The Smithsonian Institution wants to replace the proposed aquatic garden bridges outside the National Museum of African American History and Culture, now under construction, with a low, reflective wall.

The change, part of a package of design alterations to the $500 million museum scheduled to open in 2015, was cleared by the Commission of Fine Arts in July and is up for approval at next month’s meeting of the National Capital Planning Commission.

Detractors said the bridge design was inappropriate for the site along Constitution Avenue. The modified design features reflective surfaces of polished stone. Approval of the final site development plans are on the agenda for the planning commission’s meeting Sept. 12.

Smithsonian spokeswoman Linda St. Thomas called the adjustments a routine part of the construction process. “Because this was a speed-up project, they are doing the design and the construction as they go,” St. Thomas said.

In addition to disagreement about the aquatic garden, there was a concern about the water table at the site, so engineers “needed to make a change while excavating, and this costs more money,” St. Thomas said.

Ground was broken on the five-acre site, next to the Washington Monument, last year. Congressional funding will account for half of the museum’s $500 million cost.

The museum is requesting the final $50 million in federal money in next year’s budget and by June had raised $140 million from individuals, corporations and nongovernmental sources, said Lonnie G. Bunch, founding director of the museum.

Other changes included moving a cooling tower and redoing a fountain design on the building’s south side.

St. Thomas said the museum’s design process is more than 65 percent completed, adding that the change is not related to the congressionally mandated cuts known as the sequester, which have slashed $41 million from the Smithsonian’s budget.