TLDR;
This lecture covers the causes and types of soil erosion. It begins by outlining the primary causes, such as the destruction of natural protective cover, improper land use, and then classifies soil erosion into geological and accelerated erosion, further dividing accelerated erosion into water and wind erosion. The lecture details the various types of water erosion, including raindrop splash, sheet, rill, gully, and stream bank erosion, and explains wind erosion. It concludes by highlighting how human activities exacerbate soil erosion through deforestation, construction, and traditional cultivation practices.
- Destruction of natural protective cover and improper land use are primary causes of soil erosion.
- Soil erosion is classified into geological and accelerated erosion.
- Water erosion includes raindrop splash, sheet, rill, gully, and stream bank erosion.
Introduction [0:10]
The lecture introduces the topic of soil erosion, focusing on its causes and types. It is the second lecture in a series on Soil and Water Conservation Engineering. The lecture series will progress from the causes and types of soil erosion to factors affecting it, the effects of soil erosion, soil erosion mechanics, and finally, water erosion control measures.
Causes of Soil Erosion [1:00]
The primary causes of soil erosion include the destruction of natural protective cover, often due to indiscriminate deforestation. Deforestation leads to a loss of plant canopy, reducing the interception of rainfall and the dissipation of kinetic energy. This results in raindrops hitting the soil surface with greater force, making the soil more vulnerable to erosion. Overgrazing of vegetative cover similarly exposes the soil surface, increasing the risk of erosion from rainfall impact. Forest fires, predominantly occurring in summer, also contribute by destroying both the canopy and surface vegetation, creating conditions highly susceptible to soil erosion.
Improper Land Use [3:27]
Improper land use is another significant cause of soil erosion. Barren land, left without cultivation, lacks protective cover, making it vulnerable to raindrop impact and wind erosion. Improper cultivation practices and cropping patterns can also expose the soil surface, increasing erosion risk. Cultivating along land slopes creates channels for water flow, which gains velocity and kinetic energy, enhancing its erosive capacity. These forms of improper land use, whether barren land, poor cultivation, or slope cultivation, all increase soil susceptibility to erosion.
Types of Soil Erosion [5:36]
Soil erosion is broadly classified into geological erosion and accelerated erosion. Geological erosion, also known as normal or natural erosion, is a natural process involving the formation and loss of soil at a balanced rate. It is a slow, continuous process responsible for creating topographical features like stream channels and valleys. Accelerated erosion, on the other hand, is anthropogenic or man-induced, activated by human activities that alter natural cover and soil conditions. Accelerated erosion is much faster than natural soil formation, leading to visible environmental damage. Accelerated erosion is further divided into water erosion and wind erosion, based on the agents causing the erosion.
Geological Erosion: Grand Canyon and Marble Rocks [6:24]
Geological erosion, a natural process, is responsible for the formation and loss of soil in a balanced manner. It is a slow and continuous process that often goes unnoticed, shaping topographical features like stream channels and valleys. A prime example of geological erosion is the Grand Canyon in Arizona, USA, formed by the Colorado River over millions of years, displaying rocks aged from 200 million to 2 billion years. In India, the Marble Rocks of Jabalpur, Madhya Pradesh, showcase geological erosion where the Narmada River carves through white marble, creating a 3-kilometer gorge.
Accelerated Erosion [9:44]
Accelerated erosion is anthropogenic, or man-induced, resulting from human activities that disrupt natural cover and soil conditions. Activities such as deforestation, improper land cultivation, and leaving land barren contribute to this type of erosion. Unlike geological erosion, accelerated erosion occurs at a much faster rate than natural soil formation, making it easily visible and leading to significant environmental damage.
Water Erosion: Types and Processes [11:29]
Water erosion, a type of accelerated erosion, is further classified into five types: raindrop splash erosion, sheet erosion, rill erosion, gully erosion, and stream bank erosion. Raindrop splash erosion is the initial stage, where raindrops impact bare soil, breaking up soil aggregates and splashing individual particles. Sheet erosion involves the uniform removal of soil in thin layers during overland flow, often unnoticed until significant topsoil loss occurs. Rill erosion forms shallow channels due to continuous overland flow, common in bare agricultural land and easily removed by tillage. Gully erosion, a more severe form, develops when rills are not addressed, creating larger channels that cannot be smoothed out by normal cultivation. Stream bank erosion involves the scouring and undercutting of soil along stream banks due to water flow.
Raindrop Splash Erosion [12:30]
Raindrop splash erosion is the initial stage of soil erosion, primarily causing soil detachment and disintegration. When raindrops hit bare soil with high kinetic energy, they cause a splash effect, breaking up soil aggregates and dispersing individual soil particles. This process also clogs soil pores, reducing infiltration and increasing runoff, which further carries away loose soil particles.
Sheet Erosion [14:24]
Sheet erosion is the uniform removal of soil in thin layers during overland flow. As water flows over the surface, it carries away the most fertile soil particles, which contain essential nutrients and organic matter. This process often goes unnoticed until significant topsoil has been lost, making it a slow but dangerous form of erosion. Overgrazed and cultivated soils are particularly vulnerable to sheet erosion.
Rill Erosion [16:22]
Rill erosion occurs when overland flow forms shallow channels known as rills. Water concentrates its flow through these rills, causing further soil erosion. This type of erosion is common in bare agricultural land, overgrazed land, and freshly cultivated land. Rills are small enough to be removed with primary tillage implements, making this an intermediate stage between sheet and gully erosion.
Gully Erosion [18:13]
Gully erosion is a severe form of soil erosion that develops when rills are not addressed, leading to wider and deeper channels. These gullies cannot be smoothed out by normal cultivation and represent an advanced stage of erosion. The process involves waterfall erosion, where overland flow undercuts the gully head, and channel erosion, where water flowing through the gully erodes the bed and sides, causing wall collapse.
Stages of Gully Erosion [20:51]
Gully erosion progresses through four stages: formation, development, healing, and stabilisation. The formation stage involves the conversion of rills into gullies through downward scour of the topsoil. The development stage includes upstream movement of the gully head and enlargement in width and depth due to waterfall and channel erosion. The healing stage begins when vegetation starts growing in the gully. Finally, the stabilisation stage is reached when vegetation covers the gully surface, preventing further development.
Classification of Gullies [23:31]
Gullies can be classified based on shape and size. Based on shape, gullies are either U-shaped or V-shaped. U-shaped gullies form when both topsoil and subsoil have similar resistance to erosion, resulting in a uniform shape. V-shaped gullies occur when the subsoil is more resistant than the topsoil, leading to greater erosion at the top. Based on size, gullies are classified as small, medium, or large, depending on their depth and the drainage area they serve. Small gullies have a depth of less than 1 meter and drain less than 2 hectares. Medium gullies range from 1 to 5 meters in depth and drain 2 to 20 hectares. Large gullies are greater than 5 meters deep and drain over 20 hectares.
Stream Bank Erosion [26:00]
Stream bank erosion involves the scouring and undercutting of soil below the water surface in a stream. This process is influenced by water currents that erode the bed and sides of the stream. Additionally, water flowing over bare soil on the stream banks can carry soil particles into the stream, contributing to erosion. Human activities, such as removing vegetation or overgrazing on the banks, can exacerbate this type of erosion.
Wind Erosion [27:48]
Wind erosion is the removal of soil from an area due to wind, primarily occurring in low rainfall areas where soil moisture is low. Dry, loose soil particles are more susceptible to being blown away. Light winds can roll soil particles along the surface, while strong winds can lift large volumes of soil into the air, creating dust storms.
Human Impact on Soil Erosion [29:35]
Human activities significantly affect soil erosion. Deforestation removes canopy cover, exposing soil to raindrop impact and flowing water, increasing erosion. Residential and commercial construction often leaves large amounts of loose soil exposed. Traditional cultivation practices, such as ploughing and tillage, can also leave soil exposed, making it vulnerable to erosion from rainfall and overland flow.
