Debris flows

Debris flows are one of the most dangerous and hazardous processes due to their devastating power, high velocity flow and poor predictability. Comprising a mixture of fine and coarse materials, mud and water, debris flows have the ability to pick-up and transport fallen trees, boulders along with debris of bridges and other structures. In so doing, they may travel with velocities of 15 to 60 kilometers per hour. They occur commonly in mountains and highlands in human climate, though they are rather frequent in Mediterranean and semi-arid climate. 

Sources of debris include weathered, fractured and altered rocks accumulating on headwalls and steep slopes, soil blankets and veneers, glacial and volcanoclastic deposits along with channel bedload material and even man-made fills. The intense precipitation or snowmelt saturates debris, resulting in greater instability of weathered or eroded material. Just another rainfall, or rock fall, or seismic shock can be a trigger factor for flow movement. 

Debris flows classified by the movement rate, or velocity, and by the material they consist of. They are also can be divided into channelized flows and hillslope flows; debris flows and mudflows; debris and earth avalanches. Velocity of movement is critical parameter, as it determines potential hazard (and potential response). The impact of debris flows also varies depending on material fluidity and debris fan location.

Forest harvesting is proved to intensify weathering and increase erosion. Cable logging resulted in soil destruction and augment surface runoff. Forest roads and skid trails are also frequent sites of debris flows in many regions (The Himalayas, the Carpathians, and The Pacific Coast Range in the U.S. are examples).

The most debris flows prone regions are:

• The Alps and Pyrenees, the Greater Caucasus and Lesser Caucasus, Apennine mountains, Tatras, Scotland
• Many regions in Asia – the Himalayas; Indonesia; Philippines; Japan; Shanxi, Sichuan, and Yunnan provinces of China; the Tian Shan, Pamir, Ile Alatau and other mountain ranges in Central Asia.
• Many regions in Latin America — The Peruvian, Colombian and Chilean Andes; Cordillera de la Costa; Central American volcanic ranges
• The states of California, Oregon, Washington, Utah, Idaho, Colorado, and the province of British Columbia in North America
• New Zeeland and Star Mountains (Papua New Guinea)

Lahars are rapidly flowing, devastating torrential flows of debris, mud and water. Easily destroying, eroding and burying obstacles in their path, lahars can travel long distances. With the potential to flow at speeds 50-70 km/h (up to 110 km/h), and distances of more than 100 kilometers, lahars can cause catastrophic damage.

Lahars are special forms of debris flows that can be induced by volcanic eruptions, heavy rains after eruptions, or sudden melting of mountain glaciers, firn and snow.  

Lahars can cause catastrophic destructions and kill thousands. The largest known event, which occurred in 1985 in Colombia, was connected to Nevado del Ruiz eruption. Several lahars engulfed the town of Armero, killed more than 20,000. Other well-known tragic events include lahars at  Mount Pinatubo, Philippines (1991), Casita Volcano, Nicaragua (1998), Mayon Volcano, Philippines (1897, 1981),  Kelut, Indonesia (1919), and Calbuco, Chile (1917).

Zesmill’s team of geologists, geomorphologists, hydrologists and surveyors is experienced in debris flow studies and monitoring. Zesmill provides:

• geomorphologic and landscape interpretation of satellite and aerial images
• LIDAR surveying, DEM extraction and analysis in semi-arid and alpine areas where vegetation obscuring effect is minimal
• morphometric – hydrometric terrain analysis
• field geological and geomorphological mapping
• debris flow and deposits landform identification
• hydrological studies and measurements, runoff calculating
• analysis of supply-limited and supply-unlimited basins
• GIS-modeling based on geological, geomorphologic and hydrologic characteristics
• GIS-based modeling of debris flow motion
• monitoring of the debris flow prone streams
• detail mapping of the debris flow prone regions

Debris flow hazard analysis requires a combination of remote sensing methods, fieldwork (including study of landforms and detail analysis of bedrock and surficial deposits), meteorological and hydrological observations, and GIS modeling.

Dozens of debris-flow mitigation measures have been developed. Active countermeasures include check dams, wing walls, debris flow breakers, slot barriers and beams along with methods for river discharge control, slope stabilization. Passive debris flows mitigation measures include early warning systems, land zoning and planning, and hazard mapping.

Zesmill have performed several projects for debris flow assessment and hazard zone mapping as well as pre-feasibility studies of debris-flow prone valleys across Europe and Asia, including: Muota river valley, Switzerland; Arta region, Greece; Demergi, Ulu Özen and Derekoyka river valleys, Crimea; Trans-Balkan highway pre-feasibility studies; debris flow maps for Environmental Atlas of Asia.