The Effectiveness of Jetty Construction and its Impacts on Shoreline Changes Along the Western Coast of the Upper Gulf of Thailand

Regarding a high investment cost of each coastal protection project, the essential functions of coastal protection structures perform in protecting coastal infrastructures including the impact of such structures on their surrounding should be monitored to provide a prototype verification of the engineering design. The objectives of this study were to monitor the impact of the construction of the Cha-am jetty, the second largest jetties in Thailand, on shoreline change along the western coast of the Upper Gulf of Thailand. The Cha-am jetty was constructed in 1968, and the GENESIS software was used to predict the changes on the Phetchaburi shoreline due to the jetty. In this study, four shoreline positions along the study area during the period 1967-2018 were digitized from aerial photographs and satellite images using Geographic Information System (ArcGIS) version 10.6. The Digital Shoreline Analysis System (DSAS) was used to analyze the distance and rate of shoreline changes along the Cha-am coast, Phetchaburi province, where the 1,200 m-long jetty system has been installed. The actual shoreline positions were compared to those predicted by the numerical model (GENESIS) to evaluate the effectiveness of the jetty system. The results from the study indicated that the jetty system was still functioning. The comparison between actual and predicted shorelines, the GENESIS mostly overestimated shoreline change of about 0.1-3 times for the upcoast of the jetty system and up to 95 times for the downcoast of the structure. It also found that the GENESIS provided a better shoreline prediction for short-term (30 years) than that for the long-term (50 years). The causes of model uncertainty probably due to the lack of measured oceanographic data such as wave, current, and wind data for coastal processes simulation. Besides, human activities such as coastal protection measures by the private sector may alter the shoreline response to the jetty system. As the GENESIS performance had more than 3 times of the errors in simulating shoreline changes on the western portion of the Upper Gulf of Thailand, the information from this study should be utilized in designing future similar structures in this region.