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Futo Journal Series (FUTOJNLS) e-ISSN : 2476-8456 p-ISSN : 2467-8325 Volume-4, Issue-1, pp - 316 - 325 www.futojnls.org

Research Paper July 2018

Assessing the Vulnerability of South Soil to Gully Erosion

*Nwagwu, O. J.1, Madubuike, C. N.1, Okereke, C. D.1, Wole, R.4, Chilakpu, K. O.1 1Agricultural and Bio-Resources Engineering, Federal University of Technology Owerri, . 4Nigeria Geological Survey Agency, Enugu-Agidi Road, , Nigeria. *Corresponding Author’s Email: [email protected] Abstract Assessing the vulnerability of Awka South soils to gully erosion was conducted using ten soil samples from the gully erosion sites in the study area. Geotechnical investigations were carried out to enable the determination of the susceptibility (resistance of the soils) to gullying. Sampling was carefully conducted which included both lateritic soils and fresh bedrock where possible. The findings from the study showed that the soils are alkaline with pH ranging from 7.6 – 9.5 and the organic matter content was low, which ranged from 0.55 - 7.0 %. Average low dry soil density of 1.32 mg/m3 and a negligible average cohesive strength of 7.5 KN/m2 were recorded. Shear strength values are generally low, and angle of internal friction of 26.1o.The results also showed that 75% of the analyzed soils from the gullies and bedrocks are silty sand and clayey sand. Others ranged between well graded sand, silty and organic clays of low plasticity. The grading of the cohesionless soils in the study area based on geotechnical parameters revealed that Awka, Ifite-awka and Okpuno, Amawbia soils are highly susceptible to gully erosion because of their Nanka sands geologic formation whereas Isiagu, Nise, Nibo, Mbaukwu and Umuawulu have moderate susceptibility because of Imo shale.

Keywords: Susceptibility, gully erosion, geotechnical parameters, erodibility

1. Introduction

Soil erosion which is a systematic removal of soil, including plant nutrients, from the land surface by the various agents of denudation occurs in several parts of Nigeria under different geological, climatic and soil conditions. Ofomata (1985), Lal (1990) and Eze (2002) hold similar views of this landscape sculpturing process. Thus, while it is true to observe that soil erosion is one of the most striking features on the land surface of South-eastern Nigeria, especially in Awka South of Anambra State and Imo State, only rare occurrences of the phenomenon are recorded in some other States of the Federation. Although soil erosion is long-term, it is also a natural process that has been largely responsible for shaping the physical landscape through distribution of the weathered materials produced by geomorphic processes (Huber, Prokop, Arrouays, Banko, Bispo, Jones, Kibblewhite, Lexer, Möller, Rickson, Shishkov, Stephens, Toth, Van Den Akker, Varallyay, Verheijen, & Jones, 2008). It is the wearing off of material on the surface of the land by wind, water, or gravity. There are three types of soil erosion, namely sheet erosion, rill erosion and gully erosion. Ofomata (1978) resolved factors of soil erosion in Nigeria into two components; physical (geologic or

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natural) and anthropogenic (human or accelerated) factors. This soil erosion in our environment is primarily due to the reckless removal of top soil and activities that tend to concentrate runoff. It is thus not necessarily due to high erodibility of soils, but rather as a result of our unwillingness to help these soils sustain our development. Gully erosion is the removal of soil along drainage lines by surface water runoff. Once started, gullies will continue to move by head ward erosion or by slumping of the side walls unless steps are taken to stabilise the disturbance. (http://agriculture.vic.gov.au/agriculture/farm- management/soil-and-water/erosion/gully-erosion,1999). From the Studies and Investigation by past agencies such as the task force on soil erosion control, the Anambra State Erosion Control Authority, Nigeria Geological Survey Agency, and National Geo-hazard Monitoring Centre on erosion control in Anambra State discovered that about 30% of the gully erosion problems are attributable to poor agricultural practices while 70% are attributable to poor civil/road construction works without adequate provision for drainages. Gully Erosion is, however, a threat in the landscape of Anambra State. The topography of the area as well as the nature of the soil contributes to speedy development and spreading of gullies in the area. Presently, over 70% of the lands of Anambra State are being ravaged by erosion, over 20% of its farmlands have been lost to gullies, and over 30% more are endangered by massive rill and sheet erosion. (Anambra State Government Ministry of Housing and Environment, 2001). In this study the susceptibility of Awka South Soil to gully erosion was assessed and classified for land use management and erosion control in the areas.

2. Materials and Method 2.1. The Study Area

Figure 1: Map Of Anambra State And Awka South (Google Map Data 2017 )

Awka South is a local government in Anambra State shown in Figure 1. It covers a total land area of about 73 square kilometers. Awka South has a population of 189,049 which comprises Awka town and settlements such as Amawbia, Ifite-Awka, Isiagu, Mbaukwu, Nibo, Nise, Okpuno and Umuawulu. (NPC, 2006). It is a prominent urban commercial centre of Anambra State, Nigeria. The study area lies within latitude 6°101N and 6°151 N of the Equator and longitude 7°021E and7°071E of the Greenwich Meridian. Awka lies in the rain

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forest zone of Nigeria with a mean annual rainfall of 1828 mm and with an average temperature of 27° C. The relative humidity ranges between 85 and 100% during the rainy season and less than 70% during the dry season. The pattern of rainfall is bimodal, the first peak occurring in June-July, and the second in September with a little dry spell in August. (Wikipedia Encyclopedia 2015). In the geology, Awka in Anambra State lies within the Anambra Basin whose sedimentary rocks are made up of Nkporo Shale, the Mamu Formation, the Ajali sandstone and the Nsukka Formation as the main deposits. On the surface, the dominant sedimentary rocks are the Imo Shale, which is a sequence of grey shales with occasional clay iron stones and sandstone beds. The Imo Shale underlies the eastern part of the State, particularly in , , and Oruma North Local Government Areas. Below the Imo Shale is the Ameke Formation, which includes Nanka Sands (laid down in the Eocene).

2.2. Data Collection Geotechnical investigations were carried out to enable the determination of the susceptibility (resistance of the soils) to gullying. Sampling was carefully conducted (Plates1-4) to obtain representative samples which included both lateritic soils and fresh bedrock where possible. A total of ten (10) samples were collected from across the project area.

Plate 1: Sampling a gully site section for laboratory analysis Plate 2 is a gully at Awka from which soil samples were collected and showed that the gulliable unit is generally poorly graded, often dominated by uniformly and gap graded soils. This indicates the dominance of fine and medium grained sand sizes and its variability in the distribution of sand grain sizes across the study area.

Plate 2: A section of Nanka Sands at fomer Ikenga Hotel along old Enugu road, Awka

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Plate 3: Gully erosion destruct access road and threatening the federal high court at Awka

Plate 4: Illustrates the fact that a gullied tract of land is obviously an economic loss.

Plates 3 and 4 showed the other gully sites from which the samples were also collected. Each of the representative soil samples from gully erosion sites were analysed by standard methods (Smith, 1978) to establish the geotechnical parameters presented in the results.

2.3. Data Analysis The data on soil properties and soil loss for the various locations in the study area were analysed by standard statistical procedure using computer software, Ms Excel.

3. Result and Discussion

The results of the studied geotechnical parameters are presented in Table 1. The Moisture content of soils from the gully sites in the study area ranged between 5.82% and 47.44%. The dry density ranged from o.93 mg/m3 to 1.56mg/m3. A normal range of bulk densities for clay is 1.0 to 1.6 mg/m3 and a normal range for sand is 1.2 to 1.8 mg/m3 with potential root restriction occurring at ≥ 1.4 mg/m3 for clay and ≥1.6 mg/m3 for sand. A low dry density generally signifies loose sub-grade and high bulk density results in the water runoff.

The Shear strength values are generally low, with average cohesion of 7.51 KN/m2 (3.55- 10.78) and angle of internal friction of 26.1. The plasticity indices were obtained from samples that passed through No. 425 British Standard sieve gauge and lie between 3.9% and 26.7%. The atterberg limits can be used to distinguish between silt and clay. The

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plasticity index (PI) is a measure of the plasticity of a soil. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid limit and the plastic limit (PI = LL-PL). Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic tend to have little or no silt or clay.

Table 1: Result Table of the soil properties Location Moisture Bulky Dry Organic Ph L.L P.L P.I C(Kn/M2) Content Density Density Matter (%) (%) (%) (mg/m3) (mg/cm3) Content (%) Amawbia 21.16 1.828 1.5087 9.40 7.60 26.00 16.30 9.70 7.03 Bottom OkpunoAnambra 9.14 1.46 1.338 2.55 8.03 31.90 18.48 13.42 7.33 State Judiciary Bottom OkpunoAnambra 9.32 1.57 1.436 3.90 7.80 23.00 19.06 3.94 8.37 State Judiciary Top

Umuko-Isi Awka 13.35 1.572 1.39 7.00 6.8 33.00 19.87 13.13 3.55 Top Okpuno Federal 6.40 1.44 1.353 5.00 8.46 _ _ _ 7.60

High Court Top

Okpuno Federal 9.48 1.402 1.28 4.40 9.50 35.00 19.99 15.01 7.07

High Court

Bottom Isiagu Top 17.80 1.41 1.197 14.55 8.09 46.00 19.03 24.10 9.49

Isiagu Bottom 47.44 1.37 0.93 21.30 8.50 36.40 20.82 2.8 7.03 Nibo Top 15.51 1.32 1.14 0.35 8.4 42.00 17.90 24.1 6.88 Umuawulu Top 5.82 1.26 1.19 15.00 8.02 21.00 18.2 2.8 10.78 19.60

The average results show that 75% of the analyzed soils and bedrocks are silty sand and clayey sand based on the Unified Soil Classification Scheme (USCS) (Bell, 1983). Others range between well graded sand, silty and organic clays soils of low plasticity with average low dry density of 1.32 mg/m3. Table 2 shows the volume of soil loss due to gully erosion at each location in the study area.

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Table 2: The Result of Soil Loss Estimates in the Study Area Location Gully Soil Loss volume (m3) Length(L) (Lxwxd) Width(W) Depth(D) Administrative Block L = 500m W =15 M 75000m3 D =10 M OkpunoAnambra State L = 300m 162000m3 Judiciary W =30 M D =18m Retreat Pastoral And L =800m Conference CenterOkpuno D = 18m 432000m3 W = 30m Ifite Behind Melochi Hotel L =200m D = 5m 10000m3 W = 10m Opposite Melochi Hotel Along L =1000m Ekwulobia Road D = 5m 60000m3 W = 12m Umuoka-Isi Awka L =500m D = 15m 187500m3 W = 25m Behind Federal High Court L =700m Awka D = 12m 732m3 W =20m L =100m 600m3 Isiagu Opposite Anambra D = 2m State W = 3m NiboIfite L =100m D = 5m 125m3 W = 25m Umuawulu L =50m D = 3m 1500m3 W = 10m Amawbia L =200m D = 4m 14400m3 W = 18m

Total volume of soil loss is 943,857 m3 in Awka South local government area of Anambra State which amounts to 0.75% of the total land area of the study area of 73 square kilometers. Both dry and wet sieving was conducted to determine the grading of the cohesionless soils. Table 3 and Figure 3 showed the particle size distribution curves and Table 4 and Figure 4 showed the liquid limit, Table 5 showed plasticity index of sand samples from the gullying unit of the study area, namely the Nanka Formation.

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Table 3: The wet sieve analysis of Nanka sand of Amawbia

Percentage fines =47.28%

120

100 80 60 40

Sieve Sieve (mm) Size 20 0 0.010 0.100 1.000 10.000 Percentage Pssing (%) Fig 3: The graph of the wet sieve analysis of Nanka sand at Amawbia

These parameters indicated the susceptibility of the soils to easy buoying on water saturation. Cresswell and Hamilton (2002) reported that It is generally desirable to have soil with a low BD (<1.5 g/cm3), the high bulk density of 1.5mg/m3 are prone to erosion.

Table 4: The liquid limit of Nanka sand at Umuoka–Isi Awka Container Number 25 19 18

No. of blows, N

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LL = 33.00 %

Fig 4: The graph of the liquid limit of Nanka sand at Umuoka–Isi Awka

The critical value of bulk density for restricting root growth varies with soil type (Hunt and Gilkes, 1992) but in general bulk densities greater than 1.6 g/cm3 tend to restrict root growth (McKenzie et al., 2004). Sandy soils usually have higher bulk densities (1.3 –1.7 g/cm3) than fine silts and clays (1.1 – 1.6 g/cm3) because they have larger, but fewer, pore spaces.

Table 5: The High Plasticity Index of Nanka Sand at Umuoka-Isi Awka

Moisture Content W= PL (%)

From the geotechnical parameters of the soil samples carried out for different communities in Awka South; Awka, Ifiteawka and Amawbia is highly susceptible because of the Nanka sands and Isiagu, Nise, Okpuno, Nibo, Mbaukwu and Umuawulu is moderately susceptible because of Imo shale. The soils are deep red to reddish brown loamy sands (red earth) and are loose and easily eroded into gullies such as in the cuestas and other elevated areas underlain by sandstones and shales of the Ameke formation and the Nanka Sands. The Nanka Formation, relative to the area it underlies (only parts of Anambra and Imo States), has the largest density of gullies in Nigeria. Its exposure (Plate 2) commonly consists of poorly consolidated sandstone usually overlain by its derivative lateritic soil which is also poorly consolidated.

As reported by Brown (1984), soil loss due to erosion from the surface of Earth amounts to 25 billion tones per annum. This loss is affected through deflation by air masses in motion, sheet wash and rilling by running water as well as gullying which causes even more loss than just from the surface layers. The main impact of the various types of erosion operating in the project area is soil loss involving deeper than the surface and near-surface horizons.

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The removal of top soil is prevalent on slopes with scanty vegetal cover. This results in loss of nutrients and impoverishment of the soils. Thus in many farms studied seen during the field work , the prevalent colour of yam mounds and cassava ridges was reddish brown or brownish red, that is lateritic soil from which the top humus content had been stripped off. With such poor soils as shown in appendix, agricultural yield is often not worth the labour. The consequence is rural to urban population movement. Even more significant is the loss of land due to gullying. This is by far a more serious impact of erosion since it means a complete deprivation of access to the gullied area and its immediate precinct which is usually also under threat and must not be abandoned (Plate 3). From all the forms of erosion operating in the project area the soil loss may amount to hundreds of thousands of tonnes per annum. Such loss is transmitted through gullies, flood paths and the general surface to rivers which serve as conduits or conveyor belts to the River Niger and hence to the Atlantic Ocean (Plate 4) which shows the base level area of a gully, is an example of such alluvial in transit. An additional effect is the siltation and pollution of water sources caused by temporary deposition of such sediment load which is generally in transit. To appreciate the enormity of the problem of land loss an attempt is hereby made to estimate the area of land lost to gulling per drainage system as shown in Table 2.

The direct damages of erosion in Awka South include:

i. The loss of productive farmland (valuable topsoil) as in Plate 4 ii. The undermining of structures such as bridges and culverts, (Plate 3) iii. Siltation of watercourses and surface water sources. iv. Overall reduction in water quality of rivers and streams. v. The washing out of lanes, roads and fence rows as in Plates 3 and 4 vi. Restriction of land use.

4. Conclusion

Awka South local Government Area falls within the administration jurisdiction of Anambra State. The area is seriously affected by gully erosion as a result of urbanization and lack of adequate attention to environmental conservation and this led to this study on the vulnerability of Awka South soils to gully erosion. The geotechnical analysis shows that 75% of the analyzed soils of the gully sides and bedrocks are silty sand and clayey sand. From the geotechnical parameters Awka, Ifiteawka and Okpuno, Amawbia soils are highly susceptible to gully erosion because of the Nanka sands but Isiagu, Nise, Nibo, Mbaukwu and Umuawulu are moderately susceptible because of Imo shale. The highly percentage fines indicate the susceptibility of the soils to easy buoying on water saturation. The result of the study provides useful information for proper and well-designed gully erosion measures in land management in Awka South Local Government of Anambra State, Nigeria.

References

Anambra State Government Ministry of Housing and Environment (2001). Project profile and proposal for the control of Ndiabo erosion menace at ugwunano village, umunze in local government area. Unpublished. Bell, F.G. (1983). Fundamentals of Engineering Geology, 377 – 400 Matchett, B. & Moyer, S. (2000) , Soil Erosion and Conservation

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McKenzie N. J, Jacquier D.J, Isbell R. F & Brown, K.L (2004) australian soils and landscapes. An illustrated compendium. CSIRO Publishing: Collingwood, Victoria. Brown, L.R. (1984) .Conserving soil state of the world. New York, Norton, 53 – 75. Cresswell H. P. & Hamilton (2002). Particle size analysis. In: Soil physical measurement and interpretation for land evaluation. (Eds. N. J. McKenzie, H. P. Cresswell and K. J. Coughlan) CSIRO Publishing: Collingwood, Victoria. 224-239 Eze, H. I. (2002), Impact of soil erosion on bio-physical and socio-economic resources in the upper Ebonyi River basin of Enugu State, Nigeria. Unpl. P.hD Theses, Env.Mgt.Programme Enugu State University of Science and Technology, Enugu, Nigeria. Http://agriculture.vic.gov.au/agriculture/farm-management/soil-and-water/erosion/gully- erosion published 1999, accessed on 15th August ,2017 Https://En.Wikipedia.Org/Wiki/Awka_South Accessed On 5th May,2017 https://www.google.com.ng/maps/Awka_South accessed on 16th Nov.2017 Huber, S., Prokop, G., Arrouays, D., Banko, G., Bispo, A., Jones, R.J.A., Kibblewhite, M.G., Lexer, W., Möller, A., Rickson, R.J., Shishkov, T., Stephens, M., Toth, G. Van Den Akker, J.J.H., Varallyay, G., Verheijen, F.G.A. & Jones, A.R. (2008). Environmental assessment of soil for monitoring: Volume I Indicators and Criteria. EUR 23490 EN/1, Office Lal, R. (1990). Soil erosion in the tropics: Principles and management MC Graw Hill Inc. U.S.A. Ofomata, G. E. K. (1985). Soil erosion in Nigeria: The views of a geomorphologist. Inaugural Lecture Series. No 7, University of Nigeria Press Nsukka Ofomata, G. E. K. (1978). Soil erosion in Southeastern Nigeria: The view of a geomorphologist, Inaugural lecture series, University of Nigeria Nsukka.

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