Long-Time Shoreline Monitoring of the Vellar Estuarine Complex, Southeast Coast of India: Using Multispectral Satellite Data

INTRODUCTION

India has a long coastline of 8 054 km endowed with a variety of coastal ecosystems such as man-groves, coral reefs, sea grasses, lagoons, salt marshes, and estuaries and now disturbed and threatened due to the rapid increase of population and developmental activities along the coast. Most of the human settle-ments in the country taking place along the coast are in and around the estuaries, deltas, and shorelines, and the shoreline change is an issue of great concern in coastal zone management.

Shoreline, the boundary between land and sea, keeps changing its shape and position continuously due to dynamic environmental conditions, as the shore is associated with waves, tides, winds, storms, sea-level change, and the geomorphic process of erosion, accre-tion, and human activities (Kumar and Jayappa, 2009). In the case of rivers and estuaries, the change in shore-line feature also depends on long-term changes in water flow and development of sandbars in the mouth during nonrainy seasons. Besides, manmade activities, unex-pected floods, tsunamis, cyclones, and earthquakes also play a crucial role in determining the profile of the coastal areas. Shoreline also depicts the recent geologic formations and destructions that have happened in the coastal areas.

Satellite remote sensing combined with GIS tech-nologies proved to be one of the cost-effective tools for the continuous monitoring of the shoreline dynamics (Nobi et al., 2010; Kuleli, 2009; Dinesh Kumar et al., 2007). Remote sensing provides with a synoptic view of the terrestrial landscape and is used for inventorying, monitoring, and change detection analysis of environ-mental and natural resources (Seeber et al., 2010; Narumalani et al., 1997). Despite several such studies on various coasts and estuarine mouths of the country, limited studies (Murthy and Pari, 2009; Singarasubramanian et al., 2009; Pari et al., 2008; Rajamanickam et al., 2006; Cho et al., 2004) have been carried out along the Vellar estuary on coastal land-use, geomorphology, and sedimentology of the region and all these studies analyzed the shoreline for a shorter duration. However, due to change in water flow, monsoonal pattern, catchments, and developmental activities, there are considerable changes seen along the estuary. Consider-ing this, the present study was carried out to track the path of Vellar estuary over a period of 38 years (1970–2008) and also to detect the impact of tsunami on the mouth of the Vellar estuary.

MATERIALS AND METHODS

Study Area

The Vellar River originates from the Shervaroy Hills in Salem District (Tamil Nadu, India) and opens into the Bay of Bengal at Parangipettai (11°29′N and 79°46′E) after forming estuary. The Vellar estuary is connected to the Coleroon estuary in the south as it joins the sea near here. The inlet of Vellar is exposed to high energetic waves, inducing large sediment transport rates and shoreline changes. Tides in Vellar estuary are semidiurnal in nature and their influence extends up to a distance of 18 km upstream from the mouth. The average width of the estuary is 200 m, and at the mouth, it varies between 100 and 600 m de-pending on the season (Pari et al., 2008).

Satellite Data

Considering the availability of cloud-free image-ries and the cost and nature of the work, in the present study, multispectral data of Landsat TM (1991) and IRS-P6 LISS III (2004, 2006, and 2008) were used to understand the shoreline changes in the Vellar estuarine complex (Table 1). The Survey of India toposheets (No. 58 M/11) of 1970 was used as the base map for com-parison.

Table  1.  Characteristics of the satellite data used for the study

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Data Analysis

Visual interpretation technique was employed for the delineation of the shorelines of the different years. Satellite data are geocoded using Survey of India to-posheet and ground control points were collected dur-ing the fieldwork using GPS (Oregon 300) of the study area and using Erdas Imagine 9.2 software. Based on image characteristics, tone, texture, pattern, and asso-ciation, the shorelines were mapped. By compiling data from field studies, Survey of India maps, and imagery, the final shoreline maps of the Vellar estuarine complex were produced.

RESULTS

The study has clearly brought out the shoreline changes that were taking place in the Vellar estuary and adjoining coast over a period of 38 years (from 1970 to 2008; Figs. 1a–1e). For the easy description of the changes that occurred on the shoreline, the results have been mainly described with respect to five prominent locations of the study area viz. Maduvangarai (southern side of the Vellar estuary near B. Mutlur), Centre of Advanced Study in Marine Biology (CASMB), Mu-dasalodai, MGR Thittu, and Mulukuthurai by taking the Survey of India toposheets as the base. During 1970, the estuary was found to open with the sea in the same line as it flows, with only very less sand accumulation in the mouth (Fig. 1a), whereas, in 1991, the estuarine mouth started to shift toward the northern side and a large sandbar was formed along the estuarine mouth. The sandbar increased from its initial position during 1970 toward north and reached up to Annankoil (Fig. 1b). Larger geomorphologic variations were also seen in the small islets in the estuarine complex between-Mudasalodai and Mulukuthurai. Land accretion was observed in the estuarine areas near MGR Thittu. Is-lets (1–3) were found growing between 1970 and 1991; due to the growing nature of these islets, these three islets became two, along with a clear change in the river flow path in the Chinnavaikal region by shifting it flow path toward the western side. More importantly, the width of the Chinnavaikal has become drastically reduced between 1970 and 1991.

Figure  1.  Shoreline feature of Vellar estuary and adjoining coast at various periods: (a) 1970, (b) 1991, (C) 2004, (d) 2006, and (e) 2008.

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The sandbar formed due to the deposition of sediments by the action of waves and tides remained the same as observed in 1991 with minor accretion in the Annankoil area during 2004 and erosion occurred in the estuarine side of MGR Thittu (Fig. 1c). Sharp changes in the shoreline were seen in the upper region of the Vellar estuary path near Maduvangarai. Clear changes in estuarine direction were visualized over different periods (as seen in Figs. 2a–2d). Estuarine path deviated toward the northern side, whereas, at the Marine Biological Station, the estuary direction had shifted toward the southern side, which is seen in the shoreline maps of 1970 and 2004. It was also impor-tant to note the formation of new islets (Islets 4 and 5) in the estuary mouth (Fig. 1d).

Comparing the shoreline maps of 1970 and 2004 in the GIS platform clearly showed that, at CASMB and Mudasalodai, the shoreline had been shifted to-ward the southern side with an accretion of 127.4 m at CASMB and with 499.7 m toward the estuarine mouth at Mudasalodai, whereas erosion observed in the re-maining areas (Table 2). It was also evident that the islets of the Chinnavaikal increased in their size.

Table  2.  Shoreline changes recorded at different periods

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Although not much variation in the estuarine path was observed during pre-tsunami (2004) and posttsunami (2006) periods, the morphology of the estua-rine mouth was altered drastically. The sandbars formed over a period of 34 years were completely al-tered and only small pockets of sand deposits remained. Sandbars were disrupted and the estuarine mouth re-mained open as per the original orientation of the estua-rine mouth noticed in 1970 with few small islets.

However, the sandbar started growing further to-ward the north in the subsequent years, revealing clearly the influence of the tides in the formation of sandbars in the Vellar estuarine mouth. When the shoreline maps of 1970 and 2008 were compared to understand the current status of the shoreline, it was noted that the sandbar extended toward north, although it was disturbed during the tsunami (26 December, 2004). When the 1970 shoreline was compared with that of 2008, the land area near Mudasalodai had grown toward the seaward side and the Chinnavaikal creek, which passes through Mudasalodai and Mulukuthurai, became very narrow (Fig. 2a). The northern bank of the Vellar estuary near CASMB was shifted toward the southern side with an accretion of 43.3 m and similar such trend was also observed in Mudasalodai with a large accretion of 547.4 m, whereas erosion was seen in Mulukuthurai with an approximately 379.3 m land area, given way for larger creek formation. An increase of 945.9 m accretion was observed along the seaward side of MGR Thittu, whereas 271.3 m shore was found eroded in the estuarine side of the MGR Thittu, as re-vealed by the 1970 and 2004 shoreline maps (Fig. 2b).

Figure  2.  Change in shoreline recorded at different periods. (a) 1970 and 2008; (b) 1970 and 2004; (c) 2004 and 2006; and (d) 1970–2008.

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The shoreline maps of 1970, 1991, 2004, 2006, and 2008 were overlaid and are shown in Fig. 2d. This map clearly revealed the changes that occurred along the estuarine path over the period of 38 years from 1970 to 2008. When viewing the overlaid shoreline map, severe erosion was noticed in the seaward side of the MGR Thittu and accretion, along the estuarine mouth.

DISCUSSION

Surveying the morphology of river channels, es-tuaries, and coastal shorelines and the level of physi-cal habitat degradation due to human activity has tra-ditionally been monitored and helped to understand the changes in the natural habitats of the region. Re-motely sensed digital imagery provides with a potential means of gaining synoptic coverage of estuarine and river systems and monitoring change, which is comparatively cheaper on a long-term basis (Rainey et al., 2003; Leuven et al., 2002; Muller et al., 1993) as field surveys are local in extent, time-consuming, and relatively costly. Remote sensing images of 20–30 m resolution are good enough in mapping the shorelines of estuaries and rivers of 200–600 m width (Gilvear et al., 2004) as that of the Vellar estuary. For small- and medium-sized rivers (approximately 20–200 m wide), airborne remote sensing, incorporating high-resolution advanced sensors and improved temporal/spatial flexibility, may be a more suitable approach for map-ping and monitoring the changes (Milton et al., 1995).

It has been observed from the present study that both erosion and accretion had occurred along the Vellar estuary and adjacent coastline toward the sea-ward side and clear river path shift had also taken place along the river course. Significant changes in the shoreline had been observed in a time lag of 38 years, from 1970 to 2008. When shoreline profiles were ob-served using different years' satellite data and Survey of India toposheets, depositional and erosional areas have been revealed in the Vellar mouth, adjacent coastline near MGR Thittu. Rapid changes of ero-sional and accretional features, narrowing of Chin-navaikal creek, and expansion of islets in this creek have drastically altered the river path. Sedimentation and reduction in the water flow could be the major reasons for this change. As a salt wedge estuary (Levinson, 2010), Vellar is characterized by large river discharge and weak tidal forcing, which triggers higher sedimentation process during monsoon period. In addition, depth of the creek was reduced, which has become a major problem to the fish trawler operators of this region. Such sedimentation from the river dis-charges and decrease in estuarine depth due to north-erly littoral currents during monsoon and postmon-soon were also reported from Krishna estuary of this coast (Kumari and Rao, 2009).

The present study found that there is a shift in the river path toward the north in the Maduvangarai re-gion between 1970 and 2008. This lends support to the findings of Rajamanickam et al. (2006) and this shift of the courses was correlated to the presence of pa-leochannels on either side of the Vellar and Coleroon river complex and change in estuarine bathymetry also causes significant variation in estuarine morphology (Prandle, 2006). This could be due to the increase/ decrease in water flow during different seasons due to excessive rain and flood and reduced flow of fresh water in the summer, which was a characteristic fea-ture of coastal belt of the Vellar. The accretion re-ported presently just opposite to CASMB (43.3 m) over the study period might be due to siltation pro-moted by the mangrove development along the north-ern bank of the Vellar estuary. Due to this, the north-ern bank was extended toward the southern side.

In 1970, sandbar was found only for a shorter length along the southern bank tip of the Vellar estuary, whereas it grew to a larger extent over a period of 38 years and the estuarine mouth had shifted toward the northern side of the estuary. The formation of sandbar could be due to the Aeolian and marine processes that took place in the coastal areas. Tide and littoral drift might have caused the deposition of large quantities of sand in the estuarine mouth and erosion can happen in the estuaries due to strong tidal currents and low up-land fresh water discharge (Ramanadham and Varada-rajulu, 1973). In 1991, a larger and long sandbar was found in the mouth than that of the present one. This suggests that the river flow during 1991 could have been less and it could have made the marine processes (wave, current, and tide) stronger than that of the river runoff. It is worth mentioning that significant estua-rine siltation is most likely to occur via the entrain-ment of fine marine sediments (Prandle, 2004).

Although the sandbar was seen with only little morphologic modification up to 2004, the Indian Ocean tsunami that struck this coast altered the sand-bar of the Vellar estuarine mouth. It is clearly visible from the satellite imagery of 2006 with significant changes in the estuarine mouth. The velocity of the tsunami wave was reduced by the dunes present in the Protonovo (Parangipettai) coast and the wave altered the geomorphologic features of the coast (Singarasubramanian et al., 2009; Paul, 2006). In addition, spit formation was observed in the Vellar river mouth, caused due to the siltation by the northerly littoral drift, resulting in the complete closure of the Vellar estua-rine mouth (Murthy and Pari, 2009; Pari et al., 2008).

Mapping of the morphologic changes after epi-sodic events such as tsunami or storm surge is vital in assessing their impact on coastal morphology. Such information will be useful in evolving strategies for responding to such events in the future (Pari et al., 2008). Not much variation in the estuarine shores has been observed due to tsunami, as the force of tsunami waves has dissipated by transporting the large volume of water up to longer distances of the river and also by the mangroves grown along the Vellar estuarine banks (Kathiresan and Rajendran, 2005). In 2008, the estua-rine mouth has regained the sandbars, indicating the lesser activity of the river flow and higher activity of the marine processes.

Continuous changes in the shoreline observed during the present study in the Vellar estuary can be ascribed to many reasons mainly because of natural phenomenon. Changes in the estuarine dynamics can be induced by many factors including strong tidal os-cillations, water circulation, tidal flow, estuarine to-pography, mass input due to freshwater discharge, and meteorologic forcing (Pylee et al., 1990). The Bay of Bengal experienced two directional long shore sedi-ment transports influenced by both southwest and northeast monsoons and transporting large volume of sediment from north to south. This sediment transport largely influenced the geomorphology of the east coast of India by triggering erosion and accretion at differ-ent locations. Such geomorphologic changes are very prominent along the estuarine mouths like Vellar. Po-tential littoral sediment transport along the southeast-ern coast of India was recorded and the net erosive nature of the study area was due to the prevalence of waves and wave-driven currents cause long shore drift of sand along the beach and offshore from south and southeast directions (Saravanan and Chandrasekar, 2010). Construction of dams and bridges in the river can also alter the flow of water and hinder the path of the river that can cause changes in the direction of water flow and promote erosion (Pilkey and Theiler, 2002; Frihy, 2001). Construction of bridges, overall reduction in water flow over the years, and annual flooding events are playing a major role in change in estuarine path of Vellar.

Accretion of the sand in the river line can be caused due to land runoff, whereas, in the coastal ar-eas toward the sea, littoral drift causes the deposition of the sand. Accurate demarcation and monitoring of shoreline (long-term, seasonal, and short-term changes) are necessary for understanding the coastal process (Murali et al., 2009). In this direction, the present study has paved the way to detect the long-term changes that occurred to the Vellar estuarine path, mouth, and adjoining coastline.

CONCLUSION

The present study found a clear change in estua-rine path and the adjoining coast of the Vellar estuary between 1970 and 2008. Indian Ocean tsunami also brought out apparent changes in the estuarine mouth. However, the changes were only temporary and changed to its near original shape recorded before tsunami (2004). The data generated by the present study would help the administrators of the region to take adequate management plans for future develop-mental activities.

ACKNOWLEDGMENTS

We thank Dr. T Balasubramanian, Director and Dean, Centre of Advance Study in Marine Biology, and the authorities of Annamalai University for the support and encouragement and for providing the fa-cility to carry out this work. We also thank the Uni-versity of Maryland Global Land Cover Facility (http://ftp.glcf.umd.edu/index.shtml) for the Landsat satellite data.