588 JOUR.GEOL.SOC.INDIA, VOL.92, NOV. 2018 JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.92, November 2018, pp.588-595 Coastal Morphology and Long-term Shoreline Changes along the Southwest Coast of India L. Sheela Nair*, R. Prasad, M. K. Rafeeque and T. N. Prakash National Centre for Earth Science Studies, Akkulam, Thiruvananthapuram - 695 011, India *E-mail: sheela.lnair@nic.in ABSTRACT The part of southwest coast of India extending from Poovar in the south to Kasaragod in the north is considered as one of the highly dynamic coastal areas of Indian peninsula. Over the years due to rapid urbanization as well as other natural and anthropogenic activities, the coast is under severe pressure which in turn has reduced the percentage status of healthy / stable coast. Unscientific shoreline protection methods adopted without conducting appropriate studies to assess the suitability of the said method to a particular coastal stretch has often led to negative impacts. As a result, many areas that were once stable have turned eroding and in certain cases, the observed extent of erosion is severe warranting immediate protection measures. In this context, a study was carried out to assess the long-term shoreline changes along the southwest coast and to decipher the causative factors responsible for these changes. Accordingly, a 46 year period from 1968 to 2014 was studied using multi-dated shoreline images and Survey of India (SOI) topographic charts. The DSAS software (USGS) is used to compute the rate of shoreline changes along different sectors of the coast and accordingly the entire coastal stretch is classified into 7 classes depicting the present status (stable / dynamically stable / unstable) of the coast. The analysis revealed that almost 60 % of the coastline is eroding with about 29 % showing an accreting trend. INTRODUCTION The part of southwest coast of India which extends from Poovar in the south to Kasaragod in the north is one of the highly dynamic coastal stretch of the Indian peninsula. It covers about 90 % of the Malabar Coast (historical) which extends from Goa to Kanyakumari. It comprises of a narrow strip of coastline which forms part of the coastal plain between the Western Ghats on the eastern side and Arabian Sea towards the west. The variations in coastal geomorphology settings of the coast make it different from other coastal areas of the Indian peninsula which is quite evident from the observed spatial and temporal changes in wave climate and related coastal processes (Sajeev, 1993). But over the years, coastal development activities related to tourism, recreation, industries and spurt of other activities linked to globalization along this part of the coast have led to the deterioration of the coastal environment with erosion being one of the major issues (Mallik, 1987). As a result many of the world famous beaches along the southwest coast are on the verge of extinction and this in turn has affected the marine ecosystem which includes the flora and fauna, as they are very much dependent on the coastal environment. In this context an attempt has been made to study the long-term shoreline changes along the southwest coast of India. STUDY AREA The study focuses on the 590 km long coastal stretch which extends from Kasaragod in the north to Poovar in the south. It covers the entire coastal stretch of State of Kerala which lies sandwiched between the Lakshadweep Sea on the west and the southern part of the Western Coastal Plains which skirts the Indian subcontinent with the Western Ghats on the eastern side (Fig. 1). The latitude / longitude co-ordinates for the northern and southern boundaries are 12°45' N, 74°51' E and 08°17' N, 77°05' E respectively. The coastal stretch has varying morphological features as we move from south to north. Generally, the coastal landforms of the Kerala are composed of sandy beaches, cliffs, rocky headlands, spits, estuaries, lagoons, barrier beaches, etc., depending on the geology of the location. COASTAL GEOMORPHOLOGY The average distance of the coast from the Western Ghats is around 100 km and the coastline is generally oriented in the NNW to SSE direction. The 590 km long Kerala coast is intercepted by 41 rivers which originate from the Western Ghats and flow towards the west. The rivers eventually debouch into the Arabian Sea through inlets and these inlets are connected to the sea via estuaries/lagoons. There are 48 inlets along the Kerala coast out of which 20 are permanent, whereas the remaining 28 are seasonal (remain open only during the monsoon period of June – September). The seasonal inlets mostly remain closed during the fair season and this can be attributed to spit formation at the inlets due to longshore sediment transport induced deposition. The seasonal inlets are normally cut open during monsoon to discharge the heavy surface runoff from the rivers into the sea to reduce the flooding risk within inland and coastal areas. It is observed that along the southwest coastal stretch, the river mouths with untrained inlets exhibit a seasonal as well as spatial shift in their position depending on the magnitude and direction of the longshore sediment transport. In addition, the overall sediment discharge from these rivers through estuaries / lagoons to the inlets / outlets is also a deciding factor in the observed spatial shift of the inlet locations. Over the years, the contribution due to this component has become insignificant at several locations as there is hardly any discharge of sediments taking place mainly due to damming of rivers, river sand mining etc. (Padmalal and Maya, 2014). But the monsoon period with heavy rainfall during which the surface flow and currents are maximum is an exception. The drastic reduction in the supply of sediments through rivers and other sources onto the beach has badly affected the stability of the south-west coast in recent years (Black et al., 2008). Of the various factors that affect the sediment supply it is observed that anthropogenic activities play a major role (Sheela Nair et al., 2007; Kuntae and Wagle, 2001). Geology and Coastal Landforms The coastal plains are generally low lying areas with elevation < 8 m above the MSL (Fig. 2). It has been reported that Kerala coast comprising of beaches, alluvium, barrier flats, shell deposits were formed during late Quaternary period (Soman, 1997). Unlike the east coast of India, this part of the west coast is devoid of deltas which DOI: 10.1007/s12594-018-1072-x | 0016-7622/2018-92-5-588/$ 1.00 © GEOL. SOC. INDIA