Effects of Urbanisation on Channel Morphology: The Case of Ekulu River in Enugu, South Eastern Nigeria

Abstract

This study focuses on the effects of urbanisation on the morphology and hydraulic geometry of the Ekulu River in Enugu, southeastern Nigeria. Measurements of channel properties were taken along a 25 km stretch at 45 sites grouped into three sections: section A, upstream of the town (15); section B, within the town (15); and section C, downstream of the town (15). Spatial interpolation techniques were used to predict channel morphometric properties within and downstream of Enugu on the basis of upstream relationships. The independent variable in the analysis was drainage area, used as a surrogate for discharge. In section A, channel parameters increased systematically downstream with basin area. Within the town, channel parameters exhibited sudden dimensional increases. Based on sectoral relationships, measured channel dimensions showed moderate increases above the expected values, with average enlargement ratio indices of 34 per cent, 91 per cent and 65 per cent in width, channel capacity and depth, respectively, between sections A and B. Between B and C the channel exhibited average reduction ratio indices of 21 per cent and 27 per cent in capacity and depth, respectively, and an average enlargement ratio index of 17 per cent in width relative to the expected value. The increase in channel dimension within the town is explained by: (1) the confluence with a third‐order tributary; (2) the poorly structured, loose, incoherent and highly erodible channel bank materials; (3) high volumes of runoff generated from the impermeable urban surfaces; (4) urban hydrological routing due to urban drainage; (5) human traffic across the river banks; and (6) the huge volume of garbage and solid wastes dumped into the channel.

Urban transformation of river landscapes in a global context

“…Many urbanizing streams, however, were still undergoing adjustment at the time of study. These were characterized by channel reductions (Leopold, 1973;Hollis and Luckett, 1976) reflecting high sediment loads (Fox, 1976;Douglas, 1985b), variable downstream channel geometries (Ebisemiju, 1989a,b;Odemerho, 1992;Chin and Gregory, 2001;Jeje and Ikeazota, 2002;Laronne and Shulker, 2002), and rapid geomorphic changes (Arnold et al, 1982) including channel enlargement (MacRae, 1997;Trimble, 1997). These results reinforce the notion that, even though new equilibriums following urbanization are theoretically possible (Leopold et al, 1964;Graf, 1977) and observed in some examples (Fig.…”

“…8), and that the magnitude of these changes has generally been smaller than in temperate environments. Although a dramatic case of channel enlargement was documented along the Ekulu River within the urban area of Enugu (capacity ratio = 1.91, width ratio = 1.34, depth ratio = 1.65; Jeje and Ikeazota, 2002), most other studies in Nigeria reported smaller urban river channels. These include downstream sections of the Ekulu (capacity ratio = 0.79; Jeje and Ikeazota, 2002), channels along the headwaters of Elemi River through the village of Igede (capacity ratio = 0.81; Ebisemiju, 1989a), and in the Ikpoba River in Benin City (Odemerho, 1992).…”

“…In addition to construction activities that introduce vast quantities of sediment into stream channels (Douglas, 1974), bank erosion is a significant source of sediment (Douglas, 1985b) under high intensity rainfall (e.g. Gupta, 1984), even though humid tropical rivers may be expected to exhibit lower erosive characteristics (Jeje and Ikeazota, 2002) because of high silt-clay contents. Thus, channel widening has been documented along the Sungai Anak Ayer Batu downstream of the town of Jalan Damansara (Douglas, 1985b).…”

“…Similarly, dramatic channel erosion in a southern California river system was still taking place some 40-50 years following development, furnishing two-thirds of the total sediment yield (Trimble, 1997), although land use may not have been constant in this area. Except for one fully urbanized basin (Ebisemiju, 1989a,b), streams investigated in Nigeria were also estimated to require more than 50 years to adjust to new hydrologic conditions (Ebisemiju, 1989a,b;Jeje and Ikeazota, 2002). MacRae and DeAndrea (1999) further calculated that channel enlargement may take 50-75 years to complete once development starts, supporting the conclusion that most fluvial systems may take several decades to adjust to the process of urbanization and reach a new equilibrium, if such equilibrium can be observed at all.…”

Impact of Urbanization on Channel Morphology: Some Comments

“…These ratios indicate much larger increase compared with what has been reported elsewhere where, typical channel enlargement ratios range from 1.0-4.0 (Gregory, 1987a). Data from humid tropical areas of Nigeria in Ekulu river show a capacity ratio of 0.79, (Jeje and Ikeazota, 2002), Elemi River show a capacity ratio of 0.81, (Ebisemiju, 1989) and the Ikpoba River has a capacity ratio of 1.2 (Odemerho, 1992). The variation observed between the Jakara channel and others necessitates the following comment as to the possible causes and direction of inquiry.…”