SPATIAL DIMENSION OF DRAINAGE CHANNEL RESPONSES TO

1 SPATIAL DIMENSION OF DRAINAGE CHANNEL RESPONSES TO URBANIZATION IN A TROPICAL CITY BY Ajewole Davies Ojo Email:davojoiiI12@yahoo.com.Ajewole.od@unilorin.edu.ng Department of Geography, University ofilorin PMB 1515, Ilorin, Kwara State, Nigeria Being a paper presented at the 51st Annual Conference of the Association of Nigerian Geographers (ANG) held at Kogi State University, Anyigba from i h to 11th March, 2010

2 ABSTRACT Spatial interpolation techniques were used in examining channel responses of some drainage basins under different stages of urbanization to land use in /lorin, Nigeria. Data used were generated from direct hydrographic measurements of basin's width and depth. Morphometric characteristics of the studied basin such as area and length were computed from Iforin NW topographical map on scale 7:50,000. Descriptive statistical techniques were subsequently used in analyzing the data collected. Result obtained revealed that channel changes in the downstream segments below the urbanized area are substantially larger than predicted from natural channel. So, also, different spatial patterns were exhibited downstream in the studied basins which reflects difference in Landuse characteristics in the basins. Thus, various Landuse management techniques were suggested towards curbing the menace of Landuse degradation problems in the study area. Introduction A drainage basin represents the land area within the total catchment area of a major river system. It represents the soil, rivers, water bodies and vegetation within the catchment area. The drainage basin thus represents the segment of earth surface set apart from adjacent segments by a clearly defined boundary. One of the most important characteristics of a drainage system is that, it is an open system where energy supply for its maintenance and preservation is represented by input of solar radiation. This is balanced by constant removal of material and energy represented by river discharge and re-radiation. The uses to which man however put various aspects of the drainage basins considerably interrupt the tendency of a drainage basin to achieve equilibrium within its components. This is because, various actions of man such as agricultural activities, civil construction works, deforestation process, bush burning, overgrazing drainage blockage, poor waste management; all resulting from increased population pressure do most of the time manifest inform of soil erosion, flood, drought and desertification. Thus, Hollis (1988) identified five ways in which urbanization; resulting in the imposition of impermeable surface and their related activities within a drainage basin affect the natural functioning of hydrological cycle; these include: (i) The replacement of vegetated soil with impermeable surfaces (ii) Augmentation of the channel network by storm water sewers which occurs once every downpipe and road gully is served by a drain where a limited channel network used to operate. (iii) Building activities during the construction of houses, roads and bridges (iv) Encroachment on river channel by embankment, reclamation and river side roads.

3 (v) The rainfall climatology of urban areas that is affected by the greater aerodynamics surface configuration of buildings compared to rural landscape, the promotion of convection by urban heat island, and the burning of fossil fuels. Any catchment area undergoing urbanization thus experiences the aforementioned and the resultant effect is an increase in runoff discharge and sediment generation (Anderson, 1969; Jeje and Nabegu 1982; Jeje and Agu 1990; Oyegun, 1980; Jimoh, 1994; Nabegu, 2005). Though, rates of sediment yield from a watershed depends on basin geology, topography, land use, nature of storms and stream flow which produce and transport the sediments (Nabegu, 2005); increase in sediment production have been reported from previous empirical, experimental and theoretical studies to have profound effect on catchment channel form (Leopold and Miller 1956; Gregory and Walling, 1973; Walling, 1979; Walling, 1983; Odemerho, 1984). Type of load and the type of deposits are therefore associated with a particular channel shape. Thus, wide channels have often been noted to have higher bedload than deep channels (Schumm, 1963). This according to Gregory and Walling (1973) was the basis of a tentative classification of alluvial river channels based upon the type of load (bed load, mixed load, suspended load) and channel stability (eroding, depositing, stable). Relationships thus exist between the forms of stream channels and the processes which operate within them as expressed by water discharge and sediment characteristics. In line with the above, this investigation thus seeks to study the spatial pattern of channel responses to urbanization in a fast growing urban settlement of liorin. Results from the study will provide some understanding of the various interactions occurring within the basins particularly in relation to nature of sediment being generated in the study area. This will thus aid in suggesting appropriate land management techniques towards reducing rate of land degradation in the city. According to Gregory and Walling (1973), understanding of the controls upon the hydraulic geometry of stream channels is important because future changes in basin processes, instigated by modifications of climate or of basins characteristics, could have implications for the size and shape of river channels The study area liorin, the capital city of Kwara State is the study area in this investigation and the study basins are Alalubosa, Agba, Aluko and Okun drainage basins (Fig 1). liorin lies on latitude 8 30'N and longitude 40 35'E. The climate of the city is humid tropical

4 characterized by wet and dry seasonswith annual rainfall of about 1200mm (Olaniran, 2002) and annual temperature range of between 25-29 0 C (Oyegun, 1983).,.'-. '---- _... '" -4'01.. " "'0.0 '\ / / ~--.- " ".., Do", ",,, \ \ \ l I { "' /... - -. OO N ~ Ball n.area boundary -erlvar o I N A 2KIII I Fig. 1. Map of liorin, showing the four drainage basins as the study area Source: Oyegun (1986) liorin is located on a transiting zone between the northern and southern region of Nigeria. [lorin is about 300km from Lagos, 600km from Abuja, the Federal capital city of Nigeria and 60km from Ibadan, the largest city in Africa. The city is covered mainly by ferruginous tropical soil on crystalline acidic rock (Areola, 1978). The soil type has both sandy and clayey deposits lying on each other. The predominant Landuse in liorin is residential though substantial percentage area of the town is interspersed with vegetation. Urbanization process in the city is however fast replacing the natural surfaces in the town with artificial (paved) surfaces with its consequent effects on runoff generation and sedimentyield (Olaniran, 1983; Oyegun 1987; Jimoh, 1997; Jimoh and Iroye, 2009). Aluko drainage basin is the most urbanized among the studied basins. Its percentage built-up area is 97.8% and vegetation is virtually absent in the basin (Fig. 2). Agba drainage basin is the most forested; housesin the basin are widely interspersed with vegetation. There is also large expanse of vegetated land in reserve while grasses

5 'Ji,gvre:U UmlIlIse dlluifllimgn tlf IIclJinham IDfel!ite Im:llsery showing the Ivllt stv~jed Lmins 'siiutlil: ItEctAS, 0.4,U Ke 12flot} i;i t covered a substantial area of the basin. Okun drainage basins compares well with Agba, though with higher percentage built-up area. Alalubosa is the forth basin under investigation. The basin is dotted with trees, farms and fallow areas. The four basins range in size between 5.6 and 17.1 sqkm.

6 Materials and methods Direct hydrographic measurements of channel width and depth were carried out in the four drainage basins investigated using spatial interpolation techniques. This involves the division of each of the four studied basins into two reaches (i.e the upstream and downstream segments). The upstream segments are the natural unmodified parts of the stream segments where human interference is very little. The downstream are the modified parts of the drainage basins where urbanization process has brought about changes in land use pattern. Six (6) bankfull cross sectional measurement were taken in each of the total eight (8) segments of the river channels in the four studied basins (i.e four segments each in both Supstream and downstream segments of the drainage basins). Morphometric parameters of the studied basins such as area and distance were extracted from liorin NW topographical map of scale 1:50,000, using an earlier demarcated map of the study area by Oyegun (1986) as base map. Data collected were subsequently ana lysed using both descriptive and inferential statistical techniques Spatial paherns of basin channel variation 3a-d. The spatial patterns of channel variation in the studied basins are as shown on Fig.

7 ;r.".",_r""'"..' 0 /.)I.~ -y~-.,+.~ +. e. fig. 3ar Spatl1ltvlIl1afionof Olrun,draffia,ge ctlam!;lll101promelrie!l fig. 3c: SpatIal Villlalfoll of AIUIrolfralna~chann"l morpholl1stries...:...lk~1.~ ~ WiJlk...l'<.",..,.".., $ / ~~. ~.~ ow ~..",t.. " -:~~~I I.. ~ ",. "''1' 0"' l ~ s ' + '11 'i t ~ Vi H U ~~fp} fig, lm Spatfahariatil)n of Alalubos.a djajna!jll'~allnbi Ultlrpnometrfies Okun, Aluko and Alalubosa drainage basins exhibits similar pattern in width dimension with distance up to 6km from their sources before sudden and continuous increase in width dimension with distance in the three basins. However, the width dimension in the fourth basin, Agba, exhibits an undulating pattern over a distance of 7.5km from source before a sudden and continuous rise in the variable with distance. This observation support previous findings which have linked increase in channel width with distance (Knighton, 1981). River's erosive power and capacity for carrying load increase with distance. Thus, active erosion which is influenced by river load results in lateral corrosion of river channel which subsequently widen the river valley. There is however, a great difference in pattern of depth variation in the studied channels when compared with width. While Alalubosa drainage basin exhibits an undulating pattern which range between 0.8m and 4m; Okun drainage channel maintained two fairly constant depth values throughout the entire 13km channel length in the basins. Depth of 1m was first observed in the basin over the first 6km channel length. This value subsequently dropped to 0.65 at 7km distance from the river source, a value which is maintained over the remaining 5km channel length in the basin. Channel depth in Aluko drainage basin initially remained fairly uniform over a distance of 4.5km from the

8 river source: the channel however assumed an increasing undulating profile throughout the remaining 8km channel length in the basin. Reduction in channel depth with distance can also be linked to the nature of Landuse activities in the basins. This is because, the sediments being produced from active lateral erosion induced by both the load and flow frequency of the -river are subsequently dropped at the river bed, thus, silting-up the river channel. Table 1: Downstream changes in channel characteristics observed in the studied basins Basins Dimension Mean Mean Mean Enlargement majorly affected upstream downstream downstream ratio (%) by channel values value predicted value 'enlorqernent measured (m) from upstream reading (m) Agba Width 0.65 0.74 4.66 716.92 Alalubosa Width 1.50 3.46 14.31 954.0 Aluko Width 1.45 3.73 14.2 979.31 Okun Width 0.57 0.80 4.58 803.51 Source: Authors fieldwork (2003) Table 1shows the observed changes in channel characteristics of the studied basins. Most of the channel changes occurred in width dimension with enlargement ratio ranging between 71 6.92% observed in Ag ba and 979.31 % observed in Aluko. The channel cross section measured were higher than predicted in all the basins. This fact thus support findings in previous studies which linked increase in channel width with distance (Knighton (1981)). However, the values of percentage enlargement ratio observed in the studied basins reflect the pattern of Landuse activities in the basins. While Aluko drainage basin which is the most built-up basin recorded the highest percentage enlargement ratio of 979.31 %, Agba which is the most forested recorded the lowest value of 716.92% enlargement ratio. This, thus demonstrate a relationship between Landuse characteristics and processes operating within the drainage basins (Oyegun, 1986j Jimoh & Ajewole 2008j Iroye, 2008). Channel width enlargement has been observed to have a direct relationship, not only with the amount of bedload transport (Schumm, 1963), but also with the degree of urbanization (Leopold, 1968). Urbanization processes do not only leads to increase in flood magnitude, but also increase in frequency of flooding. Thus, streams in built-up basins of Aluko and Alalubosa drainage basins tend to adjust their channel dimensions to changes in flow frequency, hence the enlargement of their channels at higher rates than streams in Agba and Okun drainage basins. In the same vein, the comparative

9 higher percentage vegetation cover in Agba and Okun drainage basins may not have only reduced the proportion of gross precipitation that reach the ground surface, but might as well caused a reduction in the rate of pediment production which could have aided in widening the channel width (Jimoh, 1994). The general high values of enlargement ratio observed in the four studied basins may not be unconnected with Landuse abuses in the study area. As a result of urban growth accompanied by high population density, waste generation and disposal has become a serious issue of concern in the study area. People's poor attitude towards refuse disposal which make them dump refuse into flowing water whenever it rains (Adedibu, 1983; Jimoh, 1994; Olorunfemi, 1995; Iroye, 2008) encourages lateral erosion in the river channels. While light materials such as leaves, nylon, broken plastics, woods thrown into gutters flow into the rivers, heavy materials such as stones and iron debris are used in abrading the channel sides thereby resulting not only in increment in channel width, but also reduction in channel depth due to debris pile up. Management implication of research findings Though, local factors, including rock type, superficial deposits, soils and vegetation cover may naturally influence channel form, the extent of their influence may to a greater extent depend upon the type of Landuse pattern in such a basin. According to Gregory and Walling (1973), Landuse change and modification can give rise to very marked contrasts in catchment response because the process equilibrium is often drastically upset. Areas of forest clearance and logging activity have been found in many regions to exhibit increased flooding, and particularly increased sediment generation which are subsequently used in enlargement of natural channel form. The precise magnitude of channel change due to changes in Landuse activities can be demonstrated by reference to several specific examples where urban watersheds have been compared with rural areas, or where the changes induced by the development of an urban area have been monitored (Cohen, et. al. 1968; Oyegun, 1980; Jeje and Nabegu 1982; Nabegu, 2005) Landuse describes human uses of land or the immediate actions that modify or convert land cover (Zerbock, 2005). Thus, while land cover is the observed physical and biological cover of the earth's land such as vegetation or man- made features, Landuse is the total of arrangement; activities and input that people undertake in certain land cover type (FAO/UNED, 1999). Both Landuse and land cover changes affect catchment runoff process which ultimately influence the channel form. Landuse and land cover changes are

10 brought about by population, level of affluence, technology, political economy, political structure and attitudes and values (Niehoff et. al 2002). Attempt to solve land degradation problems caused by improper Landuse activities in liorin calls for comprehensive mpnagement of the studied basins. Good watershed management aims at retaining more rainfall on the land and improves the river regime by reducing flood peaks and sediment transport. This can be carried out through afforestation programme, controlled urban development, improvement in farming techniques and legislation against indiscriminate waste disposal. Conclusion This study has shown that there is a direct relationship between the form of stream channels and the process which operate within them as expressed by water discharge and sediment characteristics. Thus, to reduce the incidence of land degradation caused by channel width enlargement, landuse activities in the basins should be strictly monitored and controlled. Thus, indiscriminate tree felling and people's poor attitude towards refuse disposal which make them dump refuse into flowing water when it rains must be stopped.

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