Seven bridges over Interstate 93, seen at the center of this overhead view, are due for weekend repairs this summer in what state officials say could have been a five-year project. The lowest bridge affected is at Route 16.

Two projects with radically different impacts were described Monday to city councillors easing back in to a post-holiday political calendar: traffic improvements promising little effect on Cambridge and sewer and storm drainage improvements promising the opposite.

The projects were described in an informal roundtable format, with no votes taken.

The sewer separation project set to begin in summer will tear up and rebuild streets in an area roughly the size of Fresh Pond — and across Route 16 from Fresh Pond in the Huron and Concord Avenue neighborhood — in three steps to be complete in 2016.

The work is intended to end flooding in the area and ensure only clean water reaches Boston Harbor, but it will have side effects good and potentially bad, including the discovery of problems with homeowners’ sewage systems that could demand the flushing of up to $20,000 from a resident’s bank account.

But don’t panic: “I haven’t seen instances where it’s cost more than a couple thousand dollars,” said Owen C. O’Riordan, city engineer and assistant commissioner of engineering, answering Mayor David Maher’s questions toward the end of the discussion.

And certainly not every homeowner will have to do anything.

As roads are torn up and pipes put in to take only sewage, removing it from the current everything-in-one pipes running beneath the city, homeowners will be reconnected on the city’s dime. But when digging up systems to hook up to the new line, workers may find problems: intruding roots in the pipes from one house, a partial collapse in the pipes from another building.

“Sometimes we find pretty wild things that have occurred,” said Richard Rossi, the deputy city manager. The existing systems were built between 1880 and 1940.

Since it’s far cheaper to fix things while the road is already dug up, people should get in touch with contractors as soon as they learn of a problem, O’Riordan said. The city maintains a list of licensed contractors ready to work.

“We don’t push people that hard,” he said, estimating homeowners could have weeks or months to fix a problem.

Neighborhood benefits

In the meantime, residents will be consulted in redesigning their neighborhoods to say how wide streets should be and where steps should be taken to slow traffic; where sidewalks might be extended to wrap around trees, providing a walkway and avoiding removal; and whether there’s a need for “rain gardens,” planted areas designed to absorb rainfall and runoff and help prevent floods.

There are already separated water and sewer pipes under Route 16, also known as Fresh Pond Parkway, and similar work is ongoing in Whittemore Avenue, according to O’Riordan and other Public Works presenters.

Next up is the so-called Huron A area, with Route 16 to the west and Brattle Street to the south, where design is under way and construction begins in summer. The Huron B area, which goes to Appleton Street on the east, begins design in a year and construction in the summer of 2013. Finally comes the Concord Avenue area, which goes as far north as Bay State Road and Garden Street and will be designed in January 2013 and dug up in the fall of 2014.

The project’s total cost is about $100 million, with $60 million paid by the state and the rest from local water and sewer fees, said Lisa Peterson, the city’s Public Works commissioner.

When the project is done, the city should also have better-quality drinking water and better water pressure — and be halfway to the goal of total sewer separation, with the timeline for 100 percent completion in the far, hazy future. Still, O’Riordan said, “I don’t think you’ll find anybody else as aggressive as we are” in pursuing sewer separation, in part a legal obligation from the Massachusetts Water Resources Authority (because Alewife Brook is a tributary to the Mystic River and Boston Harbor) set in 1985. “What we’re doing is over and above that.”

Separating sewage, which will be sent to Deer Island for treatment, should eliminate 50 million gallons per year of combined sewage that overflows into the brook from the current single pipe.

“Everything is fine when it’s a dry day just like today and the sun is shining. But when we have rain or snow melt,” water that collects from parking lots, sidewalks, roofs and so on all go into the same pipe, said sewer maintenance engineering project coordinator Catherine Woodbury. “That pipe is finite in size, and what happens is … it ultimately overflows and gets discharged untreated into the receiving waters. Depending on the size of the rain event, [the water] will find its way back onto the streets and basements of people’s homes and businesses.”

Speedy bridge repairs

Also discussed Monday was the “Fast Fourteen,” a state plan to repair seven Interstate 93 bridges — 14 roads altogether, since traffic flows in two directions — all in the upcoming summer. Rapid deterioration in the bridges from Route 16 all the way up to Valley Street calls for equally rapid weekend construction, Massachusetts Department of Transportation officials told the councillors.

The rush in what could be a five-year project means the project is going ahead without the state’s usual elaborate studies. But the new bridges are intended to last for 75 years, the officials said.

The other goal is to keep traffic flowing , which calls for no work on the Fourth of July, no closings of the interstate at rush hour, a goal of never closing more than one local road at any time and extensive traffic advisories to be issued well into Connecticut.

People will be directed to other transportation options, including with a sophisticated and flexible project website, and could even stick with those options beyond the construction period. “If you’re headed to the beach, there are buses and trains that go to the beach,” said Eliza Partington, a state transportation engineer. “So why not get out of your car, save a little money, avoid construction and hopefully develop some good habits over time?”

Ideally, commerce and tourism will be unaffected by the roadwork, Partington said.

Although there were no specific studies to point to, state officials said Cambridge would face “minimal direct impact.”

City Manager Robert W. Healy — who uses the interstate for his daily commute — agreed.

“Cambridge does not become a cut-through if you’ve made the mistake of not using an alternate route and chose to stay on 93,” Healy said. “I don’t see a lot of interference in Cambridge traffic.”