Natural hazards and climate change = Riesgos naturales y cambio climático

              Natural Hazards
& Climate Change
Riesgos Naturales
y Cambio Climático
Editors
Santamarta Cerezal, Juan C.
Hernández-Gutiérrez, Luis E.
Arraiza Bermudez-Cañete, Mª P.
Editores
Santamarta Cerezal, Juan C.
Hernández-Gutiérrez, Luis E.
Arraiza Bermudez-Cañete, Mª P.Natural Hazards
& Climate Change
Riesgos Naturales
y Cambio Climático
Edited by
Santamarta Cerezal, Juan C.
Universidad de La Laguna, Canary Islands, Spain
Hernández-Gutiérrez, Luis E.
Gobierno de Canarias, Canary Islands, Spain
Arraiza Bermúdez-Cañete, Mª Paz
Universidad Politécnica de Madrid, SpainNatural Hazards & Climate Change
Riesgos Naturales y Cambio Climático
© 2014 The Authors.
Published by: Colegio de Ingenieros de Montes
Calle Cristóbal Bordiú, 19 28003 Madrid
Phone +34 915 34 60 05
colegio@ingenierosdemontes.org
Depósito Legal: TF 565-2014
ISBN: 978-84-617-1060-7
211 pp. ; 24 cm.
1 Ed: july, 2014
This work has been developed in the framework of the RECLAND Project. It has been funded by the European Union under the Lifelong Learning Programme, Erasmus Programme: Erasmus Multilateral Projects, 526746-LLP-1-2012-1-ES-ERASMUS-EMCR, MSc Programme in Climate Change and Res­toration of Degraded Land.
How to cite this book;
Santamarta, J.C., Hernández-Gutiérrez, L.E., Arraiza, M.P., (ed.) (2014).Natural Hazards & Climate Changue/ Riesgos Naturales y Cambio Climático. Madrid: Colegio de Ingenieros de Montes.
Designed by
Alba Fuentes Porto
This book was peer-reviewed
This book is intended for educational and scientific purposes onlyContents / Contenido
Preface / Prólogo.................................................................................................................... 5
Part 1. Introduction and Basic Concepts /
Parte 1. Introducción y conceptos básicos
Chapter 1 / Capítulo 1
Natural Hazards, an Introduction: Floods, Earthquakes and Tsunamis. Luis E. Hernández-Gutiérrez ...................................................................................................................................9
Chapter 2 / Capítulo 2
Geological Hazards: Volcanic Eruptions. Luis E. Hernández-Gutiérrez .................................25
Chapter 3 / Capítulo 3
Landslide Hazards. Luis E. Hernández-Gutiérrez.................................................................... 41
Chapter 4 / Capítulo 4
Environmental Restoration. Juan C. Santamarta, Jonay Neris, Jesica Rodríguez-Martín.....53
Chapter 5 / Capítulo 5
Sediment & Erosion Control, Future Challengues. Juan C. Santamarta, Jesica Rodríguez-
Martín.......................................................................................................................... 65
Part 2. Case Studies and Applications /
Parte 2. Estudio de Casos y Aplicaciones
Chapter 6 / Capítulo 6
Cambio climático e incendios de 5ª generación. Néstor Padrón Castañeda, Jesús Barranco
Reyes..........................................................................................................................................81
3
4
Chapter 7 / Capítulo 7
Forest ecosystems, sewage works and droughts – possibilities for climate change adaptation. Gálos B., Antal V., Czimber K. and Mátyás Cs.........................................................................91
Chapter 8 / Capítulo 8
Polluted Soil Restoration. Henn Korjus .................................................................................105
Chapter 9 / Capítulo 9
Análisis espacial de la evolución de los cambios de uso de suelo y vegetación, mediante tele­detección y SIG en el tramo medio del rio Jarama. Bernabe A. V., Riesco J.A.; Giménez M.C. y García J.L. ...........................................................................................................................113
Chapter 10 / Capítulo 10
Estudio del daño estructural y del posterior refuerzo de un edificio afectado por asientos induci­dos por la subsidencia causada por un descenso del nivel piezométrico. Esteban Díaz, Pedro Robles, Roberto Tomás ...........................................................................................................125
Chapter 11 / Capítulo 11
Determinación de deformaciones milimétricas del terreno mediante geodesia astronómica.
Itahisa González Álvarez, Antonio Eff-Darwich Peña, M. Jesús Arévalo Morales............... 143
Chapter 12 / Capítulo 12
Forestry and Field Plant Production Technologies in Environmental Life-Cycle Thinking.
András Polgár, Judit Pécsinger, Edit Pintérné Nagy, Veronika Elekné Fodor, János Rumpf, Katalin Szakálosné Mátyás, Attila László Horváth, Tamás Bazsó..........................................155
Chapter 13 / Capítulo 13
Efectos económicos de la valorización de los lodos obtenidos en los procesos de depuración de aguas residuales. Ruiz-Rosa, I.,Rodríguez-Gómez, L.E., García-Rodríguez, F.J............. 175
Chapter 14 / Capítulo 14
Simple Geospatial Data Collecting Methods for Environment Change. Tamás Bazsó, Péter Primusz.................................................................................................................................. 189
Chapter 15 / Capítulo 15
La lucha contra el cambio climático en el Derecho español: el ejemplo de la legislación de cos­tas. Luis Javier Capote Pérez ..................................................................................................1995
Preface / Prólogo
Los desastres naturales de origen geológico, metereológico o antrópico (erupciones volcáni­cas, terremotos, inestabilidad e laderas, inundaciones, huracanes, tsunamis, incendios), no sólo suponen un serio riesgo para los habitantes de nuestro planeta, sino que representan una amena­za real que influye de manera negativa en el desarrollo económico y social de una región. Su conocimiento y estudio contribuyen a la mitigación del riesgo y puede suponer un coste ínfimo frente a las enormes pérdidas materiales y humanas de su no consideración. La formación en materia de riesgos naturales de los residentes en zonas vulnerables, de los agentes sociales, de la comunidad científica y de las autoridades, es fundamental para la reducción y prevención de los efectos de estos desastres naturales.
La morfología y estructura actual de La Tierra es el producto de multitud de procesos dinámicos, desarrollados a lo largo de miles de millones de años, entre los que se encuentran las erupciones volcánicas, los movimientos corticales verticales y horizontales y otros procesos geológicos e incluso extraplanetarios, que han modelado la superficie terrestre lentamente o aceleradamente en ocasiones. En esta publicación se presenta, con intención de aportar a la sociedad una herramienta más para el conocimiento e intervención ante este tipo de procesos
La actividad humana contribuye notablemente a la degradación ambiental, provocando la aceleración de los fenómenos naturales adversos e incrementando los riesgos, especialmente los relacionados con la estabilidad de laderas, inundaciones. Procesos como los incendios, la deforestación, la modificación de cauces y cuencas, el uso intensivo del suelo, la urbanización de llanuras de inundación y canales hídricos naturales, etc, incrementan la intensidad y la prob­abilidad de ocurrencia de los desastres naturales.6
Entre los objetivos de esta publicación se encuentran, por un lado, proporcionar habilidades en la comunicación social, ya que permiten el desarrollo de la conciencia, la difusión y la sen­sibilización y el debate acerca de los riesgos presentes en una región; y por otro, proporcionar información teórica y técnica en diferentes áreas relacionadas con el tema de los riesgos natu­rales, tanto en el ámbito de la prevención como en el de la intervención.
El contenido aquí presentado va dirigido a un público diverso, desde los especialistas en ciencias sociales, a los expertos en ciencias naturales y exactas y tecnólogos. Permite una visión integral y no fragmentada, donde se combina la capacidad de interpretar los datos cuantitativos con metodologías de evaluación cualitativa, así como la adquisición de herramientas de análisis e intervención para el diagnóstico y la definición de líneas de acción en caso de desastres natu­rales. También se consideran los aspectos jurídicos y económicos, ya que son fundamentales en todo el proceso de planificación y ejecución de acciones.
Dr. Juan C. Santamarta Cerezal Dr. Luis E. Hernández GutiérrezPart 1
Introduction and
Basic Concepts
Natural Hazards
&
Climate Change
Parte 1
Introducción y
Conceptos Básicos
Riesgos Naturales
y
Cambio Climático9
NATURAL HAZARDS & CLIMATE CHANGE
RIESGOS NATURALES Y CAMBIO CLIMÁTICO
Santamarta Juan C., Hernández-Gutiérrez L.E. & Arraiza Bermúdez-
Cañete Mª. Paz (Ed.)
ISBN 978-84-617-1060-7
CHAPTER/
CAPÍTULO
1
Colegio de Ingenieros
de Montes (Ed.)
Natural Hazards, an Introduction:
Floods, Earthquakes and Tsunamis
Luis E. Hernández-Gutiérrez a*
a Consejería de Obras Públicas, Transportes y Política Territorial. Gobierno de Canarias, Spain
Abstract
The best-known geo-hazards occur suddenly, such as earthquakes, tsunamis, volcanic eruptions,
landslides and floods. These can be catastrophic and cause great damage to people and objects. How-ever,
coastal and soil erosion, slow landslides, natural radiation or land subsidence are much slower
processes. These are difficult geo-hazards to discern because sometimes a lifetime is not a sufficient time
interval for them to take place. They rarely cause fatalities and therefore do not usually generate media
headlines, though they can cause important economic losses.
© 2014 The Authors. Published by Colegio de Ingenieros de Montes http://www.ingenierosdemontes.org
Peer reviewed
Keywords; Risk; Earthquakes; Volcanic Eruption; Environmental Secourity
* Corresponding author name. Tel.: +3-492-263-3088 ext. 208
E-mail address: luisenrique.hernandezgutierrez@gobiernodecanarias.org
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
10
1. Floods
The dictionary defines a flood as a great flowing or overflowing of water, especially over
land not usually submerged.
Floods are caused by sudden changes in water level, so the level exceeds its natural confine-ment
and covers a portion of land not previously covered.
This is a natural process that occurs in river systems on a regular basis by the melting of
snows, heavy rain or coastal storms, which can cause an increase in water level over a coastal
plain. These causes are often the product of other natural processes, such as hurricanes and cy-clones
accompanied by heavy rains, volcanic eruptions capable of melting the snow suddenly,
and so on. Apart from natural processes, human influence is in many cases the cause of floods
and other acute effects.
Figure 1. Missouri River flooding (USA) on July 30, 1993 (Source: U.S. Geological Survey)
Floods are the main threat to humanity among the geological hazards. Every year millions
of people are affected, and for many countries they have become the most destructive geologic
process.
1.2 Factors involved in flood formation
The destructive power of a flood is mainly due to two factors. First, there is the power of ero-sion
and transport of material by the water when a rise in its level occurs. Secondly, there is the
fact that floodplains in their morphology and natural wealth provide very favourable conditions
for human settlements.
Flooding can happen anywhere, but certain areas are especially prone to serious flooding.
There are two types of factors involved in flood formation:
Luis E. Hernández-Gutiérrez 				 Riesgos Naturales & Cambio Climático
11
a) Conditioning Factors
• Morphology of the land: The flat configuration of the ground facilitates the expansion of the
water layer; sudden changes in slope favour sudden increases in the velocity of water and its
concentration
• Terrain: The lithological composition of the soil determines its drainage and erosion capac-ity,
this determines whether rivers may carry more or less load during a period of overflow
• River Morphometry: River systems may have different morphologies: braided, meandering,
rectilinear, which can determine the velocity of water, overflow preferential areas, etc
b) Triggering factors
• Weather: The intensity of rainfall or melting snow may exceed the capacity of drainage sys-tem
and cause an overflow
• Seismic: Earthquakes can trigger tsunamis that can cause severe flooding in the coastal zone
• Deforestation: The lack of a well-developed vegetation cover increases water runoff on the
ground.
• Obstruction of the bed: This can occur when waste, trunks or tailings act as a stopper, ob-structing
the water and causing flooding. These blockages can also be caused by the passage
of lava flows
• Paving and bed confinement: These lead to an increase in the speed of runoff and reduce (or
cancel) the infiltration of water into the subsoil. Moreover, these favour the deposition of
materials on the channel bottom, which then fill and collapse over time thus increasing the
topographic level where water circulates
1.3 Flood effects
The primary effects of floods are those due to direct contact with the floodwaters. These are:
• Transport of particles due to higher water velocities, enabling them to transport larger parti-cles
as suspended load. Such large particles include not only rocks and sediment, but, during
a flood, could include large objects like automobiles, houses and bridges.
• Massive amounts of erosion can be accomplished by floodwaters. This erosion can under-mine
bridge structures, levees and buildings causing their collapse
• Water entering human built structures causing water damage. Even with minor flooding of
homes, furniture is ruined, floors and walls are damaged and anything that comes into con-tact
with the water is likely to be damaged or lost. Flooding of automobiles also results in
damage that cannot easily be repaired.
• More sediment carried as suspended load due to the high velocity of floodwaters. When the
floodwaters retreat, velocity is generally much lower and sediment is deposited. After the
retreat of the floodwaters, everything is usually covered with a thick layer of stream depos-
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
12
ited mud, including the interior of buildings
• Flooding of farmland resulting in crop loss. Livestock, pets, and other animals are often
carried away and drowned
• The water often drowns humans that get caught in the high velocity floodwaters
• Floodwaters can concentrate garbage, debris and toxic pollutants that can cause secondary
effects of health hazards
Secondary effects refer to those that occur as a result of the primary effects (tertiary effects
are the long-term changes that take place). Among the secondary effects of a flood are:
• Disruption of services
 Drinking water supplies may become polluted, especially if sew-erage
treatment plants are flooded. This may result in disease
and other health effects, especially in under developed countries
 Gas and electrical service may be disrupted
 Transportation systems may be disrupted, resulting in shortages
of food and cleaning-up supplies. In under developed countries,
food shortages often lead to starvation
Figure 2. Road blocked by floods (Source: Civil Defence, New Zealand [2007])
Long-term effects (tertiary effects) of floods include:
 Changes in the location of river channels as the result of flooding,
new channels develop, leaving the old channels dry
 Destruction of farmland by sediment deposited on farmland (al-though
silt deposited by floodwaters can also help to increase
agricultural productivity)
Luis E. Hernández-Gutiérrez 				 Riesgos Naturales & Cambio Climático
13
 Job losses due to the disruption of services, destruction of busi-ness,
etc. (although jobs may be gained in the construction indus-try
to help rebuild or repair flood damage)
 Increase in insurance rates
 Corruption from misuse of relief funds
 Destruction of wildlife habitat
2. Earthquakes
One of the most frightening and destructive phenomena of nature is an earthquake. We can
define an earthquake as a shaking and vibration on the surface of the Earth resulting from un-derground
movement along a fault plane or from volcanic activity.
Frequently, earthquakes occur due to sudden, violent shifting of tectonic plates, which are the
earth’s outermost layer of crust and upper mantle. Due to the heating and cooling of the rock be-low
these plates, convection occurs. This results in the movement in the overlying plates, which
releases stress that accumulates along faults: a fault is a deep crack that marks the boundary
between two of these plates. The brittle outer part of the Earth crust fractures along faults. Most
earthquakes happen near the boundaries of tectonic plates, both where the plates spread apart
and where they grind together. In the process of breaking, vibrations called “seismic waves” are
generated. These waves travel outward from the source of the earthquake over the surface and
through the Earth at varying speeds. These vibrations cause the entire planet to quiver.
Figure 3. Earthquake shock wave preserved in rail tracks (photo from Civil Defence, New Zealand
[2010])
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
14
Some common causes for earthquakes include volcanic eruptions, meteor impacts, under-ground
explosions and collapsing structures (such as a collapsing mine), rock falls, and land-slides,
but this section will discuss only the main cause: tectonic earthquakes.
Earthquakes are mostly generated deep within the earth’s crust, when the pressure between
two plates is too great for them to be held in place. The underground rocks then snap, producing
a fault and sending out shock waves called seismic waves.
The location where the earthquake starts is called the focus or hypocenter. From here, waves
start to spread out in all directions. The location above it is called the epicentre. The epicentre is
the point on the surface where the waves hit first and the earthquake is the strongest (the most
damage is done).
Figure 4. Hypocenter and epicentre
Figure 5. Earthquake world map location (Source: U.S. Geological Survey)
Luis E. Hernández-Gutiérrez 				 Riesgos Naturales & Cambio Climático
15
80% of the world’s earthquakes occur around the Pacific Ocean, near the east coast of Asia
and the west coast of America. Japan has over 2,000 earthquakes every year, and California and
South America are also very active earthquake zones. In fact, the edge of the Pacific Ocean is
known as the “Ring of Fire” because there are so many active volcanoes in this region.
2.1 Earthquake measurements
Earthquakes are measured by their magnitude and intensity. The magnitude indicates the
amount of energy released at the source (or epicentre) and is measured by the open-ended Rich-ter
Scale. The intensity of an earthquake at a particular area indicates the violence of the earth
motion produced there by the earthquake.
Table 1. Modified Mercalli Intensity Scale
I. Instrumental Generally not felt by people unless in favourable conditions.
II. Weak Felt only by a few people at most, especially on the upper floors of buildings. Delicate-ly
suspended objects may swing.
III. Slight
Felt quite noticeably by people indoors, especially on the upper floors of buildings.
Many do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibra-tion
similar to the passing of a truck. Duration estimated.
IV. Moderate
Felt indoors by many people, outdoors by few people during the day. At night, some
awaken. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like
heavy truck striking building. Standing motor cars rock noticeably. Dishes and windows
rattle alarmingly.
V. Rather strong
Felt inside by most, may not be felt by some outside in non-favourable conditions.
Dishes and windows may break and large bells will ring. Vibrations like a large train pass-ing
close to house.
VI. Strong
Felt by all, many people are frightened and run outdoors, walk unsteadily. Windows,
dishes, glassware broken; books fall off shelves; some heavy furniture moved or over-turned;
a few instances of fallen plaster. Damage slight.
VII. Very Strong
Difficult to stand; furniture broken; damage negligible in building of good design and
construction; slight to moderate in well-built ordinary structures; considerable damage in
poorly built or badly designed structures; some chimneys broken. Noticed by people driv-ing
motor cars.
VIII. Destructive
Damage slight in specially designed structures; considerable in ordinary substantial
buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys,
factory stacks, columns, monuments, walls. Heavy furniture moved.
IX. Violent
General panic; damage considerable in specially designed structures, well-designed
frame structures thrown out of plumb. Damage great in substantial buildings, with partial
collapse. Buildings shifted off foundations.
X. Intense Some well built wooden structures destroyed; most masonry and frame structures de-stroyed
with foundation. Rails bent. Large landslides.
XI. Extreme Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
Numerous landslides, cracks and deformation of the ground.
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
16
XII. Cataclysmic
Total destruction – Everything is destroyed. Lines of sight and level distorted. Objects
thrown into the air. The ground moves in waves or ripples. Large amounts of rock move
position. Landscape altered, or levelled by several meters. In some cases, even the routes
of rivers are changed.
The original scale for measuring the severity (intensity) of earthquakes (table 1) was com-piled
by the Italian Seismologist, Guiseppe Mercalli, in 1902. It has gone through a number
of revisions since then. The Mercalli Scale relies on how much damage is caused by an earth-quake.
It is determined from reported effects of the tremor on human beings, furniture, build-ings,
geological structure, etc. Many places have adopted the Modified Mercalli Scale (MMS),
which classifies earthquake effects into twelve grades.
When a fault slips suddenly in an earthquake, it releases energy in the form of seismic
waves. Sensitive instruments capture these waves; a seismogram is a recording of the shakes
and jolts of these passing seismic waves. Seismology is the scientific study of earthquakes and
the propagation of elastic waves through the Earth.
A seismogram has patterns that can be matched and decoded to learn about how earthquakes
affect the world. A seismograph or seismometer is the measuring instrument that creates the
seismogram. Almost all seismometers are based on the principle of inertia: a suspended mass
tends to remain still when the ground moves. The relative motion between the suspended mass
and the ground will then be a measure of the ground’s motion.
On a seismogram from an earthquake, the P-wave is the first signal to arrive, followed by the
slower S-wave, then the surface waves, which produce the devastating effects. The arrival times
of the P- and S-waves at different seismographs are used to determine the location of the earth-quake.
Given that we know the relative speed of P- and S-waves, the time difference between the
arrivals of the P- and S-waves determines the distance the earthquake is from the seismograph.
Figure 6. Seismogram
Luis E. Hernández-Gutiérrez 				 Riesgos Naturales & Cambio Climático
17
We know the earthquake’s magnitude from the height of the waves, and we can figure out
when and where the earthquake happened from the time the waves arrive at different places.
The individual earthquake shapes the exact pattern of the wiggles: how deep it was, which di-rection
the fault moved, and what kinds of rocks the waves travelled through.
The magnitude of the earthquake is measured on the basis of ground motion recorded by an
seismograph and is related to the amount of energy released by an earthquake. This is expressed
by the Richter Scale.
The Richter scale is a scale designed by A. Richter to measure the strength or magnitude of
the shock waves produced by an earthquake. The scale is measured in steps from one upward.
Each successive unit is ten times more powerful than the one before. Therefore, an earthquake
that measures 7.0 on the Richter scale is 1000 times more powerful than an earthquake measur-ing
4.0. The severity of an earthquake can be evaluated on this scale as follows:
• Slight Magnitude up to 4.9 on the Richter scale
• Moderate Magnitude 5.0 to 6.9
• Great Magnitude 7.0 to 7.9
• Violent Magnitude 8.0 and more
However, the Richter magnitude is only accurate for measurements of earthquakes taken up
to about 500 km distance. Therefore, seismologists have developed a system called “moment
magnitude,” which takes into account the actual area of fault ruptured and gives a more consis-tent
measure of earthquake size across the spectrum.
2.2 Damage caused by earthquakes
Earthquakes can cause massive damage and destruction. Earthquakes strike suddenly, vio-lently,
and without warning at any time of the day or night. If an earthquake occurs in a popu-lated
area, it may cause many deaths and injuries and extensive property damage.
As for damage caused by earthquakes, the following aspects must be considered:
• The effects of an earthquake are strongest in a broad zone surrounding the epicentre
• Earthquake vibrations last longer and are of greater wave amplitudes in unconsolidated sur-face
material, such as poorly compacted fill or river deposits; bedrock areas receive fewer
effects
• The worst damage occurs in densely populated urban areas where structures are not built to
withstand intense shaking
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
18
• The waves can produce destructive vibrations in buildings and break water and gas lines,
starting uncontrollable fires
• Surface waves can seriously affect roads, bridges and other communication lines
• An earthquake can trigger mudslides, which slip down mountain slopes and can bury habi-tations
below
• A submarine earthquake can cause a tsunami, a series of damaging waves that ripple outward
from the earthquake epicentre and flood coastal cities
2.3 Prediction and prevention of earthquakes
Earthquakes cannot be predicted, although areas most at risk can be identified. The buildings
in these areas can then be modified to withstand earthquake shocks. Buildings constructed in
earthquake-prone areas have to meet extremely strict building regulations.
Scientists are continuously thinking of ways to try to reduce earthquakes’ power. Although
there are no guarantees of safety during an earthquake, identifying potential hazards ahead of
time and advance planning can save lives and significantly reduce injuries and property damage.
The risks that earthquakes pose to society, including death, injury, and economic loss can be
greatly reduced by:
• Better planning, construction and mitigation practices before earthquakes happen
• Providing critical and timely information to improve response after they occur
In earthquake-prone areas, populated areas need to take measures to protect themselves
against the effects of earthquakes and to reduce deaths and losses, such as the following ones:
• Earthquake drills should be conducted frequently in earthquake-prone settlements, so that
people are familiar with emergency procedures during an actual earthquake, reducing death
tolls
• Adequate shelters, medicine and food should also be provided in the settlement to handle the
after affects of the earthquake
• Disaster plans and civil defence units should also be well maintained to ensure efficient res-cue
actions after a disaster strikes
• Seismographs, machines that can detect earthquakes, should be utilized to predict potential
earthquakes, alerting authorities to evacuate the people as soon as an earthquake threat is
reported
• Tsunami warning systems are also important in coastal areas prone to earthquake in order to
reduce great loss of life and damage to property when the waves roll in
• The earthquake risk can be reduced by micro-zonation, which is the identification of sepa-rate
individual areas having different potentials for hazardous earthquake effects
Luis E. Hernández-Gutiérrez 				 Riesgos Naturales & Cambio Climático
19 19
• Architects are also designing earthquake-proof buildings, constructing on rock instead of
gravel, or on soft sand or clay. Large structures are made with strong frameworks of steel or
reinforced concrete, so that the frame stands firm even if the ground is shaking.
3. Tsunamis
Tsunamis are a series of enormous waves created by an underwater disturbance, such as an
earthquake, landslide, volcanic eruption, or meteorite. Tsunami is a Japanese word: ‘tsu’ mean-ing
harbour and ‘nami’ meaning wave.
A tsunami is generated by an impulsive disturbance in the ocean or in a small, connected
body of water. The waves sometimes inflict severe damage on property and pose a threat to
life in coastal communities. In the open ocean, a tsunami is less than a few centimetres high,
travelling at ~800 km/hour (the speed of a commercial jet airplane) with wave energy extending
from the surface to the ocean floor. As the tsunami approaches the coastline, the wave energy is
compressed into a much shorter distance, creating potentially large destructive waves that pose
a threat to life in coastal communities.
If the disturbance is close to the coastline, local tsunamis can demolish coastal communities
within minutes. A very large disturbance can cause local devastation and export tsunami de-struction
thousands of miles away.
Figure 7. An aerial view of Minato, Japan, a week after a 9.0 magnitude earthquake and subsequent
tsunami devastated the area (Source: NOAA/NGDC, Lance Cpl. Ethan Johnson, U.S. Marine Corps)
Since 1850 alone, tsunamis have been responsible for the loss of over 420,000 lives and
billions of dollars of damage to coastal structures and habitats. Most of these casualties were
caused by local tsunamis that occur about once a year somewhere in the world. For exam-
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
20
ple, the December 26th, 2004 tsunami killed about 130,000 people close to the earthquake that
caused it and about 58,000 people on distant shores.
3.1 Tsunami formation
A tsunami is different from a wind generated surface wave on the ocean. From the area where
the tsunami originates, waves travel outwards in all directions. Once the wave approaches the
shore, it builds in height. The topography of the coastline and the ocean floor will influence the
size of the wave. There may be more than one wave and the succeeding one may be larger than
the one before. This is why a small tsunami at one beach can be a giant wave a few miles away.
Tsunamis are caused by different reasons:
• Sudden movement of the ocean due to earthquakes
• Landslides on the sea floor and land slumping into the ocean
• Large volcanic eruptions
• Meteorite impact in the ocean
a. Earthquakes
The most destructive tsunamis are generated from large, shallow earthquakes with an epi-centre
or fault line near or on the ocean floor. The high seismicity of such regions is caused by
the collision of tectonic plates. Large earthquakes on the seafloor, when slabs of rock move past
each other suddenly, cause the overlying water to move. When a great earthquake ruptures, the
faulting can cause vertical slip that is large enough to disturb the overlying ocean, thus generat-ing
a tsunami that will travel outwards in all directions. The resulting waves move away from
the source of the earthquake event, spreading destruction along their path.
b. Landslides
Less frequently, tsunami waves can be generated from displacements of water resulting from
rock falls, icefalls and sudden submarine landslides or slumps. Such events may be caused
impulsively from the instability and sudden failure of submarine slopes, which are sometimes
triggered by the ground motions of a strong earthquake. Major earthquakes are suspected to
cause many underwater landslides, which may contribute significantly to tsunami generation.
In general, the energy of tsunami waves generated from landslides or rock falls is rapidly dis-sipated
as they travel away from the source and across the ocean, or within an enclosed or
semi-enclosed body of water, such as a lake or a fjord.
Luis E. Hernández-Gutiérrez 				 Riesgos Naturales & Cambio Climático
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c. Volcanic eruptions
Less common are tsunamis initiated by volcanic eruptions. Although relatively infrequent,
violent volcanic eruptions also represent impulsive disturbances, which can displace a great
volume of water and generate extremely destructive tsunami waves in the immediate source
area. These occur in the following ways:
• Destructive collapse of coastal, island and underwater volcanoes result in massive landslides
• Pyroclastic flows, which are dense mixtures of hot blocks, pumice, ash and gas, plunging
down volcanic slopes into the ocean and pushing water outwards
• A caldera volcano collapsing after an eruption causing overlying water to drop suddenly
d. Meteorite impact
No documented tsunami has ever been generated by an asteroid or meteorite impact. How-ever,
clearly, the fall of these bodies into the earth’s oceans has the potential of generating
tsunamis of cataclysmic proportions. Scientists studying this possibility have concluded that
the impact of a moderately large asteroid, 5-6 km in diameter, in the middle of a large ocean
basin, such as the Atlantic Ocean, would produce a tsunami that would travel all the way to the
Appalachian Mountains covering the upper two-thirds of the United States. On both sides of
the Atlantic, coastal cities would also be wiped out by such a tsunami. An asteroid of 5-6 km in
diameter impacting between the Hawaiian Islands and the West Coast of North America would
produce a tsunami which would wiped out the coastal cities on the west coasts of Canada, U.S.
and Mexico and would flood most of the inhabited coastal areas of the Hawaiian islands.
3.2 Effects of tsunamis
When a tsunami travels over a long and gradual slope, it has time to grow in wave height.
This is called shoaling and typically occurs in shallow water less than 100 m in depth. Succes-sive
peaks can be anywhere from five to 90 minutes apart. In the open ocean, even the largest
tsunami are relatively small with wave heights of less than one metre. The shoaling effect can
increase this wave height to such a degree that the tsunami could potentially reach an onshore
height of up to 30 m above sea level. However, depending on the nature of the tsunami and the
nearshore surroundings, the tsunami may create only barely noticeable ripples.
Some tsunamis can be very large. In coastal areas, their height can be as great as 10 m or
more (30 m in extreme cases), and they can move inland several hundred metres feet. All
low-lying coastal areas can be affected.
Areas are at greater risk if they are less than 10 m above sea level and within a kilometre of
the shoreline. Drowning is the most common cause of death associated with a tsunami. Tsuna-mi
waves and the receding water are very destructive to structures in the run-up zone. Other
hazards include flooding, contamination of drinking water, and fires from gas lines or ruptured
tanks.
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
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The force of some tsunamis is enormous. Large rocks weighing several tons along with boats
and other debris can be moved inland hundreds of feet by tsunami wave activity. Homes and
other buildings are destroyed. All this material and water move with great force and can kill or
injure people.
The largest tsunami wave ever observed anywhere in the world was caused by a rock fall in
Lituya Bay, Alaska on 10th July 1958. Triggered by an earthquake along the Fairweather fault,
an approximately 40 million cubic meter rock fall at the head of the bay generated a wave,
which reached the incredible run-up height of 525 m (~1750 feet) on the opposite side of the
inlet. An initial huge solitary wave of about 180 m (600 feet) raced at about 160 kilometres per
hour (100 mph) within the bay, debarking trees along its path. However, the tsunami’s energy
and height diminished rapidly away from the source area and, once in the open ocean, it was
hardly recorded by tide gauge stations. Only two people died and three boats destroyed in
Lituya Bay. In nearby Yakutat Bay, a 6.1 m run-up was measured and three people died.
One of the largest and most destructive tsunamis ever recorded was generated on 26th Au-gust,
1883 after the explosion and collapse of the volcano of Krakatoa, in Indonesia. This
explosion generated waves that reached 40 m, destroyed coastal towns and villages along the
Sunda Strait on both the islands of Java and Sumatra, killing 36,417 people. It is also believed
that the destruction of the Minoan civilization in Greece was caused by the explosion/collapse
of the Santorin Volcano in the Aegean Sea in 1490 B.C.
3.3 Prediction and prevention of tsunamis
Tsunamis can occur at any time, day or night. Predicting when and where the next tsunami
will strike is currently impossible. Once the tsunami is generated, forecasting tsunami arrival
and impact is possible through modelling and measurement technologies.
Although a tsunami cannot be prevented, the impact of a tsunami can be mitigated through
community preparedness, timely warnings and effective responses.
Tsunami warning systems provide warnings of potential tsunami danger in the oceans by
monitoring earthquake activity and the passage of tsunami waves at tide gauges. However,
neither seismometers nor coastal tide gauges provide data that allow the accurate prediction of
the impact of a tsunami at a particular coastal location. Monitoring earthquakes gives a good es-timate
of the potential for tsunami generation, based on earthquake size and location, but gives
no direct information about the tsunami itself. Tide gauges in harbours provide direct measure-ments
of the tsunami, but the tsunami is significantly altered by local bathymetry and harbour
shapes, which severely limits their use in forecasting tsunami impact at other locations. Partly
because of these data limitations, some tsunami warnings are considered false alarms because
the tsunami that arrives is too weak to cause damage.
The recent development of real-time deep ocean tsunami detectors and tsunami inundation
models have given coastal communities the tools they need to reduce the impact of future tsu-namis.
If these tools are used in conjunction with a continuing educational programme at the
community level, at least 25% of the tsunami related deaths might be averted.
Luis E. Hernández-Gutiérrez 				 Riesgos Naturales & Cambio Climático
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References
Boll, J. Types of Volcanic Eruptions. Article in http://www.geology.com/volcanoes
Francis, P. (1993). Volcanoes. A Planetary Prespective. Oxford Univerity Press Inc., New York.
443 pp.
Highland, M.L. & Bobrowsky, P. (2008). The Landslide Handbook - A Guide to Understanding
Landslides. U.S. Geological Survey, Reston, Virginia. 131 pp.
Web of Civil Defense, Ministry of Civil defense & Emergency Management, New Zealand
Government: http://wwwcivildefense.govt.nz
Web of Department of Conservation, State of California: http://www.conservation.ca.gov
Web of Geoscience Australia, Australian Government: http://www.ga.gov.au
Web of Instituto Geográfico Nacional, Ministerio de Fomento, Gobierno de España: http://www.
ign.es
Web of ITIC, International Tsunami Information Center: http://itic.ioc-unesco.org
Web of NOAA, National Oceanic and Atmospheric Administration, U.S. Department of Com-merce,
USA Government: http://www.noaa.gov
Web of USGS, U.S. Geological Survey, U.S. Department of Interior, USA Government: http://
www.usgs.gov
Schuter, R.L & Krizek, R.J. (1978). Landslides. Analysis and Control. National Academy of
Sciences. Washington, D.C. 234 pp.
25
NATURAL HAZARDS & CLIMATE CHANGE
RIESGOS NATURALES Y CAMBIO CLIMÁTICO
Santamarta Juan C., Hernández-Gutiérrez L.E. & Arraiza Bermúdez-
Cañete Mª. Paz (Ed.)
ISBN 978-84-617-1060-7
CHAPTER/
CAPÍTULO
2
Colegio de Ingenieros
de Montes (Ed.)
Geological Hazards: Volcanic Eruptions
Luis E. Hernández-Gutiérrez a*
a Consejería de Obras Públicas, Transportes y Política Territorial. Gobierno de Canarias, Spain
Abstract
The Earth is a dynamic planet in which geological processes take place, both internally and external-ly.
These processes can cause harm to humans and their environment. Society must make use of current
scientific and technical knowledge to try to prevent risk and intervene in an emergency. Extreme geolog-ical
processes have occurred ever since the Earth was formed. There are, of course, few risks when rivers
overflow or volcanoes erupt in uninhabited areas. However, if humans and their activities are affected,
we speak about geological risk. Geological processes are ubiquitous phenomena, and we have to live
with them. Risks are minimized if we maximize awareness of them. It is difficult for citizens to perceive
geological hazards in an area where generations have lived safely because most of these risks do not
occur in a time scale that allows them to be perceived during the experience of a lifetime.
© 2014 The Authors. Published by Colegio de Ingenieros de Montes http://www.ingenierosdemontes.org
Peer reviewed
Keywords; Risk; Earthquakes; Volcanic Eruption; Environmental Secourity
* Corresponding author name. Tel.: +3-492-263-3088 ext. 208
E-mail address: luisenrique.hernandezgutierrez@gobiernodecanarias.org
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
26
1. Introduction
Volcanism is the manifestation at the surface of a planet or satellite of internal thermal pro-cesses
through the emission at the surface of solid, liquid, or gaseous products. Volcanology is
the scientific study of volcanic phenomena. Strictly speaking, it refers only to the surface erup-tion
of magmas and related gases, and structures, deposits, and other effects produced thereby.
A volcano is an opening in the Earth´s crust through which magma or gases of magmatic origin,
or both, issue.
1.1 Types of volcanic eruptions
Volcanic eruptions are usually explosive in nature, producing fragmented rocks from erupt-ing
lava and surrounding local country rock. Some eruptions are highly explosive and produce
fine volcanic ash that rises many kilometres into the atmosphere in enormous eruption columns.
Explosive activity also causes widespread ash fall, pyroclastic flows, debris avalanches, land-slides,
pyroclastic surges, and lahars. Explosivity is usually the result of gases expanding within
viscous lava. Another mechanism for explosions from volcanoes occurs when surface water or
ground water enters a magma chamber. These eruptions are likely when a volcano occurs in a
wet area or in the sea.
The character of a volcanic eruption is determined largely by the viscosity of the liquid phase
of the erupting magma and the abundance and conditions of the gas it contains. Viscosity is, in
turn, affected by such factors as the chemical composition and temperature of the liquid, the
load of solid crystals and xenoliths it carries, the abundance of gas and whether the gas is dis-solved
or separated as bubbles.
Eruptions can be effusive, where lava flows like a thick, sticky liquid, or explosive, where
fragmented lava explodes out of a vent. In explosive eruptions, ash and gases may accompany
the fragmented rock; in effusive eruptions, degassing is common but ash is usually not.
Volcanologists classify eruptions into several different types. Some are named after particu-lar
volcanoes where the type of eruption is common; others concern the resulting shape of the
eruptive products or the place where the eruptions occur. Here are some of the most common
eruption types:
• Hawaiian Eruption
• Strombolian Eruption
• Vulcanian Eruption
• Plinian Eruption
• Lava Domes
• Surtseyan Eruption
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a. Hawaiian Eruption
In a Hawaiian eruption, fluid basaltic lava is thrown into the air in jets from a vent or line of
vents (a fissure) at the summit or on the flank of a volcano. The jets can last for hours or even
days, a phenomenon known as fire fountaining. The spatter created by bits of hot lava falling
out of the fountain can melt together and form lava flows, or build hills called spatter cones.
Lava flows may also come from vents at the same time as fountaining occurs, or during periods
where fountaining has paused. As these flows are very fluid, they can travel miles from their
source before they cool and harden.
Hawaiian eruptions get their names from the Kilauea volcano on the Big Island of Hawaii,
which is famous for producing spectacular fire fountains. Two excellent examples of these are
the 1969-1974 Mauna Ulu eruption on the volcano’s flank, and the 1959 eruption of the Kilauea
Iki Crater at the summit of Kilauea. In both these eruptions, lava fountains reached heights of
well over a thousand feet.
Figure 1. Hawaiian eruption. In a Hawaiian eruption, fluid lava is ejected from a vent as fire foun-tains
or lava flows. Kilauea Volcano, Hawaii, USA. (Source: D.A. Swanson, U.S. Geological Survey
[1969])
b. Strombolian Eruption
Strombolian eruptions are distinct bursts of fluid lava (usually basalt or basaltic andesite)
from the mouth of a magma-filled summit conduit. The explosions usually occur every few
minutes at regular or irregular intervals. The bursting of large bubbles of gas causes the explo-sions
of lava, which can reach heights of hundreds of metres. These bubbles travel upward in
the magma-filled conduit until they reach the open air.
This kind of eruption can create a variety of forms of eruptive products: spatter, or hardened
globs of glassy lava; scoria, which are hardened chunks of bubbly lava; lava bombs, or chunks
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of lava a few centimetres to a few metres in size; ash; and small lava flows (which form when
hot spatter melts together and flows downslope). Products of an explosive eruption are often
collectively called tephra.
Strombolian eruptions are often associated with small lava lakes, which can build up in the
conduits of volcanoes. They are one of the least violent of the explosive eruptions, although they
can still be very dangerous if bombs or lava flows reach-inhabited areas. Strombolian eruptions
are named after the volcano on the Italian island of Stromboli, which has several erupting sum-mit
vents. These eruptions are particularly spectacular at night, when the lava glows brightly.
Figure 2. Short bursts of glowing lava, created from the bursting of large gas bubbles at the sum-mit
vent of a volcano classed as a Strombolian eruption. Stromboli volcano, Aeolian Islands, Italy.
(Source: Andrew Hague, Istockphoto.com)
c. Vulcanian Eruption
A Vulcanian eruption is a short, violent, relatively small explosion of viscous magma (usu-ally
andesite, dacite, or rhyolite). This type of eruption results from the fragmentation and ex-plosion
of a plug of lava in a volcanic conduit, or from the rupture of a lava dome (viscous lava
that piles up over a vent). Vulcanian eruptions create powerful explosions in which material
can travel faster than 350 metres per second (800 mph) and rise several kilometres into the air.
They produce tephra, ash clouds, and pyroclastic density currents (clouds of hot ash, gas and
rock that flow almost like fluids).
Vulcanian eruptions may be repetitive and go on for days, months, or years or they may
precede even larger explosive eruptions. They are named after the Italian island of Vulcano,
where a small volcano that experienced this type of explosive eruption was thought to be the
vent above the forge of the Roman blacksmith god Vulcan.
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
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Figure 3. Vulcanian eruption. Relatively small but violent explosions of viscous lava create columns
of ash and gas and occasional pyroclastic flows. Santiaguito volcanic dome complex, Guatemala
(Source: Jessica Ball, Geology.com [2009])
d. Plinian Eruption
The largest and most violent of all the types of volcanic eruptions are Plinian eruptions. They
are caused by the fragmentation of gassy magma and are usually associated with very viscous
magmas (dacite and rhyolite). They release enormous amounts of energy and create eruption
columns of gas and ash that can rise up to 50 km (35 miles) high at speeds of hundreds of metres
per second. Ash from an eruption column can drift or be blown hundreds or thousands of miles
away from the volcano. The eruption columns are usually shaped like a mushroom (similar to
a nuclear explosion) or an Italian pine tree; Pliny the Younger, a Roman historian, made the
comparison while viewing the 79 AD eruption of Mount Vesuvius, and Plinian eruptions are
named after him.
Plinian eruptions are extremely destructive and can even obliterate the entire top of a moun-tain,
as occurred at Mount St. Helens in 1980. They can produce falls of ash, scoria and lava
bombs miles from the volcano, and pyroclastic density currents that raze forests, strip soil from
bedrock and obliterate anything in their paths. These eruptions are often climactic, and a volca-no
with a magma chamber emptied by a large Plinian eruption may subsequently enter a period
of inactivity.
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Figure 4. Plinian eruption sends columns of pulverized rock, ash and gases that rise miles into the
atmosphere in a matter of minutes. Mount St. Helens in Washington State, USA. (Source: Austin Post,
U.S. Geological Survey [1980])
e. Lava Domes
Lava domes form when very viscous, rubbly lava (usually andesite, dacite or rhyolite) is
squeezed out of a vent without exploding. The lava piles up into a dome, which may grow by in-flating
from the inside or by squeezing out lobes of lava (rather similar to toothpaste coming out
of a tube). These lava lobes can be short and blobby, long and thin, or even form spikes that rise
tens of metres into the air before they fall over. Lava domes may be rounded, pancake-shaped,
or irregular piles of rock, depending on the type of lava they are formed from.
Lava domes are not just passive piles of rock; they can sometimes collapse and form pyro-clastic
density currents, extrude lava flows, or experience small and large explosive eruptions
(which may even destroy the domes!) A dome-building eruption may go on for months or years,
but they are usually repetitive (meaning that a volcano will build and destroy several domes be-fore
the eruption ceases). Redoubt volcano in Alaska and Chaiten in Chile are currently active
examples of this type of eruption, and Mount St. Helens in the state of Washington spent several
years building several lava domes.
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
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Figure 5. Lava domes are piles of viscous lava that are too cool and sticky to flow far. Mount St. Hel-ens
in Washington State, USA (Picture from: Lyn Topinka, U.S. Geological Survey [1985])
f. Surtseyan Eruption
Surtseyan eruptions are a kind of hydromagmatic eruption, where magma or lava interacts ex-plosively
with water. In most cases, Surtseyan eruptions occur when an undersea volcano has finally
grown large enough to break the water’s surface; because water expands when it turns into steam,
water that comes into contact with hot lava explodes and creates plumes of ash, steam and scoria.
Lavas created by a Surtseyan eruption tend to be basaltic, since most oceanic volcanoes are basaltic.
The classic example of a Surtseyan eruption was the volcanic island of Surtsey, which erupt-ed
off the south coast of Iceland between 1963 and 1965. Hydromagmatic activity built up
several square kilometres of tephra over the first several months of the eruption; eventual-ly,
seawater could no longer reach the vent, and the eruption transformed into Hawaiian and
Strombolian styles. More recently, in March 2009, several vents of the volcanic island of Hunga
Ha’apai near Tonga began to erupt. The onshore and offshore explosions created plumes of ash
and steam that rose to an altitude of over 8 km (5 miles) and threw plumes of tephra hundreds
of metres from the vents.
Figure 6. Lava erupting through water creates the dramatic plumes of scoria and billowing ash-and-gas
clouds of a Surtseyan eruption. Surtsey Island, Iceland. (Picture from: NOAA, National Oceanic and
Atmospheric Administration, USA [1963])
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2. Volcanic Hazards
Volcanoes can be exciting and fascinating, but also very dangerous. Any kind of volcano is
capable of creating harmful or deadly phenomena, whether during an eruption or a period of
quiescence. Understanding what a volcano can do is the first step in mitigating volcanic haz-ards,
but it is important to remember that even if scientists have studied a volcano for decades,
they do not necessarily know everything it is capable of. Volcanoes are natural systems and
always have some element of unpredictability.
Volcanologists are always working to understand how volcanic hazards behave, and what
can be done to avoid them. Here are a few of the most common hazards, and some of the ways
they are formed and behave. (Please note that this is intended as a source of basic information
only, and should not be treated as a survival guide by those who live near a volcano. Always
listen to the warnings and information issued by your local volcanologists and civil authorities.)
2.1 Lava Flows
Lava is molten rock that flows out of a volcano or volcanic vent. Depending on its composi-tion
and temperature, lava can be very fluid or very sticky (viscous). Fluid flows are hotter and
move the fastest; they can form streams or rivers, or spread out across the landscape in lobes.
Viscous flows are cooler, travel shorter distances and can sometimes build up into lava domes
or plugs; collapses of flow fronts or domes can form pyroclastic density currents (discussed
later).
Most lava flows can be easily avoided by a person on foot, since they do not move much
faster than walking speed, but a lava flow cannot usually be stopped or diverted. As lava flows
are extremely hot - between 1,000-2,000°C (1,800 - 3,600° F) - they can cause severe burns
and often burn down vegetation and structures. Lava flowing from a vent also creates enormous
amounts of pressure, which can crush or bury whatever survives being burned.
2.2 Pyroclastic Falls
Pyroclastic falls, also known as volcanic fallout, occur when tephra (fragmented rock rang-ing
in size from millimetres to tens of centimetres) is ejected from a volcanic vent during an
eruption and falls to the ground some distance from the vent. Falls are usually associated with
Plinian eruptive columns, ash clouds or volcanic plumes. Tephra in pyroclastic fall deposits
may have been transported only a short distance from the vent (a few metres to several kilome-tres),
or, if it is injected into the upper atmosphere, may circle the globe. Any kind of pyroclastic
fall deposit will mantle or drape itself over the landscape and decreases in both size and thick-ness
the farther away it is from its source.
Tephra falls are usually not directly dangerous, unless a person is close enough to an erup-tion
to be struck by larger fragments. However, the effects of falls can cause damage; ash can
smother vegetation, destroy moving parts in motors and engines (especially in aircraft) and
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
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scratch surfaces. Scoria and small bombs can break delicate objects, dent metals and become
embedded in wood. Some pyroclastic falls contain toxic chemicals that can be absorbed into
plants and local water supplies, which can be dangerous for both people and livestock. The
main danger of pyroclastic falls is their weight: tephra of any size is made up of pulverized rock
and can be extremely heavy, especially if it gets wet. Most of the damage caused by falls occurs
when wet ash and scoria on the roofs of buildings cause them to collapse.
Pyroclastic material injected into the atmosphere may have global as well as local conse-quences.
When the volume of an eruption cloud is large enough, and the cloud is spread far
enough by wind, pyroclastic material may actually block sunlight and cause temporary cooling
of the Earth’s surface. Following the eruption of Mount Tambora in 1815, so much pyroclastic
material reached and remained in the Earth’s atmosphere that global temperatures dropped an
average of about 0.5 °C (~1.0 °F). This caused worldwide incidences of extreme weather, and
led 1816 to be known as ‘The Year without a Summer’.
2.3 Pyroclastic Density Currents
Pyroclastic density currents are an explosive eruptive phenomenon. They are mixtures of
pulverized rock, ash and hot gases that can move at speeds of hundreds of miles per hour. These
currents can be either diluted as in pyroclastic surges or concentrated as in pyroclastic flows.
They are gravity-driven, which means that they flow down slopes.
A pyroclastic surge is a dilute, turbulent density current that usually forms when magma
interacts explosively with water. Surges can travel over obstacles like valley walls, and leave
thin deposits of ash and rock that drape over topography. A pyroclastic flow is a concentrated
avalanche of material, often from a collapse of a lava dome or eruption column, which creates
massive deposits that range in size from ash to boulders. Pyroclastic flows are more likely to
follow valleys and other depressions, and their deposits infill this topography. Occasionally,
however, the top part of a pyroclastic flow cloud (which is mostly ash) will detach from the flow
and travel on its own as a surge.
Pyroclastic density currents of any kind are deadly. They can travel short distances or hun-dreds
of miles from their source, and move at speeds of up to 1,000 kph (650 mph). They are
extremely hot, up to 400°C (750°F). The speed and force of a pyroclastic density current com-bined
with its heat mean that these volcanic phenomena usually destroy anything in their path,
either by burning or crushing or both. Anything caught in a pyroclastic density current would be
severely burned and pummelled by debris (including remnants of whatever the flow travelled
over). There is no way to escape a pyroclastic density current other than not being there when
it happens!
One unfortunate example of the destruction caused by pyroclastic density currents is the
abandoned city of Plymouth on the Caribbean island of Montserrat. When the Soufrière Hills
volcano began erupting violently in 1996, pyroclastic density currents from eruption clouds and
lava dome collapses travelled down valleys in which many people had their homes, and inun-
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dated the city of Plymouth. That part of the island has since been declared a no-entry zone and
evacuated, although it is still possible to see the remains of buildings that were knocked over
and buried, and objects that were melted by the heat of the pyroclastic density currents.
2.4 Directed Blast
The May 18, 1980 eruption of Mount St. Helens did not initially produce an eruption col-umn.
Instead, the initial eruption was a directed blast. This blast was a result of depressurization
triggered by an earthquake-initiated landslide on the north flank of the volcano. The area affect-ed
by the directed blast extended for over 19 miles from the volcano. Everything within eight
miles of the directed blast area was either swept way or destroyed. Topography in this area had
no affect on the movement of material in the directed blast cloud. Between 8 and 19 miles from
the volcano, trees were flattened and resembled toothpicks aligned in a uniform direction on
surrounding hillsides. Material in the blast cloud was somewhat channelized within this zone.
Over 19 miles from the volcano, trees were seared black due to hot gases. Material from the
initial blast cloud itself was very hot ranging between 100 and 300 degrees C.
Several people were killed by the directed blast of Mount St. Helens. Mount St. Helens is
not the only volcano that has erupted with a directed blast. Its twin, Bezymianny in Kamchatka,
Russia, also erupted in this way. Current research shows that directed blasts are not uncommon.
The eruption of Mount St. Helens alerted scientists to the warning signs and hazards of such an
eruption. Knowledge of directed blast eruptions would help in the future, so that warnings can
be given to people in areas that might be affected by such a blast. Monitoring of volcanoes with
seismographs and instruments that indicate ground deformation can help identify hazardous
zones and indicate areas of possible safety.
2.5 Lahars
Lahars are a specific kind of mudflow made up of volcanic debris. They can form in a num-ber
of situations: when small slope collapses gather water on their way down a volcano; through
rapid melting of snow and ice during an eruption; from heavy rainfall on loose volcanic debris;
when a volcano erupts through a crater lake; or when a crater lake drains because of overflow
or wall collapse.
Lahars flow like liquids, but because they contain suspended material, they usually have a
consistency similar to wet concrete. They flow downhill and follow depressions and valleys,
but they can spread out if they reach a flat area. Lahars can travel at speeds of over 80 kph (50
mph) and reach distances dozens of miles from their source. If they are generated by a volcanic
eruption, they may retain enough heat to still be 60-70°C (140-160°F) when they come to rest.
Lahars are not as fast or hot as other volcanic hazards, but they are extremely destructive.
They will either bulldoze or bury anything in their path, sometimes in deposits dozens of feet
thick. Whatever cannot get out of a lahar’s path will either be swept away or buried. Lahars
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
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can, however, be detected in advance by acoustic (sound) monitors, which give people time to
reach high ground; they can also sometimes be channelled away from buildings and people by
concrete barriers, although it is impossible to stop them completely.
2.6 Gases
Volcanic gases are probably the least showy part of a volcanic eruption, but they can be one
of an eruption’s most deadly effects. Most of the gas released in an eruption is water vapour
(H2O), and relatively harmless, but volcanoes also produce carbon dioxide (CO2), sulphur
dioxide (SO2), hydrogen sulphide (H2S), fluorine gas (F2), hydrogen fluoride (HF), and other
gases. All of these gases can be hazardous - even deadly - in the right conditions.
Carbon dioxide is not poisonous, but it displaces normal oxygen-bearing air, and is odour-less
and colourless. As it is heavier than air, it collects in depressions and can suffocate people
and animals who wander into pockets where it has displaced normal air. It can also become
dissolved in water and collect in lake bottoms; in some situations, the water in these lakes can
suddenly ‘erupt’ huge bubbles of carbon dioxide, killing vegetation, livestock and people living
nearby. This was the case in the Lake Nyos in Cameroon, Africa in 1986, where an eruption of
CO2 from the lake suffocated more than 1,700 people and 3,500 livestock in nearby villages.
Figure 7. Geochemical monitoring station in El Hierro, Canary Islands, Spain (Source: Instituto Volca-nológico
de Canarias, INVOLCAN)
Sulphur dioxide and hydrogen sulphide are both sulphur-based gases, and unlike carbon di-oxide,
have a distinct acidic, rotten-egg smell. SO2 can combine with water vapour in the air to
form sulphuric acid (H2SO4), a corrosive acid; H2S is also very acidic, and extremely poisonous
even in small amounts. Both acids irritate soft tissues (eyes, nose, throat, lungs, etc.), and when
the gases form acids in large enough quantities, they mix with water vapour to form “vog”, or
volcanic fog, which can be dangerous to breathe and cause damage to the lungs and eyes. If
sulphur-based aerosols reach the upper atmosphere, they can block sunlight and interfere with
ozone, which have both short and long-term effects on climate.
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
36
One of the nastiest, although less common gases released by volcanoes is fluorine gas (F2).
This gas is yellowish brown, corrosive and extremely poisonous. Like CO2, it is denser than air
and tends to collect in low areas. Its companion acid, hydrogen fluoride (HF) is highly corro-sive
and toxic; it causes terrible internal burns and attacks calcium in the skeletal system. Even
after visible gas or acid has dissipated, fluorine can be absorbed into plants, and may be able to
poison people and animals for long periods following an eruption.
2.7 Volcanic Earthquakes
Earthquakes related to volcanic activity may produce hazards, which include ground cracks,
ground deformation, and damage to buildings and other structures. There are two general cate-gories
of earthquakes that can occur at a volcano: volcano-tectonic earthquakes and long period
earthquakes.
Earthquakes produced by stress changes in solid rock due to the injection or withdrawal of
magma (molten rock) is called volcano-tectonic earthquakes. These earthquakes can cause land
to subside and produce large ground cracks. These earthquakes can occur as rock moves to fill
in spaces where magma is no longer present. Volcano-tectonic earthquakes do not indicate that
the volcano is about to erupt, as they can occur at anytime.
The second category of volcanic earthquakes is long period earthquakes, which are produced by
the injection of magma into surrounding rock. These earthquakes are a result of pressure changes
during the unsteady transport of magma. When magma injection is sustained, a number of earth-quakes
are produced. This type of activity indicates that a volcano is about to erupt. Scientists use
seismographs to record the signal from these earthquakes. This signal is known as a volcanic tremor.
Figure 8. Location of volcano-tectonic earthquakes in El Hierro, Canary Islands, Spain (Source: Insti-tuto
Geográfico Nacional, Government of Spain)
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
37
Figure 9. Volcanic tremor of La Restinga Volcano, El Hierro, Canary Islands, Spain (Source: Instituto
Geográfico Nacional, Government of Spain)
People living near an erupting volcano are very aware of volcanic earthquakes. Their houses
will shake and windows rattle from the numerous earthquakes that occur each day before and
during a volcanic eruption.
Volcanic tremors warn of an impending eruption so that people can be evacuated to areas
of safety. The volcanic tremor signal has been used successfully to predict the 2011 submarine
eruptions of El Hierro, Canary Islands, Spain. Volcano-tectonic earthquakes can cause damage
to manmade structures and landslides. To prevent damage from being done, structures should
be built according to earthquake standards, building foundations should be constructed on firm
ground and not unconsolidated material, which may amplify earthquake intensity, and buildings
should be constructed on stable slopes in areas of low hazard potential.
2.8 Volcanic hazard prevention
The problem with volcanoes is that, though there may be similarities between volcanoes,
every volcano behaves differently and has its own set of hazards. This is why it is important for
scientists to study and monitor volcanoes. Many active volcanoes near populated areas have not
been sufficiently studied to assess their risk.
When scientists study volcanoes, they map past volcanic deposits and use satellites to look at
volcanic features, ash clouds, and gas emissions. They also monitor seismic activity, ground de-formation,
and geomagnetic, gravimetric, geoelectrical and thermal changes at a volcano. They
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
38
study and monitor volcanic gases and monitor the temperature, flow rate, sediment transport,
and water level of streams and lakes near the volcano.
By studying volcanic deposits, scientists can produce hazard maps. These maps indicate the
types of hazards that can be expected in a given area the next time a volcano erupts. Dating of
these volcanic deposits helps determine how often an eruption may occur and the probability of
an eruption each year. Monitoring of a volcano over long periods of time will indicate changes
in the volcano before it erupts. These changes can help in predicting when an eruption may
occur.
Figure 10. Hazard Map of basaltic lava flows of Tenerife, Canary Islands, Spain ( Source: Instituto
Geográfico Nacional, Government of Spain)
Figure 11. Hazard map of more than 10 cm ash covering of Tenerife, Canary Islands, Spain (Source:
Instituto Geográfico Nacional, Government of Spain)
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
39
3. References
Boll, J. Types of Volcanic Eruptions. Article in http://www.geology.com/volcanoes
Francis, P. (1993). Volcanoes. A Planetary Prespective. Oxford Univerity Press Inc., New York.
443 pp.
Highland, M.L. & Bobrowsky, P. (2008). The Landslide Handbook - A Guide to Understanding
Landslides. U.S. Geological Survey, Reston, Virginia. 131 pp.
Web of Civil Defense, Ministry of Civil defense & Emergency Management, New Zealand
Government: http://wwwcivildefense.govt.nz
Web of Department of Conservation, State of California: http://www.conservation.ca.gov
Web of Geoscience Australia, Australian Government: http://www.ga.gov.au
Web of Instituto Geográfico Nacional, Ministerio de Fomento, Gobierno de España: http://www.
ign.es
Web of ITIC, International Tsunami Information Center: http://itic.ioc-unesco.org
Web of NOAA, National Oceanic and Atmospheric Administration, U.S. Department of Com-merce,
USA Government: http://www.noaa.gov
Web of USGS, U.S. Geological Survey, U.S. Department of Interior, USA Government: http://
www.usgs.gov
Schuter, R.L & Krizek, R.J. (1978). Landslides. Analysis and Control. National Academy of
Sciences. Washington, D.C. 234 pp.
41
NATURAL HAZARDS & CLIMATE CHANGE
RIESGOS NATURALES Y CAMBIO CLIMÁTICO
Santamarta Juan C., Hernández-Gutiérrez L.E. & Arraiza Bermúdez-
Cañete Mª. Paz (Ed.)
ISBN 978-84-617-1060-7
CHAPTER/
CAPÍTULO
3
Colegio de Ingenieros
de Montes (Ed.)
Landslide Hazards
Luis E. Hernández-Gutiérrez a*
a Consejería de Obras Públicas, Transportes y Política Territorial. Gobierno de Canarias, Spain
Abstract
Landslides are defined as the downward and outward movement of slope-forming materials, nat-ural
rocks, soils, artificial fills, or combinations of these materials. Landslides are a serious geologic
hazard, common to almost every region of the world. Landslides occur throughout the world, under all
climatic conditions and in all terrains.
© 2014 The Authors. Published by Colegio de Ingenieros de Montes http://www.ingenierosdemontes.org
Peer reviewed
Keywords; Natural Hazards; Slope; Disasters; Environmental Secourity
* Corresponding author name. Tel.: +3-492-263-3088 ext. 208
E-mail address: luisenrique.hernandezgutierrez@gobiernodecanarias.org
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
42
1. Introduction
Landslides cost billions in monetary losses and are responsible for thousands of deaths and
injuries each year. Often, they cause long-term economic disruption, population displacement,
and negative effects on the natural environment.
Although gravity acting on an over-steepened slope is the primary reason for a landslide,
there are other contributing factors:
• Erosion by rivers, glaciers or ocean waves creates over-steepened slopes
• Rock and soil slopes are weakened through saturation by snowmelt or heavy rains
• Earthquakes create stresses that make weak slopes fail. Earthquakes of magnitude
4.0 and greater have been known to trigger landslides
• Volcanic eruptions produce loose ash deposits, heavy rain, and debris flows
• Excess weight from accumulation of rain or snow, stockpiling of rock or ore, from
waste piles, or from man-made structures may stress weak slopes and other struc-tures
to failure
1.1 Types of movement
A landslide is a downslope movement of rock or soil, or both, occurring on the surface of
a rupture - either a curved (rotational slide) or planar (translational slide) rupture - in which
much of the material often moves as a coherent or semi-coherent mass with little internal de-formation.
It should be noted that, in some cases, landslides may also involve other types of
movement, either at the inception of the failure or later, if properties change as the displaced
material moves downslope.
Landslides can be classified into different types based on the type of movement and the
type of material involved. In brief, material in a landslide mass is either rock or soil (or both);
the latter is described as earth if mainly composed of sand-sized or finer particles and debris if
composed of coarser fragments. The type of movement describes the actual internal mechanics
of how the landslide mass is displaced: fall, topple, slide, spread, or flow. Thus, landslides are
described using two terms that refer respectively to material and movement (that is, rockfall,
debris flow, and so forth). Landslides may also form a complex failure encompassing more than
one type of movement (that is, rock slide-debris flow).
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
43
a. Falls
Falls are landslides that involve the collapse of material from a cliff or steep slope (fig.1).
Falls usually involve a mixture of free fall through the air, bouncing or rolling. A fall type land-slide
results in the collection of rock or debris near the base of a slope. Separation occurs along
discontinuities, such as fractures, joints, and bedding planes, and movement occurs by free-fall,
bouncing, and rolling. Falls are strongly influenced by gravity, mechanical weathering, and the
presence of interstitial water.
Figure 1. Schematic of a rockfall (Source: U.S. Geological Survey)
b. Topples
Toppling failures (fig. 2) are distinguished by the forward rotation of a unit or units
about some pivotal point, below or lower down in the unit, under the actions of gravity
and forces exerted by adjacent units or by fluids in cracks. Topples can consist of rock,
debris (coarse material), or earth materials (fine-grained material). Topples can be com-plex
and composite.
Figure 2. Diagram of a topple (Source: U.S. Geological Survey)
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
44
c. Slides
A slide is a downslope movement of a soil or rock mass occurring on surfaces of rupture or
on relatively thin zones of intense shear strain. Movement does not initially occur simultane-ously
over the whole of what eventually becomes the surface of rupture; the volume of displac-ing
material enlarges from an area of local failure.
Two slide movements can be distinguished, rotational and translational.
• Rotational landslide: This is a landslide on which the surface of rupture is curved upward
(spoon-shaped) and the slide movement is more or less rotational about an axis that is paral-lel
to the contour of the slope. The displaced mass may, under certain circumstances, move
as a relatively coherent mass along the rupture surface with little internal deformation. The
head of the displaced material may move almost vertically downward, and the upper surface
of the displaced material may tilt backwards toward the scarp. If the slide is rotational and
has several parallel curved planes of movement, it is called a slump
Figure 3. Diagram of a rotational landslide (Source: U.S. Geological Survey)
• Translational Landslide: The mass in a translational landslide moves out, or down and out-ward
along a relatively planar surface with little rotational movement or backward tilting.
This type of slide may progress over considerable distances if the surface of rupture is suf-ficiently
inclined, in contrast to rotational slides, which tend to restore the slide equilibrium.
The material in the slide may range from loose, unconsolidated soils to extensive slabs of
rock, or both. Translational slides commonly fail along geologic discontinuities, such as
faults, joints, bedding surfaces, or the contact between rock and soil. In northern environ-ments,
the slide may also move along the permafrost layer
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
45
Figure 4. Diagram of a translational landslide (Source: U.S. Geological Survey)
d. Spreads
A spread is an extension of cohesive soil or rock mass combined with general subsidence
of the fractured mass of cohesive material into softer underlying material. It may result from
liquefaction or flow (and extrusion) of the softer underlying material. Types of spreads include
block spreads, liquefaction spreads and lateral spreads. Lateral spreads usually occur on very
gentle slopes or essentially flat terrain, especially where a stronger upper layer of rock or soil
undergoes extension and moves above an underlying softer, weaker layer.
Figure 5. Diagram of a lateral spread (Source: U.S. Geological Survey)
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
46
e. Flows
A flow is a spatially continuous movement in which the surfaces of shear are short-lived,
closely spaced, and usually not preserved. The component velocities in the displacing mass of
a flow resemble those in a viscous liquid. Often, there is a gradation of change from slides to
flows, depending on the water content, mobility, and evolution of the movement. There are five
basic categories of flows that differ from one another in fundamental ways:
• Debris flow: A form of rapid mass movement in which loose soil, rock and sometimes organ-ic
matter combine with water to form a slurry that flows downslope. They have been infor-mally
and inappropriately called “mudslides” due to the large quantity of fine material that
may be present in the flow. Occasionally, as a rotational or translational slide gains velocity
and the internal mass loses cohesion or gains water, it may evolve into a debris flow. Dry
flows can sometimes occur in cohesion less sand (sand flows). Debris flows can be deadly as
they can be extremely rapid and may occur without any warning
Figure 6. Diagram of a debris flow (Source: U.S. Geological Survey)
Figure 7. Debris flow deposit destroys houses in El Hierro, Canary Islands, Spain
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
47
• Lahar (Volcanic Debris Flows): The word “lahar” is an Indonesian term. Lahars are also
known as volcanic mudflows. These flows originate on the slopes of volcanoes and are a
type of debris flow. A lahar mobilizes the loose accumulation of tephra (the airborne solids
erupted from the volcano) and related debris
Figure 8. Diagram of a lahar (Source: U.S. Geological Survey)
• Debris avalanche: Debris avalanches are essentially large, extremely rapid, often open-slope
flows formed when an unstable slope collapses and the resulting fragmented debris is rapidly
transported away from the slope. In some cases, snow and ice will contribute to the move-ment.
If sufficient water is present, the flow may become a debris flow and (or) a lahar
Figure 9. Diagram of a debris avalanche (Source: U.S. Geological Survey)
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
48
• Earthflow: It can occur on gentle to moderate slopes, generally in fine-grained soil, common-ly
clay or silt, but also in very weathered, clay-bearing bedrock. The mass in an earthflow
moves as a plastic or viscous flow with strong internal deformation. Susceptible marine clay
(quick clay) when disturbed is very vulnerable and may lose all shear strength with a change
in its natural moisture content and suddenly liquefy, potentially destroying large areas and
flowing for several kilometres. Size commonly increases through headscarp retrogression.
Slides or lateral spreads may also evolve downslope into earthflows. Earthflows can range
from very slow (creep) to rapid and catastrophic. Very slow flows and specialized forms of
earthflows restricted to northern permafrost environments are discussed elsewhere.
Figure 10. Diagram of an earthflow (Source: U.S. Geological Survey)
2. Effects of landslides
Landslide effects occur in two basic environments: the built environment and the natural
environment. Sometimes there is intersection between the two, for example farming land or
forestland that is being logged.
2.1 Effects of Landslides on the Built Environment
Landslides affect manmade structures whether they are directly on or near a landslide. Res-idential
dwellings built on unstable slopes may experience from partial damage to complete
destruction as landslides destabilize or destroy foundations, walls, surrounding property, and
aboveground and underground utilities. Landslides can affect residential areas either on a large
regional basis (in which many dwellings are affected) or on an individual site basis (where only
one structure or part of a structure is affected). Furthermore, landslide damage to individual
property’s lifelines (such as trunk sewer, water, or electrical lines and commonly used roads)
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
49
can affect the lifelines and access routes of other surrounding properties. Landslides affect com-mercial
structures in much the same way as residential structures are affected. In such cases,
consequences may be great if the commercial structure is a commonly used structure, such as a
food market, which may experience an interruption in business due to landslide damage to the
actual structure and (or) damage to its access roadways.
2.2 Effects of Landslides on the Natural Environment
Landslides have effects on the natural environment such as on:
• The morphology of the Earth’s surface; mountain and valley systems, both on the continents
and beneath the oceans are the most significantly affected by downslope movement of large
landslide masses.
• The forests and grasslands that cover much of the continents
• The native wildlife that exists on the Earth’s surface and in its rivers, lakes, and seas
Landslides negatively affect forests; grasslands and wildlife, with forest and fish habitats
being the most easily damaged either temporarily or even occasionally destroyed. However,
because landslides are relatively local events, flora and fauna can recover with time. In addi-tion,
recent ecological studies have shown that, under certain conditions, in the medium-to-long
term, landslides can actually benefit fish and wildlife habitats, either directly or by improving
the habitat for organisms that the fish and wildlife rely on for food.
3. Prediction and prevention of landslides
Understanding the characteristics of the specific type of landslide hazard is vitally important
to consider when planning or adopting appropriate mitigation action to lessen the risk of loss
and damage. The type of landslide will determine the potential speed of movement, likely vol-ume
of displacement, distance of run-out, as well as the possible effects of the landslide and the
appropriate mitigation measures to be considered.
Vulnerability to landslide hazards is a function of a site’s location (topography, geology, and
drainage), type of activity and frequency of past landslides. The effects of landslides on people
and structures can be lessened by total avoidance of landslide hazard areas or by restricting,
prohibiting, or imposing conditions on hazard-zone activity. Local governments can accom-plish
this through land use policies and regulations. Individuals can reduce their exposure to
hazards by educating themselves on the history of past hazards of a desired site and by making
Luis E. Hernández-Gutiérrez 	 Natural Hazards & Climate Change
50
inquiries to planning and engineering departments of local governments. They could also hire
the professional services of a geotechnical engineer, a civil engineer, or an engineering geolo-gist
who can properly evaluate the hazard potential of a site, built or unbuilt.
Although the physical cause of many landslides cannot be removed, geologic investigations,
good engineering practices, and effective enforcement of land use management regulations can
reduce landslide hazards.
There are various mitigation methods for various types of landslide hazards:
3.1 Soil Slope Stabilization
Stability increases when ground water is prevented from rising in the slide mass by:
• Directing surface water away from the landslide
• Draining ground water away from the landslide to reduce the potential for a rise in ground-wa-ter
level
• Covering the landslide with an impermeable membrane
• Minimizing surface irrigation
• Placing a weight or retaining structures at the toe of the landslide or removing mass (weight)
from the head of the slope
• Planting or encouraging natural growth of vegetation can also be an effective means of slope
stabilization
3.2 Rock fall Hazard Mitigation
Rock fall is common in areas of the world with steep rocky slopes and cliffs. Commonly,
these are mountainous or plateau areas, whether in coastal areas or among isolated rock forma-tions.
Rockall causes extraordinary amounts of monetary damage and death, the former mostly
by impeding transportation and commerce due to blocked highways and waterways and the lat-ter
as direct casualties from falling rocks. Diverting paths and highways around rock fall areas
is sometimes imple- mented but is not always practical. Many communities post danger signs
around areas of high rock fall hazard. Some methods of rock fall hazard mitigation include
catch ditches, benches, scaling and trimming, cable and mesh, shotcrete, anchors, bolts, dowels,
and controlled blasting.
Luis E. Hernández-Gutiérrez Riesgos Naturales & Cambio Climático
51
3.3 Debris-Flow Hazard Mitigation
Due to the speed and intensity of most debris flows, they are very hard to stop once they have
started. However, methods are available to contain and deflect debris flows primarily through
the use of retaining walls and debris-flow basins. Other mitigation methods include modifying
slopes (preventing them from being vulnerable to debris-flow initiation by using erosion con-trol),
revegetation, and the prevention of wildfires, which are known to intensify debris flows
on steep slopes.
3.4 Landslide Dam Mitigation
Many problems arise when landslides dam waterways. Dams caused by landslides are a
common problem in many areas of the world. Landslides can occur on the valley walls of
streams and rivers. If enough displaced material (rock, soil, and (or) debris) fills the waterway,
the landslide will act as a natural dam, blocking the flow of the river and creating flooding up-stream.
As these natural dams are frequently composed of loose, unconsolidated material, they
are often inherently weak and are soon overtopped and fail due to erosion. When breaching hap-pens
quickly, the backed-up water rushes down the waterway, potentially causing catastrophic
downstream flooding.
References
Highland, M.L. & Bobrowsky, P. (2008). The Landslide Handbook - A Guide to Understanding
Landslides. U.S. Geological Survey, Reston, Virginia. 131 pp.
Web of Civil Defense, Ministry of Civil defense & Emergency Management, New Zealand
Government: http://wwwcivildefense.govt.nz
Web of Department of Conservation, State of California: http://www.conservation.ca.gov
Web of Geoscience Australia, Australian Government: http://www.ga.gov.au
Web of USGS, U.S. Geological Survey, U.S. Department of Interior, USA Government: http://
www.usgs.gov
Schuter, R.L & Krizek, R.J. (1978). Landslides. Analysis and Control. National Academy of
Sciences. Washington, D.C. 234 pp.
53
NATURAL HAZARDS & CLIMATE CHANGE
RIESGOS NATURALES Y CAMBIO CLIMÁTICO
Santamarta Juan C., Hernández-Gutiérrez L.E. & Arraiza Bermúdez-
Cañete Mª. Paz (Ed.)
ISBN 978-84-617-1060-7
CHAPTER/
CAPÍTULO
4
Colegio de Ingenieros
de Montes (Ed.)
Environmental Restoration
Juan C. Santamartaa*, Jonay Nerisb, Jesica Rodríguez-Martínc
a Área de Ingeniería Agroforestal, Universidad de La Laguna, Ctra. Geneto, 2, La Laguna, 38200 Tener-ife
(Canary Islands), Spain
b Vicerrectorado de Internacionalización y Excelencia, Universidad de La Laguna, C/Viana , 38200, La
Laguna, Canary Islands, Spain
c Ingeniera de Caminos Canales y Puertos, Urb. Jardines de Guajara, 1,La Laguna 38296, Tenerife,
Canary Islands, Spain
Abstract
Environmental restoration initiates or accelerates the recovery of an ecosystem which has been
degraded, damaged or contaminated by human activity or natural agents. Environmental restoration
projects may focus on restoring the environment or mitigating the negative environmental impacts of
other projects or actions.
© 2014 The Authors. Published by Colegio de Ingenieros de Montes http://www.ingenierosdemontes.org
Peer reviewed
Keywords; Ecosystem; Degraded lands; Restoration
* Corresponding author name. Tel.: +3-492-231-8550
E-mail address: jcsanta@ull.es
Juan C. Santamarta, Jonay Neris, Jesica Rodríguez-Martín 	 Natural Hazards & Climate Change
54
1. Introduction
1.1. Concept and types of degradation
An area classified as degraded has been subject to alteration or modification of its natural
state due to either natural causes (fires, floods, storms or volcanic eruptions) or direct or indirect
human activity.
Two concepts should be bear in mind to classify an area as degraded (Gómez Orea D, 2004):
a. The conservation value of the space as a structure.
• Negative value with regard to different viewpoints, such as ecological, scientific, cultural,
scenic, productive, etc
• Inferior value compared to the ecosystem’s climax value
b. The function it serves for society.
• Absence of function due to degradation
• Unsatisfactory function
When it comes to designating a degraded area, thoughtfulness and flexibility must be used.
As to estimate degradation the following considerations must be taken into account:
• Different degrees of degradation presented: extensive or intensive.
• Different points of view that can be taken into account: ecological, scientific, cultural, sce-nic,
productive etc.
• The extent of the area affected: total or partial.
Figure 1. Hydrologic restoration at Negev Desert in Israel
Juan C. Santamarta, Jonay Neris, Jesica Rodríguez-Martín Riesgos Naturales & Cambio Climático
55
Restoration is an activity that begins or accelerates the recovery of an ecosystem. It can be de-fined
as the combination of actions carried out with the aim of reversing or reducing the damage
to an area. The restoration process is an attempt to reach a situation similar to the original state.
However, this does not consist of replacing the elements one by one, but of imitating these elements
in such a way that they work together in a similar manner to that of the original situation (it is more
important to discover the way the whole system works than to study each piece separately).
An ecosystem has recovered (and therefore has been restored) when it has sufficient biotic
and abiotic resources to continue its development without further help. Thus, the end of the
restoration process can be defined as the recovery of the essential ecological elements, and par-ticularly
by the state of soil processes that allow the maintenance of a stable biology in balance
with the climate.
The ecological restoration of a degraded area and the development of the required engineer-ing
methods should begin by achieving land stability and soil recovery.
2. Historical background
Ecological restoration actions have increased in recent years, as environmental policies have
slowed the rate of environmental degradation in many parts of the world.
Ecological restoration is a scientific discipline that has recently emerged due to the increas-ing
need to restore damaged ecosystems. Natural habitats across the world have been severely
impacted due to habitat destruction, urban sprawl and direct damage due to industrial contam-ination
of soils and aquatic resources.
2.1 Environmental impact
The environmental impact is the positive or negative effect on the environment caused by
the development of a specific project. It involves effects resulting from human actions, such as
construction projects, industrial, agricultural or transport activity, energy supplies, etc. The en-vironment
is often the main target of human impacts, meaning not only the physical-chemical
and biological components of the environment, but also visual, cultural and socio-economic
components. According to the International Association for Impact Assessment (IAIA), the
Environmental Impact Assessment (EIA) is defined as “the process of identifying, predicting,
evaluating and mitigating the biophysical, social, and other relevant effects of development
proposals prior to major decisions being taken and commitments made” [International Associ-ation
for Impact Assesment 1999]. The European Union [European Commission 2012] defines
EIA as “a procedure that ensures that the environmental implications of decisions are taken into
account before decisions are made.”
Juan C. Santamarta, Jonay Neris, Jesica Rodríguez-Martín 	 Natural Hazards & Climate Change
56
It consists of predicting these effects, developing prevention and/or mitigation actions and
including them into the decision-making process as a new factor affecting the suitability of the
project studied. It is both a technical tool to study the effects of planned actions (projects, pol-icies,
plans and programmes) and unplanned events (natural disasters and conflicts) and a legal
and institutional procedure related to the decision-making process (International Association
for Impact Assesment, 1999).
The EIA concept arose in the 60s because of the increased awareness by developed countries
regarding the impact of human activities on health and the environment. The adoption of legal
and institutional procedures to include environmental impact in the decision-making process
occurred later on in that decade. The first legal development of this idea was the National En-vironmental
Policy Act (NEPA) in the USA on 1st January, 1970. After this date, many different
developments were made in most countries around the world and at different scales (national,
federal and regional levels). In 1985, the European Union (EU) approved a Directive on EIA.
As a first step, EIA addressed only impacts that result from specific actions, such as projects.
However, taking into account the influence of policies, plans and programmes on the develop-ment
of these projects, EIA was also applied to this strategic level of decision-making. This
change in perspective led to the development of the Strategic Environmental Assessment (SEA)
to ensure the sustainability of strategic decisions. The EU adopted this change and developed
environmental assessment of plans and programmes in 2001.
The main objectives of EIA are [International Association for Impact Assesment 1999]:
• To provide information about the effects of a specific action or unplanned event
• To ensure that this information is incorporated into the decision-making process
• To improve public participation and information in the decision-making process
• To develop prevention and mitigation strategies to avoid or reduce environmental impacts
• To promote the sustainable development and preservation of the environment
Juan C. Santamarta, Jonay Neris, Jesica Rodríguez-Martín Riesgos Naturales & Cambio Climático
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Figure 2. Forest restoration at Athens (Greece)
2.2 Restoration techniques
Before carrying out an environmental restoration project, two key questions should be an-swered
on which the success or failure of the project will depend: the first question is to in-vestigate
what the causes were that led to the degradation and the second is how to repair this
degradation.
Good planning is achieved based on knowledge from different sources of information, such
as the ones mentioned below:
• Topographical survey of the area to be restored and its surroundings
• Environmental valuation of the area: climate, soil and landscape, etc
• Project consequences: finding out the impact of the project on the environment regarding
existing activities, legal restrictions, etc
• Needs of future users
• Function of the restored area
• Time period for the development of the restoration project and its duration (useful lifespan)
• Cost of investment to determine the viability or if necessary, the funding required
• Maintenance: the restoration does not end with the completion of the works, maintenance to
avoid future problems must also be considered
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The main uses of human-induced degraded lands after restoration are the following:
i. Agricultural or forest use
Agricultural use has been the most common use for restoration of mining sites when the
land has a gentle topography. The limiting factors for this use are the drainage and the chemical
pollution of the soil.
Forest use is an alternative to agricultural use when the steepness of slopes is higher and the
soil quality is poor. The critical factors for this use are the physical characteristics of the soil,
availability of nutrients and the existence of toxic substances.
ii. Nature conservation and wildlife parks
When land has been abandoned for long periods of time, natural colonisation is very ad-vanced
and it is then that society puts pressure to begin restoration. In these cases, it is essential
to carry out botanical and faunistic studies to determine the evolutionary state of the area, its
quality and fragility and, thus, be able to decide what the final use of the area will be.
iii. Industrial and urban use
These uses are highly suitable for degraded areas close to urban areas which can cater for
urban and commercial activities. Before beginning to draft a restoration project in these areas,
local council offices should be contacted and existing planning programmes consulted in order
to check their compatibility with potential uses. An important aspect to be considered is the
existence of good access to the area, as urban or industrial use implies a high traffic density.
iv. Recreational use
Abandoned land located in residential areas of natural or cultural interest can be suitable for
recreational activities associated with the enjoyment of nature and education. The objectives
that need to be achieved to ensure the success of a restoration project for recreational uses are
exclusivity, environmental responsibility, a balanced integration, economic feasibility and flex-ibility.
v. Sanitary environment uses
The most common sanitary uses include:
• Landfills and rubbish dumps
Any activity involving excavations can restaurated to install controlled landfill sites. In these
cases, it is important to consider the permeability of the soil and subsoil.
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• Water reservoirs and supplies
Previous excavation activities involving deep hollows, these can be restored as water
reservoirs for a variety of uses (fire extinction, water supplies, etc.) depending on their char-acteristics.
Figure 3. Landfill restoration in La Gomera (Canary Islands, Spain)
3. Restoration models in different types of spaces
First, a study should be carried out into the approach for each restoration project, as the final
appearance of the space to be restored depends on this. The main approaches that can be adopt-ed
when restoring a degraded space are listed below:
1. Rehabilitation: on occasions, this term is confused with restoration. There is a key differ-ence;
rehabilitation does not imply achieving the original state of the land.
2. Restoration: the recovery of the initial ecosystem before degradation took place.
3. Replacement: an economic alternative through which a balance is sought without eliminat-ing
the elements that have led to the degradation.
4. Reform: only nature acts on the degraded area by natural succession.
5. Revegetation: attempts to establish stable vegetation cover (native or foreign species).
When choosing one of these approaches for a restoration project, it is worth noting that none
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of them are better or worse than the others, rather it depends on the circumstances of each space.
The fact that they could also be complementary should not be ruled out, above all in large spac-es
and ones that require separation.
3.1 Restoration of landfills
One of the oldest methods to manage waste has always been to dump it at various sites
without any kind of control, not far from the populated areas where it was generated (near to
roads, abandoned quarries, etc.). This uncontrolled waste disposal leads to a range of problems:
presence of rats and insects, risk of fires, presence of unpleasant smells, water and air pollution,
lack of aesthetics and environmental degradation.
These spaces, in the same way as controlled landfills, need to be restored to improve the
environmental quality of the area.
The basic restoration actions for these sites involve:
• Removal of waste and earthworks to mitigate the impacts of the previous excavation activi-ties
on the topography of the area
• Provision of vegetation that facilitate landscape integration
• Revegetation using species suitable for the environment of the site
• Fencing off the perimeter to avoid new dumping
Figure 4. Quarry restoration
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3.2 Restoration of areas affected by civil works
The construction of linear infrastructures in any area generates impacts on the components
that modify that area and its surroundings. Thus, it is necessary to carry out a programme to be
able to restore lost components.
When dealing with these kinds of infrastructures, it is important to bear in mind that new
constructions should get integrated into the landscape where they are built.
Linear structures are defined as ones that possess some of the following characteristics: join
two or more fixed points, cross a range of areas, are artificial and their construction responds to
a need as they provide a public service.
Linear infrastructures are classified into:
• Roads
• Oil and gas pipelines
• Railway lines
• Power lines
• Irrigation channels
• Telephone lines
Roads play an important role in landscapes; this is why the current chapter focuses on this
kind of infrastructure.
Roads present a series of environmental problems that are described below:
• Barrier effect
• Land use modification and occupation
• Noise
• Construction of new infrastructures and buildings
When faced with these alterations, the main measures for environmental restoration in these
affected areas are:
• Installation of sound barriers
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• Slope stabilisation
• Landscape integration
• Wildlife paths
Figure 5. Debris resulting from the excavation of a mine in Tenerife (Canary Islands, Spain)
3.3 Restoration of quarries
The main environmental problems caused by the exploitation of mining resources are:
• The effects on the landscape, such as hollows formed and rubble present in artificial shapes
which contrast with the original landscape. The colour of the mine waste contrasts with other
tones present. The landscape is deeply visually impacted and are sometimes interrupted due
to variations in topography
• The land use change and soil properties with the removal or occupation of fertile soil. The
alteration of the soil properties is due to the construction of infrastructures and land occupa-tion
and to the compaction caused by heavy machinery passing over.
• Water pollution due to the increase in solids in suspension and the dumping of waste
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The main operations that should be considered when beginning the restoration of quarrying
sites are:
• Dismantling and demolition of buildings and facilities
• Filling hollows
• Stabilizing rubble heaps and tailing dumps
• Earthworks to restore the original landscape topography
• Planting vegetation covering
References
Cátedra de Proyectos de la ETSIM, (2011); Apuntes del curso de restauración de espacios degrada-dos.
Tomos I y II. Universidad Politécnica de Madrid.
Clewell, A.F., and J. Aronson. (2007); Ecological restoration: principles, values, and structure of an
emerging profession. Island Press, Washington, DC.
Donald A. Falk, Margaret A. Palmer, & Joy B. Zedler (2006); Foundations of Restoration Ecology.
Society for Ecological Restoration International. Tucson, AZ.
European Commission. Environmental Assessment. In HTTP://EC.EUROPA.EU/ENVIRON-MENT/
EIA/HOME.HTM. 2012.
Gómez Orea, D. (2004); Recuperación de Espacios Degradados. Ed. Mundiprensa. Madrid.
Hall M., (2010); History of environmental clean up and restoration. World Environmental History.
(EOLSS).
International Association FOR Impact Assesment. Principle of Environmental Impact Assessment
Best Practice. In Special publications. Fargo, USA, 1999, p. 4.
Mitsch, W.J., and S.E. Jorgensen. (2004); Ecological engineering and ecosystem restoration. Wiley,
Hoboken, NJ.
Morrison, M.L. (2002); Wildlife restoration: techniques for habitat analysis and animal monitoring.
Island Press, Washington, DC.
NASA. NASA images. In HTTP://WWW.NASA.GOV/MULTIMEDIA/IMAGEGALLERY/IN-DEX.
HTML. 2012.
Perrow, M.R., and A.J. Davy. (2002); Handbook of ecological restoration. Cambridge University
Juan C. Santamarta, Jonay Neris, Jesica Rodríguez-Martín 	 Natural Hazards & Climate Change
64
Press, Cambridge, UK.
Santamarta Cerezal, J.C., Naranjo Borges, J. et al. (2012); Ingeniería forestal y ambiental en medios
insulares. Métodos y Experiencias en las Islas Canarias. Ed. Colegio de Ingenieros de Montes en Ca-narias.
Tenerife.
Santamarta Cerezal, J.C. et al. (2012); Hidrología y Recursos Hídricos en Islas y Terrenos Volcánic-os.
Métodos y Experiencias en las Islas Canarias. Tenerife
SER. (2004); The SER International Primer on Ecological Restoration, Society for Ecological Res-toration
International Science & Policy Working Group. Society for Ecological Restoration Internation-al,
Tucson, AZ.
65
NATURAL HAZARDS & CLIMATE CHANGE
RIESGOS NATURALES Y CAMBIO CLIMÁTICO
Santamarta Juan C., Hernández-Gutiérrez L.E. & Arraiza Bermúdez-
Cañete Mª. Paz (Ed.)
ISBN 978-84-617-1060-7
CHAPTER/
CAPÍTULO
5
Colegio de Ingenieros
de Montes (Ed.)
Sediment & Erosion Control, Future Challengues
Juan C. Santamartaa*, Jesica Rodríguez-Martínb
a Área de Ingeniería Agroforestal, Universidad de La Laguna, Ctra. Geneto, 2, La Laguna, 38200 Tenerife
(Canary Islands), Spain
c Ingeniera de Caminos Canales y Puertos, Urb. Jardines de Guajara, 1,La Laguna 38296, Tenerife,
Canary Islands, Spain
Abstract
Erosion is a natural process of a physical and chemical nature that degrades, destroys and transports
rock and soil of the Earth’s crust. This process can be accelerated, modified or corrected by anthropic action.
Soil erosion is one form of soil degradation along with soil compaction, low organic matter, loss of
soil structure, poor internal drainage, salinization, and soil acidity problems. These other forms of soil
degradation, serious in themselves, usually contribute to accelerated soil erosion.
© 2014 The Authors. Published by Colegio de Ingenieros de Montes http://www.ingenierosdemontes.org
Peer reviewed
Keywords; ; Degraded lands; Restoration; Sediment; Hydraulic action
* Corresponding author name. Tel.: +3-492-231-8550
E-mail address: jcsanta@ull.es
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1. Overview of erosion processes
The majority of erosion processes are the result of a combined action of various factors, such
as heat, cold, gases, water, wind, gravity and animal and plant life.
Soil erodibility is an estimate of the ability of soils to resist erosion, based on the physical
characteristics of each soil. Generally, soils with faster infiltration rates, higher levels of organic
matter and improved soil structure have a greater resistance to erosion.
The erosion process depends on the following factors:
ER= f (R, G, S, V)
o R =Factor that depends on the quantity and intensity of rainfall
o G =Factor that depends on the slope and topography of the ground
o S = Factor that depends on the physical and chemical properties of the ground
o V = Factor that depends on the characteristics of the vegetal cover
Soil erosion may be a slow process that continues relatively unnoticed, or it may occur at an
alarming rate causing serious loss of topsoil. The factors that cause erosion in an area are:
• Torrential rainfall
• Overgrazing
• Over-exploitation of water resources
• Change of land use, urbanisation and civil infrastructure
• Surface mining
• Salinization of ground
Figure 1. Surface erosion and soil loss
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1.1 Torrential rain
On occasions, torrential rainfall occurs: this rainfall leads to important erosion processes. Both
rainfall and runoff factors must be considered in assessing a water erosion problem. Runoff can
occur whenever there is excess water on a slope that cannot be absorbed into the soil or trapped
on the surface.
The main factors that are involved in the generation of solid and liquid runoff that reach
rivers, and are then transported by them, are related to the characteristics of the precipitation
and the area affected (slope, vegetation), the water erosion and the dynamics of the runoff. The
amount of runoff can be increased, if infiltration is reduced due to soil compaction and crusting.
1.2 Overgrazing
Another key aspect is overgrazing, especially when the carrying capacity of grazing land is
exceeded. The carrying capacity is defined as the number of animals that can graze by hectare or
the number of hectares required by each animal so as not to overgraze.
We can simply state that overgrazing is subjecting the land to a greater number of livestock
than it can support, leaving livestock to graze for longer than is recommended or allowing
livestock to graze at unsuitable times, which does not allow the grass or bushes to reproduce.
This eventually leads to erosion and on a large scale to desertification. Another problem is the
removal of soil by the trampling of livestock. Overgrazing facilitates desertification in semi-arid
areas.
The solution to this problem is regulation of livestock grazing, as well as the demarcation of
areas and zones sensitive to erosion, such as slopes, always within the established limit of the
grazing land’s carrying capacity.
1.3 Overexploitation of water resources
The immediate consequence of the overexploitation of water resources is the salinization
of these resources; this fact significantly affects the land and its processes, with the result
being erosion. In addition, the overexploitation of coastal aquifers leads to marine intrusion
and worsens the quality of the water extracted, which in many cases is used to irrigate crops
producing soil problems.
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1.4 Changes in land use, urbanisation and civil infrastructure
The change of land use or the abandonment of tradition farming practices are important
factors in the acceleration and increase in erosion. Furthermore, certain infrastructure projects,
such as building of roads, airports, tourist resorts, among others have contributed to the
frequency and development of erosion processes leading to barrier effects of the infrastructure
and sealing impermeabilization effects on the ground. The loss of vegetation, more roofs,
paving and clearing of woodland and grassland without proper conservation management
create erodible land areas, speed up runoff and remove areas available for rainfall infiltration.
Vegetation removal from land areas further accelerates erosion and siltation.
1.5 Forest fires
Forest fires are one of the main causes of erosion and destruction of the soil, especially when
the first autumn or winter rains are torrential.
During a fire, the undergrowth disappears, allowing elements that were fixed to be moved,
so much large-diameter necromass, stones and, above all, rolling pine cones are able to create
secondary flashpoints normally beyond the first line where human efforts are trying to control
the fire.
Figure 2. Preventive actions in the forest after a wildfire
One of the most serious and immediate consequences that occur after a forest fire is the
dragging of ash and bare soil towards rivers. This can be catastrophic for populated areas and
towns near the forest.
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In addition, forest fires generate an important distortion in the accumulation of carbon in
soil. This element, the main component of organic material, plays a key role in soil fertility,
water retention, and resistance to erosion.
As for hydrological implications, a forest fire generates a significant reduction in infiltration.
Thus, when the first rains arrive following a forest fire, the runoff on burnt soil can double or
even triple as a result of the volume of solids in suspension, and the impermeability and lack of
infiltration capacity of the soil surface.
Erosion and soil effects of a forest fire can be classified as follows (Contreras et al, 2007):
• Less soil aggregation
• Reduction in organic material
• Loss of nutrients
• Reduction in surface roughness
• Increase in surface runoff
1.6 Surface mining
Mining activity removes soil or leaves it unprotected from water, as well as changing the
dimensions and shape of the land. Aggregate extraction also causes great environmental and
landscape impact.
Figure 3. Erosion caused by surface mining works
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1.7 Soil salinization
Salinization is the accumulation of soluble salts of sodium, magnesium and calcium in soil to
the extent that soil fertility is severely reduced. This soil problem leads to an excessive increase
of water-soluble salts in the soil. The accumulated salts include sodium, magnesium, potassium
and calcium, chloride, sulphate, carbonate and bicarbonate.
Salinization on the soil surface occurs where the following conditions occur together:
• The presence of soluble salts, such as sulphates of sodium, calcium and magnesium in the
soil
• A high water table
• A high rate of evaporation
• Low annual rainfall
One of the effects of salinization is that salts in the soil increase the efforts required by
plant roots to take in water. High levels of salt in the soil have a similar effect as droughts by
making water less available for uptake by plant roots. Salty groundwater may also contribute to
salinization. When the water table rises, the salty groundwater may reach the upper soil layers
and, thus, supply salts to the rootzone.
1.8 Water erosion
The process of water erosion begins with rain that falls on the soil breaking it up, subsequently
runoff is formed, a laminar flow from the land higher up the slope is created, which flows
downwards in small rills that transform quickly into large gullies that are difficult to correct and
deal with.
The appearances of gullies are closely connected to inappropriate land use practices. Gulley
erosion is a reflection of surface erosion and is the most extreme result of this erosion. This type
of erosion is preceded by other processes (sheet and rill), due to the increase in runoff volume
and speed.
The erosion process is considered to be one of the most serious worldwide environmental
problems, associated, to a large degree, with the loss of forest cover. The way erosion works is
by detaching material, transporting it (by water, wind…) and finally depositing it. Water erosion
can also occur at depth; this effect is related to large displacements of land by the hidden
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action of water filtering down. Water lubricates land and creates the conditions necessary for
displacement by gravity. Materials slide by shearing when a certain angle of slope is reached
Another effect of water erosion, especially when caused by torrential rainfall, is when the
soil becomes saturated with water after many days of rain. This ends up provoking landslides
on slopes: the consequences and size of these depend on the angle of the slope on which they
occur.
Figure 4. Surface water erosion
1.9 Wind erosion
Wind erosion is the loss of the soil surface layer by the wind action. This is a selective
process because it affects only particles on the soil surface and depends on grain diameter.
Wind erosion is a major geomorphological process in arid and semi-arid areas. The rate
and magnitude of soil erosion by wind is determined by factors such as particle sizes: very
fine particles can be suspended by the wind and then transported great distances; fine and
medium-size particles can be lifted and deposited. Other factors are regional climate and
wind. The speed and duration of the wind has a direct relationship with the extent of soil
erosion. Vegetation cover is also important, as the lack of permanent vegetation cover in
certain locations has resulted in extensive erosion by wind.
Climatic factors lead the erosion process, but other factors such as deforestation, bad
agriculture practices, urbanisation, etc. also affect wind erosion. It is a recurrent and
progressive process that occurs constantly and its effects increase over time. It has three
different stages:
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• Detachment: The soil surface particles are picked up from the bulk soil. Grain size diameter
and wind velocity play an important role in this phase, which is selective depending on these
parameters
• Transport: Detached particles are carried from their original place by the wind. As in the
detachment stage, transport depends on the wind velocity and grain size. It may occur by:
 Surface creeping: when larger particles roll along the ground, which
is responsible for 50-70% of particle transport
 Saltation: when intermediate-sized particles are lifted a short height
and then dropped and bounce across the surface, which causes 30-
40% of wind erosion
 Suspension: when small particles are lifted by the wind and carried
for long distances, only 5-25% of particles are carried this way
• Deposition: particles transported are sedimented due to the loss of wind power
2. Erosion control measures
Remedial actions consist of site control to prevent off-site migration of surface water,
sediments and contaminants. Land disturbed by construction activities requires precautionary
measures to reduce soil erosion and sedimentation. A construction site, for example, must be
investigated for a wide range of conditions, including ground water level, surface drainage and
subsurface ground conditions.
Conservation measures can reduce soil erosion by both water and wind. Tillage and cropping
practices, as well as land management practices, directly affect the overall soil erosion problem,
although other measures might be necessary. For example, contour ploughing, surface water
works, land grading can be used to reduce water ponding, erosion and to promote vegetation.
Dry dams, strip cropping, or terracing may also be considered. To manage surface drainage at
construction sites other methods can be used. The aim of these methods is to reduce erosion, water
ponding and runoff of sediments and pollutants onto downslope land and downstream water or
streams. It is necessary to understand the application of interception or diversion methods, such
as ditches, berms, down pipes, flumes, terraces and benches and sediment and detention basins.
Practices for surface stabilization are also useful and include synthetic membranes, vegetation,
and land grading and soil bioengineering for slope protection.
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The design of structures to control erosion combines various disciplines; surface hydrology,
geotechnics and structural design. These disciplines provide the basic information that is
required to determine the conditions for the foundations and the magnitude of the forces that
are going to act on the planned structure.
The structure, itself, does not present many technical problems. Its complexity lies in
its location within an area, this is why it is absolutely essential to carry out a hydrological
study (hydromorphology, flow and slope…) of the area to determine the water erosion and
estimate approximately the amount of sediment that the river transports to obtain the optimum
compensation slope.
Furthermore, the great velocity that torrents of water can reach complicates the management
and mitigation of water erosion effects even more.
2.1 Control of water erosion by civil works
Civil works carried out in forest areas have the aim of improving the water regime of ravines
and watercourses, the appropriate maintenance and design of water infrastructure, such as dams
and reservoirs, the fight against erosion, the conservation and protection of land, as well as the
increase in infiltration of the land.
The design of appropriate structures in each micro watershed should be carried out after
knowing the results of hydrological and geomorphological studies of the stretch that will be
affected by such hydrotechnical works. The results of these studies will predict the future
development of the water current. They provide estimates of the magnitude of average, minimum
and flood flows, minimum, maximum and average levels, potential floodplains, speed of flows,
sediment transport capacity, undermining and accumulation of sediments. Without this prior
information, the remedial measures will not be designed and properly constructed.
Figure 5. Gabion drop structure to reduce erosion
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Contention dikes for surface sediments are basically small gravity dams whose function is to
keep back sediments that flow along the beds of ravines or gullies when these carry water. Their
design is very straightforward, but their contribution to the maintenance of other hydraulic
infrastructures, downstream, such as soil/land conservation is crucial in areas with steep slopes
and torrential rainfall.
a. Transverse structures to control torrential flows
These structures operate like small dams. Their main purpose is to reduce the speed of
water flow in a specific stretch, upstream from the construction. They act as control structures.
However, they can fail due to bad foundations or because of undermining generated immediately
downstream. This is why there is often an absorption basin built of the same material as the
dike.
In general, gabion dikes and hydraulic masonry are the best structures to achieve the
objectives mentioned above. It is worth highlighting their easy structural calculation and design,
as a gravity dam (useful height, drop height, stability verification, energy dissipation, etc). The
gabion drop structure is a very useful way to stabilise the dam spillway.
b. Linear hydraulic structures
Linear hydraulic structures complement the previous ones, for example, walls and breakwaters
placed on the sides of ravines prevent material being dragged by runoff from the sides and to a
certain degree channels the ravines and gullies. Longitudinal structures in ravines are generally
used at the intersection of two streams with the aim of reducing the energy of both at this
critical point, where flows and material are added together. Poor construction or inadequate
maintenance of surface drainage systems, uncontrolled livestock access and farming too close
to both stream banks can also lead to bank erosion problems.
Channelling of ravines, even when applied to torrential flows are more related to civil
engineering works than environmental or forestry ones. It is used to channel water mainly in
areas where the river flows through populated areas or near the mouths of ravines. In Denmark,
these types of structures are carried out from a bioengineering perspective, substituting the
concrete wall with plantations and green covering supported by breakwaters in a fairly natural
way and without a noticeable scenic or environmental impact.
Finally, small hydraulic structures should be mentioned, small dikes made of stone or
biological material, such as logs or faginas, are crucial for restoration following fires.
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2.2 Erosion control after wildfires
Erosion control that is normally carried out following a wildfire aims at having a double
effect. First, the scenic aesthetics need to be restored, and second the burnt fuel needs to be
removed through collection by small dikes or log erosion barriers that help limit soil erosion.
For this reason, living trees should never support small hydraulic structures in order to prevent
horizontal continuity between live and dead fuel.
Figure 6. Use of ordinary material to reduce soil loss after wildfire
Regarding the aesthetic effect, more political than technical criteria are often applied.
However, it should be borne in mind that the traces of a fire are not easily removed and therefore
adequate planning and a detailed description of the actions that need to be taken should be
made. This can ensure that any actions that are carried out will be less harmful than the fo