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1.1 BACKGROUND TO THE STUDY
Flooding is a general temporal condition of partial or complete inundation of normally dry areas from overflow of inland or tidal waters or from unusual and rapid accumulation of runoff (Jeb and Aggarwal, 2008). Floods are the most common natural disasters that affect societies around the world. Dilley et al., (2005) estimated that more than one-third of the world‟s land area is flood prone affecting some 82 percent of the world‟s population. The reason lies in the widespread geographical distribution of river flood plains and low-lying coasts, together with their long standing attraction for human settlement. Floods are natural phenomena, but they become a cause for serious concern when they exceed the coping capacities of affected communities, destroying lives and damaging property. They affect settlement of all types, from small villages and mid-sized market towns and service canters to major cities and metropolitan. In many regions of the world, people moving from rural areas to cities, or within cities, often settle in areas that are highly exposed to flooding thereby making them highly vulnerable if there is no flood defence mechanism (Jha et al., 2012).
Urban flooding can be coastal, fluvial or pluvial or even a combination of these types of floods. Coastal flooding is caused by extreme tidal conditions that occur because of high tide levels, storm surge and wave action. Fluvial (River related) flood occurs when the discharge of a river exceeds the capacity of the river channel to contain it. While pluvial flood takes place when the rainfall rate exceeds the capacity of storm water drains to evacuate the water and the capacity of the ground to absorb water (Ball et al., 2008). Pluvial flooding often occurs unexpectedly in locations not obviously prone to flooding and with minimal warning and is not well understood by the general public – hence the term „invisible hazard‟ (Houston et al., 2011). Pluvial flooding is a
characteristic of urban areas where large areas of impervious ground exist and inadequate drainage systems abound. As urban growth increases, the impervious surface area also increases; thereby rendering populations vulnerable to water inundation as natural streams and human-made drainage fails to cope with increased runoff subsequent to heavy rainfall (Youssef and Pradhan, 2011).
Urban pluvial flooding frequency is expected to increase not only due to urbanization but also to expected climate changes (Ugarelli et al., 2011; Simes et al., 2014). This type of flooding can happen virtually anywhere and has the potential to cause significant damage and disruption in highly urbanized areas, where the density of properties, critical infrastructure and population is usually high. The volumes involved and the risk related to pluvial flooding often result in consistent economic losses and consequent damage in the long term due to the high frequency of this kind of event (Freni et al.,2010). Related consequences of pluvial flooding mainly consist of economic losses such as damage to buildings and their contents and to infrastructure and intangible damages due to traffic delays, road, public and commercial function closures, and evacuation of people. Globally, the economic cost of extreme weather events and flood catastrophes is severe, and if it rises owing to climate change, it will hit poorest nations the hardest consequently; the poorest section of people will bear the brunt of it. It is therefore, urgent that the vulnerability of developing countries to climate change is reduced and their capacity to adapt increased at national, regional and community levels (UNFCC, 2007).
Excess water in itself is not a problem rather, the impacts of flooding are felt when this water interacts with natural and human-made environments in a negative sense, causing damage, death and destruction. The thing that makes natural floods a disaster is when flood waters occur in areas populated by humans and in areas of
significant human development. Otherwise, when left in its natural state, the benefits of floods outweigh the adverse effects (Bradshaw et al., 2007). Although generally, flooding is a bane to most people, floods can be quite beneficial. Actually, nature benefits more from natural floods than from not having them at all. The experience of flooding for a rural agriculturalist and an urban slum dweller will be very different: while to the farmer the flood is a natural force to be perhaps harnessed or endured for the long term benefits it may bring, for the urban dweller flooding is at best a nuisance and at worst a disaster which destroys everything she or he owns (Jha et al., 2012).
Floods regularly account for nearly one-third of all global disasters arising from geophysical hazards (Smith and Ward, 1998). They now appear to be more prevalent and destructive than centuries ago and are projected to increase both in frequency and amount of devastation in the future (Parker, 2000). Moreover, more people are now living in flood prone areas. Despite efforts in many countries to restrict development in floodplains there is substantial evidence that exposure to floods is growing rapidly as human occupation of floodplains intensifies in many parts of the world (Jha et al., 2012). According to UN-Water (2011) floods, including urban flood is seen to have caused about half of disasters worldwide and 84% disaster deaths in the world was attributed to flooding. They are some of the most frequent and costly natural disasters in terms of human suffering and economic loss in the United States and world-wide (Mason 1995; Smith and Ward, 1998; Parker, 2000). Death and destructions due to flooding continues to be all too common phenomena throughout the world today, affecting millions of people annually. Flooding is one of the major natural disasters which disrupt the prosperity, safety and amenity of the residents of human settlements (Jha et al., 2012).
Flooding is a global phenomenon which causes widespread devastation, economic damages and loss of human lives (ibid). Records of the devastating impact of flood around the world abound. In May 2008, floods triggered by torrential rains killed dozens of people across China, while thousands of others were victims of landslides caused by the downpours. Elsewhere in the United States of America, the Mississippi River caused damages put at several millions of dollars when it over flew its banks, flooding some cities, towns, farmlands and major industrial installations over a distance of about 250km and ravaging Iowa before it heaped downstream (Aderologba,2012). Several flooding events have occurred in Europe over the last decades with heavy rainfall as the main cause of flood. ). In several parts of the Netherlands, intense rainfall in autumn of 1998 caused damage to 2470 houses, 1220 premises and 350 governmental agencies (Jak and Kok, 2000). Similarly, in the summer of 2007 the City of Hull (United Kingdom) suffered from severe pluvial flooding, causing damage to over 8600 houses and 1300 premises (Coulthard and Frostick, 2010). African nations too have had their own share of flood disasters. For example in West and Central Africa more than 1.5 million people were affected by floods between the months of July and August 2012 following heavy rainstorm in which more than 11,400 homes were destroyed in Senegal; more than 94,000 in Chad and 24,000 homes in Niger (UN-OCHA,2012).
In Nigeria, the pattern is similar with the rest of world. Flooding displaces more people than any other disaster, perhaps because about 20 per cent of the Nigerian population is at risk of flooding (Etuonovbe, 2011). Flooding is therefore a perennial problem in Nigeria that consistently causes deaths and displacement of communities. For instance, in 2010, about 1,555 people were killed and 258,000 more were displaced
by flooding (Babatunde 2011).Similarly in 2012, floods claimed 361 lives, and displaced 2.1 million people( Tokunbo and Ezigbo,2012).
Mararaba Gurku sub-urban area of Nasarawa state adjoins the Federal Capital Territory of Nigeria. It has recorded series of flooding events in recent times. On the 6th of July 2012, heavy rainstorm accompanied by torrential flood claimed two lives in the White house area and submerged many residential houses and shops beside Kabayi bridge in Mararaba Gurku, Nasarawa state (Itodo, 2012). Similarly, heavy rainfall on the 14th of July 2012 resulted in flooding of street and destruction of properties in Mararaba Gurku neighborhood as seen in plate 1.1 and 1.2 (appendix I and II). Another incident occurred on the 27th of September, 2012 when an overnight downpour that lasted for three hours flooded and submerged 50 houses and seven cars in Mararaba Gurku (Ume, 2012). Over the years serious floodings have occurred much more frequently in the last twenty-five years in Nigeria (Gobo and Abam, 1991) and there is no reason to believe that it will not continue to be a problem. There is therefore the need to be prepared to respond to dangers of floods as they are happening and to protect the public health and safety during these emergencies. The focus of this study therefore, is on rainfall-induced flooding; causes, hazards and approaches to mitigate flooding in Mararaba Gurku sub-urban area.
1.2 STATEMENT OF THE RESEARCH PROBLEM
According to Smith and Ward (1998), most floods in the humid tropics result directly or indirectly from climatological events, either extremely heavy or prolonged rainfall. This in recent times happens every year in Mararaba sub-urban area of Nasarawa state, Nigeria. Urban flooding is considered as one of the most immediate and serious environmental problem confronting municipal authorities in developing countries. It is indeed a critical environmental problem or major hazard that is
continuously affecting effective functioning of urban environment, especially in the areas of sustained infrastructure and services, which are germane to sustainable livelihood. When severe floods occur in areas occupied by humans, they create natural disasters which involve the loss of human life and property plus serious disruption to the activities of large urban and rural communities (Smith and Ward, 1998). The economy can also be severely affected by flooding as businesses may lose stock, patronage, productivity and disruption to utilities; and transport infrastructure can be adversely affected on a wider area. Adeosun (2012) reported that the Federal Ministry of Environment stated that the negative effect of climate change, which manifested in a wave of flooding across the country in recent times, is capable of derailing the process of the actualization of the Millennium Development Goals (MDG).This indeed is a big threat to the progress already made in eradicating extreme poverty and disease. The damage caused by urban flood is on the rise because of the devastating effects of floods, it is important that we consistently study flood characteristic and impact, so that appropriate disaster risk reduction strategies can be put in place to reduce the impact of floods.
Over the years, despite annual Seasonal Rainfall Prediction (SRP) by the Nigerian Meteorological Agency that there would be irregular flooding in many parts of the country, most part of the country has continued to suffer from the devastating effect of floods. The nation's response to the flood has been anything but articulate and comprehensive. All we have done is to react after the disastrous events to provide relief to the unfortunate victims, and then we wait for another deluge. Nothing has been done to ensure that the hazard is prevented and its associated risk is reduced to the barest minimum (Jeb and Aggrawal, 2008; Orok, 2011). Reduction of risk of flooding will depend largely on the amount of information on floods that is available and knowledge
of the areas that are likely to be affected during a flooding event. The lack of accurate hydro meteorological data affects the uncertainties associated with flash flooding events. There is therefore an urgent need to introduce mitigation measures to ensure that these areas are protected so that flooding is minimized. This calls for the use of modern day techniques to identify measures that will help government and relief agencies in the identification of flood prone areas; which in turn will help in planning against flooding events in the future.
Several researchers have examined the issue of flooding particularly with the utilization of Remote Sensing and Geographic Information System (GIS). Okoduwa (1999) applied Geographic Information System (GIS) in the prediction of urban flooding in Benin City, Nigeria. This was achieved by creating a digital database of selected variables such as land use, land cover and soil strength. The software used was Arcview 3.1 and the overlay technique in GIS was used for analysis. The result of the analysis showed high, medium and low flood prone areas. Ishaya, et al., (2009) exploited the use of remote sensing and GIS tools to created digital terrain maps and flood vulnerability maps of Gwagwalada in Abuja showing the areas that were highly vulnerable, vulnerable, less vulnerable and free from flood hazard. The results obtained from their study showed that areas lying along the banks of River Usuma are most vulnerable to flood hazards with the vulnerability decreasing towards the northern part of the town. Jeb and Aggarwal (2008) in their studies aimed at flood inundation hazard modeling of the River Kaduna used Gumbel‟s Extreme value distribution statistical method of analyzing flood data and GIS to estimate the extent of flood inundation for different return period. The result of their study showed mapped areas (Ungwan Guza, Kawo new extwnsion, Ungwan Dosa, Badarawa, Malali e.t.c) along River Kaduna that fall under areas prone to threat of severe flood for different return periods. Similar
studies by Ndabula et al., (2012) focused on urban flood plain encroachment of Kaduna metropolis used GIS operation to digitizing the topo map of the study area in polygon shape file and overlaying it with the digital elevation map (DEM) of their study area to delineate flood plain boundary. The result of their study showed that the highest extent and rate of encroachment was recorded by communities in the proximity of the Central business district (CBD) such as T/Wada, Ungwan. Rimi, Barnawa etc.
Although these researchers have assisted in providing some information about floods inundation, encroachment and flood prone areas in Nigeria; they have not fully assessed flood hazards in all the flood prone zones in Nigeria. More so, very few methodologies have focused on local floods in small urban watersheds and the rainfall runoff generated there off, especially in urban areas where there are no major river. Some reasons behind this fact are the complexity of the flooding processes in urban areas and lack of advanced technological methods for capturing geographical data (Ishaya et al., 2008; Ishaya and Ifatimehin, 2008; Ifatimehin et al., 2009; Freni et al., 2010; Orok, 2011). Furthermore, flood studies have only been carried out in areas where appropriate data for research can easily be acquired. There is still much left to be done with regards to studies on flooding in unguaged sub-urban areas like Mararaba Gurku. This research is one of the few attempts to address issues of floods in sub-urban environment, using the GIS tool to analyze the interl-inkages of various factors that are contributing to persistent floods in Mararaba sub-urban area. This study seeks to answer the following specific questions that relate to flooding in Mararaba Gurku sub-urban area which are:
i. What is the pattern of land use land cover of the study area?
ii. What is the relationship between rainfall and runoff in the area?
iii. Which areas of the sub-urban environment are prone to flooding?
iv. What type of approach can be adopted to reduce the risk of floods in the study area?
1.3 AIM AND OBJECTIVES OF THE STUDY
The aim of this study is to assess the 2012 flooding in Mararaba sub-urban area and also identify parts of the study areas that are prone to flooding using GIS based tool. Specific objectives are to:
i. derive land use and land cover (LULC) information of the study area
ii. determine the rainfall-runoff relationships of the study area
iii. identify flood prone areas using remote sensing and GIS techniques
iv. determine a strategic approach for flood risk reduction in the study area
1.4 JUSTIFICATION OF THE STUDY
Urban flooding poses a serious challenge to development and the lives of people, particularly the residents of the rapidly expanding towns and cities in developing countries. Mararaba sub-urban area of Karu local government area is no exception. As population and land values increase, the effect of uncontrolled runoff becomes an economic burden and poses a serious threat to health and well-being of citizens (Bari and Hasan, 2001). Due to serious flooding and it attendant problem in some areas of Mararaba sub urban area, flood control is a vital issue. More so the lack of or scarcity of reliable recorded hydro - meteorological data is another serious problem, which planners and researchers face for the analysis of the hydrology of urban watersheds. Thus , the use of new tools , like remote sensing and GIS , to generate supporting land based data for flood control and water resources in watershed planning is invaluable.
A detailed understanding of the flood hazard relevant to different localities is crucial for implementing appropriate flood risk reduction measures such as
development planning, forecasting, and early warning systems (Jha et al, 2012). Thus it is envisaged that the findings of this study will be beneficial to urban and infrastructure planners, risk managers and disaster response or emergency services personnel during extreme and intense rainfall events. Furthermore, public and private businesses, private house owners, housing corporations can decide whether it is worth taking out a flood insurance policy or spending additional money on private flood protection measures. The conclusions of this study can provide additional information to the existing knowledge in the field of flood assessment.
1.5 LIMITATION OF THE STUDY
The limitations of the research presented in this study are largely related to the existing problems of data availability in hydrological models and the computational difficulties of SCS Curve Number (CN) Model. There was insufficient rainfall information as the nearest rainfall station is at Abuja relative to that at Lafia the state capital which is at a greater distance from the study area. Since there was no known rain gauge station in the study area as at the time that this study was undertaken, interpolated rainfall record from Abuja meteorological station was adopted for this study. As such the result of the rainfall runoff and flood hazard prone area may not be taken as definitive of the actual situation of the study area.
SCS (CN) model is simple and easy to use model that requires a limited amount of data. However, there are some limitations (NRCS, 1999):
(a) Time independence, for a constant CN, a given amount of rainfall produces a set amount of runoff regardless of the rate of rainfall or how fast the soil can infiltrate water. The lumped system does not identify what part of loss is infiltrated. More so, the equation does not contain an expression for time and, therefore, does not account for rainfall duration or intensity.
(b) Fixed initial abstraction: For a constant CN, the soil moisture or soil properties are not used. While these may be a factor in initially selecting CN values, they are not used in the runoff computation. Thus the accuracy of runoff estimates is reduced for small amount amounts (0.5inc) of rainfall.
(c) One of the challenges for hydrologist is to perform analysis for unguaged watersheds because model result from unguaged watershed may have error when basin characteristics such as geography, land use and soil types are significantly different. Thus the need for SCS (CN) parameter calibration for direct runoff estimation.
1.6 SCOPE OF THE STUDY
This study is focused on the assessment of 2012 flood hazard mapping and risk management in Mararaba Gurku sub-urban area in Karu L.G.A of Nasarawa State. The present study is confined primarily to the hydrologic aspects of surface runoff and does not include the hydraulics of channel flow and damage assessment of flooding which are extensive subject on their own merit beyond the scope of this study. The study is however limited to Pluvial flooding (surface runoff) estimation and flood hazard in the study area. Remote Sensing and GIS would be used as a tool to derive the land use/land cover of the study area, determine the rainfall runoff and identify flood prone areas within the study area.
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