Floodplains and FloodsFloodplains and Floods

Key idea: Key idea: River floods are naturally River floods are naturally

occurring events that sometimes occurring events that sometimes threaten human populations.threaten human populations.

Learning Objectives:Learning Objectives:

1.1. Describe the features of a Describe the features of a floodplain.floodplain.

2.2. Explain why floods occur.Explain why floods occur.

3.3. Summarize natural and artificial Summarize natural and artificial methods of flood prevention and methods of flood prevention and control.control.

Features of a FloodplainFeatures of a Floodplain

• The The floodplain floodplain of a river is the valley that of a river is the valley that is covered with water when the river is covered with water when the river overflows its banks. overflows its banks.

• As the river approaches the base level, its As the river approaches the base level, its gradient and velocity decrease. The river gradient and velocity decrease. The river current is more easily deflected sideward, current is more easily deflected sideward, and it course becomes winding. and it course becomes winding.

• The banks and the valley walls are slowly The banks and the valley walls are slowly eroded by the river and valley floor widens eroded by the river and valley floor widens into a floodplain.into a floodplain.

FloodplainFloodplain

FloodplainFloodplain

MeandersMeanders

• A river flowing through a floodplain typically A river flowing through a floodplain typically forms broad curves, called forms broad curves, called meanders. meanders.

• Meanders form because the erosion is most Meanders form because the erosion is most rapid on the outside of a river bend. In rapid on the outside of a river bend. In time, the channel is deepened there. time, the channel is deepened there.

• Swift currents send the water across the Swift currents send the water across the channel and downstream into the opposite channel and downstream into the opposite bank. The process repeats itself, forming a bank. The process repeats itself, forming a series of meanders.series of meanders.

How Meanders FormHow Meanders Form

Meanders Change Meanders Change ConstantlyConstantly• Meandering rivers change all the time. As a Meandering rivers change all the time. As a

meander swings wider and wider, it becomes a meander swings wider and wider, it becomes a loop that the river can break throughloop that the river can break through

• The river usually breaks through during a flood, The river usually breaks through during a flood, forming a cutoff. The river then deposits mud and forming a cutoff. The river then deposits mud and silt at the ends of the abandoned meander. silt at the ends of the abandoned meander.

• A curved body of water formed when the deposits A curved body of water formed when the deposits separate the meander from its river is an separate the meander from its river is an oxbow oxbow lake.lake.

• During a flood the river carries large amounts of During a flood the river carries large amounts of sediments and deposits them on the banks of the sediments and deposits them on the banks of the river, forming river, forming natural levees.natural levees.

Meanders and Oxbow LakeMeanders and Oxbow Lake

FloodsFloods

• Floods are naturally occurring events Floods are naturally occurring events which temporarily relieve the water and which temporarily relieve the water and sediment overload of a river channel.sediment overload of a river channel.

• Most floods occur due to long lasting Most floods occur due to long lasting rains, rapid melting of snows, or both. rains, rapid melting of snows, or both.

• Small rivers, with narrow floodplains Small rivers, with narrow floodplains can overflow after one heavy rain, can overflow after one heavy rain, creating a creating a flash flood. flash flood.

Floods Created by Large Floods Created by Large Rivers Rivers

• Large rivers such as the Mississippi, Large rivers such as the Mississippi, Ohio, Missouri, create floods after Ohio, Missouri, create floods after many days of steady rainfall over many days of steady rainfall over large parts of their vast drainage large parts of their vast drainage system. Also, floods occur in the system. Also, floods occur in the spring time, when the ground is still spring time, when the ground is still frozen, increasing run-off, and snow frozen, increasing run-off, and snow thaws rapidly. thaws rapidly.

Humans Activities and Humans Activities and FloodsFloodsAlthough most floods have natural Although most floods have natural

causes, some human activities can causes, some human activities can worsen or even create floods. Such worsen or even create floods. Such activities are:activities are:

• Building dams for reservoirsBuilding dams for reservoirs• Urban development increases the runoffUrban development increases the runoff• Removal of vegetationRemoval of vegetation• Agricultural activitiesAgricultural activities• Modifications of the course of the river Modifications of the course of the river

Flood in Caracas Venezuela Flood in Caracas Venezuela (Dec. 19(Dec. 19thth 1999) killed over 1999) killed over 10,00010,000

Flood in Cedar Rapids, Iowa in Flood in Cedar Rapids, Iowa in June 2008June 2008

Flood Prevention and Flood Prevention and ControlControl• People rely on many different People rely on many different

measures to control and prevent measures to control and prevent floods. Each of these measures has its floods. Each of these measures has its own advantages and disadvantages. own advantages and disadvantages.

• One good way to that people can One good way to that people can lessen the impact of flooding is to lessen the impact of flooding is to restore natural flood protection that restore natural flood protection that human activity has altered. human activity has altered.

Flood PreventionFlood Prevention

• If vegetation has been removed, If vegetation has been removed, replanting helps to control runoff. replanting helps to control runoff.

• Dams are built to control floods, but in Dams are built to control floods, but in long run they affect the natural balance of long run they affect the natural balance of the river, because they trap in all the river, because they trap in all sediments. Without deposition, the erosion sediments. Without deposition, the erosion accelerates downstream, and this worsen accelerates downstream, and this worsen floods. Dams severely alter the rivers floods. Dams severely alter the rivers ecosystems.ecosystems.

Flood Control DamFlood Control Dam

Spillways Spillways

• Spillways are channels that are parallel to Spillways are channels that are parallel to the main course of a river. the main course of a river.

• At critical points on the floodplain the At critical points on the floodplain the water is guided into the spillways, and water is guided into the spillways, and help control floods.help control floods.

• On the lower Mississippi River there are On the lower Mississippi River there are numerous channels running parallel to the numerous channels running parallel to the main river through the back swamps that main river through the back swamps that then flow into the Gulf of Mexico.then flow into the Gulf of Mexico.

Flood Control on Mississippi Flood Control on Mississippi RiverRiver

Flood Control on Mississippi Flood Control on Mississippi RiverRiver