WASHINGTON — U.S. ports are worried about how ever-growing cargo ships and an expanded Panama Canal will affect their volume and revenues in the future, prompting calls by port authorities and civil engineers for increased investment in their infrastructure.

“Post-Panamax” ships, the new, larger cargo vessels that will be able to traverse the canal after its expansion in 2015, currently have a relative dearth of U.S. ports to choose from. The larger vessels require channel depths of at least 50 feet in order to enter ports fully laden with cargo, according to the U.S. Army Corps of Engineers.

Seattle, Oakland, Los Angeles and Long Beach ports all have 50-foot channels on the Pacific Ocean, while Baltimore and New York City ports either have, or will soon have, the required depth.

New York, however, must contend with the fact that the Bayonne Bridge is too low to accommodate some vessels. The bridge spans a heavily traveled tidal strait that connects Newark Bay with Upper New York Bay.

As a result, there have been repeated calls for investment in U.S. port infrastructure.

Earlier this month, the American Society of Civil Engineers released a report highlighting what it estimated to be a nearly $16 billion gap between the roughly $14 billion expected to flow into marine infrastructure between now and 2020 and the $30 billion the ASCE said the nation’s port system needs to stay competitive in a post-Panamax world.

“Needed investment in marine ports includes harbor and channel dredging, while inland waterways require new or rehabilitated lock and dam facilities,” the group said in a release.

The ASCE report, part of a series titled “Failure to Act,” concluded that preparation for post-Panamax vessels is a clear necessity, particularly in the Southeast and in the Gulf of Mexico.

“The nation’s port system is in danger of being non-competitive at several key ports in the Southeast and Gulf ... due to the slow and complex process of project delivery for critical dredging projects — especially those that will allow key ports to participate in offering services that depend on serving larger bulk and container vessels that will call on U.S. ports once the expanded Panama Canal opens in 2015,” that report warned.

“If we don’t close the investment gaps, everyone is going to feel the negative impacts because we are on course to lose more than one million jobs and more than $1 trillion in personal income by 2020,” ASCE president Andrew Herrmann said after the report was released.

An Army Corps of Engineers report released in June also highlighted the need for land-based infrastructure capable of supporting the increased tonnage, including more large cranes and sufficient road and rail networks for transporting goods once they leave a ship.

What is less clear is how the readiness or lack thereof for the big ships will ultimately affect port revenues and creditworthiness, analysts said.

“At this point, it’s a potential impact, with a capital P,” said Maria Matesanz, senior vice president of global project and infrastructure at Moody’s Investors Service. “That being said, the potential impact is difficult to gauge.”

Moody’s port rating methodology, which the agency is in the process of revising, takes into account the factors influenced by post-Panamax readiness. Ports typically borrow against revenues from docking fees and other money related to how busy they are. But ports also receive considerable state and federal money for operating expenses.

The dredging funds needed to keep port channels deep come from a federal pool of taxes imposed on shippers. Even though port revenue amounts are “closely tied to cargo volume,” under Moody’s methodology, that is somewhat offset by the practice of many ports of requiring frequent customers to pay a certain minimal annual fee for port access.

Increased volume is a major hallmark of the post-Panamax ships, which the Panama Canal Authority said are typically 200 feet longer and capable of almost three times the storage: 12,000 twenty-foot equivalent units (TEUs) as opposed to 4,500 for pre-expansion vessels.

They are also nearly 200 feet tall above the waterline, according to the Army Corps of Engineers, which is why New York and New Jersey officials have begun a project to raise the Bayonne Bridge.

“I don’t think there’ll be a huge credit impact in terms of volume,” Fitch Ratings’ analyst Emma Griffith said.

There are too many unknowns surrounding the canal at this time, Griffith added, noting that it’s still not certain how much money it will cost vessels to use the expanded canal and at what U.S. ports those ships will call when the canal reopens.

Most East Coast ports are planning, or already undergoing, some kind of expansion, she said.

The alarm sounded by the port authorities and the ASCE about the potential for some U.S. ports to lose competitiveness could hold water, though.

“That’s certainly a concern,” Griffith said. “That’s why all these ports are pursuing these enhancements.”

Moody’s analyst Kristina Cordero said her agency is aware, but not spooked, about the potential impact on U.S. port credits.

“I don’t think we have a concern,” she said. “We know that this is happening.”

The Panama Canal expansion, originally announced in late 2006, has fallen a year behind schedule from its original 2014 target completion date. If more delays occur, it could be even longer before the economic and credit impact on U.S. ports becomes clear.

“We won’t actually know until it opens,” Griffith said.

John Martin, who runs the economic consulting firm Martin Associates, said the canal expansion has gotten a lot of “hoopla” since it was announced, but is only a symptom of the increasing sizes of ships.

“You really have to focus on the whole picture of international trade,” he said.

Martin said U.S. ports face a valid challenge of losing competitiveness without further development, because it could limit their ability to serve as a first port of call for fully-loaded post-Panamax ships.

Some of those ships could still come in, he said, but only after unloading cargo at other ports first. To focus simply on the canal expansion is “naive,” he said.

“This is a very complicated and complex issue,” Martin said.

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