Oregon State University

Dam Planning Module Glossary

a collection of terms with definitions to be used in the dam planning and operations module

Bulletin 17B

Guide which describes the data and procedures for computing flood flow frequency curves using systematic stream gaging records. The guidelines in Bulletin 17B fit a Pearson Type III frequency distribution to the logarithms of instantaneous annual peak flows.

U.S. Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency, Bulletin 17-B of the Hydrology Subcommittee: Reston, Virginia, U.S. Geological Survey, Office of Water Data Coordination, [183 p.]. [Available from National Technical Information Service, Springfield VA 22161 as report no. PB 86 157 278 or from FEMA on the World-Wide Web at http://www.fema.gov/mit/tsd/dl_flow.htm ]

confidence interval

A confidence interval refers to a range of values that likely contains a population parameters, such as discharge. It provides a means for reporting the level of uncertainty and reliability of an estimate.

Synonyms: confidence intervals
conveyance capacity

The flow rate at which water can be transported by a stream, canal, aqueduct, etc. without exceeding the bankfull stage or causing levee damage (in the U.S., generally measured in cubic feet per second (cfs)).

design flood

the flood, either observed or synthetic, which is chosen as the basis for the design of a hydraulic structure.

Equation 1

The exceedance probability, or probability that the event is equaled or exceeded in any given year, associated with each observed annual peak flow equals:

Equation 1: m / (n+1)

where n = the number of values in the dataset and m = the rank (with the largest flow of the dataset ranked 1).

Equation 2

The return interval (in years) is the inverse of the exceedance probability:

Equation 2: (n+1) / m

where n = the number of values in the dataset and m = the rank (with the largest flow of the dataset ranked 1).

exceedance probability

The probability of exceedance describes the likelihood of a specified flow rate (or volume of water with specified duration) being exceeded in a given year. For example, an event having a 1 in 100 chance of occurring in any single year will be described as the 1% annual exceedance probability event.

flood control pool

portion of a reservoir designated to hold flood waters.

flood frequency analysis

the calculation of the statistical probability that a flood of a certain magnitude for a given river will occur in a certain period of time.

objective release

flow that can be safely conveyed by the downstream channel.


United States Army Corps of Engineers

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