A Check on Tidal Street Flooding

Our Lowcountry city of Charleston has an intricate relationship with water – how we enjoy the harbor, rivers, and ocean must balance with how we plan for and accommodate weather events. One issue that we have long struggled with is that of tidal flooding in Charleston, often termed “sunny day” or “nuisance” flooding for the intrusion of brackish water not associated with rainfall.

While we are accustomed to the presence of standing water on some streets during a full moon, events in the past two years have brought this situation to a head – something had to be done about the increasing severity of tidal flooding.

A flooded street in downtown Charleston with stormwater flowing into a drain, with the steeple of St. Philips Church in the background

A City Designed for Tidal Flooding

The peninsula lies at the confluence of the deep Cooper River with the tidal Ashley River, combining to form Charleston Harbor. The height of these waters are coerced by phases of the moon, with higher tides brought about by full and new moons. Locals can glance at the marshes and tell you that it looks like a full moon high tide, often called a “king tide.” Increasingly often, these king tides inundate the streets of downtown Charleston with brackish waters from the harbor and rivers.

As Charleston established through the 17th and 18th centuries, the city and its built environment were designed and constructed with tidal flooding in mind. The city was established on high ground and as it grew into lower lying areas, homes were constructed with elevation. 

While the phenomenon of sunny day street flooding is nothing new for us, we in Charleston are seeing the effects of climate change occur right before our eyes with every full moon. A labyrinth of drains and tunnels under Charleston are intended to move stormwater out of the city, and here lies the culprit of tidal flooding: the water infiltrates the drainage system in the wrong direction, causing brackish water to seep up from the storm drains into the streets.

The interior of a brick arch drainage tunnel in Charleston

Stopping the Backflow

Charleston’s intricate network of underground aqueducts serve to  sweep and discharge deluges of rainwater from the city, and are diligently maintained and improved by the city. You can understand why this poses a conundrum – the very system designed to keep Charleston high and dry has become a point of vulnerability from tidal flooding.

The solution turned out to be remarkably simple: check valves.

Similar to smaller applications in plumbing systems, check valves, or backflow preventers, do exactly that – they permit water to flow in only one direction. A mechanism in the pipeline stays open when water flows out of the city, but a high tide pushing water into the aqueducts forces the valve to close shut.

First Steps

We have been cautiously optimistic that the installation of check valves were going to have a positive impact on tidal flooding in Charleston. With an initial focus on areas more prone to sunny day flooding – mostly in Harleston Village near Colonial Lake – the City has already installed a number of check valves in the stormwater tunnels along Ashley Avenue and Rutledge Avenue. Those of us that frequent downtown have already experienced a noticeable improvement: areas that had been regularly inundated during king tides are now high and dry, and tests indicate the check valves are holding strong.

All indications are that the City of Charleston intends to expand its use of check valves throughout the stormwater drainage system, including in West Ashley. It certainly appears that robust deployment of these backflow preventers would improve nuisance flooding throughout low lying areas, though we are well aware they are part of a much larger effort to address flooding problems in Charleston.

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