Storm drainage pipes carry rainwater from developed surfaces to detention facilities or receiving waters. Undersized pipes cause flooding, erosion, and property damage. Oversized pipes waste money on materials and excavation. The design process determines how much water arrives at each pipe inlet and selects a pipe size that carries that flow at an acceptable velocity.
The Rational Method (Q = CiA) is the standard approach for small drainage areas under 200 acres. It estimates peak runoff from rainfall intensity, drainage area, and a runoff coefficient that reflects how much rain becomes runoff versus infiltrating into the ground. This guide covers the Rational Method, Manning equation for pipe capacity, material selection, and the design criteria that municipal reviewers check.
The Rational Method: Q = CiA
The Rational Method calculates peak runoff flow rate: Q = C × i × A, where Q is in cubic feet per second (CFS), C is the dimensionless runoff coefficient, i is rainfall intensity in inches per hour, and A is the drainage area in acres. The formula assumes rainfall intensity is uniform over the entire drainage area for a duration equal to the time of concentration.
Runoff coefficients represent the fraction of rainfall that becomes surface runoff. Impervious surfaces have high C values: asphalt pavement 0.90 to 0.95, concrete 0.85 to 0.95, rooftops 0.85 to 0.95. Pervious surfaces have lower values: lawns on clay soil 0.25 to 0.35, lawns on sandy soil 0.10 to 0.20, wooded areas 0.05 to 0.25. For mixed-use areas, calculate a weighted average C based on the percentage of each surface type.
A 5-acre commercial site that is 80% impervious (C=0.90) and 20% landscaped (C=0.30): weighted C = 0.80 × 0.90 + 0.20 × 0.30 = 0.78. With a 10-year rainfall intensity of 4.5 in/hr and 5 acres: Q = 0.78 × 4.5 × 5 = 17.6 CFS.
Q (CFS) = C × i (in/hr) × A (acres)
Common runoff coefficients (C):
Pavement/roofs: 0.85–0.95
Gravel surfaces: 0.50–0.70
Lawns (clay): 0.25–0.35
Lawns (sand): 0.10–0.20
Woods/forest: 0.05–0.25
Storm Drain Pipe Sizing Calculator
Size storm drain pipes using the Rational Method (Q=CiA) and Manning's equation. Calculate required pipe diameter, flow velocity, and pipe capacity for stormwater drainage.
Rainfall Intensity from IDF Curves
Rainfall intensity (i) comes from Intensity-Duration-Frequency (IDF) curves specific to your location. These curves relate rainfall intensity to storm duration for various return periods (2-year, 10-year, 25-year, 100-year). NOAA Atlas 14 provides point precipitation frequency estimates for anywhere in the United States.
The storm duration used in the Rational Method equals the time of concentration (Tc) — the time for water to travel from the most hydraulically remote point of the drainage area to the design point. Shorter Tc means higher intensity and higher peak flow. For small developed sites, Tc ranges from 5 to 15 minutes.
Most municipal storm drain design uses the 10-year or 25-year return period for minor systems (pipes and ditches) and the 100-year return period for major systems (flood routes and detention). Check your local design standards — requirements vary significantly between jurisdictions.
Time of concentration is calculated as overland flow time plus channel flow time plus pipe flow time. For developed sites, overland flow is typically the longest component. The FAA formula, Kirpich equation, or NRCS method can estimate Tc depending on local standards.
Manning Equation for Pipe Flow Capacity
The Manning equation calculates flow velocity in a pipe: V = (1.486 / n) × R^(2/3) × S^(1/2), where V is velocity in feet per second, n is the Manning roughness coefficient, R is the hydraulic radius in feet (pipe area ÷ wetted perimeter), and S is the pipe slope in feet per foot.
For a full circular pipe: R = D/4, where D is the inside diameter. Flow rate Q = V × A, where A is the pipe cross-sectional area. Combining: Q = (1.486 / n) × (D/4)^(2/3) × S^(1/2) × (πD²/4).
Manning n values by pipe material: smooth concrete 0.012 to 0.013, corrugated HDPE (smooth interior) 0.012, corrugated metal pipe (CMP) 0.022 to 0.027, PVC 0.009 to 0.011, reinforced concrete pipe (RCP) 0.012 to 0.013. Lower n means less friction and higher capacity for the same pipe size and slope.
Example: 18-inch RCP (n=0.013) at 1% slope: R = 1.5/4 = 0.375 ft. V = (1.486/0.013) × 0.375^(0.667) × 0.01^(0.5) = 114.3 × 0.522 × 0.1 = 5.97 fps. Q = 5.97 × π × 0.75² = 10.6 CFS.
V = (1.486 / n) × (D/4)^(2/3) × S^(1/2)
Q = V × (π × D² / 4)
Manning n values:
PVC: 0.009–0.011 | RCP: 0.012–0.013
Smooth HDPE: 0.012 | CMP (annular): 0.024
Minimum Velocity and Pipe Sizing Criteria
Minimum velocity prevents sediment deposition inside the pipe. Most design standards require a minimum full-pipe velocity of 2.5 to 3.0 feet per second. Some jurisdictions allow 2.0 fps for pipes that carry only roof drainage. Below minimum velocity, sediment settles and accumulates, gradually reducing pipe capacity.
Maximum velocity prevents pipe erosion and excessive energy at outfalls. Concrete pipe handles up to 15 to 20 fps. Corrugated metal pipe should stay below 10 to 15 fps. HDPE pipe handles 10 to 15 fps depending on wall profile. Check manufacturer recommendations for specific products.
Standard pipe sizes: 12, 15, 18, 21, 24, 27, 30, 36, 42, 48, 54, 60, 66, 72 inches. Many jurisdictions set a minimum pipe size of 12 inches or 15 inches for public storm drains. Private site drains may use smaller sizes down to 6 or 8 inches.
Minimum cover over the pipe crown varies: typically 12 to 24 inches for reinforced concrete under roads, 18 to 36 inches for HDPE, and 24 to 36 inches for CMP. Insufficient cover leads to pipe deflection or crushing under traffic loads.
Pipe Material Selection and Longevity
Reinforced concrete pipe (RCP) is the most common storm drain material for public infrastructure. It handles heavy traffic loads, resists corrosion in most soils, and has a design life of 75 to 100 years. Available in strength classes I through V for different loading conditions.
Corrugated HDPE (high-density polyethylene) with smooth interior is popular for private site development. It is lightweight, easy to install, and has a lower Manning n (0.012) than CMP. Concerns about long-term deflection and joint integrity have led some municipalities to restrict or prohibit HDPE in public right-of-way.
Corrugated metal pipe (CMP) is cost-effective for temporary and rural applications. Its high Manning n (0.024) means it needs to be larger than RCP or HDPE for the same flow capacity. Corrosion limits life to 25 to 50 years depending on soil and water chemistry. Aluminized and polymer-coated CMP extends life.