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Downloaded from the United States Geological Survey Prioritization for Conservation and Management, Landsliding (USGS) International Journal of Innovative Research and Advanced Studies (IJIRAS) ISSN: 2394-4404 Volume 3 Issue 12, November 2016 Fluvial Morphometry Of The South Western Drainage Systems, Ghana Amofa-Appiah Juliana PhD Candidate, Dept. of Geog & Reg. Planning, UCC Abstract: An attempt is made to obtain baseline morphometric information to understand the hydro-geomorphic behaviour of the South Western River System which includes the Pra, Tano, Ankobra and Bia. The River system is located between latitudes 5oN and 7.7oN and longitudes 0.3o W and 3.2oW and has a total drainage area covering about 22% of the land area of Ghana. Aside it being endowed with the nation’s richest evergreen Forest which supports the Agriculture and Lumbering sectors, the South Western River System hosts some very important resources of the country like Gold, Bauxite, Diamond and Oil reserves. In this present study, GIS and RS tools have been employed to evaluate 25 morphometric parameters. Some thematic maps including Lithological and elevation maps have been prepared using ArcGIS software. From the study, it has been found that the Pra is of the 8th and highest order while the Ankobra and Bia have the 5th and Lowest order. Based on the bifurcation ratio, lithology has had and influence in the drainage development of the various river systems especially the Bia and the Tano. Calculated form factors, circularity ratio and elongation ratio indicate that the Tano and Bia basins are relatively more elongated than the Pra and Ankobra basins. Keyword: Hydro-geomorphic, morphometric, bifurcation ratio, lithology I. INTRODUCTION the lack of necessary equipment, inaccessibility or remoteness of the few gauge stations and inadequate funding as well as Water is at the center of everything but not all countries man power. This situation does not enhance solutions to the have easy access to it. Most countries especially in Africa annual hydro-geomorphic mishaps such as, low flows and have been relying on neighboring/riparian countries for major their attendant power shortages, floods, erosion, siltation, water resources such as hydro-energy for the powering of their sedimentation and other denudational as well as aggradational various economies because of the lack of such natural processes. resources (Rivers). Ghana has in a long while been one Integrated Water Resources Management (IWRM) have country providing such services to Burkina Faso, Togo, Mali been set by the Water resources commission and are being and Cote D’ivore due to the abundance of surface water implemented in an attempt to help control these many bodies notable among which are the Volta, Tano and Pra problems bedeviled with the numerous river basins in Ghana. rivers. Upon all the abundance of these water resources, the IWRM is a systematic process for the sustainable country has in often times experienced Power and even water development, allocation and monitoring of water resources use shortage to such an extent of industries shutting down and in the context of socio-economic and environmental people being laid off. Such problems under normal objectives. Executing such measures disjointly from data on circumstance shouldn’t have been the issue of a country the hydro-geomorphic behavior of these rivers will lead to a blessed with numerous surface water bodies if watershed no problem solved. management issues had been taken critically. Hydro-geomorphic processes occurring within a drainage Watershed management is a major problem in Ghana due basin can best be understood, under the above scenario, by to insufficient and unavailable flow data which arises due to analyzing the morphometry of the various drainage basins. Page 307 www.ijiras.com | Email: [email protected] International Journal of Innovative Research and Advanced Studies (IJIRAS) ISSN: 2394-4404 Volume 3 Issue 12, November 2016 This is because every drainage basin is unique and Tano flows through the international boundary between Ghana complicated in its own way since broad variations in climate, and Côte d’Ivoire before entering the Aby-Tendo-Ehy lagoon hydrology, geology, and vegetation impart strongly on the system wnich is an inlet to the Atlantic ocean and also in Côte regional morphology and dynamics each particular river d’Ivoire. system (Hicks and Griffiths, 1992). Morphometry is the The topography of the south western basin system is measurement and the mathematical analysis of the characterised by some relatively flat lands to peaks. Some configuration of the earth surface, shape and dimensions of the major tributaries include rivers Abu, Amama, Boin, Disue, landforms (Agarwal, 1998). Thus, by analyzing a basin’s Gaw, Kwasa, Sumre, Suraw and Totua all of the Tano; Anum, Morphometry, the geology, climate, topography, Birim and Offin of the the Pra; Huni, Bonsa and Mansi of the geomorphology and structure of the basin can be expressed- Ankobra; based on which some level of planning and management execution measures could be done. Various hydrologic phenomena such as erosion characteristics and soil/subsurface properties can be connected with the physiographic characteristic of the basin including the size, shape, slope and length of tributaries (Rastogi et al, 1976). Horton (1945) and Strahler (1964), the pioneers of these parameters have grouped them under linear, areal and relief aspects. Prominent among these parameters are the bifurcation ratio; is a linear measure of how one basin order discharges into another and is relevant in hydrograph time relation according to Chorley (1969)—as bifurcation ratio reduces so as the incidences of flood increase. Drainage density describes the interaction between climate and geomorphology (Rodriguez-Iturbe & Escobar, 1982); drainage intensity measures the ability of a basin to discharge its water Figure 1 and this has implication on flood management; basin shape describes the time of peak and concentration and sinuosity measures the meandering nature of a basin and this has III. MATERIALS AND METHODS implications on sedimentation, erosion and water quality (Chow, 1964). The study used the Aster gdem (version 2image, Morphometric analysis is eseential in watershed downloaded from the United States Geological Survey prioritization for conservation and management, landsliding (USGS). It is a Digital Elevation Model (DEM) with a 30m analyses, river basin evaluation and ground water quality. This resolution has an overall accuracy of around 17-m at 95% study will measure basin variables in the south-western confidence level and a horizontal resolution on the order of drainage basin system to assess hydro-geomorphic attributes 75-m Again using Aster DEM in hydrological studies of the basins to ascertain the dynamics of the various sub- comparatively is able to help in the performance of more basins within the system. advanced analysis since it comes with the quality assessment file provided by the Aster DEM. The DEMs are indexed N07W003, N07W002, N06W003 and N05W003 for the Tano II. STUDY AREA basin; N04W003, N05W002, N05W003 and N06W003 for the Ankobra basin; N06W003, N07W003 and N06W04. The south-western basin system is located at the extreme In addition to the soft copy of survey of Ghana toposheets south-western corner of the Ghana and consists of four on a scale of 1:50000 indexed (0501a, 0502d, 0601a, 0601c, principal rivers including the Bia, Tano, Ankobra and Pra. 0602a, 0602b, 0602c, 0602d, 0603b and 0603d) for the Pra Approximately, this basin system is located between latitudes basin; (0501a, 0501b, 0501c, 0602a, 0603a, 0603b, 0603c, 5oN and 7.7oN and longitudes 0.3oW and 3.2oW. their drainage 0603d, 0702c, 0703c, 0703d) for the Tano basin; area covers about 22% of the land area of Ghana. (0403b,0503b, 0503d, 0502a, 0502c, 0602c, 0603d) for the This basin system comes strongly under the influence of Ankobra basin and (0703c, 0603a, 0603c, 0604d, 0604b) for the moist south west monsoons and has a double maixma the Bia basin. DEMs’ were reprojected from Clarke (1866) to rainfall regime peaking in May/June and September/October the Ghana meter grid. These DEMs’ were mosaicked in Erdas with annual rainfall in the range of 1137-2156mm. Its imagin (9.2). The since the toposheets had been layered vegetation consists of tropical rain forest and moist semi- already, the hydro layer containing the river networks were deciduous forest. Soils of the tropical rainforest are forest just reprojected to the Ghana meter grid and used. oxysols, which is acidic, and that of the semi-deciduous forest Geological data was also obtained from the Ghana is forest ochrosols, which is alkaline and well drained. Geological Service to analyze the stratigraphy of the basins. The basin is underlain by pre-Cambrian formation, and is For meaningful morphometric analysis, the DEM was classified into Birrimain and Tarkwaian rocks. Apart from the reconditioned so that elevations direvt drainage towards the Ankobra and Pra which empties into the gulf of guinea in vector information (topo data). This involved a lot of ArcGis Ghana, the Bia enters the sea in La Cote d’Ivoire whiles the functionalities. Hydrologic terrain analysis were done after the Page 308 www.ijiras.com | Email: [email protected] International Journal of Innovative Research and Advanced Studies (IJIRAS) ISSN: 2394-4404 Volume 3 Issue 12, November 2016 reconditioning to fill sinks, calculated flow direction and flow IV. RESULTS AND DISCUSSION accumuilation. This also involved watershed delineation and stream ordering. All these were done with the Arc Hydro LITHOLOGICAL CHARACTERISTICS pluck-in of the arc map version10.1. the first step in any morphometric analysis is the stream ordering and stream In order to understand the behaviour and form of the numbers. These were done in the Arc map and the other rivers, a stratigraphic map has been prepared from the parameters were calculated based on these results using geological map sourced from the Geological Service of established formulae established. Ghana. The principal rock formation running through all the individual basins was the Birrimian Supergroup.
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