Healdsburg has seen periodic flooding in the past, but the deluge that hit downtown last week and overwhelmed Foss Creek occurs less frequently than once in 100 years, city officials said Monday.
“I can’t ever remember this much water in this amount of time,” said Councilman Gary Plass, a 58-year resident of Healdsburg. “It’s something really unprecedented for our community.”
In a 24-hour period that ended Thursday afternoon, Healdsburg experienced a whopping 8.4 inches of rain. About 6 of those inches fell in a 12- to 14-hour period, City Manager David Mickaelian told the City Council.
“What people have to remember is when you receive that volume of rain in that period of time, it just overwhelms the system,” he said. “It’s too much water in a short amount of time and that’s what we saw.”
Two large detention ponds designed to catch overflow from Foss Creek quickly reached capacity and spilled over. The result was widespread downtown flooding that sent water into a multiblock area of Healdsburg Avenue and Grove Street, turning the Safeway parking lot into a lake and inundating some cars to their windows and higher.
Despite preparing for the storm with sand bags and plastic tarps, at least two dozen businesses had inside water damage along a two-block stretch of Healdsburg Avenue north of the plaza as the water invaded, sometimes ankle deep or more. Some businesses in an industrial park on Mill Street also suffered the same fate.
The parking lot behind City Hall was waist-high, forcing water into the city’s planning and building department and forcing the closure of adjacent City Hall. The Fire Department also was flooded, including sleeping quarters. Firefighters have had to make due with sleeping in the fire engine bays the past several nights, but the city was planning to bring in trailers for them.
In an interview Monday, city engineer Brent Salmi said the Foss Creek flooding was “far more than a 100-year storm.”
He said a Federal Emergency Management Agency map showed some areas that were flooded last week weren’t even supposed to flood once in a century.
“We didn’t expect to see flooding at City Hall, flooding at the Safeway store, flooding on Mill Street. It wasn’t supposed to happen according to FEMA maps,” he said. “If indeed the FEMA maps were correct, this was more than a 100-year storm.”
The map, last revised in 2010, is expected to be accurate, since it is relied upon by insurance companies to assess flooding potential for homes and businesses.
In his post-mortem on the storm, Mickaelian said the Russian River flow jumped from 700 cubic feet per second to more than 40,000 cubic feet per second within 18 hours.
Although the swollen Russian River stayed within its banks, Foss Creek could not.
City officials are still assessing the damage, although Utility Director Terry Crowley said roughly 150 businesses were affected, meaning they were either damaged, or lost access because of road closures.
As floodwaters receded Thursday, Mickaelian declared a local emergency, which he explained later could make businesses or residents eligible to receive state or federal aid if it becomes available.
Mickaelian acknowledged Monday that a preliminary $15 million damage estimate was “speculation” and it could wind up being less.
But he said it will depend in part on how much inventory was damaged in some flooded businesses.