Charleston is making a relatively small investment, less than half a million dollars so far, to keep some streets from flooding on sunny days.

The strategy involves installing new check valves, a specialized insert into a drainage pipe that lets rainwater flow out toward the harbor but prevents high tides from flowing back in.

And it's apparently working.

Ryan Glushkoff works from his home on Ashley Avenue, across from Cannon Park, where five of these new check valves were placed last month.

"Since the installation, we haven’t seen it flood once despite there being that crazy moon last week," he said. "We didn’t see any flooding from that.”

A neighbor, Adam Sumner, agreed.

"After having our house flooded by Irma, losing two cars and enduring three years of constant worry every time it rained, it has been a pleasant surprise to see the city actually do something to help the problem," he said. "The jury is still deliberating, but, so far, especially with the recent super moon, we have remained dry."

Despite this success, the new valves are no silver bullet to the city's well-publicized drainage woes.

"All they do is prevent tidal intrusion," said Frank Newham, a senior engineering project manager with the city. "If it rains and you've got a high tide, you're still going to have flooding."

Eleven of the first 17 valves were recently installed downtown, while six others were installed along Rebellion Road and William Ackerman Lane in West Ashley's South Windermere neighborhood.

Newham said the city soon plans to test whether similar valves might reduce sunny day flooding around Morrison Drive, near Sanders Clyde Elementary; at Barre and Wentworth streets; and around Drake and South streets near Wraggborough Homes.

He said it's easy to test to see where new check valves might work: Its contractor simply clears out a drainage line and inserts a rubber balloon into the pipe, blocking any flow. Testing is done on only dry days during relatively high tides. If the test shows a positive result, then a check valve is ordered.

"This is something we need to explore all over the city," Newham said. "I'm going to keep doing this until somebody tells me to stop."

In June, the National Oceanic and Atmospheric Administration reported that Charleston saw 50 days of tidal flooding in 2016, snarling traffic, sending water up toward homes and posing a general nuisance.

That number was up from 38 days in 2015 — the previous record — and NOAA's figures for 2017 are expected to show another high number, if not another record, when they're released later this year.

As NOAA oceanographer William Sweet said in June, "Charleston keeps breaking its historical record for high-tide flooding."

Some of the areas most affected by sunny day flooding are the low-lying streets around Colonial Lake, where eight check valves already have been put in place.

The city already has begun engineering work for a major underground drainage tunnel project there that would ultimately solve the problem and improve drainage when a heavy rain arrives at high tide.

Known as Calhoun West, the project would replace the city's current shallow network of drainage pipes with a deep tunnel and pumps. That project is many years away; the city has not identified all its financing sources for a job expected to cost about $200 million.

Meanwhile, the check values are much cheaper. The hardware pieces cost between about $3,000 to $10,000 each.

Each valve is custom-made to fit snugly inside an existing drainage pipe. Newman said the city's total cost for replacing two check valves along Murray Boulevard was $50,000 each, while it spent only $39,000 to install five new valves west of Cannon Park.

"If we can spend $40,000 to keep Ashley Avenue from flooding, that's huge," he said.

Check valves have been around for a few decades, but Newman said the city is using a new version that is better. The older version was a duckbill-shaped rubber tip at a pipe's outfall, but Newham said those would deteriorate over time from being exposed to the sun's UV rays. The newer ones never see the light of day.

The old check valve at Rutledge and Murray Boulevard famously failed after Hurricane Matthew, preventing flooding around Colonial Lake from draining back into the harbor when the storm passed and the tide dropped. City crews had to bring in pumps to dry out the streets.

Reach Robert Behre at 843-937-5771. Follow him on Twitter @RobertFBehre.