HERO E&C has added two tools to H&H Tools, its free set of calculation aids for drainage and floodplain work, for the people who prepare drainage plans and the people who review them. They run in a web browser with no account and no fee.
Pre/Post Development Comparison
A drainage report usually comes down to one table: flows before and after development, return period by return period. Pre/Post Development Comparison builds that table three ways.
- NRCS. Peak flow by the TR-55 Graphical Peak Discharge method, runoff depth and volume for each condition, from the 2-year to the 100-year storm with NOAA Atlas 14 rainfall for a Texas location. With a target release you enter, it adds the preliminary storage estimate of TR-55 Chapter 6.
- Rational. Peak flow only, each condition with its own weighted C and the intensity for its own time of concentration. The method gives no runoff volume, so this mode shows none.
- Numbers from a report. Paste the flows of an existing report by outlet and event. The page lines them up and works out the differences, without recomputing any flow.
The results are numbers side by side. The target release is yours to enter, or to choose as the pre-development flow; it is never applied unless you choose it. The storage figure is a preliminary estimate, not a routed result, and the page says so.
Texas Drainage Criteria Directory
The first question on a new site is often where the local criteria are and which edition is current. The Texas Drainage Criteria Directory links to the documents published by Texas cities, counties, flood control districts and state agencies, with the edition each publisher shows and the date we checked. For the documents we have read, it also points to the section for four common items. It does not say which document applies to a project: the reviewing agency decides that.
Also updated: Culvert and Gutter & Inlet
Culvert now handles roadway overtopping by the FHWA HDS-5 procedure, identical barrels, and a channel normal-depth tailwater. Gutter & Inlet now follows a run of inlets down a grade with the bypass carried from one to the next, a sag inlet at the end, and clogging scenarios, by FHWA HEC-22.
Tested against reference cases
The storage estimate on the new page is compared with the worked examples in NRCS TR-55 Chapter 6 and FHWA HEC-22, and the comparison itself with hand calculations of the published equations. The cases and the largest differences are on its Verification tab, and limits and known gaps are on each page. These are calculation aids: the results are not a sealed engineering document, and the engineer of record remains responsible for the design.
What comes next
Detention routing is planned next: a stage-storage curve, outlet structures, and routing an inflow hydrograph through a pond.
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.