part 6 classic flood plains. flood plains flood plains are those alluvial valleys that are...
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![Page 1: Part 6 CLASSIC FLOOD PLAINS. FLOOD PLAINS Flood plains are those alluvial valleys that are periodically subject to inundation by flooding of a natural](https://reader036.vdocuments.us/reader036/viewer/2022062320/56649d7e5503460f94a61be8/html5/thumbnails/1.jpg)
Part 6Part 6
CLASSIC CLASSIC FLOODFLOODPLAINSPLAINS
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FLOOD PLAINSFLOOD PLAINS
• Flood plains are those alluvial valleys that are Flood plains are those alluvial valleys that are periodically subject to inundation by flooding of periodically subject to inundation by flooding of a natural river. 75% of the sediment deposited on a natural river. 75% of the sediment deposited on our continent is flood plain silt. our continent is flood plain silt.
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• Natural levees accumulate along the banks of rivers and streams that periodically flood, as sketched here.
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• Upper sketch shows a typical cross section through the lower Mississippi River. Note how the channel surface is higher than the surrounding flood plain.
• Lower sketch show the areal distribution of overbank flood sediments with proximity to the source channel through hydraulic sorting
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• Low gradient rivers are commonly typified by extreme sinuosity, due to entropy (conserva-vation of energy); shown in upper figure
• Flood plains adjacent to low gradient rivers are periodically subjected to inundation, as in lower figure
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Flood StageFlood Stage
• Satellite images comparing conditions before and during the 1993 flooding of the lower Missouri River near its confluence with the Mississippi River just above St. Louis
• Note the enormous land area that was inundated
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Asymmetry of Asymmetry of channel channel
affects bank affects bank under cuttingunder cutting
• Low gradient channels are seldom straight
• Their curvature causes the channels to be over-steepened on the outer side of sharp bends, causing bank undercutting
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• Low gradient channels meander to conserve energy through a process called entropy.
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• Oxbows are lakes are old meander bends that are truncated and isolated when the river cuts back into its own channel, as sketched here.
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Sinuous Low Sinuous Low Gradient Gradient StreamStream
• Oxbows and cutoffs are common features of sinuous, low gradient channels
• Very treacherous to build on these
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Abandoned Abandoned channelschannels
• Engineering geologic map of the confluence between the Ohio and Mississippi Rivers near Cairo, Illinois
• Abandoned channels are shown in blue while the active channels are white