PROVIDENCE — It rained so hard Thursday morning that for the first time all year, the state’s massive, $359-million combined sewer overflow tunnel finally filled to capacity and allowed untreated sewage to spill into the city’s rivers.

At one point Thursday morning, sewage and storm water were measured roaring into the tunnel at a rate that would have amounted to 680 million gallons a day, if it continued all day. Good thing it didn’t, because that’s more than 10 times what the tunnel can handle.

Between 7 and 8 a.m., nearly an inch of rain came down. That helped bring the total for the previous 24 hours to nearly 3 inches.

The storm also brought the first bad day for the tunnel since a major storm came through in December, soon after it opened.

The overflows Thursday are a counterpoint to what generally seems to be considered a success.

Since the first of the year, the CSO tunnel has captured 740 million gallons of sewage and storm water and sent it to the Fields Point plant for treatment.

Before the CSO was completed, the sewer system averaged 60 to 70 overflows annually. The system was designed to handle everything up to what is termed a three-month storm: 1.65 inches of rain in 6 hours.

Despite all the rain in June — there were 17 days of measurable rain in Providence, tying a record set in three other years — there were no overflows all year until Thursday.

In fact, even though the state closed 13 bathing beaches to swimming and two ponds to shellfishing Thursday, many of those closures were due to the pollutants swept off the land by rainfall rather than by sewage.

Point Judith Pond and Winnapaug Pond in South County were closed to shellfishing because sampling in 2005 showed bacteria levels would rise to unacceptable conditions after significant rains, according to Joseph Migliore, at the state Department of Environmental Management. Neither is affected by sewer plant discharges, and other ponds have remained safe.

Aimee Parris, beach coordinator for the state Health Department, said that despite the wet June, there have been fewer beach closings this year than in previous years, and she suspects that is due to the CSO being on-line. Storm-water pollution of the beaches remains a significant problem, she said.

Some observers are already crediting the CSO with improvements to the Providence River and upper Narragansett Bay. They say the water is clearer and there are fewer closed beaches and shellfishing areas.

“In general, the river has looked terrific this year,” said Save the Bay’s baykeeper, John Torgan, who gets out on the water about two days a week. “There is a lot less plastic and litter and trash, and the water color is generally better already.”

Those were some of the goals when federal authorities began pushing in the late 1970s for solutions to the pollution that occurred every time storms overwhelmed the combined storm and sanitary sewers built a century ago in Providence.

The state, with voter approval, took over the system in 1980 and began studying ways to collect polluted wastes from the dozens of overflows along the city’s rivers.

It considered digging up all the sewers, disrupting virtually every street in the city. It also considered mini-treatment plants along the rivers, but those too would cause short-term disruption and long-term risks with their tanks of chlorine and other chemicals.

Finally, engineers settled on creating a massive underground reservoir, drilling a 30-foot-wide tunnel through three miles of bedrock 300 feet under the city. The tunnel would collect overflows that would later be pumped up to the treatment plant.

The new system went into operation in November. The tunnel can hold 65 million gallons. Officials at the Narragansett Bay Commission agreed in advance to test that capacity on Thursday. Little did they know.

On Wednesday the tunnel collected 35 million gallons of wastewater. Overnight it was pumped down to 27 million gallons. But on Thursday, it filled up to more than 60 million. Everything worked fine, but at some point in the morning, the gates were closed and the system was allowed to overflow.

Next year, the commission expects to start Phase II of the CSO project, spending $200 million on big interceptor pipes along the Woonasquatucket and Seekonk rivers. Later, it expects to spend $350 million on Phase III, connecting Central Falls and Pawtucket to the Bucklin Point treatment plant in East Providence with a system of pipes and tunnels.

Meanwhile, several state agencies this year plan to step up their efforts to help coastal communities reduce storm-water pollution.

The DEM has published a manual on storm-water runoff and its staff are visiting communities and helping to prepare local plans to reduce the problem.

For more information on beach conditions, go to www.ribeaches.org.

For weekly reports on Bay conditions, go to www.dem.ri.gov/bart/

To learn more about the CSO tunnel, go to www.narrabay.com.

For more information about storm water pollution go to www.ristormwatersolutions.org.Beach closings

•Because of high bacteria counts, the Rhode Island Department of Health Thursday recommended closing the following beaches to swimming:

 Town beaches in Barrington, Bristol, Warren and North Kingstown, as well as beaches at Camp Fuller in South Kingstown; City Park, Oakland Beach and Goddard Memorial State Park in Warwick; Peabody’s Beach in Middletown; Sandy Point Beach in Portsmouth; and Scarborough State Beach (both north and south) in Narragansett.

 Beaches at Camp Canonicus in Exeter and Conimicut Point in Warwick are still closed.

To check on the status of beaches, call (401) 222-2751 or visit the beach closures page of the Health Department’s Web site.

plord@projo.com