The streets around City Dock in Annapolis flooded again Friday, closing the Spa Creek Bridge connecting the Eastport neighborhood with downtown for several hours. It was yet another reminder to Lisa Craig that she's in a race against time to protect one of Maryland's oldest communities from the rising waters of the Chesapeake Bay.

"We've probably doubled the number of nuisance flooding events in the past several years," said Craig, director of historic preservation for the city of Annapolis.

With school groups and tourists congregating on the city's picturesque waterfront Monday, she recalled that a few weeks earlier, some shops and eateries along Dock Street had to close because of flooding from a high tide abetted by heavy rains.

Maryland's capital since 1695, downtown Annapolis boasts the largest concentration of 18th-century Georgian architecture in the nation, according to the Historic Annapolis Foundation. But many of those historic structures are within the city's 100-year flood plain, which was last inundated in 2003 when Tropical Storm Isabel swept up the bay.

A report to be released Tuesday by the Union of Concerned Scientists says Annapolis' historic district, the Naval Academy and the new Harriet Tubman Underground Railroad National Monument in low-lying Dorchester County are among dozens of historic sites nationwide at risk from rising sea level, flooding and worsening wildfires as the climate changes.

"We're heading into Memorial Day weekend, and everybody's thoughts are turning to summer vacations," said Adam Markham, director of climate impacts for the Massachusetts-based environmental group. "Millions of people are going to be visiting national parks and historic sites, and we thought it was a good time to alert people to the threats they're under."

Recent federal and international reports on the effects of climate change overlook what is happening to places of historical and cultural significance, Markham said.

"We need a more comprehensive survey of the threats, and then we're going to have to make some pretty hard choices about how to deal with them," he said.

The report discusses risks of flooding and storm damage at landmarks such as Jamestown in Virginia and New York's Ellis Island, which was closed for more than a year after Superstorm Sandy hit in fall 2012.

But it also highlights one of the nation's newest historic sites. In 2013, President Barack Obama designated 25,000 acres of Blackwater National Wildlife Refuge as well as some private land in Dorchester a national monument honoring Harriet Tubman for her role in leading slaves to freedom. The proclamation was the culmination of years of planning and effort, said Alan Spears, director of cultural resources for the National Parks Conservation Association.

"We finally get the area protected, and now we see we've got a new threat," Spears said.

The water level in the largely undeveloped area has risen more than 10 inches in the past 70 years, the report said, and it's projected to increase up to 15 inches more by 2050.

Sea level is rising along the Mid-Atlantic coast faster than the national average, and Dorchester is the most vulnerable of Maryland's counties, according to William Boicourt, an oceanographer with the University of Maryland Center for Environmental Science. Isabel inundated half the county, he said, and much of the land south and west of his laboratory at Horn Point is "one big marsh."

Maryland is building a 15,000-square-foot visitors center on an adjoining 17-acre state park, also dedicated to Tubman and the Underground Railroad. Scheduled to open in 2016, the center is being built off the ground to keep it from being flooded, Markham noted.

It's more challenging to protect historic structures. At the Naval Academy in Annapolis, Isabel caused more than $120 million in damage with water rising 61/2 feet above average, the report said. The academy has taken steps to limit damage from future flooding, it noted.

Downtown Annapolis poses a different challenge, as most of the historic properties there are privately owned, yet they anchor the capital's economy.

"It's one of the main reasons tourists come here, because we have a 300-year-old historic district," Craig said.

There are 180 buildings within the 100-year flood plain, some dating to the Colonial era. Many are at risk from 8-foot storm surges, she said. A sampling of 11 properties showed an estimated cost of $24 million to replace, she said. It took the city more than a decade to get its Market House back up and running after Isabel, she noted.

As much damage as flooding can do, Craig said, "we're not looking at building a sea wall.'' Obstacles include cost, logistics and the reluctance of many to interfere with the downtown's "million-dollar view" of the water, she said. Nor is it practical to look at elevating such old structures.

Instead, Craig said the city is working with federal and state officials to develop what she called a "tool kit" of steps historic property owners can take to keep water out, or at least limit the damage if they can't stay dry.

"We're looking at putting in the hands of property owners the tools," Craig said, "to protect and hopefully minimize the effect of flooding in the event of a Sandy-type hurricane."

tim.wheeler@baltsun.com