Definition - What does Natural Levee mean?
Natural levees are embankments formed naturally after a river floods and recedes. Friction with the floodplain during a flood causes a decrease in the velocity of the river leading to the deposit of the material that the flood water is carrying. The deposits in natural levees contain mud, sand, and stones and are formed such that they slope away from either side of the river or flood plain.
Raised mounds are formed near the riverbank where the larger material is deposited, and the gently sloping sides are a result of the smaller particles deposited away from the riverbank. The water’s natural movement creates a levee by pushing the sediments aside as is seen in the banks of a river which are higher than the riverbed. Since levees form naturally parallel to the flow of the river, they also help direct the flow.
Levees form an important part of the United States infrastructure. Breaching of a levee can cause economic as well as personal losses.
Pipelines can be installed through such natural formations without disturbing them by using trenchless construction methods like horizontal directional drilling (HDD). HDD is launched from the surface and can be steered and guided below the earth’s surface in an arc to come out clear on the other side. This ensures that the embankments, slopes, or vegetation surrounding the levee are not disturbed in any manner.
Trenchlesspedia explains Natural Levee
Natural levees are commonly developed by alluvial streams flowing on flood plains. The levee is located immediately adjacent to the channel and forms a low wide ridge, which in some cases are so small that they cannot be seen. Where large levees are formed, the bed is also aggraded such that the bed of the channel is as deep as the channel was before the levee formed.
Some Processes of Levee Formation
There are several processes involved in natural levee sedimentation including the amount of inundation within the floodplain bottoms. When the water exits the channel and flows onto the floodplain there is a sudden drop in the flow velocity. This results in the immediate deposition of coarser sand and silt which is transported along the floodplain surface as bedload. The coarser sediments that travel along as bedload do not cover much distance as they encounter resistance from floodplain vegetation.
Sometimes the floodplain bottoms have been previously inundated by sources such as groundwater or local precipitation. When the overbank flood deposits are transported into them, a dominant turbulent diffusive mechanism is observed in the levee formation. This process occurs in floodplains that have flood basins with high water levels adjacent to the channel. When a flood occurs, a steep lateral sediment concentration gradient is produced between the channel and the flood basin and the sediment quickly falls out of suspension resulting in steep natural levees.
During a flood event, if the floodplain bottoms are not considerably inundated, the floodwater flows down the flank of the levee to low-lying land not significantly inundated. This results in natural levees with lower gradients and a curvilinear morphology.
The rate of sedimentation depends on the amount of sediment and the size of the sediment. The characteristics of the flood such as seasonality, duration, frequency, and magnitude also affect the sedimentation process. It takes hundreds to thousands of years for natural levees to form and is studied using soil analysis and by examining soil and sediments exposed at channel cutbanks.