What a French Drain Does

A French drain is a subsurface system consisting of a perforated pipe surrounded by gravel, installed in a sloped trench. Water enters the pipe through its perforations and travels to a designated outlet, typically a dry well, retention area, or connection to an approved discharge point. Because it operates below the surface, a French drain is particularly effective at intercepting groundwater and subsurface moisture, which makes it a strong option for high water table conditions common near Wilmington's tidal creeks and marshland.

French drains typically cost more than swales, generally running $20 to $35 per linear foot for a standard exterior system, with foundation perimeter installations costing more due to deeper excavation near the structure. Installation involves trenching, laying pipe, and backfilling with gravel, all of which adds labor cost compared to a surface solution, though most of that cost reflects the precision required rather than the materials themselves.

What a Swale Does

A swale is a shallow, sloped channel graded into the landscape that guides surface water across a property toward an approved discharge point using nothing more than the shape of the land. Unlike a French drain, a swale doesn't require pipe, gravel, or a trench dug into the subsurface, which generally makes it the more affordable option, typically costing $0.50 to $1.00 per square foot.

Swales work best when a property has at least a modest existing slope and the drainage problem involves surface water moving across a wide area rather than groundwater seeping up from below. They also tend to blend more naturally into a landscape, since a well-designed swale can look like a subtle depression rather than an obvious drainage feature.

The Trade-Offs Worth Considering

French drains excel at handling groundwater and high water table conditions, and because they're buried, they don't take up visible yard space or require ongoing landscaping maintenance beyond occasional flushing to prevent sediment buildup. Their downside is higher upfront installation cost and, if something goes wrong underground, a repair can require re-excavating the trench, which adds cost down the line if the original installation wasn't done correctly.

Swales cost less upfront and are simpler to install, but they require a usable slope to function and need periodic regrading or maintenance as they can erode, silt in, or lose effectiveness over time if not properly maintained. They're also less effective for handling groundwater specifically, since they only manage water moving across the surface.

Neither option is universally better. In many cases, a property benefits from a combination of both, using a swale to manage surface runoff across a larger area while a French drain handles a specific groundwater problem near a foundation.

How to Decide Which Fits Your Property

The right choice depends on what's actually causing your drainage problem. If water is seeping up from below, pooling in a low spot regardless of surface grading, or affecting a foundation directly, a French drain is usually the better fit. If water is running across your yard from a higher point during rain and you have room to grade a visible channel, a swale is often the simpler and more cost-effective solution.

A drainage assessment during or shortly after a rain event is the most reliable way to determine which type of water movement is actually causing your problem, since guessing based on symptoms alone can lead to installing the wrong solution.