MILPITAS, Calif. _ Thousands of residents returned to their soggy homes Thursday as San Jose officials worked to mop up the worst flooding event in the city's recent history and pinpoint the reason why water rose so much faster than they expected.
City officials have acknowledged that they were caught off guard by the scale and severity of the flooding, which began earlier this week when winter storms caused Santa Clara County's largest reservoir to overflow and flood the creek that cuts through the heart of the city.
As they have tallied the damage, officials have offered some preliminary explanations for why Coyote Creek overflowed: The information they had on the capacity of the creek's channel was not accurate, and debris may have caused blockages.
On Thursday, San Jose Assistant City Manager Dave Sykes said Coyote Creek was still too bloated for officials to be able to examine it for damage. But scientists with the U.S. Geological Survey offered up a surprising new explanation for the water's rapid rise: The drought.
"Five years of drought has allowed vegetation to grow in this channel, and choke up and fill the channel," said Anthony Guerriero, a USGS hydrologic technician. "So when the flows came, that vegetation then displaced the water, causing the water to get even higher than expected."
As he took flow measurements near the mouth of the creek in Milpitas, Guerriero explained that the bottom of Coyote Creek had become overgrown with vegetation during the five years of dry weather. As a result, the mass of vegetation on the creek bed kept growing instead of being pushed out to sea during annual winter rains.
The glut of debris ultimately helped to raise floodwaters, Guerriero said, explaining: "If I had a glass of water and I started throwing rocks in it, the water elevation is going to rise."
That was little consolation to the roughly 3,800 residents who remained under mandatory evacuations Thursday. Sykes said he believes some won't be able to ever truly return home because their property was so badly damaged. He estimated that the cost of destruction would tally several million dollars.
City Councilman Tam Nguyen said at least 350 homes were damaged in his district alone.
Some in his district typically reside in homeless encampments, he added. "After Sunday," he said, "I don't know where they are going to go."
The good news was that by Thursday, water had receded into the creek. As a result, officials lifted mandatory evacuations orders for more than 10,000 residents and turned their attention to helping the displaced re-enter their homes.
City crews were working to pump flood water out of the Rock Springs area, which was among the first neighborhoods to get inundated with floodwater. Sewers there were still out of service, though, so the city was delivering portable toilets to the neighborhood as a temporary fix.
Half a dozen roads also remained closed as of Thursday morning.
With more rain on tap for the weekend, Sykes pledged that city officials would be "keeping a very close eye on the creek."
The trouble actually began in Morgan Hill about 22 miles south of downtown San Jose, where Santa Clara County's largest reservoir has been filling with storm water for weeks.
To compensate, Santa Clara Valley Water District officials have been releasing as much water as possible through Anderson Reservoir's main outlet since January. But an unrelenting conveyor belt of storms filled the reservoir faster than officials could safely let water out.
By Saturday, the reservoir had reached its capacity and water began flowing down its spillway. Another round of heavy rain exacerbated the situation Sunday night and Monday morning, boosting the flows into Coyote Creek.
The first major flooding occurred Tuesday in the Rock Springs area of San Jose. Firefighters had to paddle on rafts and wade through a chest-deep deluge to rescue hundreds of residents trapped in homes and in trees.
A damaged levee even allowed water to flow onto U.S. 101 on Tuesday, forcing its temporary closure. More than 300 residents stayed overnight at evacuation centers.
The creek crested to a height of 13.6 feet at a South San Jose river gauge point on Tuesday evening _ nearly 4 feet above flood stage. The record recording prompted a National Weather Service meteorologist to call the situation a "once-in-a-100-year flood event."
USGS officials said the force of the flooding water was so powerful that it even damaged or destroyed some of the agency's equipment on the creek bed, including bending a steel pole that had once served as a visual marker of the river height.
According to an automatic gauge that has monitored the creek since 1991, the highest it had ever reached was 13.1 feet. This week's flood sent the creek rising to a height of 18.1 feet early Wednesday.
The San Jose flood, Guerriero said, should be a warning to other cities that long-standing estimates of how much water a river or creek can handle can sometimes be wrong.
"Rivers and streams are dynamic and changing. They're almost like living beasts," Guerriero said. "The sediment comes and goes, the vegetation comes and goes over time. All of that displaces the water."
