Urban drainage and landcover change: a case study of Upper Ala River basin in Akure, southwest Nigeria

Introduction: Developing countries, including Nigeria and many other sub-Saharan Africa are still developing programs that tend to encourage or provide growth poles factors, such as institutions, planned cities, and conversion of a settlement to administrative centers with the intention of attracting population increase through immigration. The growth pole areas often become nuclei for urban development in any of the three main models of urban development. One of the parts of the environment that become affected by urban development is the river basin, often due to competition with space for built-up areas, a situation that is often exacerbated by population increase and climate variability. A river basin is a defined unit area with topographic, hydraulic and hydrological unity; which can also be referred to as an identifiable planning region. Poorly monitored and controlled urban growth is a threat to the riparian ecosystems in many developing countries. Studies have shown that data availability is a major challenge to understand the impact of such growth on drainage basins. As at early ۱۹۹۰s when the study area became the administrative capital of a state (Ondo), its population surged due to migration because of the focus of government to establish their ministries and agencies in the new capital city. The National Population Commission reported ۴% population increase in the study area, and as such, pressure on previously unoccupied and protected areas becomes unexpected. Studies in Islamabad, Pakistan and in North-West Delhi, India also indicated increase in built-up areas around protected areas and within drainage basins due to population increase into administrative capital cities.Materials and Methods: This study examined the extent of urban growth and the impact on a river basin in one of the administrative capital cities in southwestern Nigeria, using freely available Landsat datasets. Specific objectives were to assess vegetal and topographic changes in the status of riparian vegetation and other land cover, as well as the impact of urban growth on the river basin. Data included multi-date Landsat images.Results: The Ala River in Akure, southwest Nigeria, whose vulnerability was investigated in this study, has experienced a reduction in basin area and parts that were previously classified as water bodies were later overgrown by built-up and related human activities. Physical observation of the river plain reveals that apart from bridges that were constructed to ease road transportation across the river, no important event was available to protect the basin from intrusion and attendant exploitation, at the time of the surveys. The former studies have argued for the protection of flood plains, noting that areas close to the plains are vulnerable to flood disasters. Building regulations that include the set-back rules around major rivers are rarely observed due to compromised implementation strategies that typically discouraged non-biased enforcement. In addition, parts of the downstream station of the river have been converted to dumpsites and the hitherto perennial streams in the area have either become intermittent or dried off. Also, an increase in surface temperature as well as increased built-up areas within the basin may be associated with the dryness of some of the river tributaries. In general, the wetland ecosystem in the study area has given way to poorly planned built-up and few shanty settlements. Results showed increase in built-up areas (۴۸.۱%), decline in vegetal cover (۷۱.۲%), loss of vegetal greenness (< -۰.۳۳) and increased land surface temperature (۰.۲۳-۰.۲۵oC).Conclusions: The study concluded that the Landsat images with ground surveys can provide reliable results for ecosystem monitoring in the area. Conscious sustainable urban planning and strategies for the sustenance of the urban wetlands as well as policy towards urban greening are recommended. Future studies will be focused on t hydro-morphometric analysis of the river basin.

کلیدواژه ها:

Akure ، Southwest Nigeria ، Drainage Basin ، Landcover change ، Land surface Temperature ، Urban Growth ، Impervious surface ، Water productivity