Rising sea levels along the Atlantic coast - including tidal waters like the Delaware River as far north as Morrisville - will require increasingly ambitious shore protection that may be impossible or unwise to sustain, concludes a report released yesterday in a national environmental journal.

Some of the very bulkheads or other structures built to protect the land may actually jeopardize the coast's most valuable ecosystems - its wetlands and tidal marshes, which provide flood protection, wildlife habitat, and improved water quality, the researchers said.

"You have species that might like to do something on that land - birds wanting to forage on beaches, horseshoe crabs wanting to lay eggs," said the report's lead author, James G. Titus, the Environmental Protection Agency's sea-level-rise project manager since 1982. "These structures that are continually getting built to protect people's homes tend to eliminate those systems."

"The idea of wetland ecosystems getting 'squeezed out' between encroaching seas and existing or future development and armored shorelines is the key message," said Christopher Linn, a coauthor of the study and senior environmental planner at the Delaware Valley Regional Planning Commission.

The report adds a new layer - a projection of where current land-development policies are likely to lead - to numerous studies that have looked at how low the coastal land is and how high the sea might rise.

"Sometimes, there's the implication that all this land will be underwater," Titus said. "We're saying that land won't be underwater because people will do things."

Linn said current estimates predict sea levels will rise up to a meter by 2100, or about three inches or more a decade, but estimates keep changing as more is learned about polar ice.

Most communities do not have specific plans for how they will deal with rising sea levels. They can choose between two main options: protect shorelines or retreat.

In the absence of such plans, the researchers looked at 131 land-use policies from Florida to Massachusetts, and projected where "business as usual" might lead.

It will lead to bulkheads, dikes, and similar structures made to hold back the sea.

Along the entire stretch of the coast, about 60 percent of land within one meter of the tides is developed or going to be developed. And, therefore, shore protection is likely to certain, Titus and his colleagues concluded.

In New Jersey, shore protection appears to be a foregone conclusion along 86 percent of the 137-mile Atlantic coast.

Along the 60-mile Pennsylvania "coast" - the tidal Delaware from the Delaware state line to Morrisville - all but 24 miles are likely or certain to be protected.

The researchers did not look at the cost of stabilizing shorelines. But looking at it all together, it's a lot of potential bulkheads.

"We're just trying to get people discussing it and thinking about these issues," Linn said.

Or, Titus said, "Which land do we want to protect? How do we want to hold back the sea?"

Linn said the issue was particularly timely in this region, as redevelopment plans for riverfront areas move forward.

The researchers said barriers were not without consequences. They may fail - and dramatically - as New Orleans proved during Hurricane Katrina.

Also, they limit the ability of systems such as wetlands to adapt by growing higher through deposition of more sediment or creeping horizontally inland as hydrology changes.

Indeed, because of the dramatic effect of bulkheads to wall in wetlands instead of letting them adapt, researchers conclude that an Army Corps of Engineers practice of basically rubber-stamping bulkhead permits should be reconsidered.

The authors said that taking the effects of blocking sea-level rise into account may put the corps' policies in violation of the federal Clean Water Act, which essentially prohibits filling wetland areas, but doesn't prevent someone from building a dyke on dry areas next to them.

"What this paper says is that when we put a physical footprint on the ground, we're limiting our options," said Margaret Davidson, director of the NOAA Coastal Services Center in Charleston, S.C.

"There will be some communities where we will spend a lot of money bulkheading," she said. "Those will be expensive places. But we need to think about coastal restoration, not just to take it back to the glory years of the 1950s or whatever you think your baseline is. We need to think about remediating, restoring, or creating in advance of sea-level rise."

The research was funded by the EPA between 2002 and 2007 but was never published in the scientific literature. With EPA permission, Titus and his coauthors incorporated facets of the research into a report and submitted it for peer review to the journal, Environmental Research Letters, where it was published yesterday.

"We're hoping people will look at the maps and ask themselves: 'Do we really want to protect all of these areas that are shown as likely to be protected? Are there any areas that also should be protected? And are there areas we'd really rather not protect and let the wetlands migrate in?' " Titus said.

Danielle Kreeger, science director for the Delaware Estuary Project, said that two-thirds of the marshes in the bay were already considered "degraded." She said their best shot at surviving was to migrate inland as the water rises, which would mean coming up with programs to limit development in their path, for instance.

Along the Jersey Shore, the potential for damage to homes comes not only from the ocean overwashing the barrier island but perhaps even more from flooding from the bays.

"When we talk about shore protection," Titus said, "it's realistic to think about barrier islands being protected, but it's going to require people to elevate their lands and their houses as well."

He recently did just that. He has a bayfront vacation home on Long Beach Island, and he raised it two feet, adding a foot of sand to the ground around it.

See the report and online resources, including detailed maps and state summaries, via http://go.philly.com/sealevel

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com.