An improved $200 million drainage system is coming to 17th and Folsom streets to protect against flash floods that have menaced residents and businesses for more than a decade. The catch: it will take six years to build.

The San Francisco Public Utilities Commission will dig a 3,500-foot-long tunnel that is roughly 12 feet in diameter. It will begin near Treat and Harrison streets and run to the existing sewer boxes near Mission Creek. An additional 9,000 feet of sewer improvements will be made in the area, primarily to route water to the tunnel.

The improvements are meant to help prevent flooding during a storm that delivers 1.3 inches of rain within three hours — a level of rainfall delivered by a so-called “five-year storm.”

The Folsom project is part of $700 million in capital improvements for high-flood-risk areas. The commission will spend a total of $1.6 billion on more improvements over time.

The Folsom project is still in the planning phase, but construction is expected to begin in 2020 and wrap up in 2023.

The improvements aimed at 17th and Folsom will not protect against storms larger than a five-year storm, said Stefani Harrison, a project manager with the city’s utilities commission, who on Wednesday gave a presentation at a Board of Supervisors’ Public Safety and Neighborhood Services Committee hearing on the flooding issue.

“Even when all these projects are built, the same neighborhoods will still have risk of flooding in storms that are larger than [a 5-year] storm,” Harrison said.

Which is why the utilities commission is offering residents and businesses grant assistance for making suggested improvements on their property. In certain cases, the commission might be willing to buy a piece of property from a landowner at market value.

The utilities commission will be offering grant assistance to residents and businesses who make plumbing modifications, elevate their structures, and install wet and dry floodproofing on their buildings.

But at the meeting, some supervisors felt the grant program was not enough.

“I look at the grant program — raising houses, doing wet and dry protection — and wiping it down, it’s just not attractive,” said District 8 Supervisor Jeff Sheehy, referring to the fact that some residents might have to wipe down floodwater filth in wet-proofed areas of their homes after a flood.

Sheehy then probed deeper into the suggestion that the PUC purchase properties from residents whose homes experience flooding. He referred, especially, to the homes on Cayuga Avenue near Balboa Park that have, in the past, become inundated with sewage water after large storms.

“We are definitely interested in purchasing property,” said Harlan Kelly, general manager of the utilities commission, noting that the commission is working with the city attorney, and that it would buy homes at “fair market value.”

That includes homes in the 17th and Folsom area.

The home-acquisition program is in its early stages, Harrison wrote in an email, “but the City is willing to have discussions with interested/willing property owners who have experienced flood damage, as any acquisition would be on a case-by-case basis.”

“You’ve laid out a clear set of interventions that you’re willing to make and to work with homeowners that are affected, up to purchasing the property,” said District 9 Supervisor Hillary Ronen. “Echoing my colleagues, that’s probably going to be the right option if individuals are willing to sell.”