Causes of Flooding arei) climaticii) terrestrial processesiii) human activity

CASE STUDY: Quebrada San Julian Drainage Basin

Vargas coastal region,Venezuela Dec 1999

Acknowledgements:

serc.carleton.edu/files/NAGTWorkshops/health04/MarinPoster.ppt

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

Venezuela Floods

Date: Mid December, 1999, CaracasDead: 20,000-30,000Affected: 600,000Damage: $15bn

What happened: A fortnight of torrential rains caused flooding and landslides in and around the capital, Caracas, in what was described as the country's worst natural disaster.

Response: It took five days for the International Red Cross to launch an appeal for $2.8m to fly in medical supplies. The World Bank offered $150m one week after the disaster. Some 25 countries, including Cuba, had offered help by December 19. About $27.8m had been pledged by the international community by March.

Easterly trade-winds can force moist air masses upslope over the coastal ranges and precipitate large rainfall volumes, creating conditions for high-magnitude debris flows and flash floods.

Climatic Causes – wet antecedent conditions plus torrential rain = flash flood

An unusually wet period in 1999 resulted in rainfall accumulation at sea level on the Caribbean coast of 293 mm for the first 2 weeks of December, followed by an additional 911 mm of rainfall from December 14 -16

Fifteen days of constant and intense rainfall in Venezuela culminated on 16 December 1999 in extensive flooding and massive landslides in seven northern states of the country.

Rivers overflowed their banks and swept through poor districts in the capital city of Caracas

The Venezuelan coast is extremely steep and rugged. The crest of the Sierra de Avila reaches 2,700 m within about 6-10 km of the coast. The rivers and streams of this mountainous region drain to the north and emerge from steep canyons onto alluvial fans before emptying into the Caribbean Sea. In Vargas little relatively flat area is available for development with the exception of the alluvial fans.

TERRESTRIALPROCESSES

Note the grey alluvial fans and streaks of the canyon beds (e.g. leading to Caraballeda). Red steep slopes – the only flat ground is the alluvial fans and canyon floors = DANGER

Oblique aerial photograph of Vargas state showing abundant debris-flow scars, fresh sediment deposited in river canyons and on alluvial fans and areas of deposition of mud in the sea.

Quebrada San Julián upstream of Caraballeda showing evidence of recent debris flows and flash floods. Note the high slope angles, large numbers of debris flow scars and abundance of new alluvium and colluvium in the channel bed and fan surface.

The flash floods move a lot of debris – water and sediment move rapidly through this drainage basin. Landslides occur down the valley sides inputting debris to the river.

The upstream drainage area is about 28 square kilometers but relief in this drainage basin is approximately 2 kilometers, providing tremendous energy to the fluvial system--with slopes as steep as 50 percent. The streamflow accelerates as it passes through the narrow canyon visible in the foreground.

Boulder transported by debris flow of December, 1999, Caraballeda. Sediment size ranged from clay and sand to boulders as large as 10 m in diameter. Sediment and debris including massive boulders were deposited up to several meters thick across large sections of alluvial fans in Caraballeda

Debris flow scars upstream of Caraballeda. Most of the fine material (sand, silt, clay) derived from these debris flows was transported offshore. The coarse-sized gray and white sediment that now covers the canyon floor is derived from bedrock exposed on hillslopes and channels upstream.

Oblique view to south prior to December disaster, showing El Avila mountains, Quebrada San Julián drainage basin, and coastal urbanization near Caraballeda, Vargas state, Venezuela. The settlements are growing on dangerous alluvial fan ground.

TERRESTRIAL PROCESSES MEET HUMAN ACTIVITY

Aerial view of Los Corales sector of Caraballeda. Debris flows and sediment-laden flash floods destroyed or damaged most structures on this alluvial fan.

These resorts (and shanty towns behind) had been developing in the dry riverbeds and on steep unstable hillsides outside the larger city as part of a wide-scale population shift from farming regions to urban areas, especially those migrating from countries such as : Colombia, Ecuador, Peru and others in search for better jobs.

HUMAN FACTOR

Aerial view of Caraballeda looking southwest showing newly opened channels in foreground and center right of photograph

Because most of the coastal zone in Vargas consists of steep mountains rising abruptly from the Caribbean Sea, the alluvial fans provide practically the only flat areas on which to build. Rebuilding and reoccupation of these areas requires careful mapping of hazard zones to avoid future disasters.

Caraballeda looking north showing massive deposition of coarse sediment delivered by debris flows and flash floods

Carmen de Uria: this narrow canyon and alluvial fan has no areas that are not either susceptible to flooding or to landslides. The loss of life in this community was great.

Carmen de Uria: The density of housing in the center of this photograph was equal to that visible now on the left side. A combination of debris flows that transported massive boulders, and flash floods carrying extremely high sediment loads were the principal agents of destruction

Socioeconomic losses due to slope failures are great and apparently are growing as the built environment expands into unstable hillside areas under the pressures of expanding populations and urban development.

View of San Francisco de Galipán in the El Avila National Park. These 38 debris flow scars in otherwise undisturbed forest suggest that the storm of December 1999 was of unusually high magnitude. i.e. LITTLE HUMAN IMPACT HERE BUT LANDSLIDES STILL OCCURRED = bad storm.

Human activities disturb large volumes of earth materials in construction of buildings, transportation routes, canals, and communications systems, and thus have been a major factor in increases in damages due to slope failures, and increasing the extent of natural disasters such as the 1999 Venezuelan Flood

Debris-flow scars, remains of houses and two-lane highway on Caribbean coast.

For decades, there has been rapid, uncontrolled, unregulated immigration from the rural areas of extreme poverty to, and around, Caracas and other cities seeking employment, first, in the expanding oil industry and, then, in tourism. Indeed Venezuela's weak economy depends almost entirely on oil and tourism for its foreign revenue. Vargas state has 500,000 workers who service the tourist industry, or commute each day to Caracas from the shanty towns and ranchos in the hills surrounding the capital. "Corrupt politicians and planners" turned a blind eye to such developments, where up to 350,000 workers existed, often without electricity, running water or main drainage.