Part 6Part 6

CLASSIC CLASSIC FLOODFLOODPLAINSPLAINS

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.

• Natural levees accumulate along the banks of rivers and streams that periodically flood, as sketched here.

• 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

• 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

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

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

• Low gradient channels meander to conserve energy through a process called entropy.

• Oxbows are lakes are old meander bends that are truncated and isolated when the river cuts back into its own channel, as sketched here.

Sinuous Low Sinuous Low Gradient Gradient StreamStream

• Oxbows and cutoffs are common features of sinuous, low gradient channels

• Very treacherous to build on these

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