HydroCAD® Stormwater Modeling - Since 1986
Why use the event lookup tables?
The event lookup tables are the easiest way to define the rainfall events in a HydroCAD project. After selecting your project location (typically by county), the lookup table defines the rainfall distribution and rainfall depth for each return period. This allows HydroCAD to automatically create a set of rainfall events for the current project.
What lookup tables are provided with HydroCAD?
The first event lookup table was implemented in HydroCAD 9.1, and used TP-40 rainfall depths in conjunction with the traditional NRCS Type I, IA, II and III rainfall distributions. Originally distributed as NRCS-Rain, this table has been renamed to TP-40-Rain to better identify its contents.
In HydroCAD-10 build 14, a second lookup table was added based on the latest Atlas-14 data. In addition to providing updated rainfall depths, the new Atlas-14-Rain table also references the latest NRCS rainfall distributions (e.g. NOAA_A/B/C/D, NRCC_A/B/C/D, and MSE3/4) which are gradually replacing the traditional Type I, IA, II and III distributions. Since the table includes both old and new distributions, you are free to choose the distribution that best suits your modeling objectives and stormwater regulations.
How do I use the event lookup tables?
Step 1: Open the Settings|Calculation screen, select the Rainfall tab, and click "Load Events from...Lookup Table". This will open the event lookup screen shown at the top of this page.
Step 2: Select the desired "Event Lookup File" from the drop-down list.
Step 3: Select your state from the drop-down list, or manually scroll down to your state.
Step 4: Select (click) the line for your specific county and click OK. HydroCAD will display an event confirmation box.
Step 5: If more than one Storm Distribution is available for your location, select the desired distribution for your project. If you need help selecting the correct distribution, consult your stormwater regulations or see the state-specific guidance below.
Step 6: Click OK to create the rainfall events for the current project.
Tip: You can review your event settings at any time by clicking the "View All" button on the Rainfall tab.
Lookup table details
The TP-40 lookup table is based primarily on data from the NRCS WinTR-55 program, with corrections made for rainfall distributions and other anomalies. Although this table will continue to be supported for studies that require the older TP-40 rainfall data, future updates will be minimal.
The new Atlas-14 lookup table represents a compilation of data from the latest release of WinTR-55 and many other sources. It will be updated as needed to incorporate new data from state and federal agencies as they continue the full conversion to Atlas-14. Although rainfall depths have generally been updated to Atlas-14, many states have not yet developed new Atlas-14 rainfall distributions, and continue to use the older Type I, IA, II, and III distributions. Please see the table below for details on rainfall distributions for specific states.
Details for lookup table "Atlas-14-Rain"
Rainfall distribution comments
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