A floodplain is a mostly flat area of land bordering a river that is subjected to periodic flooding. It is made up of silts and sands, which have been deposited over many years by the river. Whilst some deposition takes place within the river channel, the bulk of the sediment is deposited when the river floods. The reason for this is that as the river spreads over its floodplain, there is a sudden increase in both the wetted perimeter and the hydraulic radius. This results in an increase in friction, causing a decrease in velocity and the deposition of material previously held in suspension. The heaviest sediment will be deposited closest to the river, thus forming what is known as levees. A prominent slope known as the bluff line often marks the edge of the floodplain.
Once over the bank, the water lies for several days or weeks as a thin sheet on the flat land before it evaporates or drains away, leaving behind a fresh layer of sediment.
The reason that the name ‘floodplain’ is used, is that in this area the river will frequently burst its banks in times of high rainfall. Therefore sediment is deposited on the flood plain during times of flood, which enriches the soil. This makes flood plains very fertile land.
As meanders migrate down-valley, the floodplain may be broadened by the meanders, which continue eroding the valley sides. Also, successive flooding will cause the floodplain to build up in height. If the sea level falls and the river begins to cut into its own bed, the old floodplain may be left perched above the channel to form a river terrace. Many settlements have been built on terraces because they are less likely to be prone to flooding.
Benefits of floodplains:
Floodplains have many benefits, below are some of the main ones:
- Provide temporary storage for floodwaters
- Provide ideal settings for wetlands
- Improve water quality
- Offer green space that can be used as buffers, greenways or other functions
- Provide important habitat for wildlife
- Contain fertile soil ideal for crops
- Flat, so good for housing
Disadvantages of floodplains:
The main problem caused by floodplains is a large danger of flooding. If a flood occurs, there can be many disastrous consequences such as loss of crops, property and even life. It is very costly to repair the damage caused by floods and also for investing in flood protection schemes. However, many people in the world live on floodplains or in the immediate vicinity of them. It is therefore necessary to manage floodplains effectively.