1
Q
What is subsidence?
A
- nearly vertical downward movement of Earth’s surface
- the speed of subsidence can vary from very slow to rapid
- the subsiding area can be a variety of shapes
- it is not usually life-threatening but is one of the most widespread and costly hazards
2
Q
What causes subsidence?
A
- associated with the dissolution of limestone, marble, or gypsum
- other causes: thawing of frozen ground, compaction of sediment, shrinking of expansive soil, earthquakes, deflation of magma chambers
3
Q
What is karst?
A
- an irregular landscape containing many depressions
- it results from the dissolution of rock
- some rocks dissolve in the vicinity of water
- karst is most evident in areas of limestone because this is the most common of the dissolving rocks
4
Q
What causes limestone dissolution?
A
- water containing CO2 is acidic and thus aids in the dissolution process
- chemical reactions leading to dissolution:
water + CO2= carbonic acid
carbonic acid + limestone = calcium carbonate
-limestone dissolution can result in the lowering of a surface by 1cm per decade
5
Q
What results from limestone dissolution?
A
- the process produces empty voids over a range of sizes
- over time, continuous voids may produce caves
- at voids near the surface, sinkholes may develop
- a surface containing many sinkholes is referred to as a karst plain
- happens in Florida
6
Q
What is a ground water table?
A
- the underground depth at which point the ground is saturated with water
- during drought, this can be low
- changes depending on conditions
7
Q
What are the two types of sinkholes?
A
- solution sinkholes: formed by acidic groundwater
- collapse sinkholes: formed by collapse of the surface
- concerned about sinkholes during dry times because the water table goes downa nd its only air which can’t support it
- can reach several hundred metres in diameter
8
Q
What is a cave system?
A
- a system of caves may evolve as limestone voids continue to enlarge
- seeping water deposits calcium carbonate resulting in the formation of flowstones, stalactites, stalagmites, and columns
9
Q
What is flowstone?
A
-general term for an accumulation of calcium carbonate precipitated from water in a cave
10
Q
What is stalactite?
A
-a deposit of calcium carbonate that extends downward from the roof of a cave
11
Q
What is stalagmite?
A
-a deposit of calcium carbonate on the floor of a cave
12
Q
What is a column?
A
-a feature formed when a stalactite and stalagmite join together
13
Q
What is a tower karst?
A
-large rock pillars are the remnants of a highly eroded landscape
14
Q
What are disappearing streams?
A
-a stream on the surface that disappears into a cave
15
Q
A
16
Q
What are springs?
A
-an area where groundwater discharges at the surface
17
Q
What is permafrost?
A
- permafrost forms in areas where the annual mean temperature is below 0 degrees celsius
- no trees in continuous permafrost (continuous- over 80% of the ground is frozen)
18
Q
What is a talik?
A
-area beneath lake without permafrost
19
Q
What is thermokarst?
A
- an irregular surface produced by thawing permafrost
- in some areas, a layer of soil near the surface refreezes in winter and thaws in summer
20
Q
What is piping?
A
- this is caused by groundwater percolating through loose sediment
- groundwater can carry slit and sand laterally through the soil to a discharge point (a spring)
- piping creates underground tunnels that could collapse to form depressions and ravines
21
Q
Describe sediment compaction with fine sediment
A
- land may slowly compact over time as water in the soil pores decreases
- at river deltas this compaction is typically balanced by new sediment deposited from the river (therefore subsidence would not occur)
- however concrete levees built to prevent flooding can also prevent new sediment from being deposited (eg. Mississippi River Delta)
22
Q
Describe sediment compaction with collapsible sediment
A
- some soils have large amounts of pore space and soil grains that are loosely bound together
- large amounts of infiltrating water can dissolve minerals which aided in holding the soil together
- widespread areas of the soil are then prone to collapse
23
Q
Describe sediment compaction with organic sediment
A
- these are associated with wetlands (marshes, swamps, etc.)
- the soil contains a large amount of water and if this water is lost, there is a great reduction in thickness
- droughts combined with water extraction for agriculture and urban development has resulted in the Florida Everglades subsiding by up to 3m over the past century
24
Q
What are expansive soils?
A
- these soils easily expand during wet periods and shrink during dry periods
- they are common in clay-rich soil
- as water in the soil dries, dessication cracks may be left behind
- damage is evident in the form of cracks in concrete and asphalt
- soil in Southern Ontario is expansive
25
26
What regions are at risk for karst landscape?
-Karst landscapes are found in approx 25% of NA
27
What are the effects of subsidence?
Sinkhole formation: natural and artificial fluctuations in the water table trigger most sinkhole collapses, sinkholes can cause considerable damage to highways, utility pipelines, and sewage lines
Groundwater contamination: caves create direct access between surface water and groundwater, this access can make groundwater vulnerable to contamination, particularly where polluted streams flow into caves and fractures
Permafrost thaw: melting of permafrost can cause roads and railways to buckle and buildings to fracture, homes in permafrost areas are often built on stilts
Changes in soil volume: the shrinking and swelling of expansive soils have similar effects as the freezing and thawing of permafrost, soil contraction produces deep cracks and soil expansion creates an irregular land surface
28
What is being done to prevent loss of wetlands?
- concrete levees along the Mississippi River are restricting natural flooding events in the wetlands along the river
- these floods could provide a natural service function by depositing sediment
- therefore the levees are preventing wetlands from being replenished with new sediment
- this along with the extraction of groundwater is leading to subsidence
- 25% of wetlands in Louisiana have disappeared because of subsidence
- disappearing wetlands have caused species extinction and are leaving New Orleans more vulnerable to flooding from hurricanes
- 50 year $14B wetland restoration effort began in 2002
29
How is flooding linked to other natural hazards?
- flooding is a concern in areas that are rapidly subsiding
- permafrost thaw and changes in soil volume can lead to landslides
- climate change is enhancing droughts in some areas leading to soil dessication
- the polar regions of the Earth are currently warming the most due to climate change; this is leading to rapid permafrost melt
30
What happened with groundwater extraction in Venice and Pisa?
- at one time the cities of Venice and Pisa were subsiding by up to 2mm per year due to groundwater extraction
- Venice is built on 17 islands surrounded by canals
- There are 78 flood gates to prevent water from entering the city from the Adriatic Sea
31
What are natural service functions of karst and caves?
- nearly 25% of the world's population obtains drinking water from karst landscapes
- cave systems are popular tourist and recreation areas
- unique creatures have adapted to live in cave environments
32
How do humans interact with subsidence?
- people living in karst areas can enhance existing subsidence problems through many actions
- the best example of this is by extracting groundwater from the soil in dry areas
- underground mining of coal and salt have led to subsidence
- salt in ON is mined from rock located under Windsor and Goderich
- Constructing buildings on permafrost land and levees along rivers have both led to subsidence
- poor landscaping practices can cause subsidence in areas with expansive soils
33
What are ways subsidence hazards can be minimized?
- when pipelines are constructed in permafrost areas, special precautions are necessary
- the Trans-Alaska Oil Pipeline was specially engineered to prevent permafrost melt
34
What are adjustments to subsidence?
- widespread areas of NA are underlain by karst and thus simply preventing development in these areas is not possible
- subsidence is unpredictable but detailed mapping of soil and rock type is important to identify higher-risk locations
- Geophysical surveys by Ground Penetrating Radar and drilling and soil testing are required before laying building foundations
35
What are rivers?
- rivers are part of the hydrologic cycle
- they transport water that has fallen onto land toward the oceans
- if precipitation does not infiltrate into the soil, it flows along the surface (this is called runoff)
- eventually the runoff will reach a stream (if it does not evaporate first)
- streams are tributaries of rivers
36
What are drainage basins?
- every river has a drainage basin (the area of surrounding land that is drained by that river)
- drainage basins can also be defined for lakes
- in that case, it is defined as the area of land that drains into that lake
37
38
What are divides?
- divides form the boundaries of drainage basins
- a divide is a ridge of higher land that separates which way rivers flow
39
What is a continental divide?
- the continental divide is located along the spine of the Rocky Mountains
- it separates which ocean the streams and rivers will eventually drain into
40
What is the river gradient?
- it is determined by calculating the change in elevation over the distance the river flows
- the slope of a river is shown on a longitudinal profile
- in higher elevations, the gradient is often steep (eg. at the headwaters of the river)
- the gradient is most gentle where the river reaches base level (where it empties into an ocean or lake)
41
What are types of river transport?
Bed Load: materials that roll, slide, or bounce along the bottom (eg. sand, pebbles)
Suspended Load: silt and clay particles carried in the water, it accounts for 90% of the total load of most rivers
Dissolved Load: material derived from chemical weathering of rock and sediment within the drainage basin
42
What is discharge?
- the volume of water flowing past a point in a river over a specific period of time
- Q=VA
Q= discharge, V= velocity, A= cross sectional area
43
How are rivers involved in erosion and deposition?
- rivers play a major role in sculpting the landscape
- the amount of erosion and deposition changes depending on velocity and sicharge
- when the river slows at its mouth, it deposits either an alluvial fan (on land) or a delta (in water)
44
What are channel patterns?
-streams and rivers flow in channels; there are 3 types of channel patterns:
Braided: a large number of channels are evident
Anastomosing: two or more channels that have intervening islands where sediment is temporarily stored. Islands are more stable than those in braided channels and are formed of silt or clay that may support vegetation.
Meandering: a single channel that is shaped like a snake
45
Describe braided channels
- the numerous islands between these channels are made of sand and gravel and lack vegetation
- these channels tend to be wider and more shallow than the other types
- braided channels often develop in an area where there is abundant bed load from melting glaciers
46
What are meandering channels?
- meanders are bends in a channel
- velocity is greatest on the outside of the curves thus causing erosion and the formation of a cutbank
- slower water on the inside of the curves causes deposition and the formation of a point bar
- over time, the meanders get wider and the river lengthens
- meanders are bends in a channel
- a floodplain in the area of flat land adjacent to a channel that is subject to natural flooding
- during a flood, a meandering river may avulse (water leaves the channel and cuts a new channel that forms a shorter route to a meander bend)
47
What is a pool?
-a deep section of a river where water moves slowly
48
What is a riffle?
-a shallow section of river where water moves rapidly over a gravel bed
49
What is an avulsion?
- during avulsion, the river abandons a looping section of the old channel
- the abandoned section is called an oxbow lake
50
What are oxbow lakes?
- the meandering Mississippi River has created hundreds of oxbow lakes
- this has created geographic anomalies because state borders were originally defined by the river
51
What determines the magnitude and duration of a flood?
- precipitation in the drainage basin
- rate at which precipitation infiltrates the soil
- presence of a snowpack
- amount of moisture already in soil
- speed of surface runoff into a river
- ice jams on a river
- most floods in NA occur in spring
- this is a result of ice jams, a melting snowpack, and rain
- a flood officially begins when a river achieves bankfull discharge (the discharge at which point water first flows out of the channel)
52
What is the stage?
- the level (height) of the river surface
- flood stage: the level the river surface must reach in order for it to cause property damage
53
What is a hydrograph?
-a graph showing changes in discharge or changes in stage over time
54
What is a recurrence interval?
-the average time between floods of a certain size
55
What are regions at risk for flooding?
- flooding can occur along any stream or river and is therefore the most widespread natural hazard
- developing countries suffer a greater loss of life due to the lack of monitoring capabilities
56
What are natural service functions of flooding?
- floodplains are very fertile lands
- periodic deposits of nutrient by floods enriches the soil for agriculture
- floods flush out debris from channels thus having a positive effect on aquatic ecosystems
- floods keep the surface of river deltas above sea level and can sustain wetlands by adding sediment
57
How do humans interact with flooding?
- urbanization increases the magnitude and frequency of floods
- urban areas have impervious land cover and storm sewers that can quickly transport water to rivers
- this decreases the lag time between a heavy rain event and flooding
58
How is the flood hazard minimized?
Physical barriers: these include both earthen and concrete levees, floodways, and storm-water retention basins
-breaks in levees can cause high energy flows of water that quickly inundate an area
Channelization: this involves straightening, deepening, or widening existing river channels
- it is used to control floods and maintain navigable river channels
- it generally has negative impacts on fish and plant habitats
59
What is channel restoration?
- the objective is to return streams that have been modified back to their natural state
- the stream is allowed to meander and bends are stabilized with large stones known as riprap
60
How do people adjust to flood hazard?
- floodplain management plans are developed at the municipal level of government
- cities and towns prone to the hazard often have detailed flood hazard maps
61
What is floodplain regulation?
-the aim is to develop zoning bylaws that minimize flood damage
62
What is the Flood Damage Reduction Program?
-this program was developed by the Canadian federal government in 1975
- it operates under 2 main agreements among federal and provincial governments:
- disaster assistance will not be provided for a development built in a designated flood-risk area
-provinces and territories will ensure that municipalities zone on the basis of flood risk
63
Expansive soils expand and contract due to changes in what?
-water
Natural Disasters
flashcards
Decks in class (11)
# Cards
