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Journal · Charleston infrastructure

A Check on Tidal Street Flooding

BY BRYAN McELVEEN
Published February 22, 2018 · Updated August 2026

Charleston has spent centuries figuring out ways to deal with water from every direction – often finding that managing one source makes the problem worse elsewhere. A remarkably simple solution is now being applied on the Peninsula and elsewhere to address an ancient conundrum.

A flooded street in downtown Charleston with stormwater flowing into a drain and the steeple of St. Philip's Church in the background

Charleston has a complicated relationship with water. The harbor, rivers, and ocean have been part of the city’s appeal from the start; they are also sources of danger that must be accommodated.

One recurring result is tidal flooding – often called sunny day flooding or nuisance flooding – when brackish water covers streets without a storm. We have movable barricades permanently stationed at problem intersections in Harleston Village, upper King Street, and elsewhere. These high water events are becoming more frequent and reaching farther into the streets, so the obvious question was how to keep the tide from traveling backward through infrastructure built to carry water the other way.

What this addresses

Check valves block tidal backflow through storm drains. They do not eliminate rainfall, storm surge, groundwater, or compound flooding.

The tide comes back

The peninsula sits between the Ashley and Cooper Rivers, where both meet to form Charleston Harbor. Astronomical tides rise higher around full and new moons; the especially high tides are commonly called king tides.

At ordinary water levels, gravity carries stormwater through drains, pipes, tunnels, and outfalls toward the harbor and rivers. During a sufficiently high tide, that relationship can reverse. Seawater enters an outfall, pushes into the drainage system, and finds the low points of the street network.

The old brick tunnels are a useful reminder that drainage is not a new Charleston preoccupation. The direction in which the water travels, however, can still produce surprises.

A historic brick drainage tunnel carrying water beneath Charleston

One way out

Charleston’s drainage network presents a municipal conundrum – the same pipes that move rainwater out can give tidal water a route back in.

The answer is remarkably simple: a check valve, or backflow preventer. It allows water to move in only one direction, so when stormwater is flowing toward an outfall, the valve opens. When a high tide pushes against it from the harbor side, reverse pressure closes the valve.

That one-way simplicity is both its strength and its limitation. A check valve can stop seawater from backing into the drainage system, but a catch-22 arises when a storm coincides with high tide: the valve must remain closed just when stormwater has the least room to drain.

How an in-line stormwater check valve responds to the tide Two-panel diagram. At low tide, stormwater flows from a street drain through an open check valve and into the harbor. At high tide, the valve closes and blocks seawater from moving backward through the pipe, while rainfall may have to wait for the tide to recede. LOW TIDE · VALVE OPEN HIGH TIDE · VALVE CLOSED Street drain Runoff drains to the harbor Gravity pushes water through the open valve. Street drain Tidal backflow is blocked Reverse pressure closes the valve. Rainfall can still wait on the tide. HARBOR HARBOR How an in-line stormwater check valve responds to the tide A two-part vertical diagram. At low tide, runoff moves from a street drain through an open check valve and into the harbor. At high tide, reverse pressure closes the valve and blocks seawater from entering the drainage system. LOW TIDE · VALVE OPEN Runoff drains toward the harbor. Open valve Gravity carries stormwater out. HIGH TIDE · VALVE CLOSED Tidal backflow is stopped at the outfall. Closed valve Rain may still wait for the tide to recede.

At low tide, runoff drains toward the harbor. At high tide, reverse pressure closes the in-line valve and blocks seawater. Rainwater may still have to wait for the tide to recede. Diagram based on the City of Charleston’s Check Valve Program.

First steps

We were cautiously optimistic that check valves would make a practical difference in areas especially prone to sunny day flooding. The City’s early work concentrated around Colonial Lake in Harleston Village and South of Broad, with valves installed in stormwater lines along Ashley and Rutledge Avenues.

Those of us who frequented downtown had already noticed a change: streets that had been regularly covered during king tides were now staying dry (more or less), and the early tests suggested the valves were holding.

Workers handling a large black pipe containing a check valve, preparing to install it under a street in Charleston.

2026 update

Where Check Valves Fit Now

Charleston’s check valve program continued after this article was published. As of August 2026, the City reports more than 22 new in-line valves on the Peninsula and in West Ashley, with additional locations under consideration.

These devices are useful against tidal water entering storm drains from the wrong direction. Rainfall and storm surge are separate problems, and rain arriving at high tide may still drain slowly because the receiving water is already elevated – or because a closed valve is correctly holding seawater out.

Check valves therefore belong inside a much broader floodwater strategy that also addresses drainage capacity, pumps, tunnels, groundwater, storm surge, and behind it all the sea levels rising. The overarching strategy has become more complicated, but fortunately valve mechanics did not.

Current program details: City of Charleston Check Valve Program