What is a Flood Frequency?

The 100-year flood event is the most publicized flow event, primarily due to the extent of damage, flood-insurance regulations, and building codes. However, higher frequency events affect the community more often and at a potentially greater cost.

A 100-year flood has a 1 chance
in 100, or a
1-percent probability of happening in
any given year.

The chance of a given flow being exceeded once in any given year is expressed as a probability (Garcia, 1997). Recurrence intervals, or return periods as they are also known (and thus referred to in this web site), is the average interval of time, expressed in years, within which the given flood will be equaled or exceeded once (Langbein and Iseri, 1960), at a particular location. Thus, a flood having a return period of 100 years (a 100-year flood) has a 1 chance in 100, or a 1-percent probability of happening in any given year. This is a statistical computation using as many years of data as possible (10 years of annual peak streamflow data is the recommended minimum) and does not mean that a flood of this magnitude will happen only once every 100 years. Otherwise stated, the occurrence of a flood of a given return period does not affect the probability of such a flood occurring again the next year. Flood-frequency values will change either up or down as more data is collected and the flood frequency is recalculated.

Bank-full discharge is predicted to occur for most alluvial streams, on average, once every 1.5  years (Leopold, 1994). Out-of-bank flooding is inferred from flood-frequency analyses to occur, on average, once every 2.3 years (with a 40 percent chance of occurring in a given year); thus, frequently affecting development located near or adjacent to the main-stem Carson River. Larger flows, such as the 2006 New Year's flood (a 10-25 year flood) increase the risk and frequency of flood-related damages to property and infrastructure when development occurs on vulnerable floodplain areas.

Out-of-bank flooding is predicted to occur for most streams on average once every 2.3 years

Flood-frequency data has been provided in this web site for streamflow gaging records that exceed 10 years. These data are point estimates (annual peak flows measured at specific locations on the river). No interpretations of flood extent (how much of the floodplain is inundated) are provided in this web site with the exception of already published maps. However, using the historic photo archive for specific dates and locations, the user will obtain a qualitative appreciation of the extent of flooding in the Carson River Basin over the last century. Areas affected by past floods have been mapped by FEMA and are available to the public (http://msc.fema.gov). These maps provide approximate boundaries of areas subject to inundation by a 100-year flood.

On this web site, each gaging station includes a Hydrologic Data Page that includes an annual peak flow graph (Figure 1). The annual peak flow graph shows the highest streamflow measured for each year of record. The year for the flood event selected is highlighted in the graph. The annual peak flow data is also available in tabular format by clicking on the NWISWeb data table link. Figure 2 is an example of the resultant flood-frequency graph for a specific gaging station. Flood-frequency values are provided in the table below the graph with the “Q” values representing streamflow for the following return periods in years: 2.3, 5, 10, 25, 50, 100, 200, and 500. The streamflow values are in cubic feet per second.

Flood-frequency values, in cubic feet per second (cfs), used in figure 2.

References:

Garcia K.T., 1997, January 1997 flooding in northern Nevada—Was this a “100-year” flood?: U.S. Geological Survey Fact Sheet FS-077-97, 4 p.

Langbein, W.B., and Iseri, K.T., 1960, General introduction and hydrologic definitions, Manual of Hydrology, Part 1. General Surface-Water Techniques: U.S. Geological Survey Water-Supply Paper 1541-A, 29 p.

Leopold, L.B., 1994, A View of the River: Harvard University Press, Cambridge, Mass., p. 135. Book description and ordering information available at <http://www.hup.harvard.edu/catalog/LEOVIE.html>

Wright, J., 1996, Addressing Your Community's Flood Problems, A Guide for Elected Officials: Association of State Floodplain Managers, Inc. and Federal Interagency Floodplain Management Task Force, 54 p. available online at <http://www.fema.gov/pdf/fima/fema309.pdf>