U.S. Geological Survey Water-Supply Paper 2375
National Water Summary 1988-89--Floods and Droughts:
Precipitation is plentiful in Alabama. Annual precipitation ranges from about 50 inches in the Tennessee River Valley area to about 65 inches in the gulf region and averages about 55 inches statewide. Precipitation distribution varies seasonally, annually, and geographically. The Gulf of Mexico and the subtropical Atlantic Ocean are the principal sources of moisture for the State, and the Pacific Ocean is a lesser source (fig. 1). Most winter precipitation results from frontal systems and cyclone development in the gulf. Summer precipitation results mainly from thunderstorms and occasional tropical cyclones, including tropical storms and hurricanes.
In addition to the oceans, important moisture sources include local and upwind land surfaces, as well as lakes and reservoirs, from which moisture evaporates into the atmosphere. Typically, as a moisture-laden ocean airmass moves inland, it is modified to include some water that has been recycled one or more times through the land-vegetation-air interface.
The major causes of floods in Alabama are (1) intense precipitation and high coastal waters associated with hurricanes, tropical storms, and tropical depressions; (2) thunderstorms; and (3) slow-moving or stationary frontal systems. The probability of flooding increases during the spring when rivers and creeks, already swollen from spring runoff, receive additional rainfall. Severe weather and large quantities of precipitation can be produced when warm, moist air from the gulf converges with cold, arctic air from the north.
Droughts are a cumulative result of numerous meteorological factors. Most droughts in Alabama begin with decreased precipitation during the winter and spring, when soil moisture is being recharged. Adequate soil moisture is important during early summer because mid- to late summer is the time of least precipitation and greatest evapotranspiration.
If a subtropical high-pressure cell, called the Bermuda High, and a weak jetstream persist over the State, then the stable, subsiding air inhibits the normal development of airmass thunderstorms. Longwave troughs positioned west of the Bermuda High divert storm tracks either north or south of the region. The combination of decreased precipitation and cloudiness, increased solar radiation, and extreme heat resulting from decreased evaporation dries and hardens the soil. Should rainfall occur, the hardened soil will hinder recharging of the soil moisture.
MAJOR FLOODS AND DROUGHTS
Floods and droughts are natural characteristics of streams. Floods attract immediate attention because of their sudden and destructive effects on lives and property. In contrast, droughts usually are not felt until after long periods of deficient rainfall and unrestrained water use.
Major floods and droughts, discussed herein, are those that were areally extensive and had significant recurrence intervals greater than 25 years for floods and greater than 10 years for droughts. These major events, and those of a more local nature, are listed chronologically in table 1; rivers and cities are shown in figure 2.
To portray the intensity and duration of floods (fig. 3) and droughts (fig. 4), six streamflow-gaging stations were selected to represent a cross section of basin drainage-area size (342-1,675 square miles) and geographic distribution and to represent basins that have little if any regulation, diversion, or channelization. Streamflow data are collected, stored, and reported by water year (a water year is the 12-month period from October 1 through September 30 and is identified by the calendar year in which it ends).
Flood-producing rains in Alabama are associated with two types of storms: frontal systems and tropical storms. The former occur every year, usually between November and April, and produce steady rainfall over large areas. Major flooding in Alabama has been produced by rains associated with broad cyclonic storms embedded in frontal systems. Tropical storms, which generally occur between July and November, are less frequent but commonly produce torrential rains when movement is inland from the Gulf of Mexico. The intensity of hurricanes generally decreases as they move inland and dissipate. The areal extent and severity of major floods, the magnitude of annual peak discharge, and the theoretical 10- and 100-year recurrence intervals at the gaging stations are shown in figure 3.
Before the 1930's, flood information was obtained from a small network of continuous-record gaging stations on some of the larger streams in the State. A report by the U.S. Army Corps of Engineers (1965) provides information on hurricanes from 1900 through 1960,
Historical information indicates that the flood of April 1886 on the Alabama River near Montgomery was the largest since settlement of the area in 1814. A peak stage of 160.6 feet was determined from floodmarks.
The flood of 1906 was caused by a hurricane that originated in the Caribbean and moved inland at Pascagoula, Miss., on September 27. The storm surge exceeded 10 feet at Gulf Shores. The flood of 1916 was caused by rainfall from a hurricane that originated in the Caribbean and moved inland at Gulfport, Miss., on July 5. Flood damage was $3.5 million in the Mobile area, where winds of 128 miles per hour and a record high-water stage of 10.8 feet were recorded (U.S. Army Corps of Engineers, 1965).
Table 1. Chronology of major and other memorable floods and droughts in Alabama, 1886-1988
A storm in March 1929 resulted in one of the largest rainfalls on record in southeastern Alabama. This storm, which was centered at Elba, produced 20 inches of rain on March 15; total precipitation was 29.6 inches in 72 hours. Flood discharges had recurrence intervals greater than 100 years over a nine-county area. Isolated flooding in northern Alabama associated with this system resulted in a peak discharge of the Flint River near Chase (fig. 3, site 6) that had a recurrence interval greater than 25 years.
During February-March 1961, a succession of low-pressure systems from the Gulf of Mexico moved northward and produced several intense storms. This series of storms caused rainfall totals of 16-18 inches in central Alabama. Flooding was severe on the Alabama and Black Warrior Rivers following moderate flooding on some of their tributaries. The Alabama River at Selma had a record peak discharge that had greater than a 100-year recurrence interval, and the river remained above flood stage for 17 days (Bames and Somers, 1961).
Hurricane Canaille moved across the Mississippi coast during August 1969, then moved northward through Mississippi. Many lives were lost, and property damage was extensive in Mississippi; however, coastal Alabama was less severely affected.
Torrential rains within a 48-hour period caused severe flooding in the upper Tombigbee and Tennessee River basins during March 14-16, 1973. Rainfall was greatest in northwestern Alabama, where Hamilton received 10.5 inches. Peak discharges had greater than a 100-year recurrence interval on the Buttahatchie River, a major tributary to the upper Tombigbee River in Alabama; however, flooding was moderate on other tributaries in the basin (Edelen and Miller, 1976). This flood was the largest of record for the Flint River near Chase (fig. 3, site 6).
Record floods occurred on streams in central Alabama during March-April 1979. A series of early spring storms in the region had produced extremely moist antecedent soil conditions. Average rainfall during the period in the Tombigbee River basin upstream from Livingston was more than 8 inches: the maximum was 17.3 inches at Pickensville. The combined flow of the Tombigbee and Black Warrior Rivers produced the maximum known flood since 1874. Peak discharges were the greatest of record on the Tallapoosa River at Wadley and the Sucamoochee River at Livingston (fig. 3, sites 2 and 5). The flood destroyed a bridge over the Tallapoosa River. Total damage for the flood was about $75 million (Edelen and others, 1986).
Hurricane Frederic, in September 1979, was one of the more intense hurricanes of record to enter the United States mainland. In terms of property damage, Hurricane Frederic was by far the most destructive; total damage was about $2 billion (Scott and Bohman, 1980a,b).
Alabama has had five major droughts since recordkeeping began--1929-32, 1938-45, 1950-63, 1980-82, and 1984-88. A network of 20 long-term gaging stations was used to define the areal extent and recurrence interval of the droughts. Six of these gaging stations, which had record lengths ranging from 50 to 65 years, were selected to show the intensity and duration of droughts.
The graphs of annual departures (fig. 4) from the long-term average discharge for a gaging station identify periods of streamflow deficit or surplus. The selected hydrologic droughts included in this report lasted more than 1 year and had substantial adverse effect on agriculture and industry.
The severity and areal distribution of the hydrologic droughts identified in figure 4 differed across the State. Areas most severely affected by drought were north-central, 1929-32, and statewide, 1938-45, 1950-63, 1980-82, and 1984-88. The maps (fig. 4) show the areal extent and recurrence interval for these identified droughts.
The drought of 1929-32 had a recurrence interval greater than 25 years in the Mulberry Fork basin (fig. 4, site 4); in other river basins in east-central Alabama, the recurrence interval was 10-25 years. In the Flint River basin (fig. 4, site 6) of the Tennessee River Valley area, the drought had a recurrence interval of 25 years.
The drought of 1938-45 was statewide but was most severe in the northeastern part of the State, where recurrence intervals were greater than 25 years (fig. 4). Runoff during 1941 averaged about 50 percent of the annual average for gaging stations in the Mulberry Fork and Flint River basins (fig. 4, sites 4 and 6). In the rest of the State, the drought had a recurrence interval of 10-25 years.
Included in the sustained drought of 1950-63 was a severe drought during 1954-55. In terms of areal coverage and severity, 1954 is the most extreme drought year on record in Alabama. The annual-departure graphs (fig. 4) for sites 1-5 show a generally negative departure from 1950 to 1955. Runoff during 1954 averaged about 50 percent of the annual average of 21 inches for the four gaging stations in the Mobile River basin. In southern Alabama, precipitation for 1954 was less than average each month, and the annual total of 34.4 inches was the smallest in 71 years of record (U.S. Weather Bureau, 1955). The drought of 1950-63 had a recurrence interval of 44-60 years for gaging stations in the Mobile River basin. In the Flint River basin (fig. 4, site 6) of northern Alabama, the drought had a recurrence interval of 27 years.
Statewide, an extended period of greater than average rainfall began about 1970, lasted about 10 years, and produced a steady upward trend in the annual-departure graphs. Rainfall again became deficient in 1980, however, and in 1981 the rainfall deficiency across the State ranged from 5 to 14 inches. The drought of 1980-82 affected most of Alabama and had recurrence intervals of 10-25 years. Greater than average rainfall during 1983 resulted in recoveries in streamflow at the six gaging stations until about mid-1984 (fig. 4).
Beginning in mid-1984, all gaging stations show significant negative annual departures (fig. 4), some continuing through 1988 (sites 1-4). The drought of 1984-88 affected the entire State to some degree but was most significant in the eastern one-half. Recurrence intervals ranged from 30 to 50 years in eastern Alabama to 10 to 25 years north of this area. The recurrence interval for a narrow area in west-central Alabama and most of the Conecuh River basin in southern Alabama was 10-25 years. Maximum rainfall deficiencies in east-central Alabama ranged from 21.1 to 25.3 inches during 1986 (National Oceanic and Atmospheric Administration, 1986). Similar unofficial rainfall deficiencies were reported for 1987. Emergency water measures and restricted reservoir releases were implemented. During the most intense period of the drought, August 1988, daily water supplies were being transported to 15 communities in east-central Alabama. Crop losses during 1986 are reported to have been about $500 million (John Trotman, U.S. Department of Agriculture, oral commun., 1986).
Contingency planning for floods or droughts and corresponding responses require coordination and cooperation of all levels of government-Federal, State, county, and local. Responsibilities are defined for flood-plain management, flood-warning systems, and water-use management during droughts.
Flood-Plain Management.--Increased economic growth and development in Alabama during the past two decades have contributed to a growing awareness of the need for flood-plain management in the State. Since September 1979, the Office of State Planning and Federal Programs has functioned as the State coordinating agency governing the development and construction of buildings, structures, roads, and other facilities on flood plains.
Flood-Warning Systems.--Flood warning, protection, and abatement are largely the responsibility of Federal agencies such as the U.S. Army Corps of Engineers, Tennessee Valley Authority, National Oceanographic and Atmospheric Administration, and Federal Emergency Management Agency. The National Weather Service operates a flood-warning network in Alabama that includes about 25 sites. The Alabama Power Company operates an extensive network of 17 sites that have telemetry gages in the Coosa and Tallapoosa basins.
Water-Use Management During Droughts.--Alabama does not have a comprehensive water-management plan. Because of the abundant water resources and sparse population, competition for the resource has been minor, and little responsibility has been assigned for the water-related issues of floods and droughts.
Drought was not a major concern until the drought of 1984-88 (especially in 1986), the severity of which raised the awareness of the public and local and State governments. Water use during the drought was managed jointly by Federal (U.S. Army Corps of Engineers, Tennessee Valley Authority), State, and local governments and a public utility (Alabama Power Company). Because of this experience, concerned officials now are more aware of drought conditions and better able to coordinate drought-management activities.
Prepared by Hillary H. Jeffcoat, J. Brian Atkins, and D. Briane Adams, U.S. Geological Survey; "General Climatology" section by Steven F. Williams, Assistant Alabama State Climatologist
FOR ADDITIONAL INFORMATION: District Chief, U.S. Geological Survey, 520 19th Avenue, Tuscaloosa, AL 35401