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Geotextile sandbags

Our low lying areas are vulnerable to the impacts of sea level rise. We are active in piloting the use of Geotextile sandbags as a way to mitigate the impacts of sea level rise as an option for coastal protection, particularly our outer island communities.

Rarotonga has had a significant number of rock seawalls built over the past 30 years. These are built to protect the ever-increasing number of holiday accommodations constructed along the coast to meet the demands of the rapidly expanding tourism sector. These seawalls require large boulders to be quarried from the inland of Rarotonga, transported to the coast, and placed along the beachfront. The quarrying is environmentally damaging to the inland and the completed seawalls can also cause additional erosion due to “end effects” at each end of the wall. Erosion also occurs as the waves hit the solid walls and bounce back, scouring out sand on the beach. A number of the beaches in front of resorts are now completely gone at even a mid level tide.

This project sought to identify an alternative to these solid rock walls. It was implemented in Rarotonga, at the Avana Harbour. It involved placing geo-textile sandbags, each weighing over 1 ton, for approximately 45m along the coast to help protect against erosion caused by climate change impacts. These bags are placed on top of a geo-textile matting, which helps prevent the bags from being undermined by over-topping wave action. This particular structure starts at four bags high at the northern end and decreases to a single bag as it approaches the Avana Stream. It has the additional benefit of providing a much more stable platform for the fishermen to access their small fishing boats that are moored in the area.

This intervention is what could be termed a hybrid "nature-based solution" for climate change. It is a combination of an engineered wall of man-made geo-textile bags, filled with natural sand dredged from the site. As a further addition to this pilot, we filled one of the bags with ground glass from the Rarotonga landfill, as a way to demonstrate a possible use of the waste glass and reduce the impacts of sand mining. This man-made structure was backed up with the planting of beach vines, vetiver grass and native trees behind the geo bags to provide a 2nd level of natural defence. When eventually the wall fails, as all walls will in the event of a severe cyclone, the damaged bags can be easily removed. We will only be left with sand on the beach, rather than chaotic shambles of scattered rocks, as has already happened at some sites around Rarotonga.

Story Information:

Country: Cook Islands

Topic: Nature Based Solutions

Photo or video credit: Te Ipukarea Society

Text Credit: Alanna M (with video)

Date : 14 July 2023

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