Landslide Susceptibility Modelling in Selected States Across SE. Nigeria

: Identification of landslide and mapping are essential for landslide risk and hazard assessment. This paper gives information on the uses of landsat imagery for mapping landslide areas ranging in size from safe area to highly prone areas. Landslide mitigation largely depends on the understanding of the nature of the factors namely: slope, soil type, lineament, lineament density, elevation, rainfall and vegetation. These factors have direct bearing on the occurrence of landslide. Identification of these factors is of paramount importance in setting out appropriate and strategic landslides control measures. Images for this study was downloaded by using remote sensing with landsat 8 ETM and aerial photos using ArcGIS 10.7 and Surfer 8 software, while Digital Elevation Model (DEM) and Google EarthPro TM were used to produce slope, drainage, lineament and elevation. From the processed landsat 8 imagery, landslide susceptibility map was produced and landslide was category into various class; low, medium and high. From the study, it was observed that Enugu and Anambra state ranges from high to medium in terms of landslide susceptibility, Imo state ranges from medium to low.


INTRODUCTION
is still rather restricted, although it is a region where Landslides (LS) are one of the most dangerous annual rainfall, increasing population pressure and geological phenomena that pose important danger to deforestation and extreme rainfall make most areas in modern developed societies, since they are closely Southeast Nigeria very sensitive to LS.This is a region connected to human and property losses and have characterized by steep slopes, intense rainfalls, deep important economic impact on the regional economy, LS soil profiles and populations highly vulnerable to is caused by major factors such as; geological, geohazards [17][18][19][20][21][22].Okagbue, [23]; Okagbue, [22] further morphological, physical and human factors [1].LS are stated that LS in southeast Nigeria is influenced by a among the most widespread hazards causing million of number of factors such as; moisture, pore pressure, dollars in damage and thousand deaths and injuries topography and human activities.Although, it is clear around the world.Although Africa does not appear as a that landslides have caused damage in severe part LS hotspot at a global scale, nevertheless, the limited of the study area, but their assessment, prediction number of scientific publications [2][3][4][5][6][7][8][9][10][11][12][13][14] 1a and b where LS occurred in success has been achieved so far.In this paper, we Anambra State.
Table 1: Lithostratigraphic framework for the Early Cretaceous-Tertiary period in southeastern Nigeria (after [31]) Location and Accessibility: The study area is located in season [25].The wet periods are characterized by Southeastern Nigeria it covers a total of three states; moderate temperatures and high relative humidity, while Anambra, Enugu and Imo State.It lies between latitude the dry periods have high temperatures and lower relative 5°10'N-7°10'N and longitude 6°35'E -7°55' E (Fig. 1).
humidity [25].The study area falls in the southern Guinea The notable rivers and Lakes that were within found Savannah, where dense forests are few and far apart.within this study area are River Oji, Niger, Imo, Nike Lake, Such forests are found in low-land areas particularly Anambra, Idemili, Njaba and Oguta Lake.The area lies where population pressure is less on land.The study area between the Anambra and Niger River basins and has a is largely dominated by short grasses at the elevated drainage pattern that is dendritic in nature.The major areas with thick forest of tall trees in the valleys and along river in the area is River Niger that runs Southeast with drainage channels.the help of tributaries, as other rivers in the area runs from southeast to northwest axis [8].See (Fig. 2) below.
Geology Setting: The study area lies within the

Topograhy and Climate of the Study Area:
The study area Delta Formation.lies within the humid tropical climate.It is characterized by two seasons: the rainy and dry seasons.The rainy season Anambra Basin: The Anambra Basin is one of the begins toward the end of March or early April and Nigerian's most important sedimentary basins and continues until the end of October or early November, comprises an almost triangular shaped embayment while the dry season is between November and March.
covering an area of about 30, 000 Km [26].The sequence Between December and February, the northeast of depositional events demonstrates a progressively trade winds sweep through the region, bringing huge deepening of the Anambra basin, from lower coastal plain dusts, fog and ultimately reducing visibility.The mean shoreline deltas to shoreline and shallow marine deposits.monthly temperatures vary from 22 to 28°C in the wet Sedimentation in the Anambra Basin commenced with the season and between 28 and 32°C in the dry season.
Campanian -Maastrichtian marine paralic shales of the Annual rainfall (based on 2012 precipitation amount) Enugu/Nkporo Formations Obaje, [27].The fluvio -deltaic ranges between 1500 and 2100 mm [24].The average sandstones of the Ajali and Owelli Formation which lie on monthly rainfall for a 30-year period ranges from less the Mamu Formation constitute its lateral equivalents in than 1 mm in the dry season to about 300 mm in the rainy most places (Table 1).
sedimentary area of Nigeria it underlain selected Niger

Southern Benue Trough (SBT):
The tectonic history of contribution of the soil and rock to LS may depends SBT, Southeastern Nigeria dates back to the Pre Albian significantly on the similarity of the response of the times.According to Burke, et al. [28]; Nwachukwu, [29] country rock in their study area and its derivative soil and see Table 1 of landslide occurrence).

MATERIALS AND METHODS of landslide occurrence).
For the purpose of this study, the research was susceptibility level of landslide occurrence).divided into four stages namely: data capture, database Very steep slope 25. 54° to 66.48° (has dangerous generation and modelling.The Google EarthProTM aerial susceptibility level of landslide occurrence).imagery was employed in obtaining the spatial coordinates of landslides that could not be accessed in Van Ranst and Shu [13] stated that slope geometry the field.In the database generation, the following between the ranges of 2.00 -68.70° contributes to loss of materials were used; ASTER 30m resolution digital soil and slope failure, as it can trigger landslide.Slope elevation model: an ASTER 30m resolution global DEM geometry is considered as one of the major factor that was developed for the study area.The slope angle, slope trigger landslide, across most part of the world.aspect, elevation and curvature thematic maps were generated from the DEM.Landsat 8 imagery: Landsat 8 Landuse/Landcover Thematic Map: The classification and ETM imagery was acquired from the United States image interpretation indicated that there are five major Geologic Survey site (www.earthexplorer.usgs.com).landcover/landuse patterns in the study area.The five This was used in the extraction of land cover and types of landcover/landuse are: rivers, trees, built-up lineament thematic maps of the study area.Google areas, grass/farmland, soils and Rocks (Fig. 5).Previous EarthProTM.: The Google EarthPro TM was used in the studies has shown that s built-up areas as well as grass extraction of road network and drainage maps of the study and farmland are been encroached upon as a result of area.The modelling and validation stages was carried out human activities the area becomes a major trigger for with the use of the ArcGIS version 10.7 and Microsoft environmental hazards such as erosion in southeastern Excel respectively.

RESULT AND DISCUSSION
Lineament Buffer: Lineaments are the linear Soil and Rock Type: From Fig. 3. Ete, Enugu-Ezike, Opi, faults, fractures, ridges, major discontinuities etc [33].Aku and part of Udi fell within deep porous red soil Lineament indicates the zone or plane of the weakness of derived from sandy deposits, Ikem, part of Eha Amufu and structural feature.Hence, the probability of landslide Amagunze fell within red and brown soils derived from occurrences increases with increase in the closeness of sandstone and shale deposits, area such as Awgu and lineaments [34].From Fig. 6 shows the spatial distribution Adani fell within reddish and brown gravely and pale soil, of the mapped landslides with respect to distance to places such as Orodo, Nkwerre, Ihuo, Ogbo-Uwu, Ihaila, drainage.Area that has a highly compacted lineament will Agulu and selected part of Enugu-Ukwu, Oguta, Ozubulu be described as area prone to landslide.While area that and Nkwerre fell within deep porous red soil derived from has with low lineament may not be easily prone to sandy deposits.Okagbue, [18] further stated that the landslide (Fig. 6).Lineament Density: Lineaments are the linear indicates the areas of high lineament density with range morpho-tectonic features of the terrain which include at 0.0817201 km .Therefore, the sum of susceptibility faults, fractures, ridges, major discontinuities [33].levels from very low, low to moderate lineament density Lineament indicates the zone or plane of the weakness of classes reveal that more than two thirds of total landslide structural feature.As the probability of landslide frequencies occurred on these low ranks of lineament occurrences increases with increase in the closeness of density classes (Fig. 7).lineaments [13].From the lineament density map shown in figure 8 the highest susceptibility level with pale red at 1 Elevation Factor: Studies have shown that landslides to 1.51541 km was found to be areas of low lineament tend to occur more often with an increase in elevation topography falls within undulating and plain and also Anbalagan,et al. [36] stated that LS hazard evaluation susceptible to landsliding.Where the slope is less than and zonation mapping stated that an elevation ranges of 100m no landslide occur therefore susceptibility level is between 196.48 m to 735.08 m in maps are said to be areas lowest.prone to landslides. Figure 10 shows the different The landslide susceptibility was classified into elevation ranges in relation to landslide in the study area.three category; low, medium and high landslide At an elevation range of 350m to 550m within Nsukka, it was observed that Nsukka, Udi and Oji-River in Udi and Oji-River axis the topography is Enugu state showed high to medium susceptibility mountainous with high elevation level susceptible to levels.This is followed by Anoacha, Orumba North and landslide.
Aguata in Anambra state which shows a moderate At an elevation range of 200m to 350m within susceptibility level to landslide occurrence least on Anaocha, Orumba North and Aguata axis the the map is Ideato North, Ideato South and Orlu in Imo topography is mostly undulating and plain and that falls between medium to low level in susceptibility equally prone to landslide.level.
susceptibility zones as shown in Fig. 9. From study The resultant susceptibility zonation maps were with ease there are movement of soil particles down re-classed into three susceptibility zones namely; slope with increase in velocity of motion of the water.high, medium and low zones respectively.The landslide While the presence of clay materials presence in soils of density distribution shows an increase in the spatial (Enugu and Anambra) serve as a gliding plane for distribution of landslides from the low to the high landslide within the area.susceptibility classes in the landslide susceptibility map produced.The landslide prone areas fall within REFERENCES the high to moderate susceptibility zones in both landslide susceptibilities.In conclusion the research 1. Alexoudia, M.N., S.B. Manolopouloub and revealed that area where LS occurred is area with soil T.H.T. Papaliangas, 2010.A Methodology for in high clay content (Enugu and Anambra state) and Landslide Risk Assessment and Management.areas with loosed soil such as sandy soil (Imo state).
Journal of Environmental Protection and Ecology, However, the soil in Imo state is said to be loosed, 11(1): 317-326.
problems because as water flows through the soil
. The SBT originated as a failed arm of the rock mantle to sliding forces.triple junction rift-ridge system, which led to the separation of Africa from South America during the The Slope Angle Thematic Map: Slope was grouped into Aptian/Albian.The opening of these arms started in mid six category based on angle range.From (Fig. 4) it was Aptian in the Southern Atlantic by crustal stretching and observed that the slope angle in the study area ranges downwarping, accompanied by the development of from 0° to 66.48°.coastal evaporites basins.It reached the Gulf of Guinea by Thus the susceptible level to landslides of the slope late Albian and extended north east, to form the ranges from; Benue-Abakaliki Trough.However the north east-south west (NE-SW) trending Benue-Abakaliki Trough is Fair slope 2.34° to 4.95° (faciliates the frequency of thought to be the result of the Pre-Albian rifting of the landslide occurrence) African Shield, prior to the opening of the south Atlantic Moderate slope 4.95° to 8.60° (has susceptibility level [30].

Fairly moderate 8 .Fig. 3 :Fig. 4 :
Fig. 3: Spatial distribution of Soil and Rock Type across the study area

Fig. 9 :
Fig. 9: Landslide susceptibility Map of the Study Area and our field and management are still not well understood, as experience suggest that LS are responsible for acute available information indicates that erosion is amongst problems in Equatorial Africa During the second half of one of the factors that trigger LS in the region [17].the 20 century the number of damaging landslides Preliminary studies indicate that while rainfall event is these events and the possibility of LS with similar However, research on landslides in southeastern Nigeria magnitudes reoccurring in these areas is high [17].LS is a widespread phenomenon.Steep slopes, high Fig.1a and b: Landslide Points at two different locations Anambra and Imo State Southeastern Nigeria