GEOHAZARDS 2

LANDSLIDES

general term for rapid downslope movements/failure, with potential to kill people along their path

Mass Wasting

Is even more general and includes slow movements (creep, slow flows)

Mass wasting / slope failure / landslides

all downslope movement of rock and sediment that occurs at the surface of the earth (also underwater) in response to gravity. Speeds range from extremely fast to extremely slow.

Gravity

Main driving mechanism of mass wasting/land slides

Basis for Classification of Landslides

Based on the type of materials involved, speed, and type or mechanism of downslope movement

Range Speeds of Landslides

Extremely fast to Extremely slow

Types of Downslope Movements

Fall
Topple
Flow
Slide
Subsidence

Flowage

Downslope movement type when soil and water involved

Sliding

Downslope movement type when material slides along a slip plane. Occurs when there is a distinct zone of weekness between the slide material and the more stable underlying material.

Types of material involved in Landslides

ROCK, SOIL (Earth materials 80%>2mm (coarser), Debris 2mm<20% )

ROCK

a hard or firm mass that was intact and in its natural place before the initiation of movement.

SOIL

an aggregate of minerals and rocks that either was transported or was formed by the weathering of rock in place.

Fall

Rock material falling vertically. Common in rugged terrains where slopes are oversteepened by erosion, water, or road cuts.

Topple

Toppling of material. Forward rotation and movement of rocks/ice/regolith out of a slope at a point or an axis. Common where there is a pronounced vertical or steeply dipping discontinuity in a rock formation.

Rotational Slide or Slumping

Type of sliding with a curved slip surface

Translational slide

Type of sliding with planar slip surface

Spread

Landslides that commonly form on gentle slopes of flat slopes that have rapid, fluid-like flow movements similar to water. Common in loose sediments consisting of sand, clay, and silt.

Flow

Mobilized soil materials due to rain. Or lahar flows/ mudflows when volcanic material is mobilized

Creep

Very slow, nearly inpercebtible movements on gentle slopes.

Complex

Landslides characterized by a mixture of types

Angle of repose

Maximum slope angle at which the material is stable. More cohesion means greater angle of repose

Driving forces of landslide

mass, gravity, pore pressure. Induces downward movement

Resisting forces of landslide

inertia, friction, cohesion. Tends to keep materials intact along a slope

mass, gravity, pore pressure

Driving forces of landslide

inertia, friction, cohesion

Resisting forces of landslide

Capillary Force

Cohesive force holding partices together in pore space mostly occupied by air. Allowing greater angle of repose.

Pore Pressure

Force exerted by large amount of water occupying all pore spaces. Keeps particles apart and reduces cohesion

Water, aside from exerting capillary force in small amounts or pore pressure in large amounts, adds mass to wet materials. This increases the gravitational pull on the slope which favors mass movement

Water, aside from exerting capillary force in small amounts or pore pressure in large amounts, adds mass to wet materials. This increases the gravitational pull on the slope which favors mass movement

Effect of slope angle

Inclination increases tendency of particles to move along a slope due to gravity instead of remaining still due to resisting forces

Factor of Safety (F.S.)

Determines slope stability

Formula for Factor of safety

FS = Resisting Force/Driving Force

Resisting Force (RF)

Laboratory measure of rock strength and cohesion

Driving Force (DF)

Trigonometrically calculated force (mostly from gravity) due to angle of slope

If F.S.>1, then RF>DF and the slope is stable

If F.S.>1, then RF>DF and the slope is stable

Most modern building codes require a F.S. of 1.5 or greater

Most modern building codes require a F.S. of 1.5 or greater

Water, Earthquakes

Triggers of Landslides

Submarine Landslides in Hawaii

Largest and most far-reaching landslides on Earth

Human activities that increase the frequency of landslides

Slope over-steepening

(Slope-leveling for building and additional weight of building on lower part of slope can lead to slope failure)

(Human impact on mass wasting) to produce more level surfaces is one mechanism which can lead to slope instability and mass wasting.

Addition of water

(Human impact on mass wasting) through irrigation, septic systems, artificial ponds or leaky in-ground pools, etc., loosens up slope material and adds weight, promoting slope failure

Devegetation

(Human impact on mass wasting) concentrates surface runoff and enhances erosion, resulting in steeper slopes. Reduced interception and evaporation increases amount of water in ground. (Also, construction of logging roads result in oversteepening.)

Scars; barren or lack of vegetation
Disrupted topography
Historical records of past avalanche
Records of the character of past volcanic activity – debris avalanche
Tilted vegetation (in creep)
cracks in driveways, garage floors, freestanding brick or concrete walls, or buildings; cracks in walls or ceilings

Recognizing past mass movements/landslides

1. Identify potential hazard
- Map unstable areas; consult a geologist
- Avoid landslide-prone areas; zoning

2. Slope reduction and revegetation

3. Use of retention structures
   - vertical piles on the toe of landslides
   - buttress such as gabions or retaining walls
   - rock bolts to stabilize rocky slopes

4. Removing fluid

5. Chemical treatment

What can be done to prevent (or correct) landslides?

Landslide Hazard Map

indicates the possibility of landslides throughout a given area

it may be as simple as a map that uses the locations of old landslides to indicate potential instability, or as complex as a quantitative map incorporating probabilities based on variables such as rainfall thresholds, slope angle, soil type, and levels of earthquake shaking.

an ideal landslide hazard map shows not only the chances that a landslide may form at a particular place, but also the chance that it may travel downslope at a given distance.

an ideal landslide hazard map shows not only the chances that a landslide may form at a particular place, but also the chance that it may travel downslope at a given distance.

Landslide Inventory Map (Lanslide Hazard Maps)

Show the location of past landslides

Landslide Susceptibility map

Describes the relative likelihood of future landslide based on intrinsic properties of a locality.

Prior failure, Rock or Soil Strength, Steepness of Slope

(Lanslide Hazard Maps) Three important site factors for Landslide Susceptibility

Landslide Risk Map

Show landslide potential and expected loses to life and property should a landslide occur. Combines landslide hazard map with

Mitigation Measures

Slope stabilization: keeping the water out
Slope stabilization: revegetation (Geotextiles (synthetic soil cover) are used to stabilize slope along with revegetation)

Slope Stabilization: retaining structures (Wire mesh, Geogrids, Wire Baskets)
Slope stabilization: ground anchoring

Shelters (sheds are used to divert slides from the road (and people on it))

Slope stabilization: keeping the water out
Slope stabilization: revegetation (Geotextiles (synthetic soil cover) are used to stabilize slope along with revegetation)

Slope Stabilization: retaining structures (Wire mesh, Geogrids, Wire Baskets)
Slope stabilization: ground anchoring

Shelters (sheds are used to divert slides from the road (and people on it))

Mitigation Measures

Vertiver Grass

Perenial Clump grass usually used for soil and water conservation.

Watershed

An area of land where all of the water that falls in it and drains off of it goes to a common outlet. Can be as small as a footprint or large enough to encompass all the land that drains water into rivers that drain into Chesapeake Bay, where it enters the Atlantic Ocean.

Floodplain

Area adjacent to rivers developed due to the frequent overbanking of water, experiencing frequent floods

Coastal Flooding, River FLooding

Two Types of Flooding

Excessive rainfall
Limited river channel capacity (Width, Depth, Gradient)
Dam failure

Causes of River Flooding

RUN OFF

water from rainfall that remains on the surface of the Earth and eventually flows into streams

Runoff = Precipitation - Infiltration - Interception - Evaporation

Runoff = Precipitation - Infiltration - Interception - Evaporation

Natural Levees

build-up along the river bank due to deposition of suspended sediment during floods with each flood the levee is built higher and discharge must be higher for the next flood to occur

Floodplains

alluvial surface adjacent to a channel that is frequently flooded

Riverine flooding

Overtopping river banks

More concrete means less infiltration and less/no trees for evapotranspiration. Rainwater mostly becomes runoff

Why urbanization and flooding go together?

Yearly, the country experiences an average of twenty (20) tropical cyclones a year, seven to eight are destructive, including other weather hazards such as thunderstorm, heavy rainfall, southeast and northwest monsoons, cold front and ITCZ; is a host to more than 400 volcanoes, twenty-two (23) of which are active, together with active faults and trenches that are potential sources of earthquakes.

Yearly, the country experiences an average of twenty (20) tropical cyclones a year, seven to eight are destructive, including other weather hazards such as thunderstorm, heavy rainfall, southeast and northwest monsoons, cold front and ITCZ; is a host to more than 400 volcanoes, twenty-two (23) of which are active, together with active faults and trenches that are potential sources of earthquakes.