ASDSO Dam Safety Toolbox

Flood Hydrology

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Extreme flood do occur. Learn more at

Hydrology is essential not only in the design of a dam, but also in the feasibility study of a dam and in assessments of the dam into the future as the state-of-the-practice evolves. The three main components of flood hydrology, according to the United States Bureau of Reclamation (USBR) are “(1) hydrometeorology related to probable maximum precipitation determinations, (2) probable maximum flood hydrograph determinations, and (3) statistics and probabilities relating to the magnitude and frequency of flood flows”.[1]

“One of the most common causes of dam failures is the inability to safely pass flood flows. Failures caused by hydrologic conditions can range from sudden failure, with complete breaching or collapse of the dam, to gradual failure, with progressive erosion and partial breaching”.[2] Therefore, hydrologic studies are among the most important sources of information when it comes to dam safety.

The applicability and acceptability of various hydrologic methods varies widely across the US. Familiarity with the primary runoff generating mechanisms based on the geology, topography, meteorology, and seasonality of the region are paramount to inflow design flood determination. Guidelines and regulations related to hydrologic methods and the inflow design flow also vary based on regulatory jurisdiction.

Calibration to historic events can be helpful when determining the basin's response to precipitation events. However, it is important to note that 1) basin response is nonlinear, and 2) in most cases, precipitation and streamflow gage data is not available for extreme events. A model calibrated to an event with a 100-year return period may not appropriately simulate watershed response during a much larger magnitude Probable Maximum Flood event.



Learn more about the occurrence of extreme floods at

Best Practices Resources

Technical Release 210-60: Earth Dams and Reservoirs, NRCS

Selecting and Accommodating Inflow Design Floods for Dams (FEMA P-94), FEMA

Design Standards No. 14: Appurtenant Structures for Dams (Ch. 2: Hydrologic Considerations), USBR

Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 8- Determination of the Probable Maximum Flood, FERC

Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420), USACE

Flood Hydrology Manual, USBR


Dam Owner Academy: Extreme Rainfall Events

Technical Seminar: HEC-HMS with Application to Dam Safety


Revision ID: 7365
Revision Date: 07/21/2023