Discrepancy found in Cape flood data

It’s Shannon Hulst’s job as flood plain specialist for the Cape Cod Cooperative Extension to make complex flood predictions comprehensible to homeowners and municipalities.

When First Street Foundation released its nationwide flood risk tool last week, Hulst drilled down into the data. When she compared the number of properties threatened by flooding and potentially listed in the federal flood plain, she found that Barnstable County, along with Bristol and Plymouth counties, appeared to run counter to what the First Street tool discovered for the rest of the state and the country.

In their analysis, First Street concluded that the Federal Emergency Management Agency underestimated the number of homes at risk from a 100-year storm by 65% in Massachusetts. Similarly, First Street said FEMA flood plain insurance rate maps underestimated the number of homes at risk of flooding by a total of nearly 6 million properties nationally, a 70% difference.

But on the Cape and adjacent coastal counties, the First Street flood risk tool found the number of threatened properties was lower than what was projected on FEMA’s flood plain maps, Hulst said. In Dennis, for example, Hulst found that current FEMA maps show over 3,100 structures are in the flood plain, but the new First Street tool shows that in 2030 there are only 410 structures at risk. To homeowners, being in the flood plain is a critical distinction, and Hulst anticipates that towns and homeowners might use the new data to ask for a reassessment of their vulnerability.

“That’s unusual for First Street stories, and I’m really, really curious about why,” Hulst said.

The National Flood Insurance Act of 1968 established a program to insure properties against flooding. FEMA quantifies risk for those in the flood plain as having a 1% chance of getting flooded by a major, once-in-a-century storm, which translates into a 26% chance over the span of a 30-year mortgage.

FEMA flood plain mapping determines who has to have flood insurance and can also be used to require more stringent building codes and other regulatory actions. For instance, renovations or repairs to a home in the flood plain amounting to more than 50% of the home's market value must meet higher federal flood standards, such as raising the structure on pilings and relocating utilities from basements to areas above projected flood heights.

High premiums coupled with high deductibles caused property owners on the Cape and elsewhere to protest the 2014 FEMA update to flood plain maps along the East Coast that added many properties that hadn’t been considered to be at risk on prior maps.

FEMA is tasked with updating flood maps every five years, but a 2017 audit by the U.S. Office of the Inspector General found that it had failed to meet its goal of having 80% of maps updated, and even failed to meet a revised baseline of 64%, with just 42% of maps updated by 2016. The federal agency also has been criticized for not taking into account climate change projections for rising seas and the potential for stronger storms, something the First Street Foundation did incorporate into its new flood risk tool.

“That is true. They (FEMA) are focused on that 1% storm. They are not paying attention to bigger, more devastating storms,” Hulst said.

It’s something the First Street Foundation analysis did include, said MIT climate scientist Kerry Emanuel, who serves on the First Street advisory board. He wasn’t quite sure why there was a discrepancy in the Cape data.

Local coastal scientists say it’s a monumental undertaking to analyze each property in the country for flood risk, as the First Street tool does. While First Street has a point of data for every 30 square meters, the Cape already has flood risk analysis that is more detailed, with data points every square meter.

“It’s a very coarse, or gross, scale and they are looking at general trends. It’s not the high degree of resolution we currently have,” said Greg Berman, coastal processes specialist at Woods Hole Sea Grant. Plus, the Cape is undergoing a flood risk evaluation on an even finer scale with the Cape Cod Cooperative Extension, Barnstable County Department of Health and the Environment, Cape Cod Commission and the Center for Coastal Studies working to map storm tide pathways, the low spots that allow water to penetrate inland, causing millions of dollars in damage, as occurred in Provincetown, Barnstable, Chatham and other Cape communities in two storms in January and March of 2018. This effort "ground-truths" maps by going to the properties and taking measurements.

“We do it street by street,” Center for Coastal Studies coastal geologist Mark Borrelli said. It’s something that is too expensive and time-consuming on the national scale, Borrelli said, and reliance on computer models sometimes yields conflicting results that have to be resolved.

“It’s what often happens. You ask questions, both models get refined and it’s a good thing,” he said.

One conclusion common to both FEMA and First Street models is that the Cape has a significant flood risk.

“We are surrounded by water,” Hulst said. The Cape is laced with low-lying areas that are vulnerable to inundation on even high tides, especially if combined with wind and waves from a coastal storm. Plus subsidence — the peninsula is inexorably sinking as our soil compacts — and sea rise due to global warming add inches on top of high tides that can overwhelm the defenses man and nature have built. In the 2018 floods, sea rise contributed to the extensive flooding in downtown Provincetown, adding inches that helped overtop the low sand berm around the lip of the harbor, Berman said.

Hulst pointed out that not all flooding is coastal. The ocean off the Cape is warming faster than just about anywhere else in the world, and that means the air is correspondingly warmer and can hold more moisture, which is released in heavy downpours rather than steady rain.

“Our stormwater systems were designed for earlier times,” said Hulst, and flooded roadways and basements are a result. The former house of correction, now a county facility, is set high up on a hill, but suffered flooding in its basement in 2017, Hulst said.

Big storms, such as hurricanes, torrential rains, even high tides, threaten properties, and experts caution that even those outside flood plains are vulnerable.

“It can happen, so no one is risk-free,” she said.