**4.9 Coastal erosion**

*Soil Moisture Importance*

topsoil surface.

quently results in ravine formation.

**4.7 Slip erosion or landslip erosion**

**4.8 Stream bank erosion**

**4.5 Ravine formation**

**4.6 Tunnel erosion**

factors affecting gully erosion are shear stress of flowing water and critical shear stress of the soil. The further erosion of gullies results in ravines formation. Based

a.**U-shaped gullies:** These types of gullies are usually formed in alluvial soils where the characteristics of both the surface and subsurface soils are similar.

b.**V-shaped gullies:** This is the most common shape of gully erosion which occurs in the areas where the subsurface of soil is more resistant than the

It is referred to as a network of deep and narrow gullies that flows parallel to each other while linking with the river system. Mismanagement and non-judicious use of land result in enlargement of rills and gullies and eventually lead to ravine formation. Abrupt changes in elevation of the river bed and the adjoining land surface, deep and permeable soil with high erodibility, sparse vegetation, and backflow of river water during the recession period causes severe bank erosion which conse-

It is the sub-soil erosion through runoff flow in channels while surface soil remains intact. Tunnel erosion is also known as pipe erosion and commonly occurs in arid and semiarid regions where the soil permeability for water varied with the soil profile. The further widening and deepening of tunnels form large gullies which degrade the productive agricultural lands. Soil with erodible characteristics, having sodic B horizon and stable A horizon are highly prone to tunnel erosion. Runoff flow through natural cracks and animal burrows initiates tunnel formation by infiltrating thorough dispersible subsoil layers. Seepage, lateral flow, and interflow are key indicators of tunnel erosion. It alters the geomorphic and hydrologic characteristics of the affected areas. Management practices for tunnel erosion are ripping, contour farming, vegetation including trees and deep-rooted grasses with proper fertilization and liming, consolidation of surface soil, and diversion of concentrated runoff.

It is the downward and outward movement of slope forming materials composed of natural rocks and debris from sloppy lands. It is also known as mudslide or mass erosion. This type of erosion mostly occurs in hilly regions having watersaturated soils slips down the hillside or mountain slope. Banks along highways, streams, and ocean fronts are often subject to landslides. The large masses of land slip down which destroy the vegetation and degrade the productivity of lands. The slope can be stabilized through developments of diversion drains, contour trenches,

The scouring of soil material from the stream bed and cutting of stream bank by the action of flowing water is known as stream bank erosion. Streams and rivers change their direction of flow by cutting the bed from one side and depositing the sediment to the other side of the stream. Flash floods enhanced the stream bank

crib structures, geotextiles, kutta—crate structures, and retaining walls.

on the size, depth, and drainage area, gullies can be classified as:

**28**

Sea level is incessantly rising due which can increase the frequency of occurrence of natural disasters like the tsunami in the coastal areas in the future. Such natural hazards produce strong water waves which can severely erode the seaside areas. It is projected that the erosion rate will be higher in coastal regions in the coming years. The anthropogenic activities leading to coastal erosion are port construction, destruction of mangroves, and beach and river bed mining [23].
