9.2 Hazards resulting from mass movements

Question 1

Mass movements as hazards

Answer 1

A mass movement which affects people and their property is a hazard, especially when it is unexpected and large-scale but even a single rock fall can be hazardous.

· Rapid mass movements generally cause the greatest loss of life whilst slower, down-slope movements create significant economic costs.

· Mass movements are usually triggered by natural processes eg earthquakes, intense and/or prolonged rainfall and snow melt. However, human mismanagement is a prime cause of slope instability

· Estimates of their impacts vary widely: Em-dat database – 9000 people died in the 1990-1999 decade but Durham Fatal Landslide Database estimates that between 2002 and 2007, there were 44,000 fatalities.

Hazardous impacts depends on nature of mass movement. This includes:

· Type of material
· Scale
· Speed
· Duration
· Frequency of occurrence

Question 2

Hazardous impacts of different mass movements

Answer 2

Question 3

Label this mass movement

Answer 3

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Question 4

Distribution of mass movement hazardsq

Answer 4

Mass Movement hazards are not evenly distributed in space or time.

Although their frequency is controlled by tectonic and climatic setting, the hazard they create is related to population and infrastructure.

They are also seasonal – they peak every year between June and September. This is due to the effect of the monsoon season in SE Asia when changes in pressure and wind direction and higher rates of evaporation bring intense rain triggering mass movements in the Himalayan regions of Nepal and India. El Nino and La Nina with increased rainfall, tropical cyclones (hurricanes)

Many of the mountainous areas of SE Asia and South America have also undergone rapid development. To increase trade, many roads have been built to connect remote rural villages. This has seen poor construction and slopes destabilised with more mass movement deaths resulting. Low and middle income countries seem to be disproportionately affected but even Montecito in California saw 43 deaths in 2018.

Distribution – most deaths have occurred in geographically distinct areas:

• Pacific Rim
• Central America
• Caribbean
• Mainland China
• South East Asia
• Southern edge of Himalayan arc

Why?

• Hilly or mountainous terrain – because they are a result of gravity
• Active tectonic processes operating – earthquakes and faults trigger mass movements or weaken rock so make them more likely eg Philippines Leyte landslide of 2006 triggered by 2.6Mg earthquake and 80% of the major landslides in China occur on tectonically active flank of the Tibetan Plateau. In volcanic areas, ash combined with water may result in increased pore water pressure, loss of cohesion, added lubrication and weight leading to formation of a lahar (mud flow)
• Intense rainfall events (associated with tropical cyclones, El Nina/La Nina events, monsoon weather patterns) – saturation, pore water pressure, lubricated slip planes, additional weight so that shear stress becomes greater than shear strength and the safety factor is reduced and the slope fails eg 2001 Storm Michelle triggered debris flow of 2-3km and in 1998 Mitch caused a mudflow killing > 1500 people in the town of Posoltega in Nicaragua, La Nina caused 30,000 deaths in Vargas, Venezuela in 1999. Climate also determines depth of regolith as increases rate of weathering if rain and temps are high as in tropical areas or if cycles of frost weaken slopes due to frost shattering eg regolith maybe upto 30m deep in Malaysia
• Large populations of poor people who live in unsafe conditions and are vulnerable eg forced to live in informal settlements on steep slopes due to rapid urbanisation and limited land area in vulnerable areas eg large increase in fatalities in HK in 1970s due to growth of illegal settlements by immigrants on steep slopes

NB sometimes a combination of factors are at work to trigger the mass movement

Human activities can increase landslide risk

• Increasing slope angle – eg cutting through high ground – slope instability increases with slope angle
• Placing extra weight on a slope – eg new buildings – this adds shear stress on the slope
• Removing vegetation – roots may bind soil together and deep-rooted vegetation in shallow soils binds it and interception. If removed the slope may experience more water and less cohesion – increased pore water pressure, more lubricated slopes so less cohesive and added weight and overland flow
• Exposing rock joints and bedding planes through excavation– increases speed of weathering and depth of regolith
• Agriculture, logging, construction, mining – all destabilise fragile slopes and change the amount of water entering the soil as interception is reduced which impacts infiltration rates – can lead to more mass movements along with sheet wash, create manmade slopes eg coal spoi tips etc from the waste material.

HICs

• Deaths in HICs comparatively low eg Italy has highest fatality from slope failure in Europe at average of 60 per year and 80% of these are in fast-moving events.
• Avalanches in mountainous recreation areas cause 150 deaths per year. These occur in arctic and temperate areas with slopes of over 20º. Despite 10,000 occurring in the USA each year, only 1% affect life or property. The European Alps are more affected as populations are denser than in the American Rockies 1999 38 dead in Alps at Galtur and Valzur in Austria
• Economic losses are high – In USA, Canada and India, estimated costs of landslides exceed $1billion per year, direct damage caused by landslides in Italy between 1945 – 1990 exceeded $15 billion in Italy
• Even in LICs, economic costs are high - Annual repair cost for roads in the Caribbean alone is $15million

Question 5

Mass movement causes Wales eg

Answer 5

South Wales -prone to landslides eg Tylorstown, Rhondda 2020 Eg of how human factors combine with physical factors (geology) and triggers (heavy rainfall) to cause landslides.
Multitude of coal tips around south Wales, a legacy of the area’s industrial history – this loose, unconsolidated material has created steeper slopes which have added stress. Some will mask hidden springs as in the Aberfan disaster which caused 144 deaths in 1966. A spring flowed through the sandstone and removed clay from the toe of the slope which steepened it. 100000 cubic metres of spoil travelled at 30km/hr South Wales former mining towns and villages are especially at risk such as Tylorstown in the Rhondda and which saw significant landslips in 2020 following a low pressure system, Storm Denis - 60,000 tonnes of spoil to slid down a hillside putting residents in Tylorstown at risk. Welsh government has revealed there are still almost 2,500 disused tips. Most have been concealed by nature and largely forgotten about but 327 are still classed as “higher risk”, meaning they need to be regularly inspected for signs of movement, although do not necessarily pose an “imminent” threat. As well as the risks posed by former coal tips, the natural geology of Wales makes the area prone to slips, according to the British Geological Society (BGS). Landslides are caused by a combination of geology, the shape and steepness of the slopes and some sort of trigger, which in most cases is heavy rainfall. Wales is prone to landslides because it often has this combination of risk factors There are several engineering options to help reduce the risk of landslides, according to the British Geological Survey. These include retaining walls, covering slopes with vegetation that will anchor soil to the surface or even spray-on concrete (shotcrete). “Spray-on concrete has been used in places such as Hong Kong, but is less popular in Wales. Planting trees is a cost-effective way of anchoring soil and promoting other vegetation, but that does take time and vegetation can lead to retention of water which may add stress to the slopes.

Question 6

Brumadinho Dam, Brazil 2019

Answer 6

Physical trigger – water from spring fed into the dam increasing pore water pressure, reducing friction between soil particles so they lose cohesion and shear strength and there is more lubrication, and weight leading to landslide risk.
Human- large mining dam which built up steep waste tips to 80m, steepening slopes and reducing strength. Not well-monitored and water backed up, saturating the sediments 284 killed downstream from slide of mining waste

Question 7

Pallu, Sulawesi, Indonesia 2018

Answer 7

Mg 7.4 earthquake shook steep slopes in this elevated and steep region
Human – rice cultivation in the area had raised the water table and so increased pore water pressure leading to landslides 4300 people died in area of rice cultivation

Question 8

Montecito, California 2018

Answer 8

Causes complex – combination of climate and vulnerable human population. Human- since 1960s, planners have allowed homes to be developed in narrow valleys and floodplains of foot of Ynez Mountains even though areas identified by Federal Emergency Management Agency as being at risk from debris flows (flow of water mixed with sand, pebbles, soil and large boulders where the flow consist of more than 50% sand)

January 2018 – intense rainfall at 150mm/hr – mixed with loose soil, sand, boulders etc to produce debris flow of 5m depth Local topography funnelled the flows into valleys with settlements killing 23 people , injuring 160 people and destroying 400 homes. Flowed 3km to the Pacific coast and cut off main coastal road Highway 101

Long term Physical Triggers – drought from 2011-2017 killed over million trees creating negative water balance Dry conditions and dead vegetation created ideal conditions and fuel for more intense wild fires on the steep slopes.

Short term triggers This cleared the slopes of vegetation so less interception of rainfall as well as leading to increased surface runoff and sheet wash leading to gullying and accumulation of unconsolidated material which would lead to the debris flow. This areas was especially susceptible to this as it is comprised loose , unconsolidated sands and gravels from alluvial fans of rivers. Baked soils further reduced the infiltration of rain when it fell.

Human Triggers – settlements built in the Montecito canyon in areas previously identified on maps as vulnerable to debris flows. People refused to evacuate since they had already done so for the fire – evacuation fatigue Happened at night so would not have heard the evacuation signs Creeks and debris traps had not been cleared from past flows and had grown over as preferred by residents so overtopped.

Evacuation orders were issued by National weather Service but to start with these were only voluntary and only notified people who had signed up to mobile alerts. Later in the evening, a mandatory evacuation order was issued but 17 of the 23 people who died lived outside the hazard mapped evacuation area Could it have been mitigated against? Yes! Residents were protected by debris dams and traps built in the 1960s but these were too few in number and had not been cleared so dams were breached and overtopped - in 1980s & 1990s there had been protests about clearing them.

Question 9

Snow avalanches overview

Answer 9

Avalanches are mass movements of snow and ice.

Where and why do they occur?

· Avalanches can happen wherever there is snow lying on ground of sufficient angle. - 22-90º but most common on slopes 35-45 º

· Snow builds up in layers over the winter. As snow settles and stabilises and re-freezes, it becomes known as neve. This is mostly the case on slopes that have enough sunshine and a cycle of warming/melting/cooling/freezing to allow layers to be fused together.

· If newly fallen snow falls on top of this, especially in winter, it may slide off the top of this frozen layer.

· Likely where snow doesn’t stabilise / loss of cohesion between layers of snow

· Where shear stress (weight + slope angle) > shear strength

Similar to landslide where soil particles are held together by friction

Snow gets its strength from the interlocking of snow crystals and cohesion caused by electrostatic bonding of snow crystals. The snow remains in place as long as its strength is greater than the stress exerted by its weight and the slope angle.

BUT the process is complicated by the way that snow crystals constantly change and the slope evolves over time.

Changes occur in:

1. Over lying pressure – how much snow
2. Compaction by falling snow
3. Temperature changes – diurnal as well as seasonal and over longer time periods
4. Movement of meltwater through the snow which cause the crystal structure of snow to change

All of these can make the crystal structure unstable and move downslope as an avalanche if slope angle, convexity etc are all right.

Question 10

Factors which determine where and when avalanches occur

Answer 10

1. Aspect and avalanche risk in Northern Hemisphere

On north-facing slopes, the risk is higher as more skiers and the fresh snow does not have chance to stabilise as lack of sun inhibits this – no melting or freezing.

This pattern is common in the temperate latitudes of the northern hemisphere. Avalanches are more common on north-facing slopes because they are more shaded and therefore colder, which allows

snowfall to remain unconsolidated and weaker for longer – lower shear strength. When more snow falls, these unconsolidated layers can act as planes of weakness on which snow above can slide

East facing slopes catch sun only in the morning when temperatures are colder while west facing slopes catch the sun in the warm afternoon. Consequently, east facing slopes are colder than west facing slopes.

HOWEVER - In wet snow conditions due to strong sun, it’s just the opposite of a dry snowpack: south and west facing slopes will usually produce more wet avalanches than the more shady slopes.

2. Seasonal impact on slopes and avalanche risk

SLOPES EVOLVE THROUGH THE YEAR - In Spring, partially thawed snow often moves, especially triggered by skiers so risk is heightened late in the day on south facing slopes. In spring, the risk on north facing slopes become less important as the days are longer and the sun is higher so north facing slopes are not so much in the shadow and receive some sunshine and so there is greater stability caused by melting and freezing cycles which cause the layers to bond together increasing the shear strength of the slope.

3. Wind

Wind usually blows up one side of a slope or mountain (the windward side), and down the other (the leeward side). Blowing up the windward slope, wind will “scour” snow off the surface, carry it over the summit, and deposit it on the leeward side. What this does is pack snow unevenly on the leeward side, making it more prone to avalanche as greater weight is added to the slope- this wind-loading of slopes increases shear stress.

Wind loads slopes 10x faster than snow falling as precipitation alone.

This adds weight (shear stress) and can be a critical factor if a slope is already unstable. In the Northern Hemisphere, storms generally move from west to east due to prevailing trade winds. Consequently, the leeward slopes are most often the northeast, east, and southeast facing slopes. These slopes become easily wind-loaded and will more readily avalanche. These slopes are also unstable due to aspect and colder temperatures which means that there are fewer freezing and thawing cycles which means less fusing of layers and so reduced shear strength.

Many ski areas are built on slopes with these orientations and must use prevention measures to counteract the natural avalanche conditions that build up on these slopes.

4. Gradient

Avalanches occur on slopes of 22-90◦ and most slab avalanches occur on slopes of 25-45◦ . Convex slopes are most at risk – the maximum point of convexity is subject to fractures and stresses that can cause the starting point of an avalanche.

They tend to occur in the same places over and over again because of these controlling factors so for this reason, it is easier to hazard map them and manage eg using explosives to remove overlying layer of loose snow.

Question 11

Avalanche characteristics

Answer 11

· Average speeds = 40-60km per hour but have reached upto 200km per hour in Japan.

· Loose avalanches comprising fresh snow – occur soon after snow fall

· Slab avalanches occur at later date when snow has compacted and has formed some cohesion with the older snow – these are usually much larger avalanches and cause more destruction

Question 12

Avalanche triggers

Answer 12

Often caused by sudden rise in temperature that causes melting; the meltwater lubricates the slab and makes it unstable. These slab avalanches occur most often in spring when the snowpack is large (a lot of snow added previously to the accumulation zone so increased shear stress) and temperatures are rising so strength reduced. Occurrence of avalanches in French Alps – Spring time is important as plenty of snow has accumulated and temperatures are rising causing melting and lubrication.

Question 13

Avalanche key facts

Answer 13

• Most serious threat to human life and property in alpine environments
• Kill more than 150 people each year worldwide; most of these are skiers and snowboarders
• They reach speeds of over 80miles an hour within 5 seconds! (Remember hazard profiles – speed of on set)
• 93% of avalanche victims die within 15 minutes and this is from suffocation
• Avalanches are most likely to occur 24 hours after heavy snowfall
• If powerful enough, an avalanche can also contain rocks and trees that they pick up as they move down the mountainside

Question 14

Hazardous nature of avalanches

Answer 14

There are several ways to classify avalanches; generally the denser the snow is at source, the more destructive the avalanche (Figure 6). Slab avalanche. The most dangerous and destructive, when a weak layer of granulated snow loses cohesion with the slab above and the top layer slides down the mountain in a single slab, before breaking into a churning mass of snow blocks. travels ahead of the avalanche, which can flatten trees and damage buildings. Dry slab causes almost all deaths. Slab avalanches are the most feared and occur when a large block of snow breaks off as a slab and slides downhill. This often happens when snow has been lying on the ground for a long time. This type of avalanche travels more slowly, at around 50 to 140 km/h (30 to 90 mph) but the huge weight of snow involved can bury buildings and people. The avalanche often breaks up into smaller slabs as it moves, which may set off further avalanches Loose snow avalanche. These occur after a fresh snowfall. They start from one point and are generally not very destructive. Powder avalanche. Slab and loose snow avalanches can become airborne if moving fast enough over an obstruction or cliff. A blast wave will precede the snow, which appears as a billowing cloud. Loose snow or powder avalanches usually start after steady snowfall, when fresh snow is lying over an older layer. Once the avalanche has begun, more and more snow is gathered up as it moves downhill like a giant snowball travelling at between 95 and 240 km/h (60 and 150 mph). A very dangerous blastwave of air. Wet avalanche. Rapid melting of the snow pack causes it to flow down the mountain as a slushy flow. These do not move very fast but are very destructive. These occur when snow is melting and becoming slush. They move at about 5 to 8 km/h (3 to 5 mph), and are much more predictable. However, they still have enough weight to knock down any buildings or bridges that are in the way.

Question 15

Classification of avalanche

Answer 15

1. Airborne or ground hugging
2. Type of breakaway – from a point formed with loose snow which leads to top layer moving or from an area formed of a slab where whole slope breaks away
3. Position of sliding surface – whole snow cover or just the surface layer
4. Form of the avalanche – is it channelled in cross section or open

Question 16

Avalanche management

Answer 16

Are avalanches predictable? Avalanches are not easy to predict.

Recurrence intervals:

Some avalanche paths are only active once every few hundred years; others are active in almost every storm. However, we can predict avalanches to a certain extent as they occur in particular places at particular times of year for reasons. There are always observable signs of instability before an avalanche happens - recognised all too often after the fact: in 90% of all avalanche accidents, the avalanche trigger is the victim (or someone in the victim’s party) and 90% of the time, a slab avalanche is involved.

Seasonality and prediction - Are there more avalanches in Spring? Spring conditions may be the time of year when there are the most avalanches, but once a regular cycle of melting and freezing sets in, predicting the stability of the snowpack is more evident than during the cold winter months. During the freeze phase, the snowpack is at its strongest. Melting during the day and freezing at night is a classic springtime process. In these conditions, after a good freeze at night, the slopes are very stable first thing in the morning. Then, as the sun rises (warming east facing slopes first), the ice skeleton holding the snowpack together melts and the slope eventually becomes much less stable.

Weather and prediction: Wind is the most common cause of acute instability. Wind can deposit snow 10 times faster than snow falling during storms. Wind erodes snow from the upwind side of obstacles and deposits snow on the downwind (lee) sides. We call this “wind loading”. The added weight from this “wind loading” also causes avalanches. If the weight of new snow is added faster than a buried weak layer can adjust to its load, then it fractures and forms an avalanche.

Rain or melting of the snow surface can also cause avalanches. For instance, rain on new snow almost instantly causes avalanches. Strong sun or warm temperatures can also cause melting of the snow and creates wet avalanches. Large wet avalanches occur because of a decrease in strength of a buried weak layer as a result of water dissolving the bonds between the snow grains.

Risk Assessing Avalanches for hazard mapping

· Steepness: Almost all avalanches occur on slopes between 35 and 45 degrees. An ‘expert’ slope at a ski resort is usually around 35 degrees maximum

· Anchors: Trees and rocks that stick up through the snowpack can help to hold the snowpack in place. But the anchors need to be fairly thick to be effective. For instance a thick, mature grove of evergreen trees anchor the slab quite effectively while a sparse grove of trees has very little effect

· Aspect with respect to wind: Recently wind-loaded, steep slopes are almost always very dangerous while recently wind-eroded slopes are usually fairly safe so weather forecasting and monitoring is essential

Question 17

Avalanche management diagram

Answer 17

Question 17

Avalanche managment case study

Answer 17

Chamonix:
· Avalanche bulletins giving warnings of risks: Produced by Meteo France - provide a daily avalanche bulletin from November onward. It is separated into 3 areas for the Haute Savoie region. Chamonix is covered in the Mont Blanc area
· The App FatMap also gives a good indication of which slopes are in the potential avalanche risk range, however it does not guarantee they are safe.

· The best way to be prepared when going off-piste or entering the back country is to do an avalanche awareness course provided by local mountain guides

· Avalanche transceiver – if you do become buried- Also known as a Beeper or Avalanche Beacon this device is worn by each member of the group. Whilst worn it sends out a silent pulse. In the event of a burial the victims beeper continues to send out a signal while the remaining members of the group turn their beacons from transmit to receive. Using the information sent back from the buried beeper the group are able to locate the approximate position of the victim under the snow. All transceivers work on the same frequency and all makes and models are compatible with each other.

Avalanche equipment to carry: There are various pieces of off piste equipment available to skiers and snowboarders. The bare minimum that a person must carry with them is a transceiver, a shovel and a probe but there are some other supplementary options available too.

· Air Bag System (ABS) An ABS is a backpack that houses two large inflatable bags that can be deployed by pulling a cord. In the event of an avalanche the victim pulls the cord and triggers the compressed gas canister housed in the backpack. The inflated bags then keep the victim on the surface of a moving avalanche eliminating the chances of burial.

· Avalung – allows buried skier to breathe under the snow

Question 18

Galtür Avalanche, Austria February 23, 1999

Answer 18

Killed 31 people Worst Alpine avalanche in 40 years 170,000 tons of snow deposited

Causes: heavy snowfall (4m in area); freeze-thaw conditions => weak layer on top of existing snowpack + subsequent deposition of snow; high wind speeds

Question 19

Rigopiano, Italy 2017

Answer 19

29 people died in Italy after an avalanche buried a hotel in the central town of Rigopiano. They were reported to have assembled on the ground floor on Wednesday awaiting an evacuation after a series of earthquakes had rocked the area during the day, but it was delayed because of the bad weather (multiple hazard). Then the avalanche hit.

Question 20

Bondo, Switzerland 2017 Rock Fall and Debris Flow

Answer 20

Rockfall 3 million m3 of rock material
The resulting rock avalanche fell onto a glacier below, which released 0.6 million m3 of water. The rock debris transformed by water into a semi – liquid debris flow. In 11 minutes, this mixture of water, sediment and boulders flowed 6.5 km to the village of Bondo within 11 minutes so little time to evacuate even if warnings given. The first debris surge brought about 50,000m3 of sediment to the village.

Physical triggers/causes Bondo village below Mount Piz Cengalo, with a summit of 3369m and steep relief – gravity will impact as slopes unstable. Physical weathering of the valley due to altitude and temperature cycles weakened the face and caused rock fall along with the impacts of climate change and melting glaciers and permafrost The glacial water added to the hazard by mixing with the rock debris to create a debris flow which flooded the valley destroying homes, infrastructure, roads and bridges

Human triggers/causes Switzerland’s largest mass movement event for more than a century. Perception of risk therefore lowered as no collective memory . The village of Bondo had been built 1000 years ago at the foot of the Piz Cengalo so vulnerable to mass movement (Degg). Climate change :In the past 120 years, temperatures in the Alps have risen by just under 2C – twice the global average. Climate change has a direct effect on the ice masses and the retreating glaciers contribute to the stability of the Alps both because they can support rock and because they protect the mountains from precipitation. If water seeps into the mountain, that can lead to erosion and weathering– and rockfalls. Permafrost – rock and soil that is permanently below freezing – is the same. Once it is no longer frozen, water can seep in, working like a wedge in the rock. The result are complex chain reactions, so-called hazard chains that can include not only rockfalls but flooding and mudslides.

Management Before the event – too remote and dangerous to survey. For most of the Alps there are no alarms or movement monitors. However, this was the biggest loss to life from a mass movement in Switzerland so management since installed to monitor and warn – cannot prevent Since the event management includes: Water table levels are monitored. Should it swell to a dangerous level, early warning system installed - four sirens will warn residents to evacuate and traffic will be barred from entry. A radar and a seismic station have also been installed to detect any seismic triggers.

Impacts on people Primary: 100 residents fled homes and belongings and were displaced for 4 months village was partially buried by mudflows several metres deep. bridge destroyed and roads blocked. 99 buildings damaged, and a third of them were destroyed. Damage to infrastructure cost $41.5 million 8 hikers died Secondary and Longterm: 2 years later electricity and water supplies were still provisional Carpentry shop was biggest employer in village but that was still shut two years later

Greatest impacts on property - private and public roads, electricity and water supplies which were impacted for two years - long term . Minimal deaths and only affected hikers - poor perception of risk as not local, didn’t have warnings, low density population in village but biggest employer (carpentry factory) closed.

Falls - are sudden and can be very dangerous as happen without warning. However, they contain no water so their aerial extent is more limited and impacts are more localised although this will depend on size of material and how fast and far they bounce. As this is an aerial hazard until it reaches the ground, the impact will be more limited however in the case of Bondo, the fall became a debris flow and more destructive hence impact on property.

Question 21

Vargas , Northern Venezuela 1999

Answer 21

Mt Avila = mudslides buried large parts of 300km stretch of central coast

Physical triggers/causes Mt Avila – steep, unstable slopes steeper than angle of repose First two weeks December = unusually high rainfall (PPT 40-50% above normal levels) - increased pore water pressure, loss in internal strength as less cohesion and more lubricated. Water also added weight so shear stress on slope etc Old debris from previous slides increased instability of slopes Geology – thin clay soils over deep weathered metamorphic bedrock added to instability and created slide planes especially where bedding planes dipped downslope

Human triggers/causes Humans intensified risk: Vulnerable population -Adding to the devastation, Vargas State had experienced high population growth and development since the previous 1951 disaster, thus increasing the toll of casualties Corrupt politicians + previous government allowed shanty towns to grow in steep mountain valleys outside Caracas. Added weight to slopes so increased stress and lowered safety factor As many coastal towns had been built on old debris from previous slides, adequate government planning could have prevented this disaster. Over reliance on structural management defence -although the debris flow line was lined with concrete, the flow left the channel bend decimating town of Carablleda. Created false sense of security and perception of risk hampered by lack of collective memory – this was the worst disaster for 200 years 70% of Venezuela’s population lived in this small region on the coast which was affected – The alluvial fans built as sediments from floods and debris flows exit their channels and meet the oceans provide the only extensive flat surfaces along the mountainous coastline of north-central Venezuela. As such, many of them have been extensively developed and urbanized. This high population density increases the risk to life and property from flash flood and debris flow events. before the event, the government had already been trying to redistribute population away from the hazardous areas near the coast.

Management Before the event: concrete channels guiding debris flows in this areas but the mass left the channel as they were overtopped. Over-reliance on these caused false sense of security and complacency After the event: Landslides seen as inevitable and recurring here so Towns that were completely ruined and were swept out to the sea were not rebuilt (such as Carmen de Uria) and instead were turned into parks and other outdoor facilities

Impacts on people 10,000 to 50,000 killed – 10% of the population. Over 150,0000 people were left homeless (States of Vargas and Miranda worst affected) International airport Caracas was closed and Maiquetia sea port damaged – communications severed in region including coastal highway.. $3 billion economic damage and tourism industry affected Longterm impacts: Thousands were homeless 9 years after and value of surviving property had gone down 70%. Hundreds of containers Maiquetia seaport damaged too and long term trade impacted Public services, like water, electricity, phone lines, and land transportation (roads and bridges) completely disappeared in some places. There were no supplies of food and water for months as crops destroyed, so most of the population had to be evacuated. Looting occurred everywhere, forcing the military to implement martial law for more than a year

Vargas = buried 8-10 metres deep of mud In the long term, towns that were completely ruined and were swept out to the sea were not rebuilt (such as Carmen de Uria) and instead were turned into parks and other outdoor facilities 60% of coastline significantly altered coastline in Vargas

Question 22

Mudslide , Campania in Italy 1998

Answer 22

Mudslide in May 1998 that swept through towns and villages in the region of Campania

Physical triggers/causes Rainfall – year’s worth of rainfall in two weeks but this was just a trigger Geologically unstable area – steep relief – above 35-45⁰, active volcanoes eg Etna and Vesuviuis with seismic activity to trigger movements and past pyroclastic deposits which are unconsolidated, lacking cohesion and easily remobilised by rain or rivers of which there are many fast flowing examples. This is due to soil liquefaction. One of the most vulnerable areas to landslides – 1173 serious landslides recorded since 1892 and yet building has been prolific and unregulated in vulnerable areas showing – vulnerability partially due to human factors

Human triggers/causes River Sarno had dwindled to a trickle of water and the riverbed was cemented over for development The clay soils of the surrounding mountains had been rendered loose due to forest fires and deforestation - fires also often set by developers who wanted to develop the land Population pressure due to rapid economic development and urbanisation in the 1960s in area of steep relief meant that houses were built up hillsides identified as landslide risk zones - over 20% of the houses were built without permission on a 2m thick layer of lava from previous Vesuvius eruption which combined with heavy rainfall to remobilise the pyroclastic deposits into mudslides. Environmentalists also pointed to the burning of trees and brush to plant commercial crops and the uncontrolled expansion of towns and villages, with parts of streams and river beds disappearing under concrete and asphalt and drainage channels often clogged with rubbish and building waste. An estimated 217,000 illegal houses built without planning permission in Italy – many of them on cemented over rivers which become torrents during storms and lead to mass movements One factor thought to have contributed to the unstable mountainsides was the replacement of chestnut trees, which have large root systems that help hold the ground together, with hazelnut trees, which are more profitable but have much smaller root systems.

Management Before – no management: unrelenting examples of environmental degradation exemplified by the deforestation of the mountain slopes, the neglect and/or the elimination of the traditional flood and landslide control systems, the numerous human-caused forest fires, the uncontrolled grazing of the mountain slopes, and extensive illegal building. All these activities went on for decades and on a large scale unchecked by the government who had a low perception of risk. landslide early warning system named SANF (an Italian acronym for national early warning system for rainfall-induced landslides) forecasts the possible occurrence of rainfall-induced landslides in Italy The system uses (i) rainfall measurements from a network of about 3000 rain gauges, (ii) rainfall forecasts at different time intervals, (iii) probabilistic rainfall thresholds and (iv) a susceptibility map derived at national scale. But the problem continues :
Landslides in Ischia in Campania region in 2022 due to rainfall and past illegal building

Impacts on people Primary: Killed 300 people Hardest hit was Sarro, a town with 35,000 people that had a year’s worth of rain in the two week period before, exacerbating the effects of the mudflow $500 million in economic losses including rebuilding schools, hospitals, key infrastructure etc

Environmental impacts Rivers clogged up with mud 2m deep Hospitals and schools were destroyed

Question 23

Leyte Landslide, Philippines 2006

Answer 23

A ridge above the village of Guinsaugon located on the Philippines Fault collapsed causing a rockslide-debris avalanche event that released an estimated 15 million m3 of material

Physical triggers/causes Steep slopes : the Philippines formed as a range of fold mountains with steep sides, which means that gravity can more easily overcome inertia so that material falls or flows downslope formation of these mountains is the result of tectonic forces that caused rock layers to buckle and fault – so there are many lines of weakness which make these mountainous areas susceptible to rock falls and landslides. Several of the mountains are volcanoes that have built up layers of solidified lava and ash that not only form steep sides but are made of relatively unconsolidated material (unlike most other rocks they have not been formed under intense pressure of overlying layers of material or oceans). 10 days of heavy rains (2,000mm in 10 days), linked with La Nina (The excess water lubricated the fault lines in the steep hills above the village and increased the pore water pressure between the soil particles, causing less friction to hold particles together, cohesion lost and soil was lubricated) - Minor earthquake a possible trigger (M2.6) breaking the last bonds holding the slope together– located on the Phlippines fault which may have weakened the slope along with continuous weathering

Human triggers/causes Deforestation - Logging: many people from Southern Leyte blamed deforestation by logging companies as a major cause of the landslide. Tree-cover has continued to be removed due to growing population pressure in the area which has forced people to clear and build on and farm land further and further up the steep hillside Mining - civil authorities in Southern Leyte have argued that increased mining operations in the area are to blame. Gold, silver, bauxite, nickel and copper are all found in the uplands and Government legislation since the mid 1990s has led to an increase in activity to stimulate economic growth. These operations may have further destabilised the slopes. Human factors have mainly reduced the slope stability factor increasing the vulnerability of the slope. It was then the physical factors such as heavy rainfall and a minor earthquake than triggered the destabilised slope to fail and collapse in a landslide and debris flow

According to the Philippine government’s Mines and Geosciences Bureau (MGB), up to 80% of the country’s total land area is prone to landslides, placing the country fourth in terms of landslide risk after Indonesia, India, and China. Moreover, rapid population growth, which leads to increased urbanization, deforestation, and unplanned land-use development, all together contribute to increasing the vulnerability of areas that are already predisposed to landslides in the Philippines. PHIVOLCS is involved in managing landslide risk including early warning systems, hazard mapping, use of greening techniques to retain water and reduce sheet wash erosion , shotcrete , grading of slopes, gabion walls etc Awareness and preparedness are the most effective prevention and mitigation measures against possible threats of landslide community.

Impacts on people Primary: 1,126 died as the village of Guinsaugon was buried, including 246 pupils at an elementary school - Guinsaugon’s 375 houses buried to a depth of 10 metres. Biggest death toll from landslides in the Phillippines Roads and bridges to the village were blocked or washed away so aid was slow to arrive.

Impacts on the environment 15 million m3 of material, burying Guinsaugon’s 375 houses to a depth of 10 metres.

Question 24

Kwai Chung, New Territories, Hong Kong 1993

Answer 24

Landslide
*No warning signs
*Slope failure adjacent to Cheung Shan Estate bus terminal, Kwai Chung, New Territories in Hong Kong
*1 death, 5 injuries

Physical triggers/causes NATURALLY PRONE (relief / heavy rain / variable geology, deep weathering of granites by chemical hydrolysis of the feldspars in granite )rainy season between May and September. The wet tropical climate combines with the underlying geology and human behaviour to create unstable slopes
*200mm rain (79mm in one hour) – rainfall 6% higher than average that year
*Saturated colluvium (unconsolidated volcanic sediments) remobilised – soil liquefaction Hong Kong’s topography is a function of its variable geology, which is dominated by granite and volcanic rocks (underlying 85% of the land area). The highest point is 957m and steep hills are a common feature. Victoria Peak is the highest point on Hong Kong island itself at 552m. some geographers believe that vegetation actually increased the risk as trees held back many of the smaller landslides and allowed the larger ones, and washout, to occur.

Human triggers/causes Population pressure and rapid urbanisation following economic growth in the 1970s and 1980s has exacerbated the issue and caused development of manmade slopes – catalogue of slopes records over 60,000 manmade slopes In addition, the lack of space in Hong Kong encourages man-made alterations to the landscape – cutting then loading slopes and development further up steep slopes beyond angle of repose. Most slopes over 30º and have been steepened by excavation and building works which have reduced shear strength and added shear stress.

Management Warnings given Geotechnical Engineering Organisation (GEO), a Hong Kong government department dedicated to slope safety.
*HK$600 million each year on landslide prevention
*Private owners are required by law to carry out the necessary slope repair works.
*Controlled drainage
*Reinforcement of soil +planting long-rooted grass (mulching: turf enforcement) *Spread public awareness – what landslides are and what to do
*Warning and emergency services have also been installed for improved public safety. *Extensive network of automatically recording rain gauges throughout Hong Kong that provide real time rainfall data for the issue of public Landslip Warnings. *Landslip Warning would generally be issued if the 24-hour rainfall was expected to exceed 175mm, or the 60-minute rainfall was expected to exceed 70mm, over a substantial part of the urban area (HKSS, 2005). Success: The overall landslide risk arising from old substandard man-made slopes to the whole community of Hong Kong has been reduced to about 50% of the risks that existed in 1977 (CSB, 2005). Described as most successful mass movement management

Impacts on people Primary: Hong Kong has a history of tragic landslides. In the 50 years after 1947, more than 470 people died, mostly as a result of failures associated with man-made cut slopes, fill slopes and retaining walls. On top of casualties, these events could result in considerable economic loss and disruptions to social activities.
*More than US$45 million (HK$350 million) of direct economic losses were reported due to tropical cyclones, rainstorms, floods and landslides during the last ten years from 1994 to 2004 (Lam, 2004). 1993 six injuries and one fatality, 204 squatter huts were evacuated (53 temporarily and 151 permanently), seven buildings were temporarily evacuated, and 225 sections of road, pedestrian pavement and access were blocked.

Question 25

Mass movement Prediction And warnings

Answer 25

As is the case with earthquakes, prediction is often very limited and notoriously hard to predict . However, monitoring of weather conditions can give some warning in cases of those mass movements caused by snow melt, monsoon rains, tropical cyclones. Avalanches may also be better predicted through analysis of slope angle, aspect, wind and snow fall

· The GEO in HK is continuously updating, maintaining and disclosing the ‘Catalogue of Slopes’, which contains information of some 60,000 sizeable man-made slopes covering their geometry, geology and formation history.
· To better alert public about potential landslide danger, in 2005 the GEO also introduced the landslide potential index, a statistical model used to warn when there is the highest potential for fatal and severe landslide incidents.
· However, in a survey conducted of people living in areas of Hong Kong prone to landslides found only 10% of the 1,834 people interviewed had even heard of the landslide potential index.
· warning system is raised during heavy periods of rainfall to alert the public of potential landslide danger and broadcast across radio and TV news stations at regular intervals, together with advice on precautionary measures the public should take, such as keeping away from steep slopes and cancelling non-essential appointments. Motorists are advised to avoid driving in hilly areas or on roads with landslip warning signals.
· The GEO combines several data sources to decide on whether to issue the warning. Real-time rainfall data is gathered by a network of 120 rain gauges installed across the city, while rainfall for the coming few hours is forecast by the Hong Kong Observatory. These are compared with data showing the correlation between past recent rainstorms and landslides to determine the need for the warning.
· Unlike the landslip warning, which is issued ahead of rainstorms, the landslide potential index estimates the risk of landslides directly after major rainstorms have ended. It uses the city’s network of more than 120 automatic rain gauges to measure the intensity and location of the rainfall, and combines this with analysis of historical landslide records and the distribution of slopes to predict how many landslides could occur.
· There are sensors on at-risk slopes. These sensors include things like GPS monitors that show if a slope is moving and by how much, or how much moisture is collecting beneath the surface. That data is beamed to a central hub in near real-time.
· The GEO are looking to implement a new extreme weather and landslide early warning system, which it hopes to introduce by 2024. The new system aims to increase the forecast lead time from three to six hours, by using artificial intelligence and machine learning technology to study which rainfall patterns most seriously affect Hong Kong, and where these occur.
· The current warning system tells people to stay home if rainfall reaches 70mm (2.8in), but does not advise them on other actions, such as how far they need to move away from hill slopes. The new app will try to deliver more detailed practical advice, including via an app which sends people early warnings.

Avalanches - However, we can predict avalanches to a certain extent as they occur in particular places at particular times of year for reasons. There are always observable signs of instability before an avalanche happens network of professional snow pack observers, which have to be constantly trained. All snow and weather data gathered within a certain area is analysed at special data-processing centres (such as avalanche centres that are responsible for writing public avalanche bulletins; those bulletins identify, through a standardized danger scale, zones with different avalanche hazard (or risk, according to the adopted terminology). The bulletins are then publicised through all possible means (such as internet, mobile phones, radio, TV, leaflets etc.) so that the majority of people interested may read them before going out In 1993 The European Avalanche Warning system was introduced – universally recognised

Question 26

Mass movements Community preparedness

Answer 26

· One of Hong Kong’s government interventions is to ensure that the private owners of slopes take responsibility for slope safety. If a slope owner does not comply with the regulation, prosecution will lead to a HK $50,000 fine, and to imprisonment for up to one year. · They have focused on warning systems that alert city officials, first responders and the population of the high probability of rainfall-induced slope failures, setting up the world’s first regional landslide warning system. · This warning system is raised during heavy periods of rainfall to alert the public of potential landslide danger and broadcast across radio and TV news stations at regular intervals, together with advice on precautionary measures the public should take, such as keeping away from steep slopes and cancelling non-essential appointments. Motorists are advised to avoid driving in hilly areas or on roads with landslip warning sign · Furthermore, slopes across Hong Kong also have signs on them which signify if they have a history of landslides and are potentially at risk In the last decade, some developing countries have set up community-based systems to manage hazards - these are perceived as a cost-effective landslide risk mitigation measure due to their relatively simple monitoring networks. Such systems in places like Chittagong in Bangladesh and the favelas of Rio de Janeiro directly warn the population of the possible occurrence of landslides during heavy rainfall and to invite them to move to safer places. This is often called ‘the last mile’ management and ensures that warnings actually reach individuals who may not be well-connected and ensure they act on it despite their individual perception of risk. As in Hong Kong, the interaction of technical and social aspects is paramount in these systems “Often people in the communities, specifically the ones living in areas that have not experienced landslides, do not move to safe places if warnings are issued or sirens activated.”

However, people need to heed the warnings Montecito, California 2018 People refused to evacuate since they had already done so for the wild fires – evacuation fatigue Happened at night so would not have heard the evacuation signs Creeks and debris traps had not been cleared from past flows and had grown over as the greenery was preferred by residents due to their poor perception of risk compounded by lack of collective memory about past events Evacuation orders were issued by National weather Service but to start with these were only voluntary and only notified people who had signed up to mobile alerts. Later in the evening, a mandatory evacuation order was issued but 17 of the 23 people who died lived outside the hazard mapped evacuation area Could it have been mitigated against? Yes! Residents were protected by debris dams and traps built in the 1960s but these were too few in number and had not been cleared so dams were breached and overtopped - in 1980s & 1990s there had been protests about clearing them.

Question 27

Mass movements Hazard Mapping

Answer 27

Some preventative measures may be taken with avalanches but in most cases it is the avoidance of settlement below unstable slopes which is key to their successful management. Therefore, hazard mapping, planning regulations or land zoning potentially vulnerable areas needs to be put in place and the avoidance of human activities that overload or undermine slopes. With careful analysis and planning along with appropriate stabilisation techniques, the impacts of mass movement may be reduced. Past events can be studied to evaluate their magnitude and frequency. Mapping and testing soil and rock properties determines their susceptibility to destabilising processes. Human factors may also be studied such as potential economic losses, population of affected area , potential damage to property etc. Maps showing areas likely to be affected are important tools for land-use planners to ensure that settlements and key infrastructure are not built in at-risk areas and / or evacuation plans and routes may be put in place

Hong Kong’s catalogue of slopes and landslide potential index along with warnings and more structural measures has been very successful; deaths from landslides have dropped substantially since the mid-1980s, and Hong Kong has not seen a single landslide fatality since 2008. It is also estimated that the risk from landslides has been reduced by 50% since 1977. Managing avalanches depends on hazard mapping; Avalanche zoning in North America is based on concepts from Switzerland. The Swiss defined three categories of avalanche zoning in the 1950s following a severe avalanche winter of 1950-51. In Switzerland, the “Red” or “High” Hazard Zone is defined as an area with either frequent avalanches (average return period of 30 years or less) or areas where reference impact pressures are ~600 psf or greater. Occupied structures are generally prohibited in the “Red” zone. However, despite the frequency of mass movements being controlled by their climatic and tectonic setting, human factors have contributed hugely to their devastting impact – rapid economic growth followed by urbanisation and population pressure has caused humans to move to unstable slopes which are well documented. In Italy in the Campania region before 2018 landslides: Population pressure due to rapid economic development and urbanisation in the 1960s in area of steep relief meant that houses were built up hillsides identified as landslide risk zones - over 20% of the houses were built without permission on a 2m thick layer of lava from previous Vesuvius eruption which combined with heavy rainfall to remobilise the pyroclastic deposits into mudslides. An estimated 217,000 illegal houses built without planning permission in Italy – many of them on cemented over rivers which become torrents during storms and lead to mass movements Even in the most developed parts of the world like Montecito, California, planners since the 1960s have allowed homes to be developed in narrow valleys and floodplains of foot of Ynez Mountains even though areas identified by Federal Emergency Management Agency as being at risk from debris flows . Furthermore, in the 2018 landslides and debris flows, Evacuation orders were issued by National weather Service but to start with these were only voluntary and only notified people who had signed up to mobile alerts. Later in the evening, a mandatory evacuation order was issued but 17 of the 23 people who died lived outside the hazard mapped evacuation area
The risk also needs to be known about if it is to be successfully hazard mapped - Multitude of coal tips around south Wales, a legacy of the area’s industrial history – this loose, unconsolidated material has created steeper slopes which have added stress. Some will mask hidden springs as in the Aberfan disaster which caused 144 deaths in 1966. A spring flowed through the sandstone and removed clay from the toe of the slope which steepened it. 100000 cubic metres of spoil travelled at 30km/hr South Wales former mining towns and villages are especially at risk such as Tylorstown in the Rhondda and which saw significant landslips in 2020 following a low pressure system, Storm Denis - 60,000 tonnes of spoil to slid down a hillside putting residents in Tylorstown at risk. Welsh government has revealed there are still almost 2,500 disused tips and yet most have been concealed by nature and largely forgotten about but 327 are still classed as “higher risk”.

Question 28

Mass movements Structural and non- structural measures to modify the event and mitigate against impacts

Answer 28

Some minor actions can be taken locally such as afforestation or greening, drainage of unstable areas to avoid their saturation or buttressing slopes through pinning, netting or regrading. However, major events that are triggered by tectonic activity or the impact of cyclonic rain, have little means of prevention or amelioration These attempts may focus on structural means such as pinning, netting, regrading, gabions and drainage or planting vegetation to bind the soil surface and reduce water content . They may have limited use in preventing the mass movement.

· HK - remote monitoring of landslide barriers at the toe of hills in Hong Kong. The authorities have built barriers in many places which are designed to prevent debris sliding onto main roads and residential areas. Remote sensors have been installed on the barriers which send a notification to tell the authorities when a slide seems to have happened. The systems can even take and send photographs so the authorities can assess the level of the impact. · The GEO has also built many ‘retaining walls’ across the island at the foot of slopes that are made up of gabion cages designed to keep slopes in place and allow free drainage of rainwater

Maintenance of slopes: · Since heavy rainfall and surface runoffs are contributing to slope failure in Hong Kong, it is vital to remove excess water from slopes. Surface draining systems and protective covers are two methods used to protect slopes. · The Drainage Services Department of Hong Kong finished constructing the Hong Kong West Drainage Tunnel in 2012 in response to extensive flooding and land sliding 2005, 2006 and 2008. · Po Shan Drainage Tunnel drains water and is linked to groundwater monitors since this was the main trigger leading to soil liquefaction in the PO Shan Road landslides of 1972 · Man-made slopes are one of the main methods of slope stabilisation used in Hong Kong. These contain drains to intercept and direct water away from the slopes. The slope is usually protected from infiltration and the erosive effects of water by impermeable hard covers. · Greening techniques refer to the use of natural vegetation to reduce the risk of mass movements. There are three main types of greening techniques that are used in Hong Kong: · The mulching system provides a protective cover that makes it possible for natural vegetation to grow on the slope; a natural vegetative cover is able to grow through the mat, securing it in place. · The use of long-rooting grass is a fast and cost-effective system to cover man-made slopes. This system is applied by drilling planter holes into a hard cover. The drilled hole is then filled with soil mix and fertilisers, and finally the long-rooting grass is planted within. · The fibre reinforced soil system is constructed by mixing polyester fibre into sandy soils. This mixture is capable of resisting tension. Despite the success of these measures. They can be limited by perception of risk (both individual and collective at the national level). The dams and traps used in Montecito did little to protect the settlement in 2018 since they had become overgrown with vegetation which and had not been cleared of past debris. When FEMA did try to clear them in the 1980s, local residents protested at the loss of greenery which had taken over them!