HERO E&C has added Detention Routing to H&H Tools, its free set of calculation aids for drainage and floodplain work. It routes inflow hydrographs through a pond by the storage-indication method and runs in a web browser with no account and no fee.
What goes in
- Inflow. Paste up to four hydrographs from HEC-HMS, WinTR-55, SWMM or a spreadsheet, each with a name and a source. Or build one from a drainage area, a curve number and a time of concentration, with the NRCS dimensionless unit hydrograph and an NRCS Type II or III storm or an Atlas 14 alternating-block storm for a Texas location.
- Pond. Elevation and surface area, with storage by the conic section or the average end area, or elevation and storage directly.
- Outlets. Orifices, sharp-crested and V-notch weirs, a riser, a barrel by the FHWA HDS-5 procedure, and broad-crested spillways, with one constant tailwater.
What comes out
For each inflow: peak inflow and outflow, the time of the peak, the highest water, the largest storage, freeboard to the embankment you enter, and the time to drain 90% of the peak storage. With a target release you enter, the difference is set beside it as a number. The page also shows the hydrographs, the outlet rating, the volume balance, and how much the peak changes when the time step is halved.
If the water rises above the top of your stage-storage table, the page stops and says so. It does not extend the table for you.
Checked against HEC-HMS
HERO E&C ran the same ponds and inflows in USACE HEC-HMS 4.13, including a spillway that overflows, an orifice under tailwater, and a pipe barrel, and compared peak outflow, highest water, storage, time of peak and drain time. The unit hydrograph is compared with HEC-HMS as well, and the outlet ratings with the worked examples in FHWA HEC-22. The cases and the largest differences are on the Verification tab; what is not verified is on the Limits tab.
It works with the Pre/Post Development Comparison: a comparison can pass its post-development numbers and targets to Detention Routing, where you add the pond.
This is a calculation aid: you choose the pond and the outlets and read the result. It does not size them for you and does not state what your criteria require. The results are not a sealed engineering document, and the engineer of record remains responsible for the design.
Try it 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, detention design, or HEC-HMS and HEC-RAS models, HERO E&C can help.