Flood, rains overwhelmed drains, sewers

Publication Date: Wednesday Feb 11, 1998

Flood, rains overwhelmed drains, sewers

City says storm drains not designed for situation; sewers backed up too

While much of the flood damage in Palo Alto was the result of a wave of water from San Francisquito Creek, much of Palo Alto was flooded in a much less dramatic fashion: The storm drains backed up, slowly filling streets with water.

City officials say the storm drains and sewer systems worked as they were supposed to last Tuesday, but the system wasn't designed for the amount of water dumped on the city last Tuesday.

"(The storm drain system) was not built for this sort of scenario," said Joe Teresi, senior engineer in Palo Alto's Public Works Department. "It is designed to hold water that falls in that drainage area."

Teresi said if the creek had not overflowed, there would have only been isolated pooling of water in Palo Alto.

Although the storm drain and sewer systems are not connected, the backup of the storm drains affected the sewers during the flooding, according to city officials. The backed-up storm drains sent water into the street, where it then went down the sewer system. As a result, some unlucky residents had raw sewage back up into their streets.

The storm drain system in Palo Alto works by the force of gravity, except in a few low-lying areas, such as underpasses and land close to the bay. Water falling on the street is captured by catch basins beneath the grates on street curbs, and travels down storm pipes 12 inches to 96 inches in diameter. The system is like a tree, Teresi said.

"There are a number of trunk lines fed by smaller lines," he said.

The network of pipes takes the water and dumps it into the creeks, the main arteries of the storm drain system.

This system works fine on most rainy days, but the storm drains couldn't handle the overflow from San Francisquito Creek and the deluge of rain the city received that day.

The night of the flood Palo Altans reported seeing the water bubbling out of the catch basins, and in some cases manholes, and eventually flooding their streets, cars and even homes.

Backups most often occurs in lower, flatter areas in the city, close to the bay, Teresi said. There are seven pump locations in Palo Alto.

"When the creek water is above the level of the overspill, the water backs up," said Joe Teresi. "The pumps did not malfunction, there was so much water that the pumps were not able to keep up."

Teresi said the pump at Colorado Avenue, on Matedero Creek, pumps 240 cubic feet per second. In comparison, the flow of San Francisquito Creek the night of the flood has been estimated to have been as high as 7,100 cubic feet per second at its peak.

The rainfall in Palo Alto the night of the flood was 2.23 inches, according to the National Weather Service. But Palo Alto's drainage system, and San Francisquito Creek in particular, drain more than just the city.

Teresi said San Francisquito Creek drains a 45-square-mile watershed, which stretches to Skyline Drive and includes Portola Valley and parts of Woodside and Menlo Park. Stanford University's runoff drains down Matadero Creek.

Once standing water is on the street, it will get into the sewer system, said engineering manager Roger Cwiak of the Utilities Department.

"When water spills into the street, it will come through the (sewer) manholes," Cwiak said.

Water coming in manholes, as well as water seeping through cracks in pipes and joints, can back up the sewer system.

Jim Bewlie, manager of the South Bayside System Authority wastewater treatment plant in Redwood City, said if the backup is great enough in a low-lying area, the sewer water will push off the manhole cover, spilling sewage into the street. Cwiak said there could be other ways the manholes were opened.

"It appears that to help flooding in some local areas people removed manhole covers to drain the street," Cwiak said. He acknowledged it was possible that water backing up in the system could have pushed the covers off.

"We know that covers were removed because large pieces of debris--pipes and wooden boards--were found at the treatment plant," Cwiak said. "Things that would be too big to get into the small holes in the manhole covers."

Cwiak said another way sewage may spill is if someone unscrews the lids of sewage clean-out pipe outlets, which are attached to every house. Once off, it can be hard to screw them back on if dirt or something else gets on the threads, Cwiak said.

So much flood water entered the sewer system that Palo Alto's sewage treatment plant--which normally processes 25-30 million gallons per day--was running at capacity, 80 million gallons per day, for 40 straight hours Cwiak said. Cwiak said so much water got into the system, because manhole covers were removed.

The system handles sewage from Palo Alto, Stanford, Mountain View, Los Altos Hills and Los Altos.

Bewlie said an increase in flow of two- to three-fold through the sewer system is common in a big rain storm.

"Ground water will seep in the network when ground water rises above the level of the pipes," Bewlie said. "Also, some people will have illegal roof gutters, or basement sump pumps draining into sewer lines because it's more convenient."

The SBSA plant in Redwood City was pumping at capacity for most of Monday, Tuesday and Friday last week, Bewlie said. Normal, dry day flow in the SBSA plant, which serves about 200,000 people from Belmont south to the county border, is about 30 million gallons per day, Bewlie said, and the flow was at 68 million gallons per day on Tuesday.

Palo Alto lacks one feature that the SBSA plant has. Diversion ponds allow the SBSA plant to store sewage when it reaches capacity. When the sewage level drops, the stored sewage can be pumped back into the plant for processing.

--Charlie Breitrose

Map of the storm drains and sewers of Palo Alto

Informational graphics of how drains exeeded capacity and why there was sewage contamination

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