1.1 Background to the Study
The acronym GIS stands for geographic information system, it is a system which allows a capture, updating and displaying of a number of previously unconnected dataset bringing them into a common reference system for spatial analysis from which relationship can be identified and decisions made (Sowton 1991).
Floods are probably the most recurring, widespread, disastrous and frequent natural hazards of the world. India is one of the worst flood-affected countries, being second in the world after Bangladesh and accounts for one fifth of global death count due to floods. Nigeria is not an exception and indeed the study area as series of flooding have affected inhabitants and disrupted socioeconomic activities. Flood management and control are necessary not only because floods impose a curse on the society, but the optimal exploitation of the land and proper management and control of water resources are of vital importance for bringing prosperity in the predominantly agricultural based economy of this diversely populated country. This cannot become technically feasible without effective flood hazard maps (Pradan, 2004)
Flood hazard mapping and flood inundation modeling are the vital components in flood mitigation measures and land use planning. Advances in geospatial technologies (GPS, Remote sensing and GIS) has enabled the acquisition of data and analysis of the river basin for flood mitigation projects (either structural or non-structural) in a faster and more accurate manner. GIS facilitates integration of spatial and non-spatial geographical data such as rainfall and stream flows. Other information such as flood maps, infrastructures and land use, social and economic information can be inventorized for future use. Flood maps prepared using satellite images of real flood events and information from the ground are useful for flood damage assessment, future flood mitigation planning. Flood vulnerability maps produced using GIS enabled effective tools for land use planning, flood management and development control. Therefore, an attempt has been made to apply modern techniques like Geographical Information System (GIS) for the management of flood in Port Harcourt, the study area, as it has been proved that the use of Geoinformatics to manage the databases, maps, and photos improves the ease of the analysis (Wisner et al., 2004; Noson, 2002).
Floods are among the most destructive acts of nature. World-wide, flood damages to agriculture, houses and public utilities amount to billions of dollars each year in addition to the loss of precious human and cattle lives (R.K.Khanna, 1989). In majority of cases, ‘flooding’ is caused by a river over-spilling its banks. This can be due to excessive precipitation, combined with inadequate channel capacity. Over-spilling can also occur due to obstruction in or aggradation of the river bed. Flooding can also result from inadequate water way at rail and road crossings, or when there are encroachments in the flood plain. Flooding can also occur at confluences of streams when the main river is in high stage and backs up into the tributaries and areas there about.
Flood risk is among the most severe risks on human lives and properties, and has become more frequent and severe along with local economic development. As the watershed becomes more developed, it also becomes more hydrological active, changing the flood volume, runoff components as well as the origin of stream flow. In turn, floods that once occurred infrequently during predevelopment periods have now become more frequent and more severe due to the transformation of the watershed from rural to urban land uses. The forecast and simulation of floods is therefore essential for planning and operation of civil protection measures and for early flood warning. The effects of Global warming has also accelerated the melting of ice, which in turn increased the water level and resulting in the occurrence of flooding. Flooding causes a lot of damage, which is usually, characterize by loss of properties and even leaving families homeless.
The 2012 Nigeria floods began in early July 2012, and have killed 363 people and displaced over 2,100,000 people as of 5th November, 2012. From May to September, Nigeria has a rainy season and suffers from seasonal flash floods. These flash foods are sometimes lethal, especially in the rural areas or overcrowded slums, where drainage is poor or does not exist at all. On 2 July 2012, many Nigerian coastal and inland cities experienced heavy rains, and residents of Lagos were "gasping for breath" due to the flooding. In addition, there was a gridlock on major roads, causing people to cancel or postpone appointments they may have had. Thousands of stranded commuters had to pay increased fares for the few bus drivers who were willing to risk travelling on the roads, and construction of work by the Nigerian government on the inner Oke-Afa Road took a "heavy toll."
In mid-July 2012, flooding in the Ibadan metropolis caused some residents at Challenge, Oke-Ayo, and Eleyele to flee from their residences and save their lives. The flooding also prevented some Christians from attending churches in the morning, while a few bridges caved in. In late July 2012, at least 39 people were killed due to flooding in the central Nigerian Plateau state. Heavy rainfall caused the Lamingo dam to overflow near Jos, sweeping across a number of neighborhoods in Jos, and approximately 200 homes were submerged or destroyed. In addition, at least 35 people were missing, while ManasiePhampe, the head of the Red Cross in the state, announced that relief efforts were ongoing. The floods left 3,000 people homeless, many of whom are taking refugee in government buildings in Jos. In mid-August, flooding killed at least 33 people in central Nigeria's Plateau state. Over 12,000 people were affected by the flooding in six districts of the state, while hundreds were rendered homeless (Wikipedia).
Port Harcourt was not an exception, as a heavy rainfall weekend wreaked havoc in Port Harcourt, Rivers State, destroying property and rendering families homeless (The Vanguard Nigeria). The perennial flooding has caused a lot of damage and untold hardship to the inhabitants of the study area. The study therefore seeks to assess the vulnerability of the study area to flooding using Geoinformatics techniques.
1.3 Aim and Objectives
The aim of this research is to employ Geoinformatics(Geographical Information System (GIS), remote sensing and cartographic) techniques to generate flood vulnerability map showing buildings and roads that are susceptible to flooding in the study area of Port Harcourt at different buffer distances.
The following specific objectives were pursued in order to achieve the above aim:
1.4 Significance of the Study
The recent flooding that occurred in various part of Nigeria makes this research expedient, to allow for action to prevent such disaster from occurring. The significance of this research work cannot be overemphasized as it has tremendous importance and positve impact on almost all sectors of the economy, government departments and the society at large. These significance include;
1.5 Scope of the Study
The scope of the study is limited to Port Harcourt which is the study area and its immediate environment. The study only took into consideration buildings that are vulnerable to flooding at selected buffer distances of 20 meters, 30 meters and 50 meters; and roads that are susceptible to flooding at selected buffer distances of 60 meters, 80 meters and 100 meters.
1.6 Study Area
1.6.1 Location
Port Harcourt (Igbo: Diobu, Iguocha or Ugwuocha; Pidgin: "Po-ta-kot") is the capital of Rivers State, Nigeria. It is situated in ObioAkp, Rivers, Nigeria, its geographical coordinates are Latitude 4° 47' 21" North, and Longitude 6° 59' 55" East and its original name (with diacritics) is Port Harcourt. It lies along the Bonny River and is located in the Niger Delta. According to the 2006 Nigerian census Port Harcourt has a population of 1,382,592.
1.6.2 Climate
Port Harcourt features a tropical monsoon climate with lengthy and heavy rainy seasons and very short dry seasons. Only the months of December and January truly qualifies as dry season months in the city. The harmattan, which climatically influences many cities in West Africa, is less pronounced in Port Harcourt. Port Harcourt's heaviest precipitation occurs during September with an average of 370 mm of rain. December on average is the driest month of the year, with an average rainfall of 20 mm. Temperatures throughout the year in the city are relatively constant, showing little variation throughout the course of the year. Average temperatures are typically between 25°C-28°C in the city.
1.6.3 Geology/ Soil
Port Harcourt lies on the recent coastal plain of the eastern Niger Delta. Its surface geology consists of fluvial sediments. This includes the recent sediments transported by Niger River distributaries and other rivers, such as Andoni, Bonny and New Calabar. These materials deposited as regolith overburden of 30m thickness are clays, peat, silts, sands and gravels. The depositional sequence exhibits massive continental sand stones overlying an alternation of sandstones and clays of marginally marine origin, but eventually grading downwards into marine clays. Sands, by far, form the largest group of rock types in Rivers State, while mud constitutes all the polluted brackish waters of the riverine areas. However, peat constitutes the various vegetal and animal remains that lie in bogs and shallow pits. The gravel and pebbles form the last unit of the subsurface rock type, and are usually found at the base of the river channels. There are three major soil groups in Rivers State, namely: the marine and fluvial marine sediments; the mangrove swamp alluvial soils; and freshwater brown loams and sandy loams. The marine and fluvial marine sediments are found in the wet coastal region. The soils are organic in nature and essentially sandy in texture. Some consist of mud mixed with decayed organic matter. The mangrove swamp alluvial soils are found in the northern part of the coastal sediments zone. They are brownish on the surface, sometimes with an unpleasant and offensive odour. The soils of the swamps are rich in organic matter in the top layer, but contain too much salt especially in the dry season. The third soil group, the brown loams and sandy loams are found in the fresh water zone of the delta. The levees which form the common land forms of this zone are made up of rich loams at their crests, changing to more acidic and more clayed soils along their slopes.
1.6.4 Vegetation
The "upland" area was originally occupied by rainforest which has been drastically modified by human activities. In most places, economic trees, particularly oil palm, have been preserved and thus the sobriquet for this vegetation as "oil palm bush." The riverine area is divisible into three main hydro-vegetation zones namely, the beach ridge zone, the saltwater zone and the freshwater zone. The beach ridge zone is vegetated mainly by fresh water swamp trees, palms and shrubs on the sandy ridges and mangroves in the intervening valleys or tidal flats. The saltwater zone is the tidal flat or swamps vegetated by the red stilts rooted mangrove (Flhizophoraracemosa) and two other species of mangrove. The outliers of raised alluvial ground or coastal plain terrace within the swamps are vegetated by tall forest tree species and oil palm. The freshwater zone is mainly the Upper and Lower Delta floodplains of the Niger, having fresh water forest trees which are the edaphic variants of the rainforest. The Abura tree, oil palm, raffia palm, shrubs, lianas, ferns and floating grasses and reeds are the typical vegetation.
1.6.5 Hydrology
Rainfall in Port Harcourt is seasonal, variable, and heavy. Port Harcourt is a city of physical difficulties, such as low lying terrain riddled with an intricate system of natural water channels; too much surface water and a high rainfall; uninhabitable mangrove swamps and some parts of the state suffer from inaccessibility due to seasonal flooding.
1.6.6 Socio-economics activities
Port Harcourt is the heart of Nigeria’s Oil industry with virtually all major multi-national oil companies being represented there until recently owing to security threats from Niger Delta militants and Cultists. The economic activities of Port Harcourt include manufacturing such as food processing, car assembly, manufacture of paper products, paints, petroleum products refinery and road construction, metal works and cement making, enamelware, bicycles, furniture, and soap making. Services include legal services, hospitality, medical, Educational and engineering services. Extractive industries also exist such as mining of coal, tin and petroleum, oil and gas liquefaction. Also agricultural and agro based businesses exist in the city some of which are logging and timber processing, tobacco processing and cigarette making, plastic moulding and the manufacture of rubber based products like tyres, bands and tube and glass making. Imports are mainly automobiles, electronics, textiles and processed food. Sometimes rice, millet and meat and other agricultural produce are imported as well. Then numerous small scale enterprises like consumer retailing, artisanship and transportation businesses also thrive in the city. Various government organs such as the Nigerian Ports Authority, NNPC and the customs play various regulatory roles in the local economy. The main industrial area of the city is Trans Amadi. Residential areas include Port Harcourt Township known locally as "Town", G.R.A phases 1-5, Abuloma, Amadi-ama, Amadi Flats, and Borokiri.
Crime rate in Port Harcourt Nigeria is one of the highest. The worst cases so far have been crimes committed by criminal gangs disguising their illegality as freedom struggles for the marginalized locals of the oil rich Niger Delta Region. Other than these and congestion, Port Harcourt Nigeria is largely a peaceful and sociable city. With many recreational and fun spots, Port Harcourt can boast of a number of 5 star hotels such as the Meridien Hotel, Presidential Hotel among others. Also newly open entertainment centres with cinemas, night clubs, bars and restaurants can be found in the city.
1.7 Limitations of the Study
This research work was not without some limitations. The major limitations of this study was that the study area was very wide and so difficult to get all the attributes for roads and buildings with available map not providing all the names of the roads and rivers; and the usage of buildings.
These limitations however were overcame by proper digitization of the satellite imagery, giving code names to roads and rivers were the names were not known and generalizing buildings.
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