Question

How are flood plains formed?

Answer 1

Hint: A floodplain or flood plain or flood plain is an area of land adjacent to a stream or river that stretches from the banks of its canal to the base of the enclosing walls of the valley and experiences flooding during periods of high discharge. The soils usually consist of clays, silts, sands, and gravel deposited during floods.

Complete Answer: Most floodplains are formed by deposition on the inside of river meanders and by overflow. Wherever the river meanders, the flowing water erosions the river bank on the outer side of the meander, while the sediments are simultaneously deposited in a point bar on the inside of the meander. This is described as lateral accretion, as the deposition builds the point bar laterally into the river channel. Erosion on the outside of the meander usually closely balances deposition on the inside of the meander so that the channel shifts in the direction of the meander without significantly changing the width of the meander. The point bar is built to a level very close to that of the river banks. Significant net erosion of sediment occurs only when the meander cuts into the higher ground. The overall effect is that, as the river meanders, the level flood plain is mainly composed of point bar deposits. The rate at which the channel shifts varies greatly, with reported rates ranging from too slow to as much as 2,400 feet (730 m) per year for the Indian Kosi River. The overflow occurs when the river is flooded with more water than the river channel can accommodate. Flow over the banks of the river deposits a thin layer of sediment on the floodplain, which is the largest and thickest in the vicinity of the canal. This is described as a vertical accretion since the deposits are building up the floodplain upwards. Flood plains are formed when the meander eroses sideways as it travels downstream. When a river breaks its banks, it leaves behind layers of alluvium (silt) which are gradually being built up to create the floor of the plain.

Note: Floodplains can be formed around rivers of any kind or size. Even relatively straight stretches of the river are capable of producing floodplains. Mid-channel bars in braided rivers migrate downstream through processes similar to those in point bars of meandering rivers and can build up a floodplain.