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Earth Science - Lecture 35.docx

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Earth Sciences
Sergei Basik

November 26, 2013 FLOODS AND HUMAN INTERACTIONS Development Effects on Floods  Early towns established along rivers for access to water and transportation  Severe alterations of landscape through urbanization, logging, grazing, forest fires  Added sediment load to rivers  Increased size and damaging nature of floods Urbanization  Increasing urbanization promotes increasing numbers of flash floods and higher flood levels  Wherever roads and buildings cover landscape, water is forced to run off rapidly into nearby streams (sometimes through concrete channels)  Driving through flooded roadway can be dangerous (even fatal) because: o Force of water against car can wash it downstream o Deep erosion may not be visible underwater o Difficult to escape from submerged and sinking car Fires, Logging, Overgrazing, and Mining  Deforestation of watershed by fire, heavy logging or overgrazing caused increased sediment load  Streams become choked with sediment, braided, steeper  Fire: o Removes soil protection o Decreases soil permeability  Clear-cut logging: o Methods remove brush o Form skid trails and logging roads that focus downslope drainage and add sediment to streams  Grazing: o Removes surface vegetation o Increases soil erosion  Mining: o Can change sediment load o Disrupts the structure of the channel Bridges  Raise roadway above 100-year flood level  Partial dam across floodplain with narrow open channel under roadway bridge  Deep water under bridge: o Flows faster o Causes erosion o Undermines pilings supporting bridge November 26, 2013 Levees  Levee: o Artificially raise riverbanks o Most common response to protect area from flooding  Almost always built on top of original natural levee at edge of stream channel  Usually made with locally available, fine-grained sediments that are easily eroded in floods  Higher-quality levees mix coarse gravel or face with riprap to resist erosion  Levee breach can lead to avulsion: redirection of main channel through breach into lower path from which river can not escape  Failures can also occur from: o Bank erosion from river currents or waves o Slumps into channel o Piping or seepage through levee, sometimes reaching surface as sand boil  Every time a levee is built, also reduces width of flood-flow part of river and raises water level during flooding  Levees act to: o Eliminate storage of floodplain o Constrict flow and r
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