Geology and Geotechnical Investigations of the Anambra Basin, Southeastern Nigeria: Implication for Gully Erosion Hazards

Background Geologic and geotechnical conditions of soils where Nanka and Ajali Formations outcropped in Anambra Basin, Southeastern Nigeria were investigated and accessed. This was done using detailed mapping and mechanical soil laboratory tests to unravel the genesis and continued expansion of gully erosion in the study areas. Field study revealed that gully erosions are more pronounced in the study area with poor vegetation cover and a high degree of slope steepness. Grain size analysis revealed that the soils of the Nanka Formation have an average sand content value of 90.90% (sandy) and silt content value of 3.0% (low ne portions). The plasticity index of the ne portions indicates that the soils are weak plastic, with a mean value of 5.29%. The soils have an average cohesion value of 0.30kg/cm 2 indicating a very weak cohesion. The soils are highly permeable; with an average value of 2.67x10 -3 cm/sec. The compaction test further revealed that the soils are loosely compacted. The soils for the Ajali Formation have an average sand content value of 95.10% (sandy) and silt content value of 1.43% (low ne portions). The plasticity index of the ne portions indicates that the soils are weak plastic, with a mean value of 2.70%. The soils have an average cohesion value of 0.30kg/cm 2 indicating a very weak cohesion. The soils are highly permeable; with an average value of 2.70x10 -3 cm /sec. The compaction test revealed that the soils are loosely compacted. gullies in recent times. The genesis and continued expansion of gully erosion in the area is mainly linked to the geology, topography, human activities that are poorly planned, and geotechnical properties of the soils. The soil surface is also accessible to rainfall and run-off due to scanty vegetation/plant cover in the areas of study. The geotechnical properties of these areas determine their susceptibility to gully erosion (gorges) which are advancing into canyon proportions. Detailed mapping, plastic limits, low liquid limit, low plasticity, the high proportion of sands, high permeability, the shear strength, and the very loose compactness of soils from the Nanka Formation and Ajali Formation shows that the geological conditions and geotechnical composition of the soils were responsible for the initiation and propagation of the gully erosion in the study areas. The generated slope maps, digital elevation models, and land cover/land use maps from NASA SRTM 30m data exactly show the inuence of slope, elevation, and poor land use/land cover on the genesis and development of gully erosion in the study areas. Low plastic, low cohesive, and very loose compactness of soils are in line with the works of (Onwuemesi 1990) in Nsukka and its environs where Ajali Formation outcropped and (Igwe and Egbueri 2018) in Anambra Basin. The inter-bedded shale in the Nanka Formation changes in volume resulting in alternate wetting and drying thereby enhancing gulling. Also, the interbedded shale increases in volume when wet and becomes sticky and plastic during the rains. This is in line with the considerable study by (Egboka et al. 1983; Egboka and Okpoko 1984; Igwe and Egbueri 2018; Egboka Based on the independent studies by (Egboka et al. Egboka Okpoko Igwe Egbueri Egboka and conrmed by this study, such interbedded shale formed a dry thick layer during the dry season causing contraction of clay and eventually led to soil fracture which is also conveyed to the sandy units. The shale is soaked with water after rainfall; the clay minerals swell up and establish a susceptibility to slide. The thick layers of sand by the plastic shale usually slide down-dip in the gully with the shale acting as a lubricator. The characteristics of the Nanka Formation, rainfall-runoff, long hilly and steepness of the slope, poor land cover, low plasticity, a high proportion of sands, high permeability, the shear strength, loose compactness of soils, and human activities enhanced the initiation and continued expansion of gully erosion in the study area. These features led to carving, piping, and landslide resulting in a step-like gully cross-section (Fig. 14) that is displayed in the majority of the affected areas where the Nanka Formation outcropped. Different colours of the soils ranging from light grey, white, pink in the study area where Ajali Formation outcropped shows different heat-releasing and heat-absorbing capacity of the soils. These reveal the non-co-existence of expansion and contraction of the soils leading to structural damages of the Ajali Formation. The friability of the Ajali Formation, loose structure, low degree of diagenesis, poorly bonded mechanism, low compressive strength, the rapid disintegration of soil during rainfall and run-off, low soil fertilization, long hilly and steepness of the slope, poor land cover, and faulty land usage, low plasticity, a high proportion of sands, high permeability, the shear strength, the very loose compactness of soils, and human activities contributed to initiation and continued gullying erosion expansion. These features result in slumping and sliding movements in the affected areas. A schematic representation of the gullies in the study area shows a convex gully cross-section (Fig. 15). The danger of this badland degradation was found in all the gully sites where gorges are about 3.2m.


Background
Gully erosion is an episode that is devastating the scenery of southeastern Nigeria and one of the most threatened global environmental hazards. The gully started in the middle of the nineteenth century, about 170 years ago with the initiation and propagation of narrow channels which rapidly widened by erosion into major gullies, gorges, and canyon proportions (Egboka and Okpoko 1984). The rate of gully growth in southeastern Nigeria is estimated at 20-50 m/year (Egboka and Okpoko 1984). Over 2,800 active erosion sites comprising of over 1000 in Anambra, 300 in Imo, 500 in Abia, 500 in Enugu, and 500 in Ebonyi states were relayed by World Igbo Environmental Foundation (WIFE) (Ojukwu 2018). Gully erosion is a well-de ned water-worn channel (Monkhouse and Small 1978; Abdulfatai et al. 2014). It involves detachment and transport of soil particles by natural agents such as gravity, running water, ice mass, wind, freezethaw, and anthropogenic from the upland topmost units (Fernandez et al. 2003; Gela 2011; Ashiagbor et al. 2013; Okengwo et al. 2015). This results in sediment deposit at the river networks leading to river morphological changes and reservoir sedimentation problems (Fernandez et al. 2003; Gela 2011; Ashiagbor et al. 2013; Okengwo et al. 2015). A lot of sediments are transported which culminates in siltation and clogging up of reservoirs, river channels and reduce their potentials for water storing capacity. Based on the studies by (Brice 1966;Abdulfatai et al. 2014), gully erosion has been extended to include a drainage channel that transmits transient ow, steep side, steeply sloping or vertical head mu er with a width greater than 0.3m and a depth greater than 0.6m. Gully development in the Moldavian Plateau of Romania was studied using the Caesium 137 technique, and it was discovered that 57%of the gullying occurred during the cold season and 43% occurred during the warm season (Ionita 2006). A gully erosion susceptibility assessment and management hazardprone area was investigated in India using multivariate additive regression splines (MARS), exible discriminate analysis (FDA). Random forest (RF) and support vector machine as well as eld surveys (Gayen and Bai 2019). They built a gully erosion susceptibility model, which is useful for land managers and policymakers, as they initiate remedial measures and erosion hazard mitigation in prioritized areas (Gayen and Bai 2019). Geotechnical assessment of soil in erosion prone-zone was carried out in Thekkumalai Mountain foot, Kanyakumari District Tamilnadu to identifying the geotechnical parameters that in uence soil erodilibility, such that suitable soil stabilization can be ascertained (Subash et al. 2016). Geotechnical investigation and assessment of earthquake factors were carried out at Hurghada City, Red sea, Egypt (Ismaiel 2018) and it was discovered that the allowable bearing capacity of the investigated soils ranged from 1.5 to 2.5kg/cm2, and therefore recommended deep pile foundations in the region. Soil erosion model at Densu River Basin in Ghana using revised universal soil loss equation (RUSLE) and geographic information system (GIS) tools were developed to estimate the annual loss and found out that 88% of the basin has low erosion risk, and 6% moderate erosion risk. They further stated that erosion risk is high on 3% and severe on 3% of the basin (Ashiagbor et al. 2013). Rainfall-runoff slope length and steepness and land cover management have been used for soil erosion modelling in southeastern Nigeria (Egboka et al. 2019). They described slope length as the distance from the source of runoff to the point where either deposition begins or runoff enters a well-de ned channel that may be part of a drainage network. The geologic setting, tectonic and upliftment, geotechnical properties of soil, mining activities, farming, deforestation, and overgrazing operations have been widely reported (Brice 1966; Egboka and Okpoko 1984; Igwe and Orji 2019) to be the major causes of gully erosion in southeastern Nigeria. The Agulu-Nanka-Oraukwu gullies, Anambra State have been studied by (Egboka et al. 1983). They found out that acids are produced during oxidation and reduction in the physiochemical and weathering environment leading to erosion and gullies. (Onwuemesi 1990) investigated the hydrogeophysical and geotechnical properties of soils in Nsukka and its environs and discovered that the areas were prone to gully erosion due to low plasticity and very loose compactness of the soils. Gully erosion in southeastern Nigeria: the role of soil properties and environmental factors has been well discussed by (Onwuemesi 1990). The characteristics and erodibility potentials of soils from different geologic formations in Anambra State, southeastern Nigeria has been investigated by (Igwe and Egbueri 2018). The causes, consequences, and control measures of gully erosion in southeastern Also, few studies on gully erosion have been conducted at a large spatial scale because of its time demanding and challenges encountered during the studies.
The differences in susceptibility to gully erosion within the sedimentary formations have not been adequately studied. The adopted methods for controlling gully erosion in southeastern Nigeria are inappropriate. This is because gully erosion continues to originate and expand in southeastern Nigeria.
These necessitated the eld campaigns and mechanical soil laboratory analyses of sediments in the study areas to provide detail geological and geotechnical information on the origin and continued expansion of the erosion gully. The lithological and geotechnical characterization of sedimentary lithologies could contribute to highlighting the predisposing role of such landscape to the initiation of gully erosion hazard. The differences in susceptibility to gully erosion in the study areas will identify. The digital elevation models (DEMs), slope maps, and land cover/land use maps will be generated from National Aeronautics and Space Administration (NASA) Shuttle Radar Topographic Mission (SRTM) -30m data and displayed to show the in uence of elevation, slope and on the genesis and development of gully erosion in the study areas. The results of this study shall further help in recommending designs for appropriate control and precautionary measures. To minimize and control the menace of gully erosion, the Nigerian government established Gully Erosion Control and Commission (GECC), a statutory body vest with the responsibility to prevent, manage, control, and redress erosion and for related matters in the affected States. The World Bank-assisted agency, the Nigeria Erosion and Watershed Management Project (NEWMAP) is also assisting in sustaining the erosion control beyond 2021.
Location and Geological Descriptions of the Study Areas.
The study area lies in southeastern, Nigeria (Fig. 1). The studied gullies on the exposed Nanka Formation falls within latitudes 06 0 02 1 N -06 0 05 1 N and longitudes 06 0 43 1 E -07 0 30 1 E (Fig. 2) in Anambra State while the studied gullies where Ajali Formation exposed falls within latitude 06 0 44 1 N -06 0 55 1 N and longitude 07 0 17 1 N -07 0 27 1 E (Fig. 3) in Enugu State. In both areas, there is undulating topography, the thick forest along the low river valleys, and isolated rounded hills of discontinuous resistant beds where Guinea savannah vegetation occurs. Geological descriptions of the study areas have been well documented and published (Simpson 1954;Reyment 1965;Hoque and Ezepue 1977). The dominant geological formation within latitude 06 0 02 1 N -06 0 05 1 N and longitude 06 0 43 1 E -07 0 30 1 E is Nanka Sands which consist of successions ranging from unconsolidated to poorly consolidated sands (310m thick), thin intercalation of claystone and siltstone bands, lenses and aser beds, cross-bedded, poorly sorted and medium to coarse-grained. These units are interbedded by shale-siltstone and ne sand layers (25cm thick) in a few of the gully sites. The angle of dip ranges from 4 to 6 0 in the western direction. The Nanka sand is underlain by the thick Imo Formation of Paleocene age (Reyment 1965), and overlain by the Oligocene of Ogwashi-Asaba Formation (Reyment 1965; Adeleye and Fayose 1978) (Fig. 2). Towards the northwest part of the study area, the Nanka sand is overlain by the Nsugbe Formation of Oligocene age and underlain by the Ameki Formation of the Eocene age towards the southeastern part of the study area (Reyment 1965 (Onwuemesi 1990). The red sand is underlain by whitish sands. The whitest sandstone is more susceptible to gully erosion than the red sand since the red sand is more clayey and cohesive than the white sandstone.

Field Study
A reconnaissance survey and detailed mapping of the study areas were carried out to identify gully sites, geological conditions, and the effect of length and steepness of the slope, land cover management, and human activities to delineate the origin and continued expansion of gully erosion in the study areas. The length and slope steepness (angle of slopes) were measured using the measuring tape and inclinometer respectively. The depth, width, and lateral extent of the gullies were measured using the measuring tape to determine the gully intensities. The use of the Global Position System (GPS) aided the assessment of land use and land cover change as well as the elevation in the study area. The digital elevation models (DEMs), slope maps, and land cover/land use maps of the study areas are generated from National Aeronautics and Space Administration (NASA) Shuttle Radar Topographic Mission (SRTM)-30m data to show the in uence of elevation, slope and land use/land cover on initiation and development of gully erosion in the study areas.
The sediment samples were collected with the aid of a hand auger from twenty gully sites located in different parts of the study area where two main sedimentary Formations the Nanka Sands and the Ajali Formation outcropped. At each point of collection, samples were taken randomly from the top (10cm) by removing the topsoil, gully wall, and gully oors. The sediment samples collected were carefully bagged and labelled before subjecting them to various mechanical soil laboratory tests to determine the index properties of the soils.

Geotechnical Analyses
The soil index properties were determined in the laboratory by following the various methodology discussed hereunder: The plastic limit of the sediments was determined by rolling out a thread of the ne portion of soil on a at, non-porous surface. The test was performed in accordance with procedures speci ed by  (Adeleye and Fayose 1978) for civil engineering purposes. The liquid limit was measured using the Casagrande method and the procedure described by ASTM Standard 4318 (British standard 1990; Ishaque et al. 2010). The attterbergs tests were done to determine the behaviour of the soils. The grain size analysis of the sediments was carried out using the hydrometer method. This is to determine the particle size distribution of the soils. The compaction test which shows the optimum moisture content (OMC) and the maximum dry density (MDD) was carried using a protor soil compactor. The purpose is to understand the compaction characteristics of different soils with the change in moisture content. Permeability of the sediments was determined using the falling head permeability technique described by (Munch and Douglas 1985). The purpose is to determine the hydraulic conductivity of the soils.
The shear strength of the sediment was determined using a vane shear apparatus as described (Sohne 1953). The purpose is to determine the shear properties of discontinuities in soils of the study area.

Results And Discussion
Result of the Field Study Table 1 shows the results of slope steepness (angle of a slope) and the gully intensities in the study area. The slope is the gradient or land inclination. The  (Table 1), compared with the standard slope descriptors (https://geography eldwork.com>slopeS), the degree of steepness of the gullies in the Nanka and Ajali Formations ranges from strong slopes to a very steep slope. The generated slope maps of the study areas from NASA SRTM-30m data are displayed (Figs. 4; 5). From the results obtained and the generated slope maps, it reveals an increase in speed and volume of the overland ow, rate of particle detachment as well as transportation of soil particles. It depicts that gully erosion is more pronounced in areas with high steepness. A reconnaissance and intensive eld surveys of the study areas reveals the long length and steepness of the slope, the gully intensities which vary from low to a very high degree of erosion (Table 1; Figs. 6; 7), the elevation of the study areas which ranges from < 250 to > 450m ( Table 1). The generated digital elevation models of the study areas from NASA SRTM-30m data are displayed (Figs. 8; 9) as well as the generated land cover/ land use maps from NASA SRTM -30m, which includes water resources, built-up, road network, gully area, vegetation/derived savannah and farmland (Figs. 10; 11), contributed to the origin and continued expansion of gully erosion in the study areas. It was also observed that some of the areas have a long hilly slope which increases the amount of cumulative runoff and the steepness of the slope. This increases the velocities of the runoff and exposes surface pores of the land which eventually enhances the initiation and propagation of gully erosion. The prevalence of gully erosion in the study areas also includes human activities such as bush burning, agricultural, overgrazing, deforestation, and deliberate refusal to plant / replanting of trees, faulty road constructions and drainage systems as well as nonchalant attitude of the affected community people. The Nanka Sands comprises of sequences ranging from unconsolidated to poorly consolidated sands (310m thick), thin intercalation of claystone and siltstone bands, lenses, and poorly sorted and medium to coarse-grained. These units are inter-bedded by shale and ne sand layers (25cm thick) in a few of the gully sites. The Ajali Formation comprises of predominantly medium to coarse, thickly friable, very poorly sorted to poorly sorted, and poorly cemented sandstones with some ne sand at the base. The topmost part of the Ajali Formation consists of reddish sands. Table 2 reveals the results of soil analysis carried out on soil samples from gully sites of the Nanka sands in the study area. The liquid limit (LL) ranges from 26.80-36.70% with a mean value of 30.23%, plastic limit (PL) ranges from 20.30-28.40% with a mean value of 24.94%. The plasticity index (PI) which is a measure of the plasticity of the soil is determined by the difference between the liquid limit and the plastic limit and the value ranges from 3.30-8.30% with a mean value of 5.29%. The sands and silts content range from 86.0-96.0% with a mean value of 90.9% and 1.0-5.0% with a mean value of 3.0% respectively. The compaction which shows optimum moisture content (OMC) and the maximum dry density (MDD) ranges from 8.60-11.80mg/m with a mean value of 10 Table 2 shows that the soils in the gully sites of the Nanka Formation are low plastic which signi es poor cementing and insu cient binding materials suggesting a high susceptibility to gully erosion and high instability. The low moisture content indicates a high capacity for water retention during the rainfall. The low value of cohesion and angle of internal friction results in soil cracking. Highly sandy with low silt content and very loose compaction reveals a very loose lithology. The values obtained for hydraulic conductivity ranges from (2.1-3.2) x 10 -3 cm/sec with a mean value of 2.67x10 -3 cm/sec suggesting high permeability. Based on the U.S. Bureau of Reclamation and revealed by (Surendra and Sajeev 2017), soils are classi ed as (i) Impervious: k (Coe cient of permeability) less than 10 -6 cm/sec, (ii) Semi-pervious: k between 10 -6 to 10 -4 cm/sec and (iii) Pervious: k greater than 10 -4 cm/sec. From the values obtained (Table 2), it shows that the soils are highly permeable suggesting high in ltration rates thereby giving rise to high ow velocities, high seepage pressure, and high internal erosion potentials (Okengo et al. 2015). Table 3  95.10% and 0.00-2.60% with a mean value of 1.43%. The shear strength of the soil is the maximum internal resistance of the soil to the motion of its particles by sliding or slipping. The forces that withstand shear are mainly the inter-granular friction and the cohesion force. From Table 3, it shows that the soils of the Ajali Formation exhibit low plasticity, highly sandy with low silt content, low cohesion, very loose compactness, and high permeability. According to Coulomb's law, as described by (Onwuemesi 1990), the shear strength is given by the equation S = C + tan O P where S = Shear strength, C = Cohesion, P = Effective pressure, tan O = Coe cient of friction, and O = Angle of internal friction. The vital role played by shear strength is that the friction force due to run-off and the seepage ux is only opposed by the angle of internal friction because of the very low cohesion to cohesion-less and very permeable nature of the sandy formations. Several workers including (Paterson et al. 1978;Sudicky 1987;Onwuemesi 1990) have recorded detailed reports on permeability blueprints of soil samples. Permeability is a measure of the capacity of soil to permit the passage of uids such as water and it has the dimension of velocity (Onwuemesi 1990). The environmental framework of these study areas which includes ridges and domes impede the in ltration of the rainwater. This rainwater then ows as runoff and lose the soil particles as a result of the very low shear strength of the soil. From the lithological and geotechnical characterization of the sedimentary lithologies of Nanka and Ajali Formations (Figs. 12; 13), it shows that the landscape contributed to the initiation of gully erosion disaster in the study areas.

Genesis and Continued Expansion of Gully Erosion in the Study Area
Different parts of the study areas where the two main sedimentary formations (Nanka and Ajali Formations) cropped out have continued to witness incipient gullies in recent times. The genesis and continued expansion of gully erosion in the area is mainly linked to the geology, topography, human activities that are poorly planned, and geotechnical properties of the soils. The soil surface is also accessible to rainfall and run-off due to scanty vegetation/plant cover in the areas of study. The geotechnical properties of these areas determine their susceptibility to gully erosion (gorges) which are advancing into canyon proportions. Detailed mapping, plastic limits, low liquid limit, low plasticity, the high proportion of sands, high permeability, the shear strength, and the very loose compactness of soils from the Nanka Formation and Ajali Formation shows that the geological conditions and geotechnical composition of the soils were responsible for the initiation and propagation of the gully erosion in the study areas. The generated slope maps, digital elevation models, and land cover/land use maps from NASA SRTM 30m data exactly show the in uence of slope, elevation, and poor land use/land cover on the genesis and development of gully erosion in the study areas. Low plastic, low cohesive, and very loose compactness of soils are in line with the works of (Onwuemesi 1990) in Nsukka and its environs where Ajali Formation outcropped and (Igwe and Egbueri 2018) in Anambra Basin. The inter-bedded shale in the Nanka Formation changes in volume resulting in alternate wetting and drying thereby enhancing gulling. Also, the interbedded shale increases in volume when wet and becomes sticky and plastic during the rains. This is in line with the considerable study by (Egboka et al. 1983 and con rmed by this study, such interbedded shale formed a dry thick layer during the dry season causing contraction of clay and eventually led to soil fracture which is also conveyed to the sandy units. The shale is soaked with water after rainfall; the clay minerals swell up and establish a susceptibility to slide. The thick layers of sand underlain by the plastic shale usually slide down-dip in the gully with the shale acting as a lubricator. The characteristics of the Nanka Formation, rainfall-runoff, long hilly and steepness of the slope, poor land cover, low plasticity, a high proportion of sands, high permeability, the shear strength, loose compactness of soils, and human activities enhanced the initiation and continued expansion of gully erosion in the study area. These features led to carving, piping, and landslide resulting in a step-like gully cross-section (Fig. 14) that is displayed in the majority of the affected areas where the Nanka Formation outcropped. Different colours of the soils ranging from light grey, white, pink in the study area where Ajali Formation outcropped shows different heat-releasing and heat-absorbing capacity of the soils. These reveal the non-co-existence of expansion and contraction of the soils leading to structural damages of the Ajali Formation. The friability of the Ajali Formation, loose structure, low degree of diagenesis, poorly bonded mechanism, low compressive strength, the rapid disintegration of soil during rainfall and run-off, low soil fertilization, long hilly and steepness of the slope, poor land cover, and faulty land usage, low plasticity, a high proportion of sands, high permeability, the shear strength, the very loose compactness of soils, and human activities contributed to initiation and continued gullying erosion expansion. These features result in slumping and sliding movements in the affected areas. A schematic representation of the gullies in the study area shows a convex gully cross-section (Fig. 15). The danger of this badland degradation was found in all the gully sites where gorges are about 3.2m.
Following those key ndings, agronomic technique through the use of plant cover, soil conservation, contouring, and strip cropping and tillage system should be adopted. These would ensure rainfall absorption which will, in turn, reduce the impact of rainfall on the soil. Also, engineering protections by bundling, contour trenching, terracing, and grassed waterways should as well be adopted. With the engineering protection, slope characteristics of the area will be changed in such a way that the amount and the velocity of runoff will be lowered. The soils will be protected and the surface runoff will be lessening. These practices should be adopted to reclaim the ravaged land and to further discontinue the expansion of other gully erosion potential areas in the study areas.

Conclusions
This study treated the gully erosion problem, which constitutes a serious threat to several communities in southeastern Nigeria. It is also caused by poorly planned anthropic activities. The lithological and geotechnical characterization of the Nanka and Ajali Formations revealed that the gully erosions were   Table 3: Results of soil analysis on the samples from gully sites of the Ajali Formation in the study area.