Drainage Design 101: A Complete Introduction

Whether you’re a homeowner dealing with a soggy backyard, a contractor installing a French drain, or an engineering student learning the ropes, understanding how water moves across and through the ground is essential. This guide introduces the core concepts of stormwater drainage design.

What Is Stormwater Drainage?

Stormwater drainage is the system of managing rainwater runoff from developed areas. When rain falls on natural ground, much of it soaks into the soil. But when we build roads, roofs, and parking lots, we create impervious surfaces that prevent infiltration, causing water to flow across the surface instead.

Good drainage design answers three fundamental questions:

1. How much water will run off during a storm? (Hydrology)
2. How fast will it flow? (Hydraulics)
3. Where should it go to prevent flooding and erosion? (Design)

The Water Cycle in Development

Before Development

In a natural watershed:

• 40-50% of rainfall infiltrates into the ground
• 10-20% runs off as surface flow
• 30-40% returns to the atmosphere through evapotranspiration

After Development

In a developed area with 75% impervious cover:

• 15% of rainfall infiltrates
• 55% runs off as surface flow
• 30% evapotranspires

This dramatic increase in runoff volume and peak flow rate is the fundamental challenge of stormwater management.

Key Concepts You Need to Know

Watershed (Drainage Area)

A watershed or drainage area is all the land that drains to a common point. Watersheds can be tiny (your backyard draining to a single downspout) or massive (the Mississippi River basin covering 31 states).

Every drainage calculation starts with determining the drainage area. For small sites, this might be as simple as measuring your roof. For larger projects, it requires topographic analysis to identify where water will flow.

Time of Concentration (Tc)

The time of concentration is how long it takes for water to travel from the most distant point in the watershed to the outlet. This is crucial because:

• It determines which rainfall intensity to use in calculations
• Longer Tc = lower intensity = lower peak flow
• Shorter Tc = higher intensity = higher peak flow

Typical values:

• Small parking lot: 5-10 minutes
• Residential subdivision: 15-30 minutes
• Large watershed: 1-6+ hours

Runoff Coefficient

The runoff coefficient (C) represents what fraction of rainfall becomes runoff. It ranges from 0 (no runoff) to 1.0 (100% runoff).

For mixed land use areas, you calculate a composite runoff coefficient by weighting each surface type by its area.

Design Storm

A design storm is the hypothetical rainfall event used to size drainage infrastructure. It’s defined by:

• Return period (frequency): How often, on average, this storm is expected to occur
• Duration: How long the storm lasts
• Intensity or depth: How much rain falls

Common design storms:

• 10-year storm: Used for most stormwater conveyance (pipes, ditches)
• 25-year storm: Major drainage systems
• 100-year storm: Floodplain management, emergency overflow

Two Main Approaches to Runoff Calculation

There are many methods for calculating stormwater runoff, but two dominate practice: the Rational Method and the SCS/NRCS Method.

1. Rational Method

The Rational Method is the simplest and most widely used approach for small watersheds (under 200 acres).

Where:

• Q = Peak discharge (cfs or m³/s)
• C = Runoff coefficient (dimensionless)
• i = Rainfall intensity (in/hr or mm/hr)
• A = Drainage area (acres or hectares)

Best for: Parking lots, small developments, pipe sizing

Key assumptions:

• Rainfall intensity is uniform over the drainage area
• Rainfall intensity is constant during the storm (duration = Tc)
• Peak runoff rate occurs when entire watershed is contributing
• Runoff coefficient doesn’t change during the storm

Try the Rational Method Calculator →

2. SCS/NRCS Method

The SCS (now NRCS) method uses Curve Numbers to estimate runoff and can generate a complete hydrograph showing how flow varies over time.

Where:

• Q = Runoff depth (inches)
• P = Rainfall depth (inches)
• Ia = Initial abstraction (typically 0.2S)
• S = Potential maximum retention

Best for: Detention pond design, larger watersheds, volume-based regulations

The SCS method accounts for:

• Soil type (hydrologic soil groups A through D)
• Land use and vegetation
• Antecedent moisture conditions

Try the SCS Curve Number Calculator →

Putting It All Together: A Simple Example

Let’s say you’re designing drainage for a 5-acre commercial parking lot:

Step 1: Determine the runoff coefficient The parking lot is primarily asphalt with a small landscaped island:

• Asphalt (4.5 acres): C = 0.90
• Landscaping (0.5 acres): C = 0.20
• Composite C = (4.5 × 0.90 + 0.5 × 0.20) / 5 = 0.82

Step 2: Find the time of concentration The longest flow path is 500 ft at 1% slope across the pavement. Using the sheet flow equation: Tc ≈ 8 minutes

Step 3: Look up rainfall intensity From your local IDF curve for a 10-year, 8-minute storm: i = 6.5 in/hr (varies by location)

Step 4: Calculate peak flow

This peak flow determines the size of pipes, inlets, and detention facilities needed.

Common Drainage Elements

Conveyance Systems

• Pipes: Storm sewers, culverts
• Open channels: Ditches, swales, streams
• Inlets and catch basins: Collect surface water into pipes

Storage Systems

• Detention ponds: Temporarily store water, release slowly
• Retention ponds: Permanently hold water (wet ponds)
• Underground storage: Tanks, detention chambers, large pipe systems

Green Infrastructure

• Bioretention/rain gardens: Filter runoff through soil and plants
• Permeable pavement: Allows water to infiltrate
• Green roofs: Reduce runoff from buildings

Design Standards and Regulations

Drainage design is governed by various regulations depending on your location and project type:

• Local ordinances: Cities and counties have specific requirements
• State regulations: Often address water quality and larger systems
• Federal regulations: Clean Water Act, FEMA flood requirements
• Industry standards: ASCE Manual of Practice, HEC-22, etc.

Next Steps

Now that you understand the basics:

1. Learn about Time of Concentration - How to Calculate Tc
2. Understand the Rational Method in depth - Rational Method Deep Dive
3. Explore the SCS/NRCS Method - Curve Number Method Explained
4. Try a calculation - Rational Method Calculator

References

1. American Society of Civil Engineers. (2017). Gravity sanitary sewer design and construction (ASCE Manual of Practice No. 60). ASCE Press.

2. Federal Highway Administration. (2013). Urban drainage design manual (3rd ed., Hydraulic Engineering Circular No. 22). U.S. Department of Transportation.

3. Natural Resources Conservation Service. (1986). Urban hydrology for small watersheds (Technical Release 55). U.S. Department of Agriculture.

4. Chin, D. A. (2013). Water-resources engineering (3rd ed.). Pearson.

5. Mays, L. W. (Ed.). (2010). Water resources engineering (2nd ed.). Wiley.