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Home » Hydro Meteorological Disasters » River and Sea Erosion

Causes of Erosion
Forest Fire
Coastal erosion occurs when wind, waves and long shore currents move sand from the shore and deposits it somewhere else. The sand can be moved to another beach, to the deeper ocean bottom, into an ocean trench or onto the landside of a dune. The removal of sand from the sand-sharing system results in permanent changes in beach shape and structure. The impact of the event is not seen immediately as in the case of tsunami or storm surge. But it is equally important when we consider loss of property. It generally takes months or years to note the impact of erosion; therefore, this is generally classified as a "long term coastal hazard".

Major Causes of Coastal Erosion are

Natural Causes
Action of Waves
Waves are generated by offshore and near-shore winds, which blow over the sea surface and transfer their energy to the water surface. As they move towards the shore, waves break and the turbulent energy released stirs up and moves the sediments deposited on the seabed. The wave energy is a function of the wave heights and the wave periods.

Winds acts not just as a generator of waves but also as a factor of the landwards move of dunes (Aeolian erosion).

Forest Fire
Tides results in water elevation to the attraction of water masses by the moon and the sun. During high tides, the energy of the breaking waves is released higher on the foreshore or the cliff base (cliff undercutting). Macro-tidal coasts (i.e. coasts along which the tidal range exceeds 4 meters), all along the Atlantic sea (e.g. Vale do Lobo in Portugal), are more sensitive to tide-induced water elevation than micro-tidal coasts (i.e. tidal range below 1 meter).

Near-shore currents
Sediments scoured from the seabed are transported away from their original location by currents. In turn the transport of (coarse) sediments defines the boundary of coastal sediment cells, i.e. relatively self-contained system within which (coarse) sediments stay. Currents are generated by the action of tides (ebb and flood currents), waves breaking at an oblique angle with the shore (long-shore currents), and the backwash of waves on the foreshore (rip currents). All these currents contribute to coastal erosion processes.

Storms result in raised water levels (known as storm surge) and highly energetic waves induced by extreme winds (Cyclones). Combined with high tides, storms may result in catastrophic damages such as along the east coast of India (Orissa Super Cyclone, 1999). Beside damages to coastal infrastructure, storms cause beaches and dunes to retreat of tenths of meters in a few hours, or may considerably undermine cliff stability.

Catastrophic events
Forest Fire

In addition to the daily, slow sculpting of the coast, other events like tsunamis which result in major coastal changes over very short time periods. These are referred to as catastrophic events because of the extensive damage that is caused and the unpredictable nature of the event.

Slope processes
The term �slope processes� encompasses a wide range of land-sea interactions which eventually result in the collapse, slippage, or topple of coastal cliff blocks. These processes involve on the one hand terrestrial processes such as rainfall and water

Vertical land movements (compaction).
Vertical land movement - including isostatic rebound, tectonic movement, or sediment settlement - may have either a positive or negative impact on coastline evolution. If most of northern Europe has benefited in the past from a land uplift (e.g. Baltic sea, Ireland, Northern UK).

Sea Level Rise
Forest Fire
Sea level has risen about 40 cm in the past century and is projected to rise another 60 cm in the next century. Sea level has risen nearly 110 meters since the last ice age. Due to global warming, average rise of sea level is of the order of 1.5 to 10 mm per year. It has been observed that sea level rise of 1 mm per year could cause a recession of shoreline in the order of about 0.5 m per year.

Anthropogenic Causes
Human influence, particularly urbanisation and economic activities, in the coastal zone has turned coastal erosion from a natural phenomenon into a problem of growing intensity. Anthropological effects that trigger beach erosion are: construction of artificial structures, mining of beach sand, offshore dredging, or building of dams or rivers. Human intervention can alter these natural processes through the following actions:
  • dredging of tidal entrances
  • Forest Fire
  • construction of harbours in near shore
  • construction of groins and jetties
  • River water regulation works
  • hardening of shorelines with seawalls or revetments
  • construction of sediment-trapping upland dams
  • beach nourishment
  • Destruction of mangroves and other natural buffers
  • Mining or water extraction
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