Debris flow: categories, characteristics, hazard assessment, mitigation measures By Hariklia D. SKILODIMOU, George D. BATHRELLOS Introduction Events such as landslides, volcanic eruptions, floods, and earthquakes are physical phenomena, active in geological time. These phenomena have affected the natural environment and existing biota, even before the appearance of man on earth. Nowadays, they are considered as natural hazards and an important global problem threatening human life. The aforementioned natural hazards are physical events that occur worldwide. Their cause, occurrence and evolution show notable complexity and wide variation in magnitude, frequency, speed and duration (Burton & Kates, 1963; Tobin, 1997). Natural hazards are events, capable of producing damage to the natural and man-made environment. Moreover, their impact differs from place to place and frequently these natural phenomena appear to have adverse long-term effects due to their associated consequences. When these consequences have a major impact on human system, they become natural disasters (Alcántara-Ayala, 2002). The effects of natural disasters may change the way of human life and require years of restoration efforts with particularly high costs. During the life of a human, at least one natural hazard will certainly affect his life. In 2013, natural disasters killed over 20,000 people (21,610) and the subsequent estimated economic losses were 118.6 billion US$ all over the world. Although the number of people killed by natural disasters was below the annual average 2003- 2012, and the economic damages decreased, they still affect millions of people every year (Guha-Sapir et al., 2014). On a global scale, overpopulation and urban development in areas prone to natural hazards increase the impact of natural disasters both in the developed and developing world. However, their effects are greater in developing countries (Rosenfeld, 1994; Alexander, 1995). Generally, natural disasters occur more frequently in relation to our capability to restore the effects of past events (Guzzetti et al., 1999). Therefore, in order to minimize the loss of human life and reduce the economic consequences, proper planning, and management of natural disasters are essential (Bathrelos et al., 2012). Usually a natural disaster is an uncontrollable event that humans do not expect (Fritz, 1961). Apart from the obvious effects of natural disasters (for example when a flood or a fire destroys a house), usually there are indirect effects. Although these effects may be less obvious, they are usually more harmful and can add years to the restoration period from a catastrophe. Since, complete restoration is very difficult, natural disasters are capable to change our lives forever. Greater understanding of when, where, why and how natural disasters occur, is the first step to reduce their impacts in human activities. Consequently it is very important to create awareness in natural hazards because human activities often increase their frequency, size and severity. Natural hazard prevention is a very difficult task, but the understanding of natural hazard process can be dominant tool to reduce natural disasters. So, scientific research can supply pure and applied methods to the prevention of natural disasters in terms of origin and dynamism of the physical processes (Alcántara-Ayala, 2002). Many natural hazards can be caused by the same physical event and the cumulative effect is catastrophic. In this context scientists try to understand the interactions of these natural phenomena and find ways to minimize the effects of combined hazard. Natural hazards are characterized by magnitude, frequency and aerial extent. There are various categories of hazards such as atmospheric, hydrologic, geologic, biologic and technologic. Many hazards are the result of sudden changes of the Earth’s surface and strongly related to geomorphology. In this context, geomorphic hazards can be categorized as endogenous (caused by volcanic and tectonic processes), exogenous (caused by subaerial processes), and those induced by climate and land-use change (Slaymaker, 1997). Table 1 shows the categories and the main types of geomorphic hazards. Table 1. Categories and the main types of geomorphic hazards (modified from Slaymaker, 1997). Geomorphic Hazard Endogenous volcanism neotectonics Exogenous floods karst collapse snow avalanche channel erosion sedimentation mass movement tsunamis coastal erosion Climate or land desertification use change permafrost degradation soil erosion salinization floods Definition and Origin of Debris Flow Mass movement or wasting is a movement in which bedrock, debris or soil transports downslope in a mass or block due to force of gravity. According to Varnes (1978) the types of movements are: fall, topple, slide, spread and flow. Falls take place when rocks break off and material free-falls or bounds down to the base of a cliff. Topple is a movement that is performed by the rotating of a unit or units around a point. In slides cohesive blocks or material remain relatively intact, moving along a well- defined surface of sliding. Spread is lateral extension together with shear or tensile fractures. Flows are a fluid movement of loose earth material. Even though, each of these movements could function alone, in fact many events of mass wasting can be explained by some combination of the primary types of movement (Ritter et al., 1995; Plummer et al., 2005). Particularly, flows move entirely by differential shearing within the transported mass and no clear plane can be defined at the base of the moving debris (Fig 1). In this case, the movement of loose earth material closely looks like that of a viscous fluid. The velocity in flows is greatest at the surface and decreases downward. In some cases flows are the result of a movement that begun as slide and the division between the two movements is unclear. The presence of abundant water is a basic component for the manifestation of most types of flow. Flows permits great distance of transported materials (Ritter et al., 1995). According to Varnes (1978) rock fragment flows are dry flows that can occur when rockslides or falls increase drastically in velocity and they are not a unitized mass. Fig. 1: Diagram of a flow on a slope. The direction of the movement is shown by dark arrows (modified from Plummer et al., 2005). The term debris flow is used for mass wasting in which the movement is occurring throughout the flow. According to Johnson (1970) debris flow is a gravity-induced mass wasting intermediate between landslides and water flooding. Varnes (1978) identifies as debris flows rapid mass movements of a body of granular materials, water and air. According to Hungr (2001) debris flow is a very rapid to extremely rapid flow of saturated non-plastic debris in a steep channel. Debris flows combine loose soil, rock and sometimes organic matter (variety of grain sizes -from boulders to clay) and variable amounts of water to form a slurry that flows downslope. Debris flows may originate when poorly sorted rock and soil debris became mobilized from slopes and channels by the addition of moisture. The essential conditions for debris flow are: abundant source of coarse-grained or fine-grained sediments, steep slopes, and plentiful supply of moisture along with space vegetation. The moisture is supplied by rainfall, snowmelt, and rarely by snow and ice during volcanic eruptions (Fig. 2). These conditions can be found in mountainous areas in arid, semiarid, arctic and humid areas. Small and steep drainage basins, where runoff can be concentrated and sediment source may be high, have the potential to transport large amounts of eroded material by debris flows (Costa, 1984). Fig. 2: Schematic diagram of a debris flow (modified from Highland & Bobrowsky, 2008). The speed and volume of debris flows make them very dangerous. In general, they shape rapid surge fronts and achieve peak speeds greater than 10 m/sec. Therefore, they can bare slopes, drastically change stream channels, endanger human life and cause damages in structures. Even smaller debris flows can cause damages in a mountainous area. Still their deposits can cause damages such as damming rivers or sudden river supply to a river system (Iverson, 2004). In fact, every year debris flows kill many people and cause million dollars of property damages all over the world. In Japan only, about 90 lives per year are lost from debris flows (Takahashi, 1981). Noteworthy debris-flow disasters are those in Yungan - Peru, 1970, Armero - Colombia, 1985 and Caraballeda - Venezuela, 1999 (Fig. 3), each of which resulted in more than 20,000 victims. Fig. 3: Damages from debris flow to the city of Caraballeda, Venezuela in 1999, killing about 30,000 people. (photograph by L.M. Smith, Waterways Experiment Station, U.S. Army Corps of Engineers). Categories of Debris Flows Since debris flows are rapid mass movements and a dangerous natural hazard, their categorization is an objective of many researchers worldwide. Historically, Sharpe (1938) made a differentiation between debris flow and debris avalanches. This separation was retained by Varnes (1978), who classified the flow type mass movements, based on the involved material, the water saturation and the mass velocity. At the same time the term mudflow was referred by researches for fine- grained debris flows (Grandel, 1957; Bull, 1964). Costa (1984) proposed the names lahars for volcanic mudflows and tillflows for debris flows that their materials derived in sediments on the surface of glaciers. Additionally, Pierson and Costa, (1987), proposed debris flow classification based on the sediment concentration and the average flow velocity. Hutchinson (1988) divided debris flows into hillslope and channelized varieties, which correspond to debris flow and debris avalanches of Varnes’s classification. Slaymaker (1988) mentions the term debris torrent for debris flows that originate in forested steep relief and carry as much as 60% volume of organic matter. Hungr et al., (2001) proposed the name debris flood for a very rapid, surging flow of water with debris in a steep channel.