INTRODUCTION

Record-breaking peak discharges were recorded at eight U.S. Geological Survey (USGS) streamflow-gaging stations in central Texas during December 1991 (fig. 1), and substantial peak discharges also occurred at numerous other stations. Large peak discharges during December are unusual in central Texas. The rainfall causing the flooding began on December 18, with 6-day totals exceeding 10 inches (in.) in the area of heaviest rainfall.

This report documents peak discharges and runoff volumes during December 1991. Recurrence intervals were determined for the peak discharges and runoff volumes for 1-, 3-, and 7-day periods. A recurrence interval references the approximate number of years during which a given peak discharge or runoff volume is expected to be equaled or exceeded only once. A flood of a given recurrence interval is defined on the basis of peak discharge—for example, a 100-year flood is defined as the peak discharge that has a 1-percent chance of being equaled or exceeded in any given year.

DESCRIPTION OF STORMS

Rainfall on a large area of central Texas during December 18–23, 1991, caused extensive flooding in many locations. On December 18, a warm, moist, unstable air mass spread northward from the Gulf of Mexico and overran a cold surface ridge that had settled over Texas. Simultaneously, a deep, cold upper-level trough over the southwestern United States and northern Mexico pumped tropical moisture into central and northern Texas from low latitudes across Mexico. Embedded thunderstorms produced heavy rainfall over south-central Texas and expanded northward late on December 18. This combination of moisture and movement persisted until December 23, with thunderstorms producing heavy rainfall throughout central Texas during the period (National Oceanic and Atmospheric Administration, 1991c). Lines of equal precipitation in figure 1 depict the total rainfall during this 6-day period.

Daily rainfall totals in excess of 4 in. were recorded at numerous National Weather Service rain gages. A maximum 24-hour rainfall total of 8.60 in. and a maximum 12-hour total of 7.30 in. were observed at Evant, about 60 miles (mi) west of Waco (National Oceanic and Atmospheric Administration, 1991b). The recurrence interval for these 24- and 12-hour duration rainfall amounts is 50 years (U.S. Weather Bureau, 1961).

The monthly rainfall departure from the 1951–80 normal for December 1991 was 100 percent or more for stations in some parts of the area shown in figure 1 (National Oceanic and Atmospheric Administration, 1991a). The rainfall during November 1991 generally was 50 percent or more below normal.

PEAK DISCHARGES AND RUNOFF VOLUMES

Peak discharges for floods, some exceeding known historical maximums, occurred during December 20–26, 1991, at streamflow-gaging stations in central Texas. Twenty-two stations on unregulated and regulated streams in central Texas where the December 1991 peak discharges equaled or exceeded the 10-year flood are listed in table 1. The December 1991 peak discharge exceeded the previously known peak discharge at 8 of the 22 streamflow-gaging stations.

Recurrence intervals were determined for peak discharges and runoff volumes for 1-, 3-, and 7-day periods from flow data recorded at each gaging station. Recurrence intervals were rounded to the nearest 5 years (table 1), except for those exceeding 100 years, which are shown as greater than 100 years. Peak discharges in December 1991 exceeded the 100-year flood at 5 of the 22 streamflow-gaging stations.

Peak discharge as a function of drainage area for basins throughout Texas are shown in figure 2. The peak discharges during the December 1991 floods are included to illustrate their relative magnitudes with respect to known maximum peak discharges in Texas.

Runoff volumes were computed from discharge hydrographs for the stations; for example, discharge hydrographs for two stations are shown in figures 3 and 4. Instantaneous discharge and daily mean discharge are shown for December 19–23, 1991. Runoff volumes were computed from the shaded area representing the daily mean discharge.

FLOOD DAMAGES

The flooded area in central Texas was declared a Federal disaster area on December 26, 1991. Ten deaths were attributed to the flooding. The Federal Emergency Management Agency dispensed about $43 million in Federal funds for public assistance, temporary housing, individual and family grants, disaster unemployment, and home and business loans (Graham Nance, Federal Emergency Management Agency, oral commun., 1995). The full value of flood damages could have exceeded the $43 million.

SELECTED REFERENCES

Buckner, H.D., and Kurklin, J.K., 1984, Floods in south-central Oklahoma and north-central Texas, October 1981: U.S. Geological Survey Open-File Report 84–65, 112 p.

Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency: Reston, Va., U.S. Geological Survey, Office of Water Data Coordination, Hydrology Subcommittee Bulletin 17B [variously paged].

Massey, B.C., Reeves, W.E., and Lear, W.A., 1982, Flood of May 24–25, 1981, in the Austin, Texas, metropolitan area: U.S. Geological Survey Hydrologic Investigations Atlas HA–656, 2 sheets.

Mills, W.B., and Schroeder, E.E., 1966, Floods of April 28, 1966, in the northern part of Dallas, Texas: U.S. Geological Survey Open-File Report 66–96, 74 p.

National Oceanic and Atmospheric Administration, 1991a, Climatological data, Texas, November 1991 and December 1991: National Climatic Data Center, v. 96, no. 11-12, 92 p.

______1991b, Hourly precipitation data, Texas, December 1991: National Climatic Data Center, v. 41, no. 12, 60 p.

______1991c, Storm data, Texas, December 1991: National Climatic Data Center, v. 33, no. 12, 92 p.

Schroeder, E.E., Massey, B.C., and Waddell, K.M., 1979, Floods in central Texas, August 1978: U.S. Geological Survey Open-File Report 79–682, 121 p.

U.S. Weather Bureau, 1961, Rainfall frequency atlas of the United States: Technical Paper 40, 115 p.

Wells, F.C., Schertz, T.L., and Flugrath, M.W., 1985, Effects of October 1981 flood on the quantity and quality of water in selected streams and reservoirs in the Brazos River Basin, Texas: U.S. Geological Survey Water-Resources Investigations Report 84–4055, 119 p.