HERO E&C has added four hydrology calculators to H&H Tools, its free set of calculation aids for drainage and floodplain work. They join the six hydraulics calculators released earlier this month and run in a web browser with no account and no fee.
For a Texas location, each one reads the same NOAA Atlas 14 rainfall as the Texas Atlas 14 Rainfall Map, so a time of concentration, a peak flow, or a runoff volume can start from the published rainfall for the site instead of a number copied from a table. They are calculation aids: each applies a published method to the inputs you enter, the results are not a sealed engineering document, and the engineer of record remains responsible for the design.
The four calculators
Atlas 14 IDF — NOAA Atlas 14 Volume 11 rainfall depth and average intensity for an address or coordinates in Texas: the full table of 15 durations from 5 minutes to 10 days and 10 return periods, intensity-duration-frequency curves, and a value for any duration in between.
Time of Concentration — the NRCS TR-55 segments (sheet, shallow concentrated, and channel flow), Kirpich, Kerby-Kirpich as the TxDOT Hydraulic Design Manual combines them, and the NRCS lag equation. For TR-55 sheet flow, the 2-year, 24-hour rainfall can come straight from Atlas 14.
Rational Method — Q = CiA for up to four subareas with an area-weighted C, the intensity for your time of concentration from Atlas 14 with an optional 10-minute minimum, and the peak for every return period from 1 to 1,000 years.
NRCS Runoff — curve number runoff depth and volume for up to four subareas, antecedent runoff condition, the full TR-55 curve number tables, and the TR-55 Graphical Peak Discharge method.
Also new: road drainage and annual maximum rainfall
Gutter & Inlet — gutter spread for a flow on a uniform or depressed gutter, then the flow a curb opening, grate, sweeper combination, or slotted inlet takes on grade, or the ponding an inlet in a sag needs, by FHWA HEC-22 (4th edition). It is tested against the HEC-22 worked examples.
The Atlas 14 IDF, Rational Method, and NRCS Runoff calculators now offer both NOAA Atlas 14 series — partial duration and annual maximum — for the same location; the reviewing agency’s criteria say which to use. The Texas Curve Number Screening Map can send its screening curve number to NRCS Runoff, with a reminder to check it.
How they fit together
Enter an address or coordinates once and follow the links: the Atlas 14 IDF page passes the location to the Rational Method and NRCS Runoff calculators, and the Time of Concentration page passes its result as the duration for intensity. The soil, land cover, rainfall, curve number, and gage maps — and the property records map for a parcel — link to the Atlas 14 IDF page for the same point. Coordinates stay in your browser; an address you type is sent to the U.S. Census geocoder to find it.
Tested against reference cases
Every calculator has a Verification tab. The Atlas 14 values are compared with NOAA’s Precipitation Frequency Data Server for five Texas locations — 1,425 values in both series, matching to the digit — and the time of concentration, runoff, and peak methods with worked examples published by NRCS, FHWA, and TxDOT and with hand calculations of the published equations. The table comes from the same tests that run whenever the code changes. Limits and known gaps are listed on each page.
What comes next
Detention and retention pond calculations are planned next: storage, outlet structures, and routing.
Try them on your next project
HERO E&C is a San Antonio civil and water resources engineering firm founded in 2025 and led by Seoyoung Kim, Ph.D., P.E., CFM. When a project needs a drainage study, hydrology for a development permit, or HEC-HMS and HEC-RAS models, HERO E&C can help.