Building Sea Walls in Vanuatu - Advice | National Advisory Board

Words of caution however are wise especially with gabion baskets which even if they work superbly at first will eventually rust in the marine environment and release the materials they contain. As with any seawall they tend to fail at the worst time, during bad weather when they receive unusual levels of stress. Often people in the Pacific use gabions as a “quick-fix” on a sandy or gravelly beach which is perceived to be erosive, stacked cement filled geotextile bags are another local favourite. The problem is that if long-shore transport of sediment is an important process (it very often is) and if the structure is successful and it does stop sand from moving it often means you are starving a beach down drift of sand = more erosion. Of course, I don’t know the local conditions at this location but I do know local feelings about erosion and I imagine that if everyone has decided they want a seawall and there are funds available then it’s likely a seawall will get built irrespective.

If this is the case then I would suggest the following. Look at the shape and slope of the existing beach, try to mimic its shape and slope with the gabion’s. This will take much more material and time to do but it will be less likely to cause damage, will mitigate wave energy better and will be more likely to become buried in sand again if conditions allow. Know that if you use gabions they are going to rust and may require significant work to repair / replace in the future. If you build groins this will almost certainly cause downstream erosion, if the seawall is vertical it will likely never allow the natural beach to return (and is more vulnerable to damage). Consider the “toe” of the wall (lowest edge closest to the sea) on a vertical wall this is often very vulnerable to undercutting as vertical walls reflect so much energy and this digs away sand from below (causing them to subside / collapse). Often wall designs incorporate an extended horizontal toe seaward at the same level as the substratum or sediments. Likewise, consider the height of the wall, if you match the slope of the wall with the existing beach it is easy to see the approximate height the wall needs to be by looking at the back beach berm height (highest part of the beach where only storm waves usually reach). If the wall is vertical it will likely have to be even higher because waves smash into the vertical face and can over top the wall - rather than running up a sloping face and dissipating the wave energy.

Final comments would be around consideration the conditions / problems which have led to this situation. Is there beach mining, or inappropriate engineering nearby which is causing erosion? Is this a naturally dynamic shore and thus a poor location for permanent settlement, and would it be more appropriate to use resources to move back away for the shore? Will the construction of a seawall encourage others to move closer to the shore because they believe it to now be safe? Remember any low-lying shoreline area is going to be subject to ever increasing threat over the coming years - we know sealevel is rising. How exposed is this shore to ocean waves, storms, tsunami, etc?

Hope these very general comments might be helpful in you discussions and feel free to send some photos if you like as it might give me a better idea of the situation.

Regards,

Arthur

Dr Arthur Webb

Deputy Director – Ocean & Islands Programme

(Coastal Processes Adviser)

SPC - Secretariat of the Pacific Community.

Applied Geoscience and Technology Division (SOPAC)
Postal Address: Private Mail Bag, GPO, Suva, Fiji Islands
Street Address: Mead Road, Nabua, Fiji Islands
Tel: +679 338 1377 Fax: +679 337 0040
E-mail: arthur@sopac.org