The No. 1 strategy to prevent water runoff from washing the gravel off a driveway is to install an appropriate drainage system. The characteristics of the system you need depend on the topography, but in most cases you'll want a French drain that runs down the middle or edge of the driveway with one or more branch drains to divert the collected water to a safe place. If runoff is heavy or the driveway has a depression in which water collects, you'll probably also want one or more channel drains to collect extra water. Using the right type of gravel and sloping the driveway appropriately also help keep the small stones in place.
Surveying the Problem
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If you look at the pattern of the washout, you'll get a good idea of how the water is flowing and what you need to do to contain it. Gravel may be missing from the crest of a hill and and be collecting in piles at the bottom, or you may have rivulets streaming across the driveway on a cross slope. You can control water that flows along your driveway with a French drain, and culverts can usually handle water flowing across it.
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Installing a French Drain
A French drain consists of a perforated pipe embedded in gravel inside a trench. You can dig the trench down the middle of the driveway or along the edge, but either way, the driveway must slope toward it so water drains into it and doesn't just form a different stream.
Procedure
Step 1
Clear gravel away from the section of the driveway where you're going to dig the trench and pile it up on the other side.
Step 2
Dig a 6- to 8-inch-wide trench about 12 to 18 inches deep. You'll need to rent a power trencher to do this unless your driveway is very short. Dig trenches that branch away from the main one at strategic points as topography dictates. The branch trenches should go well off the driveway, maintain a slope away from it and end on ground that slopes away.
Step 3
Fill the bottom of the trenches with one or two inches of 1/2-inch drain rock. Lay 3- or 4-inch perforated drain pipe in the middle of the main trench. Install a tee on the main pipe at the intersection of each trench that branches off and connect nonperforated drain pipe to them. Connect fittings such as tees and drainage pipes by pushing them together until they click -- no glue is needed. Lay the nonperforated drain pipe in the middles of the branch trenches to carry away the water.
Step 4
Backfill the trenches with 1/2-inch drain rock using a shovel. When the drain rock has reached the level of the driveway, lay geotextile sheet on top of it before replacing the gravel. The geotextile sheet prevents dirt and silt from seeping into the drain and clogging it.
Installing Culverts and Channel Drains
A culvert is a length of drain pipe that goes under the driveway and provides a path for water on the higher side to flow downhill. Although you can use 3- or 4-inch pipe as a culvert, remember that small-diameter pipes will quickly clog with leaves and debris and need frequent clearing. It's usually more efficient to use 6-inch or wider pipe. When digging the trench, remember to keep it sloped toward the downhill side of the driveway.
The trench you dig for a channel drain must also slope toward the driveway's downhill side. This type of drain, which consists of a grate strong enough to withstand vehicles and an outlet you connect to a drain pipe, collects water that pools on the driveway and routes it away.
Slope Toward the Edges
If you install a French drain in the middle of the driveway, then you want the trench to form a slight depression so water flows into it. If you don't have a French drain, though, you want the driveway to have the opposite slope -- the center should be slightly higher so water flows toward the edges. If the driveway shows signs of slight erosion in the middle, add more gravel and use a plate compactor to shape it with a slight hump in the middle.
Choose the Right Gravel
The driveway may be losing gravel simply because you aren't using the right kind. Gravel with round edges, such as pea gravel, won't stay in place no matter how well the driveway drains. The best driveway gravel is angled, which allows individual rocks to lock together instead of slipping around each other.