Morphometric Analysis of Yewa Drainage Basin, Southwest Nigeria

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1 Morphometric Analysis of Yewa Drainage Basin, Southwest Nigeria Olusegun ADEAGA (Correspondence Author) Department of Geography, University of Lagos, Akoka-Yaba, Lagos, Nigeria, Tel: s: Received: February 16, 2011 Accepted: March 29, 2011 ABSTRACT The paper analyzed the morphometric parameters of Yewa drainage basin with a view to understanding the relationships between the parameters and hydrogeomorphic processes in operation within the basin. Thus, this study presents an approach, in quantifying relations between geomorphometric attributes and hydrologic variables using a terrain based technique. Data used were generated from topographical maps for the derived digital elevation model (DEM), morphometric indices were automatically generated from derived digital terrain model (DTM). Result obtained indicated that the studied drainage basin exhibits a lot of irregularities with a low tendency towards single (synchronized) flow peak and low but extended peak flows. Means of managing the multiple flow peaks and drainage irregularities were suggested towards sustainable water development in Yewa drainage basin area. KEY WORDS: Morphometry, DEM, Drainage basin, Nigeria INTRODUCTION The varied degree of water production and water utilization issues demand proper understanding of drainage basin dynamics towards attaining a sound science-based water resources policy. Thus, the present inadequate hydrological data resulting from the high incidence of poorly gauged and unguaged basins call for the adoption of synthetic hydrology. Synthetic hydrology involves estimation of hydrological parameters using appropriate coefficients related to various physical features of a catchment. Such estimation is usually carried out in order to generate data needed for hydrological information through estimates of drainage basin form (Adeaga et al, 2006). Morphometric analysis involves the mathematical description of topographic form of a drainage basin, in order to explain part of the physical reality, strength and direction of fluvial and geomophological processes of a drainage region. This entails measurement of the physical properties of a basin in its integrated form. Interrelationship between the morphological variables is often an indicator of the Lagos Journal of Geo-Information Sciences (LJGIS) Volume 1, Number 1 An International LJGIS Journal of the University Volume of 1, Lagos, Number NIGERIA 1 June 2011 June, Page ljgis@unilag.edu.ng Page 31

2 degree to which the geometric and fluvial dynamic properties of a basin are related (Gregory and Walling, 1973). Thus, the concept of morphometric analysis in fluvial geomorphology provides information on the interrelationship between morphological systems and process-response system in quantitative terms within drainage basins. Many of the indices are also ratios, meaning that they can be used to characterize and compare basins of different sizes. Proper estimation of morphometric indices are easily carried-out in a defined hydrological system. Drainage basin remains an excellent example of a geomorphologic system and a fundamental unit of virtually all watershed and fluvial investigations. Drainage basins are commonly treated as physical entities, because they are discrete landforms suitable for statistical, comparative and empirical analyses. Innumerable means of numerical and qualitative description can also be carried-out with ease within the basin (Chorley and Kennedy, 1971; Strahler and Strahler, 2002). This is due to the basin being a physical process-response system open to a cascade of inputs and outputs. Hence, input and output elements can easily be measured and quantified and their interrelationships, interdependence and interconnections with the natural elements determined with ease (Strahler and Strahler, 2002). In addition, the structural concept of drainage systems favours it as a fundamental concept in topography evolution modelling and geomorphometric parameters derivation. Thus, the drainage basin represents a spatial framework for fundamental landscape studies. This is due to the well-defined boundary conditions of explicit internal connectivity provided by the systems of drainage and ridge lines in drainage basin. Unambiguous template for models structuring in geomorphology, hydrology, landscape and other fields are therefore easily attained (Jeje, 2001). In addition, the concept is important for basin hydrology since derivable geomorphometric indices are usually used as the basic indices for predicting basin system response most especially in poorly gauged and ungauged basins (Nogami, 1995). In this study, basic form parameters were measured towards evaluating the integrated operational structural network (morphology) and their relationship within the poorly gauged Yewa drainage basin. The analysis was also used to assess the potential rate of water delivery and system response to input in the drainage basin towards development of quantitative physiographic methods while describing the evolution and behavior of surface-drainage network within the basin. REGIONAL SETTING Yewa River is a transboundary (International) river with a total catchment area of about 5000km 2 (Fig. 1). It lies approximately within latitudes N and N of the Equator and longitudes E and E of the Greenwich Meridian. Climatologically, the river is located within the tropical rainy climate (Af) in accordance to Koppen s climate classification. The region is under the influence of LJGIS Volume 1, Number 1 June 2011 Page 32

3 the tropical continental (ct) and the tropical maritime (mt) air masses. Associated with the tropical maritime air mass are the southwesterly winds, while the north easterly winds are associated with the dry and dusty tropical continental air mass. The ITD separates the two air masses (Iloeje, 1976). Fig. 1: The Yewa drainage basin Yewa drainage basin falls within Nigeria s Hydrological Area VI and is within the jurisdiction of the Ogun-Osun River Basin Development Authority. The area has an annual runoff of 35.4 *10 9 m 3 or depth of runoff of 352mm per year (Federal Dept. of Water Resources, 1986) with average annual growth rate of 2.7%. The geology of Yewa Basin is largely made up of rocks of the sedimentary origin, which overlies the Basement Complex (Geology of Part of South Western Nigeria, 1964; VON, 2004) and is closely related to the topography. Yewa basin has a rapid rural-urban transformation economy with increasing per capita water demand. Important towns and villages within the basin include Badagry, Topo, Ajilete, Oke- Odan, Idiroko and Ado-odo, among others. The present state of hydrological data and water resources investigation within Yewa basin calls for concern while considering the need for a hitch free water abstraction scheme as proposed by the Ogun and Lagos State Water Corporation (Lagos State Water Corporation, 2000). The proposed scheme was conceived as a potential alternative measure to resolve the anticipated water supply problem (domestic and industrial) within the lower Ogun area and Lagos metropolitan area for the year The estimated water supply requirement is 33 and MCM for Ogun and Lagos regions, respectively. The resulting pressure on available freshwater has often resulted in mass uncoordinated surface and groundwater resource withdrawals. The uncontrolled LJGIS Volume 1, Number 1 June 2011 Page 33

4 withdrawal is worrisome, as the long-term effect of the mis-managed groundwater resources on the basin s population and their socio-economic activities as well as the basin s surface water sustenance might be severe. METHODOLOGY This work was based on terrain analysis with the basic data source being topographical map sheet as published by the Federal Survey of Nigeria (1970), at the scale of 1:50,000. The sheets are 278A N.E and 278a N.W, 278 S.E and Part of 278 S.W, 278 N.E and Part of 278 N.W & 259 S.E and Part of 259 S.W, published. The topographical data were used in the generation of digital elevation model (DEM). The method involves digitizing of the iso-elevation (contours) from topographic maps in order to generate a digital elevation model (DEM) for the basin using cubic spline interpolation technique. The cubic spline is expressed as: fi(x) a X b X c X (1) i i i 2 3 d i X where (a i, b i, c i and d i ) are actual parameters for each of the cubic spline equations X is data point. A graph model was generated from the DEM surface in order to delineate the watershed and sub-basins boundary. The D-8 drainage model was used to define the landscape properties for each individual raster cell by the evaluation of each cell and its eight (8) neighbours. The watershed and the flow line are extracted in a simple way starting from the D-8 model graph while the flow direction in D-8 drainage model is given in accordance to the principle of the maximum descent (steepest descent). The flow accumulation analysis was accomplished through the assessment of cells flowing in and out of the flow direction model. The Grid flow accumulation grid network is used to identify watershed and sub-basin outlet points for watershed delineation (Rodriguez-Iturbe et al, 1982). The D-8 drainage model was automatically corrected before deriving the basin slope, drained surfaces and length of drains and other hydrological indices. This was carried out in order to generate stream networks and watershed delineation. The generated hydrological indices make it possible to delimit all the basin slopes starting from the identified drainage system in an automatic way. Identified basins and their sub-basins with their slopes are classified in a descending order in accordance with the Bocqullion classification (IRD, 1999). From the processed automated delineation of the stream networks, cell accumulation threshold value of 100 cells for thwaleg (flow) occurrence was selected in order to achieve an adaptive sampling scheme, data structures and accurate surface interpolation technique. LJGIS Volume 1, Number 1 June 2011 Page 34

5 It should be noted that basic morphometric indices were estimated in order to assess the potential rate of water delivery and system response to input (rainfall) in Yewa drainage basin (Strahler, 1964). Parameters measured for this exercise were the basin drainage network, geometry, and dissection as well as shape indices (Gregory and Walling, 1973; Ajaegbu and Faniran, 1980; Strahler and Strahler, 2002). Furthermore, the Strahler system of stream ordering was used to arrange the streams according to a hierarchy or magnitude of orders (Strahler, 1964). RESULTS AND DISCUSSION In this section, basic interrelationship between the morphological and processresponse system within the Yewa Drainage basin is discussed. Preliminary results of a sectional comparative analysis of the digitized river network on topographical sheets 278 N.E. and Part of N.W., which shows the actual river network and the automatic generated network (Fig 2) using the specified cell accumulation shows a similar river network pattern. The geomorphometric measures are based on the automatically generated river network. The digital elevation model (DEM) of Yewa Basin (Fig. 3) reveals that the region in the upper section (Ijaka-Oke) has a fairly steep gradient with well-defined channels. In the middle and lower reaches, the channel becomes less defined and an extensive floodplain is prominent with broad bank storage. This is prominent in the lower reach due to its relatively flat topography and wide valley. The area elevation relationship as shown in Fig 4, indicates an irregular pattern from 0 50 metres with the highest areal extent of about 124km 2 recorded between metres and thereafter flattens out. Region of significant runoff contribution and geomorphometric interest falls within 38-52metres. This is due to extensive total area covered by this section of the basin. Quantitative assessment of Yewa basin shows that the total basin area is approximately 5000km 2 with axial length of 129.5km and a perimeter of 485km. The stream ordering system adopted (Strahler, 1964) shows that it is a 4 th order catchment (Table 1). The areal coverage of the stream ordering system shows an increase in areal covered with an increasing stream order (Fig. 5). For the linear features of the channel system, there exists an indirect relationship between the stream order and stream number and between the stream order and stream lengths (Fig. 6). This is in conformity with the general stream order rule (Ajaegbu and Faniran, 1980; Strahler & Strahler, 2002), which states that the number of stream segments decreases as the order increases. The correlation between stream order and stream number is while that between stream order and stream length is -0.91, depicting the existence of an inverse linear relationship between stream order and stream number and between stream order and LJGIS Volume 1, Number 1 June 2011 Page 35

6 stream length. Deviation from its general linear behavior indicates that the terrain is characterized by variation in lithology and topography. Fig. 2 : River Network of Sheet 278 NE and part of N.W Fig. 3: Digital elevation model of Yewa Drainage Basin Table 1: Basin Morphometry indices of River Yewa Stream Order Stream Number Total Length Average Length Bifurcation Ratio LJGIS Volume 1, Number 1 June 2011 Page 36

7 Stream Number Stream Length (Km) Area Covered (Sq.km) Adeaga O. (2011) Stream Order Fig. 5: Stream order areal coverage in Yewa Drainage basin Fig. 4: Area Elevation relationship in the basin Stream Number Stream Order Fig. 6: Relationship between stream lengths, number and order in the Basin LJGIS Volume 1, Number 1 June 2011 Page 37

8 The relationship between stream order (X) and stream length (Y) in River Yewa is represented by the regression equation: Y = (X). (2) Similarly, the relationship between stream order (X) and stream length (Y) is : Y = (X).. (3) The distribution of bifurcation ratios (Table 1) results in the mean ratio of 2.35 in Yewa basin and as expected, the number of segment of each order formed an inverse geometric sequence with order number. The relatively low mean ratio depicts a relatively large range of variation for different regions or for different environment dominates. This is an indication that the geological structures fairly disturb the drainage pattern. From the areal aspects of the drainage basin, the drainage density is computed to be approximately 0.14km -1. The value is much less than the standard value of 0.65km -1. The basin therefore has a highly permeable subsoil and thick vegetative cover, with low relief. Low drainage density leads to coarse drainage texture as depicted by low Texture ratio of Generally, lower values of D also tend to occur on granite, gneiss and schist regions. The chief rock type in the drainage basin is basement complex rock (Granite formation) which falls under the gneissic group of rocks. This corroborates the low drainage density observed in the drainage basin. Hence, flow within the basin has tendency towards a delayed time of concentration. Such difficulty might be due to the discrepancies in the rate at which flow is being funneled into the few available stream segments. The result is a low tendency towards evenly distributed runoff and rapid runoff into the secondary and tertiary streams. Stream frequency analysis in the basin shows that the region exhibits a stream frequency of 0.07 per km 2 and a drainage efficiency of 0.1. The drainage efficiency of a basin is the product of the drainage density and stream efficiency. This result further portrays Yewa drainage basin as having an inefficient drainage system. The length of overland flow shows that the average distance traveled by water (on the surface) before reaching a defined channel is approximately 7.25km. This indicates that the basin is susceptible to sheet flooding, delayed time of concentration in channel, hence high evaporation and infiltration. In River Yewa the values of form factor, basin circularity and compactness coefficient are 0.24, 0.23 and 2.09, respectively with elongated ratio being 0.65 for the basin shape index. The indices imply that the Yewa drainage basin has a low relief with an elongated shape; the elongation in shape implies the presence of a lot LJGIS Volume 1, Number 1 June 2011 Page 38

9 of irregularities in the basin with a low tendency towards single (synchronized) flow peak but low extended peak flows. The total basin relief is 123.5metres while the average basin slope is about 1: 9.5. Along the middle and lower portion of the basin the slope is about 1: 2.5. This depicts a tendency towards extensive floodplain and wetland formation most especially in the lower portion of the basin.. CONCLUSION Morphometry analysis eases the quantification of drainage basin system response through appropriateness of linkages between the geomorphological parameters and hydrologic behavior. Hence, understanding the past as well as estimating the future hydro-geomorphic events are made ease. This is vital towards establishing an appropriate water resources development plans in the ever-increasing poorly gauged and ungauged basins. The morphometry analysis of Yewa drainage basin depicts a basin with channel delayed runoff time of concentration and lag time. The basin is also characterized by inefficient drainage system and low tendency towards evenly distributed runoff. Thus, an effective and efficient drainage system should dominate the water system plan within the drainage basin, in order to attain a speedy flow release from the secondary and tertiary streams into primary (main stream) channels (4 th order stream). The plan should also entail means of harnessing the unsynchronized and low but extended peak flows through structural and non-structural people-oriented tools. REFERENCES Adeaga O., Lekan Oyebande; Christian Depraetere (2006): Surface Runoff Simulation for Part of Yewa Basin, IAHS Publ. 303, PP Ajaegbu, H. I. and Faniran, A. (1980): A new approach to practical work in geography, 2 nd edition, HEB Heinemann Educational books, Ibadan. Chorley, R. J., & Kennedy, B. A. (1971). Physical Geography: A systems approach. London: Prentice-Hall International Easthernbrook, D.J. (1993), Surface Processes and Landforms, Macmillian Publishing Co., New York, 325pp. Federal Dept. of Water Resources (1986): National Water Resources Masterplan, Draft Preliminary Plan, Nigeria. Gregory, K. J. and Walling, D. E. (1973). Drainage basin. Form and Process. Edward Arnold. Horton, R. E. (1932): Drainage basin characteristics. Trans. Am. Geophys. Unions. 13, LJGIS Volume 1, Number 1 June 2011 Page 39

10 Iloeje, N.P. (1976): A new geography of Nigeria metricated edition, Publ. Longman Nigeria limited, pp Institut de recherché pour le developpement (IRD) (2000): Demiurge Version 3.1Production et Traitement De MNT appliques a I Hydrologie, Institut de Recherche pour le Developpement, Paris 1999 Jeje, L. K. (2001): Drainage basin development and the environment, Geographical Perspectives on Environmental Problems and Management in Nigeria, ed. Ofomata G.E.K. and Phil-Eze, Jamoe Enterprises (Nigeria), Enugu, pp Lagos State Water Corporation (2000) : Challenges and issues of privatization of state water institutions in Nigeria; mobilizing finance for water supply in Nigeria, Lagos Water Conference 27 th to 29 th Nov., Moore, I. D., Grayson, R. B., and Ladson, A. R. (1991). Digital terrain modeling: a review of hydrological, geomorphological, and biological applications. Hydrological Processes, 5(3):3-30. Nogami, M. (1995): Geomorphometric measures for digital elevation model Annals of Geomorphology, Pike,RJ&R.Dikay (Eds). M.suppl-BD 101 pp Strahler, A. N. (1964). Quantitative geomorphology of drainage basins and channel networks. In V. T. Chow (Ed.), Handbook of Applied Hydrology. (pp. 4, 39-4,76) New York: McGraw Hill,. Strahler,A.N., and Strahler,A.H. (2002): A Text Book of Physical Geography, John Wiley & Sons, New York, Tarboton, D.G., Bras, R.L., Rodriguez-Iturbe, I On the extraction of channel networks from digital elevation data. Hydrol. Processes, 5: LJGIS Volume 1, Number 1 June 2011 Page 40