Geology

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Application Stable Isotopes in the Study of Lake Dynamics in Ziway-Shalla Basin (Ethiopla)
(Addis Ababa,University, 2003) Ayalew, Eleni
A thorough understanding of lakes hydrology is crucial before any usage of the lakes for different development works. In Ziway-Shalla basin where there is complex inlcraction between the lakes and groundwater system, stable isotope water balance method is employed in this work to estimate the grou ndwater exchange rate for the geographica lly clustered lakes of the basin. Although the stable isotope water balance method has limitation due to lack of measurement of environmental parameters, the existence of tenninal lake, Lake Abiyata in the basin gives a chance to estimate these environmental parameters that are important in groundwater quanti fi cat ion. These environmental parameters would have been difficu lt to measure. The unknown 5A, isotopic composition of the atmospheric moisture for Zi'j»'1y - Shalla basin is estimated to be -16.8%0 and the limiting isotopic composition (5•) determined to be 12.5 %0. The lakes are evaporated with respect to the present day precipitation and the surface and ground waters between the lakes are influenced by the lakes indicating the hydraulic link between surface and groundwater. The result of stable isotope water balance shows that the lakes are influenced by different degree of groundwater fl ux .The groundwater inflow to lake La ngano and Shalla is signifi cant while the inflow to lake Abiyata is minimum. The groundwater outflow from Lake Ziway is signifi cant which may become groundwater inflows to the two downstream lakes, Abiyata and Langano. Lake Langano is characterized by modest groundwater outflow next to the two terminal lakes in the basin.
Assessment of Water Quality in the Sodere - Metehara Area
(Addis Ababa,University, 2004-06) Abduletif, Ahmed; Alemayehu, Tamiru (PhD)
Awash Ri ver basin is the major resource for both domestic and agri cultural uses while groundwater a l the majority of the study area, especially from deep boreholes seems unattractive for domest ic and agricul tural acti vi ties due to high concentration fluorides and sodium ions. Therefore, due attention should be given to Awash River, both in proper utilization and conservation of its quality. The core of this research work is concemed with assessment of the qual ity of the difTerent water bodies with respect to domestic and agricultural uses and to study their temporal and spatial variability. The interpretation o f the chemical data and the Anal distribution of TDS and nitrate prevai led some sign of contamination. The major sources of the present contamination are due to intense agricultural acti vity in the area. This is high ly manifested by high concentration of nitrate in groundwater of the area. The over all agricultural as well as dri nking water quality especia lly with respect to fluoride content, the groundwater is identified in a conditions whi ch is not suitable for both agricu ltural and domestic uses. a land area of about 16 1.55ha o f Metehara sugarcane plan tation has been affected, in which 55.55ha of the land has been tota lly lost while the producti vity of the remai ning 106ha become very poor as a resu lt of insufficient mechanical operation caused by high water table and high sodium salt deposit. The drinking and agricu lt ural quality of the different water bodies was assessed on the basis of the avai lable data. Accordingly it was observed that the groundwater from deep well are not fi t for drinking due to the presence of fluoride ions in the range beyond the admissible limi t. The relative high concentration of F" ions can be attributed to the presence of hot springs, fumaro les and the acidic rocks like ryholite, tracyte and obsidian.
Eng ineering geological study of Omorati area
(Addis Ababa University, 1990-06) Mehari, Germaye; Olszewska, Prof . B. Grabowska
Engineering geological Study of an area amounting to 25km living on the western bakn of Gamo river at winch a town is proposed to be located.
Engineering Geological Characteristics of the Clay Soils of Bole Area their Distribution and Practical Importance
(Addis Ababa University, 1990-06) Ayalew, Lulseged; Olszewska, Prof . B. Grabowska
On the basis of the cement to construct new engineering objects.
Engineering Geological Mapping and Land Slide Assessment of Dessie Town
(Addis Ababa University, 2001-06) Terefe, Kefyalew; Ayenew, Tenalem (Prof.)
In order to understand the geotechnical properties of soils and rocks and the slope stability in Dessie, field traverses, digging of test pits, drilling of bore hole, vertical electrical sounding test, standard penetration test, point load test, Schmidt hammer test and various type of laboratory tests have been carried out. The purpose of this work is to ensure those geological factors affecting planning, design, construction of engineering structures related to soils and rocks and to assesses the landslides to give a mitigation and control methods related to slope gradient classification map, Geomorphological map, Engineering geological map, landslide hazard classification map, land use and land cover map have been performed. Dessie is situated in a graben formed by successive parallel faults that run in the north south direction. The geomorphology of Desire is governed by, the geology, the regional tectonic activity, erosion and depositional process. The study area is covered with alternate layers of basalt and quaternary deposits of colluvial, alluviall and residual soils. Alternate layers of stratoid and degreaded basalt separated by thin palleosoil are typical layers along the escarpments of Tosse and Azewa Gedel. The centeral part of the town is characterised by scattered hills of vesicular basalt. The thickness of colluvial and alluvial deposits vary according to the palemorphology of the garben. Three borehole were drilled during the field work in selected areas and show that the thickness of these colluvial - alluvial soils reaches about 40m. Landslides, especially induced by gully erosion and rockfalls down the slope is nowadays a common phenomena in Desire Town. About 20 landslides including rock fall prone areas have been recognised. Most of them are resulted from the lack of proper surface drainage system on the thick colluvial - alluvial sediment and rockfalls toppleing from the steep cliffs. The landslides are aggravated by natural and man made activities, such as rainfall, change in the slope gradient, human intervention, gullies and streambed erosion.
Engineering Geological Study of Awassa Town and its Surroundings
(2000-06) Geremew, Zemenu; Ayenew, Dr. Tenalem
In the present work, engineering geological study has been carried Qlll (0 cha racterise and map the engineering geological conditions for the Awassa town and its surroundings, located 275 Km south of Addis Ababa in the central sector of the main Ethiopian rift. l-lJdrometeorological. hyd rological. hydrogeological. hydrochemical and geodynamic conditions have been assessed in view of tht;:ir relevance for future development of the town as related to Civil Engineering activities and to the rise of lake Awassa, which is becoming a threat for its future development . The engi neeri ng geological characteristics of the foundation soils and rocks has been assessed on the basis of field -obse rvations, simple in situ tests, and from the results of the laboratory analysis made on disturbed samples. Besides, borehole records from existing data have been studied in order to quantify and correlate the data from the test pits. " The main rock types in the area are volcano-lacustrine sediments and acid ic volcan ic rocks including ignimbrites, pyroclastic fall deposits, rhyolitic lava flows and associated obsidians . Basaltic lava flows, scorias and basaltic hayaloclasities are also locall y exposed . However. the town is wholly founded on the volcano-lacustrine depos its. The area generally gets moderate rainfall with moderate inter annual and seasonal variabilities. Lake Awassa has recently undergone lake level rise. The main causes of this phenomenon are believed to be high rainfall and reduced evaporation. The extreme value type 1 distribut ion is used for frequency analysis of annual maximum bike levels and the lake levels for di fferent years of recurrence interval is calculated. On the bases of these calculations. a flood hazard map of the lakeshore for 200 years retu rn period is established. Water quality analysis results indicate that the water in the area has no immedi ate danger on 'l!ngineering works. The waters in the area are of dominant ly Na - HCO) type lacking high concent ration of sulphate andlor chloride. However. the groundwater genera lly flu ctuates up to 5m following rainy seasons, and may be critical for ci vil enginee ring works close to the lake . The foundation soil is dominantly a fine-grained type though coarse-grai ned soils are also found at different stratigraphic positions. Three dominant so il types are identified in the field : clay soil s, sandy silt soils and silty sand/gravel soils. The clay soils occupy rel at ively the elevated central and southeastern part of the town and the poss ible expansion area while the other types occur in almost the whole area. According to the Unified Soil Class ification system, most of the soils belong to ML (inorganic silts, ve ry fine sands, rock flour, si lty or clayey fine sands. SM (silty sands. sand - silt mixtures) and CL ( inorganic clays of low to med ium plasticity, gravely clays. sa ndy clays. silly clays , lean clays). Relatively few soil samples are in groups MH (inorganic si lts, micaceous or diatomaceous fine sands or silts, elastic silts), CH (i norganic clays of high plast icity. fal clays) and GM (silty gravels, gravel-sand-clay mixtu res) . Al l the soil types show thickness variation both laterally and vertically ranging from about 0 .85 COl to about 3.5m. Moisture content at the time of sampling ranges from 25. 1 to 29.0% for clay soils, and from 18 to - 46 % for sandy si lt and silty sand/gravel soils. The specific gravity of the soils is low, in the range from 2.2 to 2.5. The liquid limit and plast icity index of the clay soils range from 28 10 53% and from 7 to 24% respectively . The sandy silt soils have LL% in the range from 26.7 to 56% and PI % in the range from 1.89 to 25% . The soils of the area are found out to be non expansive as revealed by the simple free swell test. However , some of the sandy sil! and silty sand/gravel soils show reduction in volume as large as IX 29% . thus suspected to have a collapsible structure . These soils may collapse both under load and when nooded by water and may also result in differential seulement. Transmitting the weighl of the structure to deeper soil or rock layer by means of piles or caissons. and if not. to make the foundation as deep as possible is believed to minimise the hazard . From engineering geological point of view, the rocks outcropping in the town area are grouped in 10 two: strong (basalts and rhyolites) and weak rocks (dominantly of basaltic hayalloclastites and tuffs). From field Schmidt hammer lest result, it is found out that the fanner ones have an unconfi ned strength in the range from 310 to greater than 700 Kgfcm1, while the laner less than 100 Kgfcm2 • The strong ones are found out to be good sources for aggregates while the weaker ones are not suitable fo r most of civil engineering works . Based on the fie ld and laboratory invest igation results, Engineering geological mapping at the scale of I: 10,000 was carried out for the town area including the possible expa nsion area
Engineering geological study of Omorati Area
(Addis Ababa University, 1990-06) Mehari, Germaye; Olszewska, B. Grabowska (Prof.)
Engineering Geological Study of the Proposed Entoto Tunnel
(Addis Ababa University, 1999-06) Wolderufael, Girma; Ayenew, Tenalem (Prof)
For the devolopmant of Addis Ababa c ity's water suppl y, there needs to des ign a project of constructing two dams and a 4.8 km tran smi ss ion tunnel through the Entoto ridge, by the Addis Ababa Water and Sewrage Authority (AA WSA). On the basis of the demand to construct a detail ed engineering geological and geotechni cal investigation for the proposed Entoto tunne l, the study was ca rried out after defining a tunnel corridor of about 10 km2 The study focuses on the engineering geological in vestigation of this tunnel corridor. The study area lies in the Shoa plateau that is formed of Tertiary rocks be longing to the Trap series. It is composed mainly of tuffs and trach ytic lava flows which are called the Entoto si licics of Addis Ababa area. There a re also basaltic lava flows outcropping on the top of the Entoto silicics as patches which area products of Tarmaber basalt. Undiffe rentiated Quaternary sediments which include residual , colluvial and alluvial depos its occur in v31ious parts of the area. Aerial photo interpretation, field traverses, insitu tests of rocks, as wel l as joint analysis from core orientation and surface observation and subsequent laboratory analysis were conducted. A number of exploratory boreholes drilled along the hlllnelline indicated that a significant number of trachyte / tuff interfaces OCCUlT in the defined tunnel zone. From the borehole drilling and the geophys ical investigation data, the area of the tunnel corridor appeared to be highly fau lted and weathered. The position of the water tab le, which is above the tunne l zone, and the permeability of the rock masses were detelll1ined.Engineering geological map at the scale of I: 10,000 was prepared. The rock mass strength was determined us ing Schmidt halllmer and point load tester perf01111ed during fie ld work and laboratory in vesti gati ons. I n additi on to thi s in the fi eld , rock materi al strength was done with field identification by using geological hammer and scratching by pocket knife. The borehole data a long the tunnel line, whi ch is combi ned with surface mapping, confi rmed the general geological assessment and allowed th e choice of construction methods. The borebole data was used as a basis for detailed des ign of the tunnel. Us ing all the avail able information on the rock mass and its hydrogeological characteristics, rock mass class ifi cation schemes are developed. From all the existing methods of rock mass classification, the Rock Mass Rating (RMR) and the Rock Mass Quality Index (Q - system) are selected. Consideling all the rock mass propelties, using the above classifica ti on systems the Illost economic and suitable excavation Illethod would be by drill and blast. The respective supporting method woul d also be by bolts and shotcretes. It is hoped th at this study will provide a sound geologica l bas is durin g the design, and excavation stages of the tunneli ng. However, it should be emphas ized that detail ed geotechni cal investigation on stress conditions of the rock mass and stability analys is, and furth er works at the pOItal areas and fault zones are very im portant.Engin eering geological map at the scale of I: 10,000 was prepared. The rock mass strength was determined us ing Schmidt hammer and point load tester performed during field work and laboratory investigations. In addition to this in the fie ld , rock material strength was done with field identification by using geological hammer and scratching by pocket knife. The borehole data along the tunnel line, which is combined with surface mapping, confirmed the general geological assessment and a llowed the choice of construction methods. The borebole data was used as a bas is for detailed design of the tunnel. Using a ll the available information on the rock mass and its hydrogeological characteristics, rock mass classification schemes are developed. From all the existing methods of rock mass classification, the Rock Mass Rating (RMR) and the Rock Mass Quality Index (Q - system) are selected. Considering a ll the rock mass properties, using the above classification systems the most economic and suitable excavation method would be by dri ll and blast. The respective supporting method would also be by bolts and shotcretes. It is hoped that this study wi ll provide a sound geological basis during the design, and excavation stages of the tunneling. However, it should be emphasized that detailed geotechnical investigation on stress conditions of the rock mass and stab ili ty anal ys is, and further works at the portal areas and fault zones are very i m po Ita n t.
Evaluation of Cement Raw Materials in Wayou ( New Mugher ) Area
(Addis Ababa University, 1980-07) Lakew, Tesfaye; Asssefa, Getaneh (PhD)
The raw mater i als used in the manu f acture of P ortland cement were s tudied in the Wayu locality (New Mughe r). The study showed th. t there is lime s tone of a good quality in large aMount which justifies exploitation. The c l ay ~ate ri a l studied was fou nd ou t t o be de f icient in silica and neens a si l ica modulus correc t or . Conse quently , a sandstone body which is found b e neath the basalt was investi~ated and the investigation showed that there is enough san stone tha t can be used as a cor r ector . The study of the gyps um deposi t also revealed that sufficient amount of gypsum e x ists . A conclusion was reached to the effect that the installation of a cement p l ant is justifiedo
The Geology and Geochemistry of the Banded Iron Formation in Chago-werekalu, Wellega Province
(Addis Ababa University, 1985-10) Getahun, Abera; Valera, Roberto (Prof.)
The r ocks of the stulied area form nart of the ~ o llepa Birbir proup of the uppe r c08plex and consist of a series of me t ased i me ntary and plutonic proterozoic rocks. The winiera lo~ical assembl age indicate tha t t he metamorphism is of the r r eenshist facies. The variation diagram cons tructed for the va rious plut onic re e ks stronpl y supvest coma pmatic oripin by crysta l f rac ti ona ti on . Major and trace element ana lyses of re ore sentative samples of the Chago iron formation ? t he plutonic ann. YOllnp:er e f f us i ve rocks are presented . The Chago iron f ormation can be c las s i f i ed as nre cambrian banded iron for mation and is in J11any r espects siJrii l ar to the oxide fa cies iron format i on of Lake Superior t ype. The Cha go iron fo r mation consis t s only of Iron oxides and mi nor amoun t of sil i ca with total absence of iron silica t e, s ulfide and carbonate minerals. Trace element pe ochemis try sUPRests that t he iron f orma ti on was chemic ally prec i pitated as ox ida t e s ediments in which the principal i ron mineral, ma gne tite 1 was formed at low te mpera ture in a s ~lallow nea r shore environment .
The Geology and Geochemistry of the Banded Iron Formation in Chago-Werekalu, Wellega Province
(Addis Ababa University, 1985-10) Getahun, Aberra; Valera, R. (Prof.)
The r ocks of the stuJied area form nart of the ~ ol lera Birbir group of the upper complex and cons ist of a s eries of metasedimentary and plutonic proterozoic rocks. The l1'inieralo£"ical assemblage indicate th a t the metamorphism is of the rreenshist facies. The variation diagram constructed for the various plutonic rocks strongly suppest comagmatic origin by crystal f ractionation. Ma jor and trace element analyses of reDresentative samples of the Chago iron for mation, t he plutonic an~ YOllnper effusive rocks are present ed. TIle Chago iron formation can be classified as nrecambrian banded iron formation and is in l1'any r espects similar to the oxide facies iron formation of Lake Superior t ype. The Cha go iron formation consists only of iron oxides and mi nor aIT'ount of si l ica with total absence of iron silicate, sulfide and carbon ate minerals. Trace element peochemistry suppests that the iron f ormation was chemically precipitated as oxidate sediments in wh i ch the principal iron mineral, ma gne tite, was formed at low temperature in a shal low near shore environment .
Geology and Geochemistry of the Negash Pluton and their Metallogenic Significance, Central Tigrai
(Addis Ababa University, 1997-01) Asrat, Asfawossen; Peccerillo, Angelo (Prof.)
The Negash area is located in Central Tigrai, Northern Ethiopia. The area consists of metamorphic rocks th at are cut by some granitoid intrusions. The present investigation was directed to one of such intrusions, the Negash granitoid stock. Geological mapping at a scale of I: 50 000, and geochemical studies led to the assessment of the petro logy, geochemistry and tectonic setting of both the intrusion and the country rocks and the economic potential of the area. The meramorphic rocks are mainly low - grade metavolcanic rocks with a wide compositional variation from tholeiitic to calc - alkaline mafic to felsic composition formed in an island arc environment. There are also minor intercalations of metased imentary rocks. Hornblende - hornfels facies rocks are found in proximi ty to the intrusion. The Negash granitoid body is unde fo rmed, discordant, circular stock that consists of diorites, quartz diorites, tonalites, monzonites and granodiorites with minor aplitic, grani tic and pegmatitic dikes. Its western boundary lies along a fault line (the Suluh Fault) that forms the Suluh River valley. The intrusion is dominantly of mafic and intermediate composition rocks, even though the felsic rocks crop out extensively at the top most part of the intrusion. Geochemical and petrological data suggest that the intrusive rocks were diffe rentiated from a mafic magma by fract ional crystallization and assimilation with fractional crystallization (AFC). Hornblende was the main fractionating mineral phase. This process resulted in relatively dry residual magma, as can be evidenced from the scarc ity of pegmatites and related mineralization. Preliminary geochemical survey of the Negash granitoid stock show no significant anomalies except for Zn and Sn. The country rocks show anomalies for several elements including Au, Zn, U, Sb, As and Ba. This indicates that the intrusive magma did not play an important role in generating mineralization in the area but it furnished the heat which triggered fluid circulation in the country rocks. The geochemical anomalies for most of the elements are concentrated along the western boundary of the , intrusion, the Suluh fault line. The fault presumably provided the ease through which the metasomatic fluids enriched the area with metals. Further geological and geochemical investigations should be directed to this zone.
The Geology Geochemistry and Origin of Supplied Mineralization in Katta,Wollega Province
(Addis Ababa University, 1980-07) Mammo, Telahun; Valera, Roberto (Prof.)
The Katta rocks form part of the Birbir group of the upper compl ex s i tuated in the paleo- cal c-alkaline arc that runs northsouth across the country and which compri ses thick sequences of metavolcanic and metasedimen tary r ocks 0 The rocks are intensel y folded and faulted o Their minera l ogy and texture indicate that metamorphism is o f the greenschist facies o Logging of five bore ho l es , their correlation and subs equerr petrographic s tudies on selected samples are conducted and depth variation i n miner a logy i s presented o Geochemical studies on stre am sedime nt s , bed r ock and f l oat samples, soil and core samples , and statistical tr eatment, of the results show that there is lateral and vertical variati ons in the copper and zinc mineralizations . The geochemical s oil maps show that Cu , Zn , Pb , Ni , and Co mineralizations are concentr ated along fo liation and bedding p laneso There is low litho logical c ontro l of mi neralization o Moda l ana l ysis on the core samples s how that main mineralization is a s sociated to carbonate o Ore microscopic studies show that pyrite , chalco pyrite , magnetite , bornite , cubanite) bournonite , cuprite and sphal e r ite are the ore minerals found occuring in bands , as d i ss eminati ons and in veins o Two types of mineralization are recognized syngenetic stratabound and e pigene t ic vein and dissemina t ed types . It was the syn~enetic type that gave rise to epigenetic types as a resul t of later met a morphi(: and supergene remobilization. 'r'he epigenetic type is considered to be the main mineralization in the area o
Geophysical Studies in the Aluto Geothermal Area
(Addis Ababa University, 2001-06) Ayele, Tibebu; Haile, Tigistu (PhD)
This geophysical investigation comprises gravity and electrical resistivity methods that add new information to the geology and tectonic setting of the study area. The results show major structures, major lithologic layers, intrusive bodies and thermally affected zones fi'om the mantle-crust boundary to the surface. The radially averagedlYower-spectrurrrresult of-the Bouguer gravity marks the-crust-mantleboundary at an average depth of 27 km. The crust above this contact is classified in to three major gravi-stratigraphic 'units, each unit consisting of a statistical ensemble of gravity anomaly sources generating comparable gravity field intensities. Geologicall y, the first deepseated gravi-stratigraphic unit of relatively high-density is about IS to 17 km thickness and is interpreted as the Basement Complex (?) It is expected to be highly perforated, partially melted and assimilated with magmatic and basaltic intrusions. On top of this Precambrian Basement a 5 to 7km thick Tertiary basaltic sequence is found and this is overlaid by recent siliceous volcanic products and recent rift sediments. The regional residual maps show the most tectonically active spreading zones at a depth greater than 10 km. The band pass filter applied to the Bouguer map shows intrusions that make a gravity contrast of 18 to 20 mgaJ with the host rock. These intrusions are situated under Shala caldera and under Aluto volcanic complex. They are interpreted as magmatic intrusions or magma chambers. The tops of these magmatic intrusions are below a depth of 7 km. The major intrusion beneath Aluto is bifurcates at this depth and show twin positive gravity anomalies at SSW of Adami-Tulu townand at north of lake Langano bay. Major fractures and faulted zones (?) at about 2 to 3 km depth are mapped. The presence of these structures is crucial if at all there is ground water flow and hydrothermal fluid circulation at this depth. The mapped intrusions are considered to be the geothermal heat sources for the study area. In the electrical resistivity interpretation, the apparent resistivity maps, the psuedosections and tile_ geoelectric sections could delineate thermally affected regions, hydrothennaly-altered zones and structurally weak zones. The Aluto volcanic rocks up to a depth 200 m show high resistivity values reaching to 3000 Ohmmeter. A very conductive region shows itself below this resistivity zone. The YES interpretations confirm the existence of a fractured and very conductive zone at the location of La-3 and La-6 geothermal wells. This zone is considered as the major up flow zone (TlMariam, 1996; ELC, 1986). The apparent resistivity maps also show the increase in the conductivity of the subsurface with depth and the relative lateral resistivity di stribution of the subsurface. Based on the joint analysis of the resistivity and the gravi-stratigraphic units at an estimated depth of 0.5 to 2.5 km about six geothermal wells are anticipated. Besides, additional geophysical methods are proposed to further reinforce the anticipated wells and further confirm the outcome of this study. I Crystallographic layer is coined here to mean various layers of the Earth's crust with respect to their density contrast.
Geotechnical and Engineering Geoloqical Investigation of Sibilu Dam Site, Reservoir and Catchment Area
(Addis Ababa University, 2001-01) Hailemariam, Trufat; Ayenew, Dr. ·Tenalem
In Sibilu dam catchment geomorphological, geological hydrological and engineering geological studies were conducted 10 characterize the lim itat ion and importance of the area for the new d:un construction. Using main ly Ihe, aerial photos and lopo-maps of the area relief and landform, s lope and geomorphological maps of the catchment afC produced at the sca le of I :50,000. Topographic features clearly show that all s urfncc water converges to the reservoir. Hydrological eva luati ons also indi cate that there is no out now of groundwater from Ihe catchment through other rou les, whi ch is in fa vor of the reservoir condition Gcologlcal 11l3pS of Sibi lu catchment and reservoir area arc produced al the scale of 1.50,000 and 1 12,500 lespecti veiy. Basalt. trachyte, pyrocl astic depOS its, rhyolit e, ignimbrite and recent Quatern ary deposits arc exposed III the area The sln/ctmal featu res of the catchment arc mapped from aerial photograph in terpretations. Th ~ N\V-SE alld NE-SW ll11e:unents arc found to he the d0l111113nt lineaments in the catchment Bul the meas urcments and int erpretation of the Jomt sets orient atIons in the reservoir area, conformed that the N-S and E-W arc the dominant jOint sets over NW-SE and NE-SW. Bes ide their concentrat ion th ese Joint sets, especial[y the E-W, nrc op..:n and cont inuolls -n le rock and so ils IIni ts of the reservOIr were described usi ng the ir lithology and genetic type a.<; the main classes for engineering geologlca[ purpose But det.:ul engineering geological descn ptlons lelay on Ihe assemblage of basic engineenng behaviors of these l11a1l1 groups So eight engmccflnf; geologlca[ subgroups of the roc~ IIl11tS arc Identified by their textural, joint and compaction nalures aphanallc, scaraCIOUS, ves icular and halocrystalhne basalis; masS ive and columnar jOlilted tf:lchyte; unwcldcd friab le and welded pyroclastiC deposl1s Rod units III the area are fractured and JOlllted by different sets The strength of basaltic rock ranges from strong to extremely strong With weathcflllg grades or I to IV. Columnar trachyte show very strong nature and weathering grade II Welded VIII pyroclastic depos its and massive trachytc show medium strong nature wilh weathering grades of II and IV respect ive ly. Friable pyroclastic deposit is the only rock un it show weak mass strength. Soil units wcre described into fi ve engineering geological subgroups us ing their grain size distributions and index limi ts: clay, s ilty clay, sandy clay, gravelly clay and clayey gravels. Clays have 1·3% gravels, 3-9% sands, 25-32% silts and 57-68% clay fract ions; with 65- 106% liqu id lim it, 30-85% pl as tic lim its and shrinkage lim it 8- 15%. Silty clays have 4-5% grave ls, 6- 11% sands, 37·41% sillS and 46·5 1 % clay frac tions; with 28-34% liquid limit, 5-6% plas tic limit and 5 I ·57% shri nkage limit. S;Uldy clays h3ve 2-4% grave ls, 38·39% sands, 22·25% s ilts and 34-36% clay fra ctions; with 43-48% li quid, limit 12- 18% pl astic limit and 20-25% shrinkage limi t. Gravelly clays have 22-3 1% gravels, 13· 17% sands, 23·24% silts and 3 I -39% clay fractions. with 3 I -38% liquid limll, 6· [1% pl astic limit and 15- 19% shrinkage limit. Clayey gravels have 54-61% gr.wels, 8- 18% sands, 16% s ilts and 12- 15% clay fractions. With these deSCri ptions, It was possible to classify rock uni ts as IlIgh, Illedllllll and low roc!. mass s trength ; and so ils class ified using the UCS as lIlorganic clays of lugh to extremely high plasticity (CY, CE, ClI), organic d ay of high plasti city (all), inorganic s ilts and clays of low plasticity (CL, ML), inorgan ic sil ts of intermediate plasti city (M I) and clayey gravels, poorl y graded gra vel-sandcl ay mi xtures (GC) All these are presented oil lhe engineering geological map of the reselvoi r area at the sca le of I: 12,500 Black clay soi ls are found in flat terra ins with poor drainage conditions, while red clay soils were developed on s loppy and gentle areas with good dram age conditions. These SOils have formed over the volcanic rocks m the area. Chemi cal analYSIS shows th at the black clays have an average of 41 % sihca, 14 1% of Iota I Iron and 25.7% ofalulllma Red clays havegol an average of34% Silica. 24 5% of tot al iron and 24 9% of alumina. The average COll1pos ltion of basalts of the Ethiopia plateau (Barlh. 1962) is 44% silica. 13% of total iron and CJOIo of alumina Through this, It IS clear that the loss of si lica III black clays is s light, but more evident III red cl ays. Their phYSIcal geotechlllcal analysl"; exhibits that black clay soils show moisture conlent of 38. 75-57.2%, liqUid Ilillit of 89- I 06% and 69- 83 plas ticity index and have 57-68% clay fractions While the red clay soils with 28.6-47% moisture IX corllent, 28·70% liquid limit and 5·32 plasticity index have 34·65% clay fractions. In the pl asticity chart tIlost of the black clay soils plot near U· hne and the red clays at nnd below A·line. Comp3rison of the acti vity of these soils indicates that black clay soils arc more active than the red clay soi ls. Most of the black clay soils arc also potentially expansive while the red clays do not. The bl ack clay soils conta in montimorillonite and red clay soils contain kaolini te as the prin cipal and dominant clay minerals in the reservoir. The main concern of deal ing the clay minerals in the dam engineering is to deline th cir dispers ivity and the black clay soils, contain high Illontirnori ll onite, arc found to be pot entially dispers ive. Sibilu reservoir intersects ridges and hills, which shows small ·scal ed mass movements The tectonic and weathering reatures on these terrarns seem not to create considcrable problems for the reservOir and dam condrt ions. But the one on the Ascre rrdge hab le to cause stability and storage problems duri ng the establishment of the dam From the borehole daw it was ab le 10 IdentLfy three gcotcchn ical layers III thc dam site. sod matenals~ highl y weathered and fragment ed basalIs; and joi nted sound basalt The rock and soi l materials for the construction of the dam arc almost avail able in the vicinity of the area For thc clay core the reserVOlT soils arc cxami ned and the red clay soils arc pre ferred than the black clay saris by their geotechnical propertIes. The Buba hills basalIs fullill the requirements as the construction material for the dam In the construc tion matenals only sands for li lter material are not ava ilable III the surroundlllg area
GIS Based Groundwater Vulnerability Assessment in Akaki River Catchment, Addis Ababa (Central Ethiopia)
(Addis Ababa,University, 2003-11) Niguss, Dereje; Ayenew, Tenalem (PhD)
The studied area, Akaki River Catchment, is Iocaled at tile headwaters of the Awash River Basin. The Int010 Mountain Ranges formed by Ambo-Kassam regional fault form Ihe hydrographic divide between Awash and Blue Nile Rivers. The Akaki River Catchment has an 3pp1'oximale surface area 01 aboull464 km2 and perimeter 01216 km, and approximately bounded between SO 46' 5T' and go 13' 00" N and JaG 35' 00" and 39005' OO"E. It lias an elevation range of less than 2040 to 3,200 m above mean sea level (amsl). The studied area is made up of different volcanic rocks ranging in age from Ouaternary to Miocene, induding recent alluviaillesidual deposits. There are also regional and local faults. Differenllype 01 day soil and lac\Jslrine deposits formed from Ihe weathering 01 volcanic rocks are the dominant soil type that characterizes Ihe area. AI some localities these clay soil are either Ihin or totally absent. The mean yearly rainfall depth of the Akaki River Catchment was 1150 mm (from 1964• 1998). The arithmetic mean monthly rainfall ranges from 6 mm (in December) to 276mm (in August) for the three stations (see chapter 2) in too catchment for 35 years. The highest and icMtest meilfl monthly maximum temperature occurs in months 01 March (25 Oc) and the lowest is in the month of August (20 Oc), while the minimum mean monthly values were 8 Oc (in December) and 12 Dc ~n March) for Addis Ababa Observatory from 1951- 1998 respectively. The calculated mean annual temperature was 16 Dc. Studies show both surface and groundwater pollution iD the Akaki River Catchment; and associated it to the following major sources: industrial activities, agricultural practices, munidpall domesticl wastes, fuel stations, garages, and health centers and also to graveyard (cemeteries), quarry mining, and market areas (see chapter 4). In this chapter an accotJnt was made on possible potential pollution sources. The over view include general potential pdlutionfcontamination sources and existing potentiaigrourKtwater polluting sources identified by earlier wor1c:s in the studied area. To see spatial distribution oIgfoundwater contamination situation in the catchment the discussion was supported by groundwater quality analysis data at some selected sites in the catchment extracted from recent study. For these purpose three impol1ant groundwater quality indicators (CI, NOl, and TOS) was seleded. The population density of Addis Ababa and its environs vary from more than 400 persons per hectare to less than 25 persons per hedare. The existing land use pattems in the catchment were broadly divided into four groups as: agriOJlture/open area (69%), forest land (1 5%), urban area &its associated lISes (15%) and water body and wetland covers (0.98%) 01 the total land use/cover. The general objective of the study is to identify and map the aquifer vulnerability to pollution in the Maki River Catchment by using an empirical modeIIsystem known as DRASTIC (Aller et. ai, 1987; Evans and Meyers, 1990) 10 assess relative groundwater pollution susceplibility using hydrogeologic factors (intrinsic ralher Ihan SpecifIC or integrated approach) was adopted, with GIS there by to prepare vulnerability index maps. One of the Specific objectives of this thesis was the application or use for the first lime in Ethiopia 01 an index method of aquifer vulnerability assessment with Geograp/lic Information System (GIS). Practically and academically, the research is of interest since there was no earlier comprehensive study concerned with groundwater conlaminatiol1 proIfJdiol1. Thus, to supplement the policy makers on grovndwater rasource management and protectioo in this catchment, there must be map-based informatiol1 that indicates spatial distribution of relatively vulnerable areas that is useful in major aspects of planning (chapter 1). YVhere as, previous researchers dealt with general hydrogeology, hydrOChemistry, pollution condition, etc. in this catchment.
Hydrogeological Controls in Sandstone of Ambo Area
(Addis Ababa University, 2001-06) Mekonta, Loemessa; Alemayehu, Tamiru (PhD)
The study area is a sub catch::lent of Abay drainage biCsin wi th a total surface area and ?erimeter of 838km2 and 171km respectively. Two , :her regional drainage basins border the catchment: the Awash drainage basin to th e east and southeast, and the Ghib"- Omo drainage basin to (, e south. The :nain rock outcrops in L0.e area are the basalt:: lava flow of Tertiary and Quaternary age, trachytic lava [ ow and pyroclastic depose,s of Quaternary age . and the Mesozoic sandstone. Application of different modeling techniques shows that the sandstone is part of the Upper 5aIldstone sequence. On annual basis, the area has 1143.3mm, 809mm 2..c'ld 162.8mm of mean total rainfall, Actual Evapotranspiration (AET), and grounr:water recharge respectively. There exist various water bodies in the area: two crate~ lakes (Lake Dandi and Lake Wanchi with a total surface area of 7.4 and 4km2 re s~ectively), river , streams, and springs (both thermal and cold springs) . Hydrogeological descriptions of the lithological units in the area IS mainly based on qualitative interpretation emplo,ing various approaches such as degree of weathering and fracturing, existence/or absence of springs, vege:ation cover, soil co,'er, etc. Despite the presence of many wells there is only a limiter: well data. However , there are some wells with well data tom which some hydraulic parameters ha,'e been determined. The existing well data as well as the qualiE tive interpretation show that the main aquifer in the area are the fractured andj or weathered basaltic lava flowfoHowed by alluvial sediments along some streams, and sandstone. The first two are aquifers for cold groundwater whereas the sandstone is mainly an aquifer of the thermal/or mineral water. Some hydraulic parameters have been determined for these aquifers; accordingly, the basaltic lava flow has a hydraulic conductivity and transmissivity of 3.7 x lO·sm/s to 3.9 x 10sm/s and 97- 100 m2/day respectively. The alluvial sediments have a transmissivity of 23.9m2/day on average. Where as, the sa:1dstone has a transmissivity of 76.5m2/day. The existence of highly permeable units (intensively fractured trachytic lava flow and u nwelded tuf~ at the elevated lands of Dandi and Wan chi at the southern part of the study area favors them to be a recharge area. If comparison is made with in the limit of the study area, good ground water potential with low Total Dissolved Solids (TDS) is found to the eastern side of Ambo town. Thermal and/or mineral water in the area is structurally controlled; the alignment of the thermal springs along the nearly east- west oriented fault line in Ambo area can justify the situation. Currently, the thermal springs are limited to a given area, mainly in Ambo tov-n; but their past extent can be inferred from the travertine deposit in the area whose thermal origin can be verified from the existence of silica sinter with it.
Hydrogeological Controls in Sandstone of Ambo Area
(Addis Ababa University, 2001-06) Mekonta, Lemessa; Alemayehu, Dr. Tamiru
The study area is a sub catchment of Abay dra inage basin with a total surface area and perimeter of 838km2 and 171 km respectively. Two other regional drainage basins border the catchment: the Awash drainage basin to the east and southeast, and the Ghibe- Orno drainage basin to the south. The main rock outcrops in the area are the basaJlic lava now of Tertiary and Quaternary age, trachytic lava flow and pyroclastic deposits of Quaternary age, and the Mesozoic sandstone. Application of di ITerent modeling techniques shows that the sandstone is part of the Upper sandstone sequence. On annual basis, the area has 1143.3mm, 809mm and 162.8mm of mean total rainfall , Actu al Evapotranspiration (AET) , and grou ndwater recharge respectively. There exist various water bodies in the area: two crater lakes (Lake Dandi and Lake Wanchi with a total surface area of 7.4 and 4km2 respect ively), river, streams, and spri ngs (both thermal and cold springs). Hydrogeological descriptions of the lithological units in the area is mainly based on qualitative interpretation employing various approaches such as degree of weath ering and fracturing, existence/ or absence of springs, vegetation cover, soil cover, e tc. Despite the presence of many wells there is only a limited well data. Howeve r, there are some wells with well data from which some hydraulic parameters have been determined. The existing well data as well as the qualitative interpretation s how that the main aquifer in the area are the fractu red and / or weathered basaltic lava flow VII followed by alluvial sediments along some streams, and s81ldstone. The first two are aquifers for cold groundwa ter whereas the sandstone is mainly 811 aquifer of the thermal/ or mineral water. Some hydraulic parameters have been determined for these aqu ifers; accordingly, the basaltic lava now has a hydrau lic conductivity and transmissivity of 3.7 x 10 sm/s to 3.9 x 10 sm/s and 97· 100 m2/day respectively. The al luvial sediments have a transmissivity of 23.9m2/day on average. Where as, the sand stone has a t ransmissivity of 76.5m2 / day. The existence of highly permeable units (in tens ively fractured trachytic lava now and unwelded tuff) at the elevated lands of Dandi and Wanchi at the southern part of the study area favors them to be a recharge area. If comparison is made with in the limit of the study area, good ground wa ter potential with low Total Dissolved Solids (TDS) is fou nd to the eastern side of Ambo town . Thermal and/or mineral water in the a rea is stru ctu ral ly controlled ; the al ignment of the the rmal springs along the nearly east· west oriented fault line in Ambo area C81l justify the situation. Currently, the thermal springs are limited to a given area, mainly in Ambo town; but their past extent can be inferred from the travertine deposit in the area whose thermal origin can be verified from the existence of silica sinter with it.
Hydrogeological Investigation of the Upper and Middle Borkena River Catchment, Northern Ethiopia, Wollo
(Addis Ababa University, 2001-06) Sahele, Mesfin; Ayenew, Tenalem (Prof)
The geology, hydrology, hydrogeology and hydrochemistry of the Borkena catchment which is located on the western edge of the Afar rift was studied based on conventional geologic and hyd rogeological mapping, river di scharge measurements, water sample anal ysis and assessment and interpretation of hydrometeorological data. The main objective of this work is to understand the different lithologic unit and to characterize and group them into hydrostratigraphic unit and map them, to analyze the water balance and to study the water quality. Four hydrostratigraphic units were recognized and mapped. These are: coll uvial and river channel deposits, alluvial deposits, scoraceous and basaltic lava flow and fractured massive basalt, welded tuff and rhyolite. Generally the volcanic rocks lie with in the recharge zone wllile the Quaternary sediments lie in the discharge zone. Accordingly the hydrostratigraphic unit located on the Quaternary sediments are characterized by many springs and hand dug well where as low or rare in the volcanic regions. From storage, utilization and abstraction point of view, the colluvial and river channel deposits are the most promising units than other units as a result most of the springs, hand dug wells and boreholes are located in this hydrostratigraphic units. Generally seven rock units and three set of fau lts are identified and mapped at the scale of I :50,000. Among the rock units, scoraceous and basaltic lava flow and alluvial deposits cover more than 70 percent of the catchment.Usually the exposure of the volcanic unit is limited to the topographically elevated and steep slope regions of the catchment where as the Quaternary sediments out crop with in the floor of the graben and gentle slope areas. The oldest rock unit (Rhyolite) corresponds to the AJaji Formation (28-32Ma) while basalts are correlated to the Tarmaber Formation (26-28Ma). Structurally the N-S trending faults are more common than the E-W ones. The analysis of precipitation shows that the area experience 1028 mm of mean annual rainfall with 84 percents obtained during the rainy seasons. Potential evapotranspiration was calculated by modified Penman method and is 1339 mm per annum while actual evapotranspiration was computed using Thornthwaite and Mather Soil-Water Balance Model, (1955) and is found to be 860 mm per annum. Computation of water balance was done in two steps: one for each subcatchment and the other for the entire catchment. Accordingly the entire catchment receives 50.6 mcm (million cubic meter) of recharge per annum while the Dessie and Kombolcha Subcatchments receive 18 .2 mcm and 32.4 mcm of recharge per annum respectively. Hydrochemical analysis of water samples indicate that the river water is enriched with respect to both cations and anions than the groundwater bodies. The plotting of ionic concentration of water samples was done through Aquachem software. From the plot it is found that the river water is a sodium-calcium bicarbonate type water where as the groundwater (springs, hand dug well and borehole) is calcium-sodium bicarbonate type water.From hydrochemical analysis, measurement of TDS, EC and other variables, the water bodies are characterized by low total dissolved solids (TDS) and low electri cal conduct ivity (EC) values as a result they are classified as fresh water and their utilization for various purposes scarcely bring danger.
Hydrogeology and Hydrochemistry of Lake Ziway Area and the Surrounding
(Addis Ababa University, 1998-06) Gashaw, Haile; Barbieri, Guilio (Prof.)
The studied area is located in the northerrt part of the central s~cto r of the Main Ethiopian Rift. It is bounded by 7° 35' N & 8 12' N latitudes and 38 25' E & 39°15' E longitudes. It is covered by Pliocene to Recent volcanic rocks and lacustrine sediments. Volcanic rocks are dominantly acid ignimbrites, pumices and lavas with a few basaltic lava flows and strombolian scoriae. The maximum altitude in the area is 4080 meters above sea level at the tip of Chillalo mountain and the lowest point is 1636 meters above sea level at the western shore of Ziway lake. The effective annual depth of precipitation in the area is 936.62 mm, the actual and potential evapotranspirations are 735.87 mm and 893.64 mm respectively in the lowlands and 639.06 mm and 695.83 mm respectively in the highI,Pndp. From the total mean annual volume of water entering thlla~e about 123. 3 x 10 m of water leaves the lake as surface runoff, about 88.4 x 10m of water leaves the lake as groundwater outflow and about 643.91 x 106 m3 of water leaves the lake in the form of evaporation. ~be main aquifers in the_area are coarse_grainedJ acustrine sediments, scoria cones~ _ hyaloclastites and highly weathered and fractu red basalts and ignimbrites. Depth of ground water level ranges from less than two metres up to 130 meters and the general groundwater flow direction is towards the lake from east, west and north of the area and out of the area from the southern part of the lake . The water in the area is mainly of sodium bicarbonate type and a few are of calciumsodium bicarbonate type. The chemical analyses of ground waters of the lowlands show high total dissolved solids and high fluoride concentrations. High fluoride concentrations may result from input from fumarolic activity, water-rock interaction and low Ca concentrations. Generally ground waters from the lowlands have poor quality while surface waters and ground waters in the highlands have good chemical quality for different purposes.

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