• . Journal Ofe.AEA, Vol 16, 1999

26 PHYSICAL MORPBOMETRIC CHARACTERISTICS AND CAPACITY CURVES OF ABAYA AND CBAMO LAKES Seleshi B. Awulachew, Arbaminch Water Technology Institute Prof. Dr.-Ing. Rabil H.-B Dresden University of Technology ABSTRACT drainage sub-Basin (ACB) compris:eS mainly the two lakes, Lake Abaya & Chamo, and rivers and The purpose of the study undertaken in this paper streams whicli are Gelana, Bilate, Gidabo, Hare, is aimed at investigating the Abaya and Chamo Baso, Amesa & other small brooks and streams lakes, which are not previously studied in any entering the Abaya lake. Outflow from Abaya with meaningful detail regarding their water resources Kulfo and other streams Sile, Argoba/Wezeka and capacity. The paper discusses the bathymetry other intermittent streams are entering to Chamo survey undertaken and the resulting and an overflow from Chamo through Metenafesha morphomoteric characteristics derived as a result. joins Sermale river to join Sagan river, which The background lake map has been digitised and intern ends up in Lake Chew-Bahir at border of surveyed data has been also developed as digital Ethiopia with Kenya. The level difference between values. The digital values have been interpolated the two lakes is about 6lm. Fig. 2 shows the and ·grids of the elevation surface have been boundary maps of the two lakes 1mder generated From the digital values elevation area investigation. and elevation volume curves (capacity curves) of the two Lakes, which can describe the water Morphometry is the measurement of the form resources capacity of the lakes body, have been characteristics of lakes and lakes basins. The three developed Furthermore, immediate application of dimensional form of a lake basin and several the result as a continuation of the study is aspects of the lake dependent on klnd of highlighted in the paper. topography in which it is formed, physical means by which the lake come in to existence and the GENERAL DESCRIPTION OF THE LAKE conditions and events in the lake & drainage-basin AND THE DRAINAGE SYSTEM since its formation. Chow (1). For the two Lakes, the most important standard general and The Abaya and Cbamo lakes belong to the morphometric parameters of lakes labelled Ethiopian rift lakes system, which in tum is the according to Chow [ l ). for are investigated in this c0mponent of African Eastern Rift Valley. In east paper. Field data collection work under the Africa the drainage pattern began to change umbrella of the ACB research~ been undertaken. following upward movements and volcanic activity The methodologies of data cbllection in the field in Miocene times. startiog.25 Million years ·ago. A survey work, analysis of data and derived results wide stretch of land from Eriteria to the Zambezi are presented in the following sections. has been lifted more than 1,000 meters since the Miocene. The two edges have been raised further, THE NEED FOR BATHYMETRY SURVEY OF forming two great rift valleys. Tectonic activities in THE 'IWO LAKES and near these valleys formed series of splits in the earth's aust of which some are more than 1000 The bathymetry data for Abaya & Chamo ·Lake is meters deep and have filled with watel'. In this way not readily available. and only lake Ziway• has the Abaya and Chamo lakes as well as the other rift bathymetty data in Ethiopian RVLB Lakes, (4). valley lakes (excluding Lake Victoria) has been The absence of such data has hindered various formed. useful studies and planning which 'WOUid have facilitated the water resources developmeot of the The Abaya and Chamo Lakes are components of drainage area and the Jake body itse1( Such the Ethiopian Rift Valley Lakes Basin (ERVLB). deficiency can be observable in the studies at: (4), The ER.VLB comprises of 8 natural lakes and their (3) and odas, where amunptioo of simple tnl>utaries see Fig. l. The Figure is digital line trapezoidal Jake bodies has been adopted. Earlie.I' graph created for this study. The Abaya-Chamo approximate depth contours· of Lake Marprita . Journal ofE.AEA, Vol 16, 1999 Physical Morphometric Characteristics and Capacity Curves 27 Ziwayllke Abiyata Lake Shala Lake AwassaLakc Abaya Lake Sagan River Figure 1 Southern Rift valley Lakes Basin of Ethiopia 37.56 37.80 Figure 2 Digitized Boundaly Maps of Abaya and Cbamo Lakes with their Islands (Long. cl Lat Degrees Coordinate) Journal ofEA.EA, Vol 16, 1999 28 Seleshi B Awulachew and Habil H.-B (Lake Abaya) has been sketched by Morandini, G. ordinate is pre-registered at intervals of in 1941 as reported in Riedel [6] with few O. lminutes latitude or longitude distance measured points. Thus, for various future and as an (about 180m), the corresponding longitude or objective of the wider study o( ACB, bathymetry latitude respectively registered. At the same data of Abaya and Chamo Lakes have been time the corresponding depth from the display collected and results are presented. of the echo sounder have been registered. The bathymetty survey of the Abaya as well as the However, near banks, and where there were sudden Chamo Lake has been carried out in the first half of depth variations, records were taken at shorter 1998. Short description of the equipment and intervals. methods used, data of the survey worlc, results and conclusions on the data collection ha\1e been THE DATABASE, ADJUSTMENT .OF DATA contained in this document Full document of the & DIGITAL DATA GENERATION wtdertaken study is provided in Ref. (8). The Databa# EQUIPMENT AND METHODS USED The sur\tey data includes 20 days survey work of The equipment used in the study includes Boat, Abaya Lake in the period 2S.02 to 29.04 1998· and Echo sowtder, Global Positioning System (GPS) 9 days survey work oi the Chamo Lake in the and accessories. For details on accuracy of the period l.OS to 31.0S.1998. The data includes~ equipment, capacity, needed modifications and etc., of measurement, sound speed taken for see Ref. (8). An optimal methodology has been set measurement and details of equipment. as well as out in the field based on the first two days of data from the measuring equipment, i.e. lat.itude, reconnaissance work. The required parameters are longitude in degrees and minutes and depths were depth and corresponding location. The method of reoorded, including peculiar remarks. In tota( 4050 measurement described as follows: depth data points for Abaya and 2400 depth data points for Cbamo have been captured. These data i. GPS setting: The GPS was set, and important have been compiled and included in Ref. (8), The parameters, which includes, latitude and data routes and captured points are indicated in Fig. longitude, travelled distance. orientation, have 3 below. been set to be. displayed. Adjustment ofData ii. Echosounder setting; The setting is carried out according to the operating manual and adjusted As can be seen in Fig. 4 below, the water level in the to display shallow water depth both bathymeuy survey period is not constmt The graphically and digitally. collected data should be adju&ed using a recerence value of water level to provide computatiom iii. Echosounder calibration: the Echosounder is referenced to a common level In orcb to adjust all set and the boat was taken in to the position bathymeuy data, a common date of the maximum where sufficient depth is available for lake level, which co~ to the date Of caltl>ration. Actual depth is measured and the . investigation. is chosen and the ot1B day's data·arc F.cho sounder is adjusted to read the~ adjlmcd to this chosen date. For Abaya Lake thi$ depth at the point, which emun:s the dale concsponm to Marth 10, 1993 and f<ll' that of calibration. Calibration value for sound speed Cllamo it corresponds to May 31, 1998. in Abaya Lake' of 1500mls and lSSOm/s for Cbamo Lake provided good result For However, as the datum of these paiticu1ar dales concsponding to the datum above sea Jevd arc ia possible errors of measuremeots refer limitations section 7 below. known it is difliaJlt to fix the values~ to S3ndanl values. This would have beeia easily carried <U, bad there been a reference bench madt around iv. Measuremad-. Dwing the measmemeot first approximate orientation arc chosen based on the ~ « bad the pagjng mtiom of Mini.my of pre selected marts. While traYelling in the Wala Rcsoon7cs (MOWR) have exact refm:oc:cs IS'SOCialed to their measmanents OI' to the gauge selected orientation. a quickly changing co- locations. Journal ofEA.EA, VoL 16, 1999 Physical Morphometric Characteristics and Capacity Curves 29 6. .J' 6. I 16. ~ 6. Lon~~--) 37.45 37..50 37..55 37.80 "---~ Figmc 3 Abaya and °'3mo lakes BalhymetJy Sum:y Points, Boundaiy and Captured External Contour Line Journal of~ VoL 16, 1999 30 Seleshi B Awulachew and Hahil H.-B The temporally adjusted and corrected lake water to sediment deposition process, the boundary can levels and depth data can be mapped on top of also be further modified if new lake boundary map digitised background Ethiopian Mapping Authority can be obtainable. Such new boundaries can be (EMA) map. In order to use the existing maps as derived from satellite maps produced during or background maps common reference should be near the investigation periods. found for the map and the lake level during investigation. A triat search procedure has been developed for the two Lalce5 which can be shortly described as follows: Abaya (upper) and Chamo (lower) Lake Level Variation• In 1998 5 . 00 E 4 .00 .Ii 3 .00 l_, $ 2 . 00 ~ 1.00 0 .00 0 100 200 300 4 00 Day 1 e • g In Ing January 1 ~ Lake Level In the Investigation Period Chamo Lake Level in the lnvestigailon Period 2,10 .£ 2,05 lc 4,15 ..5 4,10:~ I 12.00 ~ 4,C!> ~ 1.95 4,00 ~ 1,90 3,95 +-----.------.----~--~ ~ 1,85 +----~---.....---~ 50 70 90 110 120 125 130 135 Days (25.02.-29.04.1998) Daya (1 .05-31 .05.1~) Figure 4 Abaya and Chamo Lakes Water Level Variations During Investigation Abaya Lake: The Abaya Lake level reached the Chamo Lake: Unlike Abaya, the Qwv.o· Lake has highest recorded level in 1998 due.to effect of El not been regenerated to the levels in 1960 and Nino, which caused heavy rainfall and runoff in 1970s from the recent impact ofEl-N"mo.

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