City can't hold back the tides: Appeal of federal flood maps really a stopgap

One map showing where Boston thinks FEMA erred: Gray is for flood areas the city agrees with, blue areas the city thinks don't deserve the designation and red the areas the city thinks should be added.

Even as they are appealing roughly a third of the 1,500 acres the federal government wants to add to maps of areas likely to flood in a "100-year" event, Boston officials are acknowledging that, ultimately, property owners of large sections of land along the coast and city rivers will have to face the costs of flood insurance because of rising sea levels.

Brian Swett, Boston's chief of environment, energy and open space, said the process used by FEMA to propose new flood zones are based on data from the past and do not include calculations of the effect of future rises in sea level - which he said could reach 6 to 16 inches in Boston by 2050. As the sea level rises, "100-year" flooding will start to happen more frequently and likely be more severe, he said.

Last year, the city approved changes in its zoning code to require developers of new buildings in or near areas that could flood to show how they are preparing their buildings for a greater risk of flooding.

In the meantime, though, the city says the proposed FEMA maps issued last year have a variety of errors that, when corrected, will strike about 500 acres from the maps. Whether or not a property is listed on a FEMA flooding map can have large ramifications for both existing property owners, who would be required to buy more possibly expensive flood insurance if they have federally backed mortgages, and for people who want to build on the land, who might have to add mitigation features to their proposed projects.

Swett said it's impossible to quantify an average dollar amount for the individual cost of flood-map designation, since each property would have its own unique characteristics that an insurer would use to determine a yearly flood-insurance rate.