Shore Processes

SHORE PROCESSES Dr. Thrivikramji.K.P. [email protected]

Introduction The shoreline, meeting place of all the three earth spheres and embedded biosphere, is most dynamic of all environments and as a consequence, the shores and beaches of the seas and very large lakes had attracted humans not only for recreation, but also for launching sites of trade, commerce and war. Naturally huge population s centers have grown to become very large metropolises of today. According to a recent estimate as much as 2/3 of the world population lives in towns and cities located along the coastal tract. Obviously, in addition to the natural processes inherent to the shoreline, to a large degree of human intrusions have also taken place for which the response of the system is quite harming. In any process several components will partake, and in here i.e., shore processes, they are land mass of the backshore, wave and currents in the water covered sea and ocean, sediment sitting at the sea bed as well as the endless ribbon of it marking the shoreline, and in the ancient backshore and wave and tide generated currents. Understanding of the shore processes is critical to the areas of defense, navigation, oil exploration, fisheries, trade and commerce and tourism. Coastal classification Unlike ordinary mortals, Geoscientist s attraction to the coasts led to proposals on classification of coasts from time to time (Johnson, 1919; Cotton, 1952; Shepard 1963; Valentine , Putnam et. al. 1960 and Inman and Nordstrom, 1971). Earlier classifications were primarily based on recent relative changes in sea level and its manifestation on the appearance of coastal land, viz., emerging coasts and submerging coasts and neutral coasts. They also identified two subclasses like deltaic and reef coasts. Shepard (1963) also added that many coasts show evidences of both emergence and submergence. In their recent proposal on coastal classification, Inman and Nordstrom (1971), both coastal tectonics and morphology have been reckoned with (Table 1) wherein 5 different Table 1. Morphologic and Tectonic classification of coasts (Inman and Nordstrom, 1971) (all numbers in percent) Morphologic class Mountainous Narrow shelf hilly Narrow shelf plain Wide shelf plain Wide shelf hilly Deltaic coast Reef coast Glacial coast Total Percentage

1.Collision coast 97.2 --

Neo 8.0 75.1

2. Trailing coast Afro -14.1

Amero ---

3.Marginal sea coast 2.5 5.6

--

15.0

46.2

1.5

--

--

--

4.0

89.3

3.1

--

--

--

2.2

77.4

--2.8 100.0 39.0

1.0 --100.0 4.6

3.4 3.0 29.3. 100.0 7.5

1.3 1.9 3.8 100.0 35.2

5.8 5.6 -100.0 8.1

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categories have been included. Based on plate tectonic two types, viz., collision and trailing coasts have been recognized. Under collision coasts two categories have been suggested. Table 2. Types of coasts 1. Mountain coasts: Shelf width <50.0 km.- Max. elevation 300.0 m rocky cliffs & pocket beaches evolve to hilly coast, and the sub-categories are:. a) Narrow shelf hilly coast: shelf width <50 km Max elevation of hills <300.0 m occasional headlands, backshore cliffs or smaller hills occasional barriers, lagoons, bay mouth bars, a.k.a. plateau coast if relief is continuous . b) Narrow shelf plain coast: shelf width <50 km shore zone deposits less extensiveoccasional low sea cliffs and low head lands. c) Wide shelf plain coast: shelf width >50 km low lying coastal plain bordered by wide shore zone barrier beaches of some kind. E.g., Amero-trailing edge shore zone d) Wide shelf hilly coast: shelf width > 50 km. wide apart headlands wider shore zone low seacliffs - more barrier beaches grade into plain coast. 2. Deltaic coast: Ancient or modern riverine deposits low lying coastal bulges extending 50 km. or more along the coast gross features unaffected by wave erosion 3. Reef coast: Shore zone with reefs of organic origin

fringing and level out of shore line or barrier reef. .

4. Glaciated coasts: Dominated by glacial erosional features like fjords and precipitous cliffs.

Geologically, coastal zone of India is characterized by rocks very differing geological ages, i.e., from Proterozoic to Recent. Though, classically coasts of India did attract only scant attention of the geoscientists, where ever it occurred a mention of either submerging or emerging type was made. This scenario went through a revolution by the release of LANDSAT imageries to the Indian geological scene and this new tool offered a newer and reliable picture of the tectonic make up of the Indian landmass, including the coasts paving the way of tectonics to enter the realm of coastal geological studies. . Following the scheme of classification of Inman and Nordstrom (1971) based on tectonics and morphology, most of the Indian coasts will fall under one of their several categories. For example the TN coasts have segments of salt marshes, coral reefs, deltas, narrow shelf plain coast, wide shelf plain coast and narrow shelf hilly coast and wide shelf hilly coast. The coastal land of TN is typically a one with narrow or wide shelf Table 3 Characteristics of shore zone Erosional coasts Sea cliffs, Stacks and river erosion during past low sea level stands

Depositional coasts Beaches, Barriers, spits, modified glacial deposits, River deltas and coastal dunes

Organic coasts Coral and algal reefs, Mangroves and salt marshes

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hilly and plain coasts and deltaic coast. Coastal land at Mahalipuram and Kudankulam and Kanyakumari can also be good examples of a wide shelf hilly coast. In fact, the land mass of TN has witnessed relatively large shifts of shore line in the Tertiary era as well as in Holocene. The marine terraces of Kanyakumari, sedimentary structural facies of Melemanakkudi and Kudankulam, the sedimentary motifs of Manappad, beach rock occurrences etc are some compelling examples of neotectonics. Physiography of TN Based on physiography, State of TN is divided into three segments, viz., highland (chiefly underlain by PC crystalline rocks and their altered equivalents), inland plains (large and expansive pediments commonly underlain by older crystalline rocks) and coastal plain (chiefly a region partly dominated by sedimentary fills of Cretaceous and early Tertiary ages and ancient beach alluviam moulded into dunes, interdune plains, and sand-sheets). Most of the coastal plain forms a cardinal part of the coastal zone. Coastal zone (CZ) Due to the huge significance of the land edging the shore due to the inbuilt advantages as well as the risks, almost all littoral nations have mandated and embarked on a Coastal Zone Management (CZM) Program. Table-1 is a subdivision of the CZ. These days, after the devastating tsunami Dec.24, 2004, coastal zone has assumed huge importance in the minds of public and politicians alike due to the vulnerability of the land area along with its people and materials. The chief aim of such a program is the scientific management by suitable interventions in this zone for the welfare and benefit of all players (settlement and utilization sans large scale long or short term adverse responses). Table 4: Coastal zone

divisions, limits & attributes

Divisions Coastal land zone, CLZ

Limits Back shore to limit of influence

Shore zone, SZ

Backshore to foreshore

Shelf zone, ShZ

continental shelf area

Attributes Wide or narrow and rugged, rolling or plain Steep or gentle; micro or meso-tidal & narrowest of all Steep & narrow or gentle& wide or deltaic

The Coastal Regulation Zone Act and the rules framed under have been put in place by the GOI and littoral states (combined length of mainland shoreline = 5700 Km) are in the processes of implementing the same. Sediment budget Sediment budget of the shore zone is a very critical element in determining the erosional or depositional nature of the shoreline in question. Komar (1976) defines sediment budget as an embodiment of the principle of conservation of mass to the sediment import or export from the beach and consequences there of. In general, it was the river system that drained the adjacent land-area that supplied sediment to the shore zone. A careful checking would say that the sediment to the shore zone is largely of terrestrial derivation, but it partly comes from offshore, partly of biogenous origin and partly supplied locally by the wearing down of cliff forming rocks along the backshore or shoreline. A portion of sediment may locally be derived from organic reefs too.

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Due to commissioning of massive river valley projects harnessing river water for flood control, hydro-power generation, irrigation and of late recreation, large volumes of water are stored in huge reservoirs depriving the shore zoe from the optimal supply of river load and hence sediment budget of the shore zone. Fluvial systems are known to derive their load mostly from 10% of the basin and most of the river work takes place during the high water stages like floods. In TN major rivers like Cauvery, Vaigai (medium in Rao s classification, basin area=2000-20,000Km2) and Thamirabarani (basin area=<2000Km 2 ) have the river net transgressing into the monsoon climate zone and hence are largely perennial in nature, where as other smaller rivers are ephemeral. The water uptake from all rivers for irrigation and drinking water needs have severely curtailed the stage and annual discharge of water volume to the ocean, consequently affecting the wash load discharge to ocean. In addition, these day s large volumes of bed sediment from river channels and flood plains are steadily removed to meet the steadily rising needs of the construction industry locally as well as in the southern districts of adjacent state of Kerala. The flourishing mineral sand industry locally exploits the modern dune sediment in many place in the east as well as in the west coast. All these indicate a lowering of receipt side of sediment budget. The near shore are removed for building and maintaining ship channels and wharfs in harbors and are stored or disposed of in areas away or outside the shore zone affecting the sediment budget. Construction of structures like seawalls and groins, also have modified/curtailed the free or natural flow of sediment. Yet another intervention negatively affecting the budget is removal of granular beach sediment for use in mineral processing, construction industry and in landfills. In fact, procedures and algorithms of several sorts are now available to estimate the sediment budget of the shore zone. Shore processes The natural processes and anthropogenic interventions taking place in the shore zone are listed in Table. 2. Without doubt the waves are the most visible and occasionally devastating phenomenon among the shore processes. Others are storms and coastal flooding, tides and infrequent tsunamis. Many of the adhoc human interventions in the CZ especially in the SZ and ShZ had adverse responses by the coastal processes. Table.5 Shore processes .Natural Wave erosion & deposition Storm surges and backshore flooding Tides: flood and ebb flows Littoral currents: up coast or down coast Eustatic rise and Neotectonic rise/fall of SL Tsunamigenic flooding

Anthropogenic Defensive structures like jetties, groins, seawalls Service structures like wharves, pipelines Modification of guts & throats of inlets Ship channels & dredge spoils Consumptive use of beach sediment like manufacturing & landfill Dumping of un/treated domestic waste and industrial effluents Sea level rise due to global warming

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Wave characteristics Besides geology of coastal land, water waves are chiefly responsible for nature of shoreline, beach erosion and accretion and long shore transport, and wave characteristics are taken into account in design and building of harbours, jetties, waterways, shore protection structures, coastal structures and other coastal works. Waves are an important source of energy for making beaches and associated placer deposits, transporting bottom sediment on shore, offshore or alongshore and inflicting tremendous quanta of energy on the coastal structures. When waves created by wind move out of the area of their creation or sea, waves with a regularity of crests and troughs appear with a more rhythmic rise and fall. Wave theories developed by Airy (1845) and Stokes (1880) describe simple waves and predict wave behaviour better where water depth relative to wave length is not too small. For very shallow water near the breaker zone solitary wave theory predicts certain features of wave behaviour. Water particles move in circular paths in deep water and elliptical paths in shallow water. Total energy of a wave system is sum of its potential and kinetic energies. While wave crest moves at angle to the bottom contours, wave in shallow water will have lower velocity than the portion in deeper water and crest will bend to align with the contours causing wave refraction. Wave erosion Coastal erosion is the erosion and removal of detritus of land or beach or even coastal dunes by wave action, tidal currents, wave generated currents or drainage. Waves are generated by storms (i.e., by atmospheric pressure differentials) or fast moving motor craft, cause erosion of the shore sediment or rock and redistribute the sediment to another site resulting in accretion. The study of erosion and sediment redistribution is called 'coastal morphodynamics'. On rocky coasts, wave erosion creates dramatic looking rock surfaces if rocks have layers or fracture zones, as a result of differential erosion typified by features such as tunnels, bridges, columns, and pillars. Table 6. Causes of coastal erosion (Shore Protection Manual, 1984) Natural Sea level rise Variability of sediment supply to littoral zone Storm waves Wave and Surge overwash Deflation Longshore sediment transport

Anthropogenic or Man made Land subsidence due to removal of subsurface resources Interruption of material in transport Reduction of sediment supply to littoral zone Concentration of wave energy on beaches Increased water level variations Removal of material from beaches.

Four main types of wave action are recognized, which is indicated by the term "HACC". Hydraulic action results when air trapped in cracks on the cliff forming rocks get compressed by waves breaking on the cliff face, as a result of which tremendous pressure is put on the rock body, forcing out pieces of rock. Over a time, the cliff face

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may develop larger cracks and/or even cave like features. The blocks and pieces of rock reaching the sea bed actively involve in the processes of attrition and corrosion. Attrition is grinding process of rocks, due to wave action, leading to smoother and rounded surfaces and reduced sizes. The broken off pieces themselves under go the process of attrition. Corrasion or abrasion is the gradual process of erosion of the cliff forming rocks when waves break on the cliff face. Though corrosion or solution by seawater is a very slow process, yet it attacks the constituent rock minerals leading to their chemical weathering and consequent destruction of the rock in the long run and especially so in respect of limestone cliffs. The ability of waves to erode cliff forming rocks depends on factors like, strength of rocks at base of the cliff, rate of removal of debris, presence of or lack of a land-fast beach, stability of foreshore, size of long shore stream of sediment and near-shore bathymetry. Tides Tides and tidal currents do play a role in transport and deposition or erosion and removal of sediment in conjunction with waves and wave generated currents. Indian shoreline falls in the various categories of tidal regimes proposed by Davis, viz., low micro- (<1.0 m), micro- (1.0 to 2.0m) and meso- (2.0-4.0 m), macro- (4.0-6.0m), mega (>6.0m) tidal regimes. Both diurnal tides and spring and neap tides do play a definitive role. Storm surges Indian shoreline especially the Bay of Bengal shore is well known for mega cyclones and associate storm surges. The Bengladesh is a country nearly battered frequently by storm surges resulting in devastation of a few hundred thousand people and property. Orissa , Andhra and Tamil Nadu coast are open to storm surges but at a very low frequency. Coastal flooding death and destruction are accompanied by large storm surges especially due to relatively high population density. In spite of various warning mechanisms, storm shelters and other measures in the event of a storm surge loss runs into a few hundred thousand rupees. In spite of being a modern, well connected, scientific society, Katrina had inflicted a great deal of woe to several thousand US citizens. Summary Geology, geomorphology, bathymetry and wave climate of the shore zone and shelf zone really matters in respect of shore sediment dynamics, which decides the nature of onshore, offshore and long shore sediment transport. Sediment budget is a crucial factor in deciding the nature of equilibrium of beaches. Large reserves of placer minerals are associate with some beaches like in Kerala, amil Nadu and Orissa. Mining of such placers have now become a moot question especially in the context of stability of the beaches and hence that of the back shore properties. Though engineering interventions are available to stabilize and protect the beaches, these are expensive and need careful design and implementation. Protecting the resources of backshore from tsunami like events are quiet elaborate and require large outlays of funds to undertake the scientific studies and data collection to design and erect suitable monitoring platforms. ---------------

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