Assessment of environmental impacts and remedies for gully erosion in Ankpa Metropolis and environs, Kogi State, Nigeria

Project Abstract

<p>&nbsp;                    <b>ABSTRACT</b>&nbsp;</p><p>The study area (Ankpa) falls within the Nigeria meteorological zone characterized by warm temperature days and moderately cool nights. Two distinct climatic divisions are demarcated. These are the dry and rainy seasons representing two broad periods of significant but contrasting variations of weather parameters, and hence geopedologic stability. Geologically, Ankpa falls within the Anambra Basin whose genesis has been linked with the development of the Niger Delta Miogeosyncline and the opening of the Benue Trough. Underlying the Benue Trough are the rocks of Anambra Sedimentary Basin consisting of Ajali Formation and Mamu Formation. Atterberg limit tests were carried out on soil samples from gully sites. Further particle size and compaction tests were made to assess the geopedologic and hydrologic causes of the gully erosion in the area and suggestions made for a lasting remedy to the menace. The plasticity index revealed that the soils are non-cohesive and non-plastic because it ranged from 1.2% to 5.2%. Sieve analysis indicate that the soil from the gully sites are within the medium to coarse grain range with low percentages of silt/clay, therefore the soil is non-plastic. The compaction test also shows that the optimum moisture content ranges from 13.50% to 15.20% while the maximum dry density ranges from 1.75mg/m3 to 1.98mg/m3 . The maximum dry density values are generally low which indicates that the soil is not compact but loose. Enlightenment and awareness of erosion control should include land use habits of the people in their agricultural practices and care of vegetation. Concrete terracing of gully affected areas is recommended to reduce the impact or the force of rain-drop. This will restrict the widening of incipient gullies. A holistic rehabilitation development program of monitoring the pedosphere to reclaim devastated land as well as to ensure a safe environment. <br></p><p> <b>Key words Ankpa, Gully Erosion, Impact, Assessment, logical, Non-plastic soil, concrete terracing, remedies Geopedologic. </b> <br></p>

Project Overview

<p> INTRODUCTION&nbsp;</p><p>Gully Erosion is an obvious and clear form of soil degradation consisting of an open incised and unstable channel generally more than 30 centimeters deep. It occurs where surface water flow has become trapped in a small concentrated stream, and begins to erode channels in the ground surface, making it wider and deeper. Uncontrolled progress of gullies results in ‘bad land’ topography and destroys the ecology and economy of the affected areas, Cavey (2006). The Ankpa metropolis and the environs include vast area of Nigeria underlain by thick, extensive sand and sandstone deposits which enhance the groundwater resources potential of the area. However, their good hydrologic properties often produce negative environmental impacts in the area. This has led to specific in-site effects of gully erosion in Ankpa metropolis and environs which has also given rise to different hazards to lives and properties of the communities. Among these are: <br></p><p> - Reduced access to land and on properties</p><p>&nbsp;- A reduction in the area of arable and other agricultural land, which become divided into smaller parcels and leads to increased farming cost.&nbsp;</p><p>
Major changes to the patterns of overland flow causing sedimentation in watercourses and leading to bank erosion problems.&nbsp;</p><p>
Increased rates of erosion where more subsoil material is exposed.&nbsp;</p><p>
Further economic losses from soil erosion are incurred by landholders and the wider community from off-site effects such as:&nbsp;</p><p>- Sedimentation and increased flooding affecting fences, farms and public roadways, railways, culverts and bridges.&nbsp;</p><p>
Sedimentation of water ways and water supplies.&nbsp;</p><p>
Increased pollution from agricultural and chemicals and animal effluent in incised water ways. <br></p><p> Notable among these has been an increase in erodibility potential, accounting for the widespread development of a history of poor ground cover due to clearing high stocking rates, repeated cultivations or decimation by fire or rabbits. Seasonal and cyclic drought, concentrated runoff from steep lands flowing into cleared drainage depressions, unstable soils in drainage lines, intense rainfall, excavated runoff caused by factors such as low levels of free cover and poor soil infiltration around Ankpa and environs (Figure 1). All these have contributed to environmental impacts and there by produced badland topography and created fears among the dwellers in the area.<br></p><p> Gully erosion is generally most highly developed where the contributing effects of land use, climate and slope interact. The western slopes of NSW feature many hot spots of erosion on susceptible soils. High rainfall also contributes to the development of many serious gullies on the eastern slopes. Okogbue (2005) in his detailed study and the factors which govern the development of gully erosion and landslides in southeastern Nigeria, suggested that gully erosion is controlled by physiography, geology, hydrogeology, and engineering properties of the soil materials. Ankpa falls within the Nigeria meteorological zone that is characterized by warm temperature days and moderately cool nights. Two distinct climatic divisions are demarcated as the dry and rainy seasons representing two broad periods of significant but contrasting variations of weather parameters, as well as geopedologic stability. The rainfall regime is very high resulting in significant reduction of average intergranular contact. Particles disaggregation rate of the soil zone thus increase, especially in the sandy formation. <br></p><p> The Ankpa metropolis and environs is one of the areas of southeastern Nigeria that have been ravaged by severe gully erosion. In this study the environmental impacts of the gully erosion were investigated and remedies are being suggested to arrest and/or minimize the menace. While the study mainly observed the environmental impacts and causes of gully erosion in the area, remedies suggested include agroforestry/maintenance of good ground cover, legislation/laws for afforestation and against deforestation, etc. Other suggestions are enlightenment and awareness campaign on erosion control, proper land use habits of the people and their agricultural practice as well as care of the vegetation. <br></p><p> <b>MATERIALS AND METHODS&nbsp;</b></p><p>Traverse method of survey was employed to gain access to sample locations. Soil samples were obtained from incipient gullies at depths of 0.5m and 3.0m, wrapped in polythene bags and taken to the laboratory for Atterberg limits determination. In addition, particle size analysis for the soil samples was carried out using the American type of standard sieve (Half-phi ASTM Stand) and a digital weighing balance. Furthermore, compaction test was done with the aid of a BS1377 mould and a 2.5 hammer. <br></p><p> <b><i>Geology of the study area&nbsp;</i></b></p><p>Ankpa falls within the Anambra Basin whose genesis has been linked with the development of the Niger Delta Miogeosyncline and the opening of the Benue Trough, Murat (1972). The stratigraphy comprises of cyclic sedimentary sequence that started in the early Cretaceous time, Reyment (1965), Marine and fluviatile sediments comprising friable to poorly cemented sands, shales, clays and limestone were deposited, with occasional coal, peat and thin discontinuous seams of lignite, du Preez (1945). The sediments have been affected by the major Santonian folding, and a minor Cenomanian folding and uplift, Murat (1972). The study area is typical of Ajali Formation or the false bedded sandstone and the Mamu Formation. The Ajali consists of thick friable poorly sorted sandstone, typically white in colour but sometimes iron-stained. Ajali sand is often overlain by a considerable thickness of red earthly sands, formed by the weathering and feruginization of the Formation. <br></p><p> The Manu consists mainly of sandstone, carbonaceous shales, sandy shales and some coalseams (figure 2) <br></p><p> <b><u><i></i></u>Causes of gully erosion in Ankpa and environs:&nbsp;<u></u></b></p><p>Some of the most likely causes of gully erosion in Ankpa and the surroundings are: - A history of poor ground cover due to clearing, high stocking rates, repeated cultivations, or decimation by fire or rabbits.</p><p>&nbsp;- Seasonal and cyclic drought</p><p>&nbsp;- Concentrated runoff, from steep lands, flowing into cleared drainage depressions.</p><p>&nbsp;- Unstable soil in drainage lines</p><p>&nbsp;- Intense rainfall&nbsp;</p><p>- Elevated runoff caused by factors such as low levels of free cover and poor soil infiltration. <br></p><p> The hydrologic components of eroding force (Fh) thus depends on the flow velocity (L/T) and the rate of increase of flow volume (V/T), given as:&nbsp;</p><p>Fh=VL/T2……………………………………………….(1)&nbsp;</p><p>where: L is distance parameter (M), V is average flow volume (m3 ) and T is time parameter(s).&nbsp;</p><p> Thus, Fh = KVL/T2……………………(2)&nbsp;</p><p>where: K represents a constant factor corresponding to an index property of the running flood specifically the density (D) substituting PV=M (mass of flowing water).&nbsp;</p><p>Fh = ML/T = (M/T.L/T)…………………………………………(3)</p><p>For flood force during erosion, M/T depends on the amount and intensity of rainfall, while L/T is slope-controlled. The effectiveness of the force further depends on the strength properties, particulate nature (lithology) and density of the bedrock or soil. <br></p>