13.4 - Floodplains & Floods

Stages of Stream/River Development Young stream Steep Slope Mature stream Moderate Slope Old stream Gentle Slope http://www.dkimages.com/discover/previews/774/206778.JPG

Stages of River Development Young streams flow fast through steep valleys. They are found in mountainous or hilly regions and may have white water rapids and waterfalls. They have high levels of energy and erode the stream bottom more than the sides.

Mature Rivers Mature rivers flows down a more gradual slope with curves called meanders. The floodplain

Old-stage rivers flow very slowly through a very broad, flat floodplain that is curved. A river in this stage mostly erodes its sides causing changes in its meanders, like the Mississippi. Meanders can eventually be cut off forming Oxbow lakes.

Meanders

What can cause the banks of the Colorado River to be so steep? Drawing B shows that the ground has actually uplifted and the river is trying to maintain its original level relative to base level. A B

River Rejuvenation A fall in sea level or a rise in the level of the land can have a dramatic effect on a river's journey to the sea. The old age stage of the river becomes perched above the sea and therefore the river, with new-found energy, has to erode through the already deposited material to create a new route to the sea. The features formed as a result of this rejuvenation will deepen and straighten the river, have more youthful features.

Causes of Rejuvenation Climatic change: Glaciations and changes in rainfall. Tectonic change: Where land is uplifted after plate movement or volcanic activity. Changes can be positive sea level rises in relation to the land, or negative sea level fall in relation to the land.

Features of a Floodplain Wide valleys often include Floodplains Erosional floodplains Depositional floodplains Meanders Cut bank and point bar Cutoffs and oxbow lakes

Floodplain Features Meanders & Oxbow Lakes Stream Terraces Braided Streams Levees Back Swamps Yazoo Tributary

Flood Plains “Click to view animation”

Flood Plains Flood Plain – is a broad strip of land built up by sedimentation on either side of a stream channel. During floods, flood plains may be covered with water carrying suspended silt and clay. When the flood water recedes, these fine-grained sediments are left behind as a horizontal deposit. The sudden decrease in velocity of water leaving the channel causes the river to deposit most of its sediment near the main channel. Progressively less sediment is deposited away from the channel. A series of floods may build up natural levees – low ridges of flood-deposited sediment that form on either side of a stream channel. “Click to view animation”

Formation of natural levees by flooding Meandering Stream Flood over banks, sediment carrying water into wide area, greater drag, lower velocity, load drops

Erosion and deposition along a meandering stream Low v Meanders get more extreme with time. Deposition at point bar keeps up with erosion at cutbank

Channel moves in direction of cutbank Pointbar advances as crossbed drapes Cut off loops silt up as oxbow lakes Meandering Stream Old cut-off loop fills with mud, an oxbow lake OxBow Floodplain http://hays.outcrop.org/gallery/rivers/arid_meander?full=1

Evolution of a Meander to Oxbow: These are bends (loops) in the river with lateral (sideways) erosion of the river channel results in the river forming a winding pattern.

Where is cutoff likely to occur here?

An oxbow lake

Stream Terraces Remnants of former floodplain River adjusts to drop in base level by down cutting

River meanders across floodplain Base level drops, or region uplifts. Terrace forms in floodplain Further incision cuts a new terrace Why? Steeper gradient causes cuttoffs, stream straighter, cuts down narrower area.

A flight of river terraces

Braided Streams Occur where bed (sediment) load is very high. Often big boulders in the stream. Many channels because mid-channel bars split the stream Adjacent to mountains in high rainfall areas

A braided river in Alaska

braiding description - This is where the river channel is broken up into a number of distributaries.

Braiding Explanation: As a river slowly meanders across its flood plain, it often deposits material in the 'middle' of its channel. Sand and shingle banks often form small islands in this way.

Formation of a Levee

New Orleans Levee - Before Katrina

New Orleans Levee Breaks - After Katrina

Natural Levee Definition: These are mounds of alluvium (material deposited by running water) piled up along the river's edge, each time the river floods.

Natural Levee

Man-Made Levee A man-made levee reinforces the bank and helps prevent flooding. By confining the flow, levees can also increase the speed of the water. The simplest is just a mound of dirt bulldozed up when there is an advancing flood. These levees are not very good and can be breached by the power of the water. Properly designed levees (that may include specially compacted earthen mounds, reinforced concrete walls or other methods of solid construction) provide protection from river flooding to areas behind them. However all levees are at risk of failure from overtopping or breaching if the flood is bigger than what the levee was designed for or if the levee hasn't been properly maintained.

Why should people go to so much trouble and expense to prevent a river flooding ?

Other Floodplain Features:

Other Floodplain Features:

Back Swamps & Floodplain On a flood plain, a marshy area where floodwater may be confined between the valley walls. Back Swamp - poorly drained oval-shaped depressions located between the natural levees and the edge of the floodplain.

Yazoo Tributary Small tributary channel that is prevented from joining the main stream channel by the presence of levees. Yazoo tributaries tend to flow on the floodplain parallel to the main stream channel.

Flooding is a natural process that occur along river flood plains in response to heavy rains and/or the rapid melting of snow in the spring. Floods are described by recurrence interval A 100 year flood is one that can occur, on average, every 100 years. Flood erosion is caused by the high velocity and large volume of water in a flood. Relatively harmless on an uninhabited flood plain. Can be devastating to an inhabited area, such as a city on the flood plain.

Flooding Flash Floods – are local, sudden floods of large volume and short duration, often triggered by heavy thunderstorms. Controlling Floods Man-made structures can mitigate the dangers of floodwaters and sedimentation to river cities.

Controlling Floods

Controlling Floods Upstream dams – can trap water from a drainage basin and release it slowly after the storm.

Controlling Floods Artificial levees – built along a river bank can contain floodwaters within the river channel.

Controlling Floods Flood walls – may be used to protect cities from flooding.

Controlling Floods Bypasses – are used along a river to reduce the discharge in the main channel by diverting water into designated basins in the flood plain.