ch 8 natural hazards

Venice Is Sinking

Subsiding

2 mm/yr. in some areas

118 small islands connected by > 400 bridges situated in a coastal lagoon

Extremely prone to flooding


Naturally occurring for millions of years

what has increased for venice

over pumping of groundwater significantly ⇡ rate of subsidence ⇡ raise buildings and streets ⇡ flood frequency

Project MOSE’s

Modulo Sperimentale Elettromeccanico (Electromechanical Experimental Module)

78 hinged flood gates
28 m high x 20 m width

Soil biology

Solid earth material that has been altered such that it can support rooted plant life

engineering soil

Any solid earth material that can be removed without blasting

Soil is produced through

weathering

weathering

Physical and chemical breakdown of rocks

Changed by residual or transported activity of soil organisms

Soils can be viewed as

complex 3-D structure consisting of packed aggregates & pore spaces

Solid, liquid, & gaseous matter in soil is continuously interacting through

chemical, physical, & biological processes

Inorganic Mineral Matter

Derived from weathered rocks
Parent material

Organic Matter

Derived from decomposed living matter

Humus

partially decomposed organic matter mixed into surface soil

Air and Water

Void spaces between solids
Contains dissolved substances

Soil Profiles

Soils studied from surface to bedrock

soil horizon

Each layer

Eluviation

material is removed from a horizon (exit)

Leaching

dissolved nutrients are removed from a horizon (exit)

Illuviation

material is moved into a horizon (in)

O horizon

opmost layer composed of organic mater
L, F, H

A horizon

humus and organic materials mixed with minerals & zone of translocation

eluviation

B horizon

mineral layer strongly influenced
Illuviation

C horizon

weathered parent material
Regolith

R horizon

bedrock

Canadian System of Soil Classification

CLORPT

CLORPT

Five factors that generally influence soil formation and affect how soils evolve

C - CLORPT

Climate

O - CLORPT

ORGANISM

R - CLORT

Relief

P -CLORPT

Parent Material

T - CLORPT

Time

Soil Color

Can be an important diagnostic tool for analyzing a soil profile, but can be misleading

O and A horizons

dark

E horizon

white

B horizon

varies from yellow-brown to light red-brown to dark red

K Horizon

almost white

Well-drained are aerated

red

Poorly drained are wet

yellow

Texture

Sand, silt & clay (size grade)
Affects ability of soil to retain H2O
Clay holds more H2O than sand

Soil Structure

Particles cling together in peds or aggregates

Weakly developed soil

A horizon directly over a C horizon (without B)
Few hundred to several thousand years old

Moderately developed soil

A overlying an argillic Bt that overlies the C horizon
More than 10,000 years old (at least Pleistocene)

Well-developed soil

Bt redder, more translocation of clay to Bt, and stronger structure
Between 40,000 and several hundred thousand years and older

Soil chronosequence

youngest to oldest

Give information about the recent history of an area

Saturated

All the pore spaces in a block of soil are completely filled with water

Unsaturated otherwise

Moisture content

Amount of water in a soil

Important to strength of soil & potential to shrink & swell

Water flow

Saturated flow if all the pores are filled with water

Unsaturated flow otherwise (more common)

Soil taxonomy

Used by soil scientists

Based on physical and chemical properties of soil profile

Useful for agricultural and land use

Engineering classification of soils

Based on particle size or the abundance of organic material

soil Erosion as a Hazard

Appears many practices are mining the soil
Could erode foundation of our civilization

Grain-by-grain removal of mineral and organic material by wind and/or water

Removal of soil material at an unacceptable rate

Removal of soil material at a rate faster than it is being produced

Vegetation often removed prior to development

Persists where protection is not a high priority

Rates of soil erosion

Most concerned with top, organically rich soil

Takes about 500 to 1000 years to form 50 mm (~2 in) of soil

Rate of soil for agricultural land is 0.05 to 0.1 mm per year

Accelerated erosion can remove centuries of soil in less than a decade

Rates measured as volume, mass, or weight

Amount removed from a location within a specified time and area

Sediment Yield

Quantity of sediment removed by overland flow from a unit area of ground surface in a given unit of time

Universal Soil Loss Equations

Only predicts the amount of soil loss from sheet or rill erosion on a single slope
does not account for additional soil losses that might occur from gully, wind or tillage erosion

Subsistence

lowering of earth surface because earthly, e sediment caption or other natural processes

Karst

Landscape resulting from the dissolution of limestone, dolostone, marble, gypsum or rock salt

Soil expansion and contraction

changes in the water content of soil
freezing and thawing

Usually not life threatening
One of the most widespread and costly natural hazards

Karst 2

Common landscape associated with subsidence
Rocks dissolved (dissolution) by surface/ground-water 2

karst plain.

A surface pockmarked with a large number of sinkholes

Dissolution/Acid Action

Sinkholes

solution

collaspe

Cave Systems

formed when dissolution produces a series of caves

Related to a fluctuating groundwater table

Groundwater seepage will deposit calcium carbonate on the sides, floor, and ceiling of the cave as flowstone, stalagmites, and stalactites

Permafrost

Ground (soil or rock) remains at or below a temperature of 0°C for a min. period of two years

Active layer

thaws and freezes annually (important feature)

Permafrost Temperature Profile

maximum known thickens of permafrost is about 1500 m in the northern Lena and Yana river basins of Siberia

Periglacial Landscapes

20 to 35% globally (Pleistocene Glaciation: + 20%)
50% of Canada
80% of Alaska

Continuous permafrost

Mean annual temperature is less than -5°C

Discontinuous permafrost

Mean annual temperature is between -5°C and 0°C

Extensive Discontinuous

Covers 50-90% of the landscape

Mean annual temperature is between -2°C and -4°C

Sporadic Permafrost

Cover is

ice lenses

The additional ice accumulates to form discrete pods or lenses of ice in the earth material

talik

layer of year-round unfrozen ground that lies in permafrost area

Permafrost Thaw - causes

permafrost thaw

Can produce subsistence especially if sediment contains large amount of ice

Some areas permafrost table has dropped close to 20 cm per year

Thermokarst

An irregular terrain or sinkholes and mounds formed by the melting of permafrost

Piping

Particles of silt and sand in the subsurface slowly carried by groundwater laterally to a spring

piping causes

Caused by groundwater creating tunnels as it percolates through loose sediments

Common in silt and sand sediments

Over time, shallow subterranean tunnels and cavities may develop to produce surface depressions and ravines

Fine sediments

Sediment compacts when
pore water is removed

Collapsible sediments

Loess and some stream deposits in arid regions are loosely bound or are water-soluble

Infiltrating water weakens bonds Compaction

Organic sediments

Wetland soils large amounts of organic matter & water

When water is drained or soil is decomposed, these soils compact

Expansive Soils

expand during wet periods
shrink during dry periods

Common in clay, shale, and clay-rich soil containing
smectite, bentonite, montmorillonite, beidellite, vermiculite, attapulgite, nontronite, and chlorite. 

Can produce desiccation cracks

Tilting and cracking of blocks of concrete and wavy bumps in asphalt can cause structural damage

Smectite

Group of clay minerals

Small crystals = large surface area to attract H20 molecules

Abundant in many clay & shale deposits

Primary constituent of bentonite

Clay shale, and soils containing have greatest potential for shrinking and swelling

Desiccation cracks

Frost Susceptible sediments

Water expands by 9% when it freezes

causes upward movement of sediment particles called Frost

Heaving

Most common in silty sediment

Ice lenses

Earthquakes

Raise or lower ground surface

1964 Alaskan earthquake

produced subsidence over tens of thousands of km up to 2 m, while other areas were elevated

Magma Chamber Deflation

Volcanic activity

As magma moves upward underneath or into a volcano, the surface of the volcano may be forced upwards

Volcano erupts, the underground magma chamber is partly or complete emptied and the surface subsides

Regions at Risk

Landscapes underlain by soluble rocks, permafrost, or easily compacted sediment

Soils that contain abundant smectite clay are susceptible to shrinking and swelling soils

Soils containing silt are susceptible to frost heaving

Effects of Subsidence and Soil Volume Change

Sinkhole formation

Groundwater Use and Contamination

Permafrost Thaw

Coastal flood and Loss of Wetlands

Soil Volume Changes

Overloading

Sinkhole Formation

Can cause considerable damage highways, homes, and sewage lines

Triggered by fluctuations in the water table

Groundwater Use and Contamination

Caves provide direct connections between surface water and groundwater

Groundwater can be vulnerable to pollution

The water table can significantly lower during droughts

Permafrost Thaw 1

Melting of permafrost has caused roads to cave in, airport runways to fracture, railroad tracks to buckle, and buildings to crack, tilt, or collapse 1

Coastal Flooding and Loss of Wetlands

Along the Mississippi Delta, this has contributed to the sinking of New Orleans

Wetlands that protect the city from storm surges are disappearing

Soil Volume Changes

Swelling of expansive soils and frost heaving

Causes billions of dollars in damage annually in North America

Vertisolics Soils

vert = “invert”

Form on parent material rich in clays (glacio-lacustrine) under grassland vegetation

Expand and shrink in response to H2O content

Deep cracks form churns up soil (self mixing)

Cool, sub-arid to subhumid grassland areas

Horizons are not obvious

Overloading - Leaning Tower of Pisa

Construction for a new bell tower began in 1173
Dug down 2 m to lay foundation
2 m of compressible clay ½ m below inadequate foundation
3 floors – sinking and leaning
Construction stopped for close to a century

Leaning Tower of Pisa . . . .

Completed 1372 (only need the bell tower), but 1.5 m off vertical

1992: first story braced with steel tendons

1993: 600 tons of lead ingots were stacked around base as counterweighs

1999 -2001: 77 tons of soil removed – back to 1883 lean
Install drains to address fluctuating water table on north side to blame for uneven sinking

Links to Other Natural Hazards

Subsidence can be an effect of earthquakes, volcanic activity, and climate change

Climate change can add to the drying of soils and the altering of the groundwater table

Subsidence may cause

flooding

Over-pumping of groundwater

Natural Service Functions

Water Supply

Aesthetic and Scientific Resources

Unique Ecosystems

Water Supply

Karst regions contain the world’s most abundant water supply

Aesthetic and Scientific Resources

Caves and karst landscapes are scenic areas that attract tourists and provide research for scientists

Unique Ecosystems

Some animal species can live only in caves
Caves also provide shelter for other animals

Human Interaction with Subsidence

Withdrawal of Fluids

Underground Mining

Permafrost Thaw

Restricting Deltaic Sedimentation

Draining Wetlands

Landscaping on Expansive Soils