Inlets are important economically too many littoral states because harbours are often located in the back bays, requiring that the inlets are maintained for commercial navigation. At many inlets, call for the great level of maintenance cost due to continuous periodic incurred by repetitive dredging of the navigation channels. Because inlets are hydro dynamically very complex, predictions of shoaling and sedimentation have often been unsatisfactory. A better understanding of inlet sedimentation patterns and their relationship to tidal and other hydraulic processes can hopefully contribute to better management and engineering design. Beaches and sediment movement along the shore have been subjects of popular and scientific interest for over a century. Beaches are popular recreation areas and of vital economic importance to many states [1]. Beaches are critical buffer zones protecting wetlands and coastal plains from wave attack. Beaches are habitat or nesting ground for many animal species, some of which are endangered. Tidal inlets are analogous to river mouths in that sediment transport and deposition patterns in both. One  feature  of coastal  areas  that is  often studied  and analyzed  are the  profiles  of  beaches,  cliffs  and coastal  landforms. These  cross sections through  coastlines  can give a  good idea as the changes that can  occur  over time at  one point  on the coast, either in the shape  of a  beach  or cliff, or on its  size  and volume. Careful  examination  of  the sediment  transport  pattern is  very  essential  to  maintain   the balance and the stability  of the coastline.

India  has a  long  coastline  of over 6000 Kms  along  the  mainland  in addition to that  of  the  Andaman  and Nicobar   islands  in the Indian ocean and Lakhsadeep islands in the  Arabian  sea. The vast  coastal zone  has been  playing  a major role  in the economic  development  of the country  since  ancient  times  by its  contribution  to sea trade,  fishing, ocean  industry  and human settlement. But  coastal   ecosystem  is under  threat  due to  anthropogenic  activities, increased  human  settlement, industrial pollution, manmade  structures etc. The geomorphology of   beach is controlled mainly by the wave characteristics and the nearshore sediment transport. The waves while propagating towards the coast would steepen and break on the shore, expending their energy. The  hydrodynamic  process  in the surf zone  is quite complex  due to  wave breaking, presence  of   longshore  and  onshore – offshore  currents, littoral  sediment transport, erosion  or  accretion of  beach   sediments  etc. Responding   to the   various   forces acting in the surf zone, the coast readjusts itself and tends to be in a   dynamic equilibrium.  Longshore current plays a predominant role in transporting the sediment in the beach. For practical purposes, the average longshore current measured in the surf zone would be sufficient for estimating the sediment transport rate.

With  regards to the measurement  of beach profiles, Delgado  and Lloyd  [2]  describe  in their  paper  one of the  simplest  methods of  measuring  beach profiles. Cooper  et al  [3]  briefly cover the  history  of  beach  profile measurement   in their paper  on the  measurement, theory  and  analysis  of  beach profiles. Pickett et al [4] describes  the application  of the  model  in the  analysis  of  the  beach  profiles  of the  Bay of  Plenty, New  Zealand. Equilibrium   profiles were created for beaches in the bay and then compared to actual beach profiles. Romanczyk  et al  [5] discussed  the physical  limitations  of  the model, in that  it only reaches  as far  the  shoreline. Krause   and  Soares  [6]  described  a beach  profile  monitoring  programmed  for the  state  of  Para  in northern  Brazil ,  which was  the investigation  of coastal  morphodynamics  to be  undertaken  in an  area  of extensive  mangrove  forest.  Lacey  and  Peck [7]  documented  morphological   changes  that  occurred  on the  beaches  of   Rhode  Island  by  observed  both  short and long term  periodic  cycles  in  beach volume   and  suggested  explanations  for these  cycles, for example   that  the annual  cycle from  high  profile  volume  in summer  to low profile  volume  in winter  was due to the   seasonal  change  in the   frequency  and  intensity  of  storms.  Saravanan and Chandrasekar [8] have studies the monthly and seasonal variation in beach Profile along the coast of Tiruchendur and Kanyakumari, India. Larson et al [9] have studied the relationship between wave action and beach profiles. The  longshore  current    velocity  varies  across the surf  zone, attaining  the  maximum  value close to the  wave  breaking  point [10]. The  sediment transport  between  Kanyakumari  to Trivandrum  is  southerly  from May to  December  and  northerly  from  January  to  April [11].

Kanyakumari  is the smallest  district  situated  in  the  Southwest  of  Tamil  Nadu, South India   with a land  spread  of  1,684 sq. Km. It  has almost  all ecosystems  - forests, wetlands, freshwater  resources, marine  etc. The coastal  ecosystem   of this  District  comprises  68 Km in length  and is studded  with  44  coastal  fishing  villages. Since  this district  is situated at  the extreme  south  of  the  Indian  subcontinent, the coastline  is  formed  nearly  by the three seas, namely  the  Arabian Sea,  the   Indian  Ocean  and  the Bay of  Bengal. But the main part of the coast faces the Arabian Sea. The  coastal  landscape of  Kanyakumari  District  is mainly  composed  of  beach ridges  of  rocky, sandy, clay in  salt pan region  and  swampy  nature in the estuarine  regions. The  68 km long  coast  has a heavy  concentration  of fisher  folk , almost  one village  per  1.5 km. Coastline of Kumari coast is exposed   to relatively  higher waves  than the rest  of the  Indian  coast  due to its direct  exposure  to  the  Indian  ocean . The oceanography  of this region is  controlled by  three different  seasons  a) southwest  monsoon  ( June  to September) b)  northeast monsoon  (October to  December) c)  fair-weather period  (February   to  May) In the Indian Ocean mixed tides are mainly semi-diurnal type. The tidal range  along  the  west  coast  of   India  increases  from  6.5 m  to  7.6 m towards  south.

The study area Thengapatanam is a beautiful town located in latitude   80 14.24’ and longitude 77010.27’ on the shores of Arabian Sea in the southern part of Kanyakumari district in Tamilnadu (figure.1). The history of this beautiful coastal town dates back to more than 2000 years to the reign of Chera Kings.  It was, then an important port of Chera Nadu, when trade relations boomed with the Middle East and the Arab world.  There was direct cargo boat service between Thengapattinam and other foreign maritime towns around the world.  Part of the culture and tradition found here was inherited through its trade relations with the Arab world.  The king Cheraman Perumal had passed through this tiny town when he went to Mecca to embrace Islam after witnessing the moon splitting miracle.  It was part of the Travancore State until 1956 before it merged with Tamil Nadu. This Muslim town has a Juma Masjid known as Valiya Palli, which is more than 1200 years old. It is a coastal town which is 35 KM away from Nagercoil, headquarters of Kanyakumari District and 45 KM away from Trivandrum, capital city of Kerala.  It shares borders with Mullimoodu and Erayumman Thurai on the western side, Panankalmukku and Mullor Thurai on the east, Amsi on the north and the Arabian Sea on the south.  It is well connected with nearby villages and town by road and waterways.  It is a flat, plain land with intermittent rocky hills - Chentapalli Parai - on the eastern side and Aarttupalli Parai on the western side. The Kovalam - Colachel Canal, popularly known as AVM Canal (Ananda Victoria Martanda Varma Canal) which passes through this town linking upto Kanyakumari was encroached by settlers.  This waterway was operation from Mandaikadu to Poovar 30 years ago.  In some places it is filled with coconut trees and suffered encroachment.  AVM Canal merges with Kuzhithurai River at Thengapattanam and known as Valiyar.  AVM canal which connects the Thengapattinam estuary of Thamiraparani River, Kanyamuari District and Neyyar River Estuary at Poovar of Neyyatinkara, Kerala.  The AVM canal runs along the west coast. The canal well served the freshwater needs of the local people. The drainage channels from the paddyfields and AVM canal are the main sources for the flow of sewage and domestic wastewater into the estuary. AVM canal is used for retting activities round the year.  The Valyar, a tributary of Kuzhithurai River known as Tamiraparani flown and merges with Arabian Sea (Photo.1). Generally river Thamravarni is a rainfed river.

During South West and North East monsoons, it overflows and causes much damage to the lives and properties of those who live on the banks of the river. During the rest of the period the river is almost dry. The alluvial soil along the coastal strip is well adapted for the cultivation of paddy and coconuts.  “Thengapatanam”, ‘Thennaipattinam” and “Thenpattinam” are the other relevant names of this town,  it is called so because of the dense coconut groves found everywhere here (Photo.2), being located on the coast of Arabian sea, and sitting on the southern tip of India in Kanyakumari district in Tamilnadu. Thengapatanam estuary is one of the minor estuaries in Kanyakumari District, Tamilnadu. It is a bar-built estuary and sand bar is a permanent barrier (Photo.3), which prevents the entry of sea water during post monsoon and pre monsoon season (Photo.4). Near shore bathymetry is depth with   19 m from   200 m distance from the shore.  The terrain comprises largely of Precambrian crystalline rocks of charnockites, khondalites and migmatitic gneisses. According to Raju and Singh [12]. Thengapatanam hinderlands have radioactivity of 1.87 Gy/h and Thengapatanam beach sector 0.64 Gy/h. The main objective of this paper is to estimate  longshore  current  and  sediment  transport   from measured  wave  data  and  Littoral  Environmental  Observation  (LEO) data and to ascertain what  changes  have occurred  in Thengapattanam  beach profile shapes   on a temporal scale using profile  data  from  December  2006  to December 2008.

The beach profiling measurements were made out at all stations at every month from December 2006 to December 2008 at the lowest tide level. So that the maximum lengths of the beach cross section was exposed for the measurement. At times inner bars were accessible and the measurements were carried out. The stake and horizon method [13] was adopted to measure the sand level at Thengapatanam (Taingapatanam) with the fairly fixed reference point. The profile direction was kept perpendicular to the coast and all the measurements made at the station.  Monthly   changes  ( December  2006  to  December  2008)  for  25 months  in  beach  levels were  measured  at  the study area  at every 5m  interval  along  a transect  from  backshore  dune  to seaward  till  1m water depth during  the low tide. Surveyor’s dumpy level   and a graduated staff   were used for   measuring beach levels. Heights  of 10 consecutive breakers were  usually  measured  and the  average of  them were  noted as significant  wave height  at breaking. Total  time required  for  10 waves  to break  was noted  using a  stop watch  and the average  was considered  as breaking wave  period. Long shore current velocity and direction were measured by using current meter.  Based on the data collected, the long shore sediment transport rates were computed using the Walton’s  equation [14].

Where,  Q is the longshore  sediment  transport  rate  in m3/year,  ρ is density  of sea water  = 1025 kg/m3, g – acceleration  due to gravity = 9.81 m/sec2,  Cf  is the  friction  coefficient  = 0.01, Hb is  breaking wave height in metre, W  is  surf zone  width  in metre, V is the measured  longshore  current  velocity  in m/sec  and  V/Vo  is the  theoretical dimensionless  current velocity = 0.4  [15].

From  the   data  collected  during  these  surveys, the  monthly  and  seasonal  beach  profile  variations  are  graphically  represented  to show  the variability  in beach  profile  configuration. From  these  data, changes  in the  volume  of  sediments  are  calculated  using  a  computer  package  to  scrutinize  the  temporal  variation  in  fig.2.a,b.

 Figure 2a: Home Page of the Sand Volume Calculation Package.

Figure 2b: Options Available in the Sand Volume Calculation Package.

Beach Profile Studies       

Typical variations in monthly beach levels at Thengapatanam beach are shown in Table 1, 2 for the years 2007 and 2008 respectively. The drawings  of beach  level variations for   25 months  from December  2006  to December  2008 for  SW, NE monsoons and fair weather seasons  during  2007,2008 years   are given  in  figures.3 – 10. Beach  level  variations  at Kolachel  were presented  earlier  by  Jena and  Chandra Mohan [16]  as  beach level was low in  September  and high in  April. Kaliasundaram et al [17] reported that the erosion was taking place at Colachel at a rate of 1.2 m/year. Relative  changes  in the  volume of sediment  per metre  length of the  beach up to   1m water depth  at  Thengapatanam  were  estimated  and presented   in  table 3 .Volume  changes  over an annual cycle  were  estimated  as   882.38m3 for the year  2007  and  710.64 m3  for the  year 2008. The Volume changes of the study area for months are given in figure.11.

The beach level   is at the lowest in July and August for both years   and at the highest in April. Normally beach levels  are at  the  lowest  in the  southwest monsoon   period  for  west coast  beaches  also found to continue their erosion  processes during the northeast  monsoon  period also . But, the  present study  established  that   the  variation  of  beach   levels over an  annual cycle  showed that the  Thengapatanam beach is highly erosional. The beach profile is highly influenced by estuary connected by Tamiraparani river mouth. The  beach width ( m) , mean  slope  angle (dig) and  total  cross sectional  area (m2)  of  the  beach  for  various months are given in table.4. The maximum  beach width  per  meter of beach  was determined  as  59.88 m  during  July 2007  and  50.1413 m during  March 2008.The minimum beach width  obtained  as  24.2323 m in July  2007 and  26.3084  m in  June  2008. The mean slope of the beach Vs. months are given in figure.12. The slope variation has maximum during June and July months and minimum   during April. From the data available, Thengapatanam is the highly erosional   coast   even though the west coast is protected by RMS walls and Groins.

Breaker Characteristics

The  variation  of  breaking wave  height, period and  the width of  a surf zone   are  measured  and presented  as Littoral environmental observations in  table 4.The maximum  wave height  is measured  as 2.5 m in June  2007  and  1.8 m in July  2008. The wave height remained below 0.8 m in the fair weather    season.  Thengapatanam has plunging breaker type except November and December which have surging wave type. The variation of wave height for different months is given in figure.13. The  wave period  persisted  high  during  the southwest  monsoon  season  months  June  to  August  and  spilling  breaker occurred during    January  and February. The relation connecting   between period Vs month is    given in figure.14. The surf zone width was larger during June to August at about 60to 100m. The surf zone width was only about 10 – 30 m during the November to February. The study of surf zone Vs month is given in figure.15.

Wave Refraction 

Wave refraction phenomenon is an important process responsible for effecting changes in coastal configuration. Based on wave atlas wave refraction pattern prepared for the Indian coast, the predominant deep water wave direction is 2100 with reference to north with period 8s.  The bathymetry points are collected by NIO was considered for extracting digital bathymetry using Arc Map software.  Numerical refraction procedure is adapted from Mahadevan [18]. Referring to Fig.16 for waves coming from 2100 during southwest monsoon, convergence of wave orthogonal or concentration of wave energy is observed along the stretch of Muttom, Kadiyapattinam and Inayam.  Immediately adjacent to the convergence zone, there is a wave divergence zone in the Colachel and Thengapattinam and south of Muttom.  The sediments, eroded in the wave convergence zone, find the way to get deposited on the beach situated along the wave divergence of region.  In southwest monsoon period the refracted wave orthogonals for the SSW direction and for the periods 8 sec is shown in Figure.16.

4.4 Longshore current

Longshore currents are generated due to waves breaking at an angle to the shoreline. The shape of the coastline, beach face slope, estuary, nearshore profile, bathymetry, presence of sand bars and shoals significantly influence the distribution of longshore currents. The measured    longshore   current velocity and direction are shown in table 4 and longshore current for months are drawn in figure.17.  The study indicates that the average longshore current velocity   remains 0.2m/sec in the fair weather season. The  variation  in the  longshore  current  velocity  and  direction  is expected to occur  due  to the change  in coastal  geomorphology   and nearshore  bathymetry. The study also indicates that the Thengapatanam beach is quite sensitive to the both (SW & NE) monsoons. The variation  in the  nearshore   bottom  topography , variation  of  breaker height, presence  of   wind set up , rip currents and manmade  structure like  RMS wall etc. plays an important  role  in the formation of   longshore  current. Longshore current was predominantly in the west direction except months of   January to April.  Anyhow the difference in the  distribution  of longshore   current  direction  could not  much   influence  the redistribution  of littoral  sediment  within the  study region,  leading  to erosion  of the  coast over an  annual  cycle.

Longshore Sediment Transport

The longshore sediment   transport rates are estimated   using   Walton’s equation and   presented in table 4. Earlier  the  annual  longshore sediment transport  rate of Colachel   from April 1995  to April  1996  was  estimated  at  0.9*106 m3/year and  net  transport was  0.3*106  by  Jena and Chandramohan [16]. Now  the  present  study  assessed  the longshore  sediment  transport  rate  of Thengapatanam coast is,  about  1.21* 106  m3/month  during  2007  and  0.577*106 m3/month  during  2008.  The long shore sediment   transport rate was lowest in November, January and February. Chandramohan, Nayak and Raju [19] reported that the annual net transport at the tip of India peninsula near Kanyakumari was negligible and southerly (west) direction. Now the present study   indicates that the net sediment transport near Thengapatanam is in the southerly (western) direction.  The presence of estuary in the Thengapatanam coast very important role in the coast. The damming across the Tamiraparani River and river bed sand mining in the estuarine region blocks the sediments to reach the coast which maintains the coast as the erosional coast in the region. Also the manmade structure RMS wall enhances the erosion at the edge of RMS wall. The sediment transport rate for various months is given in figure.18.

The direction of  transport  depends  on  factors  such  as wave climate, bathymetry, shoreline  orientation  and the  presence  of natural  or artificial  features  that deflect  waves and currents.  The  processes  which shape  an area  of coast  can be  seen  to have  an effect  on beach profile data  is studied. The  study  of  beach  profiles  over time  highlights  the  annual  variations  in the beach  shape  and the level  caused  by varying  wave  and weather  conditions.  The  correlation  between  the beach  profiles and  a number of other factors  including wave height, wave period , surf zone width,  weather  conditions  and  human interventions   are studied. The present study also brings out the nature of the wave climate acting on the coastline. The net sediment transport near Thengapatanam is in southerly (west) direction. The RMS walls put along the coast enhance the erosion recently. The present study indicates that Thengapatanam beach is erosional over the annual cycle. The heavy mineral deposits  Ilmenite presents  in the  Mid tide  region  and  Garnet  in the  Low tide  region  of the  study area.

Limitation:  The survey was undertaken during once in every month but there is no guarantee that the conditions remains same for the entire month.

The authors are thankful to The Joint Secretary, University Grants Commission, SERO, and Hyderabad for providing the financial assistance under Minor Project Scheme (MRP.812/05, Link No.1812.0).

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