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 use change

desertification

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.

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.

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).

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).

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. 2: Schematic diagram of a debris flow (modified from Highland & Bobrowsky,

2008).

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.

In this concept debris flows are broadly defined and include several varieties. However, there is not yet general agreement on classification in literature. In many cases different terms may be used to explain the same phenomena. This fact may slow down the progress in scientific research (Coussot & Meunier, 1996). The common varieties - mudflows, debris avalanche and lahars - are described in this section.

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).

Mudflows Mudflows are flows mixed of debris and water that often move down a channel (Fig. 4). This phenomenon is very rapid to extremely rapid mass movement of saturated plastic debris in a channel containing more water than source material (Hungr et al., 2001).

The mudflow is sandy mud or muddy sand and often includes 10 to 30% clay and silt (Daniels & Hammer, 1992). It occurs after intense rainfalls and quickly becomes channel to valleys, moves down like a stream but usually mudflows are more viscous. They usually move slower than a stream because of the heavy load of debris. Houses and cars are filled by mud, if they are not broken or carried away (Fig. 5). Mudflows are frequently manifested in relief where the debris is not covered by vegetation. Additionally, mudflows occur after fires that destroy slope vegetation. So, burned-over slopes are prone areas for mudflow, especially when heavy rain falls before the restoration of vegetation performed (Plummer et al., 2005).

Fig. 4: Mudflow deposit in Peruvian Andes, Perou (photograph by Plummer, 2005).

In general, the favorable conditions for a mudflow are: availability of debris susceptible to saturation, discontinuous intense rainfalls as in the case of storms or sudden snow melting, lack of vegetation cover and steep slopes. These conditions are often found in mountainous regions, volcanic slopes with ash and dust deposits and in arid denuded areas, such as deserts. Moreover, mudflows can travel for long distances even over gently sloping terrain. They are potentially very dangerous because of their high flow velocity and the long distance of travel. Mudflows may cause severe property damages, injuries and even loss of human lives.

Debris avalanche Debris avalanche is the faster variety of debris flows. Debris avalanche is a large, very rapid to extremely rapid, flow of partially of fully saturated debris on a steep slope (Fig. 6). It begins as a swallow surface slide when an unstable slope collapses and the fragmented debris continues to develop into a rapidly moving flow, but it does not move into a channel (Hungr et al., 2001; Highland & Bobrowsky, 2008).

Fig. 5: A mudflow cause damages in a house along the Toutle River, in USA. The lines on

the tree trunks and the house indicate the height of the mudflow (photograph by Dwight Crandell, U.S.Geological Survey).

Debris avalanche was described by Sharpe (1938) as a landslide with morphology similar to snow avalanche. They take place worldwide in mountainous areas with steep slopes while they are common on very steep volcanoes. They do not repeatedly occur in the same location and their deposits are unconstrained alluvial aprons. The debris avalanche has high velocities and can travel close to 100 m/sec (Hungr et al., 2001; Highland & Bobrowsky, 2008). The most catastrophic modern example is the one that buried the city of Yungan - Peru. In 1970 a debris avalanche triggering by an earthquake buried the towns of Yungay and Ranrahirca, killing more than 20,000 people (Fig. 7).

Fig. 6: Schematic diagram of a debris avalanche (modified from Highland & Bobrowsky,

2008).

Lahars The name lahar is an Indonesian term and it is also identified as volcanic mudflow. Lahars are types of debris flows and they originate on the slopes of volcanoes (Fig. 8). They are common catastrophic events in historic and prehistoric times and can be found in nearly all volcanic areas of the world (Costa, 1984).

Fig. 7: Aerial photo of debris avalanche that buried the city of Yungay and village of

Ranrahirca, Peru 1970. (photograph by Servicio Aerofotográfico National, graphics by George Plafker, U.S. Geological Survey).

The debris of a lahar can be either hot or cold. Lahars are mainly triggered by water. They can originate from rainfall, crater lakes, condensation of erupted steam on volcano particles, or the melting of snow and ice at the top of high volcanoes. Lahar also can occur from eruptions or volcanic venting which suddenly melts surrounding snow and ice and causes rapid liquefaction and flow down steep volcanic slopes at catastrophic speed. In these cases their consequences can be extremely large on human system. For example the flow can bury human settlements located on the volcano slopes while some large flows can also block up rivers causing flooding upstream (Highland & Bobrowsky, 2008).

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 (Fig. 9).

Fig. 8: Lahars develop by the 1982 eruption of Mount St. Helens in Washington, USA.

(photograph by Tom Casadevall, U.S. Geological Survey).

Fig. 9: Aerial view of Armero, Colombia, in 1985. The mudflows were destroyed the city

and were killed about 25,000 people (photograph by Darrell G. Herd, U.S.Geological Survey).

Characteristics and Failure mechanisms of Debris Flow

As already mentioned, debris flows usually include fine-grained (clay, silt and sand) and coarse-grained (gravel, cobbles and boulders) materials mixed with a variable amount of water. However these mixtures result in flows like viscous fluids. They are often of high density, 60% to 80% by weight solids (Varnes, 1978; Hutchinson, 1988).

Debris flows are potentially very catastrophic events as they cause significant erosion of the banks and the bedrock base of the stream over which they flow. In that way, they increase their sediment charge and their erosive abilities. The velocity of movement is a very important parameter since it relates to hazard intensity. According to the velocity classification proposed by WP/WLI (1995), debris flows are able to reach extremely rapid movement rates (Table 2). The density and the very rapid movement of debris flow materials lead to a mass with significant energy. Thus, the debris flows are catastrophic events capable to inflict loss of lives and significant damage (Hungr 2005; Nettleton et al., 2005).

Table 2. Landslide rates of movement (WP/WLI, 1995). Movement Rate Velocity

Class Velocity Limits Rate (mm/sec) Debris Flow

Range Extremely rapid 7

5m/sec 3m/min 1.8m/hour 13m/month 1.6m/year

5 x 103 50 0.5 5 x 10-3

50 x 10-6

Very rapid 6 Rapid 5 Moderate 4 Slow 3 Very slow 2 Extremely slow 1

From a geomorphological point of view, a debris flow is easy to recognize on the field. A typical debris flow path is divided into a source area or initiation zone, a stream transport channel or transport zone and a depositional area or deposition zone having fan morphology (Fig. 10). Usually the source area is a slope failure in the headwall or side slope of a stream channel. The slope failure may be a physical shallow slide or rock slide or the failure of man-made (road) fill. Often the bed of the stream itself may be unstable, especially during extreme discharge. The source area of a debris flow may have the following characteristics: very steep slope (>15°); abundant supply of loose debris, a source of water and spare vegetation (Hungr, 2005; Calligaris & Zini, 2012).

In some cases, a rapid initial landslide takes place and may continue downslope. This often leads to produce a flow-like motion of mass movement. Saturated materials may rapidly load to the flow path and increase the volume as well as establish a higher level of saturation. In many cases debris flows enter in preexisting stream channels and continue flowing. The slope and valley deposits frequently supply a significant proportion of the volume of debris flows. This middle part of a debris flow can be characterized as a transportation zone or transportation channel. This zone is steeper than 10o. Deposition will start when the slope angle decreases. Deposits of debris flows establish a debris fan or cone that can be referred as depositional area or deposition zone. In the proximal part of debris fan often coarse-grained materials form thick deposits, while fine and thinner deposits occur in its peripheral part (Hungr, 2005; Nettleton et al., 2005).

Characteristics of debris flow such as viscosities can vary during the path of the flow. However, additions of debris and water from lateral sources along the course of the flow may bring about changes in the character of the moving debris flow. Debris flows have often been observed to move in surges or pulses of materials separated watery inter surge flow. This is accompanied by changes in the fluid characteristics. Flow transformations are frequent along the flow path as a function of changing hydraulic conditions and distance from their source. Flows may transport along their way debris flows to stream flows (Ritter et al., 1995).

Debris flows can move for many kilometers from the source area, emerge from a channel and spread across an alluvial fan. Therefore studies of the characteristics of the source area in combination with studies of deposition zones are an essential tool for planning and preventative designs in areas prone to debris flow (Ritter et al., 1995). In many regions, alluvial fans are perfect sites for urban development because

Fig. 10: A. Diagram of the three main parts of a debris flow phenomenon: the source area,

and the transport and deposition zone (scheme modified from Highland & Bobrowsky, 2008). B. Transport and deposition zone of a debris flow in Rocky mountain National

Park - Colorado (modified photograph by R. Harwood).

they are well drained, gently sloping, and usually provide good aquifers. Additionally morphologic and sedimentological studies of debris fans can provide useful information about the determination of fan activity. Kochel (1987), based on sedimentology of debris fan, showed that catastrophic events like the 1969 Hurricane Cammile flows in central Virginia, USA, manifested at least three times in the last 11,000 years.

Factors influencing the occurrence of debris flow

Several factors may influence the triggering of a particular debris flow, worldwide. Some of these may be considered as predisposing factors, which make an area increasingly susceptible to debris flow without actually initiating it, such as debris availability in a drainage basin. Other factors may be consider as triggers that cause debris flows, such as the intense surface-water flow (Highland & Bobrowsky, 2008; Calligaris & Zini, 2012). The following sections discuss each of the parameters that may be considered to affect the occurrence of debris flows.

Morphological factors

Slope angle

Steep slopes are susceptible to debris slides and hillside debris flows. They increase the flow rate and thus, the erosive power of water flows. Particularly, a very steep slope (>150) is a favorable condition to initiate a debris flow. Debris flows have been observed to flow at slope angles above 11º, while in some cases water-supported debris flows (i.e. high water content) often flow at angles equal or above 2º (Heald & Parsons, 2005; Nettleton et al., 2005).

Slope aspect

It is a preparatory factor in the beginning of debris flows when the slope aspect and the direction of dip of a smooth rock-head profile coincide. Moreover, the aspect of the catchment, in relation to the prevailing weather systems, may tend to trap and hold rain clouds (Heald & Parsons, 2005; Nettleton et al., 2005).

Other morphological factors

Moreover, convex slopes may increase water infiltration within surface deposits leading to increased pore pressures. Therefore, shear strength decreases and the potential for further landsliding is enlarged. Additionally, when the slope changes along a stream, a nickpoint may form, leading to the supply of more debris to the flow, and further increasing its mass and erosive power (Nettleton et al., 2005).

Geological, geotechnical factors

Geology

The bedrock geology is a predisposing factor for debris flows, as they mobilize unconsolidated deposits. The lithology of the underlying bedrock is significant to the development of a mantle of superficial deposits. The long term weathering of soils and rocks make an area susceptible to failure. The products of weathering usually form a weak mantle of soils overlying the bedrock that supplies a surface along which potentially slope failure or debris flow may propagate. Moreover, bedrock with discontinuities is characterized by low shear strength. Discontinuities within the bedrock may be used by debris flow, supplying more rock debris (Heald & Parsons, 2005; Nettleton et al., 2005).

Geotechnical factors

The geotechnical behavior of soil containing cohesion, grain size, shear strength, moisture content, void ratio, relative density and permeability are significant to the occurrence of debris flows. Loose unconsolidated deposits including silt, sand, gravel, cobbles and boulders are susceptible to debris flows, e.g. moraines and fluvial deposits. The thickness or permeability of surface deposits varies and this may cause restrictions of groundwater flow and increase in the associated pore water pressure (Heald & Parsons, 2005; Nettleton et al., 2005).

Hydrological factors

Drainage network

The morphometric parameters of the drainage basin play a very important role to the debris flow manifestation. The most important parameters are: the area and the perimeter of the drainage basin, the average length, the maximum, minimum and average elevation, the average slope angle, the shape factor and the circularity rate. Moreover, the cross-sectional or longitudinal shape of a stream, its width and depth may affect the length and volume of the run-off. Another factor is the sinuosity of a stream that may absorb the energy of the flood and thus retard it. The erosion of the material along the banks of the streams has a vital impact on debris flows. This can undercut the thick deposits of saturated materials removing support from the base of the slope triggering a sudden debris flow (Heald & Parsons, 2005; Calligaris & Zini, 2012).

Ground water and Hydrogeological conditions

The depth and the location of the ground water table is an essential parameter in the occurrences of any slope instability. Additionally, the permeability of formations is a key factor in the beginning of debris flows. The interface between permeable and

relatively impermeable formations may lead to rapid increases in pore water pressures which can trigger slope failure and mobilization of debris flow (Heald & Parsons, 2005; Nettleton et al., 2005).

Meteorological factors Debris flows are typically triggered by intense meteorological events that occur in a short time period. They cause intense surface-water flow and can raise the water table reaching a critical level or conversely, when the rainfall intensity exceeds the infiltration rate a saturated layer from the surface may be created. Infiltration phenomena create an additional system of forces increasing the slope failure. The meteorological events that cause debris flows are heavy rainfall, rapid snowmelt and moisture (Highland & Bobrowsky, 2008; Calligaris & Zini, 2012).

Vegetation cover Vegetation cover may impede debris flow as it affects soil infiltration rates and the root systems serve to hold the soil in place. Changes in vegetation such as felling of forests and fires may increase surface water run-off flow rates. Damage of organic soil horizons and vegetation roots may result in surface deposits more susceptible to water infiltration. Uprooted trees can supply the power of the debris flow. Some debris flows occur after fires have burned the vegetation from a steep slope or after logging operations have removed vegetation. Forestation may be particularly important in retarding flows (Heald & Parsons, 2005; Nettleton et al., 2005; Calligaris & Zini, 2012).

Land use Land uses may influence the possibilities of debris flows. These contain agricultural uses, the presence of buildings or other man-made constructions. Land use changes and construction activities involving cut and fill such as road construction may lead to debris flows. Excavations of slopes lead to the creation of abrupt changes in slope angle. These areas may be prone to scour erosion (Heald & Parsons, 2005; Nettleton et al., 2005; Calligaris & Zini, 2012).

Landslides Debris flows are commonly activated from other types of landslides that occur on steep slopes, are nearly saturated, and consist of a large proportion of unconsolidated material (Highland & Bobrowsky, 2008).

Earthquakes and Volcanoes In several cases earthquakes trigger mass movements (Chousianitis et al., 2016). Debris flows have been triggered by seismic activity and volcanic eruptions worldwide (Calligaris & Zini, 2012).

Debris flow hazard assessment

Debris-flow hazard poses an important threat in mountainous environments throughout the world. Thus, an increasing number of government agencies and local authorities have realized that debris flows are a source of severe natural disaster and particularly dangerous to life and property (Iverson, 2004; Jakob, 2005).

Debris flows are fast-moving flows of mud and rock, and among the most numerous and dangerous types of landslides. These physical phenomena are capable of destroying homes, washing out roads and bridges, sweeping away cars, and preventing streams and roadways with thick deposits of mud and rocks. Periods of heavy rainfall or rapid snowmelt are associated the occurrence of debris flows. Especially in burned areas, a lower threshold of rainfall may start debris flows (Highland et al., 2004). Therefore, prediction of prone areas, including the potentially affected inhabited areas, is of huge significance in debris-flow risk assessment (Wang et al., 2006).

Debris flow hazard and risk management are developing fields and vary widely from country to country and from region to region. Thus a few number of countries have provide guidelines on debris flow hazard analysis or how to quantify and map debris flow hazard. For example, such countries are Austria, Switzerland, Japan and USA (Jakob, 2005).

There are many different methods of assessing debris flow for an area. Commonly, two types of assessments can be observed: (i) studies at regional scale and (ii) studies at local scale. Debris flow hazard assessments at regional scale generally apply a statistical method and Geographic Information System (GIS), simple dynamic approaches and use aerial photographs or satellite images (Guzzetti et al., 1999; Heald & Parsons, 2005; Hürlimann et al., 2006). Studies at the local scale use numerical models or field work to establish the hazard in the depositions of debris flows (Kochel 1987; Glade, 2005).

Generally, hazard assessment of debris flows is a multiple step analysis. According to Jakob (2005) the first step in any debris flow hazard analysis is the debris flow hazard recognition. This includes the study of debris fans, record of possible geomorphologic evidence of debris flow activity, interpretation of aerial photographs or satellite images, historic accounts and records. Since urbanization of fans has increased in many mountainous regions, the hazard level of potential debris flow has increased too. Fans are formed by processes that vary in space and time. Thus, the distinction of a fan if it is formed by fluvial or debris flow possesses is necessary, but it is a very

difficult task. Consequently, the recognition of potential debris flow hazard requires detailed stratigraphic analysis in combination with absolute and relative dating methods. Absolute dating methods are radiocarbon, tephrochronology and dendrochronology. Some examples of relative dating methods are lichenometry and soil development. Debris-flow occurrence leaves traces in the landscape and they are helpful to determine if a debris flow hazard exists. Such geomorphological evidences are: boulder leaves, mudlines, boulders much larger than could be moved by flow, angularity of boulders (Fig. 11). Aerial photographs and satellite imagery are essential tools in the recognition and analysis of debris flow hazard (Fig. 12). Moreover, topographic information can also be used to recognize if a specific drainage basin is prone to debris flow. Historic data of debris flows are useful in the recognition of debris flow hazard. When historic records have spatial completeness and resolution, they are very useful tools in the evaluation of temporal and spatial probability of debris flows.

Fig. 11: Large sub-angular gneissic boulders supported by a sandy matrix. Materials have deposited by prehistoric debris flow in northern Venezuela. (photograph by Wieczorek et

al., 2001).

The debris flow hazard recognition is very important in evaluation of probability of debris flow. Probability is the second step in debris flow hazard analysis and it is the possibility of debris flow to occur in the future. The frequency of debris flow is how often an event occurs, while magnitude is expressed as total debris flow volume or peak discharge (Jakob, 2005).

The probability of debris flow for a given area and for a specified time period contains the recognition of the conditions that caused the event. Factors influencing occurrence of debris flow have already been discussed and should be taken into account in this step of debris flow hazard analysis. Generally, evaluation of the temporal and spatial probability of debris flow is a complex task, which includes field work, interpretation of aerial photos and satellite images, studies of literature and historic records, etc. Temporal probability can be found from historic data, and the magnitude-frequency relationship should be presented. Several approaches such as inventory, heuristic, statistical, and deterministic have been developed to evaluate the factors influencing the occurrence of debris flow. This evaluation leads to a susceptibility map that presents the probability of the spatial occurrence for future

Fig. 12: Aerial photograph of the debris flow at Hougawachi, Minamata city Japan

(modified from Wang et al., 2006).

debris flow events. Particularly the susceptibility map identifies the potential initiation zones (Dai et al., 2002; Hürliman et al., 2006).

The third step in debris flow hazard analysis is the analysis of the run-out behavior to delimit the extension of the endangered zones. In this case three main methods have been employed to study the mobility of mass movements: empirical, analytical and numerical models. Empirical models are focused at the prediction of the run-out distance and distribution of debris flows. Analytical models explain the physical behavior of debris movement. Numerical models include simulations that express the dynamic motion of debris, and/or a rheological model to describe the material behavior of debris (Dai et al., 2002).

The outcomes of the previous two steps should be summarized in a final hazard map of debris flows. This hazard map is divided into hazard zones corresponding to different hazard degree or level. Additionally the hazard degree can be described in a hazard matrix, which includes the intensity and the debris flow return periods. The debris flow intensity is determined by the direct impacts of an event such as maximum flow depth and velocity. The probability of occurrence represents the frequency or the return period of the debris flow. An example of this approach is shown in figure 13, where a debris flow hazard map with four different hazard degrees was established by Hürliman et al. (2006). The study area was Pal drainage basin, located in Andora, Pyrenees. Large parts of the area have been chosen for urban development. The hazard map was based on a hazard matrix (Table 3) which combined the intensity of the debris flow with its probability of occurrence for return periods of < 40 years, of 40 to 500 years, and of > 500 years (Hürliman et al., 2006).

Table 3. Debris flow hazard matrix for Andora, Pyrenees (modified from Hürliman et al., 2006). Probability of occurrence

(return period, yr) <40 400-500 >500 High Medium Low

Intensity Direct impact

h>1m and v>1m/s

High High High Moderate

h>1m and v>1m/s

Medium Moderate Moderate Low

Indirect impact (after flow)

Low Low Low Very Low

Not affected areas Very Low Very Low Very Low

In some cases hazard assessment is extended to risk analyses that contain the determination of features at risk and their vulnerabilities as well as the calculation of specific and total risks. These studies may be followed by hazard or risk mitigation strategies (Jakob, 2005).

Fig. 13: Debris flow hazard map at Pal drainage basin in Andora, Pyrenees (Hürliman et

al., 2006)

Debris flow mitigation measures The final step of each debris flow hazard assessment refers to the mitigation and the reduction of the existing hazard. This task includes a combination of land use planning and technical measures to reduce loss from debris flow. This section describes some simple mitigation methods for debris flow hazards.

In several cases, man-made constructions result in an area more susceptible to debris flow and generally to mass movements. As shown in figure 14, construction activities make a slope prone to debris flow in several ways: a) undercutting the base of the slope along with removing the physical support of the upper part of the slope, b) removing the vegetation cover, c) buildings on the upper part of slope add weight to the potential slide d) extra water may infiltrate to debris. In this case preventive measures should be taken during constructions (Plummer et al., 2005).

Debris flows are very difficult to stop once they have started because of their speed and intensity. However, methods such as modifying slopes and particularly preventing them from being vulnerable to debris-flow initiation through the use of erosion control are very useful in mitigation of debris flow hazard. Moreover afforestation and the prevention of wildfires, which are known to intensify debris flows on steep slopes, can also help in the mitigation of debris flows (Highland & Bobrowsky, 2008).

An important issue for risk management is the design of mitigation measures, which can decrease the existing risk to a low level. Mitigation measures for debris flow

Fig. 14: Man-made constructions make a slope susceptible to debris flow (modified by

Plummer et al., 2005).

hazard can be distinguished in two types: active and passive measures. Active measures aim on the hazard while passive measures aim on potential damages (Huebl & Fiebiger, 2005).

Active debris flow mitigation measures influence the initiation, transport or deposition of debris flows. Some examples of constructions for mitigation debris flow are given below.

Debris flow basins: they are usually built at the base of slopes where debris flows are frequent (Fig. 15). They are used especially in areas that are vulnerable to debris-flow damage and soil and debris are stopped from flowing into structures at the base of the slope.

They should be planned to be able to hold the maximum flow volumes of an area and to prevent overtopping during a flow event (Highland & Bobrowsky, 2008).

Check Dams: they are small, sediment-storage dams. They are typically built in the channels of steep slopes prone to erosion or gullies to stabilize the channel bed. Check dams are common mitigation measures in Europe and Japan and they used to control channelized debris-flow frequency and volume. In some cases they used to control shallow slides in the source area of debris slides. In general, check dams have a high structure cost and thus are constructed where urban areas lie downslope. Debris flows are connected with channel gradients over 25° and take most of their volume by scouring the channel bed. The installation of check dams in the channels avail the following:

Fig. 15: A debris flow basin constructed at the bottom of a slope in San Bernardino,

California, USA. (Aerial photograph by Doug Morton, U.S. Geological Survey).

• to improve the incidence of failure by limiting the channel gradient in the upper

channel, • to decrease the volume of channel-stored material by checking down cutting of

the channel with subsequent gully sidewall destabilization and by providing toe support to the gully slopes.

• to store debris-flow sediment when collected in the lower part of the channel.

Check dams can be built of reinforced concrete (Fig. 16) or log cribs. Concrete dams should not be more than 8 m in height, while log crib dams must not exceed 2 m. The spacing of dams depends on channel gradient and dam height. For example, a dam 2m high and in a 20° channel with 10° sloping channel infill will be spaced every 12 m. Lateral stream erosion and scour by spillway water are the main weaknesses (Highland & Bobrowsky, 2008).

The concrete wing walls and log crib ends are essential to prevent check dams. Such constructions must be tied securely into the canyon wall and streambed to withstand backfill pressures and lateral scour (Fig. 17). Wing walls should incline at about 70% and extend 1–2 m into the banks. The dam foundations can have a minimum width of one-third the total height of the dam (Highland & Bobrowsky, 2008).

Fig. 16: Concrete check dam with low-flow center section in southern California, USA.

(photograph by Los Angeles County Flood Control District).

Debris flow retaining walls: these can be built of various kinds of materials (Fig. 18). They are designed to prevent the evolution of the debris fall, either by blocking the flow or diverting debris around a prone area (Highland & Bobrowsky, 2008).

Fig. 17: Wing walls, Waigrainer Ache, Salzburg, Austria (photograph from Huebl &

Fiebiger, 2005).

Fig. 18: A retaining wall of debris flow in Kamikochi Basin, Japan. (photograph courtesy

of Goncalo Vieira).

Slope stabilization: the slope can be cut back in a series of terraces rather than a single steep cut. This decreases not only the slope angle but the shear strength by removing overlying material. Additionally it stops loose materials from rolling to the base (Plummer et al., 2005).

Passive mitigation measures are usually used to reduce the potential losses. Examples of passive mitigation measures are given below (Huebl & Fiebiger, 2005):

• Hazard mapping: this is an important issue for disaster prevention and management. The areas prone to debris flow hazard should be taken into account in land use planning.

• Land use zoning: building regulations in hazardous areas to debris flow are useful to reduce damages to constructions. Additionally, in those areas with high hazard of debris flow, urban development and constructions should be prohibited.

Fig. 19: Slope stabilization at Filprit tertobel in 1898, Vorarlberg, Austria. (photograph

from Huebl & Fiebiger, 2005).

• Warning system: they are able to detect debris flow and automatically trigger an

alarm. Thus they are helpful in protecting urban areas, traffic routes and infrastructure locations. Additionally, they can be used to control and monitor safety measures. The most important characteristics of early warning are: data collection, transfer and management, distribution of information, decision hierarchy structure and response of panning and organization. At present, warning systems are mainly used in associations with traffic routes.

• Immediate technical assistance: this is a technical measure following a disaster, which includes excavation of buried areas, cleaning of swamped areas and reconstruction of infrastructures.

• Documentation and control: established mitigation measures should be controlled regularly. The effectiveness of the existing measures should be evaluated after an event. Thus, weak features in the mitigation measures can be recognized and additional measures can be designed accordingly.

Exercises: Debris flow hazard recognition - Debris flow hazard mitigation measures

Debris flows are fast-moving flows of mud and rock, and among the most numerous and dangerous types of landslides. These physical phenomena are capable of destroying homes, washing out roads and bridges, sweeping away cars, and preventing streams and roadways with thick deposits of mud and rocks. Periods of heavy rainfall or rapid snowmelt associate the occurrence of debris flows (Highland et al., 2004). Therefore, prediction of prone areas, including the potentially affected inhabited areas, is of huge significance in debris-flow risk assessment (Wang et al., 2006).

Generally, hazard assessment of debris flows is a multiple step analysis. The first step in any debris flow hazard analysis is the debris flow hazard recognition (Jakob, 2005). The final step of each debris flow hazard assessment refers to the mitigation and the reduction of the existing hazard. This task includes a combination of land use planning and technical measures to reduce loss from debris flow.

Debris flow hazard recognition

Objective-Methods

Debris-flow occurrence leaves traces in the landscape and they are helpful to determine if a debris flow hazard exists. The debris flow hazard recognition includes the study of debris fans, record of possible geomorphologic evidence of debris flow activity, interpretation of aerial photographs or satellite images and historic data. Moreover topographic and geologic information can also be used to recognize if a specific area is prone to debris flow.

Problems

The geological map of figure 20 illustrates an area that is located in North Greece. The lithological formations that crop out in the study area are: alluvial deposits, Plio-Pleistocene sands and gravels, gneisses and gneiss-schists, amphibolites, marbles, younger and older granitic rocks, and andesitic rocks. In the topographic map of figure 21 the same area is shown.

1. Onto what lithological formation are Kimmeria village built?

2. Based on the geological map discuss the type of lithological formations that are exposed in the village of Kimmeria.

3. Is the urban area of the Kimmeria village prone to debris flow? Why?

4. Using the information from geological and topographic maps, draw on the topographic map which streams are expected to have the higher capacity to carry solid materials. Why?

Fig. 20: Lithological map of an area in North Greece. The drainage network, the road

network, the urban area and settlements of the study area are shown.

Debris flow hazard mitigation measures

Objective-Methods

Debris flows are very hard to stop once they have started because of their speed and intensity. An important issue for risk management is the design of mitigation measures that can decrease the existing risk to a low level.

Fig. 21: Topographic map of the study area with contour interval 20m. The drainage

network, the road network, the urban area and the settlements of the study area are shown.

Problems

Figure 22 shows an area in North Greece, which covers a surface of 737,620 m2. Loose materials of varying diameter derived mainly from erosion process of the underlying lithological formations of the study area. The thickness of debris has a mean value of 0.5 m. The steep slopes of the study area favor the initiation of debris flow.

1. Calculate the potential total volume of the debris that may be moved. 2. The settlement of Palea Morsini lies downstream of the study area and covers an

area of 101,025 m2. If the total volume of debris accumulated on this surface, calculate their mean value of potential thickness.

3. Is the urban area of the Palea Morsini settlement prone to debris flow? Why? 4. Discuss mitigation measures which can be taken to protect the settlement.

Fig. 22: Topographic map of a drainage basin in North Greece. The drainage network, the

road network, and the urban area of the settlement of Palea Morsini are illustrated.

References

Alcántara-Ayala, I., 2002. Geomorphology, natural hazards, vulnerability and prevention of natural disasters in developing countries. Geomorphology 47(2), 107-124. http://www.sciencedirect.com/science/article/pii/S0169555X02000831

Alexander, E.D. 1995. A survey of the field of natural hazards and disaster studies. In: Carrara, A., Guzzetti, F. (Eds.), Geographical Information Systems in Assessing Natural Hazards, pp. 1-19. Dordrecht: Kluwer Academic Publishers.

Bathrellos, G.D., Gaki-Papanastassiou, K., Skilodimou, H.D., Papanastassiou, D., Chousianitis, K.G., 2012. Potential suitability for urban planning and industry development by using natural hazard maps and geological - geomorphological parameters. Environmental Earth Sciences 66(2), 537-548. http://link.springer.com/article/10.1007/s12665-011-1263-x

Bull, W.B., 1964. Alluvial fans and near-surface subsidence in western Fresno County, California. Professional Paper U.S. Geological Survey 437-A, 70p.

Burton, I., Kates, R.W., 1963. Perception of Natural Hazards in Resource Management. Natural Resources Journal 3(3), 412-441.

Calligaris, C., Zini, L., 2012. Debris Flow Phenomena: A Short Overview?. In: Imran Ahmad Dar (Ed.), Earth Sciences, pp. 71-90. InTech Publisher, http://cdn.intechopen.com/pdfs-wm/27588.pdf

Chousianitis, K., Del Gaudio, V., Sabatakakis, N., Kavoura, K., Drakatos, G., Bathrellos, G.D., Skilodimou, H.D., 2016. Assessment of Earthquake-Induced Landslide Hazard in Greece: From Arias Intensity to Spatial Distribution of Slope Resistance Demand. Bulletin of the Seismological Society of America106 (1), 174-188. http://www.bssaonline.org/content/106/1/174.short

Costa, J.E. 1984. Physical geomorphology of debris flows. In: Costa, J.E., Fleisher, P.J. (Eds.), Developments and applications of geomorphology, pp. 268 – 317. Berlin Heidelberg: Springer. http://link.springer.com/chapter/10.1007/978-3-642-69759-3_9#page-1

Coussot, P., Meunier, M. 1996. Recognition, classification and mechanical description of debris flows. Earth-Science Reviews 40(3), 209-227.

Crandell, D.R., 1957. Some features of mudflow deposits. Bulletin of Geological Society of America 18, 309-322.

Dai, F.C., Lee, C.F., Ngai, Y.Y., 2002. Landslide risk assessment and management: an overview. Engineering Geology 64(1), 65-87. http://www.sciencedirect.com/science/article/pii/S001379520100093X

Daniels, R.B., Hammer, R.D., 1992. Soil geomorphology. New York: John Wiley & Sons.

Fritz, C.E., 1961. Disasters. In: Merton, R.K., Nisbet, R.A. (Eds.), Contemporary

Social Problems, pp. 651–694. New York: Harcourt.

Glade, T. 2005. Linking debris-flow hazard assessments with geomorphology. Geomorphology 66, 189–213. http://www.sciencedirect.com/science/article/pii/S0169555X0400220X

Guha-Sapir, D., Hoyois, P., Below, R. 2014. Annual Disaster Statistical Review 2013: The Numbers and Trends. Brussels: CRED. http://www.cred.be/sites/default/files/ADSR_2013.pdf

Guzzetti, F., Carrara, A., Cardinali, M., Reichenbach, P. 1999. Landslide hazard evaluation: a review of current techniques and their application in a multi-scale study, Central Italy. Geomorphology 31, 181–216.

http://www.sciencedirect.com/science/article/pii/S0169555X99000781

Heald, A., Parsons J., 2005. Key contributory factors to debris flows. In: Winter, M.G., Macgregor, F., Shackman, L. (Eds.) Scottish road network landslides study, pp. 68–80. Edinburgh: The Scottish Executive. Available at: http://www.scotland.gov.uk/Publications/2005/07/08131738/17395.

Highland, L., Bobrowsky, P.T., 2008. The landslide handbook: a guide to understanding landslides. Reston, VA, USA: US Geological Survey, 129 p. http://pubs.usgs.gov/circ/1325/

Highland, L.M., Ellen, S.D., Christian, S.B., Brown, W.M., 2004. Debris-Flow Hazards in the United States. U.S. Geological Survey Fact Sheet, 176-97. http://pubs.usgs.gov/fs/fs-176-97/fs-176-97.html

Huebl, J., Fiebiger, G., 2005. Debris flow mitigation measures. In: Jakob, M., Hungr, O., (Eds.), Debris-flow hazards and related phenomena, pp 445-487. Berlin: Springer.

Hungr, O., 2005 Classification and terminology. In: Jakob, M., Hungr, O. (Eds.), Debris-flow hazards and related phenomena, pp. 9-23. Berlin: Springer.

Hungr, O, Evans, S.G., Bovis, M., Hutchinson, J.N., 2001. Review of the classification of landslides of the flow type. Environmental and Engineering Geoscience VII (3), 221-238. http://eeg.geoscienceworld.org/content/7/3/221

Hürlimann, M., Copons, R., Altimir, J., 2006. Detailed debris flow hazard assessment in Andorra: a multidisciplinary approach. Geomorphology 78(3), 359-372. http://www.sciencedirect.com/science/article/pii/S0169555X0600078X

Hutchinson, J.N., 1988. General Report: Morphological and geotechnical parameters of landslides in relation to geology and hydrogeology. In: Bonnard, C., (Ed.) Proceedings of Fifth International Symposium on Landslides, 1, pp. 3-35. Rotterdam: Balkema.

Iverson, R.M., 2004. Debris flows. In: Goudie, A. (Ed.), Encyclopedia of

geomorphology (Vol. 1), pp. 225. New York: Taylor and Francis Group.

Jakob, Μ., 2005. Debris flow hazard analysis. In: Jakob, M., Hungr, O. (Eds). Debris-flow hazards and related phenomena, pp. 411-443. Berlin: Springer.

Johnson, A.M., 1970. Physical processes in geology: A method for interpretation of natural phenomena; intrusions in igneous rocks, fractures, and folds, flow of debris and ice. Freeman, Cooper, 577p.

Kochel, R.C., 1987. Holocene debris flows in central Virginia. Reviews in Engineering Geology 7, 139-156.

Nettleton, I.M, Martin, S., Hencher, S., Moore, R., 2005. Debris flow types and mechanisms. In: Winter, M.G., McGregor, F., Shackman, L. (Eds.), Scottish road network landslides study, pp 45–67. Edinburgh: The Scottish Executive. Available at: http://www.scotland.gov.uk/Publications/2005/07/08131738/17395.

Plummer, C.C., Carlson, D.H. McGeary, D., 2005. Physical geology. New York: McGraw-Hill (10th Ed.), 617p.

Ritter, D.F., Kochel, R.C., Miller, J.R., 1995. Process geomorphology. Dubuque: W.C. Brown Publishers (3rd Ed.), 546p.

Rosenfeld, C.L., 1994. The geomorphological dimensions of natural disasters. Geomorphology 10, 27–36.

Sharpe, C.F., 1938. Landslides and related phenomena. New York: Columbia University Press.

Slaymaker, O., 1997. Geomorphic Hazards. Chichester: John Wiley & Sons Ltd, 204 p.

Slaymaker, O., 1988. The distinctive attributes of debris torrents. Hydrological Sciences Journal 33(6), 567-573. http://www.tandfonline.com/doi/abs/10.1080/02626668809491290#.V3NhDRskrIU

Takahashi, T.,1981. Debris flow. Annual review of fluid mechanics 13(1), 57-77.

Tobin, G.A., 1997. Natural hazards: explanation and integration. New York: Guilford Press.

Varnes, D.J., 1978. Slope movement types and processes. In: Shuster, R.L., Krizel, R.J. (Eds.), Landslides; Analysis and Control, pp. 11–33. Washington DC: Transportation Research Board, .

Wang, C., Esaki, T., Xie, M., Qiu, C., 2006. Landslide and debris-flow hazard analysis and prediction using GIS in Minamata–Hougawachi area, Japan. Environmental Geology 51(1), 91-102. http://link.springer.com/article/10.1007/s00254-006-0307-0

Wieczorek, G.F., Larsen, M.C., Eaton, L.S., Morgan, B.A., Blair, J.L., 2001. Debris-

flow and flooding hazards associated with the December 1999 storm in coastal Venezuela and strategies for mitigation (No. 2001-144). https://pubs.er.usgs.gov/publication/ofr01144

WP/WLI, 1995. A suggested method for reporting a landslide. Bulletin of the International Association of Engineering Geology 52, 75-78.

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