Ch 13-4 Floodplains and floods

Geology

Pages 290-292

Flood and Floodplain

• A river may overflow its bank as a flood

• This floodwater may cover part or all of the valley floor – called the floodplain

Formation of a floodplain

• As a river approaches its base level, its gradient and velocity decrease.

• The river current is more easily deflected sideward, and the river becomes more winding.

• This winding river slowly erodes away the valley walls, and a floodplain is formed.

Features of a floodplain• River tends to wind back

and forth in broad curves called meanders

• The current erodes away at the outside of the curve, and the channel is deepened there.

• Swift current then is shifted to the other side of the bank forming a series of meanders

Meandering rivers constantly change

• Rivers erode away at the outside of the curve, forming wide loops that the river can then break through during a flood.

• This forms a cutoff connecting two adjacent curves

• Sediment is then deposited in the abandoned meander, and an oxbow lake is formed.

Mississippi river channel meanders in history

Meanders helps what industry?

• When a river floods onto its floodplain, the velocity of the river slows and its sediment load is deposited. – Thick deposits of build up along side the river

banks called natural levees

• Beyond the levees the floodplain slopes away from the river, – In the lowest areas in the floodplain, swamps

may form and are called back swamps

• New tributaries may also form in the back swamp areas, and are called yazoo tributaries

Floods

• Benefits:

– Temporarily relieve the water and sediment overload of a river channel

– Deposit minerals and nutrients onto the floodplains, making these areas highly fertile agricultural areas

Floods

• Consequences:– Destructive for people living in the floodplains– Change paths of rivers

• Most rivers flood from heavy or long-lasting rain events or the rapid melting of winter snows.

• Small rivers or streams in young narrow river valleys can flood quickly from a single cloudburst creating a flash flood

• Large mature rivers do not have flash floods

• Springtime is vulnerable for flooding because the ground may still be frozen and only allows water to flow over its surface.

• Man has made dams across rivers to:– make reservoirs – Generate hydroelectric power for industry– To turn turbines for industry like grist mills– To control flooding along the entire river

Dams form naturally:

• Ice jams

• Lava flows

• Volcanic ash deposits

• Dams from landslides

• Dams formed by Beavers

Human caused floods

• Floods also are caused by failure of reservoirs to hold back water

• Removal of backswamps (wetlands) does not allow the excess water to be absorbed as a natural sponge, sending the excess water downstream

• Pavement and buildings do not absorb water

Preventing floods

Replanting vegetation in the headwaters region of the river

Returning backswamp and wetlands to its original state

Using a series of dams along the river are effective preventing floods for a while…

Problems with dams

• Sediment builds up behind a dam

• Less deposition on a floodplain means more erosion when it actually floods

• Dams will damage the natural ecosystem of the area– Sturgeon now are only able to get to the

Shawano dam, and not into Menominee area

Artificial levees

• Artificial levees can be used to limit flooding in one area, but they may cause more damage to areas downstream due to increased height of river – which in turn increases the velocity of the river.

spillways

• Spillways have been created in the lower mississippi river to avert excess water away from the main river and reduce the risk of flooding.– New Orleans is 10 feet under sea level in

some locations