Flood Risk and Vulnerability Analysis of the Lower Usuma River in Gwagwalada Town Abuja, using GIS and HEC-RAS Model

Flood Risk and Vulnerability Analysis of the Lower Usuma River in Gwagwalada Town Abuja, using GIS and HEC-RAS Model

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D. O. Balogun1*, O. O. M. Agunloye2, A. B. Mohammed3, U. Ogunbiyi4, A. A. Okewu5, E. S. Ikegwuonu6, B. Nkut7

1National Biotechnology Development Agency (NABDA), Bioresources Development Centre (BIODEC) Kano, Nigeria.

2Federal College of Land Resources Technology, Owerri, Imo State.

3Zonal Advanced Space Technology Applications Laboratory, Kano, Nigeria

4Federal College of Education, Zaria, Nigeria.

5,6,7Department of Geography, Ahmadu Bello University Zaria, Kaduna State, Nigeria.

Abstract – Globally, flooding is seen as the most common and devastating natural disaster contributing to greater economic and social losses than any other natural disaster. This study evaluated flood risk and vulnerability of Lower Usuma River in Gwagwalada Town for 5, 10, 50, and 100-year return periods deploying Geographic Information System and Hydrologic Engineering Center – River Analysis System modelling technique. The findings revealed that areas of low to moderate vulnerabilities for agricultural lands, economic/commercial, infrastructural, and residential areas have the greatest areal coverage. Areas of high and very high vulnerability for residential buildings are less northwards away from the Lower Usuma River for the various flood return periods. The result also indicates that more residential buildings and commercial establishments are at risk of flood than any other element within the environment. Areas affected by floods from this study include the settlements of Gwagwalada, Ungwan Shanu, Dobi, Dagiri, and Ungwan Dodo, Ungwan Bassa, Old Kutunku, Ungwan, Gwari, Ungwan Aguma, and Abattoir settlements. This study suggests that the government, under its capacity as Federal Capital Development Authority should impose strict restrictions on building projects along natural waterways in Gwagwalada and enforce compliance by individuals, private, and public property developers. Disaster management institutions and academic institutions should enhance their capacity in Geospatial and hydrologic modelling techniques and ensure installations of automated gauge stations and discharge recording equipment along the Lower Usuma River to minimize errors caused by human laxities.

Keywords: Flood Return Period, Flood Frequency Analysis, Environment, Gumbel Distribution, Modelling.

INTRODUCTION

Floods are regarded as a global environmental tragedy that affects the population in different dimensions. Due to its capacity to destroy lives and assets in its course, it is the most expensive and one of the deadliest natural hazards [1]. Floods can be described as the excess flow of water which exceeds a river channel's carrying capacity, drainage system, dams, and any other water bodies [2]. These excess flow in water bodies occur mostly due to several factors, but heavy rainfall is the leading cause of river flooding in Nigeria. Land use changes due to human activities such as rapid settlement development along river

flood plains can influence flood hazards' spatiotemporal pattern [3].

Flood risk is a product of both flood hazard and vulnerability to flooding multiplied by the total value of the assets exposed to the hazard [4]. Flood hazard is referred to the probability that an area with the potential of being flooded will be inundated for a given period. On the other hand, the most significant risk component is flood vulnerability and is the state of being adversely susceptible to a flood event. flood vulnerability assessment is defined as the classification of different flood risk areas in an environment to establish successful flood mitigation strategies [5]. Flood risk and vulnerability maps are essential tools in public enlightenment, disaster response planning, and flood risk management [6]. Geospatial techniques have been deployed successfully in flood hazards, risk, and vulnerability mapping in different places worldwide [7].

In tropical regions, extreme rainstorms, hurricanes, and dam failures have triggered high-magnitude floods that have resulted in severe consequences [8] [9]. However, the apparent explanation for flooding, especially in Nigerias municipalities and coastal areas, lies in the extensive distribution of low-lying coastal areas and River floodplains since these areas have quickly become a long- standing human settlement attraction [10]. Global warming has been linked to the sudden rise in the number and severity of recent floods in Jigawa, Kano, Kaduna, Katsina (in Northern Nigeria), Gombe (North-Eastern Nigeria), and most of the Southern States [11]. As a result, Nigerians have become increasingly at risk to a wide range of natural hazards with urbanization and high incidence of poverty pressing residents, despite city planning laws banning them from inhabiting fragile flood plains and unstable hillsides.

Rapid urban growth and torrential rainfall have led to the intensification of flooding within the lower section of the Usuma River in Gwagwalada town Abuja (See Plate 1 and 2) [12]. This is becoming an annual occurrence capable of causing lives, properties, and the ecosystem to be ruined inadvertently. Floods witnessed in Gwagwalada town in 2003, 2006, 2009, and 2011 among others inundated vast lands where lives and properties worth millions of Naira were lost [13]. This frequent river flooding issue is of great concern due to excess rainfall

followed by high river discharge, so preventive steps must be taken to avoid potential flood disasters cost-effectively through reliable and continuous methodologies that are scientifically relevant [14].

The issue of flooding in Nigeria was explored by several researchers such as [15], [9], [16], [17], [18] and

[13] to name a few. They deployed the use of GIS and Remote Sensing techniques to estimate the extent of flood hazard, risk, and vulnerable areas. None of these studies analyzed flood risk and vulnerability of the lower Usuma River with explicit emphasis on Geographic Information Systems (GIS) and Hydrologic Engineering Center – River Analysis System (HEC-RAS) modelling techniques. Therefore, to provide the basic knowledge that will aid disaster management agencies in preparing and maintaining the marginal lands adjacent to the flood plain along the Lower Usuma River, it is imperative to undertake this research. This will help avoid any potential disaster that may occur from any future flood events due to the nature of land use, proximity to the lower Usuma River, climate change and the likely recurrence of flood events in Gwagwalada town. Hence, this study is aimed at carrying out flood risk and vulnerability analysis of the Lower Usuma River in Gwagwalada town, and was achieved by determining flood frequency analysis using Gumbel extreme value distribution, deriving flood risk, and vulnerable zones deploying GIS and HEC-RAS modelling.

Plate 1: Streets and Buildings Inundated in Gwagwalada Town

Plate 1: Residential Buildings Flooded in Gwagwalada Town

Plate 2. School Building Inundated in Gwagwalada Town

MATERIALS AND METHODS

Study AreaTypes and Sources of DataFlood Frequency Analysis (FFA)Land Use Data ProcessingHydraulic Modelling in HEC-RAS and Post-ProcessingVulnerability AssessmentFlood Risk AnalysisVulnerable Agricultural LandsVulnerable Economic and Commercial ActivitiesVulnerable InfrastructuresVulnerable Residential BuildingsFlood Risk AnalysisPopulation at Risk to FloodElements Within the Environment at Risk to Flood

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