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  • View in gallery

    Typical gully erosion sites in Anambra State.

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    (a) Outline map of Nigeria showing the location of Anambra State and (b) generalized geological map of Anambra State.

  • View in gallery

    Mean annual rainfall of the study area for the period 1997–2007 (source: Nigeria Meteorological Agency, Awka).

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    (a) Digital elevation map of Anambra State and (b) drainage pattern on top of the Awka–Orlu cuesta [(b) was modified from Egboka and Okpoko 1984].

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    A conceptual model for the formation of the persisting gully erosion problems in Anambra State.

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    (a) Scatterplot and (b) box plot showing the distribution of gully depths (red color) and hydraulic heads (blue color) and (c) graph of gully depth against hydraulic head in Anambra State.

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    Modeled VES curves observed at gully sites (a) 15, (b) 4, and (c) 17 with their corresponding 1D models. Insert: layer parameters generated from the computer modeling exercise.

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    Composite topography and (a) hydraulic head contour and (b) gully depth distribution map of Anambra State.

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An Assessment of the Influences of Surface and Subsurface Water Level Dynamics in the Development of Gullies in Anambra State, Southeastern Nigeria

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  • 1 Department of Geological Sciences, Nnamdi Azikiwe University, Awka, Nigeria
  • | 2 Applied Geophysics Programme, Department of Physics, University of Calabar, Calabar, Nigeria
  • | 3 Department of Geological Sciences, Nnamdi Azikiwe University, Awka, Nigeria
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Abstract

Gully erosion–induced problems have been challenging the people and government of Anambra State in southeastern Nigeria for a long time. In spite of the numerous geoscientific and engineering studies so far conducted in the area, the underlying causes of these problems still remain poorly understood. In an attempt to contribute to the understanding of the underlying processes responsible for the persistent gully erosion problems in Anambra State, an integrated study utilizing hydrological, geomorphological, and geophysical data was undertaken. Results of the analyses show that bulk density, pH, and organic matter content of the soil range from 1610 to 1740 kg m−3, 5.10 to 5.30, and 0.32% to 0.46%, respectively. Particle size analyses results show that the soils are dominated by coarse sand materials (50%–68%). Variations in the Atterberg limit parameters (liquid limit, plastic limit, and plasticity index) also point to the dominance of coarse materials in the shallow subsurface. Vertical electrical sounding results capture the shallow surface as being dominated by resistive sandy materials that are underlain by lowly resistive clayey materials. Thus, the area is dominated by porous, friable, and poorly cemented coarse materials that are located on a long and steeply sloping terrain of the tectonically elevated Awka–Orlu cuesta. Both overland and subsurface flow processes are responsible for the gully erosion problems confronting the area. Human activities (e.g., deforestation, uncontrolled urbanization, and absence of requisite legislation to protect the environment) and the high elevation of the Awka–Orlu cuesta have aggravated the severity of the problems. An aggressive reforestation program particularly with native trees, promulgation of necessary legislation to protect the environment, and setting up and empowering an enforcement agency should be vigorously pursued. Also, necessary enlightenment campaigns on best agricultural practices that can reduce surface runoff in soil and water conservation may also be helpful in changing the mindset of people.

Corresponding author address: Anthony E. Akpan, Applied Geophysics Programme, Department of Physics, University of Calabar, PMB 1115, Calabar, Cross River State, Nigeria. E-mail address: anthonyakpan@yahoo.com

Abstract

Gully erosion–induced problems have been challenging the people and government of Anambra State in southeastern Nigeria for a long time. In spite of the numerous geoscientific and engineering studies so far conducted in the area, the underlying causes of these problems still remain poorly understood. In an attempt to contribute to the understanding of the underlying processes responsible for the persistent gully erosion problems in Anambra State, an integrated study utilizing hydrological, geomorphological, and geophysical data was undertaken. Results of the analyses show that bulk density, pH, and organic matter content of the soil range from 1610 to 1740 kg m−3, 5.10 to 5.30, and 0.32% to 0.46%, respectively. Particle size analyses results show that the soils are dominated by coarse sand materials (50%–68%). Variations in the Atterberg limit parameters (liquid limit, plastic limit, and plasticity index) also point to the dominance of coarse materials in the shallow subsurface. Vertical electrical sounding results capture the shallow surface as being dominated by resistive sandy materials that are underlain by lowly resistive clayey materials. Thus, the area is dominated by porous, friable, and poorly cemented coarse materials that are located on a long and steeply sloping terrain of the tectonically elevated Awka–Orlu cuesta. Both overland and subsurface flow processes are responsible for the gully erosion problems confronting the area. Human activities (e.g., deforestation, uncontrolled urbanization, and absence of requisite legislation to protect the environment) and the high elevation of the Awka–Orlu cuesta have aggravated the severity of the problems. An aggressive reforestation program particularly with native trees, promulgation of necessary legislation to protect the environment, and setting up and empowering an enforcement agency should be vigorously pursued. Also, necessary enlightenment campaigns on best agricultural practices that can reduce surface runoff in soil and water conservation may also be helpful in changing the mindset of people.

Corresponding author address: Anthony E. Akpan, Applied Geophysics Programme, Department of Physics, University of Calabar, PMB 1115, Calabar, Cross River State, Nigeria. E-mail address: anthonyakpan@yahoo.com

1. Introduction

Natural hazards are physical events that have the potential of causing severe damage and harm to man and the environment. Many natural processes, like those related to atmospheric, hydrologic, geologic, and biologic activities, and some manmade processes (especially those related to technological urbanization: e.g., road construction, poorly maintained drainage facilities, deforestation, farming, etc.) are common inducing agents for the various forms of natural hazards that affect our environment. Although the occurrence of natural disasters is a global problem on the earth surface, the impacts of natural disaster on both man and environment have been found to be dependent on the spatial location of the place of the occurrence, strength of the event, and human vulnerability in such areas (Alcántara-Ayala 2002).

Coincidentally, many developing countries are located in areas that are more prone to natural disasters (erosions, floodings, landslides, windstorms, droughts, desertification, tsunamis, volcanic and other seismic activities, etc.) than the developed countries. Many of these countries lack the needed financial, technical, and institutional capacity to tackle these problems head on, thus exacerbating the severity of natural disaster–induced problems (Tamene and Vlek 2007; Tebebu et al. 2010). Although Nigeria is a third-world country, it is comparatively free from the destructions that usually accompany the incidence of the more devastating natural disasters (e.g., earthquakes and tsunamis) (Akpan and Yakubu 2010). Rather, government at all levels and its development partners spend its resources in tackling problems associated with flooding, erosion, desertification, storm, landslide, and so on that occasionally plague parts of the country (Egboka and Nwankwor 1985; Akpan et al. 2009; Ezezika and Adetona 2011; Okoro et al. 2011).

Gully erosion is an endemic environmental problem in southeastern Nigeria (see Table 1) and Anambra State is not an exception (see Figure 1). Previous studies conducted by Igbokwe et al. (Igbokwe et al. 2008) have shown that, in Anambra State, 37% of the total landmass is severely gullied, 28% is moderately gullied, and 35% is mildly gullied. These gullies have created many problems including loss of arable land through deterioration in soil quality; loss of natural habitats; loss of plant diversity, animals, and microbes; destruction of socioinfrastructural facilities (e.g., power transmission and distribution lines, schools, roads, hospitals, etc.); and, recently, deterioration in water quality (Ezezika and Adetona 2011; Okoro et al. 2011). Several people and even communities have been relocated to safer areas as a result of erosion-induced problems in the state (Egboka and Okpoko 1984; Igbokwe et al. 2008). To sustain life through optimal food production, farmers in the area have resorted to excessive application of chemical fertilizers, pesticides, and irrigation farming. These farming practices have some undesirable effects, like increasing soil and groundwater salinity levels and contaminating the soil and water resources (Gaus and Vande Casteele 2004; Koukadaki et al. 2007; Gemitzi et al. 2009; Mohamed et al. 2009; Sikandar et al. 2010; Kirkegaard et al. 2011; Zarroca et al. 2011; Russo and Taddia 2012). Other erosion-induced environmental problems in the state include the rising trend in the sedimentation of rivers, streams, lakes, and other surface water reservoirs (De Vente et al. 2005; Huon et al. 2005; Valentin et al. 2005). The latter problem has been attributed to the high rate in which overland runoff deposited detached soil particles transported by it in the surface water bodies (Pandey et al. 2007; Tamene and Vlek 2007). Siltation of reservoirs and dams can in turn reduce the water storage capacity of dams as well as shortening the lifespan of water reservoirs (Pimentel and Kounang 1998). Studies have shown that the recent flooding problems that are currently affecting many parts of the world can be attributed to heavy sedimentation of rivers, lakes, and other surface water bodies.

Table 1.

Distribution of gully erosion sites in at different stages of development in southeastern Nigeria (extracted from Igbokwe et al. 2008).

Table 1.
Figure 1.
Figure 1.

Typical gully erosion sites in Anambra State.

Citation: Earth Interactions 18, 4; 10.1175/2012EI000488.1

Although several erosion management strategies have been tested by both government, nongovernmental organizations, and other intervention agencies, the applicable strategies are dominated by construction of check dams in critically affected areas; construction of drainage channels (see Figure 1) that terminate in the natural drains like the rivers, streams, and so on; grading, landscaping, stabilizing, planting and terracing of slopes; protection of shorelines; construction of embankments; tree planting; marsh establishment; establishment of soil conservation schemes; and so on (Igbokwe et al. 2008; Okoro et al. 2011). Recently, the government, environmentalists, and other development partners have embarked on massive educational and awareness campaigns that are aimed at sensitizing the people on the relevance of trees in reducing the incidence of gully erosion and other environmental problems in the state. Despite all these efforts, erosion-induced problems in the state have increased more than ever and the trend seems to correlate well with the increased growth in human population, human activities within the natural ecosystem, and quantity of easily erodible materials in the soil (Hudec et al. 1998; Pimentel and Kounang 1998; Hurni et al. 2005; Kometa and Akoh 2012).

Many geoscientific and engineering investigations have been commissioned by the government and some donor agencies to unravel the remote causes and impacts and possibly advise the government on the best approach to ameliorate the effects of gully erosion in the Anambra State. Several broad-based recommendations, including construction of check dams in critically affected areas, construction of drainage channels, grading, landscaping, stabilization, planting and terracing of slopes, protection of shorelines, construction of embankments, tree planting, marsh establishment, establishment of soil conservation schemes, and so on, have been made. The government and other donor agencies have been trying to implement these recommendations from their meager resources (Nwajide and Hoque 1979; Okagbue and Uma 1987; Egboka et al. 1990; Igbokwe et al. 2008). Until now, no realistic solution has been found to the problem (Igbokwe et al. 2008; Ezezika and Adetona 2011; Okoro et al. 2011; Igwe 2012). Studies so far conducted on the causes and impacts of the gully erosion in the area have basically been revolving around geological and geotechnical investigation techniques (Nwajide and Hoque 1979; Egboka and Okpoko 1984; Okagbue and Uma 1987; Egboka et al. 1990; Igbokwe et al. 2008; Igwe 2012). Although not much attention was hitherto given to the influence of subsurface flow processes in the initiation and development of gully erosion, recent research findings from locations with similar geologic, geotectonic, and geomorphic settings and problems have shown that surface and subsurface groundwater flows can be a major contributory factor in the formation and development of gullies (Poesen et al. 2003; Carey 2006; Bu et al. 2008; Moges and Holden 2008; He et al. 2010; Tebebu et al. 2010). It has therefore become necessary to integrate these newer ideas in investigating the role of hydrological processes in the initiation and development of gullies and other erosion-induced environmental problems in Anambra State. This study is aimed at integrating groundwater, soil, and geomorphic information in assessing the underlying causes of gully erosion problems in Anambra State, southeastern Nigeria.

2. Physiography and geology of the study area

The study area is located between latitudes 6°00′ and 7°00′N and longitudes of 6°45′ and 7°20′E, covering an area of about 1706 km2 in southeastern Nigeria (Figures 2a,b). It lies within the humid tropical rainforest belt of West Africa, although anthropogenic activities such as lumbering, urbanization, road construction, and other forms of deforestation-induced activities have led to the loss of the primary forest. The rainy season (April–October) and dry season (November–March) are the two major climatic conditions that exist in the study area. The recent worldwide climatic changes with wide variations in the onset and cessation of these two climatic conditions have not only aggravated the existing environmental problems but also exacerbated the problem of gully erosion and land degradation in the state (Farauta et al. 2012; Iwuchukwu and Onyeme 2012). The mean annual rainfall data covering the period from 1997 to 2007 obtained from the Nigeria Meteorological Agency, Awka, are as shown in Figure 3.

Figure 2.
Figure 2.

(a) Outline map of Nigeria showing the location of Anambra State and (b) generalized geological map of Anambra State.

Citation: Earth Interactions 18, 4; 10.1175/2012EI000488.1

Figure 3.
Figure 3.

Mean annual rainfall of the study area for the period 1997–2007 (source: Nigeria Meteorological Agency, Awka).

Citation: Earth Interactions 18, 4; 10.1175/2012EI000488.1

Anambra State is located within the Anambra Basin in southeastern Nigeria. Two major landforms that consist of topographically elevated and low-lying regions dominate the entire landscape of Anambra State (Figure 4a). The high-lying areas occupy the southern parts of the state east and south of the Anambra River, while the low-lying plains cover a greater part of the western, northern, and northeastern areas (Figure 4a). The highland area has a low asymmetrical ridge (or cuesta) that runs northwest–southeast and occupies the northern portion of the Awka–Orlu Uplands (Figure 4b). A network of rivers, streams, and lakes drains the area. The major rivers include the southward-flowing Niger River, which flows into the Atlantic Ocean, and the Anambra River, which flows southward into the Niger River (Figures 2b, 4b).

Figure 4.
Figure 4.

(a) Digital elevation map of Anambra State and (b) drainage pattern on top of the Awka–Orlu cuesta [(b) was modified from Egboka and Okpoko 1984].

Citation: Earth Interactions 18, 4; 10.1175/2012EI000488.1

Anambra Basin is a product of tectonic disturbances in the area and was formed during the Santonian upliftment of Albian sediments in the lower Benue Trough. The subsequent deposition of sediments within the basin resulted in the formation of ancient Cretaceous deltas with Nkporo shale, Mamu formation, Ajali sandstone, and Nsukka formation dominating the sedimentary deposits (Figure 2b). The distribution of these geologic formations and the general lithostratigraphic sequence has been discussed by many researchers, such as Kogbe (Kogbe 1976), Murat (Murat 1970), Reyment (Reyment 1965), and Egboka and Okpoko (Egboka and Okpoko 1984).

The existence of proerosion lithologic units and other subsurface structural discontinuities like joints and cracks and their inherent capacity to be detached and transported by direct impact of rainfall and surface and subsurface flow processes, leading to the formation of gullies, landslides, and other environmental problems have been discussed extensively in the works of Burke (Burke 1969a; Burke 1969b), Foster (Foster 1978), Egboka and Okpoko (Egboka and Okpoko 1984), Okagbue and Uma (Okagbue and Uma 1987), Egboka et al. (Egboka et al. 1990), Hudec et al. (Hudec et al. 1998), Okoro et al. (Okoro et al. 2011), and Igwe (Igwe 2012). The sediments in the area are dominated by poorly cemented sandy materials with very low organic matter content. These sediments are surrounded by northeast–southwest trending fractures that originate from the Atlantic Ocean extending into the West African subregion with isolated zones of high seismic activities and by implication areas with potential crustal instability (Adepelumi et al. 2008; Amponsah 2002; Wright 1976; Burke 1969a).

The Nanka sand, which overlies the Paleocene Imo shale in many locations in the southeast and central parts of the state, forms the main geologic unit that is extensively exposed and is characterized by the prominent Awka–Orlu regional cuesta. The Nanka formation is dominated by a generalized sequence of unconsolidated, unstable, loose, friable, and poorly cemented sands with intercalating clay/shale layers, as discussed in the works of Reyment (Reyment 1965), Nwajide and Hoque (Nwajide and Hoque 1979), Egboka and Okpoko (Egboka and Okpoko 1984), and Igbokwe et al. (Igbokwe et al. 2008). Lithologically, the Nanka sands consist of distinct units of sand, shale–siltstone, and finely laminated shale. The aquifers within this formation exist under partially to fully confined conditions and are consequently characterized by high pore water pressures. Hydraulic conductivity, transmissivity, and plasticity index in the Nanka sand formation range from 1.20 × 10−1 to 5.93 × 10−1 cm s−1, 1.15 × 10−5 to 13.05 × 10−3 m2 s−1, and 12.50% to 36.57%, respectively (Okoro et al. 2011).

The distribution and density of gully erosion sites in the state are more concentrated in the unconsolidated sand–dominated Nanka sand formation that harbors the Awka–Orlu cuesta. This is because the formation exhibits typical hydrogeological and geotechnical characteristics that are favorable to the initiation and development of gullies. According to Egboka and Okpoko (Egboka and Okpoko 1984), such properties include the dominance of loose, friable, poorly cemented, and highly permeable sands within the formation. The Awka–Orlu cuesta has a crest that stands in excess of 350 m above mean sea level (MSL) and is characterized by moderate–steep slopes with long sides. These properties are not favorable to continuous downward infiltration of surface water but rather are favorable to overland runoff flows over long distances (see Esteves et al. 2005; Mathys et al. 2005; Zhang et al. 2012; Trabucchi et al. 2012). Under such conditions, if the energy of the overland-flowing mass of water overcomes the threshold resistance of the soil particles, then such soil particles will be detached, transported, and deposited at the downward end of the slope (Billi and Dramis 2003; Larionov et al. 2004; Wells et al. 2013; Chaplot 2013).

Most of the gullies originate at variable distances from the topographically elevated Awka–Orlu sandstone-dominated cuesta and attain maximum depth downslope. The gullies are dimensionally variable, although age, amount of rainfall, and location seem to exert significant influence on their dimension among other competing factors. The length of the gullies ranges from about 25 m to 2.9 km while the width of the gullies ranges from less than 2 m in the upstream section to over 300 m. The width is usually more in the upstream section (fan) than in the downstream section. At the upstream stage, the gullies are usually V shaped in a cross section with a typical lateral coverage of less than 1 m and a thickness of just few meters. But, as the length of the gullies increase, the gully widths usually increase to a maximum size in the downstream end with the gully shape gradually changing into a rectangular (U)-shaped figure. The gully walls are usually divided into microblocks by the pressure relief cracks that run in all directions on the gully walls.

Igbokwe et al. (Igbokwe et al. 2008) and Igwe (Igwe 2012) discussed the prevailing gully erosion–inducing factors in the state to include topography, climate, vegetation, geology, soil, and human factors. Egboka and Okpoko (Egboka and Okpoko 1984) pointed out that annual dynamic fluctuations in the groundwater table are another important factor that enhances gully initiation and growth in the area. An increase in hydraulic heads can enhance gully development through an increase in flow rate. Besides, seasonal expansion and contraction of impermeable clayey (and/or shaly) materials can also increase the rate of gully development. In the process of changing in size according to the prevalent hydrological conditions, the clays and shales lose their shear strength and develop tension cracks that are usually transferred to the over- and underlying sandy materials. These tension cracks serve as pathways through which water penetrates into deeper depths in the area. Stress set up by the expansion and contraction of clay/shaly materials causes instability in the already cracked adjoining sandstone materials by causing them to slide into the gully floor (Egboka and Okpoko 1984; Porter and Trenhaile 2007). The sliding materials used to cause trees like bamboos, oil palms, cashews, and so on and even houses to fall into the gullies.

3. The conceptual model

The southern and southeastern parts of Anambra State are occupied by the topographically elevated sandstone-dominated Awka–Orlu cuesta. The crest of the cuesta is ~350 m MSL and the cuesta is drained by many rivers including the Nkisi, Idemili, Uchu, Aghomili, Mamu, and Crashi Rivers (Figure 4b). The Agulu, Uchu, and Ulasi Lakes are some of the lakes that also drain the area (Figure 4b). All the water resources in the area are usually recharged by water from rainfall that also made the flood plains of these water resources to be seasonal. Toward the crest of the cuesta, static water level (SWL) water table elevations are usually very high, paving the way for reduced water loss by evapotranspiration (Bauer et al. 2006). The reduced loss of moisture to the air by evapotranspiration usually results in increased percolation. In the rainy season, the water from heavy and continuous rains will either flow on the surface or percolate into the ground, with the former dominating because of the steepness of the cuesta. As a continuously increasing mass of water flows downhill from the crest of the cuesta, the energy of the moving mass of water usually increases beyond the threshold, binding energy of the soil particles. Consequently, the moving mass of water can easily detach the soil particles, transport them, and deposit them at the foot of the cuesta (Egboka and Okpoko 1984). Continuous detachment of soil particles around the foothill of the cuesta results in gully formation.

The continuous movement of the infiltrating water is usually truncated by the thinly bedded intercalating clayey/shaly materials existing within the Eocene Nanka sand formation. Such truncations will cause the infiltrating water to accumulate such that, when threshold conditions required for subsurface flow pointed out by Lobkovsky et al. (Lobkovsky et al. 2004) are exceeded, topographic- and hydraulic-induced flow will begin. Lobkovsky et al. (Lobkovsky et al. 2004) has presented experimentally backed discussions on these threshold conditions. Such flows usually result in subsurface-flowing water plumes emerging on the surface, especially in the rainy season, leading to the ephemeral seepage or sapping erosion in many locations. Also, at the sand–clay interface, the weight of the accumulating water will subject the underlying thin clay traps to a continuously increasing stress such that, if a critical stress point is exceeded, water can also pass through it by seepage into the underlying sandy materials. Figure 5 shows this conceptual model for the formation of the persisting gully erosion problems in the area.

Figure 5.
Figure 5.

A conceptual model for the formation of the persisting gully erosion problems in Anambra State.

Citation: Earth Interactions 18, 4; 10.1175/2012EI000488.1

4. Methodology

The study was conducted between July 2009 and March 2011. It involved surveying 20 prominent gully erosion sites that were at different stages of formation in the state. At each gully erosion site, the geographic coordinates of their spatial location coupled with their elevation above mean sea level were measured using a differential global position system (DGPS) in order to ensure reliable location of vertical distances MSL. Measuring tape was used to measure the dimension of each gully. Hydraulic heads (piezometric surfaces) above mean sea level were determined by computing the difference between the DGPS-measured elevation reading and the SWL measurement of the nearest borehole. Scatter and box plots showing the distribution of the gully depths and hydraulic heads are shown in Figure 6a. Graphical technique was adopted in quantitatively establishing the relationship between the hydraulic head and gully depth in the area (Figure 6b). The graphical results presented in Figure 6b show that hydraulic head (HH) and gully depth (GD) data in the area are related by the power expression of the form
e1
with acceptable correlation coefficient R2 of 0.50 for spatially separated data.
Figure 6.
Figure 6.

(a) Scatterplot and (b) box plot showing the distribution of gully depths (red color) and hydraulic heads (blue color) and (c) graph of gully depth against hydraulic head in Anambra State.

Citation: Earth Interactions 18, 4; 10.1175/2012EI000488.1

To assess the influence of soil properties in the formation and development of gullies in the area, 12 well-distributed soil samples were collected at accessible gully sites (Figure 4a) at depth ranges of 0.5–1.0 m using soil auger and core sampler. The samples were collected at Agulu, Nanka, Ekwuluobia, Adazi-Nnukwu, Neni, Alor, Oraukwu, Nimo, Abagana, Ukpo, Nawfia, and Awka (Figure 4a). The samples were carefully wrapped; stored in airtight polythene bags; and analyzed to determine values of bulk density, pH, particle size distribution, and Atterberg limits [liquid limits (LL), plastic limits (PL), and plasticity index (PI)] of the soil samples. The particle size analysis was performed using an American type of standard sieve (half-phi American Society for Testing and Materials stand), the pH was measured using a pH meter, and the LL was estimated using Casagrande equipment. Results of the soil analyses are shown in Table 2.

Table 2.

Results of soil analyses.

Table 2.

Depths to water table were measured at boreholes located close to the gully erosion sites using depth sounders so as to establish the impact of groundwater flow on gully erosion initiation, formation, and growth. At three gully erosion sites (see Figure 4b) where no boreholes exist in the neighborhood, the depth to the water table was estimated from geophysical investigation involving electrical resistivity studies that were performed using the Schlumberger electrode array. Three vertical electrical soundings (VES) were performed in the area using SAS1000 model of ABEM terrameter manufactured by ABEM Instrument, Sweden. Maximum current electrode spacing (AB) was constrained by the human settlement pattern and other space limiting conditions to vary between 400 and 500 m. The 500-m spread length was performed only in one settlement where fairly straight paths exist to spread the cables in order to ensure that depths above 80 m were sampled assuming that penetration depth varies between 0.25AB and 0.5AB (Roy and Elliot 1981; Singh 2005; Akpan et al. 2013). Corresponding receiving (potential electrode) separation (MN) varied from a minimum of 0.5 m at AB = 2 m to a maximum of 20 m at AB = 500 m. VES data acquisition was performed in the wet season, and therefore data quality was generally good. Where necessary, however, the electrode positions were usually wetted with water and salt solution in order to lower the contact resistance and consequently ensure good electrical contact between the ground and the steel electrodes.

5. Data analysis, results, and discussion of results

Both manual and computer modeling techniques were employed in the reduction of VES data (Zohdy 1965; Zohdy et al. 1974) to their geological equivalent. Transformation of the measured apparent resistance Ra to their corresponding apparent resistivity ρa was achieved by using
e2

The smoothed curves were quantitatively interpreted in terms of true resistivity and thickness by a conventional manual curve matching procedure using master curves and auxiliary charts (Orellana and Mooney 1966). Computer software called Resist (Vender Velpen 1988) was used to improve upon the manually interpreted results. The primary layer parameters comprising thicknesses and depths obtained from the manual interpretation phase were entered as one of the inputs into the computer modeling software. The computer software used these parameters to generate data for the model and compared the computed data with their measured counterpart. The extent of fit between the calculated and the measured data was assessed using the root-mean-square error (RMSE) in which 10% was set as the maximum accepted value. The modeled VES curves are shown in Figure 7.

Figure 7.
Figure 7.

Modeled VES curves observed at gully sites (a) 15, (b) 4, and (c) 17 with their corresponding 1D models. Insert: layer parameters generated from the computer modeling exercise.

Citation: Earth Interactions 18, 4; 10.1175/2012EI000488.1

Results from the geophysical studies show that the shallow subsurface can be represented by a four-layered subsurface structure at all the sites. The various geoelectric layers are characterized by low to very high electrical resistivity values. At VES stations 1 and 3, the electrical resistivity values of the first geoelectric layer ρ1 were observed to be higher than 2800 Ω m, while, at VES station 2, the resistivity was observed to be higher than 11 600 Ω m (ρ1 > 11 600 Ω m). The high electrical resistivity values observed at VES stations 1 and 3 were suspected to be indicative of some poorly cemented lateritic coarse sand material with thickness of about 1 m. However, at VES station 2, where the resistivity and thickness were observed to be highest (ρ = 11 689 Ω m with a thickness of about 7 m), very dry coarse and loose nearshore sandy materials were inferred to be the dominant lithologic composition. The inferred lithologic compositions are exposed everywhere on the surface.

The second geoelectric layer is characterized by moderate to very high electrical resistivity values. Electrical resistivity values of 5854, 261, and 11 880 Ω m were observed at VES stations 1, 2, and 3, respectively (see Figure 4a), with corresponding depths to bottom of about 6, 16, and 5 m, respectively. The second layer under VES station 2 was suspected to be a saturated medium sand layer (ρ = 261 Ω m) while, at VES stations 1 and 3, the lithologic composition was suspected to be the lateral equivalent of the coarse and loose sand materials observed as the first layer under VES station 2. The extensive surface exposures and cross sections at some of the gully erosion sites were used as a guide in these interpretations. At VES station 1, the coarse sand materials contain small amounts of cementing materials that caused its resistivity value to be reduced.

The third layer at all the stations is moderately resistive with electrical resistivity values of 251, 127, and 43 Ω m at VES stations 1, 2, and 3, respectively. The sharp drop in electrical resistivity value between the second and the third layers was interpreted to be indicative of the water table. Poorly sorted sandy materials with textural properties that vary from fine to medium were suspected to dominate the lithological composition of this layer. The thickness of this layer was observed to be in excess of 75 m at VES stations 2 and 3, except at VES station 1, where it was observed to be about 31 m. The fourth electrostratigraphic unit, whose depth to bottom could not be mapped from the data acquired in the field, is characterized by moderately high electrical resistivity values. At VES stations 1, 2, and 3, observed resistivities were 4908, 320, and 220 Ω m, respectively, and were inferred to be electrical resistivity responses from materials that are dominated by resistive medium–coarse sands. The observed increase in the resistivity of the last layer has been attributed by previous researchers such as Onuoha and Mbazi (Onuoha and Mbazi 1988) and Okiongbo and Akpofure (Okiongbo and Akpofure 2012) to be due to an increase in the grain size of the highly resistive sedimentary rocks in the area at depths. Around the VES station 1 point, the sediments were suspected to be partially saturated.

The bulk density of the soil samples varies from 1610 to 1740 kg m−3 (average of 1685 kg m−3 with a standard deviation of 37 kg m−3). A mean density of 1685 kg m−3 is reflective of the dominance of recent deposits of relatively unconsolidated sediments in the area. These results are comparatively lower than the mean density of 2670 kg m−3 for materials in the upper continental crust that is commonly used in geological, geophysical, and geodetic applications (Robert et al. 2011). Since the soils are dominated by lighter materials, then raindrop impact can easily detach the soil particles from their parent materials. The low density results might also be reflective of the poor bonding condition that exist between the soil particles.

The variational range of the organic matter content of the soil (0.32%–0.46%) shows that the soil in the area is poorly enriched in organic matter. This shows that the soils in the area are porous and cannot support the growth of some plants that are supposed to protect the soils from erosion. The results observed from the pH measurements shown in Table 2 show that the soils are slightly acidic. These results corroborate with the findings of Egboka and Okpoko (Egboka and Okpoko 1984), who observed that both the surface water and groundwater resources in the area are slightly acidic. The acidic matter contents of the water might have migrated into the adjoining soils, thereby causing decomposition of the cementing materials. The soils that the cementing materials have been loosened will become friable and more susceptible to erosion (Egboka and Okpoko 1984). Such soils can be easily detached and transported from one point to another by agents of erosion.

The mean values of the particle size analyses show that the soils are dominated by coarse sand materials with characteristic readings that vary slightly from 50.0% to 68.0% (average of 57.5% and standard deviation of 6.1%). These results are indicative of soils in the area having originated from a common geologic process. The lack of proper cementation within the coarse materials is captured by the low signatures of clay materials in the samples. From the results, the clay contents of the soils range from 9.0% to 15.0% (average of 10.8% and standard deviation of 3.4%), implying that the sandy materials are poorly cemented, unconsolidated, friable, and loose. These types of materials are usually cohesionless and therefore highly susceptible to erosion, especially when present in a sloping terrain since they can be easily detached by low energy surface runoff and other agents of erosion. The Atterberg limits of the soils vary narrowly from one location to the other. Values of the liquid limit ranged from 32.5% to 60.0% (mean of 42.8% and standard deviation δ of 9.6%), while values of the plastic limit vary from 19.2% to 26.3% (mean of 21.6% and δ of 2.5%). Values of the plasticity index that were computed by finding the difference between the LL and PL were observed to vary from 12.5% to 36.6% (mean of 21.2% and δ of 8.0%). PI results show that the soils consist of cohesionless materials that cannot withstand stress because of the absence of cohesion between them. Research has shown that the rate of detachment of soils particles decreases as the extent of cementation increases (Rockwell 2002). Water can easily flow through these soils as the results of the LL test suggest, but the prevailing steep slope in the area will reduce direct infiltration in favor of overland flow. As the mass of water moves downslope over a sloping cohesionless sandy material, the energy of the moving mass of water down the slope will exceed the threshold resistance offered by the loosely held sandy materials. Thus, the sandy materials can be easily detached by the moving water. Consequently, up the cuesta where the moving mass of water has lower energy, gully depths are small compared to the downhill sides where gully depths are more. This observation is in conformity with the inverse relationship between gully depths and hydraulic heads shown in Equation (1). Some of the soil samples show zero plasticity, implying that they are nonplastic.

Observed values of hydraulic head data that vary from 7 to 230 m MSL and gully depths were gridded using the minimum curvature technique. The gridded data were added in a contoured format to the political map of Anambra State using the Surfer 11 contouring software from Golden Software, Inc., United States. The Surfer contouring software deploys its built-in interpolation and extrapolation tools in contouring the data in order to quantitatively assess their spatial distribution in the state (see Figures 8a,b). The HH contours were observed to trend in a predominantly northwest–southeast direction in concordance with the Awka–Orlu cuesta. As captured in Equation (1), the HH and the GD are inversely related, suggesting that gully depths are low at the vicinity of the crest of the cuesta and increase toward the foothill of the cuesta. The majority of the rivers (Orashi, Njaba, Idemilli, Nkisi, Uchu, and Obibia Rivers in Figure 4b) that rise from the asymmetrical ridge flow downhill along kinks or (fractures) in directions that are almost perpendicular to the HH contours gradient. Many of the westward-flowing rivers empty into the Anambra and Niger Rivers.

Figure 8.
Figure 8.

Composite topography and (a) hydraulic head contour and (b) gully depth distribution map of Anambra State.

Citation: Earth Interactions 18, 4; 10.1175/2012EI000488.1

More gullies exist at locations adjacent to the tectonically elevated Awka–Orlu cuesta in the southern parts, where the hydraulic gradients are comparatively low (Figure 8b). The elevated Awka–Orlu cuesta is characterized by a steep and variable slope, which usually led to increase in the speed and volume of overland runoff. As captured in Figure 8b, GDs are higher at communities located at the foot of the cuesta, where HHs are comparatively higher than at communities located close to the crest of the cuesta. This observation is in accordance with the expectation of Equation (1), which expects GDs to be low at locations with high HH. Thus, overland flow and direct detachment processes seem to be the dominant sources of soil particle detachment since around the top of cuesta, surface runoff and raindrop energy may not be high enough to overcome the binding energy of the soil particles. But, when moving at higher speeds, the moving mass of the large volume of surface runoff can gain enough energy to easily detach the already loose and porous soil particles. Consequently, around the foothill of the cuesta, surface runoff and raindrops may have acquired enough energy by virtue of the long distances that they have traversed. Again, the consolidated nature of the cuesta might have rendered direct detachment of sand particles close to the crest of the cuesta difficult and therefore gully depths are low near the crest.

The chance of direct detachment is higher now that the natural vegetation that is supposed to provide a primary protection has now been replaced by secondary vegetation and, in some cases, the topographic surfaces are left bare (Igbokwe et al. 2008; Shakesby 2011). The detached soil particles are usually transported by surface runoff down to the foothill of the elevated areas, the valleys and the river plains where they are usually deposited. Also, soil losses will be more on longer slopes because the acceleration and accumulation of water will be higher on long-sided slopes than on small-sided slopes (Cerdà et al. 2010). On our study area, the Awka–Orlu cuesta is the highest elevated point with a crest of the cuesta standing at over 350 m MSL. Thus, topography seems to exert significant influence on the development of gullies in the area.

In locations where the gully bottom is below the water table such that subsurface-flowing water emerges in the surface as effluent seepages (springs and ponds, especially in the rainy season), gully erosion problems are usually more severe. These sapping erosion–prone areas dominate the lowly elevated areas around the foothill of the prominent Awka–Orlu cuesta where impervious Paleocene Imo shales are exposed. Thus, these locations suffer from the erosive powers of both surface (overland) and subsurface flow processes. This observation is in conformity with the conceptual model (Figure 5) and the GD–HH relationship (Figure 6). Although the erosive powers of both processes are threshold dependent, seepage erosion has the capability of creating gullies with different dimensions (Lobkovsky et al. 2004) and has been suggested to be the dominant process that created the widespread erosion features observed on the surface of Mars (Baker 1990; Aharonson et al. 2002).

The dominant orientation of most of the hydraulic head contours is perpendicular to the rivers and streams that rise from the cuesta. Most of the northwest-flowing tributaries of Anambra River usually discharge water turbulently into the Anambra River in an area where elevations are generally less that 20 m MSL (see Figure 4a). In the rainy season, large volumes of water are usually discharged rapidly and sometimes turbulently into the Anambra River by its tributaries. The large volumes of water usually discharged into the Anambra River do sometimes exceed its carrying capacity, thereby forcing the river to overflow its banks, leading to flooding of the low-lying areas. In the process of turbulent discharges, some of the discharged waters are likely to be forced into the pores of the loosely held and porous Nanka sands, leading to further dissolution of the cementing materials. Besides, the overflowing of the bank of the Anambra River is likely to cause further dissolution of the same cementing materials and transporting them to other locations when the water recedes. In the event of bank overflow, rapid and continuous infiltration of water into the pores will cause pore water pressure at the sand–clay interface to increase, leading to further weakening of the loosely held soil structure. The weakened soil structure will be influenced by both overland and subsurface flows. The materials at the sand–clay interface will be gradually transported by subsurface seepage flow processes to other locations, leading to the creation of a void. Gravitational forces will eventually cause the weakly held materials over the voids to collapse (S. E. Ekwok 2012, personal communication). The process can occur at both the head cut and at the walls of the gully. The collapsed materials will eventually be transported by both surface and subsurface flows and deposited downslope. These processes will cause the dimensions of the gullies to increase at a faster rate. Okagbue and Uma (Okagbue and Uma 1987) attributed the high rate of gully formation and expansion in the area to the weakening of the effective strength of the unconsolidated materials along the seepage faces by high pore water pressure in the area.

These observations provoked questions on the nature of the relationship existing between the properties of the vadose zone and the rate of gully expansion. Equation (1) quantitatively captures the nature of the relationship existing between HHs and the rate of GDs expansion in the area. Thus, the weakly held cementing materials will continuously suffer rapid dissolution in the low HH areas that dominate the low-lying areas, whereas the rate of dissolution will be low at the high HH endemic areas. The high expansion rate of gully dimension, especially in the rainy season when the groundwater table is closer to the surface, originates from the high pore water pressure within the aquifers in the area. As pointed out by Egboka and Okpoko (Egboka and Okpoko 1984), along the seepage faces of the gully walls, a reduction in effective strength of the unconsolidated sediments will occur because of rapid dissolution of the cementing materials. This loss of strength will eventually leave the sands in a very loose and easily erodible state. On the other hand, when the impermeable clays/shales that underlie the weekly cemented sandy materials absorb water, reaching it by the process of infiltration, it will expand in size, which causes loss in mechanical properties like the shear strength. An increase in pore pressure at the sand–clay interface can lead to increases in hydrostatic pressure, leading to further reduction in shear strength as well as the internal friction between the rock matrices (Falcini et al. 2012). The effects of these processes will manifest in massive caving in, piping, slumping, and micro-landslides, especially in the rainy season because the pore pressure pushes the weakly held, saturated, and severely cracked soil outward (Tebebu et al. 2010). In the dry season, the ambient temperature will cause the intercalating clays/shales to dry and consequently contract, leading to the formation of cracks. These cracks are pathways through which excess pressure is released from the clays/shales. These cracks are usually transferred to the overlying sandstone materials, where they can be observed in the form of both vertical and horizontal directions (Egboka and Okpoko 1984; Tebebu et al. 2010). Gravitational and topographic-induced forces will eventually act on these loosely held but severely cracked sandy materials, causing them to fall into the gully bottom. This process can occur at both the headcut as well as the sides and, as more of these materials fall, the dimensions of the gully will increase.

The mechanical and hydrological effects of vegetation cover on slope stability have been recognized and are well documented in relevant literatures (Greenway 1987; Collison and Anderson 1996; Montgomery et al. 2002; Lane et al. 2004). Although plants can exert both detrimental as well as beneficial influences on slope stability (see Table 3), the absence of vegetation cover on slopes, as is the case now in many parts of Anambra State, should be discouraged. Also, the undercutting of slope by seepage erosion, the activities of sand miners (Igbokwe et al. 2008) at the lowly elevated parts of the area, and the presence of trees close to the undercut portions increase the downward component of the acceleration due to gravity in some locations. These activities reduce the frictional angle of the soils through reduction in the shear strength of the soils and enhance the rate of gully expansion in the process (Selby 1982). On the hydrological front, the net imbalance in the removal of water from the soil by plants through the evaporative processes could lead to the retention of more water in the soil. The evaporative process is supposed to reduce pore pressure build up through the lowering of the water table. Thus, the numerous environmental problems such as gully erosion confronting many parts of Anambra State could be attributed in part to the massive deforestation that the state has witnessed in recent years.

Table 3.

How vegetation affects slope stability and erosion (modified from Greenway 1987) Here, “A” means adverse to stability and “B” means beneficial to stability.

Table 3.

6. Suggested strategies for preventing and mitigating gully erosion problems in the area

Preventive measures will continue to be the best strategy to keep erosion and other environmental problems at bay in Anambra State. Enacting and enforcing all necessary legislations that will prevent developers from erecting structures along natural drains when appropriate channeling and/or alternative drains have not been provided should be put in place. All authorities that are responsible for providing drainage and other related facilities should ensure that proper drains and water channeling facilities are provided where necessary, considering the high susceptibility of the area to erosion by surface runoff and other water-induced environmental problems. Such infrastructures where necessary will help in channeling the runoff to safe areas where it will not cause serious harm to the environment.

The destruction of the original vegetation cover has been universally acknowledged as a major facilitator of gully erosion and other environmental problems (Okagbue and Uma 1987; Egboka and Okpoko 1984; Lane et al. 2004; Tebebu et al. 2010; Ezezika and Adetona 2011). Consequently, any control and/or mitigation strategy should primarily target replanting the native plants that originally grew in the affected areas. Planting of some tropical trees like African oil beans, eucalyptus, etc., that have the potential of economically boosting evapotranspiration should be encouraged. Such trees seem to have the potential of extracting more water from ground and consequently lower the pore water pressure associated with an increase in water table (Lane et al. 2004; Tebebu et al. 2010). Also, such trees can also help in boosting the shear strength of the soils.

Other intervention and preventive measures include demarcation of river basins into catchment and subcatchment areas for the design of appropriate civil engineering and agroforestry erosion control measures. The civil engineering measures include construction of flood channels that will terminate at safe locations such as the natural drains (river banks, etc.), dams/embankments, and catch pits; and the agroforestry measures include afforestation, selection of grazing lands, adoption of sustainable agricultural practices, etc. These factors could reduce the adverse effects of slope on erosion intensity (Greenway 1987; Montgomery et al. 2002; Cai et al. 2005; Bu et al. 2008). Among the various soil and water conservation measures that are beneficial to slope stability, level terracing is found to be one of the best practices in reducing surface runoff speed and consequently check erosion problems (He et al. 2010).

Enactment of necessary legislation to establish and empower environmental protection agencies at both state and national levels may be necessary in order to enforce the applicable environmental laws. Such an agency could be charged with the responsibility of information gathering and dissemination as well as encouragement of the use of local dry grasses for roofing purposes, which usually generates less surface runoff. Other intervention measures, such as the increase in environmental education through seminars and workshops as well as capacity building by informal and formal training of environmental scientists, should be encouraged.

6. Conclusions

Anambra State is generally underlain by materials that are susceptible to gully erosion and other water-induced environmental problems such as landslides, slope failures, and so on. These problems have rendered many families homeless. Loss of life, severe environmental degradation, and loss of arable lands are commonplace. Loss of soil through gully erosion has both onsite as well as offsite effects. While the onsite effects include loss of soil fertility and lowered water holding capacity, the offsite effects includes siltation of available water bodies (Tamene and Vlek 2007) with a resultant acute water surface scarcity.

Excess water discharged from all the northwest–southeast-flowing rivers into the southward-flowing Anambra River does sometimes exceed the carrying capacity of Anambra River, thereby forcing parts of the excess water into the pores of the loosely held materials. This results in pore water pressure buildup in the shallow groundwater system, leading to severe dissolution of the poor cementing materials and leaving the subsurface in a very loose state that is susceptible to erosion by surface runoff.

Destruction of the native vegetation that could have helped remove excess water from the ground and reduce the pore water buildup has aggravated the erosion problems. Absence of basic social infrastructures like drains, slope protection, etc., that are supposed to properly direct movement of surface runoff into the natural drains where they cannot harm the environment have not helped the situation. Aggressive reforestation programs and other sensitization campaigns should be pursued by government and environmentalists.

Acknowledgments

The authors are thankful to the Nigeria Meteorological Agency, Awka, for freely making the rainfall data available to us. We are grateful to Nnamdi Azikiwe University, Awka, Nigeria, for providing us with the required facilities and also to the anonymous reviewers whose comments and criticisms have shaped this article to its present form.

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