Exam #3 - Landslides and Floods

Flood stage

The depth of water at which a river flows over its banks, potentially causing property damage.

Which of these is the definition of a "100-year flood?"

The largest flood of the past 100 years.

How common are landslides?

They occur in every state

Topographic floodplain

This area for a meandering river is defined by a significant rise in topography (landscape) beyond the extent of the meanders.

Puerto Rico Landslide, 1985

This landslide destroyed 120 houses and killed at least 129 people. The catastrophic slide was triggered by a tropical storm that produced extremely heavy rainfall.

Hydrologic floodplain

This wider depression is filled during seasonal high water. It typically has many meanders and oxbow lakes in it.

Which of these is defined by a significant rise in topography (the landscape) outside of (beyond) the extent of river meanders?

Topographic floodplain

Which of these is NOT a significant cause of flooding?

Tornadoes

soil moisture

Underground water that wets the surface of the mineral grains and organic material making up soil, but lies above the water table.

Braided Rivers

When a river is choked with more sediment than it can carry, it becomes divided into many branches.

Meandering Rivers

When a river travels across flat land; that is, it wanders back and forth across its floodplain.

Tilted layers

When soil or bedrock layers are tilted the same direction as the slope, any removal of layers from the bottom of the slope will remove support from the higher layers.

Dams

a barrier that runs across a river or stream to control the flow of water

accretion

a growth in size; an increase in amount

Caliche

a natural cement that forms in desert soils when water evaporates, leaving behind its mineral content.

Rip-rap

blanket of boulder-size rocks designed to prevent erosion at strategic locations. The rocks must be large enough that flood waters cannot move them. They are placed with care to make them stable.

Lateral spreads

commonly induced by liquefaction of material in an earthquake, move by horizontal extension, shear or fractures.

Hydrograph

diagram of the levels or amount of water flow in a river

Slides

displace masses of material along one or more discrete planes and are grouped by the way they move.

emergency managers

disseminate forecasts and warnings as well as to support their flood response activities

force

drives all mass movement is gravity

landslide

general term used to describe the downslope movement of soil, rock, and organic materials under the effects of gravity

Earthquakes

in steep landslide-prone areas greatly increases the likelihood that landslides will occur, due to ground shaking alone or shaking caused dilation of soil materials, which allows rapid infiltration of water.

Fall

masses of soil or rock that dislodge from steep slopes and free-fall, bounce, or roll downslope.

melange

materials become part of the over-riding continental crust, and we call this mixture of materials

Volcanoes

may melt snow at a rapid rate, causing a deluge of rock, soil, ash, and water that accelerates rapidly on the steep slopes of ________ , devastating anything in its path.

Flows

mobilize as a deforming, viscous mass without a discrete failure plane.

Topples

move by the forward pivoting of a mass around its base.

drainage basin

proportional to the amount of water that can enter that river, and is thus related to potential flood discharge.

Creep

refers to extremely slow movement, and avalanche refers to extremely rapid movement.

Rock

refers to hard or firm bedrock that was intact and in place prior to slope movement.

Saint-Jude Landslide

significant clay layer in the ground blocked the downward flow of groundwater, causing the materials above it to become saturated and weak. The clay itself is very weak and slippery, and so the entire mass slid down the very slight slope. This landslide occurred in 2010. Four people were killed.

100-Year Flood

statistical assessment that a given event has a 1 in 100 chance of occurring in any year. 1% chance.

River stage

the height of the water in a stream channel (water level of the flooding)

surface tension

the inherent (but small) strength that the surface of a liquid has

Cliff Retreat

the process of erosion that undercuts a cliff, thereby causing it to collapse and shift location away from the erosion

Flash flooding

typically occurs when intense rains fall faster than the ground can absorb it

Role of Special Atmospheric Conditions

unusually heavy rainfall. The pineapple express pulls in moisture from around hawaii.

Soil

used in the engineering sense to mean unconsolidated particles or poorly cemented rock or gravel. _________ is distinguished further on the basis of texture as debris (coarse rocks) or earth (fine grains).

debris flows and mudflows

usually occur in small, steep stream channels and often are mistaken for floods; in fact, these two events often occur simultaneously in the same area.

River discharge

volume of water flowing through a river channel

Slope saturation

water is a primary cause of landslides. More causes are: • Heavy rain or rapid snowmelt • changes in ground-water levels • water-level changes along the banks and shores of bodies of water.

What do we call the natural cement that forms in desert soils by repeated evaporation of water?

Caliche

Why do Las Vegas, Phoenix, and Tucson have flash floods even from small rainfall amounts?

Caliche in the soil prevents infiltration of rain

What do we call flood mitigation measures that include removing brush and trees, deepening/widening a channel, or changing the river to a more favorable course?

Channel modification

Which of these materials increases risk of landslides?

Clay

Translational

During these slides, the failure surface is more or less planar and the mass moves parallel to the ground surface.

Rotational

During these slides, the slide plane is curved and the mass rotates backwards around an axis parallel to the slope.

What is the technical term for wet, fine-grained soil?

Earth/mud

News reporters call most types of mass wasting "mudslides." What is a more accurate technical term for movement of wet earth?

Earthflow/mudflow

Major Flooding

Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.

What danger do mountain streams pose, as opposed to meandering and braided streams (rivers)?

Fast water, rocks, and high erosion

What effect does urban development have on flooding?

Floods are more severe in urban areas because of less infiltration of rain.

Role of Sedimentation

Floods can carry and deposit tremendous amounts of clay, silt, sand, and even gravel, making both recovery and mitigation of future flooding more difficult.

Ice Jam Floods

Frozen material creates a temporary dam on a thawing river

Which of these factors increases the risk of landslides?

High rainfall

What do we call a chart of time versus river discharge?

Hydrograph

Role of Urbanization

In contrast to rural and farm areas where a high percentage of rainfall and snowmelt soak into the ground, very little infiltrates in developed areas.

Why Study Landslides?

It is estimated that in the United States, they cause in excess of $1 billion in damages and from about 25 to 50 deaths each year

Why does China's Yellow River have such severe flooding problems?

Its sediment load is so high, the river cannot be contained by levees for long

What do we call a wall or berm (earth/rocks) built to contain a river or other water body?

Levee

friction and internal strength

Materials are held in place by the forces that resist gravity and shear

Weak materials

Materials with little internal strength tend to move more easily. These include shale, silt, sand, gravel, and talus (steep piles of loose rock, usually below cliffs).

Flood Mitigation

Measures aimed at preparing for floods and trying to reduce their effects, such as constructing artificial levees, strengthening bridges, raising road levels and enlarging drains.

Channel Modification

Measures may include removing brush and trees, deepening and widening the channel, and in some cases changing the channel to a more favorable route or location.

Minor Flooding

Minimal or no property damage, but possibly some public threat or inconvenience.

Angle of Repose

Moist sand is more stable than dry or saturated sand, and can sustain steeper slopes.

Combinations

More than one form of movement may occur during a failure, in which case the movement is classified as complex if movements occur sequentially and composite if they do not.

Which of these is a Federal government agency that monitors flood conditions?

NOAA

Why does coastal California have so many landslides?

Past tectonics - used to be subduction zone

Lack of vegetation

Plant roots hold the surface materials of a slope together like a blanket, stabilizing them. Landslides are common after wildfires have removed the vegetation.

Erosion

Processes by which rock, sand, and soil are broken down and carried away (i.e. weathering, glaciation)

What do we call movement on discrete planes that stays relatively intact?

Slide

Water

Slope material that become saturated with water may develop a debris flow or mud flow

Steep slopes

Soft materials are unstable at slopes greater than the angle of repose, which is a little steeper than 2:1. Anything that steepens a slope, like erosion or excavation, increases the probability of landslides.

Clay layers

Some clays also expand greatly when they are wet, which can destabilize the slope.

Moderate Flooding

Some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations are necessary.

Levees

Barriers composed of sediments made on either side of a river due to flooding.

How deep does water have to be to sweep your car off the road?

2 feet - onto the bottoms of the doors

fine sand

35`

coarse sand

40`

angular pebbles

45`

Channel

A channel is the path that a river normally flows in.

What do we call a concentrated flow of moisture from the Pacific subtropics to the U.S. west coast? This phenomenon has been responsible for the biggest west coast floods.

Atmospheric river

Rockfall

A type of mass movement in which a mass of rock detaches from a steep slope by sliding, spreading, or toppling and descends mainly through the air by falling, bouncing or rolling.

Slump

A type of mass movement that occurs when a mass of material moves down a curved slope

alluvial fan

A wide, sloping deposit of sediment formed where a stream leaves a mountain range

On a meandering river, where does the most severe erosion take place?

Along the outside of bends

Flood gage

An instrument that uses sensors to measure the height (called "stage") of river water above the river channel bottom

Discharge

An outflow of water from a stream, pipe, groundwater aquifer, or watershed; the opposite of recharge.

Which of these is not a type of landslide (mass wasting)?

Andesite

Role of Climate and Landscape

As a consequence of this evolution, soils in wet climates generally have a high capacity to absorb water, while soils in desert climates do not. Flash floods are common in deserts because nearly all of a sudden rainfall runs on the surface.

Climate

As a result, slope materials can go from completely dry to saturated in a very short time, which destabilizes them. In wet climates, surface materials can be deeply weathered (broken down chemically) into weaker materials that are unstable.

What phenomenon happens when waves erode the base of cliffs?

Cliff retreat

What do we call imperceptibly slow movement of soil or rock?

Creep