Plans are taking shape to improve stormwater drainage in the Willows subdivision this summer.

But the total estimated cost has grown to $512,000, public works staffers told the Kalispell City Council on Monday.

That’s about $65,000 more than previously estimated, partly because of a need for directional drilling and deep trenching and the need to pump water out of a construction area with shallow ground water, Public Works Director Susie Turner said.

A preliminary engineering report by Morrison-Maierle requires formal city council approval. With that and a couple of other steps, the project can go out for bid and come back to the council for a contract award and final approval of financing terms.

“We’d like to advertise in early July and start construction in the end of July or early August with completion in October,” Turner told the City Council. “We’d like to do the construction in August and September when the groundwater is low and the river is low.”

The project would overhaul the neighborhood’s stormwater detention basin. The basin has steep, eroding sides and was built in a way that makes access difficult for maintenance crews and equipment.

The project will install a new 18-inch discharge pipe to handle 100-year stormwater flows in the area. Almost two-thirds of the stormwater coming into the area is from a county-owned property south of the neighborhood.

Plans also call for an improved stormwater outfall into the Stillwater River and two hydrodynamic stormwater treatment devices to remove oil, sediment and trash before water is discharged.

City officials inadvertently approved the subdivision’s development with an inadequate 12-inch discharge pipe, so City Council members agreed to help pay for the project. When that pipe clogged with willow roots and flooded the neighborhood several years ago, it raised the stormwater drainage issue among residents.

When the neighborhood’s stormwater drainage system is brought up to city standards, the city will agree to maintain it.

Funding plans show stormwater impact fees would pay for about 10.6 percent of the estimated $411,000 construction costs. Money from city’s annual stormwater property tax assessments would pay for $228,000 of the project.

Residents in the Willows agreed to form a special improvement district to help pay for the upgrades. They would pay $20 a month of additional property taxes to pay off a 15-year, $242,000 loan from the Montana State Revolving Fund.

Reporter Tom Lotshaw may be reached at 758-4483 or by email at tlotshaw@dailyinterlake.com.