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Download the latest official USDA TR-55 Manual and learn how the NRCS Curve Number method, Time of Concentration, hydrographs, and detention basin calculations are used in modern stormwater design.
Technical Release 55 (TR-55), Urban Hydrology for Small Watersheds, is one of the most widely used stormwater design references in the United States. Developed by the US Department of Agriculture (USDA), it provides simplified procedures for estimating runoff, peak discharge, hydrographs, and detention storage requirements for small watersheds.
The original TR-55 was first issued in January 1975. The 1986 edition was a major revision incorporating research findings and practical experience from the original edition. Although more than three decades old, the 1986 Second Edition remains the latest official edition published by the USDA and continues to be referenced by municipalities, consulting engineers, and stormwater software throughout the United States.
What is the TR-55 Manual?
When land is developed, rainfall behaves differently.
Roads, parking lots, rooftops, and other impervious surfaces reduce infiltration, increase runoff, and shorten the time it takes stormwater to reach streams and drainage systems. Engineers need reliable methods to predict these changes so that culverts, detention ponds, channels, and stormwater infrastructure can be designed appropriately.
TR-55 was developed by the USDA to provide a practical engineering method for estimating stormwater runoff from small urban and urbanizing watersheds.
Rather than requiring extensive streamflow records or complex computer models, TR-55 uses measurable watershed characteristics—including soils, land use, watershed geometry, and rainfall—to estimate how much runoff will occur during a design storm.
The manual presents simplified procedures for calculating:
- Runoff depth
- Peak discharge
- Runoff hydrographs
- Detention basin storage
These procedures remain the foundation of many municipal stormwater design standards and continue to be implemented in modern engineering software. While TR-55 focuses on hydrology—estimating how rainfall becomes runoff—its results are often used as inputs for hydraulic modeling, where engineers simulate how water moves through pipes, channels, culverts, and detention systems.
(If you’re new to these concepts, read our guide to Hydrology vs. Hydraulics: What’s the difference?)
What does TR-55 calculate?
At its core, TR-55 answers four fundamental design questions.
1. How much runoff will occur?
Using the NRCS Curve Number (CN) method, TR-55 estimates the depth of runoff generated by a rainfall event based on land cover, soil type, and watershed conditions.
2. What is the peak flow rate?
Peak discharge is critical when designing storm sewers, culverts, channels, and other conveyance systems. TR-55 provides both graphical and tabular methods for estimating peak discharge from design storms.
3. What does the runoff hydrograph look like?
Peak flow alone doesn’t tell the whole story. The Tabular Hydrograph Method estimates how flow changes over time, allowing engineers to evaluate routing, storage, and downstream impacts.
4. How much detention storage is required?
Finally, TR-55 includes simplified procedures for estimating the storage volume needed to reduce downstream peak discharge using detention basins. Today, these concepts are often applied within broader Sustainable Drainage Systems (SuDS) strategies that combine attenuation, infiltration, conveyance, and water quality treatment to better mimic natural hydrology. Learn more in our guide to What are Sustainable Drainage Systems (SuDS)?
How the TR-55 workflow works
Although the calculations can appear complex, the overall process follows a logical sequence.
- Define watershed boundaries
- Identify soils and hydrologic soil groups
- Determine land cover and Curve Numbers
- Select the design storm rainfall
- Calculate runoff depth
- Estimate Time of Concentration (Tc)
- Compute peak discharge or develop a hydrograph
- Size detention storage if required
This workflow is reflected throughout the manual and provides a straightforward framework for stormwater design.
In practice, this hydrologic workflow is often only the first stage of a larger stormwater design process. Once runoff and hydrographs have been calculated, hydraulic models simulate how that water moves through drainage networks, storage facilities, and receiving waters. Our Hydraulic Modeling Guide explains how hydrology and hydraulics work together in modern engineering workflows.
Five key concepts every engineer should understand
1. Curve Number (CN)
The NRCS Curve Number method is the foundation of TR-55. Curve Numbers represent the runoff potential of a watershed based on:
- Land use
- Hydrologic soil group
- Surface cover
- Hydrologic condition
- Impervious area
Higher Curve Numbers produce more runoff.
For example:
- Forested land on sandy soils has a relatively low Curve Number.
- Commercial development with large paved areas has a much higher Curve Number.
TR-55 provides extensive Curve Number tables covering urban, agricultural, rangeland, and other land uses.
2. Hydrologic Soil Groups
Soils play a major role in determining infiltration. TR-55 classifies soils into four Hydrologic Soil Groups:
- Group A – high infiltration
- Group B
- Group C
- Group D – very low infiltration
Even highly developed sites often retain significant pervious areas, making accurate soil classification essential for reliable runoff estimates.
3. Time of Concentration (Tc)
Time of Concentration is the time required for runoff to travel from the hydraulically most distant point of a watershed to the outlet. It is one of the most influential parameters in stormwater modeling because it affects both hydrograph shape and peak discharge.
TR-55 computes Tc by summing travel times through:
- Sheet flow
- Shallow concentrated flow
- Open channel flow
Urbanization typically reduces Tc by replacing natural flow paths with streets, gutters, and storm sewers.
4. Design storms
TR-55 uses standard 24-hour design storms and NRCS rainfall distributions:
- Type I
- Type IA
- Type II
- Type III
Today’s engineers often obtain rainfall depths from NOAA Atlas 14 or local agency guidance while continuing to apply the TR-55 methodology.
5. Hydrographs
Hydrographs describe how runoff changes over time. Rather than considering only peak discharge, hydrographs allow engineers to:
- Route flows downstream
- Evaluate detention basin performance
- Compare existing and proposed development conditions
For many stormwater designs, hydrographs provide a much more complete understanding of watershed response than peak flow alone. In many engineering workflows, the hydrograph generated by TR-55 becomes the input for a hydraulic model that evaluates pipe capacity, flooding, and storage performance. Learn more in Hydrology vs. Hydraulics: What’s the difference?
When should you use TR-55?
TR-55 is well suited for:
- Small urban watersheds
- Residential developments
- Commercial developments
- Preliminary drainage design
- Detention basin sizing
For larger watersheds, complex routing, or continuous simulation, more sophisticated hydrologic models may be appropriate.
While TR-55 remains an excellent method for estimating runoff from small watersheds, modern stormwater projects increasingly incorporate Sustainable Drainage Systems (SuDS), such as rain gardens, swales, infiltration systems, and detention ponds. These features rely on the same hydrologic principles while helping reduce runoff, improve water quality, and increase resilience to more frequent extreme rainfall.
TR-55 vs. other hydrologic methods
| Method | Best suited for |
| Rational Method | Small drainage areas where only peak flow is required |
| TR-55 | Small urban watersheds requiring runoff, peak flow, hydrographs, and detention storage |
| TR-20 | Larger and more complex watersheds with detailed routing |
| HEC-HMS | Comprehensive watershed modeling and advanced hydrologic analysis |
Choosing between TR-55, TR-20, HydroCAD, and modern drainage design software depends on your project requirements, regulatory standards, and the level of analysis required. We explore these differences in detail in HydroCAD vs. InfoDrainage: Comparing TR-55, TR-20 and modern stormwater modelling.
Using TR-55 in Autodesk InfoDrainage
TR-55 was originally developed for manual calculations using printed worksheets and lookup tables. Today, modern drainage design software automates these calculations while preserving the underlying hydrologic methods, allowing engineers to spend more time evaluating design alternatives and less time performing repetitive calculations. Many organizations also combine TR-55 hydrology with hydraulic modeling in a single workflow to better understand system performance during storm events.
With Autodesk InfoDrainage, engineers can:
- Define watershed catchments
- Assign soils and land use
- Calculate Curve Numbers
- Estimate Time of Concentration
- Generate runoff hydrographs
- Size detention ponds
- Compare multiple design alternatives
Modern drainage design increasingly combines hydrologic methods like TR-55 with hydraulic modeling and SuDS in a single workflow. InfoDrainage supports this integrated approach, allowing engineers to model runoff, evaluate hydraulic performance, and design sustainable drainage features within one environment. If you’re new to SuDS, our guide What are Sustainable Drainage Systems (SuDS)? is a great place to start.
Frequently Asked Questions
Is TR-55 still used?
Yes. Many municipalities and engineering firms continue to use TR-55 for small watershed design, and many software packages implement its methods.
What is the latest version of TR-55?
The latest official edition is the Second Edition, revised in June 1986, with an Appendix A update issued in January 1999.
Is TR-55 free?
Yes. TR-55 is a U.S. government publication and is widely available for download.
What is the difference between TR-55 and TR-20?
TR-55 provides simplified procedures for small watersheds, while TR-20 is intended for more detailed hydrologic analysis of larger or more complex watersheds.
Does InfoDrainage support TR-55?
Yes. InfoDrainage supports NRCS/TR-55 hydrologic workflows alongside a range of other internationally recognized stormwater design methods.
Go deeper down the drainage hole
If you’re interested in learning more about stormwater modeling and drainage design, these guides may help:
- Hydrology vs. Hydraulics: What’s the difference? Learn how rainfall-runoff calculations feed hydraulic models and why both disciplines are essential to modern drainage design.
- Hydraulic modeling guide: Explore 1D, 2D, and integrated catchment modeling, and see how hydraulic models simulate water movement through drainage networks and floodplains.
- HydroCAD vs. InfoDrainage: Comparing TR-55, TR-20 and modern stormwater modelling: Understand the strengths, limitations, and typical applications of today’s most widely used stormwater modeling approaches.
- Free trial: Try InfoDrainage yourself by downloading a free trial for 30 days (no credit card required). Are you a student or educator? You can use our water software for free.