SUBJECT: EARTH SCIENCE PAPER: THE DYNAMIC EARTH Code : 27 TOPIC: COASTAL PROCESSES AND LANDFORMS BY PROF. A. BALASUBRAMANIAN Objectives After attending this module, the user would be able to understand the coastal geomorphic conditions, coastal processes and the landforms created by waves, tides and currents. In addition, the features of erosion, transportation and deposition made by the tides and waves are highlighted. The uniqueness of coastal areas existing as the transition zones between the land and the seas will also be understood. Introduction The subject of geomorphology deals with landforms created by various geological agents. Among the geological agents, waves and currents are very important dynamic, never ending mechanisms, acting on the coastal areas. Seas and oceans are constantly sculpturing the edges of the continents and create various landforms all along the coasts. The coastal zones are very sensitive zones. The Continental shelves and near-shore areas comprise about 5% of the area of the world. Inspite of this, two-thirds of the world’s population thrives near the coasts, all over the world. Throughout the human history, people have been attracted to settle along the coasts. It is mainly because of the moderate climate, availability of fresh marine food, commercial and recreational opportunities and occurrence of many other natural resources. Coastal Zones Coasts are an interface between the lithosphere and hydrosphere. Coastal ecosystems are good habitats for innumerable flora and fauna. Time immemorial, beaches are considered to be the best places for tourism, fishing, mariculture, water- transport, recreation, sporting, social, political and community interactions. Sun bathing and souvenir collection are the regular activities of tourists along the beaches. Coastal zones are very unique and typical land masses, bordered by the seas and oceans. Coastal zones are not static but dynamic environments. They are affected by the action of waves and tides. All shore lines are subjected to both continental and oceanic processes. Waves and Tides Waves, tides and currents are very powerful geomorphic agents. The erosional and depositional work of the sea waves can create many spectacular landscapes along the borders of the continents. Studying the coastal landforms are interesting aspects in geomorphology. Coastal Geomorphology is a major branch of geomorphology. Geomorphology of Coasts Every continent or island is bordered by a long or short coastline. Coastline is the line separating the land and sea. Coastal zones are the transition zones between terrestrial and marine habitat. They form an interface between land and oceanic natural processes. Coastal areas also are varied in their topography, climate and vegetation. Some are sandy beaches, rocky shores, with or without tidal inlets. The climate of a coast are controlled by the land and sea breezes and the humidity controlled by marine water. Waves are powerful tools for constructive and destructive activities. Hence. the geomorphology of beach, materials and processes are always not constant due to the impact of everlasting action of tides, waves and currents. If we look at the division of a sea coast and a sea shore, the shoreline represents the actual landward limit of the seawater. This is the boundary between land and water. This limit varies with reference to location and time. Along the coasts we could notice a high tide zone and a low tide zones. These are two observable waterlines upto which the tides normally Page 1 of 8 swash the coast. It happens every day. The average water level between the high tide and the low tide is considered as the mean sea level. Divisions of Coastal Zones Coastal belts are divided into three divisions as backshore, foreshore and offshore. The Backshore represents the beach zone starting from the limit of frequent storm wave, above high tide shoreline. This zone includes a wave cut terrace and a storm scarp. The nearshore (sometimes called the breaker zone) is where the waves break; the offshore zone is further out to sea and is beyond the influence of the waves. Foreshore: The Foreshore region is the region between high tide water zone and low tide water zone. It includes a beach face and a beach terrace. The surf zone exists above beach terrace. At the end of the surf zone, the breaker zone starts. The foreshore may be a sandy foreshore, shingle foreshore, muddy foreshore or a rocky foreshore. Five Major Zones There are five major zones identified along the coastal belts as: a) Zone of wind action b) Limit of wave action c) Zone of swash and backwash d) Zone of breakers e) Zone of shoaling waves. Zone of Wind & Wave Action Zone of wind action lies on the landward side and the zone of shoaling waves exist inside the sea. The limit of wave action ends along the regions of berms, which are sediment deposited zones well above the zones of swash and backwash. The greatest amount of sediment transport as beach deposit occurs within the shore and nearshore zones. Beyond the nearshore lies the offshore region. Features of Shoreline The typical features of the shoreline are a. Breaker zone b. Surf zone c. Swash zone d. Berms and e. Beach Face. Breaker zone The Breaker zone is the area where the incoming waves become unstable, raising to a peak and breaking down. Breaker zone is an important zone within which waves approaching the coastline commence breaking. The breaker zone is also part of the surf zone. The Surf zone is an important zone where the waves of translation occur after the waves break. Sand Bars are created, inside the waters, along the zone of wave breakers. The moving water masses shape the excess quantities of detritus sediments into sorted and layered deposits. Surf zone : This is the zone where the waves of translation occur after the waves break. Long shore currents occur in this zone, which run parallel to the coastline. Swash zone: This is the area where the waves backwash the materials. It is the place where up and down movement of beach materials take place. Berms Berms are the flat back shore areas on beaches. This is the Sun-bathing zone with wave deposited sediments. A berm is a bench-like feature containing sands carried shoreward by the swash. Landward of the berm is a belt of dunes built by Page 2 of 8 wind of loose sand swept off the berms. There are summer and winter berms, both are located within the zone of wave action. During the summer, accumulation of sand takes place and it forms the summer berms. The summer berm starts from the zone of swash and backwash. Bars are created inside the waters along the zone of wave breakers. Moving water shapes excess quantities of detritus into sorted and layered deposits. Beach Face A beach face is the sloping section of a beach profile below the beach berm which is normally exposed to the action of the wave swash. Beach is the basic area where much of the geological processes are happening everlastingly. A Beach is the sloping portion of the coast normally existing below the berms. This area is partly exposed by the backwash of waves (swash zone). A beach is characterised by its geometric profile and the sediments comprising in it. The parameters of a beach are 1. its geographic setting and profile 2. the beach face slope 3. volume of sediments lying within the beache and 4. their environmental conditions. Beach cycles A beach's ability to rebuild itself, makes it a formidable bastion against the sea. Whereas headlands and cliffs erode, beaches can hold their own against the anger of the sea. A beach can store sand and grow during years of good sea winds, few rains or storms and much sunshine. As the sand pump pumps the sand from the wet beach, it causes the beach to lie steeper. During years with opposite conditions, the beach can erode and lie flatter. Beaches are classified into high, low and moderate energy beaches based on these characters. Normally, high energy conditions prevail during summer months. The wave height increases after the onset of monsoon and produce significant changes in the beach morphology. Features of a coastline: The other major features of a coastline are: a) Sand dunes b) Lagoons or tidal inlets c) Estuaries. Sand dunes: Sand dunes of a beach, upto 3 mt elevation, is considered to be the zone of wind action. Dunes are accumulations of wind-blown sand. Although some dunes are bare, most of the dunes near a coast are vegetated with coastal plants. Such plants existing in a coastal dune help to stabilize the dune. Dunes are very dynamic geomorphic features. They are subjected to erosion during periods of high waves and accrete during normal wave conditions. During a storm or a large swell, waves attack and erode the dunes. This process, known as scarping, releases sand that was stored in the dune into the active beach. The influx of sand is often carried offshore to build sand bars, which help to attenuate the incoming wave energy. Lagoons Lagoons are shallow bodies of brackish or salt (sea) water partially separated from the neighboring sea by barriers of sand or shingle. The sea water can flow only through narrow openings left by the barriers. They become the coastal ponds or lakes, if they are completely detached from the sea. Estuaries Marine ecosystems also include the salt marshes and wetlands located along the shores and river mouths. Within the coastal zone, several unique habitats like estuaries, tidal inlets, and foreshore ecosystems are also included. At the mouth of every old stream and a river, there will an aquatic condition which includes the habitat of both fresh and salt waters.