Explain the formation of flood
plains
Answers
Answer:
floodplain is an area of land which is covered in water when a river bursts its banks. Floodplains form due to both erosion and deposition. Erosion removes any interlocking spurs , creating a wide, flat area on either side of the river.
Answer and Explanation:
Formation of Flood Plains
Most floodplains are formed by deposition on the inside of river meanders and by overbank flow.
Wherever the river meanders, the flowing water erodes the river bank on the outside of the meander, while sediments are simultaneously deposited in a point bar on the inside of the meander. This is described as lateral accretion, since 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 changing significantly in width. The point bar is build up to a level very close to that of the river banks. Significant net erosion of sediments occurs only when the meander cuts into higher ground. The overall effect is that, as the river meanders, it creates a level flood plain composed mostly of point bar deposits. The rate at which the channel shifts varies greatly, with reported rates ranging from too slow to measure to as much as 2,400 feet (730 m) per year for the Kosi River of India.
Overbank flow takes place when the river is flooded with more water than can be accommodated by the river channel. Flow over the banks of the river deposits a thin veneer of sediments on the floodplain that is coarsest and thickest close to the channel. This is described as vertical accretion, since the deposits build the floodplain upwards. In undisturbed river systems, overbank flow is a frequent occurrence, typically occurring every one to two years regardless of climate or topography. Sedimentation rates for a three-day flood of the Meuse and Rhine Rivers in 1993 found average sedimentation rates in the floodplain of between 0.57 and 1.0 kg/ms. Higher rates were found on the levees (4 kg/m2 or more) and on low-lying areas (1.6 kg/m2).
Sedimentation from overbank flow is concentrated on natural levees, crevasse splays, and in wetlands and shallow lakes of flood basins. Natural levees are ridges along river banks that form from rapid deposition from overbank flow. Most of the suspended sand is deposited on the levees, leaving the silt and clay sediments to be deposited as floodplain muds furthe from the river. Levees are typically build up enough to be relatively well-drained compared with nearby wetlands, and levees in non-arid climates are often heavily vegetated.
Crevasses are formed by breakout events from the main river channel. The river bank fails and floodwaters scour a channel. Sediments from the crevasse spread out as delta-shaped deposits with numerous distributary channels. Crevasse formation is most common in sections of rivers where the river bed is accumulating sediments (aggrading).
Repeated flooding eventually builds up an alluvial ridge, whose natural levees and abandoned meander loops may stand well above most of the floodplain. The alluvial ridge is topped by a channel belt, formed by successive generations of channel migration and meander cutoff. At much longer intervals, the river may completely abandon the channel belt and begin building a new channel belt at another position on the floodplain. This process is called avulsion, and takes place at intervals of 10-1000 years. Historical avulsions leading to catastrophic flooding include the 1855 Yellow River flood and the 2008 Kosi River flood.
Floodplains can form around rivers of any kind or size. Even relatively straight stretches of river are found to be capable of producing floodplains. Mid-channel bars in braided rivers migrate downstream through processes resembling those in point bars of meandering rivers and can build up a floodplain.
The quantity of sediments in a floodplain greatly exceed the river load of sediments. Thus, floodplains are an important storage site for sediments during their transport from where they are generated to their ultimate depositional environment.
When the rate at which the river is cutting downwards becomes great enough that overbank flows become infrequent, the river is said to have abandoned its floodplain, and portions of the abandoned floodplain may be preserved as fluvial terraces.