Last month's downtown flooding that brought the city of Charleston to a halt was both familiar to longtime residents and a harbinger of things to come.

When heavy rains fall at high tide, especially if the tide is higher than usual, flooded streets are practically guaranteed.

That's what happened the morning of April 28, when flooding shut down many of downtown's major thoroughfares, such as Calhoun, King, East Bay and Market streets. Floodwater also found its way onto some of James Island's primary streets, including Camp and Folly roads.

The low-lying city has grappled with poor drainage for centuries, and while it has spent tens of millions on improvements, other forces, including sea level rise and a slowly sinking land mass, make it challenging to get ahead of the problem.

Here are some basics to know about flooding in Charleston, and when the next major flood might be on the way.

Can the city flood without rain?

Peter Mohlin, metereologist for the National Weather Service, said nuisance flooding can happen when the tide rises to about 7 feet, which some call a king tide. Tides at 8 feet can undermine properties as well as exacerbate erosion.

"If it occurs with rain, the potential (for damage) is a lot worse," he said.

In 2015, Charleston experienced 38 days of tidal floods, up from an average of four days in the 1960s. Scientists have predicted tidal floods will affect Charleston for up to 180 days in 30 years, because of a 2½-foot rise in sea level expected over the next 50 years.

When are king tides expected this year?

The Weather Service predicts the tide in Charleston will reach approximately 7 feet on May 25 and 26; June 23 and 24; Nov. 4, 5 and 6; and Dec. 3, 4, and 5.

The tide might not rise to that level on each date predicted, and if it does, that doesn't necessarily mean it will cause flooding. 

"Even though that's what the astronomical forecast is ... it could be less extreme," Mohlin said. "There could be an off-shore wind, or we could be in a dry pattern when there's not as much water flowing through Charleston."

The tide is not expected to rise above 7 feet this year, he said.

What forces are making things worse?

New developments are supposed to address their own runoff, because if water-draining soil is paved, storm water has to be diverted somehow. But such work often isn't enough to handle a major event, such as a record rain or hurricane.

Meanwhile, the sea level is rising because of climate change, and the land is sinking due to the gradual tilt of North America's main tectonic plate.

What is the city doing about it?

The city currently is working on its largest drainage project ever across a 500-acre portion of the southwestern part of the peninsula, including the Septima P. Clark Parkway, also known as the Crosstown.

The $154 million, multi-year project will create a drainage system to send water to new 12-foot-diameter tunnels 140 feet underground. From there, the water will go to a new pumping station where three pumps capable of moving 160,000 gallons per minute will empty the water into the Ashley River, even when it's high tide.

Laura Cabiness, director of Charleston's Department of Public Service, said the city currently is relying on a network of older and much smaller pipes for drainage in the area, and that's why floods still shut down the Crosstown at times. The situation is likely to improve when the large tunnels are completed, and even more dramatically when they're connected to the pumps — likely in 2021. 

A similar, smaller-scale project on Market Street has three shafts and a tunnel in place that's currently connected to a pump station, but Cabiness said the area needs more inlet points where water can drain into the tunnel. That likely will be addressed when the city improves the surface-level drainage in a streetscaping project, which is expected to begin in December or January.

What about in West Ashley?

While West Ashley's main traffic arteries often don’t suffer as much as downtown’s when a heavy rain falls during high tide, some neighborhoods have experienced repeated problems.

The city is currently working to improve the gravity drainage system in the Forest Acres area near S.C. Highway 61 and Playground Road, which is expected to be completed next year. 

Meanwhile, a development moratorium is in place in the Church Creek area while engineers study the most flood-prone parts of that drainage basin over the next nine months. Their work will determine if the city's development policies adequately address drainage and what capital improvements might help. 

The city also is pursuing a federal grant to purchase and demolish 46 West Ashley homes that have repeatedly flooded, including 32 townhomes in the Bridge Pointe section of Shadowmoss Plantation.

What else needs to be done?

The upcoming Calhoun West/Beaufain Drainage Improvement Project will encompass the 300-acre drainage basin bounded approximately by King, Cannon and Beaufain streets — one of downtown's last major flooding spots. 

It's a historically flood-prone area because its low elevation and flat topography make it nearly impossible to drain water from the streets using conventional gravity systems. But work to collect data and design the new system is just beginning, and the city doesn't have a funding plan in place. 

"Just getting the funding is a process itself, and really somewhat drives the project, so it's difficult to say when it could be complete," Cabiness said. 

Another major project under way, the Low Battery seawall repair plans, will address the structure and design of the portion of The Battery wall stretching from White Point Garden to just south of Tradd Street. A team of consultants with Johnson, Mirmiran & Thompson is assessing ways to fortify and prepare the wall for the long-term effects of sea level rise, and that work could help protect the peninsula's southern edge.

Eventually, Lockwood Boulevard — the street paved mostly over marshland around the western edge of the peninsula — will have to be raised to stay above ground.

At some point down the road, Lockwood could evolve into more of a levee for that part of the city.

"We've seen the king tides topping Lockwood Boulevard," Cabiness said, "so it's going to be one of those edges of the city that's going to have to be raised and hardened to keep higher tides out."

Reach Abigail Darlington at 843-937-5906 and follow her on Twitter @A_Big_Gail.

Abigail Darlington is a local government reporter focusing primarily on the City of Charleston. She previously covered local arts & entertainment, technology, innovation, tourism and retail for the Post and Courier.