SURVEYING AND MONITORING OF EROSION

SURVEYING AND MONITORING OF EROSION SITE

SURVEY INVOLVEMENT IN EROSION CONTROL

Erosion has been described as the washing away of the top soil as a result of the actions of agents such as water and wind. Gully erosion, being the predominant in Southern Nigeria, is also a type of environmental degradation with a lot of disastrous consequences caused mainly by flood as a result of high precipitation, which is fallout of climate change. It can be determined, especially in terms of extent and scope, monitored and controlled.

 Soil erosion is the detachment of soil particles and it occurs by the action of water, wind and glacial ice as the case may be. Soil erosion may be gradual process that continues relatively unnoticed, or it may occur at an alarming rate causing serious loss of top soil, which in some cases advance further into gullies of unimaginable magnitude and consequence.

Generally erosion occurs when raindrops, rainy seasons’ runoff, or floodwaters wear away and transport soil particles. It is a complex natural process that has often been accelerated by human activities such as land clearance, agriculture, construction, surface mining, and urbanization.

Types of Erosion:

 There are many types of erosion and mentioned here are a few of them, which include: Sheet erosion, Rill erosion, Valley or stream erosion and Gully erosion. Gully erosion, being the main topic of the discus, results where water flows along a linear depression eroding a trench or gully.. It is particularly noticeable in the formation of hollow ways where prior to being tracked, an old rural road has over many years become significantly lower than the surrounding fields.  It usually begins gradually, being initially insignificant, and graduates into an unimaginable monstrous dimension of momentous consequences when ignored. In its developmental stage, it might be in form of a shallow depression following the line of a footpath, but would sooner grow and expand in both depth and bounds into a gruesome gully.

Causes of gully erosion:

 The likely causative factors of gully erosion could be either natural or anthropogenic sources  Natural factors include:

 1. Nature of soil. Some soils have high erodibilty factors hence susceptible to high rate of erosion while some soils have very low erodibilty factors and as such low rate of erosion.

 2. Topography of terrain. Rugged and steep terrains with high slope are more likely to experience high gully erosion rate than flat terrain. This is so because flood, which is an agent of soil erosion increases in velocity with high slope than on flat terrain thereby acquiring greater momentum to move displaced soil particles from one place to another.

 3. Amount of precipitation results in high volume of rainfall, which is one of the major factors of gully erosion. The higher the amount of rainfall, the higher the quantity of soil particles that are dissolved, displaced and moved away. High precipitation amounts to high level of flood in most areas. Some areas experience higher precipitation than others.

4. Land cover. Availability of tree canopies and some plants naturally break the effects of direct raindrops on the soil on the one hand and reduce the velocity of flood and its consequences on the soil on the other.

 Areas without these natural protections, experience high level of erosion and subsequent gullies.

 Anthropogenic factors include:

1. Construction projects. Actions carried out both during and after construction such as grading, clearing, and other activities that disturb the surface of the soil, alter existing topography, and remove existing vegetation, can increase erosion potential during construction.

 2. Urbanization involving road construction, building development, etc., contribute immensely to gully sites development.

 3. Sand excavation. Sand excavation along the sides of the roads by individuals who obtain permission to carry out such excavation contribute to gully developments within the region of study.

 4. Quarrying and mining activities. Abandoned pits as a result of mining activities over a long period of time develop into gullies.

 5. Increase in the areas covered by pavements and structures and a failure to incorporate into the construction design, control measures that adequately stabilize slopes, re-establish cover on exposed soils, or convey runoffs, can increase erosion long after construction is completed.

 6. Poor construction e.g. of roads without adequate provision of drainage to rightly direct the runoffs.

 7. Some other causative factors are cattle grazing, deforestation, and bad farming habits of the people and so on.

Effects of erosion:

 The overbearing effects of erosion on the environment of the host areas cannot be emphasized. Apart from being a major source of land degradation, it has been categorized as one of the major causes of environmental disaster in the state and its environs. One of these effects includes the formation of gullies. As the erosion continues to remove soil along drainage lines, the affected areas continue to deepen. The end results over a long period of years are deep gullies, which continue to enlarge if no measure is taken to check it. The monstrous nature of gullies are such that they continue to enlarge and deepen that some have gone as deep as over 100m and these have resulted in the following.

i.                  Displacement of communities/settlements.

ii.                Destruction of Natural Habitat. Gullies are so devastating that they destroy the natural habitat thereby affecting the flora and fauna in the area.

iii.              Destruction of houses. Cracking of houses and falling of buildings into gully sites are common features in the area. People have lost their investments to gully expansion in areas like Agulu, Nanka, Nnewi, etc., including the study erosion site at Omagba II Onitsha.

iv.              Sedimentation. One of the consequences of gully erosion is the resultant sedimentation. This is because as the soil is being eroded from one area caused by water runoff, it is being deposited in other areas as sediments This has resulted in the disappearance of some streams and rivers within the zone.

v.                 Land Degradation: this is one of the worst environmental problems facing many people worldwide. Over 40million are affected in Nigeria (Uchegbu, 2002). It is the general and gradual reduction of land value mainly as a result of some unhealthy anthropogenic activities on the land. When top soils are washed away by erosion, the soil looses values due to the washing away of soil nutrients by water. Also gullies on their own part create trenches and deep holes that suddenly cut off large portions of lands thereby degrading the values of such lands.

vi.              Damage to Infrastructure. Some infrastructures such as roads and concrete drainages have been broken down and some communities completely cut off.

    The role of surveying and mapping in the management and control of gully erosion.

The surveys carried out for gully control and remediation included: 

• Mapping of catchment basin and location of rills and secondary gullies contributing run off to the main gully.

 • Detailed surveys of existing control infrastructure including gutters, culverts, catch pits, drainage channels e.t.c

• Longitudinal bed profile from head to outlet of the main gully.

 • Planimetric survey of the gully head at large scale.

 • Cross sectional surveys of the gully to determine the nature of the stage of gully development i.e whether V or U shaped.