Estimating Flash Flood Severity And Strategies For Risk Mitigation In Owerri Urban

ABSTRACT

This study involved the estimation of flash flood severity and the formulation of strategies that would help combat flash flooding in Owerri urban, Imo state, Nigeria. As one of the world’s most deadly natural catastrophies, taking more lives and incurring more property damage than most experts could have predicted, it deserved a study. The study employed the use of a new variable known as flashiness to try to extrapolate the potential of an event or basin to produce a rapid and significant response to heavy rainfall. The study made use of rainfall data from NIMET, basin area        data for Owerri and other sample points, and the mean flooding rise time for Owerri. The results show that at 75% flashiness threshold, there were 15 time stamps out of the 216 on record from the years 2000 to 2018 during which the potential for a flashy rapid and significant response to heavy rainfall would have occurred. The results also reveal that as has been obsevered in the modelled study, the potential for flashiness significantly shot up when the basin area was reduced everytime. The study therefore recommends that the right amount of gauges be placed all across Owerri to ensure better analysis and estimation of flash floods and also the design of drainage systems within the city.

 Flash floods are one of the world’s most deadly natural catastrophes, taking more lives and incurring more property damage than anybody could have predicted. Floods affect and displace more people in Nigeria than any other catastrophe, despite the fact that they do not claim as many lives. The loss of lives is also accompanied by property damage, which is increased at a geometric rate. This paper thesis emphasises on estimating the flash flood severity of Owerri urban of Imo state, Nigeria, and prescribing ways of mitigating against the unflinching risks of flash flooding before they wreak any more havoc. The approach in this study also attempts to apply the principle of remote sensing and GIS in this environmental issue in a developing countries

CHAPTER 1

1.0    INTRODUCTION

1.1     Background to the study

News of natural disasters occurring around the world is almost a yearly, if not daily feature on every news platform. FAO (2015) suggests that “these natural disasters leave people dead, houses submerged, outbreaks of fire, overrun mines, crops wasted, infrastructure abandoned, migrant crisis issues and a whole lot of other series of after effects, and that between 2003 and 2013, disasters caused by natural hazards caused $1.5 trillion in damages worldwide. In developing countries alone, estimated damages from these disasters amounted to about $550 billion and affected 2 billion people. Such disasters undermine national economic growth and development goals, as well as the growth and sustainable development of the agriculture sector. Despite this, there is as of yet no clear understanding of the extent and nature of the impact of disasters.” The WHO (1971) describes a natural disaster as “an act of nature of such magnitude that it creates a catastrophic situation in which the day–to-day patterns of life are suddenly disrupted and people are plunged into helplessness and suffering, and, as a result, need food, clothing, shelter, medical and nursing care and other necessities of life, and protection against unfavourable environmental factors and conditions.” Examples of natural disasters are tsunamis, floods, earthquakes, volcanic eruptions, typhoons, hurricanes etc.

Natural disasters will never end. Natural disasters on earth will continue for as long as the earth lives. Of even more humanistic concern is the very possibility that natural disasters may be the cause of the extinction of all life on earth. As much as both concerns continue to run, it is our duty as humans to try to understand these disasters so we can think out a way to beat, control and ultimately get mankind out of harm’s way.

Floods have been described in many ways, the most notable definition is the EU’s which described floods as “as temporary covering by water of land not normally covered by water.” Aderogba (2012) described it as “the natural response of a river or stream or mere drainage valley/channel that has too much water to cope with.” Odufuwa et al., (2012) described a flood as “the most reoccurring, widespread, disastrous and frequent natural disaster of the world.” Odufuwa et al., further opines that “when these floods occur, the resultant effect can be debilitating for those who have to bear the brunt of destroyed houses, sources of living etc., this means that many have to relocate due to the inundating nature of these floods which makes it difficult for them to reside in their primary location for the while.” Floods can occur everywhere and that is what makes it dangerous. This takes its toll on properties, the vulnerable and the underprivileged (Doswell C.A, 2003, p.1).

Examples of floods are replete; on the 1^st of June 2016, vast quantities of water swept through the Bavarian town of Simbach. Following a thunderstorm and torrential rain lasting several hours, parts of Rottal district in Southern Germany were completely inundated. On February 25, 2019, the Death Valley national park, one of the hottest and driest locations in North America, experienced a once-in-a-lifetime event when an intense storm created a temporary 10-mile long lake in the park. Another suitable example is the flood that ravaged Orsu Obodo community in Oguta council area of Imo state in august 2019, in which no fewer than 2000 people including women and children were displaced following the incident.

Coastal flooding occurs along the coast in the low-lying mangrove and freshwater swamps. River flooding occurs in the flood basins of the major rivers. Flash floods are linked with rivers in interior locations, where heavy rainfall may transform them into devastating torrents in a matter of minutes. Extensive urban flooding is a phenomenon of every rainy session in Lagos, Maiduguri, Aba, Owerri, Warri, Benin, and Ibadan, especially in towns located on flat or low lying terrain where little or no provision has been made for surface drainage, or where existing drainage has been blocked with municipal waste, refuse, and eroded soil sediments.

Almost every Nigerian is vulnerable to natural or man-made calamities. Wind gusts resulting from tropical storms claim lives and damage worth millions of naira every rainy season. Thousands of hectares of farmland have been washed away by flash floods caused by heavy rainfall. Following a flood, dams frequently break. For example, in August 1988, 142 people died, 18,000 homes were damaged, and 14,000 crops were wiped off. Following a heavy flood, the Baguada Dam fell. Flooding in cities, such as the Ogunpa tragedy in Ibadan, which killed over 200 people and destroyed property worth millions of naira, is a frequent occurrence. Floods have halted business activity in a number of towns and cities around the country. Major highways, including several that connect states, are inundated, causing traffic congestion. The flooding difficulties were not present when these roads were built, and the road construction company most likely did not anticipate the problem.

Research into the causes of flooding in Nigeria done by Nwigwe and Emberga (2014) where more than 240 urban dwellers were interviewed revealed that “illegal structures on/across drainage channels, canals and more erosion passages (100%), inadequate drainage channels (100.00%) are the major substantive conservative causes of the floods. Others are land seckmation/encroachment (82.08%) collapsed dams/embankments and bridges (95.42%) construction and reconstruction (92.25%), poor physical planning (92.25%), and nature of terrain (96.24%) poor waste management (100.00%) is also a common challenge. Ocean/lagoon surge is picked by 75.58% of the respondents. The numbers reveal how often respondents chose a particular option.”

Flash floods on the other hand are among the most dangerous of all natural hazards. Flash floods, a subset of floods, are a particularly damaging natural hazard worldwide because of their multidisciplinary nature, difficulty in predicting and fast onset that limits emergency responses (Manabendra saharia et al., 2017). Since the structural preventive measures have only a very limited effect, there is a very little range of choices in tackling flash floods. Risk mitigation involves reducing the probability or the negative impact of flash floods by reducing the likelihood of it occurring or the impact of flash floods on the environment since it is so destructive that towns get inundated before anyone can respond (Ologunorisa, 2009). WHO Africa (2018) reported that in 2018, 1.9 million persons were affected by floods across 12 states in Nigeria; flash flooding, it reported, struck in Adamawa, Borno and Yobe states, displacing more than half a million from their homes, killing many, affecting IDP camps and host communities and exacerbating a cholera outbreak.

1.2     Statement of the problem

          Owerri, the capital of Imo state, is one of the towns in Nigeria experiencing the problem of flash flooding. In the past decade, the town has recorded a lot of physical developments due to high rate of urbanisation mostly from an increase in population, and this population growth has led to developments in topographically low lying areas which has resulted in the problem of flash flooding. Some neighbourhoods in Owerri such as Aladinma, Akwakuma, New Owerri have experienced problems of flooding in recent times; this increase in the intensity of rainfalls due to climate change is expected to exacerbate in the future.

Fig 1 – Flash flooding in New Owerri, Owerri

Fig 2 – Flash flooding in New Owerri, Owerri

Fig 3 – Flash flooding in Aladinma, Owerri

Research into the causes of flooding in Nigeria done by Nwigwe and Emberga (2014) show that “the average height/depth has been consistently increasing over the years from 121cm in 1980, to 142cm in 1990. 201cm in 2000 and 366cm in 2010. It is projected that it may increase to 504cm by 2020 if the spate remains unabated. Similarly the average width/spread was just 152cm in 1980. It spread may be as much as 906m in 2020. Average frequency per annum increased from 3 in 1980 to 18 in 2010; and it may become 26 per annum in 2020.see figure 2(b). Average frequency, per annum increase from 3 in 1980 to 18 in 2010, and it may become 26 per annum in 2020, see figure 1(d) summarize average duration floods; in 1980, it was three days in 2010, it became one week (7days) on the average. Duration may increase to almost two weeks (13days) on 2020 that is if the spate should remain unabated. Incidentally, flood has become almost permanent feature of some localities that is, as long as the rainy season lasts. Communities thus live with it.”

With the rise in urbanisation without dealing with the attendant result such as reduced infiltration due to an increase in developed land by increasing drainage ways and other allied factors can increase the rate of flooding of an area where it need not be.

The effects of flooding have increased in magnitude over the last three decades, resulting in losses to urban dwellers and flood victims. It is clear that irreparable havoc has been wreaked on Nigerian citizens as a result of what has become a perennial natural disaster in our cities. Apart from residences that are flooded and collapse, schools, buildings, and bridges are also prone to collapse. Markets and farmlands are flooded for weeks and are swept away in some cases. Floods had a terrible effect on more than just buildings and persons. Flash floods even more so. When schools and market centers were buried for weeks, many farmlands, both arable and agro-foresting, were swept away. When numerous bridges fell and power poles were damaged, some animals died as a result of the floods.

Flash floods in many Nigerian capital cities are primarily caused by residents’ lack of environmental awareness, inadequate (or sometimes complete lack) of spatial information on flood-prone areas, waste dumps, and the construction of buildings (both commercial and residential, including government offices) on river channels without adequate water flow measures. Flash flood hazards, particularly in metropolitan areas, are critical from both a human settlement and economic standpoint.

The following are some of the effects and they can be seen in Imo state as well as many other places where flooding and its more dangerous child flash flooding have been observed:

• Cause, exacerbate, and induce diarrheal water borne illness
• Destroy farms, food, and cash crops
• Individuals, communities, and the nation will be impoverished as a result of service disruptions and agricultural land degradation
• Annihilate human life, animal life, and property
• Destroy and damage structures such as bridges, dams, embankments, drains, highways, and railroads
• Pollute the environment
• Infestations spread, and chemicals contaminate the land and water
• Leaching and erosion of rich top soil causes soil sterility.

So far, little or no effort has been made towards estimating the severity of flash floods in these areas which will enable the formulation of strategies for risk mitigation. Recently, the estimation of flash flood severity is starting to be considered a crucial demand. This study is intended to achieve this result.

1.3     Aim of the study

This study aims at estimating flash flood severity and the formulation of strategies for risk mitigation in Owerri urban.

1.4     Objectives of the study

The objectives of this study are as follows;

1. to examine the trend of rainfall intensity in Owerri urban
2. to identify factors influencing flooding in these areas

• to determine the level of severity of flooding in these areas

1. to formulate strategies for risk mitigation of flooding in Owerri urban

1.5     Scope of the study

The scope of this study is the estimation of flash flood severity within Owerri urban between the years 2000 and 2018, and the impact of increasing rainfall and basin catchment area size on the severity of flash floods.

1.6     Significance of the study

Flash floods have been understood to be the leading cause of loss of lives and property, but the reason why certain locations are given to severe incidences of flash floods has not been investigated. This study attempts at the examination of a trend that causes immense loss and a better understanding of the factors leading to the irreparable havoc that this phenomenon causes when left unattended to. The results of this study will put a tool in the hands of those who will inevitably help to prevent more loss and protect humans and the environment from the dangers of this natural hazard.

1.7     Limitations of the study

• Time: As a result of the lack of time, I had to limit my focus to taking fewer models.
• Lack of proper structures: Since the study makes use of results gotten from gauges to ascertain accurate event-level data and the area of interest has less than 8 working gauges rather than 42 as required by the WMO, it was impossible to get data for all locations of interest.
• Unavailability of proper data to ascertain the impact of other explanatory variables that have a great impact on floods in any of the sample areas apart from monthly rainfall data and basin catchment area.
  

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  Pages:  90

  Category: Project

  Format:  Word & PDF         

  Chapters: 1-5                                 

  Material contains Table of Content, Abstract and References.