EVERETT — The city has unveiled a 10-year plan for its aging flood-prone sewer system.

The Comprehensive Sewer Plan, revealed Wednesday at a meeting of the City Council, will cost more than $320 million over the next decade, with sewer and stormwater rates nearly doubling in that time.

The City Council is to hold a public hearing on the plan and the new rate structure, then vote on both, at its Dec. 17 meeting.

The plan is the least expensive of three options the city has considered since intense flooding in 2013 prompted an examination of the combined sewer and stormwater system in the north part of the city, Public Works Director Dave Davis told the council.

Completely separating the city’s sewer and stormwater system — 145 miles of pipe — was estimated to cost up to $1 billion and take 30 years. A smaller program would have cost half as much and still would have taken 20 years to complete.

“It’s not the billion-dollar program we’ve shown you earlier — it’s not the half-billion-dollar program, either,” Davis said.

The program would increase capacity, fix deteriorating pipes, end sewer and basement flooding, upgrade the treatment plant, reduce discharges of untreated waste into Port Gardner, implement more environmentally friendly practices and programs, and make the overall system more reliable and redundant.

Portions of the combined sewer and stormwater systems will be separated as aging pipes are replaced.

Rates are set by a separate ordinance the City Council adopts as part of the annual budgeting process. The new ordinance would set rates for single-family residences at $54.42 per month starting next July 1, rising to $59.87 on Jan. 1, 2016.

Future rates would rise between 5.2 percent and 10.2 percent per year, topping out at an estimated $110 per month in 2024.

The total bill for all city utilities for a single-family home is expected to rise from $83.33 per month today to $88.99 on Jan. 1, to $92.31 July 1, and to $102.34 on Jan. 1, 2016.

Also starting July 1, the city will move from bi-monthly to monthly billing for utilities.

The sewer plan is anchored by 10 big projects, such as the $18 million “Sewer M” upgrade and separation project in the Northwest neighborhood, to be built starting in 2015, and the $11 million East Grand Project, the second phase of which will be built starting in late 2015.

Other projects will enter the design phase next year — among them a new utility and pedestrian bridge over West Marine View Drive to the waterfront ($10 million), the “Sewer N” separation project in the northern part of the Northwest neighborhood ($6 million) and a new chlorine building and upgraded pump station at the sewage lagoons ($5 million).

The latter project is intended to address a problem that occurs during extended heavy rain, when the lagoon fills faster than wastewater can be pumped out and has to be discharged directly into Port Gardner, Davis said.

“If it gets to the point where our lagoon is at maximum, we don’t have anywhere else to put it,” Davis said.

Longer-term projects in the plan include expanding the wastewater treatment plant ($87 million over 10 years), putting in more redundancy on mains and interceptor pipes in key locations, and building new earthquake-resistant buildings for the public works staff, the emergency operations center and shops.

Another element of the plan is building a stormwater storage facility on Port Gardner, which during peak events will store water to prevent it from inundating the sewer system. The city will have to purchase the land, and that might be at the site of the former Kimberly-Clark mill on the waterfront.

The city is suing Kimberly-Clark Corp. over the cleanup of the mill site, however, so a purchase would come after the lawsuit is resolved.

A cost estimate for that purchase wasn’t being disclosed, Davis told the council, because “we weren’t willing to show Kimberly-Clark what we were willing to pay for their property.”

The plan also includes costs for operations and smaller projects, such as the city’s programs for backwater valve installation, water meter installation, rain barrels, water gardens and the like. The total estimated cost for everything is $322.8 million over 10 years.

Chris Winters: 425-374-4165; cwinters@heraldnet.com. Twitter: @Chris_At_Herald.