INVESTIGATION ON APPROPRIATE MODELLING FOR INTEGRATED FLOOD RISK IN IBADAN

CHAPTER ONE INTRODUCTION 1.1 BACKGROUND OF THE S TUDY Globally, flood has received a great deal of scholarly interest throughout the years, particularly in developing countries such as Nigeria. Although there are several types of environmental problems and issues that result, including soil degradation, greenhouse effect change, ozone depletion, drought, desertification and deforestation, industrial pollution, water contamination, and waste management. Howbeit floods are among the most prevalent and widespread. The conventional flood concept is the overflowing of a water body's riverbanks, which results in an uncontrolled spread of water that exceeds the water carrying capacity of the catchment region that surrounds the water body. This causes floods (Okorafor et al., 2015). Floods, like drought and desertification, are natural hazards that arise as a result of excessive hydrological (runoff) events, according to Danumah, Odai, and Saley (2016). According to Bello and Jeb (2016), flooding is the most common natural catastrophe, with the frequency of documented flood incidents growing dramatically over the previous 20 years. According to United Nations-Water (2011), there has been a dramatic increase in the number of persons killed or badly injured by flood catastrophes, which affect an estimated 520 million people globally each year, resulting in up to 25 000 deaths in a single year. Floods are widespread across the planet, according to a history of flood occurrences provided in NASA's Earth Observatory (EO), which was cited in Bello, & Jeb (2016), with the Northern Hemisphere and the Southern Hemisphere seeing the most extreme flooding. According to Bello and Jeb (2016), extreme rainfall in Brazil, Bolivia, and Peru during December 2006 to January 2007 caused major flooding and landslides in those nations. Significantly, the essence of flood attention in study is always linked to its origins, the consequences of severe property damage and even irreparable loss of life, and the control methods. Flood is one of the key problems preventing Africa's population from overcoming poverty, and flood victims are typically urban poor (Adetunji and Oyeleye, 2018). A flood occurs when a watercourse overflows its banks and submerges adjacent regions. To add to this, Egbinola, Olaniran, and Amanambu (2015) define flood as an overflow of a body of water that submerges land. Flood, as defined by the European Union (2007) and Adetunji et'al (2015), is a temporary covering of land by water that was not previously covered by water. Flooding, according to Nelson (2018), is a natural result of stream flow in a constantly changing environment. Flooding to Nelson represents abnormally high rates of discharge, frequently resulting in inundation of land near to streams, and is generally produced by severe or protracted rains. Surprisingly, many settlements in Nigeria's coastal region, particularly in the Niger-Delta zone and Ibadan, among other places, have weak drainage systems and are thus prone to floods. Flooding threatens an estimated 25 million people, or 28 percent of Nigeria's population, who live in the coastal zone (NCAR, 2012). Flooding poses a significant risk to riverside populations and floodplains, as well as significant impacts on the environment, including aquatic fauna and flora and bank erosion (Twumasi, Merem, Ayala-Silva 2017). Therefore, risk assessment is required in order to determine the likelihood of flood occurrence and its negative implications. According to Twumasi et al. (2017), many aspects such as vulnerability and exposures must be considered in the real evaluation of catastrophe scenarios and losses. Thus , flood risk is the result of flood dangers, susceptibility, and exposure of people, which necessitates an appropriate response in order to minimize or lessen the consequences of flood. Despite the Nigerian Meteorological Agency's monthly Seasonal Rainfall Prediction (SRP) that there would be irregular flooding in several sections of the country, floods have continued to destroy the bulk of the country. The country's response to the calamity has been far from wide and eloquent. All we've done is respond to the awful events by assisting the injured, and now we're waiting for the next rain. Nothing has been done to ensure that the hazard is avoided and its associated risk is reduced to a negligible level (Nelson 2018). The availability of flood information and knowledge of the regions likely to be impacted during a flooding event will be critical in reducing the threat of flooding. According to Komolafe, Awe Olorunfemi & Oguntunde(2020), inaccurate hydrometeorological data reduces the uncertainty associated with flash floods. As a result, it is critical to adopt mitigation measures as quickly as possible to protect these areas from flooding. This involves the use of existing methodologies to establish measurements that will assist governments and humanitarian organizations in identifying flood-prone areas, which will aid in future flood planning. 1.2 STATEMENT OF THE PROBLEM In recent years, urban flooding has been identified as one of the most pressing and serious environmental concerns confronting municipal authorities in developing countries. Floods are the leading cause of catastrophe losses in Nigeria, according to the EM-DAT database (Komolafe et al., 2015b), and with the projected rise in rainfall intensity due to climate change, several river basins in the country are expected to flood. Therefore, additional examination and research of floodplains in the country is required in order to identify susceptible locations for flood risk reduction programs (Komolafe et al., 2017). Antiquing, Ibadan and its surrounds are traversed by multiple streams and are drained by three major rivers: the Ona River in the north and west; the Ogbere River in the east; and the Ogunpa River, which flows through the city with its largest tributary, the Kudeti River, which drains the city's middle half. As one of its tributaries and the primary source of drainage in the state, the river runs through the state capital. According to recent estimates (NHSA, 2016), severe rainfalls are expected in all of Nigeria's major river basins, potentially resulting in floods. These imminent floods are projected to devastate several states, including the study region. As a result, information on probable river flood features such as geographic distribution, size, extent, flood depth, velocity at distinct occurrences, and exposure susceptibility is required in disaster preparedness decision making. Komolafe et al (2020) asserts that the usage of flood hydraulic property computation is required for floodplain appraisal and natural phenomena modeling. Importantly, several studies pertaining to the flood hazard problem have been done using remote sensing and GIS. Nevertheless, researches on the appropriate modelling for integrated flood risk like utilizing GIS and hydrologic modelling IN IBADAN have not been comprehensively done in the area. As observed by Chia et al. (2015), Hydrologic/hydraulic modelling of flood enables integration of basic hydrological and physical components such as rainfall, infiltration, channel flows, evaporation, nature of soil, roughness, hydraulics etc., which determines the propagation of flood waves. On the other hand, applying GIS techniques, flood visualization can be easily generated, which could be useful for flood mitigation and planning of the basin area (Komolafe et al., 2015a;). Hydrology Engineering Centre River Analysis System and its ARCGIS plug in models are very useful and have the capability for generating flood delineation and estimating hydraulic parameters. Numerous academics have explored the topic of floods, utilizing Remote Sensing and Geographic Information System in particular (GIS). Hula, & Udoh (2015) utilized Geographic Information System (GIS) to forecast urban floods in some regions of Nigeria. This was accomplished by the creation of a computerized database of chosen factors, including land use, land cover, and soil strength. For analysis, Arcview 3.1 was utilized, and the overlay technique in GIS was employed. The research revealed locations with high, moderate, and low flood risk. Okonkwo, & Onyeizugbe (2017) also utilized remote sensing and GIS methods to build digital topography maps and flood vulnerability maps of Gwagwalada in Abuja, illustrating the regions that were very susceptible, vulnerable, less vulnerable, and flood-free. According to the findings of their study, the areas near the riverbanks of the Usuma are the most susceptible to flooding, while the risk decreases toward the northern section of the city. Although these researchers have helped provide some information regarding flood inundation, encroachment, and flood-prone areas in Nigeria, they have not exhaustively examined flood threats in all flood-prone areas in Nigeria. Moreover, relatively few techniques have concentrated on local floods in tiny urban watersheds and the rainfall runoff created there, particularly in metropolitan regions in Ibadan devoid of big rivers. Upon this backdrop, this study seeks to appropriate modelling for integrated flood risk in Ibadan. 1.3 Aim The broad focus of this study is hinged on appropriate modelling for integrated flood risk in Ibadan. 1.4 Objectives In other to achieve the aim of this research, the study was spitted into specific objectives stated thus: 1. Use geospatial technologies to generate flood hazard model in Ibadan city. 2. Identify vulnerable areas where flood hazards occur in Ibadan city. 3. Recommend ways to curb flood occurrences in Ibadan city. 1.5 Justification of the study Flood events in many capital cities in Nigeria, are mostly due to the poor consciousness of the inhabitants on environmental information, inadequate (or sometimes absolute lack) of spatial information on the flood prone areas, waste dump and construction of buildings (both commercial and residential, even public offices) on river channel without adequate measure for water flow. The study therefore attempted to produce a flood hazard model of Ibadan town with a view to identifying hotspots and providing measures to forestall flood occurrences. To this end, the study assessed land use land cover changes in the study area, examined the socio-economic and environmental impacts of the hazards on the study area, generated flood risk and vulnerability maps as well as flood hazard models of the area using the factor criteria such as soil, elevation, rainfall and land use; slope and drainage density of the basin and finally modelled river hazards by producing a river model of the main drainage channel in the area in order to predict its tendency to flood in the future. The output of this study can serve as policy and decision-making tools for future food hazard analysis, prevention and reduction plans at the research location.The study will contribute to general body of knowledge on integration modelling for flood risk locations and serve as reference material for scholars who desire conduct further research on this topic.