6 Swale Construction Mistakes To Avoid That Prevent Costly Washouts

You watch the dark clouds gather, and that familiar knot tightens in your stomach. Another gully-washer is on its way, and you can already picture the topsoil from your garden washing down the slope. Swales are supposed to be the answer—a brilliant way to turn that destructive runoff into a landscape-hydrating asset. But a poorly built swale is worse than no swale at all; it’s a ticking time bomb that can cause catastrophic washouts, undoing years of hard work in a single storm.

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Understanding Swale Function and Design Basics

A swale is nothing more than a shallow ditch with a mound of soil, called a berm, on its downhill side. The key, and the part most people get wrong, is that it’s built perfectly on a contour line—a level line across the landscape. Its job is not to move water, but to stop it.

Think of it as a long, skinny pond that fills up during a rainstorm. By capturing the runoff and holding it level, the swale gives the water time to slowly soak into the ground. This recharges the soil moisture, hydrates the plants growing on the berm, and prevents the erosive force of water rushing downhill.

This is the absolute opposite of a drainage ditch. A ditch is built with a slight slope to get water off your property as fast as possible. A swale is built with zero slope to keep every precious drop on your property. Confusing the two is the first step toward a major failure.

Failing to Survey and Mark a True Contour Line

This is the single most critical mistake you can make. If your swale is even slightly off-contour, it ceases to be a swale. It becomes a ditch that concentrates water at its lowest point, building pressure until it blows through the berm and carves a massive gully down your hill.

You don’t need expensive survey equipment to get this right. Simple tools like an A-frame level or a water level (sometimes called a bunyip) are incredibly accurate and easy to make. The process involves starting at one end and "walking" the level across the slope, placing a flag at every spot that is perfectly level with the last one. This line of flags becomes your guide for digging.

Do not trust your eyes. What looks level to you can easily have a one or two percent grade, which is more than enough to turn your water-harvesting feature into an erosion disaster. Taking the time to properly mark a true contour line is the foundation of a successful swale system. It is non-negotiable.

Placing Swales in Poorly Drained Soil Types

Swales are a brilliant tool, but they aren’t the right tool for every situation. They are designed to infiltrate water into the subsoil. If your subsoil is heavy, compacted clay that drains incredibly slowly, you might be creating a new problem while trying to solve an old one.

In soils with poor drainage, a swale can become a stagnant, waterlogged trench for days or even weeks after a rain. This can drown the roots of the trees and shrubs you plant on the berm, turning your productive system into a failure. It can also create a perfect breeding ground for mosquitoes right in your backyard.

Before you dig, do a simple percolation test. Dig a hole about a foot deep where you plan to build your swale, fill it with water, and let it drain completely. Fill it again and time how long it takes for the water to disappear. If the water is still standing there 24 hours later, you may need to consider alternative strategies like terracing or raised beds, as swales might do more harm than good.

Building Undersized or Uncompacted Berms

The berm is the dam. It’s the structure responsible for holding back potentially thousands of gallons of water. Building it too small or with loose, fluffy soil is asking for a blowout.

When you dig a trench, the soil you excavate is "fluffed up" with air. As it settles over time with rain and gravity, it will lose significant volume. A common mistake is to make a berm that is the same size as the trench, only to have it shrink and be easily overtopped. As a rule, the volume of your freshly built berm should be at least 25% larger than the volume of the trench to account for compaction and settling.

Furthermore, that berm needs to be compacted as you build it. Don’t just pile the soil loosely. After every few shovelfuls, walk on it, tamp it down, or run a lawn tractor tire over it. A firm, compacted berm is far more resistant to the erosive power of sloshing water and is less likely to slump or fail during its first big test.

Neglecting to Plan for a Level Spillway System

No matter how well you design your swale, a massive, once-in-a-decade storm can overwhelm it. You must plan for this. A spillway is the designated "emergency exit" that allows excess water to exit the swale safely without destroying the berm.

The worst thing you can do is let the water find its own exit. It will inevitably find the lowest, weakest point in the berm, breach it, and carve a destructive channel downhill. A planned spillway prevents this by giving the water a controlled path.

A proper spillway is a wide, shallow, and perfectly level section at the end of the swale, or sometimes in the middle, designed to direct overflow toward a stable area or the next swale down the slope. By making it wide and level, you encourage the water to "sheet flow" out as a gentle sheet rather than a concentrated, erosive torrent. Reinforcing your spillway with rock or establishing dense sod can add another layer of security.

Digging Trenches Instead of Shallow Swales

There’s a temptation to think that deeper is better—that a deep trench will hold more water. This is a dangerous misconception. A swale should be a wide, shallow, saucer-shaped depression, not a deep, narrow V-shaped trench.

Deep trenches create several problems. They concentrate immense water pressure at the base of the berm, dramatically increasing the risk of a blowout. They also create a safety hazard, can be difficult to cross with equipment, and can even be a drowning risk for small farm animals.

A wide, gentle profile is far more effective and stable. It spreads the weight of the water over a larger area, reducing pressure on the berm. It also maximizes the surface area between the water and the soil, which is the entire point—you want to encourage infiltration, and a wider channel does that better than a deep one.

Leaving Freshly Dug Berms Bare and Exposed

You’ve spent hours or days carefully digging on contour and compacting your berm. The absolute worst thing you can do now is walk away and leave that pile of bare soil exposed to the elements. A single hard rain can wash away your new berm before it ever has a chance to do its job.

Your first priority after shaping the berm is to cover it. This is non-negotiable. The two best options are:

• Mulch: Cover the entire berm immediately with a thick layer of wood chips, straw, or old hay. This "armor" will absorb the impact of raindrops and prevent the soil from washing away while you wait for plants to establish.
• Seed: Rake the surface and broadcast a fast-growing cover crop mix, like oats, clover, or annual ryegrass. The quick-germinating seeds will create a living root mat that locks the soil in place.

Ultimately, the goal is to establish permanent plantings like fruit trees, berry bushes, and perennial herbs on the berm. Their roots will become the permanent reinforcement that holds your earthwork together for decades. But in the short term, that bare soil must be covered immediately.

Maintaining Swales for Long-Term Success

A swale is not a one-and-done project. It’s a living system that interacts with your landscape, and it requires occasional maintenance to function properly over the long haul. Ignoring it for years is a slow path to failure.

The main tasks involve keeping the system working as designed. Silt and organic matter will naturally wash into the channel over time, slowly filling it in and reducing its water-holding capacity. Periodically, you may need to scoop this rich material out and place it back on the berm. You also need to inspect the berm for low spots caused by settling or animal burrows and add soil to bring it back to a consistent height.

The best practice is to walk your swales at least twice a year, especially after a major storm event. Look for signs of trouble: Is water overtopping the berm anywhere other than the spillway? Is the spillway clogged with debris? Is sediment building up in one area? Catching these small issues early and making minor adjustments is far easier than repairing a major blowout after the fact.

Building a swale is about shifting your mindset from fighting water to partnering with it. By avoiding these common but costly mistakes, you transform a potential source of erosion into a powerful engine for building soil, hydrating your landscape, and creating a more resilient and productive hobby farm. Get the details right, and your swales will pay you back for decades to come.