Ignimbrite Abandoned Yeka Tafo

Q23 500,000 Ignimbrite Masonry Lege Dadi

Q24 50,000 Ignimbrite Masonry Meri

Q25 500,000 Ignimbrite Masonry Bole

Q26 Basalt Abandoned Koso Ber

Q27 50,000 Basalt Aggregate Gebre Guracha

Q28 Basalt Abandoned Intoto

Natural Resources of the Area 131 Tab. 7.1 Selected sites for quarries (Part 2) Estimated Potential usage Quarry No. Type of material Location volume [m3] of material

Q29 10,000 Ignimbrite Masonry Lege Bolo

Q30 Soil Abandoned Kewo

Q31 1,000 Basalt Aggregate Dalota Economic Geology Q32 2,000 Ignimbrite Masonry Dima River

Q33 100,000 Basalt Aggregate Dubokra

Q34 75,000 Basalt Aggregate Fiche Natural Resources of the Area Natural Resources Q35 175,000 Basalt Aggregate Gebre Guracha

Q36 210,000 Basalt Aggregate Tumano

Q37 1,200,000 Basalt Aggregate Gebre Guracha

Q38 Scoraceous basalt Abandoned Keta

Ignimbrites and ash-flow tuffs are common in the map area. Ignimbrite and ash flow-tuffs are important building materials. Trachytic obsidian is locally used as a building material. This can also be easily cut and can be used for building purposes. Obsidian is reported to be used as a gemstone in some literature. Vesicular trachyte serves as good building material. Sufficient and good quality ignimbrites which can be used as masonry are exposed in numerous places within the Addis Ababa sheet. There are a number of existing quarries (Fig. 7.1) exploiting ignimbrite rock for masonry used in the construction of buildings, bridges, culverts and other engineering structures. The quarries are sufficient, good quality, workable and accessible.

Basalts, trachytes and ash-flow tuffs are common in many areas. Coarse crushing to the size of gravel is needed for raw materials for road construction. Larger sizes are also important for foundations. The local people use them for local house construction and fencing. Fresh, very strong to strong basalt is of a sufficient quantity and good quality for crushed aggregate production (Fig. 7.2). The rock is well exposed and forms high relief hills, a resistant upper scarp zone and lower valley slopes. Numerous active and abandoned basalt quarries are found in the map area. The rock is easily workable due to systematic sets of jointing. Locally, the rock is used as masonry for construction of houses and other engineering structures.

The reddish brown, sandy silty, residual soil can be a good source for burrow material. The soil (Vertisols) occurs on rounded, low relief basaltic hills. It has a limited lateral extent, and a thickness of not more than 2 m. The soil can serve as an impervious blanket in construction of dams and other water retaining structures. As it can be seen from field observation the local people make pottery products and bricks (the Gafarsa area) from the residual soil. The Vertisols are well developed at plateau area along the main roads.

Sand and gravel naturally occurring along main rivers can be used for concrete mix and gravel for water well development. There are many existing quarries of sand and gravel in the project area especially in the mentioned river valleys.

132 Natural Resources of the Area Tab. 7.2 Physical characteristics of construction material Tertiary Basalt Pyroclast Limestone Ignimbrite Trachyte sediment Schmidt median 400–700 370–690 690 350–690 565–690 [kg/cm2 ]

Point load strength 2.0–13.0 0.4–2.5 2.4–3.5 1.1–2.6 1.8–5.8 0.48 index [MPa]

Water absorption Economic Geology 0.2–6.0 0.4–2.5 2.8 9.0–20.5 2.4 42.00 [%]

Porosity [%] 2.2–3.0 2.3–2.4 7.0 19.0–34.0 5.7 29.00

Bulk density [g/cm3] 2.2–3.0 2.3–2.4 2.4 1.5–2.1 2.3 1.40 Natural Resources of the Area Natural Resources

During the engineering geological investigation of the Addis Ababa sheet, sites for mining and quarrying rocks were selected (Tab. 7.1) having favorable physical characteristics (Tab. 7.2) as construction material.

7.2 Water Resources Water resources of the area depend mainly on rainfall and other climatic characteristics, as well as the hydrological, geological and topographical settings of the study area.

There are 17 meteorological stations operated by the Meteorological Institute within the map area. Some of them have sporadic measurements, but the majority has long-term measurements. The long-term mean annual rainfall of the map has been calculated and a value of 1,250 mm/year was adopted for further calculations.

The area of the map was calculated from the hydrogeological map 1:250,000 and an area of 18,204 km2 is used for further calculation.

The area of active aquifers that store and transmit water was calculated based on the hydrogeological map. The active aquifers (Tab. 7.3) consist of the porous, mixed and fissured aquifers of the plateau and fissured and karstic aquifers in gorges.

The surface river flow measurements are performed mainly on small tributaries of the Jemma Muger, Guder, and Awash rivers in gauging stations located on the plateau, whilst measurements of the flow of the main rivers in deep valleys are very sporadic. The Abay River provides data from

Tab. 7.3 Aquifers of the area Aquifers Area [km2]

Porous 1,252

Fissured and karst in sedimentary and volcanic rocks 15,107

Fissured aquifers in basement rocks 805

Aquitards and aquicludes 1,040

Total of the area 18,204

Natural Resources of the Area 133 large part of its area and results of measurements in the gauging station at Kessie are used for comparison. The measured tributaries provide enough data on the surface flow and baseflow of the plateau and can be used for characterization of aquifers outcropping on the plateau, but characterization of deeper aquifers outcropping in deep valleys is more difficult. This fact is the main obstacle for a more precise definition of basic river characteristics and assessment of groundwater resources of the whole map sheet. The runoff characteristics vary widely and there is no clear explanation of such variation. For further calculations, the values for specific surface runoff of 15 l/s.km2 and specific baseflow of 2.0 l/s.km2 have been adopted. The assessed

Water Resources Water water resources of the sheet are shown in Tab. 7.4 based on the adopted sheet area and values of specific runoff and baseflow.

Tab. 7.4 Assessment of water resources of the Addis Ababa sheet Input Area Resources total Remark

Rainfall 1,250 mm/year 18,204 km2 21,055 Mm3/year Natural Resources of the Area Natural Resources Total water resources 15.0 l/s.km2 18,204 km2 8,667 Mm3/year 41.0 % rainfall

Renewable groundwater resources 2.0 l/s.km2 17,164 km2 1,082 Mm3/year 5 % rainfall of active aquifers

Static groundwater 5 % porosity resources of fissured 15,107 km2 22,605 Mm3 and karst aquifers 300 m thickness

Static groundwater 15 % porosity resources of porous 1,252 km2 5,634 Mm3 aquifers 30 m thickness

7.2.1 Surface Water Resources and Development The river gauge measurements show intensive moderate evapotranspiration when about 41 % of rainfall is drained as total runoff from the area, there are good water resources to be used for irrigation, electricity generation as well as for drinking water supply of people living within the area.

The surface water of the area should be primarily used for irrigation as well as for small scale electricity generation. The irrigation should be preferably applied in rivers of the eastern part of the map sheet where plateau topography dominates. The plateau area is large and the system of small dams should be constructed along rivers like (from southwest to north) the Aleltu, Duber, Weserbi, Aleletu, Robi, etc.

Existing dams in the Addis Ababa area include Aba Samuel, Lege Dadi, Gefersa, and Dire. All of these dams are used for providing water for domestic water supply with the exception of the Aba Samuel dam.

The Aba Samuel dam is located on the confluence of the two branches of the Akaki River. It is located between 20 and 45 kilometers from Addis Ababa. Originally in 1939, the dam was intended for hydroelectric power generation utilizing the nearby Akaki gorge downstream of the dam. Nevertheless, the dam was operational only until 1970 and has remained an open reservoir ever since. The reservoir area was reduced from 12,068 km2 to 1,495 km2 due to erosion related siltation and invasion by Eichhornia crassipes (http://www.birdlife.org/datazone/sites). The area around Akaki-Aba-Samuel is permanently marshy with small ephemeral lakes. The Ethiopian Electric Power Agency (EEPA) has begun rehabilitating the lake.

134 Natural Resources of the Area The Gefersa dam is located 18 km west of Addis Ababa and its catchment stretches 10 km between the Wechacha and Entoto Mountains and has a total catchment area of 5,700 ha. It was built in 1938 and a smaller dam was later constructed further upstream in 1966. The dams have a storage capacity of 6,500,000 m3 and 1,500,000 m3, respectively. The surroundings of the dam area are well vegetated but farming and animal husbandry are increasingly practiced nearby.

According to a report (Adinew, 1998) the Lege Dadi dam treatment plant was commissioned as a major phase of expansion of water supply facilities in 1970 at a location 33 kilometers east of Addis Ababa city on the large Akaki River. Further development of the water supply facilities was Water Resources Water pursued in 1986 under the Water Supply Stage II Project. The treatment capacity of the Lege Dadi plant was hence upgraded from a mere 50,000 to 150,000 m3 of water per day. In addition, the Dire Dam project was completed to provide an additional 42,000 m3 of water per day to the plant.

Dams for small scale electricity generation should be constructed on rivers at places where these rivers enter their gorges. Locations where rivers form waterfalls are particularly convenient.

Electricity generating schemes can be constructed on rivers like the Robi, Duber, Aleltu, and of the Area Natural Resources particularly on the Muger River and its tributaries in the eastern and northeastern part of the sheet, where rough topography with gorges dominates.

Considering the fact that the use of surface water for irrigation is the most important development factor for food security in the area, we can recommend about 80 % of available resources to be used for irrigation. This portion represents 5,333 Mm3/year. Considering that about 10,000 m3 of water is needed to irrigate 1 ha of land, the calculated irrigation resources represent an irrigation potential of 533,300 ha. This area represents 5,333 km2 which is about 29 % of the whole area and 50 % of arable land (moderately to intensively cultivated plateau) classified on the land use map of the area.

The area is often affected by drought periods and in several years irrigation dams will not be refilled by Kiremt rainfall. When it will happen over several years like Dejen (1965–68) and Lemi (1972–76), irrigation cannot be practiced in drought stricken areas. The meteorological observation shows that the occurrence of drought periods is not uniformly distributed within the sheet area and in the case of drought in one part of the area other parts gain a volume of rainfall sufficient for filling irrigation dams. This analysis results in the recommendation that irrigation dams are highly important for agricultural development in the area. Drought periods and spatial distribution of drought show that agricultural production in areas of adequate rainfall can support areas stricken by drought within the area without the requirement for long distance transport of food aid. It also shows that basic decisions can be made on a regional level quicker than on a federal level.

Thirteen sites were selected for construction of dams during the field work of the current project and the sites are illustrated on the engineering geology map. The primary criterion for the selection of dams was the presence of narrow closures. This was identified on the basis of a topographic map of scale 1:50,000. Accordingly, the selected sites have been analyzed with reference to the type of stream nature, reservoir area, and availability of adequate arable land. The stream nature refers to the duration of flow as perennial or intermittent, where only those qualifying to be perennial have been selected from the original location on the topographic map. Nevertheless, analysis of the discharge measurements is mostly unavailable and even if it is available it is beyond the present scope of the project. Following the identification of the potentiality of the streams, a prompt assessment of the sub-catchment with regards to areal coverage and stream networks is made in GIS.

The irrigation as well as energy potential of the area has been known for a long time. It was assessed in the framework of the Abay Water Master Plan and by various specific studies. A detailed appraisal of the area from the potential for dam construction has been considered by

Natural Resources of the Area 135 Tab. 7.5a Selected SHP sites within the area Altitude Potential ID X UTM Y UTM River Near town [m a.s.l.] [kW]

ED4 480031 1076165 2,480 Gur Debre Libanos 100

ED12 497510 1079127 2,170 Robi Gemero Lemi 400

ED13 497299 1072373 2,210 Robi Jida Weberi 400 Water Resources Water ED15 496544 1070480 2,215 Tiliku Aleletu Weberi 200

Tab. 7.5b Selected dam sites within the area during field work Altitude Date of River Construction ID X UTM Y UTM Purpose [m a.s.l.] inventory (G.C.) name material Natural Resources of the Area Natural Resources Tilku very DCh-1 485405 1045361 2,500 irrigation 19.02.2009 duber good

Tilku very DCh-2 489483 1046736 2,582 irrigation 16.03.2009 duber good

very DD-1 462373 1042170 2,368 hydropower 25.02.2009 Aleltu good

very DD-2 463632 1037492 2,427 irrigation 26.02.2009 Aleltu good

DD-3 451404 1026745 2,662 hydropower 10.03.2009 Kersa good

DF1 466508 1065128 2,501 irrigation 18.03.2009 Girar good

DIn1 427511 1048769 2,460 good

DIn2 430441 1029028 2,551 good

DIn3 430935 1028433 2,592 irrigation

Tarun et al., Nehmya (2005). A sub-catchment (Gumero) in the area has been selected as a site for testing the applicability of satellite imagery based rainfall estimation (Ymeti, 2007).

The major population of the area lives in small towns and villages and some of them do not have access to the electricity mains. Such localities may be provided with electricity through small hydropower plants on nearby potential streams. Presently, the country has three SHPs, namely Yadot, Sor and Dembi SHP sites. These self-contained systems have a total installed capacity of 6.15 MW. As reported, Debre Birhan power station located on the next map sheet generated power of less than 100 kW by utilizing a head of 50 m. This power station was functional until 1977.

The study conducted by Nehmia and Rhaghuvanchi (2005), demonstrates the use of the main channel’s nature and length; the headrace channel length; the availability of sufficient head in excess of 60 m; accessibility of sites for proper utilization of energy, and the catchment area for selecting dams in the area. These geospatial database criteria can be built by GIS. The site selection was made based on the following criteria:

136 Natural Resources of the Area 1. The channel should be perennial in nature; in EMA topo-maps 1:50,000; perennial streams are marked with blue color and the stream name is written in capital letters. 2. The catchment should have an area of at least 25 km2. This will ensure a sufficient amount of discharge for power generation. 3. The main channel should have a minimum length of about 10 km. 4. The availability of head should be at least 60 m as lower head does not have a significant power potential in small river yields. 5. The headrace channel must be less than 5 km in length. 6. The potential site should be easily accessible and should be located near population clusters Water Resources Water to ensure proper power utilization.

Out of the total number of 36 dam sites, 4 sites (Tab. 7.5a) are located in the map sheet additional to sites located during the field work (Tab. 7.5b).

7.2.2 Groundwater Resources and Development Natural Resources of the Area Natural Resources Despite the fact that river gauge measurements show moderate evapotranspiration when 41 % of rainfall is drained as total runoff from the area and about 5 % of rainfall infiltrates and appears as baseflow, there are good groundwater resources to be used for drinking water supply of people living within the map sheet. There is also the potential to use groundwater of the map sheet to support irrigation as well as drinking water of people living outside of the area. The total volume of renewable groundwater resources in the area has been assessed to be 1,082 Mm3/year, and mainly represents groundwater resources of aquifers in the plateau area.

Considering the total number of people living within the area is 4,304,735 (Tab. 1.1) the need for water supply can be nearly 32 Mm3/year. Assessment of drinking water demand was based on a calculation of 20 l/c.d (15 l/c.d rural and 22.5 l/c.d for towns with less than 15,000 inhabitants). The figure shows that recent demand represents about 3 % of renewable groundwater resources. The area aquifers can provide adequate drinking water even in the future considering the trends in population growth.

Tesfay (2001) describes water supply issues and predicts that a large number of areas fall into category of “water scarcity” areas because of an increase in population and in demands for more water for agriculture, industry and the community. This situation will be even worse in 2025 based on trends in population growth. He defined “water scarcity” and “water stress” as cases where less than 1,000 m3/year and less than 500 m3/year are available annually per capita, respectively. These limits represent 1,605 and 802 Mm3/year; however, they are not supposed to be covered only from groundwater. Comparing these limits to the overall water resources of the area of 8,867 Mm3/year, the scarcity limit represents about 20 % and the stress limit about 10 % of the overall water resources of the sheet. It is necessary to state that the limits are based on the idea of massive human, agriculture and industrial development of the area in the next 15 years.

As most of the people within the area live in small towns and villages there is a good practice to develop large springs which form regional drainage of aquifers developed in volcanic rocks for drinking water supply. Developed and well protected springs at Debre Tsigie, Fital and Ejeri having a yield of 10–30 l/s are indicators for such a development. Additional to development of large springs, water supply based on drilled wells represents the most secure water and should be applied for small towns and concentrated village settlements. Technically, it is recommended to drill wells as follows: a) In aquifers developed in volcanic rocks, it is recommended to drill wells with a depth of about 300 m. Each of the wells can yield about 7 l/s (recent average). The recent average depth of wells is 91 m with an average groundwater level at 19 m below the surface (maximum depth to groundwater level is 70 m below the surface).

Natural Resources of the Area 137 b) In aquifers developed in basement rocks with a depth of about 30–70 m. Each of the wells can yield about 1 l/s (recent average). The recent average depth of wells is 30 m with an average groundwater level at 10 m below the surface. Each of these wells can provide 86,400 l/d and can supply a small town or group of villages with about 4,320 inhabitants considering a daily consumption of 20 l/c.d.

The most difficult question will be supply to rural areas with a widely spread population. This will be necessary from some local centers with long distribution pipes. Effectiveness and cost of water supply systems for the rural population should be studied in the future. Water Resources Water

Most rural schemes (since 2005), especially gravity schemes, do not have water charges. The tariff rates of schemes with water charges range from 0.10 Birr/family/month to 6 Birr/m3 of water. Schemes with a motorized borehole source have higher rates ranging from 3 to 6 Birr/ m3 of water. The maximum recorded urban water supply tariff is 5 and the minimum is 1 Birr/m3.

Natural Resources of the Area Natural Resources Groundwater resources developed in volcanic aquifers (with max. depth of 300 m) of the area also surpass the current needs of people living within the map sheet area. It even surpasses the potential demand of water when agriculture, living standards and industry will be developed in the area. Groundwater is generally of good quality without harmful substances and can be used for drinking purposes after the supply system is secured by chlorination. There is a chance to use the groundwater of the plateau for water supply of adjacent areas – the transfer of water to the rift valley, where there is a problem with high content of fluoride, and/or transfer of water to areas where resources do not meet demand (e.g. Addis Ababa town water supply).

The first step should be to provide a safe water supply to people living within the map area. The shortage in water supply has been reported by several rural communities and also towns e.g. Fiche, Gebre Guracha, Lemi and Menagesha. In this respect it is recommended to drill wells for the water supply of small towns where the water supply relies on developed springs and/or dug wells. Usually one well with an adequate reservoir per town or village will be enough to satisfy the water demand. Some of these water resources do not represent safe water supplies as they show an increasing content of nitrates in shallow water supply systems. Deeper wells currently represent a safe type of water supply; however, they have to be protected against pollution from local pollution sources like human and animal waste (sources of pathogens and nitrates) as well as from potential industry (tanneries, textile industry, flower plantations, etc.). The minimum required distances of water supply wells and potential pollution sources should be maintained during urban and rural development. Larger schemes like well fields should be protected by more extensive protection zones delineated on the basis of modeled flow path definition and minimum required residence time of the infiltrated water. The same level of interest should also be applied to the development and protection of groundwater resources for rural communities. Rural water development should start in relatively concentrated communities where the feasibility and impact of developed schemes will be the most significant.

The second step should be taken to utilize the groundwater resources of the area for development of human resources and agriculture resources within the area as well as other areas with high demand. Building of a well field is recommended for such groundwater use. Well fields consisting of about 20 wells can provide about 150 l/s to be used locally or transferred by pipe to the end user. A map of proposed development projects shows potential sites for the location of well fields. In total, 14 well fields were located on the map with a potential capacity of 1,650 l/s. The total yield of a well field with 1,650 l/s is 66 Mm3/year and represents 6 % of the existing renewable resources of the area. Well fields were located in two lines perpendicular to the groundwater flow direction. The first line is located between Daleti and Muka Ture. The second line is located between Bicho, Gola, Shino, Inchini and Chancho. The feasibility of the proposed schemes was tested by a model (Sima et al., 2009) together with a definition of basic principles for groundwater

138 Natural Resources of the Area resources protection. Results of the steady state model revealed that the distance between the wells of the field should be 200 m. The dynamic groundwater level will be 20–30 m below the static water level when pumping 7.5 l/s from each well. The depression cones of the individual wells only slightly overlap and the distance of the strictly protected area (where the residence time for groundwater in the aquifer is shorter than 50 days) should be 250 m form the well field.

In addition to priority development of groundwater for safe drinking water supply it should be possible to select the most fertile soil to be developed by irrigation based on groundwater to

increase the stability of food supply in prolonged periods of drought. Resources Water

Development and protection of water resources of the area and the environment as a whole have a principal importance for the development of the infrastructure with subsequent impacts upon the eradication of poverty (development of irrigated agriculture, maintaining livestock during drought). Access to drinking water changes the life of women, when a shorter distance for fetching water provides more time for family care and improves the health level of the population (statistics show that 40 % of child death rates is related to water born diseases). About 15 % of of the Area Natural Resources the rural population has access to safe drinking water in the area and about 70 % of infections are related to contaminated water resources. This is a serious problem for the creation of strong farm communities capable of full time engagement in agricultural activity. It is therefore important to provide safe drinking water to rural communities. Protection of the environment, particularly prevention of soil erosion and degradation leading to food and water scarcity, is an important development aspect for rural communities within the area. This aspect is based on the importance of water retention which is of primary importance with regard to the increase in population numbers, bringing with it an increase in demands on soil use.

The question of groundwater resources in the area has yet to be answered. The development of proposals for groundwater resources in this chapter deals with volcanic and mixed aquifers forming on the plateau because there is no regional knowledge about the character of deeper aquifers developed in sandstone and particularly limestone. Springs emerging from these formations are known from the gorges of the Jemma, Muger and Abay and their tributaries. The springs show the relatively good hydrogeological characteristics (potential) of this formation but nothing is known about the yield and quality, particularly TSD and sulphate content of groundwater when encountered by deep wells. Testing of these aquifers by deep wells is a challenge for future hydrogeological investigation of the area by deep drilling.

Another important task for the future development of knowledge about the groundwater resources of the area is the monitoring of fluctuations in groundwater levels and quality. It would be necessary to drill several monitoring wells within the area for this purpose. It is recommended to drill these wells as additional monitoring equipment at climatic stations and conduct groundwater monitoring together with measurements of climate characteristics (e.g. Fiche, Muger, Ginchi).

Results of water resources assessment show that the area is rich in both surface water as well as groundwater providing a good potential for future development. From the point of view of food security it is highly recommended to make the use of surface water for irrigation and subsequent increase in agriculture production a priority. Several rivers can be used for small hydropower schemes. Considering the surface and groundwater potential of the plateau area: 1. Surface water is sufficient for irrigation of 29 % of the area (Jemma, Muger, Awash, Guder and their tributaries in plateau area) considering 10,000 m3/ha annually. 2. In the case of groundwater consumption of 22.5 l/d by the recent population the demand will represents 3 % of the assessed groundwater resources with the potential to supply people with relatively good quality drinking water.

Natural Resources of the Area 139 The potential of the area provides feasible and environmentally sound water management within the Addis Ababa area.

7.3 Human and Land Use Resources and Development There is a large human resource potential within the area. The total assessed population was 4.3 million and average urban and rural population growth is 3 %. Taking this into account the population of the area will double in the next 20–25 years. This represents a large potential of manpower for agricultural production as well as for developing industry using the area’s natural resources. Agricultural irrigation should be practiced on part of the arable land and part of the area cover classified as pasture should be used for arable land, and livestock husbandry should use more effective methods of livestock breeding.

Improvement of the health status of inhabitants using safe water supply systems and using the rest of the water resources for agricultural irrigation and possibly for small hydropower schemes Natural Resources of the Area Natural Resources and industrial development (using other natural resources of the area) will improve the standard of life and help to eradicate poverty within this part of Ethiopia. Human and Land Use Resources and Development Human and Land Use Resources 7.4 Wind and Solar Energy Development The area has a good potential for the development of solar and wind energy. It should be feasible to use the produced energy for local supply e.g. running pumps for groundwater development in well fields or for distribution of irrigation water. It could also be feasible to use this electricity for running local small businesses as grain mills, food processing and conservation industry etc.

7.5 Environmental Problems and their Control / Management Attention is paid to the eradication of poverty, protection of the environment and natural resources as well as the increase in education in this field. The study provides information for planning in sustainable economical development, other sectorial planning, control in the use of

Wind and Solar Energy Development natural and human resources and protection against natural hazards. The study concentrates on the identification and protection of water resources, soil (particularly protection of soil against erosion), protection against natural hazards and wastewater and solid waste management.

Protection of water resources should be concentrated on better practices in sanitation within towns, villages and rural settlements. Most of the surface and groundwater is good in quality and can be used directly for drinking, agricultural and industrial purposes (see Chapter 6). Indication of improper sanitation practices is reflected in the increase of nitrates from human and animal waste in shallow groundwater that is used by developed springs and dug wells. Water development practices should follow the following basic principles of protection.

1. The source of groundwater should not be drilled directly in the center of the village/town. 2. The final design of well and distribution system should prevent direct percolation of water from the surroundings of the well along its casing to the groundwater. 3. A well should be designed upstream from the groundwater flow direction in respect to existing and potential pollution sources. 4. The required minimal protection zones should be respected by land use development in the vicinity of wells/well fields. Environmental Problems and their Control / Management 5. Regular monitoring of water level and quality should be performed. 6. There should be improvements in the general application of sanitation and waste management practices.

140 Natural Resources of the Area Particularly in the case of well field development for ex-area water transfer, the well field should be protected by a defined groundwater resources protection zone designed by hydrogeologists using groundwater flow modeling.

is one of the limiting factors of sustainable development of agriculture within the area. The Vice-Minister (ENA) of Agriculture disclosed that Ethiopia is losing 1,900 million tons of soil through erosion every year. In the opening of a three-day workshop on soil fertility management, the Vice-Minister Ato Getachew Tekelemedhin said the country is losing 600 million Birr per annum due to reduced agricultural production triggered by the effects of soil erosion. If the current trend continues unabated, a sizeable farming community in the country would be forced to earn their livelihood from sources other than farming. The prominent factors for soil degradation in Ethiopia, according to the Vice-Minister, were population pressure, deforestation, poor agricultural techniques, overgrazing and drought. He noted that the Soil Fertility Initiative (SFI) launched by the World Bank and the UN Food and Agriculture Organization played an important role in preventing soil degradation in sub-Sahran countries including Ethiopia.

Addressing the workshop, Mr. Ismail Serageldin, the Vice-President of a World Bank special of the Area Natural Resources program, expressed the bank‘s readiness to support Ethiopia‘s soil fertility initiative.

There is no data on the direct assessment of soil erosion within the map area. The highest peaks, with an altitude from 3,000 to 3,500 m a.s.l. represent the most erosion-prone area of the map sheet. Many parts of the study area are very steep and dissected. About 75 % of the area has Environmental Problems and their Control / Management slopes greater than 25 % and about one tenth of the area has slopes greater than 55 %. The main physical factors of erosion are the steep gradient of the terrain and rainfall.

The human causes of soil erosion are related to Meher ploughing and seeding, the Belg harvesting seasons and the Kiremt season with the heaviest rainfall when crop cover is limited. Another human factor which contributes to soil erosion is the short fallow period (one to four years). Soil burning which destroys the organic matter content of the soul is another adverse factor.

Traditional soil cultivation and conservation techniques use ditches for drainage. The ditches run diagonally across the slope, usually with a gradient of more than 5 %. These ditches are made by ploughing deep into the ground. The spacing of the drainage ditches in a field depends on the steepness of the slope, the steeper fields having more drainage ditches than fields on gentler slopes.

The Fanya-juu soil conservation technique was introduced in 1982 due to the negative consequences of the traditional cultivation and conservation techniques. The method is also known as the converse terrace or steep back slope terrace technique. During construction of the terraces an area is dug below the bund and the excavated soil is thrown up-slope. Natural processes of erosion and cultivation remove the soil from higher parts of the terrace and deposit it in the lower parts of the terrace; hence, leveled terraces with low erosion potential are naturally developed in the last phase of bench terrace formation.

The scale of erosion in the area is alarming and the yearly average soil loss in the traditional fields was 152 t/ha with a range from 77.94 t/ha to 218 t/ha between 1983 and 1986. From the results of the plot tests the soil loss from graded Fanya-juu was 14.72 t/ha. The land in the study area depredates seven times faster than the rate of soil formation, which was assessed to be only 2/ha/year for the study area.

Suspended sediment transport measurements are not performed by the area’s river gauges; however, some data are known from the Mugher river gauging station. The catchment of the Mugher River is 489 km2 and average transport of the sediment by the river is 38,200 t per year.

Natural Resources of the Area 141 This shows annual erosion of 78 t of soil per km2. Tab. 7.6 shows values of suspended sediments transported by the Mugher River.

Tab. 7.6 Suspended sediments transported by the Mugher River Year Sediment [t/year] Mean discharge [m3/s]

1980 37,000 8,066

1981 47,500 9,706

1982 23,500 5,605

1983 37,100 8,214

1984 45,800 9,885

1985 Not measured Not measured Natural Resources of the Area Natural Resources

1986 28,600 6,773

Human and Land Use Resources and Development Human and Land Use Resources 1987 29,900 7,329

1988 47,700 10,226

1989 43,700 9,211

1990 47,200 9,762

1991 40,200 8,473

1992 29,700 6,865

Anti-erosion measures consist of several techniques. Some of the most frequent techniques can be defined as follows: Wind and Solar Energy Development 1. The highest tops of the area along watersheds and higher hills within the plateau should be reforested. 2. This area as well as parts of gorges, where reforestation is not possible, can be terraced (similar to the Konso area and/or on the slopes at the northern part of the country). 3. Retention of water in the countryside – construction of small dams for irrigation can help not only for the accumulation of water for irrigation, but also to slow down runoff after heavy rains and the accumulation of suspended material (eroded soil) in small dams. The accumulated material can be subsequently excavated and used as a fertilizer for arable land. 4. Wicker fascine – is a cheap and very simple anti-erosion measure that can be practiced in all parts of the area either separating agricultural fields of individual owners or implemented inside the field when the fields are big enough and highly prone to erosion. 5. Creation of shrubs/tree rows preventing wind erosion and slowing down surface runoff. 6. Covering artificial cuts (along roads and other constructions) by nets or geo-textile. 7. Other technical measures and agricultural practices.

The focus on soil conservation is one of the most important factors of environmentally sound land use. Soil conservation contributes significantly to food security in the area. Environmental Problems and their Control / Management Natural hazard and protection against the consequences of earthquakes, land slides, rockfalls and other hazards is important for the preservation of human lives, property and arable land.

142 Natural Resources of the Area Sheet 2 of the engineering geology map provides detailed information on the spatial differentiation in regional susceptibility to exodynamical risky processes of mass wasting. These processes include all types of slope instability like landslides, rock slides and rockfalls, all types of erosion, rock mass susceptibility to rapid disintegration by weathering, and unfavorable geomechanical-geotechnical characteristics of soil cover.

Endogenous risky process information on Sheet 2 is provided by depicting 4 earthquake epicenters with intensity according to the Modified Mercalli scale (MM) which occurred in the mapped area between 1960 and 1996, and by a prognostic map of ground shaking intensity zoning provided as a side map to Sheet 2.

Susceptibility to exogenous risks differs both in quantity and quality between the valley and plateau engineering geological provinces. The high susceptibility class occupies 56 % of the area.

The following hot-spots have been identified: Natural Resources of the Area Natural Resources • TV2 medium and high energy zones in sides of the deep erosion valleys of the main rivers and their main tributaries where repeated rockslides of all sizes, including large deep-seated ones (activating volumes in the order of millions of m3), and small to medium sized rockfalls (up to hundreds of m3). The reason is that TV2 with its low strength, high susceptibility

to weathering and increased groundwater saturation is sandwiched between more rigid Environmental Problems and their Control / Management layers of TV1 and TV3 basalt bodies providing well known, structures of weak, plastic layers

Fig. 7.3 Scars of rockfalls with volumes in the order of 103-105 m3 at the TV4/TV5 contact above Debre Libanos village

Natural Resources of the Area 143 Natural Resources of the Area Natural Resources Environmental Problems and their Control / Management

Fig. 7.4 Huge blocks from rockfalls accumulated on the slopes just above Debre Libanos and inside the village, are testament to the very high risk of rockfall in such a crowded place

between two rigid bodies highly prone to landslides. Deformation and squeezing out of the plastic layer under the overburden weight together with selectively increased removal of the weak material causes new jointing, differential settlement and dragging out of blocks of overlying rigid beds as well as destroying and pushing out of parts of the lower rigid body. A good example of the hazard to settlements and the main road to the Jemma river bridge W of Lemi. • Repeated catastrophic rockfalls with volumes of hundreds to hundreds of thousands of m3 are bound to the contact of the basal parts of the overlaying TV4 medium and high energy units with the lower TV3 units. This contact is exposed in kilometer long passages along the upper rims of the deeply cut valley sides. Slopes above Debre Libanos and above settlements under the edge of the TV4 plateau provide good examples of the potential hazard posed to such settlements (Fig. 7.3 and 7.4). • Upper sandstones are geomorphologically weak even in a human time-scale. They are highly prone to erosion; Deep rills in the rock of road cuts can form within thirty years. Moreover, during this time the rock face of the cut can become friable and suffer greatly from raveling. Steep slopes and high rock walls built in the upper sandstone by erosion down-cutting are from over-steepening in a geomorphological time scale. Therefore their slope angle can be lowered by disastrous rockfalls/rockslides as witnessed by active and fossil forms (Fig. 7.5). Extensive, very high rock walls built by relatively freshly exposed Lower sandstone are concentrated to the erosion banks of the Jemma River and on sides where the gorges of its tributaries enter the main valley. • Residual-colluvial soils and scree accumulations at the toes of quickly deteriorating rock walls can have a considerable volume and a low stability reserve. They serve as a source of destructive mud/debris flow. The risk of mud/debris flow considerably increases under conditions of high

144 Natural Resources of the Area Natural Resources of the Area Natural Resources Environmental Problems and their Control / Management

Fig. 7.5 Scar of large, fossil rockfall from a geomorphologically over-steepened rock wall in the Upper sandstone of the Jemma river valley NE from Fiche

Fig. 7.6 Crown and transportation zone of a medium sized landslide in Upper Sandstones and colluvial soils which blocked the road from Fiche to Jemma river bridge

Natural Resources of the Area 145 precipitation, or seasonally or by occasional storms. Besides local, predominantly shallow landslides, colluvial soils and scree are also activated as a part of complex medium (Fig. 7.6) or large landslides. • River flood plains have been included into risk susceptible units because of the possibility of floods. Changes in the quantity or quality of river sediments could be also used as a far-time reaching indicator of environmental changes. The observed lithological-structural changes in cuts of alluvial soils indicate the occurrence of catastrophic floods carrying substantially increased volumes of coarse materials in sub-historical times. • Colluvial-alluvial soil accumulations in marshes at the bottom of intermontane areas are rich in clays and organic matter. That makes them highly problematic for building. Moreover the bottom areas are seasonally flooded. • Generally, the clay rich soils covering tertiary volcanic bedrock are prone to high plasticity and swelling when wet. That makes them rather problematic not only for building but also as material for earth roads especially during the rainy season. • Soil erosion and protection has been address above so we can say that areas especially

Natural Resources of the Area Natural Resources susceptible to erosion are medium energy relief in residual and colluvial soil units covering volcanic rocks with high content of ash material. In case there is intensive deforestation of eucalyptus forests in these it results in a further increase in the erosion susceptibility.

Waste water and solid waste management is important for environmentally sound

Environmental Problems and their Control / Management development of the area. Appropriate management in this field protects not only the environment and soil and water resources, but also human health against exposure to harmful pathogens and chemicals.

Recent practice is to release wastewater from households as well as from industrial production directly to the environment. Wastewater used to be discharged directly to rivers without appropriate treatment. Wastewater is mixed with surface water and is used for irrigation as well as for drinking by people living downstream from wastewater discharge. People use this polluted water from the river without any knowledge about the potential harm to their health. There is little chance to educate a large number of people about the possible adverse health impact of using polluted water and that is why the waste water producers have the responsibility to treat the water to remove substances harmful for human health.

Solid waste management is not practiced in any of the visited sites within the area except in some parts of Addis Ababa. There is a need to construct more landfills to be able to practice environmentally sound waste management. The location of the landfill should be properly selected. A landfill should be located in a shallow depression which is usually filled with clayey soil with low hydraulic conductivity or the bottom of the landfill should be sealed by an impermeable layer. The distance from the town should be appropriate to prevent the water supply being polluted. An appropriate hydrogeological and environmental impact study is required for landfill siting.

Increasing environmental care and protection of natural resources will contribute to better living standards of the people living within the area and also to an increase in their working output leading to an increase in food security in the Addis Ababa area.

7.6 Touristic Potential of the Area The area has high touristic potential because sites with rich cultural and religious history, beautiful

Touristic Potential of the Area Potential Touristic landscapes and places of high naturalist, especially geo-touristic, interest can be found here. An important aspect for the development of the touristic potential of the area is its accessibility. The area is accessible by two asphalt roads, and its touristic attractions are not very far from Addis Ababa. It is recommended to develop the touristic potential of the area.

146 Natural Resources of the Area Addis Ababa – Debre Libanos – Abay gorge tour, where religious and historical places of interest are combined with the highland landscape and a unique view of Abay gorge which is about 1,300 deep. Debre Libanos is one of the most prestigious holy places in Ethiopia and was founded in the 13th century. With the long history of St. Tekle Haimanot living in the cave (between two lava flows) and the modern church (1961) Abay gorge combines history with a view of the deep valley of the Robi Gumero River and the historical Portuguese bridge with nearby waterfalls (Fig. 7.7). This unique location is only 100 km from Addis Ababa. Abay gorge is the deepest gorge in Ethiopia providing a total cross-section through tertiary volcanic rocks to sediments of Mesozoic age.

Geology is the most influential factor controlling natural scenery and landforms. Geology and erosion resulting in the formation of deep gorges has also influenced Ethiopian history which is imprinted in the rocks in many parts of the country. It seams that little attention is given by of the Area Potential Touristic the tourist industry to geological features underlying the major tourist attractions of Ethiopia. Geotourism in Ethiopia is a new product which was introduced by Asfawossen (2008) as a result of the project “Contribution of Geology to the Growth of the Tourism Industry in Ethiopia”. The first of the Area Natural Resources

Fig. 7.7 Portuguese bridge from the 16th century

Natural Resources of the Area 147 step in the opening of significant environmental and cultural resources of the area was taken by Asfawossen (2008) when he described the important geological features along Addis Ababa – Debre Sina road in the chapter “road geology”. The same tourist potential is also provided by Addis Ababa – Abay gorge road. Touristic Potential of the Area Potential Touristic Natural Resources of the Area Natural Resources

148 Natural Resources of the Area ConclusionsConclusions

Over the past 40 years natural disasters on the Ethiopian territory have increased both in frequency and intensity and have led to severe social impacts. Evidence has long suggested that disaster risk reduction has a high cost-benefit ratio. Disasters also divert a substantial amount of national resources from development to relief, recovery and reconstruction, depriving the poor of the resources needed to escape poverty. Disasters cannot be avoided but there are ways to reduce risks and to limit their impacts. The action comprises preparedness, mitigation and prevention. It aims to enhance resilience to disasters and is underpinned by knowledge on how to manage risk, build capacity, and make use of information and communication technology as well as earth observation tools. Ethiopia is prone to natural risks like landslides, rock falls, flooding and particularly drought as reflected in geological, historical as well as recent records. Two or three subsequent periods of intense drought can cause severe crop losses, famine and population displacement in the country. The country also faces an increased risk due to climate change and more extreme weather. The insufficient quality of drinking water, the natural risks and the overall degradation of the environment are all fundamental problems and contribute to an increase in the rate of migration to urban areas.

These explanatory notes to the engineering geology, hydrogeological and hydrochemical map of the Addis Ababa area provide the results of the joint Czech Ethiopian projects. The mapping activity was carried out by field groups of hydrogeologists of the GSE in framework of the project “Addis Ababa Sheet Engineering Geology and Hydrogeological Investigation” in 2009 and 2010. The mapped area covers 18,204 km2 and is inhabited by 4.3 million people.

Groundwater accumulates in porous aquifers of alluvial and elluvial origin and in fissured and karst aquifers hosted in sedimentary (particularly limestone), volcanic rocks and some basement rocks. Aquitards and aquicludes of the area consist of mudstone and gypsum strata.

There is very good potential for development of surface water for small-scale irrigation and electricity generation in the area because large and small perennial (Abay, Jemma, Muger, Guder) rivers and several intermittent rivers drain groundwater of volcanic and sedimentary aquifers. It is necessary to consider that the groundwater level in the aquifers will fall to greater depths during periods with inadequate precipitation and river flow fed by groundwater will disappear during periods of drought in most of the rivers of the Plateau area.

Groundwater is of good quality and can be used for drinking, industrial as well as agricultural purposes. Groundwater should be primarily used for drinking water supply; it should be also used for irrigation should there be clear evidence that pumping for irrigation does not lead to over pumping of the aquifer, undermining of groundwater resources and causing degradation of the aquifer. Should the aquifer be used for irrigation, monitoring wells are recommended to be drilled together with the production wells for systematic observation of changes in groundwater levels,

149 quality of pumped water and optimization of the pumping system. Mineral and thermal water of the Filwoha thermal area can be used for recreational and curative purposes.

Local pollution of groundwater by nitrates is common in rural as well as in urban areas. In the case of developed springs their surroundings should be protected against pollution because most of the springs have shallow groundwater circulation and human as well as animal waste (problem of watering animals directly from the spring) can easily and quickly penetrate the groundwater resources. This is also a problem in karst aquifers which are highly vulnerable to pollution because of their high permeability. The spring should be developed by a solid concrete box and it is preferable that the water will flow from the spring by a tube and distributed to people 10–20 m from the spring (lower position of water distribution point). The area of the protection box should be protected against the entry of people and animals; in particular animals should be completely prevented entry.

It is advisable to use geophysical investigation to select locations where the regolith is thick and volcanic, sedimentary and basement rocks are deeply fractured and/or weathered and soft for siting wells. Groundwater can be totally missing when the regional groundwater table is not reached in cases where the drilled parts of the basalt or basement are massive without joints and fissures. It is also true for aquifers in limestone where groundwater is deep and its level is controlled by the level of surface water or the level of principal springs representing the regional drainage of the area.

Groundwater resources of the area are large in volume and can be used for safe drinking water supply within the sheet area and even part of the resources can be transferred into surrounding areas with high demands for additional water resources. The water distribution wells should preferably be equipped with a system minimizing discharge of water when it is filled into containers. In the case that water is used for animal watering it should be transported by a tube and distributed to the animals about 20–30 m from the well (lower position of water distribution point – cattle bin). The area of the well head should be protected against accumulation of surface water by drainage ditches and the entrance of animals to the well’s surroundings should be completely eliminated.

The proposed development should take into consideration the protection and conservation of the natural resources of the area. Particular interest should be paid to soil conservation and groundwater protection using the appropriate agricultural methods to decrease soil erosion and to the implementation of water resource protection to protect groundwater against pollution and over pumping, particularly in rural and urban settlements where pollution by nitrates is increasing. Monitoring of environmental components, particularly surface water flow and sediment load, in gauging stations in the lower reaches of the river should be enhanced. Recent inappropriate wastewater and waste management has to be considerably improved.

The most dangerous engineering geology features identified in the area are shown on the engineering geology maps and can be characterized as follows: repeated large and small landslides and rockfalls, and the so-called slow disaster of intensive erosional striping of soil.

Despite some local and regional environmental problems the Addis Ababa area provides the potential for feasible and environmentally sound natural and human resource management.

150 ReferencesReferences

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