A GEOMORPHIC STUDY OF BEACH AND CREEK EROSION AT DEVBAG, COASTAL

ABSTRACT

As a result of natural shore processes, the boundary between land and water undergoes, a shift in position with time. This shift may be seaward with advancing shorelines (accretion) and landward with receding shorelines (erosion). These processes occur along every shoreline. The rate at which shores erode or accrete depends on the composition of the shorezone and it's exposure to erosive forces. Forces of nature acting along the shoreline and implications of structures created by man can interfere with the continuing shore processes. Wind and its predominant direction is the main cause for resultant wave direction (usually oblique to the coastline) causing shore erosion. Waves and their longshore component of wave energy, water levels including tides, storm surges and sea levels, fluctuation in river discharge of sediments are the main 'Natural Causes of Erosion'. Exposed cliff faces, ground and surface water movements, ice action, ground motion, animal activities also contribute to the coastal erosion. Interruption of littoral drift pattern (i.e. the movement of sand by waves and currents along shores), deflection of shore current patterns, removal of sediments by dragging, modification of wave regimes through reflection and diffraction around structures are the interventions of man which result in shoreline erosion. Erosion and accretion alternate both in space and time. There are two types of coastal erosion. A fast, sudden erosion of the dune front during storm surges, causing a considerable loss of sand to deeper water and slow, chronic erosion, which is not so striking, caused by sea level rise. Due to chronic erosion sand disappears from the coastal defence zone. An increase In sea leveljiis^^ay cause an Increasing chronic erosion. In that case the coastal profile adopts to the new waterlevel by moving in a landward direction.

Beach Erosion in

43% of the Indian Coastline is comprised of sandy beaches. About 11% coast is a rocky cliffed coast and about 46% consists of mud flats, swamps and marshes. The shoreline oscillates seasonally between monsoon and fair weather season and most of the beaches regain their ohginal profile by March and April. The observation however shows that 80% of the beaches do not regain their original shape in the yearly cycle and they undergo net erosion. The preliminary studies, in the coastal areas of south Gujarat confirm the beach erosion and the movement of coastline by at least 15 m towards land in last few decades. Numerous factors including change in wave character, height, length and direction, tidal variations and pecularities, storm frequency, offshore current variations, the total sediment influx variations and dynamic nature of bottom configuration affect the beach stability over a period of time. The process of coastal change is inevitable and the results of beach erosion are more expressive than those of accretion. The major role of the different factors can be analysed and efforts made to mitigate the natural and manmade causes of beach erosion which result in colossal economic losses every year. Beach Erosion along Maharashtra Coast

There are indications that coastal erosion has accelerated in recent years along coast of Maharashtra. Coastal erosion is partly a natural phenomenon and partly man made. The situation is worst at river mouths and beaches adjacent to shore parallel rivers along Coast. Behind the narrow coastal dunes are the littoral terraces which are Inhabited all along the coast from Dahanu to . Erosion of narrow dunes is evident at many places in the last decade or so, Uncheli, Dandi, Navapur, Utambare, Mirya (Ratnagiri), Revas, Navakhar, Devbag, Navbag (Vengurla), Kalval (), Tambeldeg (Deogad), are the reported sites of erosion along Konkan Coast.

Study Area

Devbag (15°57' to 16°1' N latitude and 73°29' to73°31'E longitude) is a 6 km. long sand spit about 16 km. South of Malvan on Maharashtra Coast. The spit bar is connected to main land near . Southern end of the bar abruptly ends in sea near Mobarwadi. The eastern edge of the spit is bordered by tidal stretch of river Karli. The western margin faces Arabian Sea. The tidal range along the bar is 2.2. m. at spring and 1.8m. at neap. The lowest tide level is only 0.6 m. The dominant wave height measured in field is 1.6 m. Maximum surge level is 4.6 m. Storm surges produced due to low atmospheric pressure and coincident with onshore south-west monsoon are frequent in the area. Episodic inundation due to storm surges is being experienced since 1990 in every monsoon. Though the sea is eroding the spit bar from the shore side, the problem is also from the creek side. A massive sedimentation in last few years has decreased the depth of river, resulting in the formation of sand bars, sand islands and numerous sand lenses. It has also produced strong water currents along the creek side which has resulted in collapse of creek banks.

The Problem of Beach and Creek erosion at Devbag

The area is showing a tendency of severe breaching and erosion of shorefront continuously since 1952. The earlier incidence of beach erosion in 1952 is recurrent in 1956, 1980, 1997, 1999 and

2000. In June 1995 entire spit bar from Tarkarli to Devbag i.e. from North to South was hit by monsoon waves and the platform built for drying fish was also washed away. About 6000 houses in the nearby vicinity were affected (local information). The coconut gardens and casurina plantations could not survive. In 1998 once again there was new phase of erosion. In addition to the sea side of the spit the creek side also breached mainly due to progressing tidal water and flood discharge through the creek. Cutting of creek banks on Devbag side and transformation of land to salty wasteland was a striking consequence in 1995 and 1998. According to locals, about 40 to 50 percent coconut trees were uprooted in the central part of the spit bar, especially around Vithoba Temple. Problem of breaching from creek side and erosion from seaside still continues. In fact it appears that it has become more grave since 2004 and has now become an annual phenomenon. Objectives and Methodology

Any attempt to handle the coastal problems either to arrest erosion or prevent deposition requires a thorough understanding of the factors and processes involved in the coastal geomorphological system. Here the area of research is a sand spit or sandbar (beach) at Devbag that is showing persistant tendency of erosion and breaching. The unsual severity of erosion and breaching at Devbag needs an in detail study of beach morphodynamics and creek hydrodynamics. An attempt is made here to study the spatio-temporal changes in the beach morphology, as well as beach sediments alongwith the changes that have occured in the creek from August 1993 to November 2004, which is a field study period for this work. It was necessary to consult many records pertaining to the problem existing at Devbag since 1952. The first news of beach erosion appeared in the Times of India on 26th July 1980. The other data sources used include a formline map of the study area published by Sun/ey of India S.0.1. toposheet on 1:50000 scale (Index N0.47H/8 and 48E/5 & 9), and cadastral map of village Devbag, obtained from land records office, housed at Oros, Sindhudurg. IRS 1C LISS III image acquired on 22nd September 1997 as well as Google and Wikimapia images were also used as data sources. The Naval Hydrographic chart of the Karli Creek area was procured from harbour and port division at Khar, Mumbai. The reports published and made available by CW & PRS at Khadakwasala, were also used to gather information regarding the wave and longshore data and also details of defence measures, especially the construction of anti-erosion walls undertaken at Devbag. The major part of the work comprised of field visits, field measurements, surveying and field mapping. The area was visited regularly from 1993 to 2004 either in monsoon, pre-monsoon or post- monsoon. The water and sediment samples were collected as representatives of three different seasons. Analysis of samples collected, mapping of the results obtained, preparation of 3D Model on the basis of hydrographic data was the laboratory component of present work. In all 8 beach profiles were measured from north to south in every field visit. Since the beach erosion tendency at Devbag is restricted mainly to upper areas of beach, all the elevation levels were measured from the dune to low water level. The horizontal control in terms of geographic location was used as temporary bench mark at fixed places on dune line. These fixed points were referred as back sight levels for all levelling surveys. The relative heights were obtained along survey lines from these bench marks to the low tide level at the time of observation. These were then plotted to get east-west profiles running across the beach. The contour maps showing beach levels were prepared in the 'surfer' environment. The result of mechanical analysis were used to obtain various properties, like mean, mode and median. The parameters of sand distribution, such as skewness and kurtosis were also obtained. The water samples were analysed to know the quantity of suspended sediment in the waves. The seasonal variations in the surface area as well as the sand volume between various beach levels were computed and futuristic trends obtained by applying the technique of harmonic analysis. Spit bar at Devbag is experiencing chronic erosion since 1952. Due to chronic erosion sand disappears from the coastal defence zone. In addition to the seaside of the spit the creekside also breached mainly due to progressing tidal water and flood discharge through the creek. A tendency of breaching and erosion of shorefront has intensified considerably since 1995 and which has become annual phenomenon since 2:004. An attempt is made to describe the cause and effect relationship between these factors of coastal erosion as well as creek ^sedim^tatlori^ It is a geomorphic study mainly based on field observations and field measurements. The work is presented in five chapters in addition to the introduction. Study area and method of research employed is discussed at length in chapter one. Chapter two discusses all the morphological aspects of the Devbag spit bar. The beach sediments and their characteristics are described in chapter Three. In chapter four morphology of Karli Creek is described in detail. Major observations, and trends are listed and discussed in the last chapter.