How to plan better for New England floods

Changing rainfall patterns have increased the number and severity of floods. Better forecasts and improved flood management have helped, but planners need to do more to reduce risks and boost community resilience.

As flood waters recede in rain-soaked New England, March's record-smashing storms highlight the need for planners in the region to place an increased emphasis on reducing flood risks and boosting their communities' resilience to floods.

Focus not only should be placed on nuts-and-bolts, concrete-and-rebar projects such as upgrades to roads, bridges, culverts, and municipal drainage systems. Planners need to update the basic information on rainfall intensity they use to determine the adequacy of their projects.

That's the view of several hydrology specialists, who note that changing rainfall patterns in the area alone have increased the number of floods and appears to have increased their average severity as well over the past 40 years – particularly in the last decade.

IN PICTURES: Springtime flooding in the US

The severe rainfall early this week rounded out a trio of intense rain storms during March. That sequence was unusual, acknowledges Paul Marinelli, who heads the US Army Corps of Engineers regional reservoir control center in Concord, Mass.

"But in the past five or six years we're getting to see more and more of this type of event," he adds, referring to the floods that coursed through the region.

Despite the hardship for large numbers of homeowners, business owners, cities, and towns in the areas hardest hit hardest hit by the region's floods, it could have been worse, Mr. Marinelli says. Winter snows in the region had long since melted by the time March's storms struck.

The last time the region experienced a similar one-month onslaught, more than 200 people in the region died due to floods. That was in August 1955, when two hurricanes struck within a week. In Boston, last months' rains topped the August 1955 total by 0.03 inches.

Specialists attribute the difference in fatalities to better storm and flood forecasts, along with improved approaches to flood management.

Outdated flood-risk maps

Still, several specialists say, existing estimates of flood hazards remain based on outdated flood-risk maps – something the US Federal Emergency Management Agency has been working to change with its program to improve map accuracy. But risks also are based on storm rainfall estimates tied to climate "norms" that no longer appear to be holding.

Storm numbers used for infrastructure design in the region were initially developed in 1961, then updated in 1993, explains Ellen Douglas, a professor and water-resources engineer at the University of Massachusetts at Boston.

"But a lot has changed," she says. Her research covering coastal regions of Massachusetts, New Hampshire, and Maine indicates that while the strength of extreme storms appears to be fairly constant between 1954 and 2005, "when you extend the record to 2008, extreme rainfall appears to be increasing." And the rate of increase appears to have grown between 1970 and 2008.

Long-term flood records seem to tell a similar story, notes Mathias Collins, a hydrologist with the National Marine Fisheries Service's Restoration Center in Gloucester, Mass.

He analyzed 75 years' worth of data from US Geological Survey stream-flow gauges throughout New England – but selected from landscapes that have undergone little development, which can increase the rate of run-off and intensify floods.

The data point to a long-term increase in the number of floods. But they also point to what he calls a "stepwise" increase in the frequency of above-average floods, beginning around 1970. In some cases, he says, severity is well above the long-term average. By contrast, prior to 1970, floods typically fell below the long-term average severity.

"It's not just simply a gradual upward trend over 75 years," he says. "It's more like there are two distinct periods in the record that hinge around 1970."

Is climate change a factor?

At the moment, it's unclear whether this change represents a regional hydrological effect of global warming. Other researchers have documented trends in increased precipitation over several decades in the US, which is consistent with a warming atmosphere's ability to hold – and release – more moisture.

For now, he says, a feature of natural climate variability called the North Atlantic Oscillation looks as if it may be playing a role. Since 1970, it appears to have shifted to a regime in which it favors warmer, wetter winters in the US Northeast than it experienced before 1970, Collins says. The correlation is strong enough to warrant more work on it, he says.

Such trends imply a need to redouble efforts to reduce the risk to lives and property flooding in the region presents, researchers say.

While today's "best practices" widely applied would not have prevented the kind of damage March's flooding inflicted on New England, they can help reduce losses from lesser events, according to Paul Kirshen, a former Tufts University engineering professor who now works on climate-adaptation issues at the Battelle Memorial Institute's office in Duxbury, Mass.

Mimicking natural water flows

One concept, low-impact development, aims to engineer drainage systems – from those used in homes to those running beneath entire communities – in ways that mimic natural water flows. For homes, that could in part mean giving up a thickly paved driveway for gravel or paving stones that allow more water to sink into the soil

That may not help an individual's flooding problem during a given storm, Dr. Kirshen acknowledges. But it does help on a community-wide basis.

In addition, groups ranging from the Army Corps of Engineers to local communities and environmental groups are trying to find ways to make room for a storm's flow upstream, which eases the flood levels in more-populated areas downstream.

Dam removal also plays a role, adds Brian Graber, with the environmental group American Rivers, noting that historically, many of the region's dams were built to power mills, not control floods. Old and often in poor repair, many of these dams prompt evacuations during severe storms even when people living downstream are not experiencing flooding.

IN PICTURES: Springtime flooding in the US