Drain Tile Size Calculator
Calculate the required drain tile pipe diameter from drainage area, drainage coefficient, and pipe slope using Manning's equation for agricultural tile design.
Grassed Waterway Design Calculator
Calculate the design flow for a grassed waterway using the Rational Method from drainage area, rainfall intensity, and runoff coefficient.
acres
in/hr
%
ft
ft
Design Flow (Q)⧉
96.0 cfs
Rational method: Q = C x i x A = 0.40 x 3.0 x 80
Flow Depth⧉
2.38 ft
Total depth with freeboard: 2.88 ft
Top Width⧉
14.3 ft
With freeboard: 17.3 ft
Flow Velocity⧉
5.65 fps
EXCEEDS max 5 fps - use stronger lining
Channel Area⧉
16.99 sq ft
Hydraulic radius: 1.129 ft
Froude Number⧉
0.646
Subcritical flow (stable)
Total Excavation⧉
906 cu yd
For 1200 ft waterway length
Lining Cost⧉
$2,840.00
18,936 sq ft at $0.15/sq ft
Velocity Check
Velocity
5.65 / 5 fps
Design Flow by Watershed Area
| Watershed (ac) | Peak Flow (cfs) | Relative Flow |
|---|---|---|
| 10 | 12.0 | |
| 25 | 30.0 | |
| 50 | 60.0 | |
| 100 | 120.0 | |
| 200 | 240.0 | |
| 500 | 600.0 |
Lining Material Comparison
| Lining | Manning n | Max Velocity (fps) | Cost ($/sq ft) | Est. Total Cost |
|---|---|---|---|---|
| Grass (n=0.035) * | 0.035 | 5 | $0.15 | $2,840.40 |
| Dense Turf (n=0.040) | 0.04 | 6 | $0.25 | $4,734.00 |
| Riprap (n=0.045) | 0.045 | 12 | $3.50 | $66,276.00 |
| Concrete (n=0.015) | 0.015 | 18 | $8.00 | $151,488.00 |
| Bare Soil (n=0.025) | 0.025 | 3 | $0.00 | $0.00 |
Planning notes, formulas, and examples
About the Grassed Waterway Design Calculator
Grassed waterways are broad, shallow channels seeded to grass that safely convey concentrated surface runoff through farm fields, preventing gully erosion. Proper sizing ensures the waterway handles the design storm without overtopping or eroding.
The Rational Method (Q = C × I × A) is the most common approach for small agricultural watersheds. It combines the runoff coefficient (C), rainfall intensity (I) for the design storm, and drainage area (A) to estimate peak flow.
This calculator applies the Rational Method to determine the peak flow rate your waterway must carry, then estimates the channel cross-section needed based on allowable velocity for grassed channels. Use this page to turn drainage area and storm assumptions into a starting waterway size before layout or engineering review.
When This Page Helps
Under-sized waterways overtop and erode, defeating their purpose. Over-sized waterways waste farmable land. This page helps match channel size to runoff volume before land is committed to the practice.
How to Use the Inputs
- Enter the drainage area in acres.
- Enter the design rainfall intensity in inches per hour.
- Enter the runoff coefficient (C) based on soil and cover.
- Read the peak design flow in cfs.
- Enter the allowable velocity to estimate the needed cross-sectional area.
Formula used
Q (cfs) = C × I (in/hr) × A (ac)
Required cross-section:
A_channel (sq ft) = Q / V_allowable
Where:
C = runoff coefficient (0.2–0.7)
I = rainfall intensity for design frequency and duration
A = drainage area
V = allowable velocity (3–5 fps for grassed channels)Example Calculation
Result: Q = 42 cfs; Channel Area = 10.5 sq ft
Q = 0.35 × 3.0 × 40 = 42 cfs. Required channel area = 42 / 4 = 10.5 sq ft. A parabolic channel 8 ft wide and 2 ft deep provides about 10.7 sq ft — adequate.
Tips & Best Practices
- Use a 10-year 24-hour storm for most NRCS waterway designs.
- Runoff coefficients: cultivated land 0.30–0.60; pasture 0.10–0.45; woods 0.10–0.30.
- Allowable velocity: 3–4 fps for newly seeded channels; 5–7 fps for established grass.
- Parabolic cross-sections are most common for grassed waterways.
- Include a freeboard of 0.3–0.5 ft above the design water depth.
- Maintain grass stand in the waterway; mow but do not cultivate.
Time of Concentration
The time of concentration (Tc) is how long it takes runoff from the most distant point to reach the waterway inlet. It determines which rainfall intensity to use: longer Tc means lower intensity for a given storm frequency. NRCS TR-55 provides methods to estimate Tc.
Channel Design
Once Q is known, design the channel cross-section so flow velocity stays below the allowable limit for grass cover. Use Manning's equation (Q = 1.486/n × A × R^(2/3) × S^(1/2)) with n = 0.035–0.050 for grass to check velocity and depth. Iterate until the design fits.
Maintenance
Mow waterways at least once a year to maintain a dense grass stand. Repair any rills or bare spots promptly. Re-seed damaged areas and protect newly seeded sections from runoff until established. Well-maintained waterways last decades.
Sources & Methodology
Last updated:
Frequently Asked Questions
C represents the fraction of rainfall that becomes runoff. It depends on soil type, slope, cover, and moisture condition. For silt loam cropland at moderate slope, C is about 0.35–0.45.
Use NOAA Atlas 14 or your state IDF curves. Select the design storm frequency (10-year is standard) and the time of concentration for your watershed. Intensity = rainfall depth ÷ duration.
Parabolic is most common: wide and shallow for stability. Trapezoidal is used where space is limited. Avoid deep, narrow channels that concentrate erosive flow.
No. The grass cover is essential for erosion protection. You may mow it but should not till, plant crops, or drive heavy equipment through it. Leave it as permanent grass.
Depend on climate: brome, fescue, or bluegrass in cool regions; Bermuda grass, bahia, or buffalo grass in warm regions. Use NRCS species recommendations for your area.
The Rational Method gives peak flow rate only for small watersheds (<200 ac). The SCS Curve Number method produces a hydrograph (volume and timing) and works for larger watersheds. For small waterway design, the Rational Method is simpler and sufficient.
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