Comparative analysis of the expansion rate and soil erodibility factor of some gullies in Nnewi and Nnobi, Southeastern Nigeria

The research focused on assessing the expansion rate and soil erodibility factor (K) of specific gullies located in Nnewi and Nnobi, Southeastern Nigeria. Fifteen representative gullies were studied extensively. The Grain size distribution analysis revealed that the soils are composed of gravel (5.77–17.67% and 7.01–13.65%), sand (79.90–91.01% and 82.47–88.67%), and fines (2.36–4.05% and 3.78–5.02%) for Nnewi and Nnobi respectively. The cohesion and internal friction angle values range from 1–5 to 2–5 kPa and from 29–38° to 30–34° for Nnewi and Nnobi respectively, which suggests that the soils have low shear strength and are susceptible to shear failure. The plasticity index (PI) of the fines showed that they are nonplastic to low plastic soils and highly liquefiable with values ranging from 0–10 to 0–9% for Nnewi and Nnobi respectively. Slope stability analysis gave factor of safety (FoS) values in the range of 0.50–0.76 and 0.82–0.95 for saturated condition and 0.73–0.98 and 0.87–1.04 for unsaturated condition for both Nnewi and Nnobi respectively indicating that the slopes are generally unstable to critically stable. The erosion expansion rate analysis for a fifteen-year period (2005–2020) revealed an average longitudinal expansion rate of 36.05 m/yr and 10.76 m/yr for Nnewi and Nnobi gullies respectively. The soil erodibility factor (K) are 8.57 × 10−2 and 1.62 × 10−4 for Nnewi and Nnobi respectively indicating that the soils in Nnewi have higher erodibility potentials than those of Nnobi. Conclusively, the Nnewi area is more prone to erosion than the Nnobi area.

These queries lead to the formulation of a hypothesis that attributes the observed variations to the accelerated urbanization and population growth in Nnewi 4,14-16 compared to the less densely populated Nnobi.Urban expansion often involves deforestation for commercial and residential purposes, disrupting natural drainage patterns and exacerbating surface runoff in highly populated areas like Nnewi 14,15 .In contrast, the presence of trees in Nnobi mitigates the force of surface runoff, which is the primary catalyst for gully erosion 4,14 .The roots of these trees bind the soil particles, reducing the destructive impact of surface runoff during the rainy season 4 .
Gully erosion, triggered by fast-flowing water eroding soil and creating channels, is exacerbated by increased slope instability during wet seasons, causing steep gully slope walls to fail due to heightened pore pressure levels 4,5,12,13 .The vulnerability of soil to water erosion is determined by geological, geomorphological, and climatic conditions within the area 4,13 .
Researchers such as Igwe and Fukuoka and Igwe et al. confirm that soil saturation during peak wet seasons increases pore pressure, reducing soil shear strength and rendering it susceptible to failure through liquefaction 17,18 .Additionally, intensified rainfall augments soil erodibility 4 .Egbueri and Igwe established that Eocene Nanka Sands (Imo Basin) and Cretaceous Ajali Sandstone (Anambra Basin) are highly susceptible to erosion due to their unique soil gradation 8 .This susceptibility is further exacerbated by undulating topography, leading to the formation of large, steep gullies 4 .Rainfall amplifies existing gullies as surface runoff erodes more soil 19 .The influx of pollutants from increased population and urbanization contributes to alkaline surface runoff, further eroding soil 12 .
While previous research has explored the occurrence and causes of soil erosion in southeastern Nigeria, none have ventured into comparing the expansion rate of gullies and estimating the soil erodibility factor of soils within Nnewi and Nnobi.This research aims to analyze the expansion rate and soil erodibility factor (K) of select gullies in Nnewi and Nnobi, Southeastern Nigeria.The primary objectives include evaluating the geotechnical properties impacting soil erodibility and gully intensity, calculating the gully expansion rate and soil erodibility factor (K), developing a slope failure model, and determining the factor of safety of the gully slope material.

The setting of the study area
The study area is located at Nnewi and Nnobi in Anambra State (Fig. 2).It is bound by Latitudes 6°0′ 0ʺ N to 6° 6′ 0ʺ N and Longitudes 6° 50′ 0ʺ E to 6° 58′ 0ʺ E. Major towns in the study area includes Nnewi, Nnobi, and Awka Etiti.The southern part of the study area can be easily accessed through the Nnewi-Okigwe and New Oba-Nnewi roads which run essentially Northwest-Southeast of the area.The Northern part of the study area can be accessed through the Nnobi-Nkpor road.
The area has an undulating topography comprising of gently inclined hilltops to very steep V-shaped valleys cut by seasonal drainage channels and gullies in the study area (Fig. 3).The dendritic drainage pattern in the area can be associated with the uniformity of the underlying lithology.The inherent weakness of the lithology combined with the down-cutting action of the seasonal drainages have shaped the area into a jagged topography it has today.The study area is within the humid tropical climate; it is characterized by two seasons: the rainy and dry seasons.The wet season spans from late March or early April till the end of October or early November, while the dry season is usually from November to March.The mean monthly temperatures vary from 22 to 28 °C in the wet season and between 28 and 32 °C in the dry season.The annual rainfall is between 1500 and 2100 mm 4 .The average monthly rainfall for 30 years ranges from less than 1 mm in the dry season to about 300 mm in the rainy season 20 .The wet periods are characterized by moderate temperatures and high relative humidity, while the dry periods have high temperatures and lower relative humidity 20 .
The study area falls in the tropical rainforest belt 21 , and is largely dominated by short grasses at the elevated areas with thick forests of tall trees in the valleys and along drainage channels.
The study area is underlain by loose Nanka Formation (Fig. 4) which is part of the Ameki Group.Ameki Group comprises three lateral equivalents: Ameki Formation, Eocene Nanka Formation and Nsugbe Sandstone 22 .The Nanka Formation is predominantly composed of loose and poorly consolidated fine-grained sand materials with low clay content and little or no coarse-grained aggregates 21 .It is noted as the loose sand facies of the Ameki Group 22 , outcrops over an area in excess of 1400 km exclusively east of the Niger and noted to be very susceptible to erosion.The Formation is made up of loose, tabular to planar cross bedded, flaser-bedded, fine to  medium grained sand, with a few mudrock breaks.The sand consists of sub-rounded to sub-angular grains and has an average of 5% clay content, which makes it texturally submature.It is however compositionally mature because of the absence of feldspars 22 .

Materials and methods
The materials used in this work were soils obtained from gully sites around Nnewi and Nnobi towns both in Anambra State.A total of 30 gully erosion sites were visited in both towns and 15 representative soil samples were collected: 10 soil samples from Nnewi and 5 soil samples from Nnobi.The soil samples were put in well labelled sampling bags and sealed to minimize the loss of natural soil moisture through evaporation.
The detailed field study was carried out in February 2021.It involved detailed geologic descriptions of outcropping units with emphases on their texture, colour and structures.
At every sampling point, about 5 kg of soil samples were collected after the removal of the top soil from a depth of 25-30 cm to ensure that a true and fresh representative soil sample devoid of humus and/or artificial fertilizer was obtained.These were carefully put in well-labelled sampling bags to avoid mixing up the location details.A total of fifteen (15) representative samples-ten (10) samples from Nnewi and five (5) from Nnobi were analyzed in the Civil Engineering Laboratory of the University of Nigeria Nsukka.The laboratory analysis to obtain the geotechnical and other properties of the samples such as specific gravity (ASTM D854-14), permeability (ASTM D5084-16), shear strength parameters (ASTM D3080-11)-cohesion (C) and angle of internal friction (Ø), particle size distribution, Optimum Moisture Content (OMC) using ASTM D698-12, Maximum Dry Density (MDD) using ASTM D698-12, Natural Moisture Content (NMC) using ASTM D2216-19, Atterberg limits using ASTM D4318-17-Liquid Limit (LL), Plastic Limit (PL) and Plasticity Index (PI) were determined.All were carried out in accordance with American Society for Testing and Materials (ASTM) standards for soil.The results obtained from the field and laboratory were integrated into Slope/W Geostudio 2012 software to produce a modeled diagram of the gully slope morphology and failure prediction on the slope.The integrated results are the slope height, width and slope angles obtained from the field and shear strength parameters (cohesion and angle of internal friction), unit weight obtained from the laboratory.The Morgenstern-Price limit equilibrium method was used to run the slope stability analysis.Aerial photographs from Google Earth Pro were used to monitor the erosion sites (gullies) for a fifteen year period for both Nnewi and Nnobi areas.The gully expansion rates (longitudinal and width) were determined using dimensions obtained with the ruler tool on Google Earth Pro.www.nature.com/scientificreports/Calculation of soil erodibility factor (K).The soil erodibility factor (K) was determined using the formula introduced by Jain and Singh 24 .This factor characterizes a soil's susceptibility to erosion and is integral to erosion prediction models such as the Revised Universal Soil Loss Equation (RUSLE).The calculation involved the following steps: 1. Parameters and equations: The soil erodibility factor (K) is expressed as: where F p is the particle size parameter, computed as F p = P silt (100-P clay ).P om is the percentage of organic matter.S struc is the soil structure index.F perm is the profile-permeability class factor.P clay is the percentage of clay content.

Results and discussion
Field observation of gully characteristics.The field observations of gully properties are shown in Table 1.The gullies in Nnewi were active, wider and deeper than those in Nnobi, which were mostly dormant and overtaken by tall trees and open dumps for solid wastes (Fig. 5a).The use of vetiver plants (grass) and retaining walls for soil stabilization was observed in some Nnewi locations (NNE1, NNE2, and NNE4) (Fig. 5b).Some of the houses, drainage channels and culvert were observed to have been damaged by the erosive actions of surface runoff.The soils in the study area were clean, loose, fine-medium grained lateritic sands.The little or no fines in them increases their susceptibility to erosion 21 .The gullies in the study area generally trend NW-SE with slope angles ranging from 40° to 78° for Nnewi and 45°-69° for Nnobi area respectively.The relatively higher slope angles measured in Nnewi makes the gully walls more unstable than those in Nnobi.This increases the intensity and rate at which erosion occurs in Nnewi.The steeper slopes help to reduce the soils shear strength making them susceptible to shear failure 23 .Furthermore, Nnobi lies in a relatively higher topography with gentler slopes than Nnewi.This makes surface runoffs more erosive in Nnewi area since water moves from higher elevation to lower elevation 14 .(2.36-4.05%)for Nnewi area and gravel (7.01-13.65%),sand (82.47-88.67%),and fines (3.78-5.02%)for Nnobi area.The soils in the study area generally have high sand percentage and low percentage of fines and gravel.The dominance of sand and low fine and gravel content of the soils (Fig. 6a,b) make them very susceptible to erosion since the cohesion among sand grain is little 5,14,21 .The relative higher fine percentage in Nnobi may also be the reason for less gully intensity since the cohesive forces between grains (supplied by the fine content) are higher.

Particle size distribution (grain size analysis). The particle size distribution results are presented in
In addition, the values of the coefficient of uniformity (Cu) and coefficient of curvature (Cc) ranged from 1.03-3.78 to 0.98-1.23 for Nnewi and 3.15-4.02and 1.06-1.98for Nnobi respectively (Tables 2, 3) indicating that the soils are poorly graded.According to Arora, well-graded soils have Cu > 6 and Cc ranging from 1 to 3 23 .Since the shear strength of well-graded soils are higher 23 , it follows that the soils in the study area have low shear strength.
Based on the Unified Soil Classification Scheme (USCS) as shown in Table 2, the soil types in the study area vary from poorly graded sand (SP), clayey sand (SC) and well-graded sand (SW).However, the most dominant type is poorly graded sand, which makes the area prone to shear failure because of its low shear resistance 23 .

Specific gravity.
The fifteen soil samples in the study area are predominantly sands with little gravel and fines, indicating that they possess similar range of specific gravity values.The specific gravity results (Table 4) for Nnewi and Nnobi ranged from 2.45-2.66 to 2.54-2.78respectively.This indicate the soils have similar erodibility properties 5,21 .

Coefficient of permeability.
The permeability (k) of a soil is a direct indication of the infiltration capacity of that soil 23 .The higher the permeability of the soil, the higher the infiltration capacity and ease at which they can be washed away by running water.The soils in the study area have permeability values ranging from 2.92 × 10 −5 to 6.80 × 10 −4 m/s and 2.35 × 10 −6 to 3.84 × 10 −4 m/s for Nnewi and Nnobi respectively (Table 5).According to Arora, soils with permeability values falling within the range of 10 −7 to 10 −5 m/s are categorized as moderately to highly permeable 23 .This shows that the soils in the study are moderately to highly permeable.The high permeability and the predominance of sands in the study area makes it very vulnerable to erosion  www.nature.com/scientificreports/especially at the peak of the wet season when the rainfall intensity is maximum.Furthermore, the relatively lower permeability observed in Nnobi samples (NNO2 and NNO3) was because of their higher fine content making them more resistant to erosion than Nnewi area.As rainfall infiltrates into these highly permeable soils, the pore pressure increases thereby reducing the shear strength of the material 23 .This leads to shear failure and increased rate of instability in the study area.

Compaction characteristics of the soils in the study area.
Compaction test is done to reveal how loose or dense (compacted) a soil sample is.The denser the soil, the higher the internal friction angle between the grains and the higher the shear strength of the material 19,23 .The results of the compaction test are presented in Table 6 with the compaction curves shown in Fig. 7a,b for Nnewi and Nnobi respectively.From the results, the values of maximum dry density (MDD) and optimum moisture content (OMC) ranged from 1.82-2.11g/cm 3 and 8.20-17.81%for Nnewi and 1.98-2.13g/cm 3 and 6.00-17.80%respectively.The result reveals that the soils have moderate to high OMC and low MDD.This suggests that the soils from both area are loose, lack ability for structural support (bearing capacity) hence, susceptible to erosion.However, soils in Nnewi area with lower dry  Natural moisture content and Atterberg limits.The natural moisture content (NMC) of the soil gives an indication of its shear strength and load bearing capacity 23 .The NMC values of the soils in the study area (Table 6) ranged from 2 to 4% for both Nnewi and Nnobi areas.This shows that the soils have low water saturation and thus, low cohesive attractive forces among the soil grains 26 .
The results of the Atterberg limits as presented in Table 7 reveals that the liquid limit (LL) ranges from NP-26%, the plastic limit (PL) from NP-19% and the plasticity index (PI) from NP-10% for Nnewi area while for Nnobi area, the LL ranges from NP-25%, PL ranges from NP-16% and PI ranges from NP-9%.Generally, the plasticity index (PI) of the fines showed that they are nonplastic (NP) to low plastic soils and highly liquefiable.8).From the plot, the soils are all susceptible to liquefaction and shear failure 27 .
Shear strength characteristics of the soils.The cohesion (C) and internal friction angle (Ø) are very important parameters in the determination of the shear strength of materials.According to Arora, the higher the Ø in sands, the higher the degree of interlocking among grains and consequently, the higher the shear strength (ability to withstand shear failure) 23 .Furthermore, the lower the shear strength of the material the higher their erodibility.The values of the shear strength parameters (C and Ø) of the analysed soils are presented in Table 8 and Fig. 9a,b.All the soils have low cohesion values ranging from 1-5 to 2-5 kPa and internal friction angle ranging from 29°-38° to 30°-34° for Nnewi and Nnobi respectively, which suggests that the soils have low shear strength as cohesion is a major component of shear strength equation 23 and are susceptible to shear failure 14,21,25 .
Gully slope stability analysis.The stability analysis of the fifteen gully slopes within the study area was conducted to determine their factor of safety (FoS).The FoS values fell within the range of 0.50-0.76and 0.82-0.95for the saturated condition, and 0.73-0.98 and 0.87-1.04for the unsaturated condition, for Nnewi and Nnobi respectively (Fig. 10).The behaviour of these gully slopes during the dry season was represented by the unsaturated condition, while the saturated condition represented their behaviour during the rainy season.The results of the stability analysis for both the saturated (wet season) and unsaturated (dry season) conditions were documented in Table 9.Specifically, the gully slopes labelled as NNE2, NNE6, and NNE10 corresponded to the Nnewi location (Fig. 11a), while gullies NNO1, NNO3, and NNO4 represented the Nnobi location (Fig. 11b).
From the analysis, it was revealed that all the gully slopes in Nnewi were determined to be unstable 28 presented in Table 10.In contrast, in the Nnobi area, 60% of the gullies were found to be unstable, while the remaining 40% were classified as critically stable.To provide a more specific breakdown, the locations NNO1 and NNO3 displayed FoS values of 0.94 and 0.95 during the rainy season, and 1.01 and 1.04 during the dry season, respectively.These values indicated unstable slopes for NNO1 and critically stable slopes for NNO3.This highlights the heightened hazard of the gullies during the rainy season, when the slopes are either partially or fully saturated, aligning with the findings of Igwe and Chukwu 29 .Furthermore, the decrease in FoS values during the rainy season was attributed to the rise in pore pressure and unit weight of the soil, resulting in reduced shear strength of the slope materials.This observation was consistent with previous studies by Emeh and Igwe, Igwe and Egbueri, Arora, and Nebeokike et al. 15,21,23,25 .Incorporating slope angle, it was noted that the steeper angles of the gullies in Nnewi contributed to the higher instability of their walls.This collectively pointed to the conclusion that the gullies in Nnewi were inherently more unstable than those in Nnobi, both during the rainy and dry seasons.The instability of a slope occurs when the activating shear force surpasses the resisting shear strength of the slope materials 23,30 , invariably leading to slope failure.The soil erodibility factor (K) of Nnewi area is given as: The soil erodibility factor (K) of Nnobi area is given as: The derived values indicated that the soils in Nnewi exhibited a notably higher erodibility potential compared to those in Nnobi.

Conclusion
This research work has been focused on a comprehensive analysis of selected gullies situated in the regions of Nnewi and Nnobi, south-eastern Nigeria.The central aim of this study was to delve into the expansion rate and soil erodibility factor (K) of these specific gullies, shedding light on critical geotechnical attributes that influence both soil erodibility and gully intensity.The distinct objectives that guided this investigation encompassed the evaluation of geotechnical characteristics that exert an impact on soil erodibility and gully intensity, the computation of the expansion rate of the gullies and determination of the soil erodibility factor (K), the formulation of a model for slope failure, and the assessment of the factor of safety of the gully slope material.
In the process of achieving these objectives, a thorough examination of the geotechnical properties inherent to the soils within the study area was conducted.It became evident that Nnewi, in contrast to Nnobi, is positioned on a relatively steep topography.The outcome of index and geotechnical analysis revealed noteworthy distinctions between the two regions.Specifically, Nnewi exhibited lower fine content, higher sand content, greater permeability, decreased shear strength, inadequate soil compaction, and sparse vegetation coverage when compared to Nnobi.These findings collectively render the soils in Nnewi significantly more susceptible to erosive forces generated by running water, thus explaining the escalated gully erosion observed in this area.
An essential parameter, the soil erodibility factor (K), emerged as a crucial indicator of erosion susceptibility.Notably, the calculated K values for Nnewi and Nnobi were 8.57 × 10 -2 and 1.62 × 10 -4 , respectively.This disparity  www.nature.com/scientificreports/ in K values aligns with the disparities identified in the geotechnical property analysis, thereby reinforcing the notion that Nnewi's soils possess a higher potential for erosion compared to Nnobi.

Figure 1 .
Figure 1.A gully erosion site in the study area.

Figure 2 .
Figure 2. Location and accessibility map of the study area.Created using the ArcGIS and ArcScene software (version 10.4.1).

Figure 3 .
Figure 3. 3D DEM of the study area showing drainage channels.Created using the ArcGIS and ArcScene software (version 10.4.1).

Figure 4 .
Figure 4. Geologic map of the study area.Created using the ArcGIS and ArcScene software (version 10.4.1).

2 .
Soil Structure and Permeability Class: Values were assigned to soil structure and permeability class factors based on specific classifications.For soil structure, values were designated as follows: 1 = Very fine granular soil 2 = Fine granular soil 3 = Medium or coarse granular soil 4 = Blocky, platy, or massive soil Similarly, for permeability class, values were assigned as follows: 1 = Very slow infiltration 2 = Slow infiltration 3 = Slow to moderate infiltration 4 = Moderate infiltration 5 = Moderate to rapid infiltration 6 = Rapid infiltration

Figure 5 .Figure 5 .
Figure 5. (a) Some of the Gullies in the study area A-C) Nnewi D-F) Nnobi.(b) Some of the gullies in Nnewi area (G-J) and some mitigating measures (I-J).

Figure 6 .
Figure 6.(a) Particle size distribution curve for Nnewi area.(b) Particle size distribution curve for Nnobi area.

Figure 7 .
Figure 7. (a) Compaction curves of the soil samples in Nnewi area.(b) Compaction curves of the soil samples in Nnobi area.

Figure 8 .
Figure 8. Liquefaction chart of the study area (after Seed et al. 27 ).

Figure 12 .
Figure 12.(a) Gully expansion rate analysis of Nnewi area from 2005 to 2020.Aerial photos obtained from the Google Earth Pro software.(b) Gully expansion rate analysis of Nnobi area from 2005 to 2020.Aerial photos obtained from the Google Earth Pro software.(c) Gully expansion rate analysis of Nanka area from 2005 to 2020.Aerial photos obtained from the Google Earth Pro software.

Figure 14 .
Figure 14.Rainfall data of the study area from 1971 to 2021.

Table 1 .
Gully slope locations and field observations.

Table 2 .
Particle size distribution and soil classification of the study area.

Table 3 .
21mmary statistics of particle size distribution analysis.densityaremore prone to erosion compared to Nnobi area.This agrees with the findings of Igwe and Egbueri that loose soils require very little force to detach and transport the particle21.

Table 4 .
Specific gravity of the soil samples in the study area.

Table 5 .
Permeability (m/s) of the soil samples in the study area.

Table 6 .
Compaction test result of the soil samples in the study area.

Table 8 .
The shear strength test results of the soils.

Table 11 .
Gully expansion rate analysis result.Significant values are in [bold].