The dams in the Mississippi river are a hard flood defence. There are two different types of dams on the Mississippi River, wing dams and closing dams. Wing dams are built close together with brush and stone structures that extend from the river bank to the channel and closing dams are used to block connections between the main channel and side channels of the floodplains. The advantages of building dams on the Mississippi are that they enhance the electricity generating capacity for the surrounding regions and the river won’t dry up but just waits for more water to arrive.
The disadvantage of building dams on the Mississippi is that it could cause flooding if there are too great amounts of water. The flow of major tributaries in places such as Missouri, Ohio and Tennessee has been controlled by a series of dams. The upper st Anthony falls dam is situated on the upper course of the Mississippi river. It turns the current flow into surface storage, and holds back water to form navigation pools. Levees were designed to protest the populated areas by restricting current flow to avoid floods.
They were first natural creations by the rivers fluvial processes, and were increased in size and strength to try to eliminate the flooding problem. They work by creating a higher and stronger barrier so that when the river is flooding the water is still held in place, and doesn’t spill out onto the flood plain. The extension of the levees along the Mississippi, and of its tributaries, means that water is more confines, so although it reduces flood risk upstream, due to the water rising higher and flowing faster, the flood risk downstream is increased.
Dredging can be used to create a deeper channel so that greater volumes of water can pass through, however flood risk is increased downstream. This can be seen at numerous points along the Mississippi river, and also where the upper st Anthony falls dam is, dredging has been used to create a deeper navigation pool of water. Straightening a river meander shortens the rivers course. This causes an increase in gradient and velocity, which moves floodwaters away more quickly, reducing floodrisk upstream, however increases floodrisk downstream.
On parts of the Mississippi river the riverbed has been lined with concrete to reduce friction. This restricts floods by creating a smoother channel for faster flow. Once again floodrisk is transferred downstream. Between 1934 and 1945 one stretch of the Mississippi was reduced from 530km to only 230 km. Wing dykes are rock like structures that extend from the river bank outwards onto the river channel. The purposes of these are to direct flow into the main navigation channel away from banks or side channels, to reduce erosion along the sides of the riverbed.
Wetlands act as vast areas of surface storage, preventing floodwater from reaching the main river channel. Southern Louisiana contains 40-45% of the wetlands found in the lower state, as Louisiana is the drainage gateway to the Gulf of Mexico for the lower Mississippi. The Lower Mississippi Regional Watershed drains more than 24 million acres (97,000 km?) in seven states from southern Illinois to the Gulf of Mexico. The coastal wetlands found in Rosedale, Louisiana, which stretch from the border to the Texas state line, are beneficial to humans and the regional ecosystem for a variety of reasons.
Wetlands provide flood control, water purification, storm buffer, wildlife habitat, nursery grounds for aquatic life, and recreational areas. These estuaries experience a natural subsidence of their soil materials as the mud sinks under its own weight and is replaced with water. However beginning in 1930, an extensive levee system aided by locks and dams was developed in the waterways of the lower Mississippi Regional Watershed. The levees, designed to prevent flooding along the waterways, direct drainage water directly into the Gulf of Mexico and also the river’s silt is directed from its mouth directly into the Gulf of Mexico.
So with no new accretion and with loss of structure below, the wetlands subside and are lost. Afforestation is the replanting of trees and plants along the river banks. Afforestation means an increase in interception which causes an increase in transpiration. More leaf litter increases soil moisture storage so it is less likely that there will be soil saturation and low surface runoff. This then produces a non flashy storm hydrograph, as there is a gentler rising limb, so therefore our river is very unlikely to flood.
This has occurred at the marginal agricultural land in the Lower Mississippi Alluvial Valley. In some cases levees are breached intentionally. This can be done to protect other areas or to give back land to nature. In most cases an intentional breach is not without discussion since valuable land is given up. During the Great Mississippi Flood of 1927 a levee was blown up with dynamite to prevent the flooding of New Orleans. We call these intentional levee breaches spillways.
These work by reducing the amount of channel flow by purposely allowing the water to flow onto farmland (usually farmland that isn’t owned by anybody), which increases surface storage on the floodplain. This therefore reduces discharge, which gives us a reduced risk of flooding in the town downstream, as a non flashy storm hydrograph is created, with a gentler rising limb. On the Mississippi river the Bonnet Carre Spillway has been constructed to the north or New Orleans, to divert floodwater into Lake Pontchartrain.