The right way to protect foreshore

Monday October 24, 2016 Written by Published in Environment
An example of geotextile sandbags or “geobags” providing erosion protection on a sandy beach. 16102122 An example of geotextile sandbags or “geobags” providing erosion protection on a sandy beach. 16102122

This weekly column is supplied by Te Ipukarea Society. It deals with environmental and conservation matters of interest to the Cook Islands.


TE IPUKAREA Society (TIS) is concerned that there is an increasing number of developments taking place in vulnerable coastal locations on Rarotonga. 

These developments are both at great risk of damage due to storms and cyclones, as well as being likely to add additional stress to the already suffering lagoon environment, through pollution.

The National Environment Service currently has three Environmental Impact Assessments (EIAs)  available for comment which relate to coastal development. One of these is a proposed rock revetment. The other two are coastal constructions which will likely also require rock walls to protect the new establishments. The following are just some of the issues which can arise from rock revetments:

If there is a long term underlying erosion problem (called beach recession), then the rock wall will eventually result in there being no sandy beach at the site.

During a cyclone or storm event, the hard surface of the rock wall will cause increased wave reflection and turbulence, which will result in increased erosion (beyond what would naturally have occurred), both at the revetment, and more importantly, more erosion to neighbouring properties. This is a well-known phenomenon in coastal engineering industry, known as an “end effect” of a seawall.

During a storm the sand on the beach will be eroded, and there is a considerable risk that the erosion would undermine the toe of the revetment. This would result in the rocks collapsing into the erosion hole, and then being located within the typical sandy beach area. This would become a safety hazard, and ruins the amenity of the beach.

There is also a risk to people’s own property if they build close to the revetment, that the rocks could be projected into their own buildings.

Many other alternatives to rock revetments are available which pose less risk to our coastal environment. A more environmentally friendly option is a less permanent revetment design. It is becoming common practice to construct revetments from large geotextile containers (geobags) filled with sand. These are typically 0.7m3 or 2.5m3 volume for each bag. These have a number of advantages.

They can be more easily removed if the erosion risk reduces in the future and if the revetment is damaged by waves, the bags can be cut open and removed, leaving only sand behind on the beach, not rocks.

They are also nicer and safer to sit on and walk over.

However, it’s not all good news, as geobags have some disadvantages. The bag fabric has a life span (though this is now in the decades), and they may still be displaced by waves. But at least the consequences can be undone, unlike pile of rocks that get displaced by waves and ruin the beach

Recent improvements in geosynthetic materials have resulted in increased strength and durability, allowing improved designs and performance of sand-filled container systems. Geosynthetic fabrics now combine high strengths and resistance to abrasion, puncture, tear and ultraviolet deterioration, as well as site-specific characteristics such as the ability to be coloured to match the beach sand at a project area.

Sand-filled containers are “softer” and more “user-friendly” than structures constructed of rock, concrete and steel, making them more desirable and benign in areas of natural sandy beaches, something we in the Cook Islands rely on heavily for our tourism industry.

For more information about these options to using rock walls to protect beachfront property call Te Ipukarea Society on 21144 or email This email address is being protected from spambots. You need JavaScript enabled to view it.


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