A bridge replacing the deteriorating 87-year-old structure that now spans the San Dieguito River in northern Del Mar will be constructed on-site rather than pre-cast at another locale, according to direction provided Monday, April 1, by the Del Mar City Council.
The city’s engineer and consultants recommended the “cast in place” option as providing more flexibility in architectural design and making it easier to build so the bridge meets requirements for withstanding tsunamis.
Also, council members advised the staff and consultants to build one side of the bridge at a time lengthwise, so that two-way traffic flow can be maintained on Camino Del Mar throughout the project.
The city’s main drag, Camino Del Mar is a heavily traveled stretch of the coastal highway popularly known as 101.
Council members, however, questioned whether the replacement deck’s proposed height increase of 2 feet to 3 feet over the existing, low-lying bridge will be sufficient to accommodate sea-level rise due to global warming.
The proposed design, which would bring the bridge’s deck up to a 17-foot-high maximum, was calculated based on a 100-year-flood height of 12 feet above sea level; a 5-inch rise in the Pacific Ocean’s surface in the near future; and a projected 5.5-foot rise in the Pacific Ocean’s surface over the ensuing decades.
“I’ve just seen so many sea level-rise projection numbers, when you tell me we’ve got 5 inches before our bridge is wet (in a major flood), I’m nervous,” Councilman Dwight Worden said.
The panel asked City Engineer Tim Thiele and the city’s consulting firm Kleinfelder Inc. to explore how the bridge deck could be raised.
“It’s doable, but would require a lot of additional work,” Thiele said.
While Mayor David Druker supported taking a look at a height increase, he cautioned that the numerical projections on future sea level-rise remain speculative.
As part of Monday’s bridge update, the council voted 5-0 to approve an amendment to its contract agreement with Kleinfelder.
If all goes according to schedule, Thiele said, construction could occur from 2023 to 2025. The project is expected to cost about $23 million, mostly funded by the federal government.
Construction will take from 21 months to 24 months, nearly twice the time it would take if both the east and west sides of the bridge were built at the same time.
The existing 600-foot-long bridge was built in 1932, widened in 1953 and modified in 2001. A comprehensive analysis concluded the concrete structure could not be rehabilitated and needs to be replaced.
The span is a landmark, being located at the northern gateway to Del Mar along the coast, while being straddled by the seashore to the west and the coastal railroad and Del Mar Fairgrounds to the east.
Preliminary designs call for the replacement bridge to be supported by six piers rather than 10, resulting in less intrusion on the river and water flow in and out of the adjacent lagoon.
When completed, the new bridge will have a vehicle lane, bike lane and sidewalk in each direction, among other features.