**3. The role of topography**

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‐

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‐

olution" campaign.

158 Research on Soil Erosion Soil Erosion

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

**2. Causes of Soil erosion**

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

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‐ sis and location of this particular gully site on the landscape is similar to numerous other gully sites in the region.

at Samaru in Northern Nigeria was about 3600 Jm-2. This was twice the amount recorded in southern Africa by Stocking [15]. However, the product of the kinetic energy of the storm and the maximum intensity of the rainfall during the first 30 mins of a storm (EI30) was most significantly correlated with soil loss determined on standard field plots [16]. Erosivity val‐ ues therefore have been used successfully to produce iso-erodent map of West Africa [14]. In southeastern Nigeria, Obi and Ngwu [17] characterised the rainfall regime and recom‐ mended Lal's index of Aim as having advantage over other indices of erosivity such as KE> 1 and EI30. However, Salako et al. [18] compared all the available indices of erosivity adopt‐ ed in southeastern Nigeria and came up with some modifications of existing ones. Two indi‐

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

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

161

Rainfall therefore plays very significant roles in the erosion hazard of southeastern Nigeria. The rainfall distribution, amount and intensity in combination of other environmental fac‐ tors contribute in accelerating the rate of interrill rill and gully erosion in southeastern Ni‐ geria. This is evidenced in the sense that as rainfall amount decrease northwards, the rate of

The constant deforestation of the former rainforest due to population explosion and in‐ creased agricultural activities in the region expose the bare soils to the vagaries of weather thus escalating the soil erosion problems. The implication is that the soils are frequently sub‐ ject to different degrees of erosion including accelerated erosion. Vegetation and land use are one of the most important factors in soil erosion process in southeastern Nigeria. Stock‐ ing [15] noted that vegetation acts in a variety of ways by intercepting raindrops through encouraging greater infiltration of water and through increasing surface soil organic matter and thereby reducing soil erodibility. According to Lal [19], choosing an appropriate land

In southeastern Nigeria soil erosion especially gullies are most intensive on soil on which the former growth has been disturbed, that is mostly on agricultural soils stripped of growth for reasons of infrastructural developments such as road and housing construction. Ofomata [3] showed that in the region soil erosion is connected mainly with agricultural activities and other related land use activities such as mining, road building, urbanization, industriali‐ zation and general infrastructural development. These land use activities deprive the soil surface of its vegetation and also contribute directly to sliding, slumping, interrill and rill

ces EkI30 and EkIm were recommended

I30 is 30 minutes rainfall intensity and

all types of soil erosion by water decreases.

**5. The influence of vegetation**

use can drastically curtail soil erosion.

erosion including gullying.

Im being maximum intensity computed over a 6 minutes duration

Ek is in MJha-1 (kinetic energy)

Where;

**Figure 2.** Typical gully site
