The torrential downpour in September brought by Tropical Storm Lee turned quickly into a perilous situation for the town of Occoquan.

Residents saw water gushing from Tanyard Hill Road into the town’s center, carrying garbage, debris and tree trunks along the way. It is lucky, they say, that no one was seriously hurt and that damage was minimal.

Because the town sits on the Occoquan River, it has had its share of floods and scares in the past. But September’s inundation wasn’t caused by the river but by a relatively small creek called Ballywhack that overwhelmed an area culvert, flooded back yards and ran down the hill toward the town.

It was a stark reminder of the perils the town faces when managing the storm water system, which has been an issue for years — and it speaks at least partly to why town officials and residents are afraid that a proposed nearby development, Oaks III, could further imperil the town. The Prince William Board of County Supervisors is scheduled to take up the issue Tuesday.

The Oaks III proposal isn’t a huge development. It’s a single, 32,500-square-foot office building and one house proposed for the corner of Tanyard Hill and Old Bridge roads, just outside the town. Town residents and elected officials say that they had hoped to persuade developer Ken Thompson to improve the project and that they thought he didn’t take their concerns seriously.

James Phelps lives on Union Street, which becomes Tanyard Hill Road. His house was flooded badly in September, when water rushed through his yard.

Although he is concerned about his property, he said, it’s the town he worries about most if the storm water situation worsens. He and others also are concerned about the traffic — described as a “nightmare” during rush hour — on Tanyard Hill, a two-lane road that goes directly into the town.

“I have a deep and abiding love for this area, and I’m torn about this because I want the county to grow,” Phelps said. “But . . . this is not the right project.”

If the project is approved, “I don’t believe that the town will recover,” he said.

Thompson, of Ken Thompson & Associates, who has developed in the county for decades, said the advanced storm water management system he has proposed would trap water better than the natural land does. He also said an independent traffic study called for only 11 cars traveling on Tanyard Hill — most of the traffic will go to Old Bridge Road, which connects to Woodbridge and the surrounding area, he said.

“When people are against something, they have to talk about something,” he said.

His proposal includes rain barrels, cisterns and an underground storm water management system that exceeds the town and Chesapeake Bay requirements, he said.

Further, Thompson points to the county’s long-range planning document, the Comprehensive Plan, which calls for 150,000 square feet of office space and about 40 homes on the site. His project is considerably smaller. He is promising to place about 14 acres of the site into a conservation easement with the rights held by the Northern Virginia Conservation Trust, meaning that he or another landowner could not build there, he said.

Board Chairman Corey A. Stewart (R-At Large) said that he has not made a final decision but is inclined to support the project because more commercial space is good for the economy, and the project is smaller than those proposed in past years.

“I think what everybody needs to understand is, it’s not a choice between this development and nothing,” Stewart said.

The town and county officials also tried to persuade Thompson from going forward with the development by offering a deal.

Officials said they would look to speed approvals for a parking lot Thompson wants for two other office buildings sitting adjacent to Oaks III; the lot would be located on the Oaks III site. Thompson could then place much of the rest of the property into a conservation easement, which would allow him to take advantage of state tax credits and a federal tax deduction. He could sell the land, worth less with its restricted development rights, to the county and the town, which would split the estimated $300,000 cost, under terms of the deal discussed by officials.

“If this gets approved by the Board of Supervisors, they’ll be rewarding a developer for all the types of behavior the county says they’re trying to discourage,” said Mayor Earnest W. Porta.

Thompson said the deal did not make “economic sense,” and although the county’s tax dollars were promised in discussions, he had no indication the county would be bound to any agreement.

“I’ve looked into it and evaluated it,” Thompson said. “The project does not work.”

Supervisor Michael C. May (R-Occoquan), who has been coordinating with the town and the developer, said he shares the town’s concerns but wouldn’t make a decision on the development before Tuesday’s scheduled public hearing.

Gary Savage, who owns a restaurant and inn at the town’s center, said he’s been busy running his restaurant and hasn’t paid too much attention to the proposed development. But he’s aware of its perils, he said, remembering the September flood.

“It looked like the Colorado River coming down here,” Savage said. “It was nuts.”