To build or not to build, that is the question

21 July 2014 | Article

Mo O is a coastal village in Soc Trang Province where the cultivation of watermelons is the leading income source. Watermelons are planted on sandy soil just behind an earth dike. When the dike breaks and sea water floods in, as it often does, the villagers lose most of their crop.

A storm in late 2013 caused a major breach of the dike. Against IUCN’s advice, the commune people’s committee rebuilt the dike by excavating dirt from in front of the dike on the seaward side. This made things worse because despite being behind a 200-meter belt of mangroves, the deeper water that the excavation created right in front of the dike meant that the wave energy was not dissipated and instead struck the rebuilt dike with greater force. Unsurprisingly, when the next big storm arrived, the dike collapsed.

The people’s committee had to decide whether or not to build a bigger, stronger dike using dirt trucked in from inland, a more expensive proposition. To help answer this question, IUCN hired a German coastal engineering specialist who has done similar work for GIZ in the Mekong Delta. The report the specialist prepared includes a rough cost-benefit analysis of the new dike.

The cost of building and maintaining the 1.8-km dike over 30 years is US$276,000 (as with all the costs and benefits, this is an estimate). If the dike isn’t built, the village will suffer US$443,000 in lost watermelon production. Even if a new dike is built, it will occasionally be breached and over 30 years the village will still lose US$35,000 in lost production. So the dike would prevent a net loss of US$408,000 in watermelon production over 30 years or about US$13,600/year.

This cost-benefit analysis does not take into account other benefits that a dike can provide, for example protecting residential areas from flooding or providing opportunities for faster travel along the top of the dike. It does not take into account the fact that the US$13,600/year in lost watermelon production that the dike would prevent is not net income but sales value, which excludes the cost of seeds, irrigation, labor, transport, and other inputs. Nor does it take into account changes in the price of watermelons (or alternative crops) over the next 30 years. So the actual net loss prevented by the dike could be very different from US$13,600/year.

This kind of economic evaluation is about assessing trade-offs toward achieving a goal. In this case, the cost-benefit analysis raises doubts about the value of large-scale dike construction in the face of rising sea levels, higher waves, and potentially more powerful and more frequent storms. It suggests that it would be wise to consider alternatives rather than immediately deciding to build dikes and other infrastructure to protect coastal livelihoods. A full range of options should be considered. Villagers could be trained in sea water-tolerant income generating activities including various forms of aquaculture, particularly mangrove polyculture, which produces shrimp, fish, crab, etc. and protects against storms. Or the budget could be spent on improving education to give the next generation the skills they need to prosper off the farm.

In fact, the commune decided not to rebuild the dike because of the high cost and doubts about how long it will last. Instead, the villagers will fix and maintain the dike themselves on a year to year basis and plant mangroves to better protect the dike. This kind of incremental response, which avoids spending large sums on coastal defenses that are ineffective and/or inefficient (the risk of “maladaptation”), makes sense given the uncertainties associated with climate change.

Another conclusion that can be drawn from the Mo O study is the value of mangroves as “soft” infrastructure for coastal defense. The study considered the possibility of building a dike outside the 40-hectare mangrove forest. This would cost US$963,000 over 30 years to build and maintain or US$687,000 more than the dike inside the mangroves. This is because the dike outside the mangroves would have to be twice a wide as there would be no mangroves to protect it from the waves. This implies that the wave attenuation value of the mangroves over 30 years is equal to US$687,000 divided by 40 hectares or US$567/hectare/year. This is similar to economic valuations of the storm protection function of mangroves that have been carried out elsewhere.

As the government considers investing billions of dollars in cement sea dikes along large stretches of coastline, it will have to seriously consider if this is the best use of public funds to adapt to a changing climate.

Jake Brunner - Mekong Programme Coordinator - IUCN Viet Nam