Chapter11-12 - Ch. 11-Stream and Flood Processes Causes of...

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Ch. 11--Stream and Flood Processes Causes of floods Ground conditions Non-ground conditions Types of floods Regional Flash Flood frequency, recurrence interval and probability Overview of a river/stream system Stream characteristics Discharge, load, channel pattern, gradient When sediment load or discharge is changed, a river will change its pattern or gradient to maintain a natural balance of erosion and deposition. Channel patterns and what conditions cause them Straight Braided Meandering Rivers change shape over time Floodplains: a natural part of the river system
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Floods Floods occur when precipitation or snow melt cannot be absorbed quickly enough by the ground excessive surface runoff Characteristics of the ground that contribute to surface runoff: Ground is already saturated from previous rainfall Ground is frozen Soils are hard-packed Subsurface layers of natural cements (ex: calcium carbonate)
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Floods Non-ground contributors to surface runoff Excessive rainfall (hurricanes, thunderstorms, etc) Rapid snowmelt from warmer-than-usual climate Shape of streambeds perturbed by previous flooding
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Flood Types Flash floods appear unexpectedly (sometimes only seconds of warning) Regional floods : occur after prolonged wet periods (example: Great Flood of 1993); sometimes seasonal Flash floods commonly induced by large thunderstorms in mountainous areas; surge of discharge is channelized by steep topography
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Flash Floods Highest flash-flood danger in semiarid, mountainous areas Desert surfaces tend to have hard-packed soils increases surface runoff Steep topography results in rapid discharge of surface runoff
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A Flash Flood from an Afternoon Thunderstorm Big Thompson Canyon, Northwest of Denver 1976/7/31 : 3-day celebration of Colorado’s centennial > 3,500 people in sparsely populated canyon Thunderstorms stationary over Big Thompson Canyon, dumping 30 cm of rain (3/4 of typical annual total) in 4 hrs Rapid runoff from mountain slopes became flash flood, 6 m deep wall of water moving through canyon at > 22 km/hr Highway road washed out, 139 people in cars died and more than 600 others never accounted for
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Flash flood example Venezuela Flash Flood and Debris Flow, 1999
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The Great Flood of 1993 Wet winter and spring saturated ground in northern Great Plains; heavy rainstorms continued for months Upper Mississippi River basin submerged for weeks – almost 100,000 square acres: 50 people died and more than $22 billion in damages
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The Great Flood of 1993 Tributaries backed up, even flowed upstream Largest flood on record for upper Mississippi by August Crushed rocks and sandbags used to raise level of levees Over 75% of locally- or state-built levees were damaged
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Flood Frequency and Recurrence Interval Large floods occur less often Small floods occur more frequently Each river has its own flood frequency curve
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Recurrence Intervals and Discharge
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This note was uploaded on 09/11/2011 for the course GEO 107 taught by Professor Stidham during the Spring '08 term at SUNY Stony Brook.

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Chapter11-12 - Ch. 11-Stream and Flood Processes Causes of...

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