Common non-volcanic processes by which lahars and other debris flows form are by heavy rains falling upon loose debris or by loose debris becoming saturated with water from melting snow, glaciers or heavy rains (Osterkamp et al., 1986). Water-saturated material can move downhill like wet concrete when its internal strength is exceeded.
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The high clay content (3-5 wt. % clay in the < 2 mm fraction) and sedimentary characteristics indicate that the flow was a cohesive lahar, most likely induced by collapse of a hydrothermally ...
The CVL was an enormous non-cohesive debris flow, notable for its ash-flow origin and immense volume and peak discharge which gave it characteristics and …
Mar 19, 2017 · Workability of Concrete. Workability is often referred to as the ease with which a concrete can be transported, placed and consolidated without excessive bleeding or segregation. OR. The internal work done required to overcome the frictional forces between concrete ingredients for full compaction.
A landslide is defined as the movement of a mass of rock, debris, or earth down a slope. Landslides are a type of "mass wasting," which denotes any down-slope movement of soil and rock under the direct influence of gravity. The term "landslide" encompasses five modes of slope movement: falls, topples, slides, spreads, and flows. These are further subdivided by the type …
Lahars can occur by rapid melting of snow and ice during eruptions, by liquefaction of large landslides (also known as debris avalanches), by breakout floods from crater lakes, and by erosion of fresh volcanic ash deposits during heavy rains.
Trigger mechanisms The higher up the slope of the volcano, the more gravitational potential energy the flows will have. A flood caused by a glacier, lake breakout, or heavy rainfalls can generate lahars, also called glacier run or jökulhlaup. Water from a crater lake can combine with volcanic material in an eruption.
GENERAL CHARACTERISTICS Lahar is an Indonesian term for a volcanic mudflow. These lethal mixtures of water and tephra have the consistency of wet concrete, yet they can flow down the slopes of volcanoes or down river valleys at rapid speeds, similar to fast-moving streams of water.
A lahar (an Indonesian term for volcanic mudflow) is a slurry of rock debris and water that originates on the slopes of volcanoes....LaharClast.Debris Flow.Lava Flow.Pyroclastic Flow.Tephra.Lava.Crater.Volcano.
More often, lahars are formed by intense rainfall during or after an eruption–rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys. Some of the largest lahars begin as landslides of saturated and hydrothermally altered rock on the flank of a volcano or adjacent hillslopes.
Lahar is a rampaging slurry of thick debris – pyroclastic material and ash – and water washed down by the rain from the slopes of Pinatubo. The lahar is then delivered to lowland towns and cities through rivers originating from the volcano – the Tarlac, Sacobia-Bamban, Abacan, and the Pasig Potrero Rivers.Feb 13, 2019
People caught in the path of a lahar have a high risk of death from severe crush injuries, drowning or asphyxiation. Lahars are often highly erosive to river banks and eyewitnesses should remain at a safe distance. Lahar events will cause destruction of buildings, installations and vegetation caught in their path.Jan 20, 2010
The presence of “rain-triggered” clay-rich lahar and deposits originating from a single small phreatic eruption is important because usually such clay-rich lahars are known to occur in association with large-scale sector collapse and debris avalanches.Jul 5, 2018
Definition: A lahar is a hot or cold mixture of water and rock fragments that flow quickly down the slopes of a volcano.
The two types of lahar are:PRIMARY: Primary lahars occur during a volcanic eruption.SECONDARY: Secondary lahars occur after an eruption during periods of inactivity. For example, heavy rainfalls can trigger a secondary lahar with little warning.Jan 9, 2022
The Chillos Valley Lahar (CVL), the largest Holocene debris flow in area and volume as yet recognized in the northern Andes, formed on Cotopaxi volcano's north and northeast slopes and descended river systems that took it 326 km north–northwest to the Pacific Ocean and 130+ km east into the Amazon basin. In the Chillos Valley, 40 km downstream from the volcano, depths of 80–160 m and valley cross sections up to 337 000 m2 are observed, implying peak flow discharges of 2.6–6.0 million m3/s. The overall volume of the CVL is estimated to be ≈3.8 km3. The CVL was generated approximately 4500 years BP by a rhyolitic ash flow that followed a small sector collapse on the north and northeast sides of Cotopaxi, which melted part of the volcano's icecap and transformed rapidly into the debris flow. The ash flow and resulting CVL have identical components, except for foreign fragments picked up along the flow path. Juvenile materials, including vitric ash, crystals, and pumice, comprise 80–90% of the lahar's deposit, whereas rhyolitic, dacitic, and andesitic lithics make up the remainder. The sand-size fraction and the 2- to 10-mm fraction together dominate the deposit, constituting ≈63 and ≈15 wt.% of the matrix, respectively, whereas the silt-size fraction averages less than ≈10 wt.% and the clay-size fraction less than 0.5 wt.%. Along the 326-km runout, these particle-size fractions vary little, as does the sorting coefficient (average=2.6). There is no tendency toward grading or improved sorting. Limited bulking is recognized. The CVL was an enormous non-cohesive debris flow, notable for its ash-flow origin and immense volume and peak discharge which gave it characteristics and a behavior akin to large cohesive mudflows. Significantly, then, ash-flow-generated debris flows can also achieve large volumes and cover great areas; thus, they can conceivably affect large populated regions far from their source. Especially dangerous, therefore, are snow-clad volcanoes with recent silicic ash-flow histories such as those found in the Andes and Alaska.
Lahars are among the most hazardous mass flow processes on earth and have caused up to 23,000 casualties in single events in the recent past. The Cotopaxi volcano, 60 km southeast of Quito, has a well‐documented history of massively destructive lahars and is a hotspot for future lahars due to (i) its ~10 km² glacier cap, (ii) its 117‐147‐year return period of (Sub)‐Plinian eruptions and (iii) the densely populated potential inundation zones (300,000 inhabitants). Previous mechanical lahar models often do not (i) capture the steep initial lahar trajectory, (ii) reproduce multiple flow paths including bifurcation and confluence, and (iii) often fail to generate appropriate key parameters like flow speed and pressure at the base as a measure of erosion capacity. Here, we back‐calculate the well‐documented 1877 lahar using the RAMMS debris flow model with an implemented entrainment algorithm, covering the entire lahar path from the volcano edifice to an extent of ~ 70 km from the source. To evaluate the sensitivity and to constrain the model input range, we systematically explore input parameter values, especially the Voellmy‐Salm friction coefficients μ and ξ. Objective selection of most likely parameter combinations enables a realistic and robust lahar hazard representation. Detailed historic records for flow height, flow velocity, peak discharge, travel time and inundation limits match best with a very low Coulomb‐type friction μ (0.0025‐0.005) and a high turbulent friction ξ (1000‐1400 m/s²). Finally, we apply the calibrated model to future eruption scenarios (Volcanic Explosivity Index = 2‐3, 3‐4, > 4) at Cotopaxi and accordingly scaled lahars. For the first time, we anticipate a potential volume growth of 50‐400% due to lahar erosivity on steep volcano flanks. Here we develop a generic Voellmy‐Salm approach across different scales of high‐magnitude lahars and show how it can be used to anticipate future syneruptive lahars.
Due to bleeding, water comes up and accumulates at the surface. Sometimes, along with this water, certain quantity of cement also comes to the surface. When the surface is worked up with the trowel, the aggregate goes down and the cement and water come up to the top surface.
Workability is often referred to as the ease with which a concrete can be transported, placed and consolidated without excessive bleeding or segregation. The internal work done required to overcome the frictional forces between concrete ingredients for full compaction.
Because the strength of concrete is adversely and significantly affected by the presence of voids in the compacted mass, it is vital to achieve a maximum possible density. This requires sufficient workability for virtually full compaction to be possible using a reasonable amount of work under the given conditions.
Concrete derives its strength by the hydration of cement particles. The hydration of cement is not a momentary action but a process continuing for long time. Of course, the rate of hydration is fast to start with, but continues over a very long time at a decreasing rate In the field and in actual work, even a higher water/cement ratio is used, since the concrete is open to atmosphere, the water used in the concrete evaporates and the water available in the concrete will not be sufficient for effective hydration to take place particularly in the top layer.
Air entrainment reduces the density of concrete and consequently reduces the strength. Air entrainment is used to produce a number of effects in both the plastic and the hardened concrete. These include: Resistance to freeze–thaw action in the hardened concrete.
Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist are the other situations promoting segregation of concrete. Vibration of concrete is one of the important methods of compaction. It should be remembered that only comparatively dry mix should be vibrated.
Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse. Air-entraining agent is very effective in reducing the bleeding.
Abstract#N#The best-known geo-hazards occur suddenly, such as earthquakes, tsunamis, volcanic eruptions,#N#landslides and floods. These can be catastrophic and cause great damage to people and objects. However, coastal and soil erosion, slow landslides, natural radiation or land subsidence are much slower#N#processes. These are difficult geo-hazards to discern because sometimes a lifetime is not a sufficient time#N#interval for them to take place. They rarely cause fatalities and therefore do not usually generate media#N#headlines, though they can cause important economic losses.#N#2014 The Authors. Published by Colegio de Ingenieros de Montes http://www.ingenierosdemontes.org#N#Peer reviewed
When land has been abandoned for long periods of time, natural colonisation is very advanced and it is then that society puts pressure to begin restoration. In these cases, it is essential
Some pyroclastic falls contain toxic chemicals that can be absorbed into#N#plants and local water supplies, which can be dangerous for both people and livestock. The#N#main danger of pyroclastic falls is their weight: tephra of any size is made up of pulverized rock#N#and can be extremely heavy, especially if it gets wet. Most of the damage caused by falls occurs#N#when wet ash and scoria on the roofs of buildings cause them to collapse.#N#Pyroclastic material injected into the atmosphere may have global as well as local consequences. When the volume of an eruption cloud is large enough, and the cloud is spread far#N#enough by wind, pyroclastic material may actually block sunlight and cause temporary cooling#N#of the Earths surface. Following the eruption of Mount Tambora in 1815, so much pyroclastic#N#material reached and remained in the Earths atmosphere that global temperatures dropped an#N#average of about 0.5 C (~1.0 F). This caused worldwide incidences of extreme weather, and#N#led 1816 to be known as The Year without a Summer.#N#2.3 Pyroclastic Density Currents#N#Pyroclastic density currents are an explosive eruptive phenomenon. They are mixtures of#N#pulverized rock, ash and hot gases that can move at speeds of hundreds of miles per hour. These#N#currents can be either diluted as in pyroclastic surges or concentrated as in pyroclastic flows.#N#They are gravity-driven, which means that they flow down slopes.#N#A pyroclastic surge is a dilute, turbulent density current that usually forms when magma#N#interacts explosively with water. Surges can travel over obstacles like valley walls, and leave#N#thin deposits of ash and rock that drape over topography. A pyroclastic flow is a concentrated#N#avalanche of material, often from a collapse of a lava dome or eruption column, which creates#N#massive deposits that range in size from ash to boulders. Pyroclastic flows are more likely to#N#follow valleys and other depressions, and their deposits infill this topography. Occasionally,#N#however, the top part of a pyroclastic flow cloud (which is mostly ash) will detach from the flow#N#and travel on its own as a surge.#N#Pyroclastic density currents of any kind are deadly. They can travel short distances or hundreds of miles from their source, and move at speeds of up to 1,000 kph (650 mph). They are#N#extremely hot, up to 400C (750F). The speed and force of a pyroclastic density current combined with its heat mean that these volcanic phenomena usually destroy anything in their path,#N#either by burning or crushing or both. Anything caught in a pyroclastic density current would be#N#severely burned and pummelled by debris (including remnants of whatever the flow travelled#N#over). There is no way to escape a pyroclastic density current other than not being there when#N#it happens!#N#One unfortunate example of the destruction caused by pyroclastic density currents is the#N#abandoned city of Plymouth on the Caribbean island of Montserrat. When the Soufrire Hills#N#volcano began erupting violently in 1996, pyroclastic density currents from eruption clouds and#N#lava dome collapses travelled down valleys in which many people had their homes, and inun-
of lava a few centimetres to a few metres in size; ash; and small lava flows (which form when#N#hot spatter melts together and flows downslope). Products of an explosive eruption are often#N#collectively called tephra.#N#Strombolian eruptions are often associated with small lava lakes, which can build up in the#N#conduits of volcanoes. They are one of the least violent of the explosive eruptions, although they#N#can still be very dangerous if bombs or lava flows reach-inhabited areas. Strombolian eruptions#N#are named after the volcano on the Italian island of Stromboli, which has several erupting summit vents. These eruptions are particularly spectacular at night, when the lava glows brightly.
Some common causes for earthquakes include volcanic eruptions, meteor impacts, underground explosions and collapsing structures (such as a collapsing mine), rock falls, and landslides , but this section will discuss only the main cause: tectonic earthquakes.#N#Earthquakes are mostly generated deep within the earths crust, when the pressure between#N#two plates is too great for them to be held in place. The underground rocks then snap, producing#N#a fault and sending out shock waves called seismic waves.#N#The location where the earthquake starts is called the focus or hypocenter. From here, waves#N#start to spread out in all directions. The location above it is called the epicentre. The epicentre is#N#the point on the surface where the waves hit first and the earthquake is the strongest (the most#N#damage is done).
on relatively thin zones of intense shear strain. Movement does not initially occur simultaneously over the whole of what eventually becomes the surface of rupture ; the volume of displacing material enlarges from an area of local failure.