NEW YORK (AP) — Inside tunnels threading under a Houston medical campus, 100 submarine doors stand ready to block invading floodwaters. Before commuters in Bangkok can head down into the city's subways, they must first climb three feet of stairs to raised entrances, equipped with flood gates. In Washington, D.C., managers of a retail and apartment complex need just two hours to activate steel walls designed to hold back as much as a 17-foot rise in the Potomac River.

If metropolitan New York is going to defend itself from surges like the one that overwhelmed the region during Superstorm Sandy, decision makers can start by studying how others have fought the threat of fast-rising water. And they must accept an unsettling reality: Limiting the damage caused by flooding will likely demand numerous changes, large and small, and yet even substantial protections will be far from absolute.

Sandy's toll is overwhelming. But finding the money and political will to build a proposed system of giant storm barriers at the mouth of New York Harbor will likely be very difficult. Even at a cost of up to $27 billion, such barriers would leave large parts of the region unprotected.

So government, businesses and property owners will need to consider taking smaller steps — on land — to minimize the impact of flooding, with or without sea barriers.

The good news is that many cities have already learned much about how to limit the damage from floods. Researchers are working on still other strategies, like 16-foot-wide inflatable plugs being developed at West Virginia University to seal off subways and tunnels from water.

But there's no single cure-all.

"You really have to go with a series of levels of protection. You can't just buy into one engineer's dream of building" a 5-mile-long barrier for New York Harbor, said Phil Bedient, a flood expert at Houston's Rice University whose research was key to shoring up that city's defenses after it was swamped by Tropical Storm Allison in 2001. "So you have to pick your spots carefully. And you really can't protect everything."

Sandy's destruction in the New York area highlights a host of weaknesses that must be addressed, experts said. But the region's size, density and geography will complicate the task.

"It's hard to predict what's going to happen, where it's going to happen and what magnitude, and that leads to a quandary of what makes sense to do," said William Coulbourne, a Delaware structural engineer specializing in flood plain design and construction. "New York City is unique in the number of people who live there, the age of the buildings, that it's on islands and it's a hub of U.S. commerce."

That could force people to make trade-offs they might have been unwilling to consider before Sandy. When city officials met with real estate and construction industry representatives starting in 2008 to look at making New York buildings more environmentally efficient, the conversation included whether to move flood-prone electrical equipment out of basements in apartment buildings and office towers to higher floors, said Rohit "Rit" Aggarwala, former director of the city's Office of Long-Term Planning and Sustainability.

"One of the people from the real estate industry said, 'Rit, you're crazy. That's rentable space up on those floors,' Aggarwala said. "That's the problem of thinking in the near-term of losing revenue vs. the long-term certainty of needing it."

Now New York needs a wide-ranging discussion, considering not just how to limit damage to high-rise districts like lower Manhattan that are critical to the region's function, but about whether and how to rebuild in residential neighborhoods along the shoreline, said Larry Buss, a recently retired hydrologic engineer for the Army Corps of Engineers who for many years led its committee on non-structural flood proofing.

"If you're thinking long-term," said Buss, who worked with communities along the Gulf of Mexico to build flood resilience after Hurricane Katrina, "you've got to use all the tools in your toolbox."

In the search for answers, few places may offer as many lessons as Houston's Texas Medical Center campus, which is bisected by a bayou and was swamped by intense rains in a 2001 storm. When Ed Tucker, the center's senior vice president of planning and development, watched televised footage of rescuers carrying critically ill patients down the darkened stairwells of New York hospitals during Sandy, he was struck by a terrible thought: He had seen it all before.

The floods in Houston caused a blackout, inundated medical center streets with up to 9 feet of water, and forced evacuations of patients from the district's 6,900 hospital beds, some airlifted from rooftops by helicopter. The campus sustained more than $2 billion in damage.

"Allison was a significant event for us and fortunately we learned a lot," Tucker said.

A review of the area's flood weaknesses led officials to create a list of 112 projects, including widening the bayou and building culverts that funnel water away from the campus. But many of the projects were based on acknowledging that even if planners couldn't ensure that all the water from a future storm would stay out, they could at least work to limit the damage.

TMC's member hospitals moved their electrical vaults and backup generators out of basements to areas above flood level. They rejiggered the way they used their space, rebuilding and moving facilities like research labs, many of which were destroyed by the flood, to higher floors. Scores of existing buildings were fitted with flood gates, and new buildings were built surrounded by berms. Underground tunnels were outfitted with 100 submarine doors, some 12 feet tall. The bill was $756 million, paid by the Federal Emergency Management Agency, not including millions more spent on the public works projects.

Variants of some of those flood mitigation measures could be put to work in New York, experts said, with a focus on protecting the infrastructure and centers of activity critical to its function.

— SUBWAYS AND TUNNELS: Sandy exposed the weaknesses of the 108-year-old subway system, including the large number of stations in flood-prone neighborhoods and the overall porosity of a network ventilated by thousands of grates set into sidewalks.

In recent years the Metropolitan Transportation Authority, which runs the system, has begun looking for ways to defend it from water. After flooding from a 2007 storm forced closure of part of the system, the agency spent $157 million on a host of projects, including one that closed half the 1,600 grates along a low-lying avenue in Queens, raised others and installed water-activated mechanical closing devices on still more. It also hired an architecture firm to design raised grates that double as street furniture.

But those changes were designed to prevent flooding caused by rain, not storm surge, and were limited by a capital budget with little room for projects not directly related to transportation, said Projjal Dutta, the MTA's director of sustainability initiatives.

"Sandy just upped that bar hugely," Dutta said. The agency is studying how subway systems elsewhere protect themselves from floods, including some that have installed gates or built drainage tunnels. But the MTA has not reached a decision on how to move forward, and hardening the system against a surge like Sandy's will require significant additional funding, he said.

While New York is designing raised entrances for a new subway line, it is far behind newer systems, like Bangkok, where most station entrances are raised several feet above street level.

Defending the system from a major flood will likely require numerous changes to seal off its many entry points. One answer could come from researchers at West Virginia University, funded by the Department of Homeland Security, who are developing inflatable plugs to seal off underwater tunnels in case of a breach.

A 16-foot-wide prototype was tested in the Washington, D.C., Metro system in 2008, with highly pressurized smoke proving its ability to seal off a tunnel with irregular contours, said Ever Barbero, a professor of aerospace and mechanical engineering at West Virginia who developed the plug. Barbero said plugs, which could be made to varying sizes, could also be used to seal highway tunnels like the ones that flooded in New York.

After New York was hit by Sandy, "I told my co-workers we have our work cut out for us for the next 20 years," Barbero said.

The subway system would be a particular challenge, requiring flood gates, plugs or some other closure at thousands of vulnerable openings.

"A technology like this might be useful to plug certain points but it certainly is not an end-all, be-all answer to everything," said Dave Cadogan, director of engineering for Frederica, Del.-based ILC Dover, which has a contract to manufacturer the plugs. He and Barbero estimate they are still a couple of years away from marketing the plugs, with ones similar to the prototype likely to sell initially for about $400,000.

— ELECTRIC GRID: With Sandy pounding the coast, New York power supplier Con Edison preemptively shut down three networks serving parts of lower Manhattan and Brooklyn to prevent damage to equipment. But widespread outages were prolonged after a 14-foot surge inundated the utility's 13th Street substation, swamped critical gear located just over 11 feet above sea level, and caused an explosion. Also, above-ground lines in New Jersey and New York were taken down by falling trees. Continued...