Hydrology

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All dams need adequate spillway capacity. Learn more at DamFailures.org

Hydrology is essential not only in the design of a dam, but also in the feasibility study of a dam as well as 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”.

“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”. 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 taken 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”.

Types of Evaluations

• Low Flow Conditions
• Normal Flow Conditions
• Flood Conditions
• Sedimentation
• Groundwater
• Unusual or Changing Conditions in a Watershed

Examples

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 DamFailures.org

Best Practices Resources

Technical Release 210-60: Earth Dams and Reservoirs, NRCS, 2019

Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 2- Selecting and Accommodating Inflow Design Floods for Dams, FERC, 2015

Selecting and Accommodating Inflow Design Floods for Dams (FEMA P-94), FEMA, 2013

Design Standards No. 14: Appurtenant Structures for Dams (Ch. 2: Hydrologic Considerations), USBR, 2013

Flood Hydrology Manual, USBR, 1989

Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420), USACE, 1987

Trainings

On-Demand Webinar: Hydrology 101 for Dam Safety

On-Demand Webinar: Hydrology 201 for Dam Safety

Citations:

Revision ID: 6501
Revision Date: 01/26/2023