breaks in agricultural areas focused flood-flow energy at hundreds of sites and caused

extraordinary levee damage, deep scour, and extensive sand deposition on the floodplain. Locally, breaches through railway embankments and flow diversions around railroad and highway bridges also acted to focus flood-flow energy and thereby added to the scour and deposition problem. On average, approximately 5 to 7 percent of the floodplain (5,000 to 7,000 hectares) between Glasgow and St. Louis, Mo., was seriously affected by these processes.

More than 90 percent of this damage was directly related to levee breaches in the high-energy zone. Scour holes, locally known as "blow holes" or "blew holes," were eroded into the floodplain by the high-intensity flood scour associated with these levee breaches. The larger holes are as much as 500 meters wide and 1,000 meters long. These holes typically attain their greatest depths (as much as 16 meters) just at the levee break. Stripped zones, locally cutting completely through the tilled zones of these rich agricultural lands, formed immediately downstream of the deep scour holes. Within these zones, which locally extend as far as 1.6 kilometers downstream from the largest breaches, shallow parallel channels and grooves as much as 1.5 meters deep and scattered scour holes as much as 4 meters deep and 60 meters long mark the locations of major flow paths.

Extensive thick deposits resulting from the 1993 flood are generally located downstream from large levee breaches within the high-energy floodplain zone. Moderately thick to thick sand deposits locally cover more than 30 percent of the floodplain in some bottomland areas. In contrast, moderate to thick flood deposits cover less than 5 percent of the floodplain along most other reaches. Deposits related to levee breaches are thick enough to conceal preexisting features of the floodplain surface. At the downstream margins of the scour zones associated with these breaches, lobe- to crescent-shaped sand sheets as much as 1 to 3 meters thick formed where the flood flow fanned out from the levee break. In some areas, these deposits locally extend as much as several hundred meters downstream from the scour zone margin. Less extensive deposits of thick sand were laid down in the lee of trees and buildings, in borrow pits adjacent to levees, and within the channel's riparian fringe.

J.C. Dohrenwend

is a member of the Scientific Assessment and Strategy Team and was a member of the Interagency Floodplain Management Review Committee. He is an expert in geological hazards and the applications of surficial geology, geomorphology, and remote sensing to hazards and ecosystems analyses.

B.D. Stone

is a member of the Scientific Assessment and Strategy Team and was a member of the Interagency Floodplain Management Review Committee. He is an expert in Quaternary geologic and geomorphic mapping and analysis.

The Independence Day Flood of 1994

In July 1994, parts of Alabama, Florida, and

Georgia were devastated by floods that resulted from rains produced by Tropical Storm Alberto. The storm took 33 lives and caused property damages of nearly $1 billion. Ultimately, a total of 78 counties in the three States were declared Federal flood disaster areas. Whole communities were inundated by floodwaters as numerous streams reached peak stages and discharges far greater than those of previous known floods. Montezuma and Newton, Ga., were almost entirely encompassed by floodwater from the Flint River. Many municipal, industrial, and private water systems were inundated and rendered unserviceable for several days. In Macon, Ga., the municipal water-treatment plant was flooded, and about 150,000 people were without water for 3 weeks. Highway traffic was disrupted as hundreds of bridges and culverts were overtopped and, in many cases, washed out. Roughly 1,000 bridges were closed during the flooding, and about 500 remained closed for several more days while temporary repairs were made. Stretches of Interstates 75 and 16 were closed for several days, and preliminary estimates of road and bridge damage in Georgia were about $130 million. Numerous dams failed, emptying recreational lakes and farm ponds. In Albany, Ga., sinkholes formed in some areas underlain by cavernous limestone, resulting in the destruction or condemnation of numerous homes. Thousands of acres of crops

were damaged or destroyed by rain and floodwater. About 471,000 acres of farmland in Georgia were affected by the flood; estimated damages were about $100 million.

The death and suffering caused by this storm serve to emphasize once again the high cost exacted in life and property by flood disasters and the attendant importance of preparing for, monitoring, and documenting such occurrences.

Tropical Storm Alberto grew from a tropical depression that formed off the western coast of Cuba in the Gulf of Mexico on June 30. Alberto first came over land on the morning of July 3 near Fort Walton Beach, Fla. Once ashore, the storm rapidly lost energy and was downgraded to a tropical depression by that afternoon. The remnants of Alberto drifted north to just west of Atlanta, Ga., early on July 5, changed course, and moved slowly in a southwesterly direction before dissipating on July 7. Slow movement of the storm and abundant tropical moisture combined to help produce historical rainfalls. Storm rainfall totals greater than 13 inches were common in areas of Florida, Alabama, and Georgia. The highest total

rainfall of 27.6 inches (July 3-7) and the highest 24-hour rainfall of 21.1 inches (24-hour period that ended at 7 a.m. on July 6) were recorded in Americus, Ga. The total rainfall for Americus was about 0.6 percent of the area's mean annual rainfall, and the 24-hour high was nearly 2.5 times greater than the area's estimated 100-year recurrence-interval unit for 24-hour rainfall.

The floods that resulted from Tropical Storm Alberto were no less remarkable than the rainfall that caused them. On the night of July 4 and the morning of July 5, damaging flash floods ranged from the southern suburbs of Atlanta to Macon. The peak discharge at the streamflow-gaging station on Line Creek near Senoia, Ga., which has its headwaters near Atlanta, was 2.4 times the 100-year recurrence-interval flood discharge. The maximum stage at the Senoia gage was 5.2 feet higher than any other recorded during its 30 years of operation. As the rains moved south on the night of July 5 and the morning of July 6, more destructive flash flooding occurred in the Americus area. Muckalee Creek at Americus, which probably had been affected by undetermined amounts of water released as a result of local dam failures, peaked on July 6 at a discharge about 4.0 times larger than the 100-year flood discharge. About 20 miles south of Americus, a peak discharge 1.4 times larger than the 100-year flood discharge was recorded on July 7 at the streamflowgaging station on Kinchafoonee Creek near Dawson, Ga.

As the floodwaters on small streams merged and moved downstream, larger streams, such as the Flint and the Ocmulgee Rivers in Georgia and the Choctawhatchee River in Alabama and Florida, surpassed the 100-year recurrence-interval flood discharges. The most widespread flooding was in the Flint and the Ocmulgee River basins in Georgia; floods equaled or were greater than the 100-year recurrence-interval discharge along almost their entire lengths. Floods equal to or greater than 100-year recurrence-interval discharges were recorded at all Flint River streamflow-gaging stations from Lovejoy, Ga., about 20 miles south of Atlanta, to Bainbridge, Ga., about 2.9 miles upstream of its confluence with the Chattahoochee River at the southwestern corner of the State. At the streamflow-gaging station at Montezuma, the Flint River peaked on July 8 at a stage 6.7 feet higher than the 1929 flood, which had been the largest flood of this century. Floods on the Ocmulgee River were greater than the 100year recurrence-interval discharge from Juli

Management Agency, the Federal Highway Administration, various State naturalresource and highway departments, electrical power companies, and numerous county and city officials as they worked to minimize loss of life and property. The flooding was so severe and widespread that 17 streamflowgaging stations were heavily damaged or destroyed; thus, much of the necessary data had to be gathered manually and reported by cellular telephone. At the height of the flooding, nearly 40 USGS personnel were working in the field to provide hydrologic information vital to protecting lives and property.

Even with the superb effort to collect current hydrologic information during the flood, it was impossible to visit every site where data were needed. In some instances, bridges and roadways were inundated, and floodwaters were too dangerous to risk working from boats. In other cases, personnel simply could not get to the point of interest before the floodwater receded. Immediately following the flood, field crews were dispatched to flag high-water marks along major rivers and many of their tributaries so that flood profiles could be determined and indirect determinations of peak discharge at key locations could be computed. Reconstruction of gaging stations and followup fieldwork continued well into the fall of 1994. The hydrologic information collected and

Debris-Flow Hazards

at Mount Rainier

D

ebris flows pose significant hydrologic hazards on and near volcanoes in the Pacific Northwest. Debris flows are destructive, churning masses of water, rock, and mud that travel rapidly down river valleys. They typically contain as much as 65 to 70 percent rock and soil by volume and have the appearance of wet concrete. Assessment of debrisflow hazards has been a key activity for U.S. Geological Survey (USGS) hydrologists. Debris-flow hazards near Mount Rainier, Wash., have been of particular concern because of the volcano's proximity to the densely populated Puget Sound region. The volcano also lies at the heart of frequently visited Mount Rainier National Park, so visitor safety is of further concern.

USGS investigations of giant debris flows originating at Mount Rainier have provided key natural-hazards information to public officials charged with developing comprehensive, long-term land-use plans. A preliminary report that can be consulted is Open-File Report 90-385, Sedimentology, Behavior, and Hazards of Debris Flows at Mount Rainier. A source of more information about glacier-generated debris flows at Mount Rainier is Water-Resources Investigations Report 93-4093, Geomorphic Change Caused by Outburst Floods and Debris Flows at Mount Rainier, Washington, with Emphasis on Tahoma Creek Valley.

Debris flows at Mount Rainier vary tremendously in size and may form by various processes. The smallest but most frequent ones begin as glacial outburst floods. Outburst floods originate when water stored at the base of glaciers or within pockets in glacier ice is suddenly released. Outburst floods have been recorded from four glaciers on Mount Rainier-the Nisqually, Kautz, South Tahoma, and Winthrop Glaciers. By far the most prolific producer of outburst floods, the South Tahoma Glacier released 15 between 1986 and 1992. Outburst floods from the South Tahoma Glacier occur during periods of unusually hot or rainy weather in summer or early autumn and are apparently caused by rapid input of meltwater or rainwater to the base of the glacier. The exact timing of such outburst floods is unpredictable, however.

Outburst floods become debris flows by incorporating large quantities of sediment from valley floors and walls, usually by triggering small landslides that mix with the floodwaters. Glacier-generated debris flows at Mount Rainier travel downstream at speeds of 10 to 20 miles per hour and have steep, bouldery snouts (30-60 feet high in the most constricted parts of a stream valley) followed by a churning mass of mud, rock, and vegetation. Their deafening noise is often accompanied by strong local wind, thick dust clouds, and violent ground shaking. These debris flows usually follow stream channels, but their exact paths are unpredictable. The

flow's bouldery snout commonly clogs the stream channel; the moving mass behind the snout then overtops the stream banks and cuts a new channel, perhaps through forest or across trails and roads. Debris flows that originate as outburst floods have repeatedly destroyed or damaged roads and facilities in Mount Rainier National Park. Out of concern for visitors' safety and guided by the results of USGS investigations, park managers have restricted access to certain areas.

Two sorts of giant but infrequent debris flows originating from Mount Rainier pose hazards well beyond the National Park boundaries. USGS investigations of these

Partially buried table at the picnic ground alongside Tahoma Creek at Mount Rainier after debris flow in the fall of 1986. All traces of the picnic ground have since been obliterated.