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How It Works
This calculator uses the curb-gutter form of Manning's equation to relate flow rate, surface roughness, cross slope, longitudinal slope, and flow spread width. Highway engineers use it to ensure stormwater does not encroach into travel lanes. The flow width (spread) check is the most common design use.
Example Problem
An asphalt roadway (n = 0.016) has a cross slope of 0.02, longitudinal slope of 0.01, and a flow width of 6 ft. What is the gutter flow rate?
- Q = (0.56/0.016) × 0.025/3 × 0.011/2 × 68/3
- Q = 35 × 0.001366 × 0.1 × 58.09
- Q ≈ 0.28 ft³/s
Frequently Asked Questions
What is flow spread in gutter design?
Flow spread (T) is the width of water across the gutter and roadway surface, measured in feet. AASHTO standards typically limit spread to keep it off travel lanes, often to 6–10 ft depending on road classification.
What Manning's n should I use for asphalt gutters?
Smooth asphalt uses n = 0.013; rough asphalt uses 0.016. Concrete gutters with asphalt pavement typically use 0.015. These values come from FHWA HEC-22.
How does longitudinal slope affect gutter capacity?
Flow rate increases with the square root of longitudinal slope. Steeper roads move water faster to inlets, but very flat grades (below 0.5%) can cause ponding between inlets.
Related Calculators
- Manning Equation Calculator — the general open-channel flow formula this gutter equation derives from.
- Time of Concentration Calculator — determine peak runoff timing for inlet spacing.
- French Drain Design Calculator — size subsurface drains that complement surface gutters.
- Hydraulic Radius Calculator — compute Rh for triangular gutter cross-sections.
- Hazen-Williams Calculator — size storm drain pipes downstream of gutters.
- Length Unit Converter — convert between feet, meters, inches, and other length units.