Write short notes on: 1. Saltation 2. Surface Creep in Wind Erosion

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1. Saltation

Saltation is the transportation of sand grains in small jumps by wind or flowing water. The term does not refer to salt, but is derived from the Latin saltare, to dance. Saltation is the main process that suspends soil particles in the air – particles between 0.1 and 0.5mm in diameter are lifted by wind then fall back to the ground, so they hop or bounce across the surface. The major fraction of soil moved by the wind is through the process of saltation. In saltation, fine soil particles are lifted into the air by the wind and drift horizontally across the surface increasing in velocity as they go. Soil particles moved in this process of saltation can cause severe damage to the soil surface and vegetation. They travel approximately four times longer in distance than in height. When they strike the surface again they either rebound back into the air or knock other particles into the air.

Certain conditions are necessary for Saltation:

First, a bed of sand grains must be covered by flowing air or water, as in a streambed or windy desert.

Second, this flow must be turbulent. In turbulent flow, a fluid swirls and mixes chaotically—and virtually all natural flows of water and air are turbulent.

Third, some of the eddies in the turbulence must be strong enough to lift individual sand grains from the bottom. Fourth, the turbulence must not be so strong that grains cannot settle out again once suspended.

An individual saltating grain spends most of its time lying at rest on the bottom. Eventually an eddy happens to apply enough suction to the upper surface of the grain to overcome its weight, lifting it into the current. The grain is carried for a short distance, and then allowed to settle to the bottom again by the ever-shifting turbulence. After a random waiting period, the grain is lifted, carried, and dropped again, always farther downstream.

Grains too small to settle once suspended are carried indefinitely by the current; intermediate-size grains saltate; and grains too large to saltate either remain unmoved or move by sliding or rolling. Turbulent flow thus tends to sort grains by size.

2. Surface Creep in Wind Erosion:

The large particles which are too heavy to be lifted into the air are moved through a process called surface creep. In this process, the particles are rolled across the surface after coming into contact with the soil particles in saltation. The rolling and sliding of larger soil particles along the ground surface. The movement of these particles is aided by the bouncing impacts of the saltating particles. Soil creep can move particles ranging from 0.5 to 1 mm in diameter, and accounts for 5 to 25% of total soil movement by wind. Particles greater than 0.5mm in diameter are usually too heavy to be lifted by wind. Creep is when wind rolls these particles or they are moved along the surface by bombardment by other moving particles.

 

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