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 standard slope descriptors (https://geographyfieldwork.com>slopeS), a slope value <0.3o indicate flat level slope, 0.3<1.1o depict nearly level, 1.1<3o indicate very gentle slope, 3<5o indicate gentle slope, 5<8.5o indicate moderate slope, 8.5<16.5o indicate strong slope, 16.5<24.0o depict very strong slope, 24<35.0o indicate extreme slope, 35<45.0o indicate steep slope, >45.0o indicate a very steep slope. Based on the results obtained (Table 1), compared with the standard slope descriptors (https://geographyfieldwork.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 flow, 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 field 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 fine 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 fine sand at the base. The topmost part of the Ajali Formation consists of reddish sands.
Result of the Geotechnical Analyses
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 10.62 and 1.40-2.00 mg/m with a mean value of 1.68. The shear strength of the soil is the result of friction and interlocking of particles and possibly cementation or bonding at the particles [36]. The shear strength parameters are the cohesion and the friction angle. The cohesion value obtained varies from 0.23-0.43kg/cm2 with a mean value of 0.30 kg/cm2. The shear angle of internal friction ranges from 24.00 to 32.00 with an average value of 24.70. The shear strength enhances the initiation of gully erosion by encouraging overland flows. According to (Surendra and Sajeev 2017), plasticity index (PI) = 0 indicate sand, non-plastic, and non-cohesive, >0 <7 indicate sand/silt, low plastic and partly cohesive, 7-17 indicate silt/clay, medium plastic, and cohesive and > 17 indicate clay, high plastic and cohesive. The angle of shearing resistance <280 indicates very loose compaction, 28-300 indicates loose, 30-360 suggests medium compaction, 36-410 indicates dense compaction, and >410 indicates very dense compaction (Surendra and Sajeev 2017). Table 2 shows that the soils in the gully sites of the Nanka Formation are low plastic which signifies poor cementing and insufficient 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 classified as (i) Impervious: k (Coefficient 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 infiltration rates thereby giving rise to high flow velocities, high seepage pressure, and high internal erosion potentials (Okengo et al. 2015). Table 3 reveals the results of soil samples from the Ajali Formation: liquid limit (LL) ranges from 21.40-27.10% with a mean value of 24.09%, plastic limit (PL) ranges from 20.80-30.30 % with a mean value of 26.79%. The plasticity index (PI) ranges from 0.00-5.40% with a mean value of 2.70%. The optimum moisture content (OMC) and the maximum dry density (MDD) ranges from 6.40-10.70mg/m with a mean value of 8.47mg/m and 1.10-2.80 mg/m with a mean value of 1.9mg/m. The cohesion value obtained varies from 0.20- 0.41kg/ cm2 with a mean value of 0.30 kg/cm2. The shear angle of internal friction ranges from 18.00 to 30.00 with an average value of 25.30. The values obtained for hydraulic conductivity range from (2.01-3.61) x10-3 cm/sec with a mean value of 2.70x10-3 cm/sec suggesting high permeability. The sand and silt contents range from 87.0-100.0% with a mean value of 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 = Coefficient 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 flux 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 fluids 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 infiltration of the rainwater. This rainwater then flows 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 influence 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 et al. 2019). Based on the independent studies by (Egboka et al. 1983; Egboka and Okpoko 1984; Igwe and Egbueri 2018; Egboka et al. 2019) and confirmed 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 findings, 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.