Scenario: Grading a Swale Along a Property Line

The Situation

Your neighbor’s property is higher than yours, and surface runoff from their yard flows across the property line into your yard during every rain event. The affected stretch is about 80 feet long. You want to construct a grass-lined swale along the property line to intercept the runoff and redirect it toward the street without creating a dispute with your neighbor.

The combined drainage area (the portion of the neighbor’s yard that slopes toward you plus the strip of your yard) is about 6,000 square feet. The soil is silt loam, and you can achieve about a 1% slope along the swale toward the street.

Which Calculator to Use

The Swale Calculator determines the required cross-section dimensions (bottom width, side slopes, and depth) for a vegetated channel to carry the design flow without erosion.

Walking Through the Inputs

Design Flow (Q = 0.28 cfs)

Using the Rational Method:

• Drainage area: 6,000 sq ft = 0.14 acres
• Runoff coefficient: C = 0.40 (mix of lawn and some compacted soil)
• Rainfall intensity: i = 5 in/hr (10-year, 10-minute intensity for a small area)

Q = 0.40 x 5.0 x 0.14 = 0.28 cfs

Longitudinal Slope (S = 1.0%)

The swale will slope at 1% toward the street. This is measured along the length of the swale, not the side slopes. A 1% slope is ideal — steep enough to move water but gentle enough to prevent erosion in a grass-lined channel.

Side Slope (z = 4)

A 4:1 side slope (4 feet horizontal for every 1 foot vertical) is the standard for mowable grass swales. This gentle slope is:

• Safe to mow across
• Stable for grass establishment
• Easy to maintain

Steeper slopes (like 3:1) are harder to mow and more prone to erosion before grass establishes.

Bottom Width (b = 2 ft)

A 2-foot flat bottom provides a stable base and is wide enough for a mower. Narrower bottoms tend to erode into V-shaped channels.

Vegetation Type (Grass, mowed)

The swale will be grass-lined and regularly mowed. This gives a Manning’s n of approximately 0.030 for short grass. Taller, unmowed grass (n = 0.050) would slow the water more but requires a larger channel.

Swale Length (L = 80 ft)

The full length of the property boundary where the runoff crosses: 80 feet.

The Results

The calculator shows:

• Required flow depth: Approximately 3-4 inches
• Flow velocity: Approximately 1.0-1.5 ft/s
• Freeboard available: Significant (the swale has much more capacity than needed)

With a 2-foot bottom width, 4:1 side slopes, and a total depth of 6-8 inches, this swale can handle the design flow comfortably. The velocity of 1.0-1.5 ft/s is well below the maximum permissible velocity for grass-lined channels (about 4-5 ft/s), so erosion will not be a problem.

What This Means for Your Project

This is a grading project, not a pipe project. You are essentially creating a shallow, wide depression along the property line:

• Cross-section: 2-foot flat bottom with 4:1 slopes on both sides
• Total top width: About 6 feet (2 ft bottom + 2 ft each side at 6-inch depth)
• Depth: 6-8 inches below surrounding grade
• Soil to move: Approximately 5-7 cubic yards

The swale will look like a gentle dip in the lawn — barely noticeable when mowed. Water will flow through it during rain and the grass will dry quickly between events.

Construction Steps

1. Mark the swale centerline with stakes and string
2. Excavate to the cross-section profile (rent a small skid steer for large areas, or dig by hand)
3. Verify the slope with a laser level or transit
4. Compact the subgrade
5. Add 2-3 inches of topsoil
6. Seed with a durable grass mix (perennial ryegrass + tall fescue works well)
7. Mulch and water until established

Estimated Cost (DIY)

• Topsoil (2 cubic yards): $60-100
• Grass seed (5 lbs): $25-40
• Straw mulch: $15-25
• Stakes and string: $10
• Total: $110-175 (plus your labor)

Try This Scenario

Next Steps

1. Before constructing, talk to your neighbor — a swale benefits both properties and they may share the cost
2. Check local setback requirements — some jurisdictions have minimum distances from property lines for grading work
3. If the runoff is severe, consider adding a French drain under the swale for additional capacity