EROSION CONTROL (WATERSHED MANAGEMENT)

UNIT – IV

Rambabu Palaka, Assistant ProfessorBVRIT

Learning Objectives1. Soil Erosion

a) Types of Erosionb) Factors affecting Erosionc) Effect of Erosiond) Estimation of Soil Loss

2. Measures to Control Erosion a) Contour Techniquesb) Ploughingc) Furrowingd) Trenchinge) Bundingf) Terracingg) Gully Controlh) Rock-fill Damsi) Brushwood Damj) Gabion

Soil ErosionSoil erosion is a naturally occurring process that affects all landforms. In agriculture, soil erosion refers to the wearing away of a field's topsoil by the natural physical forces of water and wind or through forces associated with farming activities such as tillage.

Erosion, whether it is by water, wind or tillage, involves three distinct actions – soil detachment, movement and deposition.

Types of Erosion1. Geologic Erosion2. Accelerated Erosion

a) Wind Erosionb) Water Erosion

i. Raindrop Erosionii. Rill Erosioniii. Gully Erosioniv. Bank Erosionv. Sheet Erosionvi. Landslide Erosion

c) Tillage Erosion

Geologic ErosionIt is a normal process, representing erosion of land in natural environment. It is caused by the effect of Rainfall Run-off Wind Topography Atmospheric temperatures Gravitation Process

Accelerated ErosionAccelerated erosion is due to man-made activities, which have brought about changes in natural cover and soil condition. The activities includes Land Preparation for raising crops Land Use patterns for building houses, infrastructures, industries, hill cutting, deforestation, lack of soil conservation and watershed management

Water Erosion The impact of raindrops causes splash erosion Run-off water causes scouring, scrapping and transport of soil particles, leading to sheet, rill and gully erosion. Flood water causes erosion of River Banks Water waves cause erosion of bank and sides of reservoir, lakes and oceans. Subsurface seepage water causes soil boiling, erosion and removal by piping. Glacial erosion causes heavy landslides

Wind ErosionIt is primarily responsible for creation and maintenance of desert areas. Finer soil particles from top soil along with organic matter and nutrients are easily detachable and removed by wind velocity. Various factors affecting wind erosion are wind velocity, rainfall, temperature, humidity, soil texture, organic matter content, water holding capacity of soil, vegetation cover etc.,

Erosion due to Wind

Erosion due to Surface Runoff

Bank Erosion

Sheet Erosion Sheet erosion is the movement of soil from raindrop splash and runoff water.

It typically occurs evenly over a uniform slope and goes unnoticed until most of the productive topsoil has been lost.

Deposition of the eroded soil occurs at the bottom of the slope or in low areas.

Rill Erosion In general, the sheet flow is carried out by very small definable channels called interrill.

Raindrops detach every layer of soil particles through splash and the detached particles are then carried through interrills by a thin layer of overland lateral flow.

When the process of soil erosion and deposition through interrill is prolonged, the interrills are widened, leading to formation of small channels called rills.

Raindrop Erosion

Gully Erosion Rills are usually smaller in size and depth. They can be destroyed by tillage operation.

If their occurrence is prolonged, rills become larger in size and can not be destroyed by tillage operation, and they are transformed into Gullies.

Large gullies and their networks are called Ravines.

Tillage Erosion

Landslide Erosion

Factors Affecting Erosion Climatic Factors

a) Rainfallb) Rainfall Intensityc) Runoffd) Temperaturee) Wind

Characteristics of Soila) Textureb) Structure & Cohesionc) Particle Sized) Organic Content

Vegetation Cover Slope of Watershed

Effect of Erosion Loss in Agricultural Production Loss of Nutrients Reduction of Infiltration Rates Reduction in Water holding Capacity Increased Cost in Tillage Operation Floods may occur in Rivers Decrease in useful life of Reservoir Survival of Wild Life Land Degradation Cost of Removal of Sedimentation Desertification

Estimation of Soil Loss The Universal Soil Loss Equation (USLE) predicts the long-term average annual rate of erosion on a field slope based on rainfall pattern, soil type, topography, crop system and management practices.

USLE only predicts the amount of soil loss that results from sheet or rill erosion on a single slope and does not account for additional soil losses that might occur from gully, wind or tillage erosion.

This erosion model was created for use in selected cropping and management systems, but is also applicable to non-agricultural conditions such as construction sites.

This was developed by United States Department of Agriculture (USDA) in the year 1961

Estimation of Soil LossUniversal Soil Loss Equation (USLE):

A = R.K.LS.C.PWhereA = Average Annual Soil Loss due to Sheet and Pile Erosion in Tons per Hectare per YearR = Rainfall & Runoff factor based on Geographical LocationK = Soil Erodability Factor depends on Texture of Soil & Organic Content in SoilLS = Length-Gradient Factor depends on Slope Length & Slope (%)C = Crop Management Factor = Crop Type Factor x Tillage Method FactorP = Erosion-Control-Practice Factor

Note: check tables at http://www.omafra.gov.on.ca/english/engineer/facts/12-051.htm

Reference

Watershed Management by

Madan Mohan Das & Mimi Das Saikia, PHI Learning Private Limited

&JVS Murty, New Age Publications