Study finds geotubing at Poonthura effective in controlling coastal erosion

A study conducted by scientists from the National Institute of Ocean Technology (NIOT), officials of the Kerala State Coastal Area Development Corporation (KSCADC) and experts in Coastal Engineering of the offshore breakwater system using geotube technology along the Poonthura coastal stretch has found that they yielded remarkable transformations in the coastal landscape behind the offshore breakwater stretch.

One of the major outcomes was the effective prevention of wave overtopping beyond the seawall, extending roughly twice the length of the offshore breakwater on the shore side. Notably, the resultant beach formation proved to be sustainable even amid inclement weather conditions. Three layers of geotubes, each with a circumference of 15 m, are placed on the 6 m to 7 m deep parallel to the seabed. Each layer of tubes are placed vertically to the beach with sufficient length in the wave approach against the conventional approach of placing parallel to the shoreline.

The installation comprises five units, each breakwater unit spanning 100 m with a fixed distance of 50 meters between them. The composite structure of the geotube breakwater below the sea level serves as a barrier, diminishing the force of large waves approaching the coastline. This design not only mitigates the risk of coastal erosion but also facilitates the accumulation of additional sand between the existing coast and the offshore breaker, fostering the development of a broader beach.

The study also revealed that the upper layer had become separated from the geotube due to the intense swaying motion of the sea, resulting in the escape of sand through it. Despite this, the investigation confirmed that the geotubes remained securely in position without any signs of drifting, primarily due to the sand firmly anchored at the four corners of the tube. The analysis also indicated that all geotubes in the bottom layer had undergone complete consolidation and stabilisation.

The State government had sanctioned ₹20 crore through the Kerala Infrastructure Investment Fund Board (KIIFB) to implement the offshore breakwater system using geotube technology. The geotubes act as an effective barrier, diminishing the strength of oncoming sea waves. The result is a gentler wave action depositing sand particles onto the shore, creating a stable and usable beach.

Due to heavy under current in the region, the placing and spreading of the geotube was cumbersome. The experienced divers earlier associated with the Indian Navy were specifically engaged for the work. The placement of geotubes in the deep sea bed posed significant challenges due to rolling caused by strong currents and wave disturbances, leading to an initial failure in the laying operation. Following detailed deliberations among the involved agencies, 250 kg concrete anchor blocks were deployed on all sides of the geotube, ensuring it was securely fastened with the assistance of divers.