ASDSO Dam Safety Toolbox


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All dams need adequate spillway capacity. 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 looking to 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.

Hydrologic Design Criteria

“Selection of an Inflow Design Flood (IDF) for a new dam or a dam undergoing significant modifications should take into account both current conditions and reasonably anticipated future development. Development within the upstream watershed can cause increased runoff and peak flows, while development within the downstream inundation area can alter hazard potential classification and specific estimates of consequences of hydrologic events… When the cost of more detailed methods such as incremental consequence analysis, a site-specific probable maximum precipitation study, or risk-informed hydrologic hazard analysis is prohibitive, prescriptive IDF criteria for High, Significant, and Low Hazard dams are recommended”.[2]

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.

Types of Evaluations


High and significant hazard dams should be designed to pass an appropriate design flood. Dams constructed prior to the availability of extreme rainfall data should be assessed to make sure they have adequate spillway capacity. Learn more at

Best Practices Resources

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

Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 2- Selecting and Accommodating Inflow Design Floods for Dams, FERC

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

Flood Hydrology Manual, USBR

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


On-Demand Webinar: Hydrology 101 for Dam Safety

On-Demand Webinar: Hydrology 201 for Dam Safety

Technical Seminar: HEC-HMS with Application to Dam Safety


Revision ID: 7172
Revision Date: 07/11/2023