Project – Drainage System Capacity, Land-Use Change and Pluvial Flood Vulnerability: A Geospatial Assessment of Selected Flood-Prone Neighbourhoods in Lagos State.

CHAPTER ONE

INTRODUCTION

Title: Drainage System Capacity, Land-Use Change and Pluvial Flood Vulnerability: A Geospatial Assessment of Selected Flood-Prone Neighbourhoods in Lagos State

1.1 Background to the Study

Flooding has become one of the most significant environmental challenges confronting rapidly urbanising cities across the world. It occurs when water accumulates in areas where the existing natural or artificial drainage systems cannot adequately accommodate the volume of water generated by rainfall, river overflow, coastal processes, or other hydrological events. Among the different forms of flooding, pluvial flooding is particularly associated with intense or prolonged rainfall that produces surface runoff faster than it can infiltrate into the soil or be conveyed through drainage channels. Unlike riverine flooding, which results from rivers and other water bodies overflowing their banks, pluvial flooding can occur in locations that are not necessarily close to rivers or coastal environments. Its occurrence is often influenced by rainfall intensity, soil characteristics, surface permeability, topography, drainage infrastructure, and patterns of urban development. In recent decades, the increasing frequency of intense rainfall events and the expansion of urban settlements have heightened concerns about urban flood vulnerability. The Intergovernmental Panel on Climate Change (IPCC, 2021) explains that climate change is intensifying the global water cycle and increasing the likelihood of heavy precipitation events in many regions. Similarly, the United Nations Office for Disaster Risk Reduction (UNDRR, 2022) identifies flooding as a major disaster risk with significant implications for human settlements, infrastructure, economic activities, and environmental sustainability. These developments demonstrate the importance of understanding the physical and human factors responsible for urban flooding, particularly in rapidly developing metropolitan environments.

The capacity of drainage systems is an important factor determining the ability of urban areas to manage rainfall runoff and reduce the occurrence of pluvial flooding. Drainage systems consist of natural and constructed channels, gutters, culverts, underground pipes, stormwater drains, retention facilities, and other infrastructure designed to collect and convey excess surface water away from developed areas. Their effectiveness depends on several factors, including channel dimensions, depth, slope, construction materials, maintenance conditions, sediment accumulation, and the volume of runoff entering the drainage network. Where drainage infrastructure is adequately designed and maintained, it can reduce surface water accumulation by facilitating the movement of stormwater into appropriate discharge points. However, when drainage channels are undersized, obstructed, poorly connected, or deteriorated, their capacity may become insufficient during periods of heavy rainfall. Butler and Davies (2011), in their work on urban drainage, explain that effective stormwater management requires drainage infrastructure to be designed in relation to catchment characteristics, rainfall conditions, runoff generation, and the hydraulic performance of drainage networks. Similarly, Fletcher et al. (2015) emphasise that urban stormwater management requires an integrated understanding of drainage systems, land development, and the movement of water through urban environments. This suggests that the capacity and physical condition of drainage infrastructure are important determinants of the extent to which neighbourhoods experience surface water accumulation during rainfall events.

Land-use change is another major factor influencing the vulnerability of urban neighbourhoods to pluvial flooding. Land-use change refers to the modification of the natural or existing use of land through residential development, commercial activities, industrial expansion, transportation infrastructure, and other forms of human occupation. As cities expand, natural vegetation, wetlands, agricultural land, and open spaces are frequently converted into buildings, roads, parking areas, and other impervious surfaces. These changes reduce the ability of the land to absorb rainfall and increase the volume and speed of surface runoff generated during storms. Impervious surfaces prevent or limit the infiltration of rainwater into the soil, causing a greater proportion of rainfall to flow across the land surface and enter drainage channels. This situation may increase pressure on existing drainage infrastructure, particularly in neighbourhoods where urban development has occurred without corresponding improvements in stormwater management facilities. Shuster et al. (2005) established that urbanisation and the associated increase in impervious surfaces influence stormwater runoff and water-related environmental conditions. Similarly, Jacobson (2011) explains that urban development changes the hydrological characteristics of watersheds through alterations in infiltration, runoff, and water movement. These findings demonstrate that land-use change is not merely a physical transformation of the urban landscape but also an important process influencing the generation and distribution of flood risk.

The relationship between drainage capacity and land-use change becomes particularly important in densely populated urban neighbourhoods where infrastructure development may not keep pace with the rate of settlement expansion. Rapid urbanisation often increases demand for residential accommodation, commercial spaces, roads, and public facilities, resulting in the conversion of undeveloped land into built-up areas. In some neighbourhoods, buildings and other structures may encroach on natural drainage paths, wetlands, and existing waterways, thereby reducing the available space for stormwater movement and storage. In addition, the construction of roads, walls, and other physical barriers may alter the natural direction of surface runoff, directing water towards low-lying locations or areas with inadequate drainage facilities. This can create conditions in which even moderate rainfall produces significant water accumulation. McGranahan, Balk and Anderson (2007) observe that urbanisation in coastal areas creates particular environmental challenges because population growth, settlement expansion, and ecological pressures frequently occur in locations exposed to multiple hazards. Similarly, Douglas et al. (2008), in their study of urban poverty and environmental risk in African cities, identify inadequate infrastructure, poor settlement conditions, and environmental degradation as important factors increasing urban vulnerability to flooding. These studies suggest that pluvial flood vulnerability cannot be understood by examining drainage infrastructure or land-use patterns independently; rather, their combined effects must be considered within the spatial and environmental characteristics of individual neighbourhoods.

Geospatial assessment has become an important approach for investigating the spatial distribution, causes, and vulnerability of flooding in urban environments. Geospatial assessment involves the use of geographic information systems (GIS), remote sensing, satellite imagery, digital elevation models, spatial databases, and other geographical technologies to collect, process, analyse, and interpret information about the Earth’s surface. These technologies allow researchers to examine the relationship between topography, land-use patterns, drainage networks, rainfall distribution, and flood occurrence across different locations. Through GIS-based analysis, drainage infrastructure can be mapped, land-use changes can be identified, low-lying areas can be delineated, and neighbourhoods with different levels of flood vulnerability can be classified. Remote sensing also makes it possible to compare satellite images from different periods to determine the extent and direction of urban expansion. Goodchild (2007) explains that geographic information science provides important analytical capabilities for understanding spatial relationships and geographical processes. Similarly, Weng (2001) demonstrates the usefulness of remote sensing and GIS in examining urban land-use and land-cover change. More specifically, Tingsanchali (2012) identifies GIS and other spatial techniques as important tools for urban flood modelling, flood hazard assessment, and disaster management. These developments demonstrate that geospatial technologies provide an effective basis for assessing the combined influence of drainage capacity and land-use change on pluvial flood vulnerability.

In Nigeria, flooding has remained a recurring environmental challenge, particularly in major urban centres experiencing rapid population growth, inadequate infrastructure, and changing patterns of land development. Urban flooding is associated with several factors, including intense rainfall, poor drainage maintenance, indiscriminate waste disposal, uncontrolled construction, settlement encroachment on waterways, and the conversion of natural drainage areas into developed land. These conditions can increase the volume of surface runoff while simultaneously reducing the capacity of drainage systems to convey excess water. The National Emergency Management Agency (NEMA) has repeatedly identified flooding as a major disaster affecting communities, residential properties, transportation networks, agricultural activities, and public infrastructure across Nigeria. Similarly, the Nigerian Meteorological Agency (NiMet) provides seasonal climate predictions and rainfall-related information that demonstrate the importance of understanding changing rainfall conditions for environmental and disaster-risk management. Adebayo and Oruonye (2012), in their examination of the challenges associated with environmental problems and urban development in Nigeria, highlight the importance of planning and infrastructure management in addressing urban environmental difficulties. Furthermore, ActionAid (2006), in its assessment of flooding and vulnerability in Nigerian communities, identifies inadequate drainage, poor urban planning, and environmental management challenges as important contributors to flood risk. These national experiences demonstrate the need for systematic assessments that identify how infrastructure limitations and changes in land use contribute to flooding in Nigerian urban settlements.

Lagos State presents a particularly important geographical setting for examining the relationship between drainage system capacity, land-use change, and pluvial flood vulnerability. As Nigeria’s major commercial and economic centre, Lagos has experienced considerable population growth, urban expansion, infrastructure development, and increasing pressure on available land resources. Its geographical characteristics, including low-lying coastal environments, lagoons, wetlands, creeks, and areas with relatively limited natural drainage gradients, create additional challenges for effective stormwater management. The expansion of residential, commercial, industrial, and transportation developments has transformed several parts of the state, with some previously open or vegetated areas being converted into impervious built-up surfaces. In certain neighbourhoods, inadequate drainage infrastructure, blocked channels, indiscriminate waste disposal, and development along natural waterways may further increase the likelihood of surface water accumulation during heavy rainfall. Adelekan (2010), in her study of vulnerability of poor urban coastal communities to flooding in Lagos, identifies the relationship between environmental conditions, settlement characteristics, and socioeconomic vulnerability. Similarly, Adelekan (2016), in examining flood risk management in the coastal city of Lagos, highlights the significance of urban development, institutional capacity, and environmental conditions in shaping flood vulnerability. These studies establish Lagos as an important location for investigating the physical and human factors responsible for urban flooding.

Despite the growing attention given to flood management in Lagos State, the interaction between drainage system capacity and land-use change requires further neighbourhood-level investigation, particularly in relation to pluvial flood vulnerability. Flood experiences are not uniform across the state because different neighbourhoods possess different topographical characteristics, drainage infrastructure conditions, land-cover patterns, settlement densities, and levels of exposure to intense rainfall. Some locations may experience frequent flooding because their drainage channels are inadequate or poorly maintained, while others may be more vulnerable because extensive land-use conversion has increased impervious surfaces and altered natural runoff pathways. In addition, the absence of detailed spatial information linking drainage infrastructure characteristics with historical land-use changes can make it difficult to identify the relative contribution of these factors to flood vulnerability. The IPCC (2022) emphasises the importance of location-specific climate risk assessments and adaptation strategies, while the World Bank (2021), through its work on urban flood resilience, highlights the need for integrated infrastructure planning and evidence-based urban risk management. Adelekan (2010) and Adelekan (2016) further demonstrate the importance of examining flood vulnerability within the specific environmental and socioeconomic conditions of Lagos. Against this background, this study seeks to undertake a geospatial assessment of drainage system capacity, land-use change, and pluvial flood vulnerability in selected flood-prone neighbourhoods in Lagos State, with a view to identifying their spatial relationships and providing evidence that can support improved urban drainage planning and flood-risk management.

1.2 Statement of the Problem

Flooding has become a recurring environmental problem in Lagos State, affecting residential neighbourhoods, commercial centres, transportation networks, public infrastructure, and the general wellbeing of residents. Although rainfall is a natural environmental process, the extent to which it produces flooding in urban areas is influenced by the capacity of existing drainage infrastructure and the physical characteristics of developed land. In several urban neighbourhoods, inadequate drainage channels, poor maintenance, sediment accumulation, indiscriminate waste disposal, and obstruction of natural water pathways may prevent the effective movement of stormwater during heavy rainfall. Consequently, water accumulates on roads, residential compounds, and other developed surfaces, creating conditions associated with pluvial flooding. Adelekan (2010) identified inadequate infrastructure, uncontrolled expansion of built-up areas, and failure to maintain existing stormwater drainage systems as important factors contributing to flood risk in Lagos. Similarly, Adeloye and Rustum (2011) established the relationship between flooding and urban planning challenges in Lagos, highlighting the importance of effective infrastructure and physical development control. The problem, therefore, concerns the extent to which existing drainage systems in selected flood-prone neighbourhoods possess adequate capacity to accommodate rainfall-generated surface runoff.

Another major problem is the continuous transformation of land-use patterns resulting from rapid urbanisation and increasing demand for residential, commercial, industrial, and transportation infrastructure. Natural vegetation, wetlands, open spaces, and other permeable surfaces are increasingly converted into buildings, paved roads, parking areas, and other impervious surfaces. These changes reduce the natural infiltration of rainfall into the soil and increase the volume of surface runoff generated during rainfall events. In some locations, development activities may also encroach on natural drainage channels, wetlands, and floodplains, thereby reducing the available pathways through which stormwater can flow. Idowu and Zhou (2021), in their assessment of land-use and land-cover change in Lagos State, documented substantial changes in developed areas, wetlands, vegetation, and water bodies over a 35-year period, establishing the importance of land-cover transformation in understanding flood hazards. Similarly, Koko et al. (2021) examined urban growth and land-cover changes in Lagos using multi-temporal remote sensing and GIS, highlighting the implications of urban expansion for flood mitigation. Despite these findings, the extent to which land-use changes have altered runoff conditions and increased pluvial flood vulnerability across selected neighbourhoods requires further spatial investigation.

Furthermore, the combined influence of drainage system capacity and land-use change creates a significant challenge for understanding variations in flood vulnerability across different neighbourhoods in Lagos State. A neighbourhood may experience serious surface water accumulation because its drainage infrastructure is inadequate, while another may experience similar conditions because extensive land development has increased runoff beyond the capacity of existing drainage channels. In some areas, both conditions may occur simultaneously, resulting in more severe flood vulnerability. However, flood assessments that consider rainfall, elevation, or land-use characteristics independently may not adequately explain how drainage infrastructure and urban development interact to influence the spatial distribution of pluvial flooding. Wahab, Ojolowo, Adelekan and Fasona (2014), in their spatial investigation of building contraventions and flood incidence in Lagos metropolis, demonstrated the importance of development along drainage channels and floodplains in explaining the occurrence of flooding. Likewise, Ibeabuchi (2023), in a GIS-based assessment of flood vulnerability in Lagos, demonstrated the usefulness of integrating spatial factors such as rainfall, elevation, slope, soil, and land use in identifying areas exposed to flood risk. These studies indicate the importance of integrated spatial assessment, but there remains a need to give specific attention to the relationship between drainage capacity, land-use change, and rainfall-generated surface flooding.

The central problem addressed by this study is the need for reliable neighbourhood-level geospatial evidence explaining how drainage system capacity and land-use change jointly influence pluvial flood vulnerability in selected flood-prone areas of Lagos State. Although previous studies have examined flooding, urban expansion, land-use transformation, and general flood susceptibility in Lagos, further investigation is required to establish the spatial relationship between drainage infrastructure conditions, changing land-use patterns, and the vulnerability of individual neighbourhoods to rainfall-generated surface water accumulation. The absence of sufficiently integrated spatial assessments may limit the ability of urban planners, environmental management authorities, and disaster-risk institutions to identify locations where drainage improvements and land-use planning interventions are most urgently required. Therefore, this study seeks to use geospatial techniques to assess drainage system capacity, analyse land-use changes, identify areas of varying pluvial flood vulnerability, and examine the relationship between these factors in selected flood-prone neighbourhoods in Lagos State. The findings are expected to provide useful spatial evidence for improved drainage planning, land-use regulation, and urban flood-risk reduction.

1.3 Objectives of the Study

The main objective of this study is to undertake a geospatial assessment of drainage system capacity, land-use change, and pluvial flood vulnerability in selected flood-prone neighbourhoods in Lagos State.

The specific objectives are to:

1. Assess the capacity and spatial distribution of existing drainage systems in selected flood-prone neighbourhoods in Lagos State.

2. Examine the pattern and extent of land-use and land-cover changes in the selected neighbourhoods over the study period.

3. Determine the spatial distribution and levels of pluvial flood vulnerability in the selected neighbourhoods using geospatial techniques.

4. Examine the relationship between drainage system capacity, land-use change, and pluvial flood vulnerability in the selected neighbourhoods.

1.4 Research Questions

The following research questions will guide the study:

1. What is the capacity and spatial distribution of existing drainage systems in selected flood-prone neighbourhoods in Lagos State?

2. What are the patterns and extent of land-use and land-cover changes in the selected neighbourhoods over the study period?

3. What are the spatial distributions and levels of pluvial flood vulnerability in the selected neighbourhoods?

4. What relationship exists between drainage system capacity, land-use change, and pluvial flood vulnerability in the selected neighbourhoods?

1.5 Research Hypothesis

The following null hypothesis will be tested at a 0.05 level of significance:

H₀: There is no statistically significant relationship between drainage system capacity, land-use change, and pluvial flood vulnerability in selected flood-prone neighbourhoods in Lagos State.

1.6 Significance of the Study

This study will be significant to urban planners, environmental management authorities, Lagos State Government, drainage engineers, disaster management agencies, residents of flood-prone communities, geospatial researchers, and future researchers.

Lagos State Government: The study will provide useful spatial information to support government efforts towards improving urban drainage infrastructure, controlling indiscriminate development, and reducing flood vulnerability. Its findings may assist relevant government agencies in identifying neighbourhoods requiring urgent drainage rehabilitation, expansion, and maintenance.

Urban and Physical Development Planners: The study will provide information on how changes in land-use patterns influence surface runoff and flood vulnerability. It will assist urban planners in understanding the environmental consequences of converting wetlands, vegetation, and open spaces into built-up areas. The findings may also support improved development control and the enforcement of land-use planning regulations.

Drainage Engineers: The research will provide useful information on the relationship between drainage infrastructure capacity and rainfall-generated surface water accumulation. It may assist drainage engineers in identifying locations where existing drainage channels are inadequate, obstructed, poorly connected, or require improvement to accommodate stormwater runoff.

Environmental Management Authorities: The study will contribute to environmental management by identifying the influence of land-use transformation, impervious surfaces, and drainage conditions on urban flood vulnerability. It may support the development of appropriate environmental protection and stormwater management strategies.

Disaster Management Agencies: Agencies responsible for disaster prevention and emergency management may benefit from the geospatial information generated through the study. The identification of highly vulnerable neighbourhoods will help support flood preparedness, risk communication, emergency planning, and the prioritisation of flood-risk reduction interventions.

Residents of Flood-Prone Neighbourhoods: The study will increase awareness of the environmental and human activities that contribute to pluvial flooding. It will help residents understand the importance of proper waste disposal, protection of drainage channels, responsible land development, and the preservation of natural water pathways.

Geospatial and Environmental Researchers: The study will demonstrate the application of Geographic Information Systems (GIS), remote sensing, satellite imagery, and other spatial analytical techniques in assessing urban flood vulnerability. It will provide useful insights into the integration of drainage infrastructure characteristics and land-use information in environmental assessment.

Future Researchers: The research will serve as academic reference material for future studies on urban flooding, drainage infrastructure, land-use and land-cover change, hydrological processes, and geospatial environmental assessment. It will also provide a foundation for further research on urban resilience and sustainable stormwater management in Lagos State and other Nigerian cities.

1.7 Scope of the Study

The study focuses on drainage system capacity, land-use change, and pluvial flood vulnerability in selected flood-prone neighbourhoods in Lagos State. It examines how the physical characteristics and capacity of existing drainage infrastructure, together with changes in land-use and land-cover patterns, influence the vulnerability of urban neighbourhoods to rainfall-generated surface flooding.

Geographically, the study will be conducted in selected flood-prone neighbourhoods in Lagos State. The specific neighbourhoods will be selected based on documented flood occurrences, availability of relevant spatial data, and accessibility of information relating to drainage infrastructure and land-use patterns.

In terms of content, the study will cover four major areas:

1. Assessment of drainage system capacity, including drainage dimensions, distribution, connectivity, condition, and possible obstructions.

2. Examination of land-use and land-cover changes, particularly the conversion of vegetation, wetlands, and open spaces into built-up areas.

3. Geospatial identification and classification of areas according to their levels of pluvial flood vulnerability.

4. Examination of the relationship between drainage system capacity, land-use change, and pluvial flood vulnerability.

The study will employ geospatial techniques, particularly Geographic Information Systems (GIS) and remote sensing, to analyse spatial information. Relevant datasets may include satellite imagery, digital elevation models, drainage network information, rainfall records, soil characteristics, and documented flood occurrence locations.

The study is specifically concerned with pluvial flooding resulting from rainfall-generated surface runoff and inadequate drainage. It does not seek to provide a comprehensive assessment of coastal storm surges or riverine flooding, although relevant geographical conditions may be considered where they influence surface water movement.

1.8 Operational Definition of Terms

Drainage System: A natural or constructed network of channels, gutters, culverts, pipes, and other infrastructure designed to collect and convey excess surface water away from developed areas.

Drainage System Capacity: The ability of an existing drainage network to accommodate and convey a particular volume of stormwater without causing excessive water accumulation or overflow.

Land-Use Change: The modification of land from one use to another through human activities such as residential construction, commercial development, industrialisation, road construction, and agricultural expansion.

Land-Use and Land-Cover Change (LULC): The transformation of the physical characteristics and utilisation of land over time, including changes involving vegetation, wetlands, water bodies, open spaces, and built-up areas.

Pluvial Flooding: Flooding that occurs when rainfall-generated surface runoff accumulates because the volume of water exceeds the infiltration capacity of the ground or the conveyance capacity of drainage systems.

Flood Vulnerability: The degree to which a particular location, population, infrastructure, or environment is susceptible to the damaging effects of flooding.

Geospatial Assessment: The application of geographical information systems, remote sensing, satellite imagery, and spatial analytical techniques to examine geographical patterns and relationships.

Geographic Information System (GIS): A computer-based system used to collect, store, manage, analyse, and display geographically referenced information.

Remote Sensing: The process of obtaining information about the Earth’s surface through satellite imagery, aerial photographs, and other sensors without direct physical contact with the objects being studied.

Impervious Surfaces: Land surfaces such as concrete, asphalt, rooftops, and paved areas that prevent or significantly reduce the infiltration of rainfall into the soil.

Surface Runoff: The portion of rainfall that flows over the ground surface when it cannot infiltrate into the soil or is not otherwise retained.

Flood-Prone Neighbourhoods: Residential or developed urban areas that experience recurring flooding or possess geographical and environmental characteristics that make them susceptible to flooding.

Project – Drainage System Capacity, Land-Use Change and Pluvial Flood Vulnerability: A Geospatial Assessment of Selected Flood-Prone Neighbourhoods in Lagos State.