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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Lahars have several possible causes: Snow and glaciers can be melted by lava or pyroclastic flows during an eruption. Lava flow out of open vents can mix with wet soil, and mud on the slope of the volcano making a very viscous, high energy lahar.
The cohesive lahar picks up debris in is way and can reach heights of 100 meters or more. They can range from ten to hundreds of kilometers downstream and take out anything in its way depending on the clay content.
During downstream movement in water-rich environments, lahars may progressively mix with water and transform to hyperconcentrated flood flows.
Factors affecting concrete workability: 1 Water-Cement ratio 2 Amount and type of Aggregate 3 Amount and type of Cement 4 Weather conditions#N#Temperature#N#Wind 5 Temperature 6 Wind 7 Chemical Admixtures 8 Sand to Aggregate ratio More ...
Lahars can occur with or without a volcanic eruption Pyroclastic flows can generate lahars when extremely hot, flowing rock debris erodes, mixes with, and melts snow and ice as it travel rapidly down steep slopes. Lahars can also be formed when high-volume or long-duration rainfall occurs during or after 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.
Lahars form when water from intense rainfall, melting snow and ice, or the sudden failure of a natural dam, mixes with this loose volcanic material, creating mudflows that can be particularly dangerous and destructive.
Lahar is an Indonesian word describing a mudflow or debris flow that originates on the slopes of a volcano. Small debris flows are common in the Cascades, where they form during periods of heavy rainfall, rapid snow melt, and by shallow landsliding.
Lahars are extremely destructive: they can flow tens of metres per second, they have been known to be up to 140 metres (460 ft) deep, and large flows tend to destroy any structures in their path.
LaharClast.Debris Flow.Lava Flow.Pyroclastic Flow.Tephra.Volcano.Lava.Crater.
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.
Definition: A lahar is a hot or cold mixture of water and rock fragments that flow quickly down the slopes of a volcano.
Strategies to mitigate the potential for damage or loss from lahars fall into four basic categories: (1) avoidance of lahar hazards through land-use planning; (2) modification of lahar hazards through engineered protection structures; (3) lahar warning systems to enable evacuations; and (4) effective response to and ...
What is a Lahar? - Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments. - A lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter.
Lahar deposit from a volcano landslide The lahar deposit contains many dark angular clasts (rock fragments) that were derived from basalt lava flows on the volcano--they were shattered, broken, and split apart during the landslide and its transformation to a lahar.
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.
During downstream movement in water-rich environments, lahars may progressively mix with water and transform to hyperconcentrated flood flows. Hyperconcentrated flood flows lack the strength and cohesion of lahars, but can carry high sediment loads with fragments supported both by turbulence and particle interactions (Pierson and Scott, 1985; Scott, 1988; Smith, 1986). Such floods can move as far as 250 km or more down valleys, and because of their high loads, will impact an entire river system such as occurred at Mount St. Helens in 1980.
For field geologists who need to interpret the origin of a layer of rock from from its field characteristics, a lahar may be defined as a debris flow composed of a significant component of volcanic materials (>25%) (Fisher and Schmincke, 1984), a descriptive definition that can be applied in the field from observations of deposits without requiring a judgement about synchroneity of volcanism. Many volcanologists prefer to define a lahar as caused by a volcanic eruption. But in ancient deposits, it is not always possible to determine if the lahar was caused directly by eruption or by remobilisation of volcanic rubble long after an eruption.
But in ancient deposits, it is not always possible to determine if the lahar was caused directly by eruption or by remobilisation of volcanic rubble long after an eruption. For geologists who study modern deposits, lahars may be defined in terms of visible characteristics of witnessed flows. The following definitions of lahar come ...
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.
Three major categories of lahars by origin are, (1) those formed by the direct and immediate result of eruptions through crater lakes, snow or ice, and heavy rains falling during or immediately after an eruption on abundant unstable loose material.
Internal resistance to flow results from (1) electrostatic forces causing cohesive resistance to flow arising from clay-water mixtures or (2) from friction caused by inertial interaction of large fragments (greater than medium silt-size) causing inertial resistance to flow or frictional resistance (cohesionless or density-modified grain flows; Lowe, 1982). The relative importance of these two causes of flow resistance depends upon the amount of admixed clay-size grains, a small amount (~5%) of which can cause major changes in flow behavior. Grains are supported in debris flows by mass effects caused by high concentration (e.g., cohesive strength, frictional strength, viscous resistance and dispersive pressure), and by turbulence and pore-fluid expulsion.
Debris flows as fluids. Debris flows are granular fluids with high bulk density, and exhibit the property of strength resulting from particle interactions due to high concentration of particles. At volume concentrations of less than about 20 or 30 percent, particle support in a solids-water mixture is mostly by turbulence, ...
Lahars are a form of mudslide or landslide that doesn’t necessary need to be triggered by volcanism . The name lahar derives from the Javanese Hindi language in Indonesia. The volcanic mud-flow in Hindi means wave. Lahars can effect all of the mountains of the world; but not all of them are prone to lahars. The appearance of lahars from the pyroclastic result create a cone shape top that covers the top of the crater that are often snow covered or contains water in a crater lake.
Lahars can result from volcanic eruption or by melting glaciers that produce flooding in the summit crater, or by heavy rainfall, or ice in the crater. In addition, water deposits that produce lahars can occur at (or) beneath the crater of the mountain. Thereby, producing glacial outburst (s) from the flooding activity, or by a volcano eruption. The eruption of pyroclastic density can flow out tens of meters per second and spread several kilometers away.
The mountains that are known for producing lahars are high threat areas such as: USA at Mount Rainier, New Zealand’s Mount Ruapehu, and Indonesia’s Galunggung mountain. Some towns near Mount Rainer are built on top of lahar. The towns are Orting, Washington State, and the Puyallup River Valley, in Washington State have been constructed atop lahars.
Mount Rainer has been investigated by scientists because of the high risk factor involved with the future eruption from this mountain. Mt. Rainier explodes every 500 to 1,000 years according to scientists and it has been 550 years since the mountain erupted. The valleys that would be hit would be
The workability of concrete contains two aspects, consistency and cohesiveness.
Concrete Workability is a property of concrete in its fresh state during mixing. It determines the ease with with casting activities are done including mixing, placing, compacting and finishing. Concrete workability is usually identified in the mixture design process according to the type of construction.
Fresh concrete is said to be workable if mixing, placing, compacting and finishing can be done with minimum effort and without concrete ingredients being segregated. In contrast, if concrete has low or poor workability, it will not flow in a smooth way into forms and it will be difficult in mixing, placing, compacting and finishing.
The desirable conditions are, a suitable temperature and ample moisture. Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the point of view of volume stability. Jeat of hydration of concrete may also shrinkage in concrete, thus producing cracks. If the heat generated is removed by some means, the adverse effect due to the generation of heat can be reduced. This can be done by a thorough water curing.
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.
These continuous bleeding channels are often responsible for causing permeability of the concrete structures. While the mixing water is in the process of coming up, it may be intercepted by aggregates. The bleeding water is likely to accumulate below the aggregate. This accumulation of water creates water voids and reduces the bond between the aggregates and the paste. The above aspect is more pronounced in the case of flaky aggregate. Similarly, the water that accumulates below the reinforcing bars reduces the bond between the reinforcement and the concrete. The poor bond between the aggregate and the paste or the reinforcement and the paste due to bleeding can be remedied by re vibration of concrete. The formation of laitance and the consequent bad effect can be reduced by delayed finishing operations. Bleeding rate increases with time up to about one hour or so and thereafter the rate decreases but continues more or less till the final setting time of cement.
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.
The hardening of concrete before its hydration is known as setting of concrete. OR
Weak or easily erodible volcanic rock layers for source of lahar material
Dilute pure lahars, contain 20% to about 60% volcanic debris by volume