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HydroCAD Stormwater Modeling - Since 1986


TR-55: Urban Hydrology for Small Watersheds

Author: USDA Soil Conservation Service (SCS)
            Now Natural Resources Conservation Service (NRCS)

"Technical Release 55 (TR-55) presents simplified procedures to calculate storm runoff volume, peak rate of discharge, hydrographs, and storage volumes required for floodwater reservoirs.  These procedures are applicable to small watersheds, especially urbanizing watersheds, in the United States."


1) TR-55 is perhaps the most widely used approach to hydrology in the US.   Originally released in 1975, TR-55 provides a number of techniques that are useful for modeling small watersheds.  Since the initial publication predated the widespread use of computers, TR-55 was designed primarily as a set of manual worksheets.  A TR-55 computer program is now available, based closely on the manual calculations of TR-55.

2) TR-55 utilizes the SCS runoff equation to predict the peak rate of runoff as well as the total volume.  TR-55 also provides a simplified "tabular method" for the generation of complete runoff hydrographs.  The tabular method is a simplified technique based on calculations performed with TR-20.  TR-55 specifically recommends the use of more precise tools, such as TR-20, if the assumptions of TR-55 are not met.


While the TR-55 manual remains a most useful reference (it contains complete curve number tables and rainfall maps, among other things) most engineers have sought out more advanced or more accurate hydrology software.

How to get it:

You can download the TR-55 Manual here. (2MB PDF format)   You can download the complete software and documentation from the NRCS Tech Tools.

For additional background information on the TR-55 calculations please see Understanding Hydrology or the "Hydrology" section of the training materials.

How it compares to HydroCAD:

HydroCAD is widely used for projects that might otherwise have been performed with TR-55.  HydroCAD directly implements some of the key features of TR-55, such as curve-number lookup and procedures for calculating time-of-concentration, in a more user-friendly graphical format.  Other aspects of HydroCAD employ more advanced techniques, such the SCS Unit Hydrograph runoff procedure, in place of the TR-55 tabular method.  HydroCAD also adds other capabilities that have no equivalent in TR-55, such as outlet hydraulics, exfiltration calculations, and a range of other features.  See the feature comparison table for details.

For most applications, HydroCAD is a preferred alternative to TR-55, and is widely accepted by reviewing agencies.  Results from the two programs will usually be similar, although the basic limitations of TR-55 (as cited above) will result in some expected differences.  For a comparison of the basic Tc and runoff calculations, see subcatchment 1S in the "Mountain View" sample project installed with HydroCAD.

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