Environmental disasters such as gullies and landslides have long been a major threat to humans and ecosystems worldwide. These hazards have recently been a significant concern in several fields of study, particularly geoscientists, environmentalists, geotechnics, and civil engineers. Gullies and landslides have similar occurrence mechanisms and dynamic/complex in mitigating, whereby one could happen in association with the other. It is believed that as gravity pulls soil materials from the region of higher elevation to lower elevation, the mechanism is termed landslide, and it is an integral part of the progression of erosion (Khamkar and Mhaske 2018). Gully erosion, on the other hand, refers to the process by which soil particles are detached from their original soil mass and mechanically transported by running water, ice, and wind (Egbueri et al. 2021). Gully erosion processes impact both the biophysical and socio-economic components of the environment. Gully formation is highly influenced by surface runoff; as raindrops strike the soil surface, soil particles are sparsely thrown apart through the air over several centimeters. As continuous exposure to high rainfall considerably weakens the soil. Over time, soil mass is broken down through weathering processes, both mechanically and biochemically (Nwajide 1992; Nazari Samani et al. 2009).
The threat of gully erosion has been experienced across southeast Nigeria. This is a conception of the nature of the geologic formations (poorly consolidated sediments) underlying the study area (Egbueri and Igwe 2020). Gully occurrence in the study area poses a threat to many environmental issues, including the loss of enormous amounts of arable land, the destruction of the transportation system and engineering structures, the abandonment of ancestral homes, the migration of communities, and the deterioration of water quality (Hudec et al. 2006; Bell, 2007; Igwe and Egbueri, 2018; Egbueri and Igwe 2020; Nebeokike et al. 2020). Research has proven that numerous geogenic factors and anthropogenic activities contribute highly to the initiation, development, and expansion of gullies. These factors may include geology, geomorphology, land use/land cover, hydrologic conditions, excavation, and mining activities (Poesen 2011; Emeh and Igwe 2017; Egbueri and Igwe 2020). According to studies, out of the five states in Nigeria's southeast, Anambra State appears to have the most gullies, with over 700. Enugu, Imo, Abia, and the Ebonyi States follow with 600, 450, 300, and 250 gullies, respectively. Most of these gullies have not been successfully controlled (Igbokwe et al. 2008; Egboka et al. 2019; Egbueri and Igwe 2020). Coincidentally, Anambra State, with the highest number of gullies (700 gullies) is underlined by the youngest, poorly-consolidated geologic formations. In contrast, Ebonyi State, with the fewest number of gullies (250 gullies), has the oldest, firmly-consolidated geologic formations (Egbueri and Igwe 2020).
Many methods such as remote sensing, geospatial models, multivariate statistical analysis, and the RUSLE model (revised universal soil loss equation) have been utilized in gully erosion studies. Each method adopted is employed upon the peculiarity of the problem intended to be solved. In this study, statistical analysis was adopted to help reveal the interrelationship of the analyzed soil geotechnical parameters. Statistical analysis was employed in this study to aid in demonstrating how the evaluated soil geotechnical parameters are interrelated. The SPSS software (v. 22) would be used to achieve this. Pearson's correlation analysis (CA) and the factor analysis (FA) tools were considered. The CA uses a correlation coefficient to correlate two variables (x) and (y), with values between + 1 and − 1. A coefficient of + 1 means a positive correlation (direct relationships), while the coefficient of − 1 means a negative correlation (indirect relationships) (Hotelling 1953). Correlations have been categorized as strong, moderate, and weak, depending on the coefficient. Thus, the larger the coefficient, the stronger the association (Egbueri et al. 2019; Nebeokike et al. 2020). The FA describes observation among variables and aims to find independent latent variables (Thompson, 2004). This study utilized the Varimax rotation method to optimize the factor loadings at eigenvalue ≥ 1. Like the CA, the FA considers factor loading low, medium, and high (Egbueri et al. 2019; Nebeokike et al. 2020).
Numerous studies have been done on the initiation and development of gullies in southeast Nigeria, including those by Nwajide and Hogue (1979), Egboka and Nwankwor (1985), Okagbue and Ezechi (1988), Okagbue (1988), Obiadi et al. (2011), Emeh and Igwe (2017); Igwe et al. (2017), Igwe and Egbueri (2018) and Nebeokike et al. (2020). Okagbue and Ezechi (1988) described gullies as catastrophic. Depths and widths far beyond several kilometers, which would be termed a canyon (Obiadi et al. 2011). Nwajide and Hogue (1979) and Okagbue (1988) quoted and believed that these gullies are caused by a combination of geogenic, biotic, and anthropogenic factors. Meanwhile, Egboka and Nwankwor (1985) argue that they can be attributed to rock's significant hydrogeochemical and geotechnical properties. Recent research on the erodibility and slope characteristics of gullies in the Udi region was conducted by Nebeokike et al. (2020), and it was noted that the soils are erodible. There is evidence that the soils of the Anambra state are erodible, which was reported in recent studies by Emeh and Igwe (2017), Igwe et al. (2017), Igwe and Egbueri (2018), and Egbueri et al. (2021). At this time, no study has given importance to assessing the influence of geotechnical and geomorphological characteristics on the erosional processes in Udi and Aguata, southeast Nigeria. Therefore, it is essential to assess the erosional processes of both geologic formations in Udi and Aguata due to their propensity for gullying in the region. The current research is focused on evaluating the geotechnical and geomorphological influence on the occurrence of gullies in Udi and Aguata, southeastern Nigeria. The study objectives are to (1) determine the gully distribution within the geologic formations; (2) compare and characterize the geologic formation and determine their erodibility characteristic; (3) identify the key factors initiating/facilitating gullying through the integration of multivariate statistical analysis (CA and FA) and (4) determine the impact of geomorphological characteristics on the gullying processes. It is believed that the information obtained from this research would contribute immensely to the mitigation planning and control of the erodible soils within the study region.