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GEOLOGY, ECOLOGY, AND LANDSCAPES 2021, VOL. 5, NO. 3, 227–240 INWASCON https://doi.org/10.1080/24749508.2020.1711637 RESEARCH ARTICLE The impact of hydrogeomorphological characteristics on gullying processes in erosion-prone geological units in parts of southeast Nigeria a,b a Johnbosco C. Egbueri and Ogbonnaya Igwe a b Department of Geology, University of Nigeria, Nsukka, Nigeria; Department of Geology, Chukwuemeka Odumegwu Ojukwu University, Uli, Nigeria ABSTRACT ARTICLE HISTORY Received 26 August 2018 Hydrogeomorphic factors were suspected to contribute to the persistent gully erosion taking Accepted 31 December 2019 place in Nanka, Ogwashi and Benin formations underlying the southern Anambra State, Nigeria. Therefore, this study investigated the impact of hydrology and geomorphology on KEYWORDS gully development and expansion in this area using integrated ﬁeld survey, hydrological, Anambra State; geoelectrical geotechnical and geomorphological approaches. Field survey and hydrological results survey; geotechnical; gully revealed that the study area is characterized by numerous surface water bodies and shallow erosion; groundwater systems. Both the surface waters and groundwater have a westward ﬂow direc- hydrogeomorphology tion, from areas of high elevations on the Nanka Formation to areas of low elevations on the Ogwashi and Benin formations. Geotechnical results revealed that the soils are permeable, weak, easily dispersible and collapsible. Geomorphological analysis showed that the area is characterized by uneven badland topography, high gully slope gradients, concave slopes, poor land-use practices, and low vegetation cover. Generally, the results of this study indicated that hydrogeomorphology and soil engineering properties substantially inﬂuence the gullying processes in the area. However, areas underlain by the Nanka Formation have higher gullying intensity than in areas underlain by the Ogwashi and Benin formations due to variations in their hydrogeomorphological characteristics. Introduction surface drainage facilities, poor land-use practices and poor vegetation cover, etc., contribute to the con- Of all the natural hazards reported from diﬀerent tinuous soil erosion and the development and expan- regions around the globe, gully erosion and mass sion of erosion gullies in this part of Nigeria (Emeh & wasting are the ones peculiar to many parts of Igwe, 2017, 2018; Igwe, 2012; Igwe & Egbueri, 2018; Nigeria. Gullies are natural geologic hazards which Nwajide, 2013; Okoyeh et al., 2014). arise due to persistent loss of soils. This geologic In the southern Anambra State, the towns especially hazard has been a serious environmental problem in exposed to this erosion menace include Uli, Okija, the southeastern Nigeria, causing the degradation of Ozubulu, Oraukwu, Nanka and Ekwulobia. These arable lands, destruction of civil engineering infra- towns are the major focus of the current study. Recent structures and underground utilities; silting and pollu- studies conducted by Obiadi et al. (2014), Chikwelu and tion of surface water bodies, and loss of estates and Ogbuagu (2014), and Igwe and Egbueri (2018)onthe resident lands, etc. However, reports have shown that gullies in the southern Anambra State majorly reported amongst the southeastern States in Nigeria, Anambra on the geological and geotechnical properties of the soils. State has the highest number of gullies, followed by In their studies, Obiadi et al. (2014) and Chikwelu and Enugu, Imo, Abia, and Ebonyi States, respectively Ogbuagu (2014) reported that the soils in this area are (Igbokwe et al., 2008; Okoyeh, Akpan, Egboka, & very weak and dispersible. They further hinted that Okeke, 2014). The variations in the gullying intensity anthropogenic activities also predispose the soils to ero- could be due to diﬀerences in causative factors and sion. Similarly, Igwe and Egbueri (2018) revealed that the types of geologic formations underlying these States. poor geotechnical characteristics of the soils and impro- Anambra State has more of the youngest and friable perlandusearethe majorcausesoferosion within the sedimentary deposits whereas Ebonyi State (with the area. However, recent research by Emeh and Igwe (2018) fewest number of gullies) has more of the oldest and has revealed that environmental pollution is also one of well-consolidated sediments (Nwajide, 2013). the major factors contributing to the high susceptibility However, diﬀerent erosion researches conducted in of the lateritic soils in part of the area to water erosion. this region reported that several factors, such as poor Consequent to the research ﬁndings of the previous soil engineering properties, inadequate road construc- authors who have worked in this region, several tion, poorly constructed and poorly maintained CONTACT Johnbosco C. Egbueri firstname.lastname@example.org Department of Geology, University of Nigeria, Nsukka, Nigeria © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 228 J. C. EGBUERI AND O. IGWE mitigation recommendations, including construction of Anambra State, Nigeria. To better accomplish the check dams in critical areas,constructionofadequate research aim, an integrated ﬁeld survey, hydrological, drainage channels, planting of vegetation cover, roadway geotechnical and geomorphological approach were grading, landscaping, stabilization of soils, terracing of used. Through this research, the authors hope to con- soil slopes, dewatering, construction of embankments, tribute to the understanding of the various factors aﬀect- and establishment of soil conservation schemes were ing soil erosion in the area. Information provided in this proﬀered (Chikwelu & Ogbuagu, 2014;Egboka & paper is important to researchers, citizens, local policy- Nwankwor, 1985;Obiadietal., 2014;Okoyehetal., makers, non-governmental organizations and the State 2014). Unfortunately, many of these remedial measures Government towards planning, management and miti- have not been fully adopted, and the adopted ones have gation of gully erosion hazards in the State. not been able to completely control or halt gully erosion within the study area. Possibly, the mitigation measures Location, geology and physiography of the fail because a comprehensive list of component factors study area contributing to the erosion processes has not been gen- ′ ′ erated. Hence, further inquiry is required to know more The study area is located within latitudes 5º 45 to 6º 10 ′ ′ of the component causative factors accelerating gullying N and longitudes 6º50 to 7º15 E, in the southeast Nigeria. in this area. In line with this thought, hydrogeomorphic Based on the geology, the area is underlain by three factors were suspected to contribute to the persistent formations, the Eocene Nanka, Oligocene-Miocene gullying in the area. The failure of previous eﬀorts to Ogwashi, and Oligocene Benin formations (Figure 1). halt the expansion of gullies in this area could be because The three formations are characteristically composed of of poor understanding of the close interactions between sands interbedded with thin layers of mudrocks and the geomorphologic and hydrological processes. ironstones (Nwajide, 2013). Studies have shown that the Previous researches from diﬀerent regions of the formations are generally extensive in the southern world have shown that the mechanical interactions of Nigeria and inherently weak and collapsible (Chikwelu surface and groundwater with landscapes could play & Ogbuagu, 2014; Egbueri, Igwe, & Nnamani, 2017; signiﬁcant roles in the development and expansion of Igwe, 2012;Igwe&Egbueri, 2018;Nwajide, 2013; gullies (Mahmood, Kim, Ashraf, & Ziaurrehman, Obiadi et al., 2014;Okoyeh etal., 2014). Despite their 2016; Moges & Holden, 2008; Ownegh & Nohtani, age diﬀerences, all the soils underlying the study area are 2004; Poesen, 2011; Rockwell, 2002; Scheidegger, predominantly composed of loose and poorly consoli- 1973; Shellberg, Brooks, Sencer, & Ward, 2013; dated ﬁne-grained sand materials with low clay content Sophocleous, 2002; Tebebu et al., 2010). Similarly, and little or no coarse-grained aggregates (Egbueri et al., several researchers have reported the inﬂuences of 2017;Igwe& Egbueri, 2018). geomorphology and hydrology on soil erodibility in In addition, the study area is characterized by uneven diﬀerent parts of the southeastern Nigeria (Egboka & topography, several surface water networks and humid Nwankwor, 1985; Egboka, Nwankwor, & Orajaka, tropical rainforest belt (Igwe & Egbueri, 2018), although 1990; Igwe, 2017; Igwe, Mode, Nnebedum, human activities including deforestation, urbanization, Okonkwo, & Oha, 2013; Nwajide, 1979, 1992; Obi, roadway construction, indiscriminate agricultural prac- Okogbue, & Nwajide, 2001; Ofomata, 1965, 1988; tice and other forms of man-induced activities have led Ogbukagu, 1979; Okoyeh et al., 2014). In some parts to the loss of the primary forest (Okoyeh et al., 2014). of Anambra State, Okoyeh et al. (2014) assessed the The mean-monthly temperatures in the study region inﬂuence of surface and subsurface water level vary from 22°C to 28°C in the wet season and between dynamics in the development of gullies. Their research 28°C and 32°C in the dry season (Igwe, 2017). Dry and indicated that hydrological processes play noticeable rainy seasons are the two seasons experienced in the role in the initiation and development of gullies and area. The rainy season lasts from April to October other erosion-induced environmental problems. whereas the dry season lasts from November to However, no previous studies integrated geomorpho- March. The study area experiences heavy precipitation, logic and hydrologic attributes in the evaluation of especially in peak of the rainy season. The average gully erosion processes within most parts of the cur- annual rainfall in the region is between 1,400 and rent study area (i.e. southern Anambra State). 2,500 mm (Eze, 2007). However, there may be varia- Therefore, this paper aims at identifying and evaluat- tions in the intensity of the rainfall depending on the ing the potential impacts of hydrology and geomorphol- prevailing topography. In such incident, daily rainfall ogy on the development and expansion of major erosion amount may increase to 20–30% of the mean annual gullies within three diﬀerent geological units (the Nanka, rainfall in the area (Igwe, 2015). The heavy rainfall often Ogwashi, and Benin formations) in the southern leads to such hazards as ﬂooding, soil leaching and GEOLOGY, ECOLOGY, AND LANDSCAPES 229 Figure 1. Geologic map of the study area showing the survey stations. erosion. In the southeastern Nigeria, the severity of using a GPS (GARMIN GPSMAP 78S series) which mass movements (especially those associated with digitally reports the latitude, longitude and elevation gully formation) has not only increased environmental of the reference point. Attributes (average widths, degradation but has also limited the mitigation options depths, and lateral extents) of the major gullies were available to environment stakeholders (Igwe, 2017). also measured during the ﬁeld survey. Research methodology Geoelectrical survey Identiﬁcation of surface waters and gully Four VES (Vertical Electrical Sounding) and ADMT attributes (Audio Magneto Telluric) surveys were carried out along the major gullies in Uli, Okija-Ozubulu, Surface water distribution, land use and land cover Oraukwu, and Nanka-Ekwulobia, in order to delineate information were acquired during the ﬁeld survey. the groundwater levels around these gullies (Igwe, 2017; Records of surface water widths, depths, ﬂow direc- Okoyeh et al., 2014). The ADMT was integrated with tions and elevations; gully slope (gradients, aspects, the VES in order to ensure the validity and accuracy of and curvatures) were also identiﬁed during the ﬁeld the latter. Because of their close proximity, one station survey. The width and depth of the surface waters was chosen for the Okija and Ozubulu gullies. The same were measured using a measuring tape and pole, applies for the Nanka and Ekwulobia gullies. respectively. Generally, the widths of the surface The VES method was based on the estimation of the waters were measured relative to the lengths of bridges electrical conductivity or resistivity of the hydrostrati- that crossed them. Likewise, the depths were measured graphic sequences. ABEM SAS 1000 (produced by standing on the bridges with long measuring pole (of ABEM Instrument, Sweden) set at “sounding stations” about 10 m). Unfortunately, some of the surface water was employed for the survey. Using the Schlumberger networks had no bridges, and thus their widths were array method, induced polarization mode being acti- roughly estimated and their depths estimated by inter- vated, current was passed into the earth from the source viewing the villagers. The ﬂow direction was acquired electrode, A, and received at the sink electrode, B, at using a Brunton compass by pointing the compass in every AB/2 from the midpoint. Measuring at IP mode the ﬂow direction of the stream and reading the cor- enabled simultaneous acquisition of IP and self-potential responding bearing, while the elevation was measured 230 J. C. EGBUERI AND O. IGWE (SP) data.Generally,ateachsoundingstation,astraight soil testing standards of the American Society for line of about 200 m (on average) was measured having Testingand Materials(ASTM)asdescribed by Kalinski a midpoint, O, for current electrode spread. The potential (2011). electrodes were set at 0.5 m from the midpoint, O, whereas the current electrodes were set at 1.5 m from the midpoint. From the 1.5 m points, the current elec- Geomorphology survey trodes were simultaneously shifted sideways up to 200 m. In order to determine the geomorphology of the Forevery AB/2 thepotential diﬀerence was then mea- study area, the GARMIN GPS was used to acquire sured by the earth resistivity meter at the corresponding the latitude, longitude and elevation of diﬀerent loca- MN/2 distance which was the distance of the potential tions under consideration. This data was also inte- electrodes. Since prior to the measurements, the array grated with the DEM (Digital Elevation Model) data method was chosen and the AB/2 and MN/2 was set to acquired by SRTM (Shuttle Radar Topographic their corresponding distances, direct measurement of Mission) from the permission of the USGS using apparent resistivity was obtained. This VES method is ArcGIS (v. 10.4) and Surfer 10 GIS modelling and similar to those outlined/described in Roy and Elliot analytical tools. Following the methods described in (1981), Singh (2005), Akpan et.al (2009), Okoyeh et al. Crozier (1984), Highland and Bobrowsky (2008), (2014), and Igwe (2017). The ADMT method, which was Igwe (2015, 2017)), the 2D, 3D, and drainage system based on the estimation of electrical potential from the maps of the study area were generated. Slope gradi- sequences measured in millivolts (mV), is similar to that ents and aspects were measured and estimated using of the VES. However, the ADMT surveys were carried Brunton compass while slope curvature was esti- out from the midpoint, O, at consistent 5 m intervals up mated using an empirical approach (based on obser- to 150 m. vation). The land use and land cover maps relatively The geoelectrical data obtained from the VES and matching our ﬁeld observations were adopted from ADMT surveys were used in creating their respective Ifeka and Akinbobola (2015). models. IP models were also created and they corre- lated well with the VES models. However, in order to reduce the number of ﬁgures produced for the height Results and discussion of water tables and for the purpose of clarity and Hydrological and geotechnical characteristics simplicity, only the VES and ADMT models are pre- sented in this paper. The hydrological and geomor- Surface water distribution and characteristics phological modelling involved the use of various The area is characterized by several surface water net- softwares in processing most of the data acquired works which drains the area (Figure 2). These streams during the ﬁeld surveys. The VES data were modelled are conﬁgured into diﬀerent drainage patterns, mostly using INTERPEX-1D (IX-1D), while the ADMT mod- dendritic and trellis drainage pattern. In the Uli area, els were created using Microsoft-Excel (v. 2016). With two streams were recorded, the Atammiri and Enyinja these, the generated geoelectric sections were used in rivers. The Ulasi-Okija, Okpu, and Okposi rivers drain delineating the hydrostratigraphic succession around the area around Okija and Ozubulu, while Nanka- the major gullies. A model showing the aquiferous Ekwulobia area is drained by the Iyeagu rivers. There layers and ﬂow direction was developed using the is also the presence of two lakes (Agulu and Atama VES data, as reported in Igwe (2017). lakes) within Agulu area which holds most of the sur- face runoﬀs within the area. In the northernmost part Geotechnical analysis of the study area, numerous streams which include Soil samples were collected during the ﬁeld survey for Nwocha, Iyi-Udo, Ezigbo, Oridike, Mvomvo, Mmaba, geotechnical analysis. A total of four samples were col- and Iyi-Ukwu rivers drain the area around Oraukwu. lected from each of the three formations, in an attempt to All these rivers and streams are tributaries to the Niger further clarify the information provided by the geoelec- River (west of the study area), which, in turn drains tric survey. Two samples were collected from Uli and into the Atlantic Ocean (south of the study area). The Okija gullies to represent the Benin and Ogwashi forma- abundance and complex conﬁguration of these surface tions, respectively. Two samples were collected to repre- water bodies is believed to accelerate the rate of sedi- sent the Nanka Formation (one from Oraukwu gully and ment loss from these gullies (Egboka & Nwankwor, the other from Nanka gully). The soils were analyzed for 1985; Igwe, 2017; Okoyeh et al., 2014; Shellberg et al., grain size distribution, moisture contents, permeability 2013). According to Egboka and Nwankwor (1985), coeﬃcients and direct shear strength. The laboratory the surface water networks as agents of gully erosion tests were done following the recommendations and are more damaging during the rainy season. GEOLOGY, ECOLOGY, AND LANDSCAPES 231 Figure 2. Drainage map showing the surface water distribution. The estimated average widths, depths, ﬂow direc- was observed that some of the gullies in the study area tions and elevations of the recorded surface water already have depths laterally on the close-ranged ele- networks are presented in Table 1. The surface water vations with some of the surface water bodies. This networks have depths in the range of 0.8–8.0 m, suggests that there could be close interactions between widths range of 8–55 m. Most of the surface water the surface water networks and the ground-water. This networks ﬂow in westward direction (Figure 2). This may be contributory to the high erosivity of the weak, conﬁrms that they are tributaries to the Niger River. It loose and dispersible soils underlying the study area. Table 1. The surface water distribution and measurements. Estimated aver- Estimated aver- Elevation Flow S/no. Surface water Town age width (m) age depth (m) (m) direction 1. Atammiri Stream Uli 8 2.0 38 Northwest 2. Enyinja River Uli 20 3.5 35 West 3. Ulasi River Okija-Ozubulu 22 3.0 32 West 4. Okpu Stream Okija-Ozubulu 10 1.5 26 West 5. Okposi River Okija-Ozubulu 20 2.0 28 West 6. Nwocha River Oraukwu 25 3.3 72 Southwest 7. Oridike Stream Oraukwu 15 2.0 78 West 8. Mvomvo Stream Oraukwu 13 2.5 76 Southwest 9. Ezigbo River Oraukwu 23 2.3 83 Southwest 10. Iyi-Udo River Oraukwu 23 1.7 82 West 11. Mmaba Stream Oraukwu 18 0.8 86 West 12. Iyi-Ukwu Stream Oraukwu 12 1.0 72 Southwest 13. Iyeagu River Nanka-Ekwulobia 25 4.0 160 West 14. Agulu Lake Nanka-Ekwulobia 55 8.0 134 Wester 15. Atama Lake Nanka-Ekwulobia 48 6.5 170 East 232 J. C. EGBUERI AND O. IGWE Groundwater level and ﬂow directions Table 3. The ADMT values. Uli Okija-Ozubulu Oraukwu Nanka- The VES and ADMT results obtained from the geoelec- ADMT ADMT ADMT Ekwulobia ADMT S/ Depth tric surveys are presented in Tables 2 and 3 respectively. no. (m) mV The VES models (Figure 3), which are, respectively, 1. 5 0.055 3.464 32.615 0 validated by the ADMT models (Figure 4), delineated 2. 10 0.131 5.429 0 0.006 3. 15 0.068 14.785 0.015 0.684 the groundwater levels and aided in modeling of the 4. 20 0.256 .294 0.018 0 hydrostratigraphic successions around the major gullies 5. 25 0.222 12.544 0 157.078 in the area. The VES curves are majorly the k-type, which 6. 30 0.190 6.335 3.605 98.257 7. 35 17.817 1.840 7.872 86.169 usually indicates saturated zones. The heights of the 8. 40 0 7.815 13.377 187.414 water tables ranged from 9–60 m. Figures 3 and 4 reveal 9. 45 7.965 11.910 0 0.724 10. 50 5.679 2.896 156.333 1.089 that the major kicks on the curves represent zones of 11. 55 4.634 1.146 0.238 0.997 saturation. The minor kicks are inferred to be intercala- 12. 60 5.026 4.457 0.366 1.192 13. 65 0.992 15.335 0 0 tions of clay and sand, with little or no saturation. Since 14. 70 6.055 16.930 0.227 0 the ADMT measures in mV, it is therefore factual to say 15. 75 0 10.721 0.287 0 16. 80 0 17.862 0.321 0 that water saturation increases with increasing mV. 17. 85 107.960 25.046 0.306 0 Both the VES and ADMT models show that the 18. 90 0.380 13.632 6.729 0 study area is generally characterized by shallow water 19. 95 0.704 28.511 5.651 0 20. 100 0.405 2.654 41.214 0 tables. It is believed that the abundance of surface water 21. 105 0.470 15.371 0.334 0 bodies inﬂuences the shallowness of the groundwater 22. 110 10.482 8.776 31.649 – 23. 115 0.513 32.125 15.433 – tables. This is because the abundance of surface water 24. 120 0.827 19.359 33.323 – networks and the shallow depth to groundwater table 25. 125 0.760 21.722 58.816 – 26. 130 0.679 37.205 0 – suggest that recharge and discharge mechanisms are 27. 135 0.796 20.690 52.604 – likely working in collaboration in the area. As a result 28. 140 0.547 0 0 – 29. 145 0.455 52.043 0.217 – of close interactions between the recharge and discharge 30. 150 0.350 16.861 0.232 – mechanisms, it is believed that the groundwater hydrol- ogy works in conjunction with the surface water hydrol- ogy to accelerate the gullying processes in the study area and indicates that topography inﬂuences the ground- (Egboka & Nwankwor, 1985). Tables 1, 2 and 4 also water ﬂow in the area. The VES for Nanka-Ekwulobia revealed that the distances between the groundwater and Oraukwu indicated that they have their upper- and the surface water bodies at the various major gully most aquiferous layers as perched aquifers (Figure 5). sites are at close proximity. This suggests that the inﬂu- This suggests that there could be the presence of ence of hydraulic activities would continue impacting aquitard inhibiting further vertical ﬂow of water on the gullying processes in the area. into subsequent aquiferous layers. During the ﬁeld Figure 5 is a model correlating the various aqui- survey, the shallowness of groundwater was con- ferous layers of the diﬀerent formations (Igwe, 2017). ﬁrmed in the Nanka gully complex as water was It was observed that the groundwater ﬂows in the observed dripping from some sand overburdens. westward direction. This conﬁrms the result from The water dripping from these sand bodies initiates the study on surface water ﬂow directions (Table 1) sheet and rill erosions. Table 2. Interpretation of the resistivity data (Based on Elkhedr, Mohamed, Ahmed, & Laust, 2004; Loke, 1999; Telford, Gilbert, & Sheriﬀ, 1977). Elevation VES name Layers Rho (Ωm) Thickness (m) Depth (m) (m) Water saturated? Uli 1 896.81 0.404 0.404 64.57 No 2 937.54 10.049 10.453 54.55 No 3 12,388.00 36.310 46.763 18.24 No 4 1065.80 13.757 60.520 4.48 Yes 5 204.62 – – – Yes Okija-Ozubulu 1 88.66 2.426 2.426 64.57 No 2 5543.60 13.640 16.066 50.93 No 3 346.29 – – – Yes Oraukwu 1 197.97 1.444 1.444 173.56 No 2 4535.20 4.437 5.881 169.12 No 3 1326.70 13.810 19.691 155.31 Yes 4 4219.00 63.345 83.036 91.96 No 5 851.12 – – – Yes Nanka-Ekwulobia 1 50.722 0.554 0.554 313.45 No 2 1069.90 2.000 2.555 311.45 No 3 499.88 7.046 9.601 304.40 Yes 4 7289.70 89.588 99.188 214.80 No 5 81.383 – – – Yes GEOLOGY, ECOLOGY, AND LANDSCAPES 233 Figure 3. The VES models for the four stations. In addition, Figure 5 further reveals that Oraukwu three of the sampled gullies have mostly sand com- and Nanka-Ekwulobia have higher elevations. This position (in the range of about 68.06% to 86.27%), matched the ﬁeld observation. These areas have higher with little binders (clays). This is consistent with gullying intensity than those (Uli and Okija-Ozubulu) what previous researches carried out in the region situated at lower topography. In other words, soil reported (Chikwelu & Ogbuagu, 2014;Egbueri et al., erosion is severe in the northeastern part of the study 2017; Igwe & Egbueri, 2018;Obiadietal., 2014). area (underlain by the Nanka Formation) than in the However, the gully at Okija-Ozubulu area has grains western part (underlain by the Ogwashi and Benin typicalofgravelsizes.Inthissample, thegravel formations). Hence, it is believed that the high topo- composition was observed to be about 83.38% graphy, westward ﬂow of the water systems, and the whereas sand constitutes about 11.47% (Table 4). shallowness of the groundwater table in the Nanka With respect to the low ﬁnes content in all the soils, Formation play a signiﬁcant role in the high gullying they are said to be vulnerable to erosive agents. The intensity in the area. moisture content of the soils shows that they were not saturated. This could be due to the low ﬁnes content. Geotechnical properties of the soils Fines, which are mostly composed of clays, have Results from the geotechnical analysis are presented higher water retention capacity than sands and grav- in Table 4. It was observed that the soils underlying els (Bell, 2007). 234 J. C. EGBUERI AND O. IGWE Figure 4. The ADMT models for the stations. Table 4. Analyzed geotechnical properties of the soils. Parameter Uli Okija-Ozubulu Oraukwu Nanka Fines content (%) 24.33 5.15 3.20 14.97 Sand content (%) 75.67 11.47 86.27 68.06 Gravel content (%) 0.00 83.38 10.53 16.97 −5 −4 −4 −5 Permeability (k, m/sec) 8.52 x 10 2.53 x 10 4.34 x 10 8.52 x 10 Moisture content (%) 2.00 2.00 2.00 3.00 Cohesion (kPa) 6 6 1 7 Angle of shear resistance () 333636 33 USCS* SC GP SW SC *uniﬁed soil classiﬁcation scheme. The permeability coeﬃcients revealed that the soils 2017;Igwe, 2015, 2017). In such a scenario, the eﬀective are permeable (Igwe et al., 2013). Nevertheless, the stress of the soils drastically reduces, leading to the order of permeability of the soils is: Oraukwu > Okija- spontaneous collapse of overlying soil units could Ozubulu > Nanka > Uli. This order is consistent with occur. This failure mechanism thereby continuously their respective USCS descriptions (Table 4). Hence, accelerates the rate of deepening and widening of the hydraulic conductivity of SW > GP > SC (Egbueri these gullies. Because the area experiences heavy rain- et al., 2017). Permeable soils often have high inﬁltration storms, shallow landslides on steep slopes could also be rate (Bell, 2007). Based on the soils’ inferred high inﬁl- a contributory factor to the gully expansion (Acharya, tration capacity and their hydraulic conductivity, the Cochrane, Davies, & Bowman, 2011). Moreover, where water regime is believed to contribute to build up of there are many surface drainage networks and bad- pore water pressure and the erosion through liquefac- lands, the washing out of sediments degraded by mass tion of the basal sand units (Bell, 2007; Egbueri et al., wasting processes would be readily facilitated. GEOLOGY, ECOLOGY, AND LANDSCAPES 235 Figure 5. Model showing the aquiferous layers. Table 5. The gully slope components. Gully slope Slope gradient ( ) Slope aspect Number of gully slopes Dominant erosion S/no. identity Formation range (direction) measured intensity 1 Uli gully Benin 18–34 Various 8 Moderate 2 Okija gully Ogwashi 20–33 Various 7 Moderate 3 Ozubulu gully Ogwashi 20–50 Various 10 Moderate to high 4 Oraukwu gully Nanka 30–70 Various 12 High 5 Nanka gully Nanka 33–85 Various 24 Very high 6 Ekwulobia gully Nanka 30–75 Various 15 High-Very high The shear strength results (Table 4)showedthat density. Out of the six gullies studied, it was observed the soils are weak and possess poor erosion-resistant that those (Oraukwu, Nanka and Ekwulobia) gullies properties. The low cohesion of the soils is also within the Nanka Formation are more vulnerable to attributed to their low ﬁnes contents. Furthermore, erosion, with respect to slope angle. Gullies with stee- the angle of shearing resistance can be drastically per slopes have higher erodibility potentials than ﬂat reduced due to rainfall inﬁltration, thus further ones. This is because the velocity and erosive impact of exposing the soils to mass wasting and erosion. surface runoﬀ tend to increase in steeper slopes (Igwe, Research has shown that the erosivity of rainfall is 2015; Igwe et al., 2013). Figure 6 shows that Oraukwu, very crucial in predicting water erosion (Laker, Nanka and Ekwulobia towns are situated on higher 2004). According to Mahmood et al. (2016), rainfall elevations than those towns underlain by the Benin inﬁltration (which often leads to saturation) and Ogwashi formations. This could be one of the increases the height of water table and decreases the reasons the rate of erosion is severe in the Nanka unsaturated shear strength of soils, due to reduction Formation than in other formations (Table 5). With in matric suction as the wetting front moves increasing sediment loss, the groundwater height through it. becomes much shallower. Slope aspect and curvature Geomorphological characteristics Field measurements showed that the slope direc- Slope gradient tions are generally diﬀerent, with directions ranging Field observations and measurements of the gully from north, east, south, to west (Table 5). slopes showed that majority of the gully slopes in the Moreover, it was observed that gully slopes having area have angle of inclination >30° (Table 5), predis- the same direction with the general (westward) ﬂow posing the soils to inﬂuence of denudation agents such direction of the surface waters appear to be more as landsliding. According to Igwe (2017), an increase vulnerable to erosion. Furthermore, ﬁeld observa- in soil slope steepness may be controlled by drainage tions revealed that these gullies are predominated 236 J. C. EGBUERI AND O. IGWE Figure 6. 2D, 3D maps showing the topography of the area. with slopes that have negative curvature conﬁgura- gullies. Results from the gully attribute estimation tion (i.e. concave in outlook) (Figure 7). Across the revealed that gullying intensity is more in areas study area, only few slopes have convex curvatures. underlain by the Nanka Formation, while those Concave slopes are predominant in the Nanka, areas underlain by the Ogwashi and Benin forma- Ekwulobia and Oraukwu gullies and these slopes tions have relatively low gullying intensity. The rea- have the highest concentration of slide scars. son for this observed variation could be because of Generally, concave slopes have the tendency to the topography, geotechnical composition of the retain more water and a reduced rate of water soils, landuse, and vegetation cover (Chikwelu & drainage. The higher the volume of water in a soil Ogbuagu, 2014;Igwe, 2012; Igwe & Egbueri, 2018). slope, the higher its load and the weaker it It was observed that the topography and slope gradi- becomes. In addition, during the ﬁeld mapping, it ent of the areas underlain by the Nanka Formation wasobservedthatmany of gullyslopeshaveten- are relatively high with average elevation value of sion cracks. These lines of weaknesses were identi- 240 m which ranges from 150 to 300 m, and mean ﬁed as potential sliding surfaces. slope value of 60° which ranges from 30° to 85° (Tables 5 and 6). Conversely, the topography of the areas underlain by the Ogwashi and Benin forma- Gullying intensity tions is relatively low with average elevation Table 6 presents the measured and estimated average values of 48 m which ranges from 40 to 55 m, and widths, depths, and lateral extents of the major GEOLOGY, ECOLOGY, AND LANDSCAPES 237 Figure 7. Various slope outlooks at Nanka-Ekwulobia gullies. Red circles and arrows show building structures at the verge of collapse. Table 6. Gully attributes and intensity measurements. Geologic Average elevation Depth of gully range Width of gully range Lateral extent Dominant erosion Gully formation (m) (m) (m) (m) intensity Uli Benin 55 5–15 7–25 ≥ 600 Moderate Okija Ogwashi 40 4–74–10 ≥ 430 Moderate Ozubulu Ogwashi 50 15–25 30–75 800 Moderate to high Oraukwu Nanka 150 20–75 65–95 1000 High Nanka Nanka 300 75–10 350–750 1200 Very high Ekwulobia Nanka 270 40–62 75–110 ≥ 750 High-Very high gentle-low slope gradients with mean value of 28° which are all underlain by the friable Nanka that ranges from 18° to 50° (Tables 5 and 6). Formation. The data for population distribution in Topography of an area have been attributed as one these towns are not known. However, ﬁeld observa- of the major factors which controls the rate of soil tion suggests that the population density and the erosion (Emeh & Igwe, 2017;Igwe, 2015;Igwe& anthropogenic activities were relatively low in towns Egbueri, 2018;Igweetal., 2013;Okoyeh etal., 2014). (Uli, Okija, and Ozubulu) underlain by the Benin and In conjunction with the topography of the area, the Ogwashi formations. The variation in population den- geotechnical properties of the soils derived from the sity is almost inversely proportional to the vegetation Nanka Formation have been ascribed to be relatively cover, with areas noted for high population density poor compared with those of the Ogwashi and Benin having low vegetation cover (Figure 8). Researchers formations (Chikwelu & Ogbuagu, 2014; Igwe & have reported that soils become more exposed to ero- Egbueri, 2018). Results from ﬁeld mapping also sion when there is poor vegetation cover (Emeh & revealed a relatively high landuse in areas underlain Igwe, 2017; Igwe & Egbueri, 2018; Igbokwe et al., by Nanka Formation compared with those of Ogwashi 2008; Okoyeh et al., 2014). and Benin formations (Figure 8). This evidence is revealed by the high-population density and high Conclusions anthropogenic activities (such as inadequate road and drainage system constructions, soil quarrying, Based on the research ﬁndings, the following conclu- disposal of solid wastes into drainage channels) sions are made: around Oraukwu, Nanka and Ekwulobia towns 238 J. C. EGBUERI AND O. IGWE Figure 8. Land use/land cover maps for (a) 1986 and (b) 2013 (modiﬁed after Ifeka & Akinbobola, 2015). (1) The study area is characterized with numerous processes in the area. However, the Nanka surface water bodies and shallow groundwater Formation has higher gullying intensity than systems. These attributes are believed to inﬂu- the Ogwashi and Benin formations due to var- ence the development and expansion of gullies iations in hydrogeomorphological characteris- in the area. Both the surface waters and ground- tics. It is indicated that with increasing rainfall waters in the study area have a westward ﬂow and inﬁltration intensity, the impacts of the direction, from areas of high elevations on the studied attributes could be more severe. Nanka Formation to areas of low elevations on the Ogwashi and Benin formations. Recommendation (2) The geotechnical properties of the soils from the three formations revealed that they are very This paper is only a preliminary eﬀort to spotlight the permeable, weak, easily dispersible and inﬂuence of various hydrologic and geomorphic character- collapsible. istics on the gullying processes in the study area. It does not present an exhaustive report on the hydrogeomorphologic (3) The study area is characterized by uneven bad- attributes pertinent to these gullies. Therefore, the authors land topography, high gully slope gradients recommend that further hydrogeomorphological studies, (dominantly > 30°), concave slopes, poor land- including the use of remote sensing, advanced GIS, soil use practices, and poor land cover. loss modeling, and soil water characteristics modeling, etc., (4) Hydrogeomorphology and soil engineering be conducted in order to enhance this study and deepen the properties substantially inﬂuence the gullying understanding of the dominant component factors in the GEOLOGY, ECOLOGY, AND LANDSCAPES 239 erosion processes. This will foster the generation of newer, water erosion. Geology, Ecology, and Landscapes, 2, better, and eﬀective mitigation measures for the gully ero- 115–126. sion menace in the area. Emeh, C. O., & Igwe, O. (2017). 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Geology Ecology and Landscapes – Taylor & Francis
Published: Jul 3, 2021
Keywords: Anambra State; geoelectrical survey; geotechnical; gully erosion; hydrogeomorphology
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