COASTAL EROSION AMD PROTECTION ALONG KARNATAKA COAST

Coastal areas are assuming greater importance in recent times in view of their strategic location for residential;, recreational and industrial activities. A need has arisen to preserve and maintain the shore against the destructive forces of nature which are more often aided by human interference- The coastline of Karnataka State is in a state of dynamic equilibrium except for areas near the rivermouths which suffer permanent erosion due to natural shifting and migration of the river mouths. Encroachment into the active zone of the beach is the primary problem. The coastal protection works adopted in the Karnataka State is mainly seawalls- Seawalls properly located and constructed have performed satisfactorily while those built in the active zone of the beach have suffered damage-  There  is  need to  enforce  strictly the  Coastal  Zone Regulation Act of the Govt- of India..

INTRODUCTION Coastal zones have certain aesthetic and practical advantages and this has resulted in the concentration of residential, recreational and industrial areas in this zone. Nearly two thirds; of the World's population lives in these areas which is often only one or two meters above the mean sea level- Most of these coastal areas are geologically young and are still being shaped by nature. People have chosen to live close to the water front either ignorant of or ignoring deliberately the risks involved,  the risks that this boundary is not stable and not clearly defined- Water gives and takes, and a state of dynamic equilibrium exists- More often the fluctuations  that  are  part  of  this  dynamic  equilibrium  are underestimated leading inevitably to disaster, loss of life and loss of property- This fragile dynamic equilibrium can often be upset by human interference- It becomes necessary that this coastal zone is managed and developed in a way that respects its natural and social significance.

Karnataka State on the Most Coast of India has a coastline of about 300 KM intercepted with a number of rivers joining the Arabian Sea. The coastal region particularly in the South is heavily populated-Certain reaches of this coastal region are facing the problem of beach erosion, river mouths migration and siltation of minor ports. Karnataka Regional Engineering College, Surathkal, located on the coast near Mangalore is an active participant in a number of Coastal zone studies and this paper highlights the investigations regarding the coastal erosion and the evaluation of the coastal protection works adopted so far.

COASTAL ZONE OF KARNATAKA STATE The Coast stretches from Karwar in the north to Mangalore in the south. The region is bordered by the Arabian Sea on the west and the Western Qhats on the east. The rivers along the coast flow westward and take an almost right angle bend and flow either northward or southward close to and parallel to the coast. Such rivers join together at some point before they empty into the Arabian Sea- The river mouths in most of the cases are found to be the confluence of two or more rivers.

The West Coast of India is under the influence of the S-M monsoon and most of the heavy wave activity that is likely to cause beach erosion is confined to   the four monsoon months from June to September.. Waves reaching upto 6.5 m in height have been recorded off the Karnataka coast- Uaves during the non-monsoon months are generally less than 1 m in height.- The predominant wave direction is M and N-W and occasionally S-M (DATTATRI, 19733)

Littoral drift data on the Karnataka Coast is very scanty and what is reported does not appear to be reliable. Often estimates of littoral drift are made based on the maintenance dredging of the entrance channels of the major ports that are located in the given area. The maintenance dredging in the outer approach channel of New Mangalore Port has varied from 0,54 million m 3//year to 3.81 million m3//year (KAMATH, 1986). But the material dredged is silt and clay, where as the beach material is fine sand with a median diameter in the range of 0.2 mm to 0..4 mm. The material dredged is generally sea bed material which gets disturbed by the high wave activity in the monsoon months and is carried by the currents to be deposited in the deeper and calmer waters of the entrance channel and the harbour basin of NMPT- Observations of the depositional and erosion tendencies adjacent to the breakwaters at NMPT indicate that the magnitude of the littoral drift is negligibly small and if any, has a bias to move to the south. This is true of most of the areas along the Karnataka  coast-

COASTAL EROSION ALON9 KARNATAKA COAST

Increasing reports of coastal erosion in the last two decades have raised a doubt whether the erosion is a recent phenomenon? Dynamic changes have occurred in this fragile 'boundary between the land and the sea, ever since the boundary was formed, but the changes have received public-, attention only now because of their increased importance and economic impact- It is also possible that increased human interference  may  have  upset  the  delicate  equilibrium established by nature- Man has no business to interfere in nature's playfields and must face the consequences of such intervention. The primary causes of coastal erosion in any given region are generally the following" 1) Direct wave action, 2) Interception of littoral drift., 3)Sea level changes, 4) River mouth changes, and 5) Sand mining-  The  question  to  be answered  is,  which  one  or combination of  these is  responsible for  the coastal  erosion reported at different locations along the Karnataka coast?

On the Karnataka coast    with negligible littoral drift,  the erosions reported particularly on the open coasts cannot be due to interception  of  littoral  drift  which causes  erosion  on  the downdrift  side,  With reference  to Karnataka  coast,  there  is relative sea level fall as a result of the upward rise of land at a faster rate than the global sea level rise (SUBRAHMAMYA and RAO, 1991). Sea level changes cannot be the cause of any erosion on the Karnataka coast as at present. Sand mining, removal of beach sand for land fills,, plinth fillings and other uses, upsets the sediment equilibrium and may result in erosion. Sand mining from the beaches is a prohibited activity under the enactment of the Ministry of Environment  but  still  clandestinely  practiced.  However,  the magnitudes of such mining activity on the Karnataka coast is small and unlikely to be a cause of erosion.  Therefore the erosion reported on the Karnataka coast must be due to direct wave activity and that due to changes in the river mouths-

The erosion that is reported is along isolated stretches and not continuous all  along the coast-  Further the erosion is of a temporary nature in the sense that the eroded beach would be fully recovered in the post-monsoon season. The only reports of permanent loss of land due to erosion on the Karnataka coast have been at the river mouths  at Ullal south of Mangalore;, and Pavinakurve north of Honnavar.  It should be remembered that the river mouths do undergo continuous cyclic, changes as a result of the complex interactions between river flow (water+sediment), waves and the tides- The only solution to this would be to stabilise the river mouths with river bank protection and breakwaters.

The erosions on the open coasts must be due to direct action of waves- Although the deep water wave characteristics may be the same along the Karnataka coast, there are local variations due to wave refractions- Coastal reaches where convergence of wave orthogonals takes place due to wave refraction are more vulnerable to erosion. Further the beaches adjacent to the coast parallel rivers are more susceptible to erosion due to higher exit gradients and higher pore pressures..

If the eroded beaches fully recover after the monsoon season is over, the question that arises and needs to be analysed fully is, where then is the problem?

When the waves approach the coast straight from the deeper waters., the beach develops an equilibrium slope which is a function of the energy of the incoming waves and the grain size of the beach sand-Higher waves cause a flatter slope and the coarser the sand, steeper will be the slope for the same wave conditions- For a given beach, the grain size is unlikely to vary much but the wave heights can vary between wide limits and this can change the beach slope and consequently the beach profile.  This explains the typical monsoon profile (the storm profile) and the non-monsoon profile (the normal profile) observed on the Karnataka coast. The sand moves onshore and offshore with the seasons and a state of dynamic equilibrium exists. At times of a severe monsoon, or a concentration of wave energy due to wave refraction,, a more than usual erosion may result, which is only a temporary inconvenience-Land owners and property owners who have encroached in this area may not take this philosophical approach. They must realise that they have encroached in the playground of nature (d*ANGREMOND;, 1993). Most of the erosions reported on the Karnataka coast is of this nature where developmental activities have been allowed in the active zone of the beach .

COASTAL PROTECTION WORKS ON THE KARNATAKA COAST Coastal erosion that threatens settlements and economic interests needs to be protected against.  There are three basic options available (GRIGGS et al, 1991):

i) Armoring, the 'hard' measure of constructing a protective structure like a sea wall, a groyne or an offshore breakwater.

ii)  Nourishment,, the "soft" measure of widening the beach by feeding the required sand to the beach.

iii)Retreat, withdrawal beyond the active coastal zone-

The best option is the retreat as envisaged in the Coastal Zone Regulating Act of 1991 wherein the Central Government has declared the coastal stretch upto 500 m on the landward side from the High Tide line as the Coastal Regulation Zone and has put a severe restriction on the activities in this zone. Whether it will be possible to strictly enforce the regulations of this act or not, remains to be seen-

The  next  best  option  is  the  'soft'  option  of  artificial nourishment, though very expensive is often preferred as it is most environment friendly. This solution is most suitable where the erosion is due to interruptions to the littoral drift- As the erosions on the Karnataka coast is primarily due to direct wave attack,  the most suitable options economically are the 'hard' options.  Among these groynes are not suitable due to lack of littoral drift and the contractor potential does not exist to build offshore breakwaters- Seawalls, therefore appear to be the best coastal  protection  method  that  can  be  used  to  protect  the vulnerable areas. Even in other countries like U.S.A., seawalls have been the most common approach to protecting either private or public oceanfront property (GRIGGS et al, 1991).

The Minor Irrigations Department of the Government of Karnataka has been adopting a standard seawall design as evolved by the Karnataka Engineering Research Station, KRS. total of about 15 KM length of the vulnerable reaches of the Karnataka coast have been protected with the seawalls.

PERFORMANCE OF SEAWALLS ON THE KARMATAKA COAST  A  significant  length  of  the  seawalls  built  have  performed satisfactorily while in a few isolated areas, the seawalls have suffered extensive damage. A detailed study of these failures have shown that these seawalls have been built in a forward position on the beach primarily because of the severe encroachment into the active zone of the beach. The best place for a seawall, if one is necessary, is at the back of the beach. Just beyond the active zone of the beach- In contrast seawalls built seaward will have constant problems related to frontal and end scour (TAIT AND GRIGGS, 1990). The failures in the present case clearly indicate frontal scour, loss of support and collapse of toe and part of the seawall.

The construction of the seawalls is generally done in the post-monsoon season when the eroded beach would have recovered fully. Construction in this period would require a significant amount of beach to be excavated to expose the back of the beach for the ideal location of the seawall- Only a strict supervision can ensure this.

To meet the ever increasing public demand for immediate protection against erosion during severe monsoon periods,, emergency works are carried out. These constructions are invariably from the top down in these adverse situations. These constructions are not only in the active zones of the beach but to upgrade them later to the standard cross-section is also very difficult. These sections will be the ones to be damaged first in the subsequent monsoons- The ends of  seawalls  and  the  gaps in  the  seawalls  for  fishing activities are the other zones where failures are more common. Seawalls properly located and constructed have withstood the test of time satisfactorily.

DO  SEAWALLS  CAUSE  BEACH  EROSION  ? This is a Question that has often been asked by Planners and Decision makers regarding whether proposed protective structure should be allowed or not- A body of opinion exists that insists that seawalls promote and accelerate the erosion process in front and adjacent to the seawalls. To answer this question, GRIGGS et al (1991) have monitored for over four years the beaches in the protected and the unprotected coastal areas in Central California, USA- Their findings are valid for a coast which is not undergoing any net retreat with time. They find that seawalls properly located had no impact on the erosional behaviour of the beaches in front or adjacent to it. Some scour was observed at the down drift end of each structure and the extent of this scour was controlled by the end section orientation,  angle of wave approach and the wave characteristics.

CONCLUSIONS Karnataka coast experiences erosion at isolated regions in the monsoon season. However these erosions do not cause any net retreat of the coast as the beach fully recovers in the post- monsoon season. But where the active zone of the beach is encroached, these erosions cause loss to property <and there is a public demand that these areas be protected- Seawalls have been adopted for coastal protection works and these have performed satisfactorily where properly located and constructed-

ACKNOWLEDGEMENTS The  author  expresses  his  thanks  of  gratitude  to  the  Minor Irrigations Department, Government of Karnataka for providing an opportunity to be actively associated with the coastal protection works undertaken on the Karnataka coast,. The views expressed in this paper are those of the author. The permission of the College authorities to be associated with this work is. gratefully acknow1 edged

REFERENCES

1d’ANGREMOND, K , 1993, "Mew Aspects of Coastal Protection and Development”, PIANC Bulletin No. 81, pp. 84-90.

2. DATTATRI, J., 1973, “Waves off Mangalore Harbour – West Coast of India”, Journal of Waterways, Harbours and Coastal Engineering Division, Proc. ASCE, 99(1), pp. 39-58.

3. GRIGGS, G.B., TAIT, J.F., SCOTT, K., and PLANT, N., 1991, “The Interaction of Sewalls and Beaches: Four years of Field Monitoring, Monetery Bay, California”, Conference on Coastal Sediments, Coastal Sediments ’91, ASCE, pp. 1871-1885.

4. KAMATH, M.M. 1986, “Maintenance Dredging Practice at New Mangalore Port”, National Seminar on Dredging, Indian Institute of Port Management, Calcutta.

5. SUBRAHMANYA, K.R., and RAO, B.R.J., 1991, “Marine Geological Aspects of Dakshina Kannada Coast”, Perspective on Dakshina Kannada and Kodagu, Mangalore University Decennial Volume, Mangalore, pp. 201-220

6. TAIT, J.F., and GRIGGS, G.B., 1990, “Beach Response to the Presence of a Seawall”, Shore and Beach, 58(2), pp.11-28.

7……………………, 1991, Project for Anti Sea Erosion Works Along Karnataka Coastal Line in Priority Reaches, Irrigation Department, Govt., of Karnataka, Bangalore.