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EARTHSC 2GG3 (199)
Lecture

Earth Science - Lecture 31 Notes.docx

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Department
Earth Sciences
Course
EARTHSC 2GG3
Professor
Sergei Basik
Semester
Fall

Description
STREAMS AND FLOODING PROCESSES Groundwater, Precipitation and Stream Flow  Some of precipitation in an area percolates through soil to become groundwater and is collected by rivers and streams  Gaining streams are fed by groundwater  Losing streams lose water into the ground Gaining and Losing Streams  Wet climates: groundwater flows into gaining streams, ensuring year-round flow  Dry climates: water from streams feed groundwater  Losing streams may dry up between rainstorms  Areas of moderate to high annual rainfall: o Groundwater continuously feeds streams o Changes in precipitation result in changes to groundwater levels but stream inflow is generally constant  Semi-arid to arid regions: o Water from losing streams sinks into ground and streams may dry up between storms o Flash floods can occur after any major or prolonged rainfall Precipitation and Surface Runoff  Runoff: liquid water that travels over the surface of the Earth, moving downward due to gravity  Some precipitation flows as surface runoff during torrential rainfall (rapid runoff is overland flow)  Ability of ground to absorb rainwater depends on: o Rate of precipitation o Permeability of soil o Extent of prior saturation o Whether ground is frozen  Some regions are more prone to heavy rainfall: o Weather patterns such as major storm systems o Higher elevations and heavy snowfall Flooding Processes  Bankfull level: level at which water spills over banks  Streams generally reach bankfull every 1.5 to 3 years  Increase in discharge during flood involves increase in water velocity, water depth and stream width Changes in Channel Shape during Flooding  Channel scour: depth of sediment eroded during floods  Flood increases water velocity  increases frictional drag on stream bottom  causes more erosion  Waning flow as flood declines  coarser material in suspension drops out  deposition raises streambed  Slow water velocity at edge of deeper channel forms natural levee of sediments: o Nearly continuous low ridge along edge of channel o May keep small floods within channel Flood Intensity  Intensity of flood depends on: o Discharge of floodwater o Rate of rise of water  Va
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