Why Your Drains Slow Down on a Sunny Day: Charleston's Tide Problem

The two hour rule

Start with the single most useful fact, because it turns a mystery into something you can predict.

The City of Charleston puts it plainly: "The height of the tide has one of the greatest impacts on how quickly stormwater will drain from the City." And more specifically, "Rain events that occur within two hours of high tide will drain significantly slower" ².

Two hours either side of high tide is the window. If your yard is holding water, the street is ponding, or the downstairs shower is slow, and you are inside that window, you have learned something real about the cause before anyone has picked up a wrench.

Why the tide reaches your drains at all

The mechanism is not mysterious once somebody explains it, and almost nobody does.

Most of the city's stormwater outfalls discharge into water that is itself tidal. So when the tide is high, the system has nowhere to put anything. In the City's words, "At high tide, much of the stormwater collection system (the pipes and ditches) is already full of sea water leaving little room for the stormwater runoff," with the result that "The stormwater that has collected on the surface has no place to go because the pipes and ditches are full and ponding occurs" ².

It can also run the other way. During high tides "seawater often enters stormwater outfalls and water can back up far enough in low lying areas to result in backflow flooding on streets, even on a sunny day" ³.

That last phrase is the one worth holding onto. Water on the street with a clear blue sky overhead is not a mystery and it is not a burst main. It is the harbor, arriving through the drain.

The sanitary side, which is the one inside your house

Here is where we have to be precise, because the two systems get talked about as though they were one and they are not.

Everything above is the stormwater system: streets, gutters, ditches, outfalls. It explains a ponding yard and a flooded intersection. It does not, by itself, explain a slow toilet.

The pipe that carries waste out of your house is the sanitary sewer, and the tide reaches that one differently, through what Charleston Water System calls infiltration and inflow.

Infiltration is when "groundwater enters sanitary sewer pipes through cracks, joint failures, holes, faulty connections, etc." ⁴. A high tide raises the groundwater table. Higher groundwater presses harder on every crack in every buried pipe in the area, including yours.

Inflow is "direct flow of stormwater into sanitary sewers via sources such as incorrect roof drain downspout or foundation drain connections, interconnections with storm sewers, holes in sanitary sewer manhole covers, etc." ⁴.

Both put water into a system that was never sized to carry it. CWS is direct about the consequences: excessive infiltration and inflow causes "increased wastewater transport/treatment costs" and "system capacity issues," and contributes to "sanitary sewer overflows (SSOs)" ⁴.

CWS also names the trend explicitly. "Rising tides increase the frequency and intensity of tidal flooding, which can lead to saltwater intrusion into gravity-based wastewater systems," and that saltwater "accelerates corrosion and reduces system capacity" ⁴.

So the chain is: tide up, groundwater up, more water squeezing into cracked pipe across the neighborhood, less spare capacity in the sewer, and less room for whatever leaves your house. Your own lateral is the last and lowest link in that chain, which is why you feel it first at the lowest fixture in the building.

Last year was quiet. This year is not forecast to be.

This is the part that changes what you should do about it now rather than in the spring.

NOAA measures high tide flooding by meteorological year, May through April. For the year that just finished, the news was good: the Southeast "experienced a median of nearly 7 flood days over the 2025-2026 meteorological year, within the predicted range of 6 to 9 days," and NOAA singles out Charleston as "experiencing their lowest flood counts since 2018" ¹.

A quiet year is easy to read as a solved problem. It is not the year you are in.

For the meteorological year now running, NOAA's outlook is that "NOAA water level stations across the country are predicted to experience a median of 7 to 12 high tide flood days, a new record" ¹. That figure is a national median rather than a Charleston number, and it is worth stating that plainly rather than borrowing a scarier statistic than the source supports.

The driver is a returning El Nino, and NOAA is specific about the timing: those conditions have "the potential to increase high tide flooding during the Northern Hemisphere's cool season (October to April)" ¹.

October to April. That window opens in a few weeks, and it runs straight through the season when most people stop thinking about water.

What is actually yours to fix

Not much of the above is your responsibility. Two things are, and both are worth knowing.

A cracked lateral is yours. The pipe from your house to the public connection belongs to you, and every crack in it is a place groundwater gets in when the tide pushes it up. A line that behaves in dry weather and misbehaves on high tides is describing itself fairly precisely. That is a camera job rather than a guessing job, and we have written about telling a fixture clog from a main line problem, which is the first thing to establish.

A downspout or foundation drain plumbed into the sanitary sewer is yours too. CWS lists "incorrect roof drain downspout or foundation drain connections" as a cause of inflow ⁴. If your gutters discharge into the sewer rather than to grade, you are loading the same pipe that has to carry your own waste away, at exactly the moment it has least room. That is a genuine and fixable own goal, and older Charleston properties have it more often than you would think.

Worth adding for completeness: a sump pump discharging into the sanitary sewer is the same mistake in a different jacket.

What a camera actually settles here is worth knowing, because a tidally influenced fault does not look like an ordinary blockage. A cable machine restores flow and tells you nothing about why the line only misbehaves on high water. A camera inspection shows the joints and cracks that groundwater is using, and where they sit relative to the water table. The useful timing is a dry spell rather than a king tide, because a line full of infiltrated water hides the very defects you are looking for.

What the city is doing, and what to report

The City has been fitting check valves to outfalls. A check valve "prevents seawater from backing up into drainage infrastructure to mitigate tidal flooding, while still allowing the outfall to drain stormwater as usual when the tide recedes" ³. Charleston "has installed over 22 new in-line check valves in West Ashley and the Peninsula and has plans for more," with the program set to extend to James Island and Johns Island ³.

If the flooding you are seeing is on the street rather than in your house, that is the City's Citizen Services desk on 843.724.7311 ³, not a plumber. Telling those two apart saves everyone a callout.

What to do when the tide is high

Practical, in the order that helps.

Check the tide before you diagnose. If the trouble tracks high water, note the times for a few days. That pattern is worth more to whoever comes out than any description of the smell.

Do not add water to a full system. If the sewer is struggling on a high tide, running laundry, dishwasher and showers at once is the worst possible timing. Wait two hours.

Keep your cleanout accessible. If it ever does back up, that is where the work starts, and hunting for it in standing water is a bad use of an hour.

Do not assume it is your pipe, and do not assume it is not. Both are common. The test is whether it behaves when the tide is out.

When to call us

Call if sewage is coming up inside the building, if more than one fixture is affected at once, or if the same backup has now happened on more than one high tide. That last one is the pattern that says your lateral has a defect the tide is finding.

A single slow drain on a king tide, clearing by itself as the water drops, is usually the system telling you it is full rather than your pipe telling you it is broken. Worth watching, not worth panicking about.

We run 24 hour emergency plumbing across Charleston, North Charleston, Mount Pleasant and Columbia. If it is not urgent and you want the line looked at properly before the cool season, book a service call and we will find out whether the tide is finding a fault in your pipe or simply filling the system it drains into.

We are a licensed South Carolina plumbing contractor, CLM.117414.