ESCONDIDO – The seismically unsound Lake Wohlford dam in Escondido will be replaced by a new dam downstream – once the city finds the funds to do the work.
Background: In 2007, Escondido found that most of the Lake Wohlford dam cannot withstand a big earthquake and needed to be reinforced or replaced.
What's changing: City officials recommended building a dam downstream from the existing structure at a cost of $19 million to $35 million. The City Council has approved studies on different ways to build the dam.
Estimated to cost $19 million to $35 million, depending on the materials used, the dam would be built without interrupting the city's water supply.
Funding is a problem now as Escondido grapples with a budget crisis, but the city will apply for state grants and probably sell bonds to pay for the construction, utilities manager Lori Vereker said.
Last week, the City Council instructed the city staff to study what kinds of materials should be used to build the dam, and how it would affect the environment.
Lake Wohlford was created in 1895 with a 76-foot-high rock dam to store water from the San Luis Rey River via the Escondido Canal.
In 1916, the city of San Diego hired rainmaker Charles M. Hatfield to end a prolonged countywide drought. He was blamed for a storm that pummelled the region that January, killing dozens of people and causing Lake Wohlford to overflow.
Eight years later, Escondido leaders decided to build up the dam, using sand and silt from the lake bed and piling it on top of the rock structure without compacting the material.
The dam's height was increased to 100 feet, and the city's forefathers thought they had eliminated the threat of flooding.
What they didn't know was that Lake Wohlford is 11.5 miles from the Elsinore fault, which is capable of a 7.5-magnitude earthquake.
Seismic tests in 1969, 1976 and 1978 showed the dam was earthquake resistant. But in 2007, a test using a different method of extracting dam material yielded a big surprise.
The top two-thirds of the dam, the portion that was built with layers of sand and silt, could liquefy in a big earthquake and flood the city.
Since the discovery, the lake has been kept at 43 percent of its capacity, with the waterline below the top of the original rock dam.
That means Lake Wohlford is being used at less than one-half of its 2.1 billion gallon capacity at a time when the city needs water the most because of drought.
Lake Wohlford supplies 30 percent of the city's water, as well as 120,000 customers in the Vista Irrigation District who live in Vista and parts of San Marcos, Oceanside, Carlsbad and the unincorporated county.
The city hired a consultant this year to study the dam's stability and make recommendations as to what can be done. In a draft report, GEI Consultants of Carlsbad outlined the following options:
Replace the sand-and-silt portion of the existing dam with rockfill.
Build a berm upstream.
Build a wall upstream.
Build a dam downstream with an earth core and a rockfill shell.
Build a dam downstream with a rockfill core and a concrete shell.
Build an earth dam downstream.
Build a concrete dam downstream.
Vereker said her department chose a new dam downstream because any work that is done upstream would require the reservoir to be emptied for two years.
“The recreational and environmental impacts are pretty extreme,” she said. “That knocks them out of the assessment.”
She recommended a rockfill or concrete dam downstream, which would take up to two years to build.
She discarded the option of an earth dam downstream because the city would have to truck in the material, prolonging construction.
Once the new dam is completed, the unstable sand-and-silt portion of the old dam will be demolished, she said.
The city will be hiring consultants to do soil tests and environmental studies soon.
“We will get a better idea of the challenges involved in doing either one,” Vereker said.
Angela Lau: (760) 737-7575; firstname.lastname@example.org