DC Department of Transportation worker, Daniel Ellis, scrapes the sewer overflow from T Street NW near the intersection of Rhode Island Ave. NW. (Sarah L. Voisin/The Washington Post)

D.C. Water and city government officials are proposing to divert runoff from severe storms into holding facilities before it threatens to inundate the Bloomingdale and LeDroit Park neighborhoods of Northwest Washington in response to flash floods last summer.

The work, estimated to cost as much as $40 million more than previous flood-relief plans, would involve converting facilities on the former McMillan sand filtration site into storage tanks capable of holding 3 million gallons of runoff each. Meanwhile, D.C. Water crews would immediately begin digging a six-block-long, Metro-size tunnel under First Street, which could store 6 million gallons of stormwater and sewage.

The McMillan storage tanks could be finished as soon as spring 2014, said George S. Hawkins, the D.C. Water and Sewer Authority’s general manager, and the First Street tunnel could be completed two years later. Engineers estimate the projects could reduce flooding depths by 20 inches.

Under previous time lines, the affected neighborhoods would not have seen significant relief until 2025.

Hawkins presented an outline of the plans to D.C. Water’s board of directors during a Thursday morning meeting. More details were discussed at an afternoon meeting of a flooding task force appointed by Mayor Vincent C. Gray (D).

Storing stormwater and a new tunnel could bring relief from flooding to the Bloomingdale and LeDroit Park areas.

“It looks like we’re getting there, like we’re almost there,” City Administrator Allen Y. Lew said before briefing task force members. “Not 100 percent, but a substantial, significant portion of the problem this area has experienced going back years.”

The area affected by the flash floods is an anomaly of geography and infrastructure. Low-lying to begin with, it is also a place where three major storm sewers’ draining points to the north and west converge into a single line that runs south under First Street to a trunk sewer running along Florida Avenue.

Floods have plagued the area intermittently for generations, but this year’s deluges were unprecedented. Four times, brief but intense storms caused significant flooding in the blocks surrounding Rhode Island Avenue and First Street. Sewage backed up into the basements or flowed from street level into about 200 households. Standing water approached two feet deep on some streets.

Storing runoff at the McMillan site, where much of the city’s drinking water was filtered and treated until 1985, is expected to relieve pressure on the First Street line during intense rains. The tunnel will provide additional relief — enough that the water from last summer’s most severe storm would have barely lapped the top of street curbs. But a full solution is not expected until the completion of a 23-foot-wide, east-west trunk sewer that would drain the First Street bore.

The proposal, in part, represents an acceleration of existing plans to build relief sewers, previously expected to be completed in 2025. Rather than wait to build the First Street tunnel, D.C. Water wants to start tunneling immediately, using the 19-foot bore to store runoff while the trunk line is built from the east.

Under the new plan, the entire project would be completed by 2022, three years earlier than previously contemplated.

Building the McMillan tanks is estimated to cost $15 million, and the First Street tunnel is expected to cost $130 million. Most of the latter figure is budgeted for as part of D.C. Water’s $2.6 billion Clean Rivers Project, which involves building massive tunnels to relieve sewage overflows into the Potomac and Anacostia rivers.

About $40 million remains to be funded. D.C. Water has been in talks with the Gray administration about covering that cost through the city budget, but District officials said Thursday that no final agreement has been reached. Hawkins told the D.C. Water board he expected the cost-sharing plan to “be consistent with what the board will approve, as well as the city.”

Complicating the relief plans is an effort to redevelop the McMillan site into a new neighborhood of residences, shops and offices. Recently, Gray issued an economic development plan that proposed a medical hub for the site, tying into the three hospitals immediately to the north.

Officials think that the flood-relief plan will not significantly alter the development plans. One filtration cell on the site’s northeast corner would capture flow from a storm sewer running along North Capitol Street; on the site’s western edge, another cell will be used to capture flow headed down First Street. Three acres at the southwest corner would be used for the tunnel-boring operation.

After the First Street tunnel is completed in 2016, the boring site would be available for development, except for a small area containing a maintenance shaft. The storage tanks could be dismantled and developed once the trunk sewer is completed.

Representatives of the development team tasked with preparing the site were skeptical of the flood-relief plan when it was floated in September, citing the fact that the 27-acre McMillan site is a protected historic landmark. But city officials think converting the old filtration tanks to hold stormwater is compatible with its historic use.

“They will be used for their original purpose, except they won’t be cleaning the water, they’ll just be holding it,” Hawkins said.

Although the sewers in Bloomingdale and LeDroit Park generally carry combined sanitary and storm sewage, the McMillan tanks would hold only storm runoff, he added. Retrofitting the tanks would involve removing the filtering sand and installing a flexible plastic liner and pump facilities.

Aside from the new infrastructure, Hawkins said, his agency will be redoubling efforts to encourage homeowners in the affected area to install backflow preventers, devices meant to prevent sewer overflows that threaten basements even if flooding never reaches street level.

The devices are crucial, he said, particularly because the larger-scale measures will not be in place before next year’s rainy season. “We are nervous about 2013,” Hawkins said. “We’ve got a year to get through.”

Only about two dozen of several hundred eligible households have applied to a rebate program thus far, Hawkins said. The program has been extended to March 31.

Teri Janine Quinn, a Bloomingdale community activist who serves on the flooding task force, said Thursday that she was “cautiously optimistic” about the proposal but was waiting to hear details on funding and implementation.

“But the key piece is, I am optimistic,” Quinn said before the task force met. “This is the closest thing to good news we’ve had in a while on this issue.” If the plan is viable, she said, “That would be a great Christmas present.”