TRIGGERING OF COASTAL EROSION AND SHORELINE CHANGES IN SANDY BEACHES CONSEQUENT TO BREAKWATER CONSTRUCTION: AN EXAMPLE OF IMBALANCED COASTAL MORPHODYNAMICS IN ARABIAN SEA ALONG SOUTH WEST COAST OF INDIA

The construction of protruding breakwaters is envisaged to create safer harbors, however, it causes alterations to the wave fields, which approach the coastline adjacent to the structures and modifies sediment movement along the coast. Here, an example of a breakwater-induced imbalance of coastal morphodynamics is presented for the Arabian Sea along the southwest coast of India, causing substantial coastal erosion and changes in the shoreline patterns.

The shore perpendicular breakwaters of Bharathapuzha River mouth at Ponnani, southwest coast of India were constructed to eliminate wave-induced long-shore sediment inputs and formation of sand bar across the entrance, thereby assuring navigability along the channel. A comprehensive shoreline change study, temporally spanning before and after the construction of the breakwaters shows significant imbalance in the natural sediment supply/transport system causing coastal erosion at one side and accretion on another side of the breakwaters. Based on erosion accretion patterns, the beach sector has been further classified into different sectors in the north and south segments of the beach. Low to high energy conditions have been reported in this region with breaker heights varying between 0.4 m and 2.3 m. Long-period waves with plunging breakers are observed during the non-monsoon season, whereas more erosive short-period waves with plunging and spilling breakers are observed during monsoon. Beach profile data indicates drastic erosion during the monsoon and accretion except in a few locations during post-monsoon. The Northern sector of the study area has more wider and stable beaches as compared to the southern sector. Shoreline changes were processed from Landsat imageries and compared using End Point Rate (EPR) and Linear Regression Rate (LRR) methods applying the Digital Shoreline Analysis System (DSAS) technique developed by the USGS. Erosion and accretion transects were obtained for every 50 m interval with annual change rates of –6.78 to 10.89 m/year for EPR and –5.67 and 11.34 m/year for LRR. A total of 1.113 sq km area has undergone erosion and 0.174 sq km has accreted. The study classifies the shoreline into different categories based on erosion/accretion patterns and highlights the causes of erosion experienced by sandy beaches due to the construction of breakwaters.