**1. Introduction**

The countries of sub-Saharan Africa are besieged by serious environmental degradation re‐ sulting in desert encroachment, draught and soil erosion due to either wind impact or very high intensive rainfall resulting in heavy runoff and soil loss. The problems have adversely affected agricultural productivity and thus casting doubt of food security in the zone. The ecological and social settings in the zone are often distorted some times leading to losses in human and material capitals. In Nigeria desertification and aridity are the major environ‐ mental problems of the Northern part of the country while the high torrential rainfall of the southern Nigeria creates enabling environment for catastrophic soil erosion in the region.

The greatest threat to the environmental settings of southeastern Nigeria is the gradual but constant dissection of the landscape by soil erosion by water. Although the incipient stages of soil erosion through rill and interrill are common and easily managed by the people through recommended soil conservation practices, the gully forms have assumed a different dimension such that settlements and scarce arable land are threatened. Therefore, gully ero‐ sion problems have become a subject of discussion among soil scientists, geographers, geol‐ ogists, engineers and social scientists. Ofomata [1] indicated that gully erosion types are the most visible forms of erosion in Nigeria mainly because of the remarkable impression they leave on the surface of the earth. Again Ofomata [2] remarked that more than 1.6% of the entire land area of eastern Nigeria is occupied by gullies. This is very significant for an area that has the highest population density 500 persons per km2 in Nigeria. Before the 1980's the classical gully sites in the region were the Agulu, Nanka, Ozuitem, Oko in Aguata area, Isuikwuato and Orlu. With the increased development activities the number and magnitude escalated thus making many government administrations within the region to set up soil erosion control with different names in different states. At the last count the Federal Govern‐

© 2012 Igwe; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Igwe; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ment of Nigeria has started showing interest in ecological problems in the country including the control of the gullies which has reached more than 600 active sites in the region. The gul‐ lies are also a visible manifestation of the physical loss of the land due to erosion. Long be‐ fore now a lot of attention has been focused on the control measurers. As early as the 1930s, the colonial government in Nigeria has undertaken a campaign of tree planting with the main objectives of controlling erosion especially on the steep slopes of upland landscapes in the region. Ever since then there has been a constant enquiry as to the causes of these cata‐ strophic erosion. Most researchers [2, 3, 4] have shown that the environmental factors of vegetation, geology, geomorphology, climate in the form of rainfall which is very aggressive in the region and the soil factor all contribute in the erosion problem and their development. The consequence of the soil erosion is loss of land for agriculture and for habitation. During some slides caused by gully formation, lives have been lost while some communities have been separated because of deep and very wide gullies that may reach in some cases 12 m deep and more than 1.5 km long like the Nanka/Agulu gully complexes or in Oko in Agua‐ ta, Anambra State. Crop yields have been reduced, thus creating problem in the "green rev‐ olution" campaign.

sects the entire land surface. These are very common site all over the geographical region of southeastern Nigeria. It is well established fact among earth scientists that a number of envi‐ ronment factors as well as pedological parameter influence the extent of soil erosion where ever it occurs globally. These factors are perhaps guided by human factors known as anthro‐ pogenic factors. Although man has helped in reshaping and preserving the earth surface yet man has also helped in causing instability of equilibrium in the natural ecology and hence the rapid spread of environmental problem such as soil erosion. Igwe [5] noted that the an‐ thropogenic factors are mainly technical factors comprising mainly of land use and tillage methods, the choice and distribution of cultures and the nature of agro-technology. In Northern hemisphere including many countries of Europe, Giordano et al. [6] showed that among the factors that encourage soil erosion are vegetation clearance, intensive harvesting and over-grazing leaving the soil bare. Other factors are soil compaction caused by heavy machinery which reduces the infiltration capacity of the soil and thus promoting excessive water runoff and soil erosion. In classical modelling works on soil erosion prediction and estimation, works by Renard et al. [7], Igwe et al. [8] among others recognised topography/ relief, rainfall and soil factors as being the main agents that determine the extent of soil ero‐ sion hazard. The soil factor represents the soil erodibility which is also a product of geology and soil characteristics. In showing how these factors influence the extent of soil erosion and gullying in southeastern Nigeria, there is going to be an attempt into discussing how these

Gully Erosion in Southeastern Nigeria: Role of Soil Properties and Environmental Factors

http://dx.doi.org/10.5772/51020

159

Hudson [9] observed that in simplest terms steep land is more vulnerable to water erosion than flat land for reasons that erosive forces, splash, scour and transport, all have greater ef‐ fect on steep slopes. Soil erosion generally is a function of slope attributes. The slope length and the amount of soil erosion have always been proportional to the steepness of the slope. Also the slope geometry of hill sides (i.e. whether convex or concave) often contribute signif‐ icantly to soil loss and gully development. In southeastern Nigeria, Ofomata [3] found that there is a positive relationship between relief and soil erosion while in southwestern Niger‐ ia, Lal [10] observed an increased severity of soil erosion as the slope changed from 5 to 15%. On a 15% slope he recorded a total soil loss of 230 t/ha/yr from bare plots as against soil loss

The topography of southeastern Nigeria according to Ofomata [2] can be classified into three relief units. These units are the plains and lowlands including all the river valleys, the cuesta landscapes and the highlands. It is observed that the uplands which are made up of highly friable sandstones yield easily to erosion and induce gullying even on slopes of about 5%. The cuestas and other highlands with somewhat stable lithology resist gullying but pro‐ vide aggressive runoff which moves down to devastate the lowland areas especially at the toe slopes and river head-waters. The popular or infamous Agulu-Nanka gully erosion sites started from the head waters of streams and slopes of Awka-Orlu Upland region. The gene‐

parameters contribute to gully erosion in other geographical zones.

**3. The role of topography**

of 11.2 t/ha/yr on 1% slope.

**Figure 1.** Location map of the reviewed area

#### **2. Causes of Soil erosion**

Soil erosion generally is caused by several factors working simultaneously or individually to detach, transport and deposit soil particles in a different place other than where they were formed. The resultant effects of this phenomenon are deep cuttings and ravine which dis‐ sects the entire land surface. These are very common site all over the geographical region of southeastern Nigeria. It is well established fact among earth scientists that a number of envi‐ ronment factors as well as pedological parameter influence the extent of soil erosion where ever it occurs globally. These factors are perhaps guided by human factors known as anthro‐ pogenic factors. Although man has helped in reshaping and preserving the earth surface yet man has also helped in causing instability of equilibrium in the natural ecology and hence the rapid spread of environmental problem such as soil erosion. Igwe [5] noted that the an‐ thropogenic factors are mainly technical factors comprising mainly of land use and tillage methods, the choice and distribution of cultures and the nature of agro-technology. In Northern hemisphere including many countries of Europe, Giordano et al. [6] showed that among the factors that encourage soil erosion are vegetation clearance, intensive harvesting and over-grazing leaving the soil bare. Other factors are soil compaction caused by heavy machinery which reduces the infiltration capacity of the soil and thus promoting excessive water runoff and soil erosion. In classical modelling works on soil erosion prediction and estimation, works by Renard et al. [7], Igwe et al. [8] among others recognised topography/ relief, rainfall and soil factors as being the main agents that determine the extent of soil ero‐ sion hazard. The soil factor represents the soil erodibility which is also a product of geology and soil characteristics. In showing how these factors influence the extent of soil erosion and gullying in southeastern Nigeria, there is going to be an attempt into discussing how these parameters contribute to gully erosion in other geographical zones.
