Debris flow: categories, characteristics, hazard assessment, mitigation measures Hariklia D. SKILODIMOU, George D. BATHRELLOS
Natural hazards: physical phenomena, active in geological time capable of producing damage to the natural and manmade environment Natural disasters : When natural hazard consequences have a major impact on human system Natural disasters: In 2013, killed over 20,000 people and estimated economic losses 118.6 billion US$ all over the world.
Geomorphic Hazard Endogenous Exogenous Climate or land use change volcanism neotectonics floods karst collapse snow avalanche channel erosion sedimentation mass movement tsunamis coastal erosion desertification permafrost degradation soil erosion salinization floods
Definition and Origin of Debris Flow Mass movements: Falls Topples Slides Spread Flows Flows: a fluid movement of loose earth material
Debris flow: very rapid to extremely rapid flow of saturated nonplastic debris in a steep channel combine loose soil, rock and sometimes organic matter and variable amounts of water Essential conditions: coarse or fine-grained sediments, steep slopes, supply of moisture, space vegetation
Debris flows: high velocities and volume, very dangerous, kill many people and cause million dollars of property damages Damages from debris flow, Caraballeda, Venezuela, 1999, 30,000 deaths.
Categories of Debris Flows Mudflows: very rapid to extremely rapid flow containing more water than source material, sandy mud or muddy sand and often includes 10 to 30% clay and silt. Favorable conditions: debris susceptible to saturation, intense rainfalls, lack of vegetation, steep slopes. Mudflow deposit in Peruvian Andes, Perou.
Very dangerous: high flow velocity and the long distance of travel. A mudflow cause damages in a house along the Toutle River, in USA.
Debris avalanche: large, very rapid to extremely rapid, flows of partially of fully saturated debris on a steep slope not move into a channel, high velocities, deposits are unconstrained alluvial apron, common on mountainous areas and very steep volcanoes.
Debris avalanche triggered by an earthquake buried the city of Yungay and village of Ranrahirca, Peru 1970.
Lahars: flows on the slopes of volcanoes, debris can be either hot or cold. Favorable conditions: water, eruptions, volcanic venting Lahars developed by the 1982 eruption of Mount St. Helens in Washington, USA.
In 1985, a large lahar occurred after the eruption of the volcano Nevado Ruiz, Colombia. The flow destroyed the city Armero and caused more than 20,000 victims
Characteristics and Failure mechanism of Debris Flow flow like a viscous fluid, high density, 60% to 80% by weight solids, significant erosion of the banks and bedrock base of the channel, increase sediment charge and erosive abilities, mass with significant energy.
1. Source area or initiation zone: slope failure in the headwall or side slope of a stream channel. 2. Transportation zone or transportation channel: flow of mass movement 3. Depositional area or deposition zone: debris fan or cone.
Factors influencing occurrence of debris flow 1. Morphological factors: slope angle, slope aspect, convex slopes, slope changes. 2. Geological, geotechnical factors: lithology and discontinuities of bedrock, geotechnical behavior of soil. 3. Hydrological factors: drainage network, ground water and hydrogeological conditions. 4. Meteorological factors: heavy rainfall, rapid snowmelt and moisture.
Factors influencing occurrence of debris flow 5. Vegetation cover: changes in vegetation, damage of vegetation roots, fires. 6. Land use: agricultural uses and urban uses, land use changes. 7. Landslides. 8. Earthquakes and Volcanoes.
Materials were deposited by prehistoric debris flow in northern Venezuela. Debris flow hazard assessment 1 st step: flow hazard recognition study of debris fans: field work, detailed stratigraphic analysis, dating methods, record of possible geomorphologic evidence of debris flow activity,
1 st step: flow hazard recognition interpretation of aerial photographs or satellite images, historic accounts and records. Aerial photograph of the debris flow at Hougawachi, Minamata city Japan.
2 nd step: probability of debris flow frequency and of magnitude debris flow, factors influencing occurrence of debris flow, temporal and spatial probability of debris flow, magnitude-frequency relationship, inventory, heuristic, statistical, and deterministic methods. Susceptibility map: the probability of the spatial occurrence for future debris flow events. 3 rd step: run-out analysis empirical, analytical and numerical models.
Hazard map of debris flows Debris flow hazard map at Pal drainage basin in Andora, Pyrenees.
Intensity Direct impact h>1m and v>1m/s h>1m and v>1m/s impact Probability of occurrence (return period, yr) <40 400-500 >500 High Medium Low High High High Moderate Medium Moderate Moderate Low Indirect Low Low Low Very Low (after flow) Not affected areas Very Low Very Low Very Low Debris flow hazard matrix for Andora, Pyrenees
Debris flow mitigation measures The final step of debris flow hazard assessment is the mitigation and the reduction of the existing hazard. Man-made constructions make a slope susceptible to debris flow.
Active debris flow mitigation measures: influence the initiation, transport or deposition of debris flows. Debris flow basins: built at the base of slopes where debris flows are frequent. A debris flow basin constructed at the bottom of a slope in San Bernardino, California, USA.
Check Dams: small, sedimentstorage dams built in the channels of steep slope. Concrete check dam with low flow center section in southern California, USA. Wing walls: essential to prevent check dams. Wing walls, Waigrainer Ache, Salzburg, Austria.
Debris flow retaining walls: built of various kinds of materials are designed to prevent the evolution of the debris fall. A retaining wall of debris flow in Kamikochi Basin, Japan.
Slope stabilization: the slope cut back in a series of terraces rather than a single steep cut. Slope stabilization at Vorarlberg, Austria.
Passive mitigation measures: used to reduce the potential loss. Hazard mapping. Land use zoning. Warning system. Immediate technical assistance. Documentation and control.
Exercises Debris flow hazard recognition
Debris flow hazard recognition
Debris flow hazard mitigation measures