Geotechnical and GIS-based environmental factors and vulnerability studies of the Okemesi landslide, Nigeria

doi:10.1016/j.regsus.08.001

Abstract

Abstract:

Landslide is a geological hazard typically associated with extreme events such as earthquakes, heavy rainfall, volcanic eruptions, changes in groundwater level, etc. This study was carried out in Okemesi-Ekiti (also known as Okemesi), Southwest Nigeria, with the purpose of using remote sensing and GIS technologies to analyze the environmental factors (grain size, direct shear strength resistance, rainfall data, wet density, surface, and slope) resulting in the occurrence of the Okemesi landslide. The study also aimed to conduct a vulnerability analysis in the study area to identify regions with a probability of landslide occurrence. The grain size analysis of the soil in the Okemesi landslide area showed that slope materials comprised 17.14% gravel, 59.31% sand, and 19.48% fines, thus the soil type could be classified as poorly graded gravely sand with a high possibility of landslide occurrence. The geomorphic characteristics of the study area was characterized by slopes ranging from 0.00° to 49.00°, while most slopes in the area were less than 8.00°. The slope aspect direction was mainly in south (157.51°-202.50°), southwest (202.51°-247.50°), west (247.51°-292.50°), and north (0.00°-22.50° and 337.51°-360.00°). The highlands were primarily bounded by the slope directions of north (0.00°-22.50° and 337.51°-360.00°), northeast (22.51°-67.50°), east (67.51°-112.51°), and southeast (112.51°-157.50°), which indicated the potential direction of mass movement. The study area can be divided into three vulnerability zones: high, medium, and low, with the area percentages of 9.00%, 61.80%, and 29.20%, respectively. The analysis suggested that the Okemesi landslide was likely triggered by rainfall, which might have weakened the physical structure of slope materials. Understanding the causes and impacts of landslides is crucial for policy-makers to implement measures to mitigate landslide hazards, protect infrastructure, and prevent the loss of life in the landslide-prone regions.

Key words: Rainfall-induced landslide, Landslide hazards, Slope materials, Rainfall, Geomorphology, Vulnerability zone, Okemesi-Ekiti (Okemesi)

Oluwakemi Bolanle AKINTAN, Johnson Adedeji OLUSOLA, Olaniyi Patrick IMOLE, Moyosoluwa Odunayo ADEYEMI. Geotechnical and GIS-based environmental factors and vulnerability studies of the Okemesi landslide, Nigeria[J]. Regional Sustainability, 2023, 4(3): 249-260.

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Item	Type	Date source	Year
Rainfall	Station	Nigerian Meteorological Agency (NiMET) in Ado-Ekiti, Nigeria (https://nimet.gov.ng/https://nimet.gov.ng/)	Monthly rainfall (2001-2017) and daily rainfall (2017)
Landsat image (Landsat 8 OLI_TIRS)	Raster	https://www.usgs.gov/	2019
Copernicus sentinel data	Raster	Alaska Satellite Facility and National Aeronautics and Space Administration (NASA)	2007
High resolution satellite imagery	Raster	Google Earth Pro	2018
Administrative map	Vector	https://www.usgs.gov/ and https://www.nasa.gov/	2010
Geology map	Raster	The Ministry of Agriculture and Natural Resources, Nigeria	1965
Soil map	Raster	Federal Department of Agriculture Land Resources and Soil Survey Division, Nigeria	2018
Study location	Vector point	Field investigation	2019

Input parameter	Influence level (%)	Field value	Field class	Weight scale
Land use and land cover type	10.00	1	Farmland	1
		2	Bare or open surface	2
		3	Forest	3
		4	Rock surface	2
		5	Built-up area	3
Reclassified slope	40.00	1	High	1
		2	Medium	2
		3	Low	3
Reclassified geology	25.00	1	High	1
		2	Medium	2
		3	Low	3
Reclassified soil	25.00	1	High	1
		2	Medium	2
		3	Low	3

Attribute	Soil sample
Attribute	A	B	C
Gravel (%)	23.00	0.26	0.03
Sand (%)	0.65	0.57	0.57
Fines (%)	0.12	0.18	0.28
Particle density (mg/m³)	2.68	2.72	2.67
Natural water content (%)	7.67	5.92	10.95
Wet density (mg/m³)	2.03	2.11	2.01
Unit weight (kg/m²)	1989	2068	1970
Collapsibility potential (%)	6.65	12.09	12.62
Direct shear test	0.20	0.25	0.16
Cohesion (×10³ Pa)	16.00	18.00	17.00

Year	Rainfall (mm)
Year	Maximum	Minimum	Mean monthly	Annual	Standard deviation
2000	214.9	2.4	102.0	1223.6	72.9
2001	217.7	0.0	92.6	1110.7	94.5
2002	279.9	0.0	134.1	1609.8	99.2
2003	335.2	0.0	123.1	1477.1	100.4
2004	275.5	0.0	74.6	895.4	86.3
2005	242.6	0.0	110.0	1320.1	71.5
2006	228.6	0.0	109.7	1316.9	79.6
2007	182.4	0.0	87.6	1051.6	61.8
2008	241.7	0.0	112.3	1347.9	85.2
2009	315.9	0.0	121.4	1456.6	91.9
2010	279.1	0.0	116.5	1397.8	79.6
2011	325.0	0.0	126.1	1512.8	95.1
2012	329.2	6.7	133.8	1605.4	102.6
2013	339.8	15.1	132.6	1591.7	105.9
2014	292.6	0.2	130.5	1566.2	94.7
2015	258.5	0.0	94.5	1133.6	85.3
2016	228.6	0.0	100.2	1201.9	80.2
2017	405.7	0.4	118.3	1419.4	129.1

Type	Area (hm²)	Proportion (%)
Built-up area	257.18	1.15
Bare or open surface	2695.69	12.01
Forest	7947.56	35.41
Farmland	8898.34	39.65
Rock surface	2643.06	11.78
Total	22,441.83	100.00