NEW YORK – There are few places in the U.S. where hospitals have put as much thought and money into disaster planning as New York. And yet two of the city’s busiest, most important medical centers failed a fundamental test of readiness during superstorm Sandy this week: They lost power.

Their backup generators failed, or proved inadequate. Nearly 1,000 patients had to be evacuated.

The closures led to dramatic scenes of doctors carrying patients down dark stairwells, nurses operating respirators by hand, and a bucket brigade of National Guard troops hauling fuel to rooftop generators in a vain attempt to keep the electricity on.

Both hospitals, NYU Langone Medical Center and Bellevue Hospital Center, were still trying to figure out exactly what led to the power failures Thursday, but the culprit appeared to be the most common type of flood damage there is: water in the basement.

While both hospitals put their generators on high floors where they could be protected in a flood, other critical components of the backup power system, such as fuel pumps and tanks, remained in basements just a block from the East River.

Both hospitals had fortified that equipment against floods within the past few years, but the water – which rushed with tremendous force – found a way in.

“This reveals to me that we have to be much more imaginative and detail-oriented in our planning to make sure hospitals are as resilient as they need to be,” said Irwin Redlener, director of the National Center for Disaster Preparedness at Columbia University’s Mailman School of Public Health.

The problem of unreliable backup electricity at hospitals is nothing new.

Power failures crippled New Orleans hospitals after Hurricane Katrina. The backup generator failed at a hospital in Stafford Springs, Conn., after the remnants of Hurricane Irene blew through the state in 2011. Hospitals in Houston were crippled when Tropical Storm Allison flooded their basements and knocked out electrical equipment in 2001.

When the Northeast was hit with a crippling blackout in 2003, the backup power at several of New York City’s hospitals failed or performed poorly.

Generators malfunctioned or overheated. Fuel ran out too quickly. Even where the backup systems worked, they provided electricity to only some parts of the hospital and left others in the dark.