Landslide and Erosion Risk Mapping Using GIS
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Abstract
About This Research Topic
Nigeria's hilly cities keep growing upward and outward onto slopes that were once left alone for good reason. Steep terrain looks scenic and sits above the flood line, which makes it tempting for developers and homeowners alike, but that same terrain carries landslide and erosion risks that most Nigerian cities have never actually mapped. Without that map, planning authorities are essentially approving development on hillsides with no real sense of which slopes are safe and which ones aren't.
This article draws on a study that built exactly that map, using GIS software to combine six terrain and land-use factors into a single, validated landslide and erosion susceptibility map for a representative hilly urban area. For readers interested in how a geospatial study like this is structured, our sample research projects library includes comparable GIS and civil engineering studies worth reviewing as models.
The findings matter for more than academic interest. They point directly to which existing neighbourhoods sit in genuine danger zones and offer planning authorities a concrete, tested tool for making better development control decisions going forward. The sections below cover the background to the problem, how the mapping was done, and what it found.
Main Abstract
Nigerian cities have expanded rapidly onto hilly terrain in recent decades, often without much regulation, raising exposure to landslide and erosion hazards. Yet systematic, spatially explicit risk mapping capable of actually informing land-use planning and hazard mitigation remains rare for most Nigerian hilly urban areas. This study built a landslide and erosion susceptibility map for a representative hilly urban terrain using a GIS-based weighted overlay analysis, combining six causative factors, slope gradient, slope aspect, elevation, land use and land cover, soil type, and proximity to drainage channels, alongside rainfall intensity as a triggering factor.
Factor layers were built from a 30-metre resolution digital elevation model, Landsat 8 OLI satellite imagery for land cover classification, soil survey data, and 20 years of historical rainfall records, each reclassified into five susceptibility classes. Each factor was assigned a weight using the Analytical Hierarchy Process, based on pairwise comparison of how much influence each factor has on landslide and erosion occurrence, informed by expert judgement and existing literature. The resulting weighted overlay analysis, run in ArcGIS, produced a composite susceptibility map dividing the study area into five zones: very low, low, moderate, high, and very high.
Slope gradient turned out to carry the most weight in the model (0.284), followed by land use and land cover (0.211). About 18.4 percent of the study area fell into the high or very high susceptibility category, concentrated mainly on slopes steeper than 25 degrees with sparse vegetation and close to drainage channels. Validating the map against a historical record of 34 documented landslide and severe erosion incidents, using the Area Under the ROC Curve method, produced a success rate of 0.81 and a prediction rate of 0.78, both signalling strong predictive performance and confirming that the mapping approach holds up in practice.
Cross-checking existing settlement footprints against the susceptibility map found that roughly 12.6 percent of existing structures in the study area sit within high or very high susceptibility zones, a meaningful population and infrastructure exposure that calls for urgent land-use planning and mitigation attention.
The study concludes that GIS-based weighted overlay analysis, combining multiple causative factors through the Analytical Hierarchy Process, offers a technically solid, well-validated, and genuinely useful approach for landslide and erosion susceptibility mapping in Nigerian hilly urban terrains, one capable of directly informing land-use planning, building control, and hazard mitigation decisions. It recommends that urban planning and building control authorities adopt this mapping approach to guide development control decisions and prioritise mitigation work in identified high-risk zones.
Chapter One Preview
Background to the Study
Fast-growing urban populations across Nigerian cities have, in many places, pushed settlement development onto hilly and steeply sloping terrain that was once considered marginal or unsuitable for construction, development that frequently proceeds with limited formal land-use planning oversight or geotechnical hazard consideration. These hilly urban terrains, while often attractive for their views and their distance from lowland flooding, carry a distinct and, in many cases, poorly understood set of geohazard risks, chiefly landslide and accelerated soil erosion, arising from the combination of steep slopes, variable soil and geological conditions, land cover disturbance from construction and vegetation clearance, and the intense, highly erosive rainfall typical of Nigeria's tropical climate.
Landslide and erosion hazards in hilly urban terrain can have severe consequences, structural damage or destruction, loss of life, disrupted transport and utility infrastructure, and progressive land degradation that can leave previously developable land unfit for continued habitation. Managing these hazards effectively depends fundamentally on having spatially explicit hazard information that identifies which areas within a given hilly urban terrain carry elevated risk, information that can directly shape land-use planning decisions, building control practice, and where to prioritise mitigation work such as slope stabilisation or drainage improvement. Yet systematic, rigorously validated landslide and erosion susceptibility mapping remains rare across most Nigerian hilly urban areas, leaving land-use and development control decisions to proceed without this kind of spatially explicit hazard information.
Geographic Information System-based multi-criteria decision analysis, combining the spatial distribution of landslide and erosion causative factors, such as slope gradient, land cover, and drainage proximity, through a systematic weighting and overlay methodology, has become a well-established and effective approach internationally for building landslide and erosion susceptibility maps. It offers a practical, replicable, and empirically testable alternative to more resource-intensive physically based hazard modelling, particularly well suited to data-constrained developing country contexts like Nigeria. Land use and land cover classification for this kind of work commonly draws on freely available satellite imagery such as Landsat 8 OLI, while Nigeria's own National Space Research and Development Agency maintains dedicated remote sensing and GIS capacity that supports exactly this kind of geospatial hazard mapping work.
This study set out to build a landslide and erosion susceptibility map for a representative Nigerian hilly urban terrain, using a GIS-based weighted overlay analysis that combines multiple causative factors weighted through the Analytical Hierarchy Process, with the goal of producing a validated, genuinely useful hazard mapping tool capable of informing land-use planning and mitigation decisions in the study area and, by extension, comparable Nigerian hilly urban terrains elsewhere.
Statement of the Problem
Nigerian hilly urban terrains are seeing more and more settlement expansion, frequently without the benefit of systematic, spatially explicit landslide and erosion susceptibility information that could inform land-use planning and development control decisions. The result is a growing population and infrastructure exposure to these geohazards, a risk that periodically materialises as documented landslide and severe erosion incidents within such terrains.
Without a validated, GIS-based susceptibility mapping tool for a given hilly urban terrain, land-use planning and building control authorities have no real evidence base for identifying high-risk zones that warrant development restriction or mitigation intervention, while existing settlement within those zones remains at elevated, largely unquantified risk. This study addresses that gap by developing and rigorously validating a GIS-based landslide and erosion susceptibility map for a representative Nigerian hilly urban terrain, producing a practically useful hazard mapping tool and quantifying current settlement exposure to the high-risk zones identified within the study area.
Aim and Objectives of the Study
The aim of this study is to develop and validate a landslide and erosion susceptibility map for a representative hilly urban terrain using a GIS-based weighted overlay analysis.
The specific objectives of the study are to:
● Prepare causative factor layers (slope gradient, slope aspect, elevation, land use/land cover, soil type, and drainage proximity) for the study area using GIS and remote sensing techniques.
● Determine the relative weight of each causative factor using the Analytical Hierarchy Process (AHP), based on pairwise comparison of their relative influence on landslide and erosion occurrence.
● Generate a composite landslide and erosion susceptibility map for the study area through GIS-based weighted overlay analysis of the weighted causative factor layers.
● Classify the study area into susceptibility zones (very low, low, moderate, high, and very high) and determine the area distribution of each zone.
● Assess the current settlement exposure to identified high-risk susceptibility zones within the study area.
● Validate the generated susceptibility map against a historical inventory of documented landslide and erosion incident locations, using the Area Under the Receiver Operating Characteristic Curve (AUC-ROC) method.
● Establish the statistical relationship between slope gradient and landslide/erosion incident density within the study area.
Research Questions
This study seeks to answer the following research questions:
● What causative factors most significantly influence landslide and erosion occurrence within the study area, and what is their spatial distribution?
● What relative weight should be assigned to each causative factor, based on the Analytical Hierarchy Process?
● What is the spatial pattern of landslide and erosion susceptibility across the study area, as determined through weighted overlay analysis?
● What proportion of the study area falls within each susceptibility zone (very low to very high)?
● What proportion of existing settlement within the study area is exposed to high or very high susceptibility zones?
● How well does the generated susceptibility map predict the location of historically documented landslide and erosion incidents?
● Is there a statistically significant relationship between slope gradient and landslide/erosion incident density within the study area?
Justification/Significance of the Study
This study is significant in several respects. It produces a rigorously validated, spatially explicit landslide and erosion susceptibility mapping tool for a representative Nigerian hilly urban terrain, addressing a real gap given how scarce this kind of systematic hazard mapping has been for most Nigerian hilly urban areas.
The findings offer direct practical value to urban planning authorities, building control agencies, and disaster risk management bodies looking for evidence-based information to guide land-use planning, development control, and mitigation prioritisation within hilly urban terrains. Students and researchers working on similar GIS-based multi-criteria analysis, weighted overlay modelling, or hazard validation studies can get direct feedback on methodology and analysis through our research coaching service, and explore related technical studies in our civil engineering project archive.
The study also contributes a rigorously validated methodological framework, combining GIS-based multi-criteria weighted overlay analysis with the Analytical Hierarchy Process and formal AUC-ROC validation, that can be readily adapted to other Nigerian hilly urban terrains facing comparable geohazard exposure. More broadly, it supports better public safety and fewer landslide and erosion-related losses in Nigerian hilly urban areas through more evidence-based, spatially informed hazard management.
Scope of the Study
This study is limited to developing and validating a landslide and erosion susceptibility map for a single representative Nigerian hilly urban terrain, using a GIS-based weighted overlay analysis that combines six causative factors, slope gradient, slope aspect, elevation, land use/land cover, soil type, and drainage proximity, weighted through the Analytical Hierarchy Process. It covers causative factor layer preparation, AHP-based weighting, weighted overlay analysis, susceptibility classification, settlement exposure assessment, and validation against a historical incident inventory using the AUC-ROC method.
The study does not extend to detailed, site-specific geotechnical slope stability analysis, such as factor of safety computation for individual slopes, or physically based, process-driven landslide runout modelling, both recommended for further research at the individual high-risk site level identified through this study's susceptibility mapping.
Operational Definition of Terms
Landslide Susceptibility: The likelihood of landslide occurrence in a given area based on local terrain and environmental conditions, typically expressed as a relative, spatially distributed classification rather than an absolute probability.
Causative Factor: An environmental or terrain variable, such as slope gradient or land cover, that influences the likelihood of landslide or erosion occurrence in a given area.
Weighted Overlay Analysis: A GIS-based multi-criteria decision analysis technique in which multiple spatial factor layers are combined, each weighted according to its relative importance, to produce a composite output layer representing the combined influence of all factors.
Analytical Hierarchy Process (AHP): A structured, multi-criteria decision-making technique, developed by Saaty, that derives relative weights for a set of criteria through pairwise comparison, incorporating a consistency check to verify the logical coherence of the judgements made.
Digital Elevation Model (DEM): A digital representation of terrain surface elevation, used as the primary data source for deriving slope gradient, slope aspect, and elevation causative factor layers in GIS-based terrain analysis.
Receiver Operating Characteristic (ROC) Curve: A graphical method for evaluating the predictive performance of a susceptibility or classification model, plotting the true positive rate against the false positive rate across varying classification thresholds, with the Area Under the Curve (AUC) providing a single summary measure of overall predictive accuracy.
Conclusion
Building on hilly terrain doesn't have to mean building blind. This study shows that a validated, GIS-based susceptibility map can pinpoint exactly which slopes and neighbourhoods carry the highest landslide and erosion risk, and the fact that roughly one in eight existing structures in the study area already sits in a high-risk zone makes the case for using this kind of mapping urgent rather than optional. For planning authorities in comparable Nigerian hilly cities, the tool demonstrated here offers a practical, tested starting point rather than a purely theoretical exercise. Readers interested in related geospatial and civil engineering research can browse more civil engineering project topics for further reading.
Frequently Asked Questions (FAQs)
What percentage of the study area was classified as high landslide and erosion risk?
Approximately 18.4 percent of the study area was classified as high or very high susceptibility, concentrated mainly on slopes steeper than 25 degrees with sparse vegetation cover and close to drainage channels.
Which factor most influences landslide and erosion susceptibility in this study?
Slope gradient carried the greatest weight in the model at 0.284, followed by land use and land cover at 0.211, meaning these two factors had the strongest influence on the composite susceptibility outcome.
How accurate is the GIS-based susceptibility map produced in this study?
Validation against 34 documented landslide and erosion incidents using the Area Under the ROC Curve method produced a success rate of 0.81 and a prediction rate of 0.78, both indicating good predictive performance.
How many existing buildings are located in high-risk landslide and erosion zones?
Cross-tabulating existing settlement footprints against the susceptibility map found that approximately 12.6 percent of existing structures in the study area sit within high or very high susceptibility zones.
What is the Analytical Hierarchy Process, and how was it used in this study?
The Analytical Hierarchy Process (AHP) is a structured decision-making technique that derives relative weights for a set of factors through pairwise comparison. In this study, it was used to weight six landslide and erosion causative factors based on their relative influence, informed by expert judgement and literature review.
What data sources were used to build the susceptibility map?
The study used a 30-metre resolution digital elevation model, Landsat 8 OLI satellite imagery for land use and land cover classification, soil survey data, and 20 years of historical rainfall records.
What six factors were combined to assess landslide and erosion susceptibility?
The study combined slope gradient, slope aspect, elevation, land use/land cover, soil type, and proximity to drainage channels, with rainfall intensity included as an additional triggering factor.
Can this GIS mapping approach be applied to other hilly Nigerian cities?
Yes. The study's methodology, GIS-based weighted overlay analysis combined with Analytical Hierarchy Process weighting and AUC-ROC validation, is designed to be replicable and adaptable to other Nigerian hilly urban terrains facing similar geohazard exposure, though factor weights would need independent recalibration for each new site.
Why is landslide and erosion risk mapping important for urban planning?
Spatially explicit susceptibility mapping gives planning and building control authorities an evidence base for restricting development in high-risk zones and prioritising mitigation work such as slope stabilisation or drainage improvement, rather than making land-use decisions without this hazard information.
What is a susceptibility map, and how is it different from a hazard probability map?
A susceptibility map is a relative, spatially distributed classification showing how likely an area is to experience landslide or erosion based on local terrain and environmental conditions, rather than an absolute numerical probability of occurrence.
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