Study Shows Green Infrastructure Provides Cost-Effective Protection Against Rising Sea Levels
A just-released study by the Bay Institute, a San Francisco based environmental organization, shows that using coastal marshland as a buffer against the San Francisco Bay is effective against rising sea levels and provides significant cost savings over other methods. The full report, “Analysis of the Costs and Benefits of Using Tidal Marsh restoration as a Sea Level Rise Adaptation Strategy in San Francisco Bay” (pdf), suggests that a “horizontal levee” that includes restored marshland is significantly less expensive than a larger levee that would otherwise need to be constructed.
As an introduction to the report, the authors note that sea levels in San Francisco Bay have rising some 7 inches over the past century and estimates of another 14 inches of sea level rise in the next half-century with a potential of 55 inches by 2100. Protecting shorelines, however, is more complex — tides, storm surge, and wave action need to be accounted for in shoreline protection calculations, as does the slow subsidence of some coastal lands and the slow seismic shifts that are also occurring. The obvious approach to addressing these concerns is to simply increase the height and bulk of current levees and flood control systems.
Meanwhile, the Bay area has lost the vast majority of coastal salt marshes that existed prior to 1850, having been developed or converted to agriculture or solar salt ponds. There has been a new focus on restoration of these marshlands, and the new study indicates why more needs to be done. A key finding:
The Bay Institute’s analysis determined that tidal marshes can significantly reduce the destructive power of storm surge. This finding suggests that shoreline flood protection is improved significantly when areas of tidal marsh exist between the developed shoreline and the open waters of the bay. Further, it indicates that by using tidal marsh in combination with a levee constructed at the landward edge of the marsh, the size of the levee can be reduced significantly while still providing the same level of flood protection as would be provided by a larger levee that was not fronted by tidal marsh.
Indeed, the study indicates that using marshlands for flood protection will save up to 50% in infrastructure costs over comparable systems. To protect 275 miles of San Francisco Bay shoreline, the savings could amount to more than $1 billion.
However, current restoration projects are not necessarily being designed with sea level rise in mind. The report has several recommendations to help marsh restoration projects survive climate impacts and to make the restoration projects more functional as flood protection. But the report warns that such projects will need to start soon so that marshland and accompanying “horizontal levee” systems will be in place before the sea level impacts are felt.
Using nature to fight nature is a cost-effective solution, the report concludes. “The study clearly finds that nature’s capital is quite tangible and can be put to much greater benefit than is currently the case.”