The city could be washing away $1.4 billion on its Manhattan flood-prevention plan because its new East River Park design is two feet too short, according to a new analysis of the controversial project.

Dutch expert Hans Gehrels said if the city doesn’t add enough fill to raise the level of the park by two feet at the outset of the project now, it will have to go back to the drawing board and rip up the park again in about 30 years.

Next year the city is expected to begin reconstruction of the 58-acre park, razing its structures and natural growth and filling it in with 8 to 10 feet of dirt to create a barrier to rising waters during hurricanes and storms. New grass and trees will be planted, and amenities restored, once the landfilling is complete. But according to Gehrels, unless the current design adds two more feet to the height of the park, everything will again have to be leveled in the future so that another layer of fill can be added.

“Including it in the current project would avoid having to remove the mature vegetation around the 2050s, when sea level will likely reach a level that the two additional feet will be needed,” the report says.

The additional fill would protect against a one-in-100-year flood through the year 2100, Gehrels notes.

Gehrels said it was also prudent to do the work now because “sea levels are rising faster than previously predicted.”

Manhattan Borough President Gale Brewer and City Councilwoman Carlina Rivera hired the Netherlands-based Deltares firm to assess the project because of community opposition to the city’s plan.

The current proposal, unveiled a year ago, was an about-face from an alternative that would have preserved the riverfront park and moved a flood barrier closer to the FDR Drive. The city contends this option will offer faster flood protection.

Such protection is seen as a necessity after Superstorm Sandy sent water surging through the area. The floodwaters washed into Con Edison’s 14th Street substation which caused power to be knocked out for much of Manhattan south of 39th Street.

But opponents argue the work will close off the park for at least three years and ruin newly upgraded features, including a refurbished track and a soccer field. Nearly 1,000 trees would be removed.

Hundreds of people rallied last month, holding a mock funeral to “bury the plan, not the park.”

Gehrels said he had trouble evaluating certain aspects of the project’s design because technical documents were not public. He said the city needs to be more transparent to “help rebuild trust and gain support of the community,” including making documentation available.

He also called upon the city to protect the area during construction, when the park is closed and trees are gone, because “a severe storm may surge into the neighborhood more easily.”

The report recommended phased construction of the park to keep parts of it open, something Mayor de Blasio announced earlier this month that the city would do in order to mollify opponents.

But some say that doesn’t go far enough.

“This plan needs to be really vetted. There’s so many unanswered questions. They really shouldn’t be proceeding to demolish this park,” said Pat Arnow, a founder of the East River Park Action group.

Brewer said she implored the de Blasio Administration to take the report’s suggestions into account “before any construction begins.”

Rivera said she was scheduled to meet with the Mayor’s Office next week “to discuss how these recommendations can best be implemented into the proposed plan.”

The City Council is expected to vote on the project last this fall.

A de Blasio spokesman said the project was “based on the best available science.”

“It will protect New Yorkers from flooding through 2100 and can be further strengthened if sea-level rise and global warming outpaces these projections. The city has participated in nearly 85 community meetings and will continue to conduct robust and transparent engagement with all stakeholders,” said Phil Ortiz.