Selecting the Right Design Storm: Return Period and Duration Guide

Selecting an appropriate design storm is one of the most consequential decisions in drainage engineering. Too small, and your infrastructure floods frequently. Too large, and you’ve overdesigned at unnecessary cost. This guide explains how to make this critical choice.

Understanding Return Periods

What Does “10-Year Storm” Mean?

A “10-year storm” has a 10% probability of occurring in any given year. It does NOT mean:

• It happens exactly once every 10 years
• It won’t happen for 10 years after occurring
• Two 10-year storms can’t happen in the same year

The Mathematics

Where:

• P = Annual exceedance probability (AEP)
• T = Return period in years

Project Life Risk

The probability that a storm will be exceeded at least once during a project’s design life:

Where:

• R = Risk of exceedance during project life
• T = Return period
• n = Project life in years

Example: 10-year storm, 30-year project life:

This means there’s a 96% chance the 10-year design storm will be exceeded at least once during a 30-year project life.

Standard Design Storm Selection

Minor Drainage System (Storm Sewers, Inlets)

Minor drainage handles routine storms with manageable consequences if exceeded.

Major Drainage System (Channels, Overflows)

Major drainage handles extreme events when minor system capacity is exceeded.

Detention and Retention Facilities

Detention requirements vary significantly by jurisdiction:

Factors Affecting Selection

1. Consequences of Failure

Higher stakes require larger design storms:

2. Economic Analysis

For some facilities, optimal design balances construction cost against damage cost:

Expected annual damage decreases as design frequency increases, but construction cost rises. The optimal design minimizes total lifecycle cost.

3. Local Requirements

Always check local standards first. Common sources:

• Municipal drainage criteria manuals
• County/regional stormwater ordinances
• State DOT standards (for road projects)
• FEMA requirements (for floodplain development)
• MS4 permit requirements

4. Upstream/Downstream Impacts

Consider the broader context:

• Upstream controls: What enters your site may already be “controlled” to a specific storm
• Downstream capacity: Don’t design for 100-year if the receiving system only handles 10-year
• Cumulative development: Future upstream development may increase flows

Storm Duration Selection

For Rational Method

Duration equals Time of Concentration (Tc):

• Calculate Tc for your watershed
• Use this duration to select rainfall intensity from IDF curves
• Critical storm duration produces maximum peak flow

For SCS/NRCS Method

Standard durations are typically used:

• 6-hour storm: Common for small urban watersheds
• 24-hour storm: Standard for most applications
• 72-hour or longer: Large watersheds, snowmelt

Duration and Intensity Relationship

Shorter storms have higher intensity but less total volume:

Rainfall Data Sources

NOAA Atlas 14

The primary source for U.S. precipitation frequency data:

• NOAA Precipitation Frequency Data Server
• Point-and-click interface
• Depths and intensities for durations from 5 minutes to 60 days
• Return periods from 1 to 1000 years
• Confidence intervals included

IDF Curves

Intensity-Duration-Frequency curves relate:

• Rainfall intensity (y-axis)
• Duration (x-axis)
• Return period (multiple curves)

Local agencies may provide jurisdiction-specific IDF curves that should be used instead of NOAA data.

Temporal Distribution

For hydrograph methods, you need not just total depth but distribution over time:

Climate Change Considerations

Increasing Precipitation Intensities

Many regions are experiencing:

• More intense short-duration storms
• Larger total storm depths
• More frequent extreme events

Current Practice

Some jurisdictions now require:

• Climate adjustment factors: 10-20% increase in design depths
• Multiple scenario analysis: Current + future conditions
• Adaptive design: Infrastructure that can be upgraded

Common Mistakes

Mistake 1: Using Wrong Duration

Error: Using a “standard” 24-hour storm for small-area pipe sizing.

Problem: Peak flow occurs with short, intense bursts, not day-long storms.

Solution: Match duration to Time of Concentration for peak flow calculations.

Mistake 2: Ignoring Major System

Error: Only designing the 10-year minor system without checking 100-year overflow.

Problem: Extreme events cause uncontrolled flooding.

Solution: Always analyze the major drainage system, even if not formally “designed.”

Mistake 3: Misunderstanding Probability

Error: Believing a 100-year storm won’t happen because one occurred recently.

Problem: Each year has independent 1% probability.

Solution: Understand that past storms don’t affect future probability.

Mistake 4: Using Outdated Rainfall Data

Error: Using rainfall data from decades-old studies.

Problem: NOAA Atlas 14 and subsequent updates provide improved, current data.

Solution: Always use the most current precipitation frequency data.

Worked Example: Design Storm Selection

Project Description

New 50-unit apartment complex:

• 15-acre site
• Urban area with combined storm/sanitary system
• Downstream channel has 25-year capacity
• Local code requires 10-year minor system
• 100-year overflow path required

Selection Process

Step 1: Identify Requirements

• Minor system: 10-year (local code)
• Major system check: 100-year (best practice)
• Detention: Match downstream 25-year requirement

Step 2: Consider Consequences

• Residential property damage if exceeded
• No critical facilities
• Basement flooding possible at lower units

Step 3: Final Selection

• Storm sewer design: 10-year (per code)
• Detention outflow: 25-year (match downstream)
• Overflow analysis: 100-year
• Consider basement protection with 25-year minor system capacity

Step 4: Duration Selection

• Rational Method (pipes): Tc = 15 minutes → use 15-minute intensity
• SCS Method (detention): 24-hour storm
• Check both short and long duration for detention critical storm

Summary

Selecting the right design storm requires balancing:

1. Regulatory requirements - What does your jurisdiction require?
2. Consequences of failure - What happens if exceeded?
3. Economic factors - What level of protection is cost-effective?
4. Project life - What risk is acceptable over the facility’s lifespan?
5. Downstream coordination - What can the receiving system handle?

Always:

• Check local standards first
• Consider both minor and major drainage systems
• Use current rainfall data (NOAA Atlas 14)
• Document your assumptions and reasoning

Related Calculators

• Rational Method Calculator
• Time of Concentration Calculator
• Rainfall Distribution Calculator

References

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

2. American Society of Civil Engineers. (2017). Design and construction of urban stormwater management systems (ASCE Manual of Practice No. 77). ASCE Press.

3. NOAA. (2022). NOAA Atlas 14: Precipitation-frequency atlas of the United States. National Weather Service.

4. Bonnin, G. M., et al. (2006). Precipitation-frequency atlas of the United States (NOAA Atlas 14, Vol. 2). National Weather Service.

5. Federal Emergency Management Agency. (2020). Guidelines and standards for flood hazard mapping. FEMA.

6. McCuen, R. H. (2016). Hydrologic analysis and design (4th ed.). Pearson.