Resources by Topic

Resources by Topic: Hydrology


Hydrology is one of the earth sciences and encompasses the occurance, distribution, movement and properties of waters of the earth. For dams, the most relevant subset of the science of hydrology is surface water hydrology and is most often used in dam engineering for estimating flood inflows to the dam and reservoir. Dams are to be designed to be capable of passing large floods and estimating the magnitude of these floods is critical to dam safety.

Floods typically evaluated in dam engineering include frequency based storms (e.g. 1-year through 500-year flood) and the Probable Maximum Flood (PMF), which is developed based upon the Probable Maximum Precipitation (PMP). The PMP is the greatest depth (amount) of precipitation, for a given storm duration, that is theoretically possible for a particular area and geographic location. The PMF is the flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in a particular drainage area.

Dams are designed or required to safely pass what is typically termed the Spillway Design Flood (SDF) or Inflow Design Flood (IDF), which typically ranges from the 100-year flood to the PMF. The selection of a SDF or IDF is usually based on the hazard category of the dam and the potential for loss of life or property damage that would result from a dam failure during a given flood.

Hydrologic analysis for estimating the SDF or IDF for dams includes:

  • Delineation of the watershed contributing to the dam
  • Developing theoretical precipitation amounts and distribution over the storm duration
  • Estimating infiltration to compute runoff volume
  • Computing runoff distribution based upon a synthetic hydrograph theory
  • Storage routing of the inflow through the impounding body (lake, reservoir, etc.)

In addition to estimating the SDF or IDF, hydrology for dam projects could include evaluation of flood protection provided by the structure. Also, many dams provide water supply and the hydrologic analysis for these structures could extend to drought hydrology for sizing reservoirs and defining releases to address environmental concerns.

Hydrologic analyses can from simplified equations and methods to complex computer models, including commonly used models developed by the US Army Corps of Engineers (USACE) and Natural Resources Conservation Service (NRCS). Geographic Information Systems (GIS) have helped engineers and scientists develop watershed parameters more quickly and accurately.

Individuals interested in hydrologic analysis for dams should develop an understanding of the hydrologic cycle and flood events and should be proficient in reading topographic maps and computer models such as the USACE HEC-HMS and NRCS SITES programs. An understanding of GIS is also recommended.


To be proficient in this area one needs to understand and/or have experience in the following areas:

  • Basic Hydrology for Dams
  • Meteorology and Rainfall Frequency
    • Precipitation – Formation, types, losses
    • Precipitation – Frequency estimates
    • PMP Theory and Studies
    • GIS-based Methods for Precipitation Estimates
    • Probability of Extreme Events
  • Precipitation Distribution Modeling
    • Methods for precipitation distribution
    • PMP Distributions
    • Site Specific PMP Analyses
    • GIS-based Methods for Precipitation Distribution
  • Watershed Modeling
    • Components of the runoff hydrograph, unit-hydrograph theory and application
    • Stream gage flood frequency analysis
    • Empirical hydrologic estimation methods and model calibration
    • GIS-based Methods for Watershed Modeling

Classroom and Web-Based Training

ASDSO Webinars
Additional Classroom Training

For additional training opportunities, search the ASDSO Training Directory.

Guidelines and Suggested References

Some of the most highly recommended resources in this area are listed below. (See also: TADS materials, listed above under "Self-paced Courses.") For additional resources, search the ASDSO Bibliography. Suggested search terms: guidelines, state programs, program management, quality control

Federal Publications

Federal Emergency Management Agency/Interagency Committee on Dam Safety

Federal Energy Regulatory Commission

National Oceanic and Atmospheric Administration

  • L. C. Schreiner, J. T. Riedel and United States National Oceanic and Atmospheric Administration. Hydrometeorological Report No. 51 (HMR-51), Probable Maximum Precipitation Estimates, United States East of the 105th Meridian, Washington, DC, June 1978, 87 pp.

U.S. Army Corps of Engineers

U.S. Bureau of Reclamation

U.S. Department of Agriculture, Natural Resources Conservation Service

Miscellaneous Publications

  • V. T. Chow. Handbook of applied hydrology; a compendium of water-resources technology (1964). McGraw-Hill.
  • D. R. Maidment. Handbook of Hydrology (1993). McGraw-Hill.


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