Why Your Retaining Wall Has a Trench (The Scour & Void Pattern) and How to Fix It

The Dilemma

A homeowner recently asked:

I’ve noticed a deep trench forming alongside my railing and retaining wall, it's eroded so much that a sprinkler head is now completely exposed. I already have buried downspouts, so I thought the runoff was managed, but the soil is still washing away. What can I do to fix this permanently?

The GardenOwl Diagnosis

The Scenario

You walk out to check the yard and find a jagged, muddy ditch running right alongside your retaining wall or house foundation. You fill it with dirt. Two weeks later, after a heavy rain, the dirt is gone, and the ditch is back—only deeper this time. In this specific case, the erosion has gone deep enough to expose the plumbing of a sprinkler head.

This is a textbook case of The Scour & Void Pattern. It ruins curb appeal, destabilizes your hardscape, and if left unchecked, can compromise the footing of the wall itself.

The Trap

The mistake most homeowners make here is treating this as a "hole" that needs filling. They grab a shovel and toss more topsoil into the void.

Here is the reality: Water is lazy, but it is heavy.

When surface water flows down a slope (even a gentle one), it eventually hits a vertical barrier—in this case, your retaining wall. Since it can't go through the wall, it has to turn 90 degrees to flow along it. When water turns a corner, it accelerates. That acceleration creates a scouring effect, acting exactly like a high-pressure hose aimed at the soil line.

If you just put loose soil back in there, you are simply reloading the cannon for the next storm. The water will liquify that soil and transport it to the bottom of the hill immediately.

The Solution (Deep Dive)

To fix this, we have to stop fighting the physics and start working with them. We need to "armor" the channel.

1. The Diagnostic Check

Before you haul any rock, check that exposed sprinkler. In The Scour & Void Pattern, we often assume rain is the culprit, but a weeping irrigation head can saturate the soil 24/7, making it structurally weak before the rain even starts. Run the zone. If that head leaks, fix it first.

2. Clean the Wound

Don't just dump rock on top of the mess. Take a square shovel and clean up the trench. You want a defined channel, about 6 to 8 inches wide and 4 inches deep, running parallel to the wall. This isn't just for looks; you are creating a "keyway" for the stone to sit in so it doesn't roll down the hill.

3. Armor with Heavy Stone

This is the most critical step. Do NOT use:

• Mulch: It floats.
• Pea Gravel: It is too light and acts like ball bearings; it will wash away.
• Decomposed Granite: It will turn to mush in a flow zone.

You need 3-to-5-inch River Rock.

Why this size? It is heavy enough that the velocity of the water coming down the hill cannot pick it up. It is round, which creates voids between the stones, allowing water to trickle down into the subsoil rather than sheeting across the top.

Fill the trench you excavated with this stone until it is flush with the surrounding grade. You are essentially building a dry creek bed that acts as a shock absorber for the water hitting the wall.

4. The Result

Functionally, the rock breaks the energy of the water. Aesthetically, it creates a clean "mowing strip" so you don't have to run your string trimmer against the stucco wall (which ruins the wall) or the wood posts (which ruins the posts). It turns a drainage failure into a crisp architectural detail.

The Diagnostic and Visualizing Safety Net

It is hard to visualize how a strip of rock will look against your specific siding or wall material. Will it look like a drainage ditch, or will it look like a high-end design feature?

This is where GardenDream saves you from heavy lifting regrets. You can upload a photo of your erosion problem and overlay different textures—river rock, cobble, or slate—to see which material complements your home's exterior before you buy a single bag of stone. It acts as a safety net, helping you spot spatial constraints and material clashes instantly.

If you want to spot hidden opportunities in your own yard, upload a photo to our Exterior Design App to get an instant diagnosis and visualize the transformation.