Garden coordinator Jeffrey Wankel tended the rooftop garden at Bread For the City in 2011. (Amanda Voisard/THE WASHINGTON POST)

Not excited by succulent shrubs or porous pavement?

Some people are — including D.C. Mayor Muriel E. Bowser and D.C. Water chief executive and General Manager George Hawkins, who on Wednesday sat next to an acre of rainwater-capturing plants and signed an agreement that will, among other things, bring more of them to the city.

The D.C. Water and Sewer Authority presides over the District’s drinking water but also its storm water and waste removal.

Under the District’s existing 19th-century system, sewage and rainwater run together underground in a “combined sewer pipe.” Sometimes, including last Monday, the network fills up and casts dirty water into the city’s rivers — and even its streets and basements.

The D.C. Clean Rivers Project aims to change all that. The modifications to the $2.6 billion project signed Wednesday, which have been under development since 2011, call for a redesign and extension of a planned Potomac River drainage tunnel, elimination of a planned Rock Creek drainage tunnel, and a large-scale plan for “green infrastructure” development around Rock Creek Park and the Potomac.

The key words there, according to D.C. Water, are green infrastructure.

“Green infrastructure” means things such as porous pavement, cisterns, rain gardens and roof gardens — like the one atop the Fort Reno Reservoir that hosted Wednesday’s news conference. All are meant to capture and clean rainwater while also cooling temperatures and, in some cases, making places look a lot prettier.

The project is not without cause for skepticism. To start, it will run five years longer than originally planned, and it won’t be completed until 2030. (Hawkins said that some structural components will be accelerated by the new plan.)

The other concern is that green infrastructure is still a relatively novel and experimental concept in American cities.

“The mathematics of how much storm water you capture in a tunnel is very simple,” Hawkins said. How much storm water does a roof garden or porous pavement capture? Well, that’s less quantifiable.

But city officials think the new plan will do that better and faster than an earlier plan that relied mostly on the development of “gray” infrastructure, or drainage tunnels. The Justice Department and Environmental Protection Agency also signed on to the new plan; the federal agencies have been involved since the project was created in 2005 as a result of a federal-city agreement to clean up local waterways.

The amended plan “brings in this modern era” and “puts the District of Columbia in a leadership role in green infrastructure in the nation,” said John Cruden, the deputy attorney general for the environment division of the Justice Department.

The amendments, Cruden added, are “coming out of citizen involvement — and we desperately want it to work. Because if it does, it will set a best practice for the rest of the United States, and will achieve something that very few other cities in the United States have done.”

Bowser’s administration added another component to the plan: the goal that 51 percent of jobs created by green infrastructure go to District residents.

To be clear: It’s a goal, not a requirement.

“We can’t legally bind D.C. Water for a requirement,” said D.C. City Administrator Rashad Young. But the plan includes various provisions that will help D.C. Water and its contractors reach the goal of hiring more D.C. residents, he said. The program will also involve green jobs training and certification opportunities for District residents.

That could mean close to 100 new green jobs for D.C. residents by 2030, according to Bethany Bezak, the project’s green infrastructure manager.

The plan will go through a 30-day period of public commentary before it is finalized, Bezak said.