Lecture 6: Subsidence Flashcards
Subsidence
-near vertical downward movement of Earth’s surface
Karst
-irregular landscape containing a lot of depressions
Types of Sinkholes
1) Solution Sinkhole:formed by acidic groundwater
2) Collapse Sinkholes: formed by collapse at surface
Ground Water Table
AKA Water table, the underground depth at which the ground is saturated with water
Flow Stone
General term for accumulation of Calcium Carbonate precipitated from water in a cave
Stalactite
deposit of Calcium Carbonate that extends downwards from the roof
Stalagmite
deposit of Calcium Carbonate on floor of cave
Column
feature formed when stalactite and stalagmite join together
Tower Karst
Large rock pillars are remnants of a highly eroded landscape
Disappearing Streams
stream that disappears into a cave
Springs
Area where groundwater discharges at the surface
Permafrost
Areas with annual mean temperature lower then 0 deg. celcius
Fine Sediment
- land compacts with low water in pores of soil
- balanced by sediment deposited from rivers
Collapsible Sediment
Soil has large amount of pore space; large amounts of water can dissolve minerals that hold soil together, and can collapse
Organic Sediments
soils contains much water; water decrease results in thickness decreasing
Expansive Soils
- easily expand/contract with we/dry periods
- dessication cracks can be left behind in clay rich areas
Effects of Subsidence
1) Sinkhole Formation
2) Groundwater Contamination
3) Permafrost Thaw
4) Changes in Soil Volume
Links of Subsidence to Other Natural Hazards
- flooding in areas of much subsidence
- permafrost or change in soil volume -> landslides
Natural Service Functions of Subsidence
- good drinking water from karst
- tourism in caves
- unique creatures in these landscapes
Human Interactions that Increase Subsidence
1) Mining Groundwater
2) Underground mining of coal/salt
3) Constructing buildings on permafrost land
4) Levees along rivers
5) Poor landscaping with expansive soils
Minimizing Risk of Subsidence
- pipelines above ground in permafrost areas
- test soil before building
- limit levees in less populated areas
- Use fill layer between building and expansive soils
- adjust to karst by knowing soil types and where risk is worse
Rivers
Carry water on land to oceans
-runoff, which is water that does not infiltrate soil, reaches streams is not evaporated
Drainage Basin
Area of surrounding lake that drains into that rive/lake
Divides
-Areas of higher land that seperates which way rivers flow
Continental Divde
-spine of rocky mountains that separate which ocean rivers flow into
River Gradient
Determined by calculating the change in elevation over the distance the river flows
Base Level
Where river empties into ocean/lake
Bed Load
materials that roll, slide, bounce along bottom of river; sand, pebbles
Suspended Load
silt/clay particles, 90% of total load
Dissolved Load
Derived from chemical weathering of rock in drainage basin
Discharge
Volume of water flowing past a point at certain time: Q=VA, Q= Discharge, V= Velocity, A=Area
Alluvial Fan
deposition of sediment from river onto land
Delta
deposition of sediment into water
Braided Channel
- large number of channels that are wide/shallow
- islands between made of sand/gravel, no vegetation
- abundant bed load from melting glaciers
Anastomosing Channel
2 or more channels with intervening islands
-Islands store temporary sediment, made of silt/clay with vegetation
Meandering Channel
single snake-like channel
- velocity of water greatest on outside[erosion], smallest on inside[deposition]
- meanders get larger and river lengthens
Floodplain
area of land adjacent to channel that is subject to flooding
Avulsion
-during a flood, water abandons the big loop and cuts an easier route, forming an ox-bow lake
Meander Scar
-dried up ox-bow lake
What does Magnitude/duration of flood depend on?
1) Precipitation in drainage basin
2) Rate at which precipitation infiltrates soil
3) Pressence of Snowpack
4) Amount of Moisture in soil
5) Speed of surface runoff in water
5) Ice Jams in river
Stage
Level of the river surface
Flood Stage
Level river surface must reach to cause property damage
Hydrograph
Graph that shows change in discharge/stage over time
Recurrence Interval
Average time between floods of certain size
Upstream Floods
-occur in upper part of drainage Basin
-produced by large amount of rainfall in small area
-occur in steep areas with little vegetation
ie: Flash Flood:
Sudden flood that causes a large amount in discharge
Downstream Floods
- produced by rain of long duration in large areas
- slow but substantial increase in discharge
- ie: Outburst Flood:
- sudden draining of lakes originally dammed by glacier or other material
Why Does Red River Flood so much
1) flows South to North
2) Impermeable Soil
3) North has been rising due to melting ice sheet= slower current
Links of Floods to Other Natural Hazards
landslides/Hurricanes/Volcanoes -> Floods
-floods -> fires/landslides through erosion
Natural Service Functions of Floods
- Floodplains are very fertile due to nutrient deposits
- floods clean debris from channels, help aquatic ecosystems
- sustain wetlands
Human Interactions with Floods
Dams: cause deposition upstream of dam, erosion downstream
Urbanization: increase magnitude/freq of floods, because ground is impenetrable and sewers bring water to rivers fast
Minimizing Floods
Physical Barriers: earthen/concrete levees, floodways, stormwater retention basins
Channelization: Straightening, deepening, widening existing channels
