Assessing the effects of urban development and climate change on flooding in the Greater Port-Harcourt watershed, Niger Delta, Nigeria
Dan-Jumbo, Nimi Gibson
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Developing countries have been rapidly urbanising over the last decades, resulting in major environmental pressures and increased vulnerability to natural disasters. A complex combination of factors, including climate change, land use change, poorly implemented regulation and a lack of integrated planning has often resulted in environmental degradation and disproportionate impacts of natural disasters affecting millions worldwide, particularly in tropical cities. The main aim of this study is to understand the effects of land-use and climate change on flooding in the Greater Port-Harcourt watershed. The specific research objectives were: to understand the historical and future land use /land cover changes; to understand the magnitude of change in hydrologic and hydraulic conditions due to land-use and climate changes; to assess the influence of different forest mitigation scenarios on peak-discharge; and to make recommendations on how to improve future planning using insights from this study. Methodologically, the post-classification change detection method was applied to examine the extent and nature of historical LULC changes using remotely sensed data. Future LULC changes were estimated by superimposing the 2060 digitised Masterplan map on the year 2003 baseline imagery. Hydrologic changes were assessed using HEC-HMS model, while changes in the hydraulic condition were assessed using HEC-RAS model. Model output was further used to map flood hazards, flood zones and damage potential. Priority areas and infrastructure at risk were identified by means of their location in flood zones and exposure to floods with high damage potential. On the extent of change, this study revealed that urbanisation and loss of agricultural land had been the dominant and intensive land use change in the watershed. Urbanisation is projected to almost double its 2003 extent by 2060 and is likely to remain the dominant force of land use change. On the nature of change, this study found that urban land was the most dynamic in terms of gross gain and net change. It exhibited the grossest gain (about 9% of the watershed) and the grossest loss leading to a high net change of about 8.6%. In fact, the most prominent transition was the conversion of agricultural land (about 422km2) to urban land, and roughly 93.3% of all conversions to urban land resulted from agricultural land. On the process of change, urban land mainly experienced a net-type of change (change in quantity), whereas changes in agricultural land was more of a swap-type of change (change in location). Importantly, the study reveals that the impact on flood flow was historically significant (about 68%) and is projected to amplify in future, however, these changes are largely attributed to increased storm size. Urbanisation is likely to have little or no impact on annual maximum peak flow at the watershed scale; however, urbanisation is projected to have a considerable impact on peak flow in a number of subbasins, which could have severe implications for flash flooding in those subbasins. Similarly, afforestation could have little or no impact on future maximum peak flow when assessed at the watershed scale. Although some subbasins experienced changes in peak flow, the effect of forest is variable. The study concludes that although the impact of urbanisation is projected to be insignificant at the watershed scale, it could also increase flood risk due to increasing developments in floodplains and channel encroachment. Priority infrastructure and areas requiring urgent flood risk management include the Port-Harcourt seaports, Onne seaport, the University of Science and Technology and cement factory. Priority areas in the Masterplan are mainly in the south (Phase 3), comprising of the Air force base and the residential area near Onne seaport. Lastly, approximately 8.1km and 189m of road and rail network are at greater risk of flooding by means of their exposure to floods with the highest damage potential. Based on this study, I have furthered understanding by showing that the transition to urban land category was dominated by net changes (i.e. changes in quantity). I have also furthered understanding by showing that substantial changes in future urban land-use may not have significant effect on flood parameters. My main contribution to knowledge is that despite the high rate of urbanisation in the GPH watershed and its minimal impact on flooding (which could be due the large size of the storm and watershed), urbanisation could still increase flood risk due to greater exposure of elements at risk in the flood plains to damaging floods. Based on the results, the study recommends that the development authorities should integrate both structural measures (mainly for flood defence around existing developments) and non-structural measures (primarily for future developments). For flood risk management research, this study recommends that conclusions about the effects of urbanisation should not be made solely on the basis of changes in hydrology and river hydraulics, however researchers should also consider the exposure of important elements at risk within the floodplains under study to better understand the effects of urbanisation. Moreover, to better understand urbanisation effects on runoff dynamics in other watersheds, this study recommends that research efforts should be concerted in understanding subbasin-scale changes given that the effects of urbanisation are more pronounced in smaller basins.