Abstract
This study was aimed at monitoring, mapping and characterisation of floodplain encroachment patterns on the River Kaduna floodplain in Kaduna metropolis, Nigeria,as part of the approach to flood hazard evaluation, flood risk assessment and effective flood and floodplain management. A Topomap of 1967 was used to extract the base built-up layer, while Landsat.TM, 1987, Spot.XS, 1995, Landsat.ETM, 2001 and Quickbird, 2006 were used to generate other built-up layers, which were extracted by digitization and converted to polygon shape files and later used for overlay analysis. A Digital Elevation Map (DEM) of the area was used for delineation of floodplain boundary. ArcGIS sorfware 9.0 operational tools was highly robust and flexible for mapping and analysis of urban growth patterns and characterisation of floodplain encroachment by communities. From the results, it was observed that the highest extents and rates of encroachment are recorded by communities in the proximity of the Central Business District (CBD) such as T/Wada, Ung. Rimi, Barnawa, Doka and the industrial layouts of Kakuri and Kudenda. These areas are the centers of highest socio-economic infrastructure which implies greater flood risk and damage potential in the event of flooding. Results also showed that about 52.83% of the urban segment of the River Kaduna total floodplain area of 48.55km2, has been encroached by built-up. As a result of this pattern of encroachment, strong institutional framework and investment towards effective floodplain management is recommended.
Highlights
Floodplain is a vital part of the river/stream ecosystem that acts as flood buffer, water filter, nursery, major center for biological life and provide fresh water for wetlands (Tadaferua, 2003)
This growth has been accompanied with simultaneous encroachment on the River Kaduna floodplain, in an increasing trend from 2.02 km2 (4.16%) in 1967 to 25.65km2 (52.83%) in 2006
This study has shown that floodplain encroachment has progressed at an unprecedented rate in Kaduna metropolis in the face of increasing urban flooding episodes following climate change responses and poor land use practices, especially in tropical developing cities and Nigeria in particular where ENSO events have been reported to account for rainfall variability (Shamshudin, 1993; Sawa, 2002; Ati, 2006)
Summary
Floodplain is a vital part of the river/stream ecosystem that acts as flood buffer, water filter, nursery, major center for biological life and provide fresh water for wetlands (Tadaferua, 2003). The morphological features of floodplains which include fertile alluvial soils, flat topography and moderate gradient, make them less costly in terms of socio-economic development and have encouraged encroachment (Jaenig, 2006). Floodplain encroachment has seriously increased flood risk and damage potential, especially of urban floods due to heavy socio-economic infrastructural development on these floodplains. Gilbert (1991) asserted that urban encroachment into floodplain alters the integration of surface- runoff with the main channel, reduces surface water storage and conveyance capabilities, reduces water quality of receiving waters and adjacent lands, in addition to other secondary effects such as depletion of water resources, cumulative impact on wetlands and pollution of downstream surface waters. Floodplain encroachment has caused urban floodplain management to become a major concern worldwide, especially the contemporary rising trends in urban flooding. The problem is even more critical in developing cities where there is poor control over land use practices within floodplains and inadequate institutional mechanism to enforce floodplain ordinances (George, 2006)
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