NATURAL HAZARDS ON ALLUVIAL FANS:


THE VENEZUELA DEBRIS FLOW AND FLASH FLOOD DISASTER


In December 1999, rainstorms induced 

thousands of landslides along the 

Cordillera de la Costa, Vargas, 

northern Venezuela. Rainfall on 

December 2-3 totaled 200 millimeters 

(8 inches) and was followed by a major 

storm (911 millimeters, or 36 inches) on 

December 14 through 16. Debris flows 

and flash floods on alluvial fans 

inundated coastal communities, 

caused severe property destruction, 

and resulted in a death toll estimated 

at 19,000 people. Because most of the 

coastal zone in Vargas consists of 

steep mountain fronts that rise 

abruptly from the Caribbean Sea, the 

alluvial fans are the only areas where 

slopes are not too steep to build. 

Rebuilding and reoccupation of these 

areas requires careful determination 

of potential hazard zones to avoid 

future loss of life and property. 

NATURAL HAZARDS ON ALLUVIAL 

FANS 

Large populations live on or near 

alluvial fans in locations such as Los 

Angeles, California, Salt Lake City, 

Utah, Denver, Colorado, Naples, Italy, 

and Vargas, Venezuela. In time scales 

spanning thousands of years, alluvial 

fans are dynamic zones of high 

geomorphic activity. Debris flows 

and flash floods occur episodically in 

these environments, and place many 

communities at high risk during 

intense and prolonged rainfall. 

Although scientists are constantly 

improving their ability to understand 

and delineate areas of high natural 

hazard, population expansion and 

development pressure have put more 

people at risk than ever before. 

Therefore, debris flow and flash 

flood hazard analysis is a critically 

Highly developed alluvial fan, Caraballeda, Venezuela, aerial view looking north. 

important area of natural hazard 

research. This fact sheet describes the 

alluvial fan environment and 

associated hazards, with examples 

from Venezuela. 

WHAT ARE DEBRIS FLOWS? 

Debris flows are fast moving 

landslides that occur in a wide range 

of environments. A debris flow is a 

rapidly moving mass of water and 

material that is mainly composed of 

sand, gravel, and cobbles, but 

typically includes trees, cars, small 

buildings, and other anthropogenic 

material. A debris flow typically has 

the consistency of wet concrete and 

moves at speeds in excess of 16 

meters per second (35 miles per hour). 

WHAT ARE ALLUVIAL FANS? 

Alluvial fans are gently sloping, 

cone- to fan-shaped landforms created 

over thousands to millions of years by 

deposition of eroded sediment at the 

base of mountain ranges. They are 

easily recognized in arid, to semi-arid 

environments such as that of the 

western United States; however, 

alluvial fans can occur in more humid 

Oblique view looking south at the Cordillera de la Costa, Venezuela. 

Coastal alluvial fans east of Maiquetia appear as lobate promontories at 

the base of the 2,000 meters (6,600 feet) high mountain range. The alluvial 

fans were heavily impacted by floods and debris flow in 1999. Image 

courtesy of F. Urbani, Universidad de Venezuela. 

U S Department of the Interior USGS Fact Sheet FS 103 01 
environments, including the northern 

coast of Venezuela. Alluvial fans may 

be highly active, where floods and 

debris flows can occur episodically at 

any location on the fan surface. Other 

alluvial fans are less active, where 

tectonic uplift and incision have 

channelized flows so that much of the 

fan is not affected during floods. The 

principal natural hazards on alluvial 

fans are floods and debris flows that 

are induced mainly by intense and 

prolonged rainfall. Floods on alluvial 

fans commonly occur with little to no 

warning, and have high velocities and 

sediment-transporting capabilities. 

Water-generated natural hazards 

on alluvial fans are typically relatively 

shallow but can strike with little 

warning, travel at high speeds, and 

carry massive amounts of sediment 

and debris. Alluvial fan flooding 

typically begins at the hydrographic 

apex, which is the highest point where 

flow is last confined 

by mountain 

valleys, and then 

spreads out as sheet 

flood, debris 

slurries, or in 

multiple channels. 

Flooding is 

characterized by 

enough energy to 

carry coarse 

sediment even at 

shallow flow 

depths. The abrupt 

deposition of Aerial view of Caraballeda alluvial fan looking south, showing an 

sediment or debris estimated 1.8 million tons of freshly deposited sediment that was spread 

during a flood may 

across the highly urbanized community by floods and debris flows in 

December 1999. 

substantially alter 

hydraulic conditions

and initiate new, distinct flow paths of perceived as too large to overflow. It

uncertain direction. The uncertainty is this flow path uncertainty that

of landslide risk can be heightened by makes debris flow and flood hazard

sediment deposition in an alluvial fan on alluvial fans extremely dangerous.

channel, resulting in rapid overbank

flooding of a channel that was 

Oblique view looking south at the Río San Julián watershed and Caraballeda 

alluvial fan, Vargas, Venezuela. Image courtesy of P. Delfiín, Venezuela Ministry 

of Environment and Natural Resources. 

HAZARD MITIGATION STRATEGIES ON 

ALLUVIAL FANS 

Two mitigation strategies can be 

implemented to protect property: 

(1) large structural flood control 

measures, such as check dams, and 

(2) avoidance of the affected area. As 

indicated by the extensive alluvial fan 

development in Caraballeda, 

Venezuela, where no check dams exist, 

neither strategy has been widely used, 

contributing to the tragedy of 

December 1999. Land-use regulations 

can be used to reduce hazards by 

limiting the type or amount of 

development in high-risk areas. 

Hazard-prone areas can be used for 

open space uses, such as parks and 

grazing. In high-risk zones where 

development and reconstruction are 

inevitable, steps such as orienting a 

building so that its length is oriented 

parallel to the direction of flow will 

minimize the width of building 

exposed to a debris flow. Additionally, 

orienting streets parallel to the 

downslope direction of the fan allows 

the streets to serve as overflow 

channels, limiting potential damage to 

structures. Monitoring, warning, and 

evacuation are non-structural 

approaches to hazard mitigation that 

reduce potential loss of life. Early 

warning systems based on weather 

forecasts and rainfall information can 

substantially improve emergency 

managers responses to warn and 

evacuate threatened communities. 

Areas such as Hong Kong, San 

Francisco, and Denver use warning 

systems such as sirens and radio 

bulletins to alert residents of 

potentially threatening conditions. 
AN EXAMPLE OF ALLUVIAL FAN 

HAZARDS: 

THE VENEZUELA DEBRIS FLOW AND 

FLASH FLOOD DISASTER 

Several hundred thousand people 

live in a narrow coastal zone north of 

Caracas, Venezuela, in the state of 

Vargas. Located at the base of steep 

mountains that rise to elevations of 

more than 2,000 meters (6,600 feet), 

the population is highly vulnerable to 

episodic rainfall-induced landslides. 

An unusually wet period in December 

1999 included rainfall accumulation 

along the Caribbean coast of 200 

millimeters (8 inches) on December 

2-3, followed by an additional 911 

millimeters (36 inches) of rainfall 

from December 14 to 16. 

The rainfall induced thousands of 

debris flows and other types of 

landslides in the coastal mountains, 

and downslope, these landslides 

coalesced into massive debris flows 

that moved rapidly through steep 

narrow canyons and onto the alluvial 

fans. Residents with homes on the 

alluvial fans described multiple floods 

and debris flows that began late on the 

night of December 15 and continued 

until the afternoon of December 16. 

Debris flow damage to apartment building 

on alluvial fan, Caraballeda, Vargas, 

Venezuela. Boulders passed through the 

first two stories of the building. 

THE PAST IS THE KEY TO 

THE FUTURE 

The landslides and 

flooding triggered by 

intense rainfall in 

December 1999 were 

not unique in this 

region or elsewhere in 

Venezuela. Sediments 

exposed along the 

banks of river channels 

reveal a record of 

prehistoric floods and 

debris flows. Historical 

This house was carried and rotated by debris flows in Caraballeda,	 records indicate that, 

on average, at least one Vargas, Venezuela, 1999. 

The landslides (mostly debris 

flows) and flash floods along the 

coastal zone of the state of Vargas and 

neighboring states in northern 

Venezuela killed an estimated 19,000 

people, caused extensive property 

damage, and changed hillslope, 

stream channel, and alluvial fan 

morphology. Residents had little 

advance warning of the debris flows 

and flash floods that struck in the 

early hours of December 16, so many 

were caught in their homes and their 

bodies were carried out to sea or 

buried by the flood debris. 

A combination of debris flows 

that transported massive boulders, and 

flash floods carrying extremely high 

sediment loads, were the principal 

agents of destruction. On virtually 

every alluvial fan along the Vargas 

coastline, rivers incised new channels 

into fan surfaces to depths of several 

meters, and massive amounts of new 

sediment were disgorged upon fan 

surfaces. Sediment and debris, 

including massive boulders as large as 

10 meters (33 feet) in diameter, were 

deposited in layers up to several 

meters thick across large portions of 

the alluvial fans east of Maiquetia. 

Hundreds of houses, bridges and other 

structures were damaged or destroyed. 

Total damage in Vargas was estimated 

at $1.9 billion. 

or two high-magnitude 

flash-flood or landslide events per 

century have occurred in this region 

since the 17th century. Spanish 

archives indicate that flash floods and 

debris flows caused extensive damage 

to 219 homes and government 

buildings and destroyed all bridges in 

La Guaira in February 1798; the 

floods and debris flows associated 

with this event were so large that 

Spanish soldiers placed cannons 

cross-wise in front of the upstreamfacing entrance of a fort to barricade 

the structure near the stream channel. 

Erosion caused by the 1999 storm exposed the 

foundation of this house and garage in 

Caraballeda. The sand, cobbles and boulders upon 

which the structure rests are an old debris flow 

deposit. 
CAN THIS TYPE OF DISASTER 

HAPPEN WHERE YOU LIVE? 

Debris flows commonly begin on 

steep, mountainous hillslopes that 

may be dry except during rainy 

weather. Excavation for roads or other 

construction can increase the 

probability of a debris flow, even on 

relatively gentle hillslopes. Although 

debris flows usually start on steep 

slopes, after a debris flow begins to 

move, it can travel long distances 

across gentle slopes. Downslope or 

downstream areas such as stream 

channels, canyon bottoms, and 

alluvial fans are particularly 

vulnerable during heavy or 

continuous rainfall, or rapid 

snowmelt. This example from 

Venezuela shows the potential for 

extreme loss of life and property 

damage where a large population 

occupies alluvial fans. The possibility 

for an event of comparable magnitude 

exists in other parts of the world 

where extensive development has 

encroached on alluvial fans. Without 

careful planning of human 

settlements, the impacts of these types 

of disasters are likely to increase in 

the future. Building communities and 

other infrastructure on alluvial fans 

has changed high-intensity natural 

hydrologic processes into major lethal 

events. As stated by the Secretary 

General of the United Nations, Kofi 

Annan, "The term ‘natural disaster' 

has become an increasingly 

anachronistic misnomer. In reality, 

human behavior transforms natural 

hazards into what should really be 

called unnatural disasters." 

These large boulders, estimated at 300 to 400 tons 

each, were transported by the December 1999 

debris flows near La Guaira. 

The mudline on these houses shows the height and shape of flood and debris flow deposits. The 

uphill street served as a conduit for water and debris thereby reducing damage to some of the 

buildings on this alluvial fan in La Guaira. 

Mud, rocks, and other debris 

filled streets and houses in 

La Guaira. 

FOR MORE INFORMATION ABOUT 

Landslides, contact the 

U.S. Geological Survey 

National Landslide Information Center 

Federal Center, Box 25046, MS 966 

Denver, CO, 80225, USA 

http://landslides.usgs.gov/landslide.html 

NLIC@usgs.gov 

Alluvial fan hazards, see the 

National Research Council Committee 

report on Alluvial Fan Flooding: 

http://www.nap.edu/readingroom/books/all/ 

The Venezuela disaster, see 

http://pr.water.usgs.gov/public/venezuela 

See also Debris-flow and flooding

hazards associated with the December 

1999 storm in coastal Venezuela and 

strategies for mitigation, 

available on the web at 

http://geology.cr.usgs.gov/pub/open-filereports/ofr-01-0144/

Cooperating Organizations

U.S. Agency for International Development, Office of

Foreign Disaster Assistance

Venezuela Ministry of Environmental

and Natural Resources

American Embassy, Caracas, Venezuela

Authors

Matthew C. Larsen1, Gerald F. Wieczorek

2

,

L. Scott Eaton3

, Benjamin A. Morgan2, and

Heriberto Torres-Sierra1

1 U.S. Geological Survey, Caribbean District, GSA Center,

651 Federal Drive, Suite 400-15, Guaynabo, Puerto Rico

00965, USA mclarsen@usgs.gov

2

 U.S. Geological Survey, Reston, Virginia, USA

3

 James Madison University, Harrisonburg, Virginia, USA

Photographs taken by Matthew C. Larsen

Layout & Design by Ruth I. Guzman