Regional Regression Peak Flow
Peak flow of an undeveloped Texas watershed from area, slope, rainfall and OmegaEM, by the USGS equations TxDOT uses, with its conditions of use.
Results
Enter your values; the answer appears here as you type.
The curve appears with a result.
- The equation
- QT = Pc Sd 10(e OM + a + b Aλ) (USGS SIR 2009-5087 Eq. 3 and Table 3; TxDOT HDM Chapter 4 Eq. 4-12 and Table 4-4, with the same coefficients).
- Slope
- The change in elevation between the two ends of the longest defined channel, divided by its length.
- Mean annual precipitation
- For the approximate center of the watershed, from TxDOT HDM Figure 4-6 or another authoritative source.
- OmegaEM
- The value of the 1-degree quadrangle that holds the site, from TxDOT HDM Figure 4-5. TxDOT says not to weight or average it when the watershed overlaps quadrangles. You read it from the map; this page does not look it up.
- Conditions of use
- From TxDOT: not under 1 sq mi; a comparison method must be used below about 5 sq mi and should be used below 10; natural basins only, with less than 10 percent impervious cover and less than 10 percent of the area controlled by reservoirs. The page refuses or warns accordingly, and flags inputs outside the range of the development data.
The calculator is tested against the cases below every time its code changes.
| Case | What | Reference | Values compared | Largest difference | Result |
|---|---|---|---|---|---|
| The table loads with the calculator. The same results are in verification.json. | |||||
Compared with the USGS example for the Nueces River at Laguna, about 145,000 cfs for the 100-year flood, to the precision printed, and with direct evaluation of TxDOT Table 4-4. The coefficients were read from both the USGS and TxDOT tables and compared. No StreamStats comparison has been run for this page.
- For natural conditions only: not urban or regulated watersheds, and not for flows after development. Use a hydrograph method there.
- OmegaEM and mean annual precipitation are read by you. USGS notes a typical uncertainty in OmegaEM of about 0.10 log10; the residual standard error of each equation is in the table.
- A regional estimate: a stream gage nearby may support a better one. USGS advises against adding peaks of subdivided watersheds with these equations.
- TxDOT does not state an upper area limit. The development data reach 9,329 sq mi.
- Where do I read OmegaEM?
- On TxDOT HDM Figure 4-5: one value for each 1-degree quadrangle of latitude and longitude. Use the quadrangle of the site.
- What is a comparison method?
- TxDOT does not define one further. It means another hydrologic estimate of the same flow, for example a TR-55 peak or a hydrograph model.
- Can I attach the calculation sheet to a report?
- You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews and is responsible for what goes into a report.
| Topic | Reference |
|---|---|
| Equations | Asquith, W.H., and Roussel, M.C. (2009). USGS Scientific Investigations Report 2009-5087. |
| Conditions of use | TxDOT Hydraulic Design Manual, Chapter 4, Sections 7 and 10, revised September 2019. |
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A free calculation aid using the published methods above. Not a sealed engineering document; the engineer of record is responsible for design. HERO E&C · TBPELS Firm F-27793.
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A free calculation aid. This calculator applies the published methods named on this page to the inputs you enter. It is not a sealed engineering document, has not been reviewed by a licensed professional for your project, and no warranty is expressed or implied. Check the results independently before using them for design, bidding or permitting.
The criteria of the reviewing agency decide which methods and values apply to a design. The engineer of record is responsible for the design.
Using this calculator does not create an engineer–client relationship. Calculations run in your browser; your inputs are not sent to or stored by HERO E&C. HERO E&C · TBPELS Firm F-27793.