Soil erosion is the systematic removal and transportation of soil particles by the erosive agents of water and wind. Several theories which have tried to explain the development of soil erosion through the primary erosional processes are recorded in some key publications (Egboka and Okpoko 1984; Casasnovas and Zaragoza 1996; Simpson 2001; Carey 2006; Ustun 2008; Ismail 2008; Ibitoye et al. 2008). Soil erosion in southeastern Nigeria are generally associated with several factors which include high rainfall intensity, poor drainage, wind action, slope instability, poor engineering and agricultural practices by earlier scholars (Egboka and Okpoko 1984; Ofomata 1988, 1989; Ofomata et al. 2009). However, anthropogenic activities tend to accelerate erosional processes in the study area. In addition however, some endogenic factors which may include the existence of some particular geological and geomorphic features such as weak zones may have serious implications (Onu and Opara 2010).
The study area is underlain by the Imo Shale and Ameki Formations. The Eocene to Oligocene aged Ameki Formation is composed of medium to coarse-grained whitish sandstone, bluish calcareous siltstone, with spotted clays and thin limestone. The lithological sections are laterally variable. The lower units are made up of fine to coarse-grained sandstone lenses, with dominantly calcareous shales and thinly bedded shaly-limestone. This Formation overlies the Paleocene Imo shale characterized by vertical to lateral lithologic variations (Uma 1989; Akpabio and Ekpo 2008). The Ameki Formation overlies the Imo Formation vertical, but the stratigraphic boundaries between the Ameki Formation and the Imo Formation have not been precisely defined. Lithostratigraphic sections in the Ameki Formation are described using two different groups (Reyment 1965; Whiteman 1982; and Arua 1986); an upper grey-green sandstones and sandyclay; and a lower unit with fine to coarse sandstones and intercalations of calcareous shale and thinly bedded shelly limestones. The incidence of high porosity and permeability in addition to the loose, friable and uncompacted nature of the Ameki formation makes the area vulnerable to erosion initiation and development (Ofomata 1989; Ananaba et al. 1991; Onu et al. 1992; Okeke and Agbasoga 2001; Okeke et al. 2011; Ibe et al. 2000).
Gully erosion is a crucial component of land degradation and desertification, which represent a serious danger to the ecosystem and environment (Onu et al. 1992; Vanmaercke et al. 2011). Gullies are described as erosional channels that are deeper than 0.5 meters and are typically brought on by concentrated water flow during and soon after a period of intense rainfall (Ogbukagu 1988; Akamigbo 1988; Ofomata 1989). Gullies typically have a dynamic nature and are influenced by the terrain, soil characteristics, vegetation, climate, and land use. Land cover and land use patterns are spatio-temporal, although topography and soil characteristics are essentially fixed over time. Although anthropogenic impacts are typically the main driver of gully erosion potentials, other erosion-prone qualities such as erodible soils, soft uncompacted subsoil, or unstable slopes do exist (Castillo and Gómez 2016). In order to assess the likelihood of gully initiation and propagation in a particular region of interest, it is crucial for sustainable land use management to have a good understanding of the dynamics of gully erosion, particularly with regard to climate change and land use dynamics (Poesen et al 2003).In general, gully erosion is frequently linked to changes in catchment hydrology, such as the removal of native vegetation and soil disturbance, and is frequently associated with land degradation brought on by anthropogenic activity (Oygarden 2003). Gully characteristics and erosion rates have been extensively studied in agricultural settings, but metropolitan areas are also known to experience high rates of erosion, particularly during engineering building (Wolman 1967). Few research have examined gully erosion in urban contexts (Castillo and Gómez 2016). Other studies detail the headcut retreat and growth of permanent urban gullies. From 1994 to 2000, Archibold et al. (2003) examined two urban gullies and recorded gully headcut retreat, widening, and deepening. They made the same conclusion about gully erosion made by Guerra and Hoffman (2006) in Brazil and Imwangana et al. (2014) in the Congo. They attributed it to changes in land use. A substantial correlation between soil texture and land usage was discovered by Adediji et al. (2013) when they studied the association between urban land surface characteristics and gully erosion in Nigeria.
Recent environmental catastrophes that have significantly altered the terrain in Southeast Nigeria include the formation and spread of erosion gullies (Okpala, 1990; Adekalu et al., 2007). This area is rapidly turning into a badland, with a deformed topography and limited land resources that are being lost to erosion every year. Since erosion has ruined many farmlands and decreased agricultural production, large portions of agricultural lands are becoming unfit for agriculture (Egboka et al. 1990). The elements that cause erosion and the creation of gullies are erosivity and erodibility. Erodibility, in contrast to erossivity, is a function of soil characteristics, topography, and land use management. Erossivity, however, is a function of rainfall, a natural occurrence that is beyond human control and modification. In Southeastern Nigeria, rainfall intensities are typically high, averaging between 100 and 125 mm/h (Obi and Salako 1995). According to Hudson (1981), rainstorms with an intensity of 25 mm/h or above are typically erosive. Gully erosion often causes major soil losses and produce large volumes of sediment which most often silt up local streams. Earlier scholars have variably linked soil erosion in southeast Nigeria to heavy rainfall, drainage, wind, slope instability, inadequate engineering, and agricultural practices (Egboka and Okpoko 1984). The soil material is typically loosen, detached, and moved from one location to another by flowing water, waves, wind, moving ice, or other geological forces and bank erosion agents. Erosion is typically induced by a set of physical and chemical processes. Although human activities in the studied area tend to speed up erosion processes, it appears that some exogenic and endogenic geological features make some portions of the region more prone to erosion than other locations (Onu and Opara 2010). Existence of specific geological and geomorphic units as well as weak zones are a few of these endogenic causes. In order to establish a potential connection between the initiation and propagation of gully erosion and some geological characteristics of the various formations in the study area, some studies have attempted to characterize soils in gully erosion-prone areas in southeastern Nigeria using geophysical data and other non-evasive techniques (Ananaba 1991; Onu and Opara 2010; Onu et al. 2012; Onu and Opara 2012).
Attempts have been made to use geographic information system (GIS) and satellite imagery to map gully erosion zones and their associated land use types (Igbokwe et al. 2008; Okereke et al. 2012; Amangabara et al. 2015; Udoka et al. 2015, 2016). However, there is a dearth of studies on the detailed geological, petrophysical and geotechnical characterization of soils within and around the study area to evaluate their susceptibility to soil erosion. A few studies only highlighted the influence of geology on the soils (Egboka et al. 1990; Ogbukagu 1988; Egboka and Nwankwor 1985) and possible tectonic influence on gully initiation and development (Ananaba 1991; Onu and Opara 2010) across the study area. This study therefore hopes to address these geological factors and illustrate the significance of these factors by carrying out a detailed characterization of one of the most affected litho-stratigraphic units – the Oligocene Ameki Formation. To investigate the factors that generally affect erodibility and erodability, geological and petrophysical data were used to assess the factors that initiate gullies in the study area. The objective of this study therefore is to characterize the soils of the study area and to infer the effect of soil properties on gully initiation, formation and development in the area.
1.1 Location, Physiography, and Geomorphology of the study area.
The study areas lie within the Tertiary sediments of the Anambra basin around Okigwe and Umuahia covering Umuda, Ude, Isingwu, Ugwuaku, Ndiakeme, and environs, Southeastern Nigeria. The area is accessible by many untarred roads and foot-paths that traversed through the area connecting the villages and nearby towns. The roads, both tarred, untarred, and the foot-paths were used as the traverse lines for data collection. The litho-units (Imo Shale and Ameki Formations) of this study are exposed at various locations within the study area. These outcrops formed the basis for the geological data. The relief is a rugged and undulating land of nearly parallel ridges arranged in linear forms and separated from each other by shallow valleys. The ridges are made up mainly of the more resistant sandstones while the valleys are underlain by the weaker and more labile clay, shale and siltstones. The drainage patterns consist mainly of dendritic and radial patterns while the vegetation cover is that of the Tropical Rain Forest.
The study area is located within the coordinates defined by latitudes 50301-50551N and longitudes 70151-70401E (Fig. 1). The study area's climate is characterized by erratic temperature changes and persistent precipitation spreading. There is a tropical wet and dry season, with the dry season starting in October and ending in March, and the rainy season starting in March and ending in October. The longest daylight hours, including November and December, are mostly experienced from January to April. At this time of year, there is an average of six hours of sunlight per day, compared to three hours during the rainy months of May through October. The region's diurnal temperatures range from 180C to 340C, while the daily mean lowest and maximum temperatures are situated between 190C and 280C, respectively (Akinsanola and Ogunjobi 2014). The estimated evapotranspiration rate falls between 1450 and 1460 millimeters per year. Similar to this, there is typically low relative humidity during the dry months of January through March and November through December, when the greatest figure of 95% is recorded. The rainy season, which lasts from April to October, has an average relative humidity value of 97%. (Iwuchukwu et al. 2018). Heavy rains typically fall between July and August, following the migration of Atlantic Ocean marine air to the north. Rainfall and sunny periods alternate due to the strong downpour's set pattern. The start of the rainy season, which lasts through October, is signaled by the month of April. The average annual rainfall ranges from 2500 to 4000 mm, with May through October accounting for 89% of the total (Iwuchukwu et al. 2018). Regularity and intensity of rainfall in the research area, along with a high rate of surface runoff, cause soil leaching and huge sheet erosion, which ultimately cause percolation and infiltration into groundwater sources (Ibe et al. 2018).