Summary of Significant Floods in the United States, Puerto Rico, and the Virgin Islands, 1970 Through 1989

C.A. Perry, B.N. Aldridge, and H.C. Ross

Abstract

This volume is a compilation of significant floods that occurred throughout the United States, Puerto Rico, and the Virgin Islands during the period from January 1, 1970, through December 31, 1989. A significant flood in this report refers to a maximum discharge (instantaneous or time averaged) that is in the top 5 percent of all the annual maximum discharges recorded or measured at streamflow-gaging stations during their total period of record. Most of these floods are approximately equal or greater than the 20-year recurrence-interval flood (0.05 probability of occurrence in any 1 year) for that station.

A summary of the most devastating floods according to amount of damage and lives lost is provided for each year for the period 1970 through 1989. Significant interstate floods are described also. For each year, a map is provided showing the States with the percentage of total streamflow-gaging stations having significant floods.

A compilation of specific data for the significant floods is arranged State by State for each of the 50 United States and Puerto Rico. Each State compilation includes: (1) a description of the general hydroclimatology and conditions that produce significant floods, (2) descriptions of climatic and basin characteristics that significantly affect maximum flows within the State, (3) tables of data that allow the reader to compare each significant flood during the period 1970 through 1989 with the maximum flood for the entire period of record at selected streamflow-gaging stations, and (4) State maps show the location of the streamflow-gaging stations.

Introduction

Maximum stream discharges for selected locations are compiled annually by the U.S. Geological Survey's State and Puerto Rico offices. Each office publishes this data along with other data, including daily flow, water-quality, and ground-water information, in the U.S. Geological Survey annual Water-Data Report series. The maximum discharges for each streamflow-gaging station are also placed in the Peak Flow File, which is maintained at each office's World Wide Web (www) computer site. However, publications were needed that compiled significant floods nationwide and provided a relative measure of the severity in a single publication. These publications became the National Flood Summary series of which this volume is a part. This Water-Supply Paper, in addition to providing a list of floods for the period 1970 through 1989, provides a description of major or significant floods and provides some information on their cause and resulting costs, damage estimates, and reported loss of life.

For this report, a significant flood refers to a maximum discharge (instantaneous or time averaged) that is in the top 5 percent of all the annual maximum discharges recorded or measured at a streamflow-gaging station during its total period of record. If the period of record contains an unregulated period and a regulated period, the record is broken into two parts. The top 5-percent maximum discharges for each period were determined and combined for that station. If the significant flood occurred during the period 1970 through 1989, that flood was included in this summary. Most of these significant floods were approximately equal to or greater than the 20-year recurrence interval (0.05 probability of occurrence in any 1 year) for that station. The listing includes the designation of whether a specific flood was unregulated or regulated.

Innumerable combinations of variable meteorologic and physiographic factors produce floods of all degrees and severity. Some meteorologic factors that affect floods are the form, amount, duration, and intensity of precipitation; the amount of previous precipitation, which would affect the moisture absorption of the soil; the air temperature, which may cause frozen soil or may determine the rate of snowmelt; and the direction of storm movement. The principal physiographic features of a drainage basin that determine floodflows are drainage area, elevation, character of soil, shape, slope, direction of slope, and vegetative or other land cover. With the exception of vegetative cover and soil preconditions, the physiographic features are fixed for any given natural drainage basin. The combination of the magnitude and intensity of meteorologic phenomena, the antecedent moisture conditions, and the effect of inherent physiographic features on runoff determines the magnitude of a flood.

Flood damages frequently are difficult to assess. Dollar amounts given in this report should be used as a general indication of flood losses rather than as definite values. Even if detailed surveys and estimates have been made, there is little consistency among methods used and types of losses included. Some estimates may exclude certain locations (such as mountainous areas) or types of loss (either insured or uninsured) or type of property (either private or public). Some estimates include traffic interruptions and flood-mitigation costs; others include strictly physical damage. Estimates may be based on replacement costs or on depreciated values. For floods not described in detailed published reports, the only damage estimates available usually are the preliminary figures contained in newspapers, National Oceanic and Atmospheric Administration (NOAA) climatological data, or other sources published shortly after the flood. A statement that a disaster declaration was issued indicates that the damage was severe and that financial aid to victims was authorized by the governmental entity making the declaration.

Some of the flood descriptions in this volume give the amount of rainfall and duration of the storm associated with the flooding. Recurrence intervals for these storms may be determined from a rainfall-frequency atlas of the United States (Hershfield, 1961) or from a simplified set of equal-rainfall maps and charts contained in a report by Rostvedt (1965).

Continuing investigation of surface-water resources within the United States is performed by the U.S. Geological Survey in cooperation with State agencies, the U.S. Army Corps of Engineers, the Bureau of Reclamation (U.S. Department of the Interior), and other Federal or local agencies. The National Weather Service, in addition to collecting and compiling data on meteorological phenomena, also collect data on stream stages in some areas.

Previous Flood Summary Reports

During the 1950's and 1960's, the U.S. Geological Survey summarized floods of each year in an annual series of Water-Supply Papers entitled, "Summary of Floods In the United States." A summary was published for each calendar year from 1950 through 1969. Water-Supply Paper 1137-I, the first in the series (U.S. Geological Survey, 1954), states the purpose of the series as being:

"To assemble in a single volume information relating to all known severe floods in the United States, whether local or of wide areal extent. For floods that are described in... other publications of the Geological Survey, or in reports by other Federal and State agencies, only very brief mention including references to the reports containing detailed descriptions, will be given here. Local floods for which no individual reports have been prepared are briefly described."

In the first volume of that Water-Supply Paper series, each flood was described in a maximum of three or four paragraphs. Later volumes contained longer articles including maps. The series was discontinued after the 1969 volume; however, in 1987 a program was begun to prepare and publish summaries for 1970 and succeeding years. Two flood summary publications (one for the calendar years 1990 and for 1991 and one for January 1992 through September 1993) were published with the longer article format. Water-Supply Papers 2474 and 2499 cover the periods 1990 and 1991 and January 1992 through September 1993, respectively. Much of the following introductory material is paraphrased from these reports (Jordan and Combs, 1997; Perry and Combs, 1998) and from the published report describing floods for 1968 (Rostvedt, 1972). This volume contains flood summaries and flood statistics for the time period from January 1, 1970, through December 31, 1989, for the 50 United States, Puerto Rico, and the Virgin Islands.

Determination of Maximum Stages and Discharges

The usual method of determining stream discharges at a streamflow-gaging station is the application of a stage-discharge relation to a known stage. This relation usually is defined by current-meter measurements made through as wide a range of stage as possible (fig. 1). If the maximum discharge exceeds the range of the current-meter measurements, short extensions may be made to a graph of the stage-discharge relation by logarithmic extrapolation, by velocity-area studies, or by the use of other measurable hydraulic factors (Kennedy, 1983).

Maximum discharges that are greatly above the range of the defined stage-discharge relation at gaging stations and maximum discharges at miscellaneous sites that have no developed stage-discharge relation generally are determined by various types of indirect measurements. In addition, adverse conditions often make it impossible to obtain current-meter measurements at some gaging stations during significant flooding. Maximum discharges at these stations are determined after the floods have subsided, by indirect methods, which involve determination of water-surface elevations from high-water marks, surveying cross sections, and computing discharge from hydraulic equations rather than from direct measurement of stream velocity by use of a current meter. Indirect methods are described by Dalrymple and Benson (1967), Hulsing (1967), Matthai (1967), Bodhaine (1968), and Benson and Dalrymple (1987).

The accuracy of indirect measurements depends upon onsite conditions and the experience of personnel who select the indirect measurement sites and make the surveys, and generally is poorer than for current-meter measurements. The indirect measurements used in determining maximum discharges for floods are not identified as such in this volume. Information as to the source and quality of discharge data in this volume can be obtained from the U.S. Geological Survey office in the State or territory in which the particular streamflow-gaging station is located.

Explanation of Flood Information

In this volume, first a summary of the most devastating floods according to the number of lives lost and the amount of damage is provided for each year for the period 1970-89. Significant interstate floods are described also. A map is provided for each year showing the percentage of total streamflow-gaging stations having significant floods in each State or territory. In addition, a list of selected references is provided for floods occurring that year.

Next, floods in individual States or territories are examined. A brief narrative of State or territory hydroclimatology is followed by selected significant flood descriptions. A map showing location of all streamflow-gaging stations where there was a significant flood of an approximately 20-year recurrence interval or greater during 1970 through 1989 is provided. Some of these maps have physiographic designations that are described by Fenneman (1946). Finally, specific flood data for each significant flood, including maximum stages and discharges for the period of record for those streams, are summarized in tables. The flood data for each State or territory are compiled in downstream order.

In an example of the summary table (table 1), the first two columns identify the streamflow-gaging station, which may be a continuous-record streamflow-gaging station, a partial-record station, or another site at which data have been obtained. The first column gives the U.S. Geological Survey permanent station number (downstream-order number). The second column gives the name of the streamflow-gaging station.

Table 1. Example of summary table presented in State or territory compilations

[mi², square miles; ft, feet above an arbitrary datum; ft³/s, cubic feet per second; --, not determined or not applicable; >, greater than; <, less than. Source: Recurrence intervals calculated from U.S. Geological Survey data. Other data from U.S. Geological Survey reports or data bases]

				Maximum stage and discharge for period of record through 1992, 1993, 1994, or 1995				Significant floods during 1970-89
Station number	Station name	Total drainage area (mi²)	Period of record	Water year	Stage (ft)	Dis- charge (ft³/s)	Date (month/ day/year)	Stage (ft)	Discharge (ft³/s)	Regu- lated during flood¹	Recur- rence interval (years)
05551212	Hypothetical Creek near Town, ST	21.0	1961-90	1961	13.1	--	02/02/72 11/30/88	12.22 12.67	4,200 5,500	N	25 25-50
05555000	Hypothetical River at City, ST	1,212	1939, 1955-94	1986	21.21	82,800	09/12/86	21.21	82,800	N	>100
06930030	Hypo River near Metropolis, ST	3,333	1919-94	1943	33.33 --	-- 99,900	12/23/76	25.55	33,000	N	25 10-25

¹Regulated during flood; N, no; Y, yes.

Total drainage area in the summary table is the total area, as measured on a flat projection map, that constitutes the stream drainage basin (enclosed by the divide). The actual drainage area contributing to runoff may be smaller than the total drainage area if the total area includes areas of extremely rapid infiltration rates that do not produce surface runoff or includes closed subbasins within the larger basin that do not have surface outlets (noncontributing areas).

The column headed "Period of record" shows the calendar years for which the stage or discharge shown in the sixth and seventh columns are known to be a maximum. For most streamflow-gaging stations, this period corresponds to the period of systematic collection of streamflow data. For other stations, written or oral history may indicate that a flood stage was the highest since people have observed the stream or was the highest since some known date. For some stations, two or more periods are given. The use of two periods separated by a comma indicates a break in the period of record. Maximum stages or discharges during the intervening period are unknown.

The fifth column shows the water year in which the maximum stage and discharge for the indicated period occurred. The sixth and seventh columns show the stage and discharge of that maximum. Separate listings are made when maximum stage and maximum discharge did not occur concurrently.

The last five columns present data for the maximum stages and discharges from January 1, 1970, through December 31, 1989. The data include the date on which the maximum occurred, maximum stage, and maximum discharge, whether the stream was regulated at the time of the flood, and, where available, the recurrence interval (RI) of the discharge. Regulation can have a substantial effect on discharge measurements, as in the case of a flood-control reservoir a short distance upstream or a flood bypass around the gaging station, or a less substantial effect, as in the case of a reservoir controlling a small part of the total drainage area.

The probability of a given discharge being equaled or exceeded in any given year frequently is used as an indication of a flood's relative magnitude and for comparison with floods at other gaging stations. The relative flood magnitude also can be expressed in terms of the percentage chance of occurrence, which is 100 times the flood probability. A third way of expressing the relative magnitude is in terms of recurrence interval, which is the reciprocal of the flood probability. A discharge that has a probability of 0.10 will be equaled or exceeded on average (over a long period of time) of once in 10 years, has a 10-percent chance of occurring in any given year, has a recurrence interval of 10 years, and is termed a "10-year flood." A 100-year flood has a probability of 0.01, will be equaled or exceeded on average (over a long period of time) of once in 100 years, has a 1-percent chance of occurring in any given year, and has a recurrence interval of 100 years. Because recurrence interval is used most commonly by Federal agencies (for example, in the context of flood insurance), it is used in this volume even though percentage chance avoids the unintended connotations of regularity of occurrence that accompany the term "recurrence interval."

Equivalence of flood probability and percentage-chance values to selected recurrence-interval values is as follows:

Probability	Percentage chance	Recurrence interval (years)
0.50	50	2
.20	20	5
.10	10	10
.04	4	25
.02	2	50
.01	1	100

In addition to probability or percentage chance of a given magnitude of discharge occurring in any 1 year, the probability or percentage chance of occurrence during a given period of consecutive years also can be calculated. Results of such calculations for selected combinations of recurrence interval and length of period are as follows:

[*, greater than a 99.9- but less than a 100-percent chance]

Recur- rence interval (years)	Percentage chance for indicated time period, in years
	5	10	50	100	500
2	97	99.9	*	*	*
10	41	65	99.5	*	*
50	10	18	64	87	*
100	5	10	39	63	99.3

Recurrence intervals computed for any given flood may differ from gaging station to gaging station because of nonuniform distribution of runoff and uncertainty in the computed recurrence values. Operational patterns for reservoirs generally are not defined adequately to permit recurrence intervals to be computed for maximum discharges on regulated streams.

Another method of indicating a flood's relative magnitude is by comparison of its maximum discharge and the stream's drainage area with values on a regional "envelope curve." A flood-envelope curve is one drawn on a graph in which maximum known discharges are plotted against the drainage area of each streamflow-gaging station (fig. 2). The envelope curve is a smooth curve drawn to equal or exceed all the plotted discharges in relation to the drainage areas. Envelope curves are given for 17 regions of the conterminous United States in Crippen and Bue (1977). This method is better than the formerly used calculation of "unit discharge" (division of the discharge by the drainage area) because unit discharges for greatly different sizes of drainage area are not comparable. Large unit discharges are common for small drainage areas but are usually rare for large drainage areas.

Selected References

Barrows, H.K., 1948, Floods, their hydrology and control: New York, McGraw-Hill Book Co., 432 p.
Benson, M.A., and Dalrymple, Tate, 1987, General field and office procedures for indirect measurements: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A1, 29 p.
Bodhaine, G.L., 1968, Measurement of peak discharge at culverts by indirect methods: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A3, 60 p.
Crippen, J.R., and Bue, C.D., 1977, Maximum floodflows in the conterminous United States: U.S. Geological Survey Water-Supply Paper 1887, 52 p.
Dalrymple, Tate, and Benson, M.A., 1967, Measurement of peak discharge by slope-area method: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A2, 12 p.
Fenneman, N.M., 1946, Physical divisions of the United States: Washington, D.C., U.S. Geological Survey, map, scale 1:7,000,000.
Hershfield, D.M., 1961, Rainfall frequency atlas of the United States: U.S. Department of Commerce, Weather Bureau Technical Paper 40, 115 p.
Hoyt, W.G., and Langbein, W.B., 1955, Floods: Princeton, N.J., Princeton University Press, 469 p.
Hulsing, Harry, 1967, Measurement of peak discharge at dams by indirect methods: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A5, 29 p.
Jennings, M.E., Thomas, W.O., Jr., and Riggs, H.C., 1994, Nationwide summary of U.S. Geological Survey regional regression equations for estimating magnitude and frequency of floods for ungaged sites, 1993: U.S. Geological Survey Water-Resources Investigations Report 94-4002, 196 p.
Jordan, P.R., and Combs, L.J., eds., 1997, Summary of floods in the United States during 1990 and 1991: U.S. Geological Survey Water-Supply Paper 2474, 257 p.
Kennedy, E.J., 1983, Computation of continuous records of streamflow: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A13, 53 p.
Langbein, W.B., and Iseri, K.T., 1960, General introduction and hydrologic definitions, in Manual of hydrology, part 1, General surface-water techniques: U.S. Geological Survey Water-Supply Paper 1541-A, 29 p.
Leopold, L.B., and Maddock, Thomas, Jr., 1954, The flood control controversy: New York, Ronald Press Co., 278 p.
Matthai, H.F., 1967, Measurement of peak discharge at width contractions by indirect methods: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. A4, 44 p.
Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., compilers, 1991, National water summary, 1988-89-Hydrologic events and floods and droughts: U.S. Geological Survey Water-Supply Paper 2375, 591 p.
Perry, C.A., and Combs, L.J., eds., 1998, Summary of floods in the United States, January 1992 through September 1993: U.S. Geological Survey Water-Supply Paper 2499, 286 p.
Rostvedt, J.O., 1965, Summary of floods in the United States during 1960: U.S. Geological Survey Water-Supply Paper 1790-B, 147 p.
---1972, Summary of floods in the United States during 1968: U.S. Geological Survey Water-Supply Paper 1970-B, 73 p.
U.S. Geological Survey, 1954, Summary of floods in the United States during 1950: U.S. Geological Survey Water-Supply Paper 1137-I, p. 957-991.
Wilson, W.T., 1942, Melting characteristics of snow and its contribution to runoff, in June 30-July 2, 1941, Hydrology Conference, Proceedings: University Park, Pennsylvania State College, School of Engineering Technical Bulletin 27, p. 153-165.

Water-Supply Paper 2502

U.S. Department of the Interior U.S. Geological Survey