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Landslide impacts and management in Nanka area, Southeast Nigeria

Landslide impacts and management in Nanka area, Southeast Nigeria The devastating impacts and mitigation measures for landslides in Nanka, Southeast Nigeria were studied. A procedure encompassing field mapping, laboratory analysis and limit equilibrium simulations were employed in the study. A total of 120 landslides in this area were documented during the field study. They were mainly shallow o o landslides with depth range of 0.2 m – 1.8 m and slope angle range of 36 -65 . This study revealed that landslides in this area occur mainly during the rainy season, and are triggered by water infiltration in slopes with high gradient, where poorly consolidated sands overlies less permeable silty clay units in the Nanka Formation. The increase in pore water pressure due to soil saturation leads to reduction of the shear strength and loss of apparent cohesion. Geotechnical laboratory analysis showed that the thin silty clay soils found in the area have very high plasticity, while the sands are non-plastic. The limit equilibrium simulations revealed that as the slope angle increases, the potential slip mass volume increases. Our study in Nanka revealed that landslides have negatively affected the topography/morphology, settlement areas, agricultural lands and vegetation/land cover of the area and many areas are also threatened by this phenomenon. Standard mechanical drainage system and biotechnical slope protection have been suggested as mitigation measures. Finally, we suggested that landslide hazard information should be incorporated into long-term plans for Nanka and other areas prone to landslides, the public should be aware of the hazards and preventive measures. Keywords: Impacts, Limit equilibrium, Mitigation, Biotechnical Introduction should be area dependent, though some management Landslide occurs in different parts of the world, under measures can be universally applied. Globally, many all climatic conditions and environments. It is a major researchers such as Dai et al. (2002), Schuster and High- natural hazard in which its global concern keeps increas- land (2003), Lateltin et al. (2005), Chen (2008), Chowdh- ing yearly. In many regions of the world, the impacts of ury and Flentje (2014) and Kazmi et al. (2017) have landslides have been cataclysmic with damages worth reported the devastating impacts of landslides and pos- several millions of dollars in monetary losses, and are re- sible mitigation measures such as slope modification, sponsible for thousands of deaths and injuries annually. bioengineering, slope reinforcement, construction of Regularly, they cause long-term economic disruption, retaining walls, check dams etc. Authors like Ogbukagu population displacement, and negative effects on the (1976) and Okagbue (1986, 1992) have reported the natural environment, (Ancuţa et al. 2007; Highland and mechanisms of landslides in Nanka area and some of the Bobrowsky 2008; Igwe, 2015). Though landslides can devastating socio-economic impacts. In this research, we occur in different parts of the world, their mechanisms, will present our latest findings on the impacts of land- impacts and management is often determined by the slides to the over-all environment, comprising the geology, hydrogeology and geomorphology of the area. It people, their livestock and farms, structures, and the means that the risk associated with landslides is area morphology of the earth’s surface in Nanka area, dependent, which implies that mitigation techniques Orumba North Local Government Area, Anambra State, Southeast Nigeria (Fig. 1). Our report will also suggest mitigation measures to manage this phenomenon. * Correspondence: unachuku@gmail.com Department of Applied Geology, Wesley University, Ondo, Nigeria Full list of author information is available at the end of the article © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 2 of 12 Fig. 1 Location of study area Geology and geomorphology of the study area monocrystalline quartz (Nwajide 1977; Egboka and The study area is part of the Campanian to Eocene Okpoko 1984; Ezechi 1987 and Okagbue 1992). Anambra basin, which was formed as a result of com- Nanka is part of a regional system of escarpments; it pressive movement along the Northeast – Southwest lies near the minor escarpment of the Awka-Orlu up- axis of the Aptian - Santonian Abakaliki basin. This tec- lands formed by the Nanka Formation (Okagbue, 1992). tonic activity resulted in the folding and uplift of the Nanka has an undulating topography ranging from gen- Abakaliki basin into an anticlinorium and formed the tly sloping lowlands to slightly steep highlands (Figs. 3 Anambra basin on the Southwest and Afikpo syncline and 4). Based on documented landslides, Nanka eleva- on the Southeast. This enforced the depocenter to move tion ranges from 170 to 373 m above sea level, with to the recently formed Anambra Basin and the Afipko average elevation of 222 m. syncline (Murat 1972; Benkhelil 1989; Kogbe 1989; Oboh-Ikuenobe et al., 2005 and Nwachukwu et al. Climate 2011). The Nanka Formation (Early Eocene) which is South-East Nigeria is generally a tropical climate with underlain by Imo Shale (Paleocene) is the main exposed two distinct (rainy and dry) seasons. The rainy season Formation in the study area (Nwajide 1980). begins in April and ends in October, while the dry sea- Nanka Formation consist of unconsolidated or poorly son begins in November and ends in March. Records of consolidated sands with thin claystone and siltstone rainfall showed high values within the months of May to bands. The sand is poorly-sorted, cross-bedded and early August when there is a rainfall break and resumes medium to coarse grained with variegated colour; yellow in late August to end of October with higher values. to brown, pink to purple to iron stained on weathered There is virtually little or no rainfall from November to surface and white to milky white on fresh surfaces March. The average monthly temperatures vary from 22 (Fig. 2). The Clay/Shale beds are dark grey to grey with °C to 28 °C in the rainy season and between 28 °C and specks of mica, pyrite and gypsum in some places. Sand- 32 °C in the dry season. Intense and short duration rain- stone consists of quartz arenites with predominantly falls characterize the outset of rainy season. Annual Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 3 of 12 Fig. 2 Geologic map of Nanka rainfall ranges between 1500 mm to 2500 mm (Monanu Nature of landslides in Nanka and Inyang 1975; Ezemonye and Emeribe 2012 and Igwe One hundred and twenty (120) landslides were docu- et al. 2013). South-East Nigeria lies in the Lowland rainfor- mented within Nanka and its neighbouring towns of est natural vegetation belt with evergreen trees in the Oko and Agulu. We have grouped them under Nanka south, and gradually gives way northward to rainfall- because they are all underlain by the Nanka Formation savannah forest characterized by trees interspersed with and have similar mechanism of occurrence. Complex grass. The natural vegetation of the study area has been gully erosions have devastated Nanka area; the most disturbed over the years through forest clearance and bush spectacular is the Nanka-Oko gully erosion, which has burning for agriculture (Ibeanu and Umeji 2003). The pre- destroyed several homes and farmlands (Fig. 5). Most of cipitation regime is fairly regular. The rainfall amount in the landslides in Nanka occur along the gully channels. the month of May appear to support the idea that intense/ The slope angles created by these gullies vary from 36 - o o short duration rainfall is a main factor in landslides trigger 65 , with average slope angle of 48 . The characteristics (Igwe et al. 2013). Rapid landslides are usually initiated of the soils, the nature of the slopes, and rainfall com- within hours of intense rainfall (Terlien 1998). bine to produce slides that vary in scale and structure. Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 4 of 12 Fig. 3 The 3D ASTER DEM revealing the geomorphologic features and landslide Areas inSoutheast Nigeria Most of the landslides in Nanka are shallow translational by rainfalls during the rainy season and the swelling debris slides, slumps and occasional debris topples (Fig. behaviour of active clay minerals in interbedded clay/ 5). Landslide depths in this area ranges from 0.2–1.8 m, shale units have been reported as the mechanism of with average depth of 0.74 m. Okagbue (1992) stated landslides in Nanka (Egboka and Okpoko 1984;Okag- that topples (rotational falling) occasionally occur in the bue 1992). Brand et al. (1984) noted that majority of area. The impermeable clay beds separating the lower landslides in Hong Kong were induced by concen- sand horizon acts as gliding plane for several large trated, short-duration rainfalls of high intensity. Ma- slumps (Ogbukagu 1976; Okagbue 1986). Many of the tric suction at shallow depths partially or completely slumps are known to occur within the near homoge- disappears when rainfall infiltrates the soil and there- neous soil overburden (Okagbue, 1992). fore, the slope may fail. The reduction in soil matric Slopes in Nanka frequently fail during short or long, suction decreases the soil shear strength, and conse- intense rainfalls mostly at the beginning of rainy sea- quentlycausesthe slopetobecomeunstableand fail son. The effects of high pore water pressures created (Li et al. 2005). Fig. 4 The 3D ASTER DEM revealing the geomorphologic features and landslide dominatedAreas in Nanka Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 5 of 12 Results from the laboratory analysis were used to per- form two-dimensional slope stability analysis using the general limit equilibrium (GLE) method integrated in the software SLOPE/W of GeoStudio for stability ana- lysis of slopes. To study the relationship between the slope angle and the sliding mass volume, we constructed a two-dimensional homogeneous slope model from Slope/W GeoStudio version (2016) software based on the analysis type of the Morgenstern-Price limit equilib- rium method. Morgenstern and Price (1965) method was preferred, because they allowed for various user-specified interslice force functions. The half-sine user specified interslice force function available in SLOPE/W was implemented to compute for factor of safety. The entry and exit function was used to find the Fig. 5 Nanka complex gully erosion slip centre and the potential failure surface. The slope material parameters needed for this simula- The triggered slides in Nanka are mainly shallow with tion are angle of internal friction, cohesion and unit short run-out distance (< 200 m). The average landslide weight. Based on geological and geotechnical data, inter- run-out distance in this area is 13.8 m. pretations were made and best possible mitigation mea- sures were suggested. Methods Field surveys covering Nanka and neighbouring areas Results and discussion were undertaken to study landslides and their impacts to Geotechnical characteristics the over-all environment. Landslides were described in Results of geotechnical characteristics of the slope mate- the field to allow classification according to Cruden and rials (Table 1) showed that samples 1, 2 and 3 are Varnes (1996), Hungr et al. (2001). The extent of the medium-fine clayey sand materials while sample 4 is landslide bodies were mapped from crown to toe of rup- greyish silty clay. The average range of coefficient of per- − 6 ture. Details of the nature and types of materials associ- meability of samples 1, 2 and 3 is 1.11 X 10 - 1.47 X − 6 − 8 ated with slope movement, slope angles, depths, width 10 , while sample 4 is 9.51 X 10 m/sec. The very low and lengths of landslide, elevation of landslide source permeability to impermeable silty clay units that sepa- and toe were also recorded. Measurements essential to rates the sand units in the area have been identified to the preparation of an adequate geologic map were taken be the gliding plane for several landslides in the area at various locations within the study area. Geological (Okagbue 1992). The average range of the angle of in- cross-sections were then made to study the morphology ternal friction and cohesion of samples 1, 2 and 3 are o o of the slides. Global positioning system (GPS) was used (28 -30 ) and (5 kPa – 8 kPa) respectively (Figs. 6, 7, to get the exact locations of landslides in the field. These 8), while sample 4 is 8 and 43 kPa respectively (Fig. 9). measurements were integrated with ASTER data to pre- Samples 1, 2 and 3 are non-plastic, but have average li- pare adequate 3D digital elevation map (DEM) of the quid limits (LL) range of 23–26. Also, the liquid limits study area revealing landslide locations. Samples were (LL), plastic limits (PL) and plasticity index (PI) of sample collected at different landslide locations and subjected to 4 are 75, 36 and 39 respectively. Sowers and Sowers laboratory analysis to obtain geotechnical data and (1970) reported that PI > 31 should be considered high parameters needed for limit equilibrium simulations. and indicates high content of expansive clay while Bell Samples were analyzed for grain size (using the (2007) classified clays with Liquid Limit (LL) range of 70– Unified Soil Classification System, USCS). A set of Brit- 90 as very high plasticity. This could explain why the clays ish standard (BS) Sieves of sizes: 4.75 mm, 3.35 mm, in this area serve as gliding plane to several landslides. 2.36 mm, 1.18 mm, 600 μm, 425 μm, 300 μm, 212 μm, From our geotechnical results, we propose that shal- 150 μm, 75 μm, and receiver (pan) was used for this low landslides in this area are triggered by water infiltra- analysis. Atterberg limits, coefficient of permeability, tion into high topographic gradient slopes where the compaction, and triaxial tests were determined in poorly or unconsolidated sands overlies the less perme- accordance with BS 1377 (1990). Results from the able clay/shale units. As the rainfall infiltrates the soil, triaxial tests were plotted on a Mohr stress diagram the matric suction at shallow depths partially or com- (MohrView) to obtain values for cohesion and angle of pletely disappears and therefore, the slope may fail. The internal resistance. reduction in soil matric suction decreases the soil shear Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 6 of 12 Table 1 Selected geotechnical characteristics of the slope materials Sample Cohesion Angle of internal friction Coefficient of permeability Liquid Plastic Plasticity Grain size number (kPa) ( ) (m/sec) Limit Limit Index −6 Nanka Sample 1 5 28 1.11 X 10 24 Np Np Clayey Sand −6 Nanka Sample 2 6 29 1.47 X 10 23 Np Np Clayey Sand −6 Nanka Sample 3 8 30 1.28 X 10 26 Np Np Clayey Sand −8 Nanka Sample 4 43 8 9.51 X 10 75 36 39 Silty Clay Np Non-plastic strength, and consequently causes the slope to become 110 m in some places with a width of over 500 m based unstable and fail (Li et al. 2005). If the soil is completely on field estimations. Landslides occur along the gully saturated, soil matric suction will disappear completely slopes especially during the rainy season, regular slides and a perched water table with positive pore-water along the slopes persistently increase the width of the pressure will develop in the soil, this will lead to a reduc- gullies and modify the morphology of the area. Yalcin tion of the shear strength of the soil due to rise in (2007) reported that earth’s surface is continually being pore-water pressure and loss of soil apparent cohesion, modified by gravitational mass movements, particularly this will consequently trigger landslide (Fukuoka 1980; landslides. The surface of the earth, both on the conti- Wieczorek 1996; Li et al. 2005; Lee et al. 2012; Igwe et nents and beneath the oceans is incessantly modified by al. 2013). internal forces and the forces of gravity (landslides). The net morphologic effect of landslides is to reduce slopes Impacts of landslides in Nanka to angles at which they possess long-term stability Landslides have impacted Nanka area negatively in vari- (Schuster and Highland 2003). ous forms and dimensions, but we will focus on the Impacts of landslides on settlement area in Nanka morphology of the land area, settlement region, agricul- have been disturbing. Okagbue (1992) reported the 1988 ture and land cover. Generally, Nanka has an undulating landslide in this area that destroyed several homes and topography/morphology ranging from gently sloping over 50 families were evacuated. Interaction with some lowlands to slightly steep highlands. Morphologically, villagers revealed that over the years, several homes have Nanka keeps evolving due to the effects of landslides been destroyed by landslide and many more are also and erosion. Gullies are widespread in Southeast Nigeria, threatened by this menace. The continuous increase of but Nanka gully complex is the most outstanding in gully erosions which gives rise to unstable slopes that fail terms of extent and natural destruction (Okagbue 1992). during short intense or long-term rainfalls have been the The complex gullies have high slope angles ranging from major cause of landslides in Nanka. Field observation o o 36 -65 based on field measurements, though in some showed some destroyed and abandoned buildings due to inaccessible areas, the slope angles were estimated to be this hazard (Fig. 10). Lives and properties worth several o o between 70 and 80 . The depth of the gullies are over millions of dollars have been lost due to the impact of Fig. 6 Nanka sample 1 Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 7 of 12 Fig. 7 Nanka sample 2 landslides on settlement areas in Nanka and its sur- difficult for land cover recovery. Majorly, landslides rounding towns like Agulu, Oko and Ekwulobia. occur in Nanka during the rainy season, which is from Impacts of landslides on agriculture in the study area is April to October, but interactions with local field guides overwhelming, many farmlands have been destroyed by and report from Okagbue (1992) revealed that landslides landslides and some have been abandoned due to threats also occur during the dry season, which is between No- posed by this phenomenon. Loss of agricultural lands and vember and March. Bush burning for agricultural pur- livestock have been reported in this area by Okagbue pose also destroys the land cover and increase the (1992). Some of the residents who are mainly farmers have potential of landslide occurrences. Generally, landslides been affected tremendously because their main source of have negatively affected the topography/morphology, livelihood have been destroyed by landslides. Also, some settlement areas, agricultural lands and the land cover in have abandoned their farming business and engaged in Nanka area. petty trading due to this hazard. Agricultural lands keep reducing yearly and there is need for urgent intervention Mitigation of landslides in Nanka so that more lands will not be lost. Because of enormous damages caused by landslides glo- Landslides in Nanka have negative impact on the vege- bally, research and government institutions have for a tation/land cover such that many areas have been left long time attempted to decrease damages caused by it bare by continuous landslides. Widespread stripping of (Yalcin 2007). Landslides often are characterized as local forest and vegetation cover by landslides has been noted problems, but their effects and costs frequently cross in many parts of the world (Shaikh et al. 2005; Yalcin local jurisdictions and may become State or national 2007). The nature of the slope materials and the long problems (Highland and Bobrowsky 2008). This is the period of time that landslides occur in the area make it present state of Nanka; the cost of repairing damages Fig. 8 Nanka sample 3 Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 8 of 12 Fig. 9 Nanka sample 4 caused by landslides is beyond the local government and similar method was proposed in East black sea region, is now a national problem. Much of the problems caused Turkey by Yalcin (2007). The drainage is for the whole by landslide in this area could have been curtailed long Nanka area, this will solve the problem of hill cuttings time ago, but due to lack of integrating landslide hazard by erosion. Once the area is well drained, the effect of information into local governments long-term plans. pore pressure in combination with gully erosions will be The Federal Government of Nigeria has spent huge drastically reduced and consequently landslide occur- amounts of naira in recent times to repair damages and rences will be reduced to the barest minimum. Highland mitigate future occurrences of landslides in this area. and Bobrowsky (2008) reported that adequate drainage Field observations showed that the mitigation plans have is necessary to prevent sliding or, in the case of an exist- failed and a better approach is needed urgently (Fig. 11). ing failure, to prevent a reactivation of the movement. Our suggestions will be based on our understanding of Well-designed drainage system will not only prevent the geology, geomorphology, hydrogeology and geotech- sliding, but will also prevent reactivation of movements nical characteristics of the area. Field observation on already failed areas in Nanka. After a good drainage revealed that most parts of Nanka do not have any system is constructed, the next step will be biotechnical drainage, while some places have poor drainage systems. slope protection. According to Gray and Sotir (1996), We suggest a standard drainage system that is intercon- nected. A well-designed drainage system will decrease the amount of moisture entering the soils and thereby increasing the shear strength and slope stability. A Fig. 10 Landslides and settlement area threatened by landsides Fig. 11 Failed mitigation of gully erosion in Nanka Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 9 of 12 Fig. 12 Nanka simulation at slope angle of 40 biotechnical slope protection consists of biotechnical Gray and Sotir (1996) asserted that soil bioengineering stabilization and soil bioengineering stabilization, both stabilization is a specialized subset of biotechnical of which entail the use of vegetation. Gray and Leiser stabilization in which live plant parts serve as mechan- (1982) said that biotechnical stabilization employs mech- ical elements in the slope protection system. Different anical elements in combination with biological elements plants that can stabilize the slopes should be planted by to prevent and arrest slope failures and erosion. While individuals and the government. Vetiver works very well Fig. 13 Nanka simulation at slope angle of 50 Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 10 of 12 Fig. 14 Nanka simulation at slope angle of 60 to stabilize slopes against erosion in many different envi- Limit equilibrium simulations ronments (Highland and Bobrowsky 2008). Bamboo The limit equilibrium simulations have been widely used trees also works exceptionally well in the stabilization of to analyze slope stability problems. A slope is stable if it slopes. Bamboo trees have been successfully used to meets a set need for a fixed period with an acceptable stabilize slopes in some parts of Oko and Amucha in safety factor. A factor of safety is defined as that factor by Southeast Nigeria. Farmers should be encouraged to which the shear strength of the soil must be reduced in plant trees and grasses that will help stabilize the slopes. order to bring the mass of soil into a state of limiting equi- Since Nanka area is dominated by shallow landslides, librium along a selected slip surface (Slope/W 2016). vegetation turfing is one of the most important mitiga- Most of the slope angles measured during the field o o tion measures for such areas (Singh 2010). Finally, we studies ranged between 40 and 60 , therefore, our limit suggest that landslide hazard information should be in- equilibrium simulations to model the slopes will have o o o corporated into long-term plans for Nanka and other three representative slope angles (40 ,50 and 60 ). The areas prone to landslides. The public should be aware of Table 3 Correlation of Landslide dimensions and slope angle the hazards and preventive measures. Nanka Correlations Length (m) Width (m) Slope angle (°) Length (m) 0.444 0.4887 Table 2 Summary of Statistics of Nanka (120) (120) Summary statistics of Nanka Length (m) Width (m) Slope Angle (°) Width (m) 0.444 0.289 Count 120 120 120 (120) (120) Average 12.457 2.16042 47.9583 0 0.0014 Standard deviation 10.233 1.33303 6.70431 Slope angle (°) 0.4887 0.289 Coeff. of variation 82.15% 61.70% 13.98% (120) (120) Minimum 1 0.4 36 0 0.0014 Maximum 40 7 65 Pearson correlation (Sample Size) Range 39 6.6 29 P-Value Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 11 of 12 Table 4 Principal components analysis of Nanka Component Number Eigenvalue Percent of Variance Cumulative Percentage 1 1.82005 60.668 60.668 2 0.713122 23.771 84.439 3 0.466831 15.561 100 slope material parameters needed for this simulation are accounted for 100% of the total variance in the data set cohesion, angle of internal friction, and unit weight with each component 1, 2 and 3 having percentage based on the analysis type of the Morgenstern and Price variance of 60.668, 23.771 and 15.561. Using their high (1965) limit equilibrium method. Morgenstern-Price is a eigenvalues, only two components were extracted as prin- general method of slices developed on the basis of limit cipal components because they accounted for 84.439% of equilibrium. It requires satisfying equilibrium of forces the total data variance. The results of PCA and Pearson and moments acting on individual blocks. correlation revealed that landslide dimensions are not cat- We decided to make the slope material parameters egorically related to the slope angle, it implies that other constant for the three simulations and vary the slope factors such as nature of the slope material, geologic struc- angles, this is to help us understand the relationship tures and hydrogeology could be more influential in deter- between slope angle and landslide dimensions, and to mining the dimensions of the landslides in Nanka area. reveal the safety map of the slope with colour map and factor of safety range. The simulations revealed that as the slope angle increases, the potential slip mass volume Conclusions increases (Figs. 12, 13, 14). This could be because of The impacts and mitigations of landslides in Nanka, slope height and slope base length obtainable in this Anambra State, Southeast Nigeria were studied. A pro- area. This means that areas with higher slope angles will cedure that involved field, laboratory and limit equilib- possibly have more potential mass volume. Our simula- rium simulations were employed in the study. A total of tions agree with our observations in the field and with 120 landslides in this area were documented during the two dimensional discrete element numerical simulations field study, they were mainly shallow landslides with by Katz et al. (2014). The factor of safety ranges at differ- depth range of 0.2 m – 1.8 m and slope angle range of o o ent slope angles revealed slight differences. The factor of 36 -65 . Laboratory data revealed that the silty clay o o o safety ranges at slope angles of 60 ,50 and 40 are sample is highly plastic with low coefficient of perme- 0.93–1.83, 0.95–1.85 and 0.97–1.87 respectively. This ability. This could explain why the clays in this area implies that areas with higher slope angles will fail serve as gliding plane for several landslides. The limit before areas with smaller slope angles under similar con- equilibrium simulations revealed that as the slope angle ditions. This is in agreement that lowering of slope increases, the potential slip mass volume increases, angles in a given area will make the slope more stable. which implies that areas with higher slope angles will possibly have more potential mass volume. Our study of Statistical analysis of landslide dimensions and slope landslides in Nanka revealed that the topography/ angle in Nanka morphology of the area have been negatively affected by Table 2 shows the summary statistics for the study area, landslides. Settlement areas and agricultural lands have the table revealed that the coefficient of variation is high been terribly devastated by this phenomenon. Houses, in both landslide length and width, but minimal in the farm lands and livestock have been destroyed by land- slope angle. The high coefficient of variation is due to slides and many areas are also threatened by landslides. the large differences in landslide dimensions observed in Continuous landslides have left many areas bare thereby the field. The results of the Pearson correlations (Table 3) having negative impact on the vegetation/land cover. revealed that there is a moderate relationship between Standard mechanical drainage system have been sug- length and width of landslides in Nanka (0.444). The gested as a mitigation measure to drastically reduce the Pearson correlations showed that there is minimal rela- impacts of landslides in this area. Biotechnical slope pro- tionship between length or width of landslides with the tection was also suggested as a mitigation measure to slope angle. This suggests that the slope angle does not help stabilize unstable slopes and curb sliding. It will really determine the length or width of landslides in the also restore the vegetation/land cover in the area. areas studied. The results of the principal component Finally, we suggested that landslide hazard information analysis (PCA) shows the loading of variables on each should be incorporated into long-term plans for Nanka component and percentage of data variance in the data and other areas prone to landslides, the public should be set (Table 4). The three components shown in the table aware of the hazards and preventive measures. Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 12 of 12 Abbreviations Highland, L.M., and P. Bobrowsky. 2008. The landslide handbook—Aguide to KPa: Kilopascal; LL: Liquid Limits; M: Metres; NP: Non-plastic; PI: Plasticity understanding landslides: Reston, Virginia, U.S. Geological Survey circular 1325, 129. index; PL: Plastic limits Hungr, O., S.G. Evans, M.J. Bovis, and J.N. Hutchinson. 2001. Review of the classification of landslides of the flow type. Environmental and Engineering Acknowledgements Geoscience 7: 221–238. Not applicable. Ibeanu, A.M., and O.P. Umeji. 2003. Aspects of the Palaeocology of Okigwe cuesta, eastern Nigeria. West African journal of Archaeology 31 (1): 17–30. Funding Igwe, O. 2015. The geotechnical characteristics of landslides on the sedimentary Not applicable. and metamorphic terrains of Southeast Nigeria, West Africa. Geoenvironmental Disasters 2 (1): 1–14. Availability of data and materials Igwe, O., W. Mode, O. Nnebedum, I. Okonkwo, and I. Oha. 2013. The analysis of The dataset supporting the conclusions of this article is included within the rainfall-induced slope failures at Iva Valley area of Enugu state, Nigeria. article and its additional files. Environment and Earth Science. https://doi.org/10.1007/s12665-013-2647-x. Katz, O., J.K. Morgan, E. Aharonov, and B. Dugan. 2014. Controls on the size and Authors’ contributions geometry of landslides: Insights from discrete element numerical simulations. IO conceived the study, participated in its design and coordination. UCO Geomorphology 24: 104–113. carried out the field/laboratory work and drafted the manuscript. All authors Kazmi, D., S. Qasim, I.S.H. Harahap, S. Baharom, M. Mehmood, F.I. Siddiqui, and M. read and approved the final manuscript. Imran. 2017. Slope remediation techniques and overview of landslide risk management. Civil Engineering Journal 3 (3): 180–189. Competing interests Kogbe, C.A. 1989. Palaeogeographic history of Nigeria from Albian times. In The authors declare that they have no competing interests. Geology of Nigeria, ed. C.A. Kogbe, 257–275. Lagos: Elizabethan Publ. Co. Lateltin, O., C. Haemmig, H. Raetzo, and C. Bonnard. 2005. Landslide risk management in Switzerland. Landslides 2: 313–320. Publisher’sNote Lee, C.C., L.S. Zeng, C.H. Hsieh, and C.Y. Yu. 2012. Determination of mechanisms Springer Nature remains neutral with regard to jurisdictional claims in and hydrogeological environments of Gangxianlane landslides using published maps and institutional affiliations. geoelectrical and geological data in Central Taiwan. Environment and Earth Science 66 (6): 1641–1651. Author details Li, A.G., Z.Q. Yue, L.G. Tham, C.F. Lee, and K.T. Law. 2005. Field-monitored 1 2 Department of Geology, University of Nigeria, Nsukka, Nigeria. Department variations of soil moisture and matric suction in a saprolite slope. Canadian of Applied Geology, Wesley University, Ondo, Nigeria. Geotechnical Journal 42: 13–26. Monanu, S., and F. Inyang. 1975. Climatic regimes. In Nigeria in Maps (ed. by, ed. Received: 16 January 2019 Accepted: 9 May 2019 G.E.K. Ofomata, 27–29. Benin: Ethiope Publ. House. Morgenstern, N.R., and V.E. Price. 1965. The analysis of the stability of general slip surfaces. Geotechnique 15: 79–93. References Murat, R.C. 1972. 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In Proceedings of the 4th inter-national symposium on Nwajide, C.S. 1980. Eocene tidal sedimentation in the Anambra Basin, southern landslides, vol. 1, 377–384. Toronto: BiTech publishers, Vancouver, B.C. Nigeria. Sedimentary Geology 25: 189–207. British Standards 1377. 1990. Methods of testing soil for civil engineering purposes, Nwajide, S.C. 1977. Sedimentology and stratigraphy of the Nank sand. M.Phil. 1990. London: British Standards Institute. Thesis, Dept. of Geology. Nsukka: University of Nigeria. Chen, C.Y. 2008. Sedimentary impacts from landslides in the Tachia River basin. Oboh-Ikuenobe, F.E., G.C. Obi, and C.A. Jamarillo. 2005. Lithofacies, palynofacies Taiwan. Geomorphology. https://doi.org/10.1016/j.geomorph.2008.10.009. and sequence stratigraphy of paleogene strata in southeastern Nigeria. Chowdhury, R.N., and P.N. Flentje. 2014. Mitigation of landslide impacts, strategies Journal of African Earth Sciences 41: 79–101. and challenges for the 21st century. 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Biotechnical and soil bioengineering slope Sea region, Turkey. Environmental Engineering Science 24 (6): 821–833. stabilization – A practical guide for Erosion control, 378. New York: Wiley. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

Landslide impacts and management in Nanka area, Southeast Nigeria

Geoenvironmental Disasters , Volume 6 (1) – May 16, 2019

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Environment; Environment, general; Earth Sciences, general; Geography, general; Geoecology/Natural Processes; Natural Hazards; Environmental Science and Engineering
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Abstract

The devastating impacts and mitigation measures for landslides in Nanka, Southeast Nigeria were studied. A procedure encompassing field mapping, laboratory analysis and limit equilibrium simulations were employed in the study. A total of 120 landslides in this area were documented during the field study. They were mainly shallow o o landslides with depth range of 0.2 m – 1.8 m and slope angle range of 36 -65 . This study revealed that landslides in this area occur mainly during the rainy season, and are triggered by water infiltration in slopes with high gradient, where poorly consolidated sands overlies less permeable silty clay units in the Nanka Formation. The increase in pore water pressure due to soil saturation leads to reduction of the shear strength and loss of apparent cohesion. Geotechnical laboratory analysis showed that the thin silty clay soils found in the area have very high plasticity, while the sands are non-plastic. The limit equilibrium simulations revealed that as the slope angle increases, the potential slip mass volume increases. Our study in Nanka revealed that landslides have negatively affected the topography/morphology, settlement areas, agricultural lands and vegetation/land cover of the area and many areas are also threatened by this phenomenon. Standard mechanical drainage system and biotechnical slope protection have been suggested as mitigation measures. Finally, we suggested that landslide hazard information should be incorporated into long-term plans for Nanka and other areas prone to landslides, the public should be aware of the hazards and preventive measures. Keywords: Impacts, Limit equilibrium, Mitigation, Biotechnical Introduction should be area dependent, though some management Landslide occurs in different parts of the world, under measures can be universally applied. Globally, many all climatic conditions and environments. It is a major researchers such as Dai et al. (2002), Schuster and High- natural hazard in which its global concern keeps increas- land (2003), Lateltin et al. (2005), Chen (2008), Chowdh- ing yearly. In many regions of the world, the impacts of ury and Flentje (2014) and Kazmi et al. (2017) have landslides have been cataclysmic with damages worth reported the devastating impacts of landslides and pos- several millions of dollars in monetary losses, and are re- sible mitigation measures such as slope modification, sponsible for thousands of deaths and injuries annually. bioengineering, slope reinforcement, construction of Regularly, they cause long-term economic disruption, retaining walls, check dams etc. Authors like Ogbukagu population displacement, and negative effects on the (1976) and Okagbue (1986, 1992) have reported the natural environment, (Ancuţa et al. 2007; Highland and mechanisms of landslides in Nanka area and some of the Bobrowsky 2008; Igwe, 2015). Though landslides can devastating socio-economic impacts. In this research, we occur in different parts of the world, their mechanisms, will present our latest findings on the impacts of land- impacts and management is often determined by the slides to the over-all environment, comprising the geology, hydrogeology and geomorphology of the area. It people, their livestock and farms, structures, and the means that the risk associated with landslides is area morphology of the earth’s surface in Nanka area, dependent, which implies that mitigation techniques Orumba North Local Government Area, Anambra State, Southeast Nigeria (Fig. 1). Our report will also suggest mitigation measures to manage this phenomenon. * Correspondence: unachuku@gmail.com Department of Applied Geology, Wesley University, Ondo, Nigeria Full list of author information is available at the end of the article © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 2 of 12 Fig. 1 Location of study area Geology and geomorphology of the study area monocrystalline quartz (Nwajide 1977; Egboka and The study area is part of the Campanian to Eocene Okpoko 1984; Ezechi 1987 and Okagbue 1992). Anambra basin, which was formed as a result of com- Nanka is part of a regional system of escarpments; it pressive movement along the Northeast – Southwest lies near the minor escarpment of the Awka-Orlu up- axis of the Aptian - Santonian Abakaliki basin. This tec- lands formed by the Nanka Formation (Okagbue, 1992). tonic activity resulted in the folding and uplift of the Nanka has an undulating topography ranging from gen- Abakaliki basin into an anticlinorium and formed the tly sloping lowlands to slightly steep highlands (Figs. 3 Anambra basin on the Southwest and Afikpo syncline and 4). Based on documented landslides, Nanka eleva- on the Southeast. This enforced the depocenter to move tion ranges from 170 to 373 m above sea level, with to the recently formed Anambra Basin and the Afipko average elevation of 222 m. syncline (Murat 1972; Benkhelil 1989; Kogbe 1989; Oboh-Ikuenobe et al., 2005 and Nwachukwu et al. Climate 2011). The Nanka Formation (Early Eocene) which is South-East Nigeria is generally a tropical climate with underlain by Imo Shale (Paleocene) is the main exposed two distinct (rainy and dry) seasons. The rainy season Formation in the study area (Nwajide 1980). begins in April and ends in October, while the dry sea- Nanka Formation consist of unconsolidated or poorly son begins in November and ends in March. Records of consolidated sands with thin claystone and siltstone rainfall showed high values within the months of May to bands. The sand is poorly-sorted, cross-bedded and early August when there is a rainfall break and resumes medium to coarse grained with variegated colour; yellow in late August to end of October with higher values. to brown, pink to purple to iron stained on weathered There is virtually little or no rainfall from November to surface and white to milky white on fresh surfaces March. The average monthly temperatures vary from 22 (Fig. 2). The Clay/Shale beds are dark grey to grey with °C to 28 °C in the rainy season and between 28 °C and specks of mica, pyrite and gypsum in some places. Sand- 32 °C in the dry season. Intense and short duration rain- stone consists of quartz arenites with predominantly falls characterize the outset of rainy season. Annual Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 3 of 12 Fig. 2 Geologic map of Nanka rainfall ranges between 1500 mm to 2500 mm (Monanu Nature of landslides in Nanka and Inyang 1975; Ezemonye and Emeribe 2012 and Igwe One hundred and twenty (120) landslides were docu- et al. 2013). South-East Nigeria lies in the Lowland rainfor- mented within Nanka and its neighbouring towns of est natural vegetation belt with evergreen trees in the Oko and Agulu. We have grouped them under Nanka south, and gradually gives way northward to rainfall- because they are all underlain by the Nanka Formation savannah forest characterized by trees interspersed with and have similar mechanism of occurrence. Complex grass. The natural vegetation of the study area has been gully erosions have devastated Nanka area; the most disturbed over the years through forest clearance and bush spectacular is the Nanka-Oko gully erosion, which has burning for agriculture (Ibeanu and Umeji 2003). The pre- destroyed several homes and farmlands (Fig. 5). Most of cipitation regime is fairly regular. The rainfall amount in the landslides in Nanka occur along the gully channels. the month of May appear to support the idea that intense/ The slope angles created by these gullies vary from 36 - o o short duration rainfall is a main factor in landslides trigger 65 , with average slope angle of 48 . The characteristics (Igwe et al. 2013). Rapid landslides are usually initiated of the soils, the nature of the slopes, and rainfall com- within hours of intense rainfall (Terlien 1998). bine to produce slides that vary in scale and structure. Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 4 of 12 Fig. 3 The 3D ASTER DEM revealing the geomorphologic features and landslide Areas inSoutheast Nigeria Most of the landslides in Nanka are shallow translational by rainfalls during the rainy season and the swelling debris slides, slumps and occasional debris topples (Fig. behaviour of active clay minerals in interbedded clay/ 5). Landslide depths in this area ranges from 0.2–1.8 m, shale units have been reported as the mechanism of with average depth of 0.74 m. Okagbue (1992) stated landslides in Nanka (Egboka and Okpoko 1984;Okag- that topples (rotational falling) occasionally occur in the bue 1992). Brand et al. (1984) noted that majority of area. The impermeable clay beds separating the lower landslides in Hong Kong were induced by concen- sand horizon acts as gliding plane for several large trated, short-duration rainfalls of high intensity. Ma- slumps (Ogbukagu 1976; Okagbue 1986). Many of the tric suction at shallow depths partially or completely slumps are known to occur within the near homoge- disappears when rainfall infiltrates the soil and there- neous soil overburden (Okagbue, 1992). fore, the slope may fail. The reduction in soil matric Slopes in Nanka frequently fail during short or long, suction decreases the soil shear strength, and conse- intense rainfalls mostly at the beginning of rainy sea- quentlycausesthe slopetobecomeunstableand fail son. The effects of high pore water pressures created (Li et al. 2005). Fig. 4 The 3D ASTER DEM revealing the geomorphologic features and landslide dominatedAreas in Nanka Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 5 of 12 Results from the laboratory analysis were used to per- form two-dimensional slope stability analysis using the general limit equilibrium (GLE) method integrated in the software SLOPE/W of GeoStudio for stability ana- lysis of slopes. To study the relationship between the slope angle and the sliding mass volume, we constructed a two-dimensional homogeneous slope model from Slope/W GeoStudio version (2016) software based on the analysis type of the Morgenstern-Price limit equilib- rium method. Morgenstern and Price (1965) method was preferred, because they allowed for various user-specified interslice force functions. The half-sine user specified interslice force function available in SLOPE/W was implemented to compute for factor of safety. The entry and exit function was used to find the Fig. 5 Nanka complex gully erosion slip centre and the potential failure surface. The slope material parameters needed for this simula- The triggered slides in Nanka are mainly shallow with tion are angle of internal friction, cohesion and unit short run-out distance (< 200 m). The average landslide weight. Based on geological and geotechnical data, inter- run-out distance in this area is 13.8 m. pretations were made and best possible mitigation mea- sures were suggested. Methods Field surveys covering Nanka and neighbouring areas Results and discussion were undertaken to study landslides and their impacts to Geotechnical characteristics the over-all environment. Landslides were described in Results of geotechnical characteristics of the slope mate- the field to allow classification according to Cruden and rials (Table 1) showed that samples 1, 2 and 3 are Varnes (1996), Hungr et al. (2001). The extent of the medium-fine clayey sand materials while sample 4 is landslide bodies were mapped from crown to toe of rup- greyish silty clay. The average range of coefficient of per- − 6 ture. Details of the nature and types of materials associ- meability of samples 1, 2 and 3 is 1.11 X 10 - 1.47 X − 6 − 8 ated with slope movement, slope angles, depths, width 10 , while sample 4 is 9.51 X 10 m/sec. The very low and lengths of landslide, elevation of landslide source permeability to impermeable silty clay units that sepa- and toe were also recorded. Measurements essential to rates the sand units in the area have been identified to the preparation of an adequate geologic map were taken be the gliding plane for several landslides in the area at various locations within the study area. Geological (Okagbue 1992). The average range of the angle of in- cross-sections were then made to study the morphology ternal friction and cohesion of samples 1, 2 and 3 are o o of the slides. Global positioning system (GPS) was used (28 -30 ) and (5 kPa – 8 kPa) respectively (Figs. 6, 7, to get the exact locations of landslides in the field. These 8), while sample 4 is 8 and 43 kPa respectively (Fig. 9). measurements were integrated with ASTER data to pre- Samples 1, 2 and 3 are non-plastic, but have average li- pare adequate 3D digital elevation map (DEM) of the quid limits (LL) range of 23–26. Also, the liquid limits study area revealing landslide locations. Samples were (LL), plastic limits (PL) and plasticity index (PI) of sample collected at different landslide locations and subjected to 4 are 75, 36 and 39 respectively. Sowers and Sowers laboratory analysis to obtain geotechnical data and (1970) reported that PI > 31 should be considered high parameters needed for limit equilibrium simulations. and indicates high content of expansive clay while Bell Samples were analyzed for grain size (using the (2007) classified clays with Liquid Limit (LL) range of 70– Unified Soil Classification System, USCS). A set of Brit- 90 as very high plasticity. This could explain why the clays ish standard (BS) Sieves of sizes: 4.75 mm, 3.35 mm, in this area serve as gliding plane to several landslides. 2.36 mm, 1.18 mm, 600 μm, 425 μm, 300 μm, 212 μm, From our geotechnical results, we propose that shal- 150 μm, 75 μm, and receiver (pan) was used for this low landslides in this area are triggered by water infiltra- analysis. Atterberg limits, coefficient of permeability, tion into high topographic gradient slopes where the compaction, and triaxial tests were determined in poorly or unconsolidated sands overlies the less perme- accordance with BS 1377 (1990). Results from the able clay/shale units. As the rainfall infiltrates the soil, triaxial tests were plotted on a Mohr stress diagram the matric suction at shallow depths partially or com- (MohrView) to obtain values for cohesion and angle of pletely disappears and therefore, the slope may fail. The internal resistance. reduction in soil matric suction decreases the soil shear Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 6 of 12 Table 1 Selected geotechnical characteristics of the slope materials Sample Cohesion Angle of internal friction Coefficient of permeability Liquid Plastic Plasticity Grain size number (kPa) ( ) (m/sec) Limit Limit Index −6 Nanka Sample 1 5 28 1.11 X 10 24 Np Np Clayey Sand −6 Nanka Sample 2 6 29 1.47 X 10 23 Np Np Clayey Sand −6 Nanka Sample 3 8 30 1.28 X 10 26 Np Np Clayey Sand −8 Nanka Sample 4 43 8 9.51 X 10 75 36 39 Silty Clay Np Non-plastic strength, and consequently causes the slope to become 110 m in some places with a width of over 500 m based unstable and fail (Li et al. 2005). If the soil is completely on field estimations. Landslides occur along the gully saturated, soil matric suction will disappear completely slopes especially during the rainy season, regular slides and a perched water table with positive pore-water along the slopes persistently increase the width of the pressure will develop in the soil, this will lead to a reduc- gullies and modify the morphology of the area. Yalcin tion of the shear strength of the soil due to rise in (2007) reported that earth’s surface is continually being pore-water pressure and loss of soil apparent cohesion, modified by gravitational mass movements, particularly this will consequently trigger landslide (Fukuoka 1980; landslides. The surface of the earth, both on the conti- Wieczorek 1996; Li et al. 2005; Lee et al. 2012; Igwe et nents and beneath the oceans is incessantly modified by al. 2013). internal forces and the forces of gravity (landslides). The net morphologic effect of landslides is to reduce slopes Impacts of landslides in Nanka to angles at which they possess long-term stability Landslides have impacted Nanka area negatively in vari- (Schuster and Highland 2003). ous forms and dimensions, but we will focus on the Impacts of landslides on settlement area in Nanka morphology of the land area, settlement region, agricul- have been disturbing. Okagbue (1992) reported the 1988 ture and land cover. Generally, Nanka has an undulating landslide in this area that destroyed several homes and topography/morphology ranging from gently sloping over 50 families were evacuated. Interaction with some lowlands to slightly steep highlands. Morphologically, villagers revealed that over the years, several homes have Nanka keeps evolving due to the effects of landslides been destroyed by landslide and many more are also and erosion. Gullies are widespread in Southeast Nigeria, threatened by this menace. The continuous increase of but Nanka gully complex is the most outstanding in gully erosions which gives rise to unstable slopes that fail terms of extent and natural destruction (Okagbue 1992). during short intense or long-term rainfalls have been the The complex gullies have high slope angles ranging from major cause of landslides in Nanka. Field observation o o 36 -65 based on field measurements, though in some showed some destroyed and abandoned buildings due to inaccessible areas, the slope angles were estimated to be this hazard (Fig. 10). Lives and properties worth several o o between 70 and 80 . The depth of the gullies are over millions of dollars have been lost due to the impact of Fig. 6 Nanka sample 1 Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 7 of 12 Fig. 7 Nanka sample 2 landslides on settlement areas in Nanka and its sur- difficult for land cover recovery. Majorly, landslides rounding towns like Agulu, Oko and Ekwulobia. occur in Nanka during the rainy season, which is from Impacts of landslides on agriculture in the study area is April to October, but interactions with local field guides overwhelming, many farmlands have been destroyed by and report from Okagbue (1992) revealed that landslides landslides and some have been abandoned due to threats also occur during the dry season, which is between No- posed by this phenomenon. Loss of agricultural lands and vember and March. Bush burning for agricultural pur- livestock have been reported in this area by Okagbue pose also destroys the land cover and increase the (1992). Some of the residents who are mainly farmers have potential of landslide occurrences. Generally, landslides been affected tremendously because their main source of have negatively affected the topography/morphology, livelihood have been destroyed by landslides. Also, some settlement areas, agricultural lands and the land cover in have abandoned their farming business and engaged in Nanka area. petty trading due to this hazard. Agricultural lands keep reducing yearly and there is need for urgent intervention Mitigation of landslides in Nanka so that more lands will not be lost. Because of enormous damages caused by landslides glo- Landslides in Nanka have negative impact on the vege- bally, research and government institutions have for a tation/land cover such that many areas have been left long time attempted to decrease damages caused by it bare by continuous landslides. Widespread stripping of (Yalcin 2007). Landslides often are characterized as local forest and vegetation cover by landslides has been noted problems, but their effects and costs frequently cross in many parts of the world (Shaikh et al. 2005; Yalcin local jurisdictions and may become State or national 2007). The nature of the slope materials and the long problems (Highland and Bobrowsky 2008). This is the period of time that landslides occur in the area make it present state of Nanka; the cost of repairing damages Fig. 8 Nanka sample 3 Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 8 of 12 Fig. 9 Nanka sample 4 caused by landslides is beyond the local government and similar method was proposed in East black sea region, is now a national problem. Much of the problems caused Turkey by Yalcin (2007). The drainage is for the whole by landslide in this area could have been curtailed long Nanka area, this will solve the problem of hill cuttings time ago, but due to lack of integrating landslide hazard by erosion. Once the area is well drained, the effect of information into local governments long-term plans. pore pressure in combination with gully erosions will be The Federal Government of Nigeria has spent huge drastically reduced and consequently landslide occur- amounts of naira in recent times to repair damages and rences will be reduced to the barest minimum. Highland mitigate future occurrences of landslides in this area. and Bobrowsky (2008) reported that adequate drainage Field observations showed that the mitigation plans have is necessary to prevent sliding or, in the case of an exist- failed and a better approach is needed urgently (Fig. 11). ing failure, to prevent a reactivation of the movement. Our suggestions will be based on our understanding of Well-designed drainage system will not only prevent the geology, geomorphology, hydrogeology and geotech- sliding, but will also prevent reactivation of movements nical characteristics of the area. Field observation on already failed areas in Nanka. After a good drainage revealed that most parts of Nanka do not have any system is constructed, the next step will be biotechnical drainage, while some places have poor drainage systems. slope protection. According to Gray and Sotir (1996), We suggest a standard drainage system that is intercon- nected. A well-designed drainage system will decrease the amount of moisture entering the soils and thereby increasing the shear strength and slope stability. A Fig. 10 Landslides and settlement area threatened by landsides Fig. 11 Failed mitigation of gully erosion in Nanka Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 9 of 12 Fig. 12 Nanka simulation at slope angle of 40 biotechnical slope protection consists of biotechnical Gray and Sotir (1996) asserted that soil bioengineering stabilization and soil bioengineering stabilization, both stabilization is a specialized subset of biotechnical of which entail the use of vegetation. Gray and Leiser stabilization in which live plant parts serve as mechan- (1982) said that biotechnical stabilization employs mech- ical elements in the slope protection system. Different anical elements in combination with biological elements plants that can stabilize the slopes should be planted by to prevent and arrest slope failures and erosion. While individuals and the government. Vetiver works very well Fig. 13 Nanka simulation at slope angle of 50 Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 10 of 12 Fig. 14 Nanka simulation at slope angle of 60 to stabilize slopes against erosion in many different envi- Limit equilibrium simulations ronments (Highland and Bobrowsky 2008). Bamboo The limit equilibrium simulations have been widely used trees also works exceptionally well in the stabilization of to analyze slope stability problems. A slope is stable if it slopes. Bamboo trees have been successfully used to meets a set need for a fixed period with an acceptable stabilize slopes in some parts of Oko and Amucha in safety factor. A factor of safety is defined as that factor by Southeast Nigeria. Farmers should be encouraged to which the shear strength of the soil must be reduced in plant trees and grasses that will help stabilize the slopes. order to bring the mass of soil into a state of limiting equi- Since Nanka area is dominated by shallow landslides, librium along a selected slip surface (Slope/W 2016). vegetation turfing is one of the most important mitiga- Most of the slope angles measured during the field o o tion measures for such areas (Singh 2010). Finally, we studies ranged between 40 and 60 , therefore, our limit suggest that landslide hazard information should be in- equilibrium simulations to model the slopes will have o o o corporated into long-term plans for Nanka and other three representative slope angles (40 ,50 and 60 ). The areas prone to landslides. The public should be aware of Table 3 Correlation of Landslide dimensions and slope angle the hazards and preventive measures. Nanka Correlations Length (m) Width (m) Slope angle (°) Length (m) 0.444 0.4887 Table 2 Summary of Statistics of Nanka (120) (120) Summary statistics of Nanka Length (m) Width (m) Slope Angle (°) Width (m) 0.444 0.289 Count 120 120 120 (120) (120) Average 12.457 2.16042 47.9583 0 0.0014 Standard deviation 10.233 1.33303 6.70431 Slope angle (°) 0.4887 0.289 Coeff. of variation 82.15% 61.70% 13.98% (120) (120) Minimum 1 0.4 36 0 0.0014 Maximum 40 7 65 Pearson correlation (Sample Size) Range 39 6.6 29 P-Value Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 11 of 12 Table 4 Principal components analysis of Nanka Component Number Eigenvalue Percent of Variance Cumulative Percentage 1 1.82005 60.668 60.668 2 0.713122 23.771 84.439 3 0.466831 15.561 100 slope material parameters needed for this simulation are accounted for 100% of the total variance in the data set cohesion, angle of internal friction, and unit weight with each component 1, 2 and 3 having percentage based on the analysis type of the Morgenstern and Price variance of 60.668, 23.771 and 15.561. Using their high (1965) limit equilibrium method. Morgenstern-Price is a eigenvalues, only two components were extracted as prin- general method of slices developed on the basis of limit cipal components because they accounted for 84.439% of equilibrium. It requires satisfying equilibrium of forces the total data variance. The results of PCA and Pearson and moments acting on individual blocks. correlation revealed that landslide dimensions are not cat- We decided to make the slope material parameters egorically related to the slope angle, it implies that other constant for the three simulations and vary the slope factors such as nature of the slope material, geologic struc- angles, this is to help us understand the relationship tures and hydrogeology could be more influential in deter- between slope angle and landslide dimensions, and to mining the dimensions of the landslides in Nanka area. reveal the safety map of the slope with colour map and factor of safety range. The simulations revealed that as the slope angle increases, the potential slip mass volume Conclusions increases (Figs. 12, 13, 14). This could be because of The impacts and mitigations of landslides in Nanka, slope height and slope base length obtainable in this Anambra State, Southeast Nigeria were studied. A pro- area. This means that areas with higher slope angles will cedure that involved field, laboratory and limit equilib- possibly have more potential mass volume. Our simula- rium simulations were employed in the study. A total of tions agree with our observations in the field and with 120 landslides in this area were documented during the two dimensional discrete element numerical simulations field study, they were mainly shallow landslides with by Katz et al. (2014). The factor of safety ranges at differ- depth range of 0.2 m – 1.8 m and slope angle range of o o ent slope angles revealed slight differences. The factor of 36 -65 . Laboratory data revealed that the silty clay o o o safety ranges at slope angles of 60 ,50 and 40 are sample is highly plastic with low coefficient of perme- 0.93–1.83, 0.95–1.85 and 0.97–1.87 respectively. This ability. This could explain why the clays in this area implies that areas with higher slope angles will fail serve as gliding plane for several landslides. The limit before areas with smaller slope angles under similar con- equilibrium simulations revealed that as the slope angle ditions. This is in agreement that lowering of slope increases, the potential slip mass volume increases, angles in a given area will make the slope more stable. which implies that areas with higher slope angles will possibly have more potential mass volume. Our study of Statistical analysis of landslide dimensions and slope landslides in Nanka revealed that the topography/ angle in Nanka morphology of the area have been negatively affected by Table 2 shows the summary statistics for the study area, landslides. Settlement areas and agricultural lands have the table revealed that the coefficient of variation is high been terribly devastated by this phenomenon. Houses, in both landslide length and width, but minimal in the farm lands and livestock have been destroyed by land- slope angle. The high coefficient of variation is due to slides and many areas are also threatened by landslides. the large differences in landslide dimensions observed in Continuous landslides have left many areas bare thereby the field. The results of the Pearson correlations (Table 3) having negative impact on the vegetation/land cover. revealed that there is a moderate relationship between Standard mechanical drainage system have been sug- length and width of landslides in Nanka (0.444). The gested as a mitigation measure to drastically reduce the Pearson correlations showed that there is minimal rela- impacts of landslides in this area. Biotechnical slope pro- tionship between length or width of landslides with the tection was also suggested as a mitigation measure to slope angle. This suggests that the slope angle does not help stabilize unstable slopes and curb sliding. It will really determine the length or width of landslides in the also restore the vegetation/land cover in the area. areas studied. The results of the principal component Finally, we suggested that landslide hazard information analysis (PCA) shows the loading of variables on each should be incorporated into long-term plans for Nanka component and percentage of data variance in the data and other areas prone to landslides, the public should be set (Table 4). The three components shown in the table aware of the hazards and preventive measures. Igwe and Una Geoenvironmental Disasters (2019) 6:5 Page 12 of 12 Abbreviations Highland, L.M., and P. Bobrowsky. 2008. The landslide handbook—Aguide to KPa: Kilopascal; LL: Liquid Limits; M: Metres; NP: Non-plastic; PI: Plasticity understanding landslides: Reston, Virginia, U.S. Geological Survey circular 1325, 129. index; PL: Plastic limits Hungr, O., S.G. Evans, M.J. Bovis, and J.N. Hutchinson. 2001. Review of the classification of landslides of the flow type. Environmental and Engineering Acknowledgements Geoscience 7: 221–238. Not applicable. Ibeanu, A.M., and O.P. 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UCO Geomorphology 24: 104–113. carried out the field/laboratory work and drafted the manuscript. All authors Kazmi, D., S. Qasim, I.S.H. Harahap, S. Baharom, M. Mehmood, F.I. Siddiqui, and M. read and approved the final manuscript. Imran. 2017. Slope remediation techniques and overview of landslide risk management. Civil Engineering Journal 3 (3): 180–189. Competing interests Kogbe, C.A. 1989. Palaeogeographic history of Nigeria from Albian times. In The authors declare that they have no competing interests. Geology of Nigeria, ed. C.A. Kogbe, 257–275. Lagos: Elizabethan Publ. Co. Lateltin, O., C. Haemmig, H. Raetzo, and C. Bonnard. 2005. Landslide risk management in Switzerland. Landslides 2: 313–320. Publisher’sNote Lee, C.C., L.S. Zeng, C.H. Hsieh, and C.Y. Yu. 2012. 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