Infiltration trenches can slow the flow of storm water off of your land, giving your soil more time to absorb it while simultaneously reducing the risk of polluting local water supplies.
One of the biggest causes of storm water runoff are the impermeable surfaces associated with homes. Rooftops and driveways make it particularly difficult for your soil to get the water it needs, directing precipitation off of your property and out into the street.
Luckily, there’s an easy way to slow the escape of precipitation. Well-placed and well-constructed infiltration or runoff trenches can keep storm water close long enough for the soil to absorb more before it flows away.
This project will take you through the steps of installing a simple infiltration trench next to your driveway or underneath the downspout of your roof’s gutters.
- Easy to intermediate
- 4-8 hours, depending on the dimensions of the trench needed
- Gravel or pebbles
- Permeable landscape fabric
- perforated PVC pipe or gutter, cut to the width of the trench (optional)
Step 1: Plan your trench
First, you’ll need to determine which way the ground slopes from the highest point of the trench you intend to dig. This can be done with a level, but an easier method might be to use your garden hose to pour water from the highest point, watching the path the water takes as it flows off of your property. That’s the direction your trench should follow. If you intend to feed the trench from a downspout connected to the gutters on your roof, start at the opening of the spout.
A width of about 6 to 8-inches should work fine in most cases. The depth of your trench will depend on the average rainfall you receive, as well as the height of the local water table at its seasonal high. If you’re not sure about either of those factors, contact your local county extension office. They may even be able to make recommendations about the dimensions of your infiltration trench.
To avoid weakening structures or contaminating your own drinking supply, don’t dig your trench within 10 feet of the foundation of a building or 100 feet from a drinking water well. Do not install infiltration trenches on inclines that require a drop of 1 foot in depth for every 7 in length.
Step 2: Dig the trench
Here’s where the elbow grease comes in. But before you start digging, a little math is called for. A width of about 6 to 8-inches should work fine in most cases. The depth of your trench will depend on the average rainfall you receive, as well as the height of the local water table at its seasonal high. If you’re not sure about either of those factors, contact your local county extension office. They may even be able to make recommendations about the dimensions of your infiltration trench.
It’s recommended that you make your trench no deeper than four feet above the water table at the height of the season. An 18-inch deep trench should be sufficient even for areas with reasonably heavy annual rainfall; in areas with relatively light rainfall, you can make your trench even shallower. If you’re not sure of the depth you need, stick with a shallow trench, no deeper than 12 inches.
In either case, be sure to call 811 to check the location of underground utilities before you dig to any depth greater than 6-inches.
Step 3: Fill the trench
Start by lining the walls of the trench with a permeable landscape fabric. That will prevent soil from the surrounding landscape from filling the gaps between stones. Fill the bottom of the trench to a depth of several inches with a layer of clean, coarse sand. Then fill the remainder of the trench with rocks or pebbles roughly 1 to 3 inches in diameter. A tamper can be used to make the stones less prone to shifting during storms, but don’t pack them too closely.
Step 4: Feed the trench
Any downspouts that might have otherwise emptied onto an impermeable surface can be redirected toward your runoff trench. To distribute the water more evenly along the width of the trench, you can affix a perforated length of pipe at the end of the downspout, burying just beneath the surface of the rocks if you find the pipe itself unsightly.
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