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"GEOMORPHOLOGICAL ASSESSMENT OF GROYNE TYPE BUNDS AND THEIR IMPACT ON VARSOLI CREEK, MAHARASHTRA" CHAPTER - 1 INTRODUCTION Coastal erosion and siltation Coastal erosion is becoming an increasing problem all over the world. Maharashtra State has 720 Km long coast, which is divided in 5 districts as: 1) Sindhudurg - 120 Km 2) Ratnagiri - 238 Km 3) Raigad - 121 Km 4) Mumbai - 114 Km 5) Thane - 127 Km. Out of the coastal length of 720 Km about 262 Km length is vulnerable to erosion (NCRMP 2008). Major uses of these coastal zones include commercial and industrial development along the coastal belt, human habitation, military defense, extraction of living and non-living resources, recreational facilities, waste disposal, in addition to the construction of port and harbors. For all this development, more and more land along the coastal belt is being used by the humans (NCRMP 2008). The coast represents the meeting point between the land and sea. Coasts are very dynamic areas and they are constantly changing. This change is due to 3 main processes such as erosion, transportation and deposition which operate at the coast. 1) Erosion Natural forces such as wind, waves and currents are constantly shaping the coastal regions. The landward displacement of the shoreline caused by the forces of waves and currents is termed as coastal erosion. Coastal erosion occurs when wind, waves and longshore currents move sand from the shore and deposit 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 change in beach shape and structure. The impact of the event is not seen immediately as in the case of tsunami or storm surge. It is equally important when we consider loss of property. It generally takes months or years for the impact of erosion to be visible. Therefore, this is generally classified as a "long term coastal hazard" (Chambers 2007). 2) Transportation The second process operating at the coast is the material transport. Material eroded by the sea is carried within the water in a number of ways, minerals dissolved from rocks are carried in solution, whilst small rock fragments, light enough to be held within the water, float in suspension. The largest rock fragments which are too heavy to be picked up by the waves, are transported by the process of traction, this is where they roll along the bed when the waves pick up enough energy. Finally, medium sized rock particles, which cannot be carried by the waves all the time, are moved by siltation. This is where during times of higher wave energy the particles are picked up and then dropped again as the wave looses its energy (Chambers 2007). 3) Deposition Material is moved up the beach by the swash at an angle which is controlled by the prevailing wind. The backwash then carries material back downs the beach at right angles to the coastline under the influence of gravity. Gradually the material is moved along the coastline, its direction being controlled by the prevailing wind direction. The final process operating at the coast is that of deposition. This is where material that is too heavy to be transported any more is left behind, building up the beach. Due to the importance of energy in transporting sand and shingle, it is the largest material that is deposited first (Chambers 2007). Material that is transported by the waves along a coastline is eventually deposited forming distinctive deposition features. There are four main depositional features developed on coasts. These are beaches, spits, bars, tombolos. Beaches are the main feature of deposition found at the coast. These consist of all the material (sand, shingle etc.) that has built up between the high and low tide mark. There are number of different sources of beach material, the main source being rivers, where fine muds and gravels are deposited at the river mouth. Other sources of beach material include longshore drift (bringing material from elsewhere along the coast), constructive waves (bringing material up the beach from the sea) and from cliff erosion. Erosion is the most significant process on the coast. It works as follows : - a) Corrasive action Waves equipped with sand, boulders and pebbles perform erosive work. Hurling of sand, shingle and boulders against the cliffs are destructive in action. These have abrasive effect. It is physical erosion caused by loose solid material during its journey. Vertical and lateral corrasion occurs on the coast. Undercutting the coastal rocks pave out into caves. Finally the ceiling of the cave collapses and the rock mass is eroded. Collapsing of the cave roof or overhanging cliff is assisted by weathering or sub aerial erosion, pull of gravity, structural weaknesses and rain action (Singl998). b) Attrition The pieces of beach material as they are thrown up against sea rocks, they are swirled up and down the beach. Due to swash and backwash of waves rock fragments are repeatedly hitted within each other and get broken and ground up. This is known as attrition. It mainly occurs due to coalescence of eroded particles among each other. c) Hydraulic action The waves that attack against cliffs and sea walls put pressure upon the coast. At the time of breaking of the waves against cliff face the air in gaps, cracks, crevices is locked up, it gets compressed and this high pressure acts like a wedge, forcing the gap and side walls apart. With the backwash the pressure is released and compressed air expands again. The compression and expansion of the air in the cracks and gaps alternate repeatedly so that, rocks are weakened and cavities are enlarged by hydraulic action. d) Solvent action Solvent action denotes chemical reaction of sea water and corrosion effects. Sea water with dissolved chemicals induces solvent action. Rocks like chalk and limestone are eaten by solvent action of sea water. Following are the major natural factors of coastal erosion :- 1) Waves Waves are generated by offshore and nearshore 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 (Chambers 2007). 2) Wind Wind acts not just as a generator of waves but also as a factor of the landward movement of dunes which is necessarily aeolian erosion. 3) Tides Tides result in the water elevation of water surface due 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). 4) 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 (longshore currents), and the backwash of waves on the foreshore (rip currents). All these currents contribute to coastal erosion processes (Chambers 2007). 5) Storms 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. Beside damages to coastal infrastructure, storms cause beaches and dunes to retreat by tenths of meters in a few hours, or may considerably undermine cliff stability. 6) Catastrophic events In addition to the daily, slow erosion of the coast, other events like tsunamis 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. 7) 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 also involve terrestrial processes such as rainfall and water. 8) Vertical land movements (compaction) Vertical land movement including isostatic rebound, tectonic movement, or sediment deposition may have either a positive or negative impact on coastline evolution. Problem of coastal erosion and deposition Coastal erosion is becoming an increasing problem all over the world. Before finding solution to stop the erosion at a specific location, the cause of erosion must be found at that location. Therefore it is important to understand how sediment is removed and transported. The different types of erosion may be split up into two groups. 1) Erosion due to natural causes 2) Erosion resulting from human interference (Perdok 2002). The problem of erosion calls for the protection of houses, cultivable lands, valuable properties, monuments etc. in the coastal belt. It is well known that the erosion of a coast is mainly due to action of waves in addition to the currents setup by the oblique attack of waves. Erosion of the coast depends on many natural factors like storm waves, nature of the beach, beach material and the shape of the coast, tidal level changes, movement and quantity of the littoral drift material. Storms may occur seasonally, sand spits may grow in a few decades and the sea level rise that involves centuries. The various natural causes have different time scales. The best natural defense against erosion is however an adequate beach on which waves expends their energy. Waves are the prime cause of beach erosion; it is natural that the protective methods are evolved so as to dissipate the energy of waves either by absorbing this energy on the beach or dissipating / diverting the same before the waves approach the beach.
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