The Situation
You have a 10,000-square-foot backyard (about a quarter acre) that floods every time it rains more than half an inch. The yard has a mix of grass lawn and a 600-square-foot concrete patio near the house. Your soil is clay loam, and water sits on the surface for hours after a storm. You want to know how much runoff your yard generates so you can size a proper drainage solution.
Which Calculator to Use
The Rational Method Calculator is the right tool for this job. It estimates the peak flow rate (how many cubic feet per second of water your yard produces during a storm) using three inputs: the runoff coefficient, rainfall intensity, and drainage area.
Walking Through the Inputs
Runoff Coefficient (C = 0.45)
The runoff coefficient represents how much rain becomes runoff versus soaking into the ground. Your backyard is a mix of surfaces:
- Grass lawn on clay soil: C = 0.35 (clay does not absorb water well)
- Concrete patio: C = 0.95 (almost all water runs off)
Since the patio is 600 sq ft out of 10,000 sq ft total (6%), the weighted average is approximately:
C = (0.35 x 9,400 + 0.95 x 600) / 10,000 = 0.39
We will round up to 0.45 to account for some compacted soil near the house and to provide a safety margin. It is always better to slightly oversize drainage than undersize it.
Rainfall Intensity (i = 4.0 in/hr)
This is the intensity of the design storm. For residential drainage, we typically use the 10-year storm — meaning the rain intensity you can expect to be exceeded about once every 10 years.
A 10-year, 15-minute rainfall intensity of 4.0 in/hr is typical for much of the eastern United States. You can look up your exact local value from NOAA Atlas 14 (search for your county).
We use the 15-minute duration because that approximately matches the time of concentration for a typical residential lot — the time it takes water to travel from the farthest corner of the yard to the drainage point.
Drainage Area (A = 0.23 acres)
Your backyard is 10,000 square feet. Since the Rational Method uses acres:
10,000 sq ft / 43,560 sq ft per acre = 0.23 acres
The Results
Plugging these values in:
Q = C x i x A = 0.45 x 4.0 x 0.23 = 0.41 cfs
That is 0.41 cubic feet per second, which equals about 185 gallons per minute. To put that in perspective, that is roughly the flow from two standard garden hoses running at full blast simultaneously.
What This Means for Your Project
With a peak flow of 0.41 cfs, you have several options:
- French drain: A 6-inch perforated pipe in an 18-inch-wide gravel trench at 1% slope can handle about 0.30 cfs of pipe flow plus additional capacity from the gravel itself. This is sufficient for your backyard.
- Surface grading: If you can regrade the yard to slope toward a discharge point, you may not need underground drainage at all. A 2% slope over 50 feet can carry well over 0.41 cfs as sheet flow.
- Catch basin and pipe: A single 12-inch square catch basin at the low point connected to a 6-inch pipe to the street can handle the flow.
Try This Scenario
Try this scenario in the Rational Method CalculatorNext Steps
- Use the French Drain Calculator to size a French drain for this flow rate
- Read our Why Is My Yard Always Wet? guide to understand all the potential causes
- Consider getting a soil percolation test to confirm your soil type — see Understanding Your Soil Type