The parks department’s plan must be approved by the Public Design Commission, whose members are appointed by Mr. Bloomberg. If the commission approves the plan, as it is expected to do, reconstructing the Boardwalk would take at least another year, said Adrian Benepe, the parks commissioner.

Some commission members said they would reluctantly embrace the synthetic wood-concrete compromise.

“I have pushed them to look at every possible wood alternative, and they have persuaded me that there aren’t wood alternatives that are practical,” said one commission member, Otis Pratt Pearsall, a trustee of the Brooklyn Museum. With that in mind, he said he would support the hybrid plan because “it is important to have the thing look as Boardwalk-y as possible.”

Another commissioner, Paula Scher, shared that sense of resignation.

“If you think we’re happy that wood is being replaced by material we find less appealing, that is certainly not the case,” said Ms. Scher, a partner at Pentagram Design. “It’s called a Boardwalk, and if you use other material, it loses its identity. I understand that, but it’s so much better to have a surface to walk on next to the beach.

“We love our icons of the past, and sometimes you can preserve them,” Ms. Scher said, but “things have changed.”

Mr. Benepe said the parks department had investigated every option, from natural woods like Douglas fir and black locust to treated woods like Southern yellow pine. They concluded that such hardwoods were neither durable enough nor, in the case of black locust, abundantly available. Opponents rejected this argument, and Michael Caruso, a forestry expert in West Virginia, said that black locust wood could be ordered in sufficient quantities.

The plastic composite, which can last 75 years, is cheaper than wood to build with and maintain, Mr. Benepe said. According to City Council member Domenic M. Recchia Jr., it costs more than $1 million a year to maintain the wooden Boardwalk.

Under the hybrid plan, it would cost the city $6.85 million to replace the 60,110-square-foot stretch of the Boardwalk from Coney Island Avenue to Brighton 15th Street. That is more expensive than concrete, but cheaper than some natural woods that could last just eight years, the parks department said.