The formation of levees and floodplains are linked and involve repeated flooding and the build-up of material during the period of flood. Under normal low conditions, the river is contained within its banks and so no sediment is available to form levees of the floodplain. However, during periods of high rainfall and discharge when the river has burst its banks, both of these features are formed.
Video showing the lower course features of the River Tay –
Video showing the lower course features of the River Severn –
In its middle and lower courses,a river is at risk from flooding during times of high discharge. If it floods, the velocity of the water increases and it overflows the banks. This results in deposition. It is usual for the coarsest (heaviest) material to be depoisted firest, froming small raised banks (levees) along the sides of the channel. Subsequent floods increase the size of these banks and further deposition on the bed of the river also occurs. This means that the river, with channel sediment build-up, now flows at a higher level than the floodplian. For this reason, the authorities sometimes strengthen levees and increase their height.
Floodplains are created as a result of both erosion and deposition, although the accumulation of river deposits suggests that they are predominately depositional features. They are relatively flat areas of land either side of the river, which form the valley floor in the middle and lower courses of the river. They are composed of alluvium – river deposited silts and clays. Over time, a floodplain becomes wider and the depth of sediment accretions increases. The width of the floodplain is determined by the amount of meander migration and lateral erosion that has taken place.
Over time, this results in the migration of meanders, leaving their scars clearly visisble on the floodplain. Interlocking spurs are eventually removed by lateral erosion in the middle course and this widens the valley. The depth of the alluvial deposits depends partly on the amount of flooding in the past, so floodplain creation is linked to extreme events.
Levee and floodplain formation