Effects of Time and Land Use Land Cover Change (LULCC) in the Dimension of the Gully Expansion and Soil Particles Loss at Ibeziako Erosion Site in Nsukka Urban, Enugu State, Nigeria

Almost all parts of the world are threatened by one type of hazards such as soil erosion, flooding, landslide, earthquake, among others. Various types of soil erosion exist which include splash, sheet, rill and gully erosion. Gully erosion has been recognized as an important environmental threat in many parts of the world and remains the world’s biggest problem affecting the lives of man, plants and animals. Gully erosion occurs in various parts of Nigeria under geologic, climatic and soil conditions. The purpose of this study is to look at flood and soil erosion in Nsukka Urban and the erosive activities of the floods along its path ways. The study used both primary and secondary data comprising measurements, oral interviews, photographs, and satellite imageries. This study adopted a mixed method approach in data analysis and representation with ArcGIS version 10.4 software as the major analytical tool used, other calculations were either manually performed or were calculated using Microsoft Excel 2007.The results of the satellite imageries analyzed, were rendered in tables and maps. The soil particles removed from the identified gully site in the study area were calculated and projection of further annual soil loss established, Original Research Article Ayadiuno et al.; AJGR, 5(2): 1-18, 2022; Article no.AJGR.87502 2 pending when the erosion problem will be addressed. The management, control of flood and soil erosion required to reduce and mitigate against the adverse effect of the gullying were also proffered.


INTRODUCTION
Earth's environment is threatened by irreversible damages. Almost all parts of the world is threatened by one type of hazard such as soil erosion, flooding, landslide, earthquake, among others [1]. Erosion as a term is the reduction in size of a thing, the act of eroding or being eroded. Erosion by its nature is one of the geomorphic processes that affect an area [2]. Soil erosion is referred to as the systematic removal of soils, including plant nutrients, from the land surface by various agents of denudation [3]. Erosion is the action of exogenic process such as water flow or wind that removes soil and rock from one location of the earth's crust and then transports it to another location where it is deposited [4]. Soil erosion is caused by water, wind, glacier, waves and gravity induced events. It is an intrinsic natural process which in some many places is increased by poor land use, poor water management practices, poor irrigation practices, over grazing, bush burning, deforestation and improper construction activity especially, drainage channel construction [5].
Various types of soil erosion exist which include splash, sheet, rill and gully erosion. Splash erosion results from raindrop energy; that is energy of the raindrop as it strikes the soil surface. They detached soil particles and destroy soil structure [6]. Sheet erosion occurs where runoff is unconcentrated but rather flows as a thin sheet over the entire surface or a good proportion of that surface [6]. Rill erosion is the removal of soil by concentrated water running through little streamlets or head cuts. Gully erosion occurs where runoff is concentrated along definite channels. The gullies lengthen by head ward erosion, also known as head-scarp retreat, and widen through basal sapping, leading to the collapse of materials on gully walls in the form of sliding and slumping [6].
Gully erosion has been recognized as an important environmental threat in many parts of the world and remains the world's biggest problem affecting the lives of man, plants and animals. About 65% of the soils on earth have displayed some form of degradation as a result of soil erosion, salinity and desertification [7]. Gully erosion occurs in various parts of Nigeria under geologic, climatic and soil conditions [8]. For the past three decades, gully erosion has been an issue of concern in the southeastern part of Nigeria in particular. The most devastating gully erosion sites in Nigeria are found in southeastern Nigeria, such as Anambra, Imo, Enugu, Ebonyi, and Abia States [9,10,11]. Gully erosion in Nigeria occurs in both rural and urban areas, and could become more severe in areas of high population in urban centers with high rainfall intensity generating high runoffs.
There have been many published and unpublished research on gullies in southern Nigeria like Ayadiuno [12]; Ndulue et al [13]; Ayadiuno et al [14]; Adedeji [15]; Egbueri and Ogbonnaya [16]; Adedeji [17]; Okengwo et al [18]; Abolade et al [19]; Okoyeh et al [20]; Aladelokun and Ajayi [21]; Amangabara and Otumchere [22]; Ikechukwu [23]; Obi and Okekeogbu [24]; Obiadi et al [25]; Nwilo et al [26]; Amah et al [27]; Amagu et al [28]; Amah et al [29]; Amos-Uhegbu and John [30]; Egboka et al [31]; Oluwatayo and Olatunji [32]; Ndukwe et al [33]; Nwunonwo [34]; Egbueri et al [35]; Nwankwoala and Igbokwe [36]; Ocheli et al [37] among others, majority of which dwelt on the dangers of the menace of soil erosion to lives and properties. However, Ihinegbu et al [38] chose to differ by looking at the socio-economic benefit of flooding in Alor Uno community in Nsukka with special focus on the Ibeziako gully site. They maintained that the economic activities being created as a result of the flood deposits were improving the livelihood of residents of the study area, and at same time warned that such economic activities were not sustainable. They therefore sued for possible mitigation choices such as appropriate policy framework, application of suitable technology to flood control and involvement of institutions with vast knowledge in environmental management and sustainable economic development.
Wagari and Tamiru [39] in their paper demonstrated the integration of RUSLE model and GIS techniques on annual soil loss quantification as well as soil erosion protection in Ethiopia. They maintained that the techniques are very powerful and are relevant in the quantification of soil loss and assessment of effective management of soil resources in a large spatial dimension rate. This study therefore looks at time dimension in relation to flood and soil erosion in Nsukka Urban, the anthropic ventures (land cover change) responsible for the increased flood generation over time and the erosive activities of the floods along its path ways, the dangers and losses so far created, with suggested measures to trap, harvest and reduce the devastations so far caused.  [41,42,6]. All these conform very well to the explanation and support of the law of terrain which states that differences in the topography is as a result of variation in geology of a place under similar climatic conditions [43].  [45] reported that the plateau landform (Nsukka area) is characterized by residual hills separated by generally wide and flat-bottomed dry valleys. The western lowland forms the northern section of the Anambra plains of the Niger landforms; it is a gentle rolling plain which slopes gradually westwards to the Niger River. Udi-Nsukka plateau falls gently towards the lowlands along the Niger Rivers. The escarpment (scarp face) of the Nsukka Cuesta, formed as a result of the resistant nature of the sandstones of the Lower Coal Measures to erosion, and tends towards the eastern lowland of the Cross River plains. In Nsukka Urban, the plateau is about 48 km 2 wide and slopes from a height of 459 m to 249 m above sea level (a.s.l) along the escarpment to about 214 m a.s.l to the west and finally merges with the lowland areas of the Anambra plains [46,47,48]. The area is covered by sedimentary formations which fall into three main geological groupings: the shale, coal measures and sandstones [46]. The soil of the study area comprises of the shallow and stony lithosols found at the steep slopes or the cuesta, the ferrallitic soils (red earth or acidic soils) found on the plateau, and the hydromorphic soils found at the depositional plains (Plates A, B, C and G). Where; P 2 is the projected population. P 1 is the unknown population census figure.
n is the number of years interval between P 1 and P 2 .

Flood path ways in Nsukka urban
Horton and others have proposed that there is an orderly development of drainage basins according to a set of principles, which Marisawa [51] calls the "laws of drainage composition." Horton noted that the number of streams of different orders in watershed decreases with increasing order in a regular manner known as the law of stream numbers [52]. There is a natural pattern of flow, whether of streams or floods. Flow directions have always been from uplands to lowlands in all directions [53], and in most cases along fault lines. The same is the case of Nsukka Urban (Fig. 2).

Source: USGS, modified by the authors (2020)
The flow pattern and flood path ways in Nsukka Urban follows after the same natural patterns of flow. The peaks of the hills in the area is where all floods begin and form the first order of flood channel and the type of erosion experiences at these area is basically splash and sheet erosion. These areas are the peaks and the bases of the hills in area like Okpa-Ogwu Hill, Edem-Ani hill, Olivert Hill, Ugwu Sister, Ugwu Ike-agwu, Agric Hill, Aso Hill, Odenigwe Hill etc. The second order of flood is where the various first order floods join together; these are found usually at the middle slope of the entire landscape of the area and they are basically drainage channels constructed at these areas especially where there are roads.

Research Design and Methodology
Primary and secondary data were used in this research work. The primary data used were obtained from field visits, which include measurements, photographs and oral interviews while the secondary data were from other existing literature and related organizations such as satellite imageries, (www.earthexplorer.org), Nigerian and African maps shapefile (www.divasgis.org; https://open.africa/dataset/africashapefiles), topographic sheets, among others. Landsat 8 Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) imageries of the selected years which require bands 4, 3 and 2 of the nine (9) spectral bands of thirty (30) m spatial resolution were used for the land use land cover change (LULCC) classification. ERDAS IMAGINE version 9.2 was also used for the radiometric, geometric corrections and image sharpening. This study adopted a mixed method approach in data analysis and representation with ArcGIS version 10.4 software as the major analytical tool used, other calculations were either manually performed or were calculate using Microsoft Excel 2007.

The gully site at the study area
There is one major gully site at the northern part of Nsukka Urban. During one of the field visits, this gully site was identified and later revisited for data collection. Measurements of the width (bases), depth (height) and the coordinates of eight (8) different points along the gully channel; from the lower (end), through the upper (head) parts of the gully (Fig. 3), were taken, using a 50 m, 7.5 m steel tapes and a global positioning system (GPS). The length of the gully was also traced using GPS logger. The cross sectional areas of the eight (8) different sampled point locations in the gully were determined as in the example below using the formula given as: where A is the cross sectional area of the gully sampled location (m

Season and Causes of Flood and Soil Erosion in Nsukka Urban
Flood in Nsukka Urban always occurs during rainy season (April to September). Flood occurs in areas like, Ugwu-nkwo, Ugwu-oye, Aku Road, Odenigbo through University Road, Obollo Road, and Obukpa Layout etc. When rain falls in these areas, flood accumulates very quickly and also disperses very quickly. In places like Ugbene-Agu, Iheuno, Amaebo, Aguneze villages in Alor Uno which is at the receiving end of all the floods in Nsukka Urban because of its location at a valley floor and as the lowest point (340 -345 m a.s.l) in Nsukka Urban, experiences flooding beyond September.

Causes of Soil Erosion in Nsukka Urban
Although soil erosion is a natural occurrence on all lands, however, certain human factors exist that can accelerate erosion, making it more noticeable and problematic. They are deforestation, bush burning, over grazing etc.
Other physical factors of soil erosion in Nsukka Urban are flood water (rainfall) and wind action.

Rainfall
This has been recognized as the most important causes of gully erosion, it is also very vital in determining the magnitude of erosion [54]. The amount, intensity and duration of rainfall together with the type of soil and nature of the surface are the main reasons for the type of soil erosion in an area [55,39,10]. The speed and velocity of the surface runoffs or flow is determined by the degree of slope of the area. The level of disintegration of soil particles is a function of rainfall erosivity.

Rainfall analysis of the study area
The rainfall analysis was done in order to determine the amount of water infiltrated, amount of runoff generated and the amount of water evaporated in the study area. It was calculated using the formula given as: Where P is total annual precipitation, I is infiltration, R is runoff, E is water evaporated with reference to the hydrograph or water budget analysis of the study done by Egboka et al [56] in Agulu in Anambra State. Anambra State shares boundary with Enugu State in the east and northeast, falls into the same climatic zone, shares the same rainfall amount and most importantly, Nsukka in Enugu State, shares the same geologic formation (Bende-Ameke/Nsukka formations) with Nsugbe in Anambra State [57]. Hence the adoption of Nsugbe general ground water budget for the hydrologic system as modified in table 1.
The values of the annual precipitation, evapotranspiration, infiltration and runoffs were adopted from that of Nsugbe [56], and was used for the rainfall analysis of the study area because both areas are under the same climatic zone and share the same climatic condition as well as geologic formations. The values of the total annual precipitation, evapotranspiration, infiltration, and runoffs for the catchment or study area are 1.715 m, 1.096 m, 0.360 m and 0.259 m respectively. To determine the initial amount of evapotranspiration, infiltration, and runoffs in percentages that occur within the catchment area, each of the variable values were divided by the value for the total annual precipitation and multiplied by 100 in other to render the outcome in percentages. It was calculated as thus: These percentage values are the assumed precipitation, evapotranspiration, infiltration, and runoffs level in the study area as at 2006. Due to the increase in roof tops, cemented and bare land cover, which reduced infiltration and increased the actual value of runoffs as at 2020, the value of runoffs were recalculated using the formula given as: Where I lcc is the infiltration level of the latest calculated cover, Y icc is the year of initial calculated cover, Y lcc is the year of last calculated cover, and I icc is the infiltration level of the initial calculated cover.
As a result of increase in land cover (Bare land and Built Up) to 59.16%, the value of infiltration was expected to reduced from 0.360 m to 0.184 m, which was added to the value of runoffs, that is 0.259 m + 0.176 m = 0.435 m, while evapotranspiration remained constant at 1.096 m. Therefore runoffs became 0.435 m (25.36%). As it is at this level, runoffs at 25.36% of annual precipitation of 1.715 m is very high, hence the high velocities and competences that are capable of washing off the surfaces, removing soil particles, carving deep gullies along its paths and depositing a large amount of debris in the wide plane north of Nsukka Urban towards Obukpa, Itchi, Nkelagu, Ebulu Mmiri, and Alor Uno (Plates A -L).  [55] modified by the authors, (2020)

Land use land cover change of Nsukka Urban
The land use land cover distributions for each study year as derived from the maps are presented in table 2. The study years were randomly selected and mapped, to depict the various change levels of the different land uses.
The study years mapped were 2005, this was the year that preceded the 2006 population census in Nigeria), followed by 2015 which was also the year that preceded the first reconnaissance visit of the gully site in the study area, and then 2020, the year that the field work of the entire study area was carried out (Fig.4).
In the analysis below ( Land cover classification (built-up area and bare lands) which are responsible for high runoffs generation recorded an unprecedented increase of 6.76% and 23.47% respectively with the period under review. This result shows that the amount of runoff generated in the study area is to be so high since there was an increase in both built-up area and bare lands from 28.93% as of 2005 to 39.61% in 2015 and to 59.16% in 2020.
The implication here is that as at 2020, 59.61% of the entire landmass of the area is either bare, cemented or roof topped, hence preventing infiltration. Low percentage of vegetation cover (8.07%) as well as intermittent baring of the ground (crop lands) during agricultural activities which results in high generation of runoffs because of the absence of materials to slow down flows and encourage much water to infiltrate into the ground. This calculations and analyses show the rationale behind high runoffs generation as seen as one and the main causes of gully erosion in the study area.

Soil erodibility
The characteristic of each unique soil is more or less susceptible to erosion. Reoccurring erosion is more for soil in the areas that have experienced erosion in the past. Soil erodibility is usually seen as the ability of the soil to be removed. In a sense fundamentally, it should be described as the propensity of the soil to loose and be carried away under the exogenic or erosivity force of rainfall, surface and underground flow. Understanding the concept of soil erodibility is useful in calculating and predicting soil loss [58,10,12].

Slope gradient
Slope gradient plays a major role in soil erosion, especially if the slope is steep. The steeper the slope, the greater the erosive force and subsequently, the amount of soil that can be erode. As the soil erodes downwards in most cases, it increases the degree of the slope steepness, which in turn creates sliding that is inimical to further erosion.

Deforestation
Vegetative cover of plants or crop residues protects the soil from rain drop and splash more than the areas with less vegetative cover. Here, human beings play a major role in soil erosion through their abuse of vegetated areas like deforestation, over grazing, building constructions etc.

Degradation of the environment
Erosion causes the soil to lose its nutrients in the environment thereby causing the vegetation around to look sickly and become unproductive. This can also affect the amount of carbon dioxide this vegetation removes from the atmosphere and the oxygen they release thereby causing deficiency in the balancing of these gases in the atmosphere.

Reduce farmers' income and farmlands
Erosion mostly occurs in agrarian areas like we have in southeastern Nigeria, removing top soils of farmlands and destroying the structure of the land, leaving the farmers in fear of going near the affected areas (gully site) and this has affected farmers in these areas.

Destroys social infrastructure
Erosion affect human life negatively, destroy properties, cut off roads, brought down electricity poles, collapsed pipe lines, culverts and bridges, etc (Plates H -L)

Endangers human health and livestock
Sediments that are deposited in water ways can contaminate the water which will become unhealthy for human and live stock to drink.

Improved soil quality
Eroded materials which are deposited when the flood recede recharge the land and improve its fertility for crop production. Sand and gravel deposits are mined and sold for constructions.

Estimating Soil Particles Loss
Eight (8) point locations of the gully were sampled. Point 1 was at the beginning of the gully, points 2 -7 were at the middle, while point 8 was at the end of the gully. It is worthy to note that the more the sampled area the more accurate the result would have been, however for accessibility and ease of measurements at these sampled points, the authors chose to reduce the number to eight (8).The following measurements as were presented in the table below were collected ( Table 3).
The gully length is 1674 m, stretching from Catherine Rest House Road to Alor Uno as depicted in red (Fig. 3). Therefore as at 2020, the total soil particles loss (SPL) from the gully site due to erosion is estimated to be 18,547.92 m 3 (1674 m * 11.08 m 2 ) or 18.548 tons.
The mean volume of soil particles lost per annum since the inception of the gully according to the respondents from 1984 to 2020 is calculated using the formula given as:
Since there are variations in the conversion from cubic meter to tonnes (metric), this study used the conversion calculator and table of 1 m 3 to 0.001 ton (http://convertwizard.com/convert-cubic_meters-to-tons). The volume of soil lost divided by the number of years which is (18,547

Control measures
Measures to control erosion can be in two ways which are preventive and curative measures.

Preventive measures
In preventive measures, there is a general saying that "prevention is better than cure". Where the incidence of erosion has not occurred, but is likely to occur, some measures are needed to be taken in order to prevent the inception of the soil erosion. Some of these measures are reducing the level and extent of degrading the forests, adopting a system of farming or crop cultivation that will always ensure that there is vegetation or foliage cover on the ground surface to prevent the ground being bared; eliminate and control the extent to which bushes are set on fire for any slightest excuse like hunting; reducing overgrazing by herders; due to the topography, adapting contour ploughing will go a long way in checking surface flow; introduction of multiple or mixed cropping and the use of cover crops effectively; checking and controlling sand mining that can expose the ground surface and trigger flow; proper channelization for easy evacuation of floods whenever it rains; constructing cemented drainages that will prevent erodibility, and introducing rain water harvesting system to trap these water at homes so that little will be released to the environment which will pose no threat [59].

Curative measures
In curative measures, actions to be taken will depend on the type of erosion involved, whether it is gully, sheet, splash, etc. If it is gully erosion, preventing the concentration of flows will be necessary so as to prevent so much runoff from getting at the gully at once; stabilization of the slopes using concrete or retaining walls among others will be useful. Combining afforestation, ridging across slopes, contour ploughing, among others in farming areas, while in urbanized areas, construction of well articulated side road drainages that will direct floods to soak-away pits designed to accommodate such floods for the time being, and the construction of concrete structures and drainage channels large enough to evacuate large volumes of flood out of the urbanized areas, areas devastated by gully erosion can be reclaimed and sand filled to the level it was before the erosion occurred with proper channelization to avoid reoccurrence. If it is on sheet erosion, reducing the extent of the ground that is bare in the area by planting trees and grasses such as Bahamas grass, and shrubs like Acioa bacteri will go a long way in protection the soil from sheet erosion [59].

CONCLUSION AND RECOMMENDA-TIONS
Several research and researchers' result and experience have shown that rainfall is the principal cause of flooding, gully erosion and siltation of streams and agricultural lands, destruction of properties, and sometimes loss of lives in southeastern Nigeria. Flooding often results from poor or absence of well articulated drainage systems. Water budget analysis and interviews extracted from residents in the study area has identified April, September and October as the months during which flood and soil erosion are at their peak, the anthropic and natural causes of gully erosion are rainfall, gradient and land cover as a result of urbanization, which has provided for tiled roads, cemented compound, roof tops that has prevented infiltrations and encouraged increased generation of runoffs. The computed data, information and results obtained from this research are very vital in the designing of suitable and sizeable drainages for flood and soil erosion control.
The authors are of the opinion that alignment of sewers and storm water drainages should follow the natural drainage patterns considering topography, land use, land cover and right of way for both the drainages and other environmental activities for cultural, social, and economic sustainability. The study therefore recommends collaboration with all the three tiers of government and their environmental management agencies in Nigeria and Nsukka Urban Municipal Development in designing and constructing an earth dam and water recycling facility in the area that the concentrated drainage passed through in the University of Nigeria, Nsukka premises, to trap, harvest and recycle this flood water. The recycled water can be treated and made available and accessible to the university community or better still to the Nigerian Fire Service in Nsukka, whose facility is incidentally located just opposite the university. In the absence of funds to put up a mechanically manned recycling facility, the flood water trapped by the earth dam may be allowed to infiltrate naturally and transmit to nearby aquifers as a source of groundwater recharge, while the silts generated can be evacuated intermittently and used as a source of nutrients to the surrounding farm lands in the suggested proposed locations.
This suggested proposed location for the earth dam and the flood water (mechanical or natural) recycling facility was selected based on the antropic and the natural providence. Anthropic in the sense that the concentrated drainage channels from the study area passed through the area (marked area in pink and yellow colours in Fig. 3) before getting to Ibeziako Street where the damages are being occurred, and natural in the sense that the area is used for agriculture by the university community. Siting the project in this location will serve multiple purpose of providing recycled and treated water for the university community, making same water available for the Nigerian Fire Service located opposite the university, providing nutrients for the university farmers, serve as a way of recharging the groundwater in the area, and also reduce the amount of flow getting to the gully site.

ACKNOWLEDGEMENT
We are thankful to TPL Ezeugwu Ndubuisi (the Chief Town Planner) and all the staff of Nsukka Town Planning Authority for making out time for us during our various visits, and also for the interactive discussions we had with them. Engr. Collins Okafor (Technologist in Charge), Soil Laboratory of Civil Engineering Department, University of Nigeria, Nsukka is of great help too in the course of this research.