HydroCAD, TR-20, TR-55 Comparison
The following table summarizes the technical similarities and differences between these three programs. HydroCAD combines the most-used capabilities of TR-20 and TR-55, plus many other techniques and features not provided by either program.
|
HydroCAD |
TR-20 |
TR-55 |
SCS unit hydrograph procedure (unlimited points), Santa Barbara Urban Hydrograph, Rational Method |
SCS unit hydrograph procedure (301 points) |
Tabular method derived from TR-20 |
|
Extensive rainfall library plus user-defined storms |
SCS type I, IA, II & III |
SCS type I, IA, II & III |
|
SCS or Delmarva U.H., plus user defined curves |
SCS Unit Hydrograph or user defined curve |
SCS Unit Hydrograph only |
|
Automatic curve number lookup and weighting |
Requires direct entry of composite curve number |
Automatic curve number lookup and weighting |
|
Curve number limitations |
No restrictions |
No restrictions |
Reduced accuracy as CN differs from 75 |
All TR-55 methods plus common channel shapes, upland method, and CN method |
Contains no Tc calculations: Requires direct entry of Tc |
Calculates Tc for sheet, shallow concentrated, & channel flow |
|
Tc limitations |
No restrictions |
Must be greater than zero |
Limited to .1 to 2 hrs |
Tc precision |
1/10 minute or 1/600 hour |
1/100 hour |
Rounded to nearest step |
Subarea limitations |
None |
None |
Must not differ by more than factor of 5 |
Within 1% of 1982 TR-20 release |
“The standard” |
Within +25% of TR-20 |
|
Muskingum-Cunge or Storage-indication method with optional routing delay (translation) |
Modified Att-Kin procedure |
No routing, separate tables for travel times of 0 to 3 hours |
|
Reach hydraulics |
Automatic calculation for common shapes or custom cross-sections |
None. Requires entry of routing coefficients. |
n/a, no reach routing |
Storage-indication method, dynamic storage-indication method, or simultaneous routing |
Storage-indication |
None, only estimates ponding effects |
|
Automatic calculations for orifice, weir, culvert, etc, used alone or combined for outlet structures |
Must be entered directly, no calculations provided |
n/a, no pond routing |
|
Using actual inflow hydrograph |
none |
Using assumed hydrograph shape |
|
By plug-flow and center-of-mass methods |
not calculated |
not calculated |
|
Automatic calculation from pond dimensions or surface areas, plus direct entry |
Direct entry only |
n/a, no pond routing |
|
Flow diversions (split flows) |
Automatic diversion of outflow from specified pond outlet(s) |
Separate “Divert” procedure based on user-defined curve |
n/a |
Interactive, on-screen, with labels and background image |
none |
none |
|
Calculation procedure |
Automatically calculated as required |
“Batch mode” calculation of entire watershed |
Manual initiation of each calculation |
English, Metric, or custom, plus split input/output units |
English only |
English only |
|
Full graphics to screen, printer, or file |
none |
none |
|
Automatic reports with headings, graphics, etc. |
Manual |
Manual |
|
Data Storage |
Automatic by job name |
Manual |
Manual |
Note: This table reflects the earlier DOS versions of TR-20 and TR-55.