Potential plans for the construction of a roundabout at Highway 305 and Day Road could involve the development of an iconic piece of open space on Bainbridge Island.

The newest design plans for a roundabout at Highway 305-Day Road call for a stormwater detention pond to be built on the undeveloped pastureland at the southwest corner of the intersection.

The scenic land is currently protected from development by a conservation easement, but according to officials with the Washington State Department of Transportation, part of the property will need to be purchased or condemned for a roundabout to be built.

A large detention pond would be constructed there, officials said, due to the desire to expand the park-and-ride lot on the other side of the highway, but also because space is needed on the west side of the highway for a 10-foot-wide piece of the Sound to Olympics Trail.

While a different design scenario would allow stormwater detention infrastructure to be built elsewhere — for example, using the park-and-ride land entirely or in concert with the adjacent right-of-way — officials said not using the protected open space property would result in higher overall construction costs for the roundabout that may doom it entirely.

The state has been studying potential safety improvements along Highway 305 between the Bainbridge ferry terminal and Hostmark Street in Poulsbo.

Highway 305 has been the subject of study since 1997, but a more detailed look at possible highway improvements began in 2017.

The 305 project has a budget of $36.5 million, and potential changes to the highway are expected to improve traffic and safety along the busy 305 corridor.

A series of 11 roundabouts along the highway has been proposed, and the Highway 305/Day Road project has been listed as one of the higher priorities in the budget for the improvements.

The Bainbridge council was briefed on the latest designs for the intersection at its meeting Tuesday night.

According to a memo by Bainbridge Island city staff for the meeting, three development options are under consideration for the new roundabout at the intersection.

“One of the key reasons that the Day Road intersection has been given the green light for improvements ahead of the other intersections along the corridor is because the intersection rankings prioritized congestion; transit, access; safety; non-motorized; and environment (in that order), and Day Road is the only intersection project that includes a significant transit enhancement — expansion of the existing park-and-ride facility,” the memo says.

“Unfortunately, the irony is that the park-and-ride expansion is exacerbating one of the project constraints — the limited ability to cost-effectively mitigate stormwater runoff,” said the memo, prepared by Public Works Director Christopher Wierzbicki.

The location of the stormwater treatment infrastructure will impact the cost of the project, and Wierzbicki noted the Department of Transportation is looking at three options:

• Converting the park-and-ride to a stormwater pond (not currently included as this configuration would likely lead to a re-ranking of the projects due to the loss of the significant transit component);

• Building a stormwater pond on the adjacent property (Option 1 — property is currently subject to a conservation easement, and will require purchase or condemnation);

• Building smaller stormwater pond on the adjacent property and making up the balance of the stormwater detention needs with an underground detention vault in the park-and-ride facility (Option 2, plus $1.6 million);

• Building two underground detention vaults — one in the park-and-ride lot, and one in the adjacent right-of-way (Option 3, plus $2.6 million).

Wierzbicki also noted in his memo that the Department of Transportation was asked that if a stormwater detention pond was indeed built on the land protected under an easement owned by the Bainbridge Island Land Trust, that the pond be built to resemble an “environmental/stormwater restoration area.”

“WSDOT has indicated that they will take this comment into consideration, but they have not yet provided further information about how this area could be constructed as a natural drainage feature,” he added.

Wierzbicki also said in his memo that all three roundabout development plans would require the acquisition of land at the southwest corner of the intersection, on land protected by the easement held by the Bainbridge Island Land Trust, so that the trail could be built for bicyclists and walkers.

“WSDOT has indicated in previous discussions that they do not foresee any other design options that would accommodate the desired non-motorized facilities without impacting the subject property on the southwest corner,” Wierzbicki said in the memo.

Wierzbicki said that the costs of Option 3, where stormwater detention infrastructure would not be built on the protected open space, may make the project cost prohibitive.

He added, however, “the Day Road improvements will reduce congestion and add non-motorized facilities, leading to significant reductions in greenhouse-gas emissions. The improvements will also double the park-and-ride facility, thereby further reducing greenhouse gas emission through the expanded use of public transit.”

After the council’s briefing this week, several members of the council said they didn’t like the proposal to put a detention pond on the protected land.

Councilman Joe Deets said he preferred Option 3, which he said was the less intrusive approach.

While Deets acknowledged that design would be more expensive, he added, “I’d really like to get this right.”

Ponds, he said, “don’t have a good reputation, lets just say,” Deets said.

Councilman Ron Peltier also noted the importance of the iconic landscape to the Bainbridge community.

“I know that people are going to be really upset when they see some excavation going on in that field,” Peltier said.

Michele Britton, the project manager for the Department of Transportation for the 305 improvements, noted during her description of the options that the eventual size of the pond wasn’t exactly known.

“It could be smaller; it could be a little bit larger; this is our best estimate,” Britton said.

Britton also said the detention pond wouldn’t be an eyesore, and displayed pictures of water-detention facilities built by the state or other entities.

“You’ve all seen some pretty ugly ponds. We’ve built some nice ones, too,” she said.

And while most WSDOT detention ponds that have been built have been surrounded by chain-link fencing, Britton said that was open to discussion.

“It’s all things we can talk about,” Britton said.