River deposition

Depositional landforms formed by flowing rivers:

Deltas ‐

• formed near mouth of streams where it meets seas or stagnant water bodies. generally formed by fine deposits.

Factors responsible for formation:

• Load carrying capacity of rivers: If the load is not carried far into the sea, it spreads and accumulates as a low lying cone forming deltas.

• Composition of sediments carried by rivers: Coarser material settles and finer fraction like silts and clays are carried down into the sea.

• Continental shelf: Broader and less steep continental shelf aids delta formation.

Economic significance:

• most fertile agriculture areas ‐ supporting the livelihood. trade and commerce, and major ports, Fish, other seafood. World's largest delta ‐ Ganges–Brahmaputra

delta densely populated.

• sorted sand and gravel which is quarried.

• centres of tourism and recreation owing to their natural diversity and richness.

Ecological: absorb runoff from both river floods and storms (from lakes or the ocean), filter water and reduces the impact of pollution flowing from upstream. important wetland habitats ‐ diverse flora and fauna, dense forests. migratory birds.

Alluvial fans ‐

• formed at foothills where streams flowing from higher level break into foot slope plains of low gradient

• generally composed of coarse sediments ‐ not distinguishingly stratified

• shape depends on the climate o in humid areas ‐ normally low cones with gentle slope from head to toe

o in arid and semi‐arid climate‐ appear as high cones with steep slopes.

Economic significance of alluvial fans:

1. Porous and permeable fan deposits ‐ primary source of groundwater, irrigation and for water supply, especially in arid and semi‐arid climates.

2. Concentrate heavy mineral particles in placer ‐ economical extraction of elements.








Threats to River Deltas

Human activities, ‐ diversion of water for irrigation and creation of dams reduce sedimentation, which can cause delta to erode away. Unsustainable mining of sand and gravel degrades river delta.

• The use of water upstream can greatly increase salinity levels as less fresh water flows to meet salty ocean water.

• Climate change and rising sea level – rising sea level flood deltas bringing in saline water and threatening wetland ecosystem. For ex ‐ 31 square miles of Sundarbans have vanished entirely due to sea level rise. Four million people on Indian side, as well as royal Bengal tigers inhabiting there are threatened.

Measures to overcome these threats

• Physical measures aimed at the management of sediment and/or at the management of water‐ full or partial recovery of a water system such as removing dikes, seawalls and dams as well as steering natural processes by creating flood areas and using sand replenishment along the coastline. Dredging can be one tool for addressing these problems.

• Adapting human behavior‐ spatial planning of deltas, which include evacuation or even relocation of people to less risky higher elevations, can be effective in reducing harm to life and livelihood caused by flooding. By identifying flood hazard and buffer zones, damage can be controlled.

Western coast of our country is a high rocky retreating coast while the east coast is a low sedimentary coast. This is the main reason behind erosional forms dominating in the west coast while depositional forms dominate in the east coast

• Along the high rocky coasts, the rivers appear to have been drowned with highly irregular Coastline. The coastline appears highly indented with extension of water into land. The hill sides drop off sharply into the water. Shores do not show any depositional landforms initially. Erosion features dominate.

• Along low sedimentary coasts rivers appear to extend their length by building coastal plains and deltas. Coastline appears smooth and occasional incursions of water in the form of lagoons and tidal creeks. Land slopes gently into water. Marshes and swamps may be abounding along coasts. Depositional features dominate.







Soil Liquefaction:

Definition: Soil liquefaction is a phenomenon whereby a saturated or partially saturated soil substantially loses strength & stiffness in response to an applied or sudden change in stress condition, causing it to behave like a ’liquid’.



• Commonly observed in shallow, loose, moderately saturated granular soils with poor drainage such as silts & gravels. (Near inland body).

• Regions which exhibit soil liquefaction are geographically dispersed & mainly located in active seismic zones.

Manifestation during seismic events

• Seismic events increase the water pressure making soil particles can readily move.

• Strong vibrations in earthquake can induce water logging which increases liquidity of soil.

• Contact forces between individual soil particles causes weakening of soil deposit which cannot sustain stresses of its load from the foundations.

• As a result, soil loses its cohesion & reduces the ability of a soil deposit to support the construction above it resulting into structural failures.

• It increases the urban seismic risk as its occurrence results into buckling of piles, quick sand effect, spreading of ground, formation of sand volcanoes, failure of retaining walls & loss of bearing capacity

Preventive measures to minimize the impact:

• Avoid construction in liquefaction susceptible soils.

• Build liquefaction‐proof structural system

• Increasing liquefaction resistance: loss to existing buildings can be mitigated by injecting grout into the soil to stabilize soil.

• Deploying soil compaction techniques: Methods such as Vibro compaction

• Earthquake Drain: They are corrugated pipe wrapped in a filter fabric installed with a vibrating mandrel in a grid pattern.