Evacuations along the Pajaro River in Monterey County last month were necessary because flooding occurred at a lower threshold than official projections suggested.
That was the message delivered to the Monterey County Board of Supervisors this week by the county’s Water Resources Agency and the Pajaro Regional Flood Management Agency, which is a joint powers agency that will take over maintenance of flood control facilities on the river later this fiscal year.
That agency plans to work with the U.S. Army Corps of Engineers on a major upgrade of the levee system in the coming years.
“Thresholds for monitoring and flood stage for the Pajaro River do not reflect current conditions within the channel and should be reevaluated by the [California Nevada] River Forecast Center,” said Mark Foxworthy, an engineer with the county Water Resources Agency.
“This presentation is critical for the community to understand how close we came to massive flooding.”Supervisor Luis Alejo
Foxworthy said projections suggested that the river would not top the levee until peak flows reached a height of 32 feet, when in fact it was breached when flows were measured at 28 feet at the Chittenden monitoring location, which they called the most important point to monitor downstream flows.
“The flows were literally at the top of the existing levee at that location,” said Lew Bauman, interim general manager for the Water Resources Agency.
Bauman said the county’s response, including the evacuation orders, was appropriate, given their observations that the hydrographs were not accurate. He said the agency would explore hiring a consultant to work with the river forecast center to get modified hydrograph projections.
Supervisor and board chair Luis Alejo sought to dispel rumors that he said were contributing to residents questioning the motivation and need for the evacuations, which lasted from Jan. 9 to Jan. 17.
“This presentation is critical for the community to understand how close we came to massive flooding,” said Alejo.
Building walls, reinforcing levees
To prepare for levee overtopping, county crews installed 3,000 feet of emergency concrete block and Muscle Wall along the upper stretch of the levee, stockpiled supplies, strengthened and made repairs to levee roads and banks, improved all-weather access for the levee and identified key areas to monitor river flows. Adjacent farmland that was flooded on Jan. 10 was monitored by county crews, Foxworthy said.
A major project by the Army Corps of Engineers to redo the levee in sections has secured increased funding in the last five years, said Mark Strudley, executive director of the new joint powers agency.
The first phase of the $400 million project will be at Salsipuedes and Corralitos creeks and is tentatively planned to start construction in 2025 for a period of two years. A segment of levee from Pajaro to Murphy Road crossing is planned for the next stage, with construction planned to start in 2027 and last one year.
He said the current infrastructure, built in 1949, only provides protection from floods anticipated to occur every five to eight years, but the new levee system will be able to withstand a 100-year flood event, which is defined as flood levels that have less than 1 percent chance of occurring each year.
“Once this project is built, there will be a significantly greater amount of protection and factor of safety built into the system,” said Strudley.
Supervisor Glenn Church said March could be another wet month and said it was critical for residents to understand that flood threats can vary along the levee.
“This is [an] important thing to get out, for people to really understand that what they’re seeing in their neighborhood isn’t really the situation just a few miles up,” he said.