STORM WATER MANAGEMENT
SEDIMENT CYCLE
What Is The Sediment Cycle?
The Sediment Cycle describes the flow of sediment particles from bedrock to suspended sediment and back again. As with the hydrologic cycle, there is no true start or end. The process is an endless flux from one state to the next.
Detachment, Entrainment, Transportation and Deposition
The process of detachment is where sediment particles are first created. Some natural process: the falling of a raindrop, high gust of wind, or movement of glacial ice, causes some particles of soil to separate from the others. Once these particles are separated they become susceptible to the next step in the process.
Entrainment involves the lifting of the separated particles and suspension in the eroding medium: water, air, or ice. Entrainment is closely related to detachment, though there are subtle differences.
Transportation involves the movement of detached and entrained particles from their original location to some new location. The speed of transportation depends on the type of medium, size of the particle and various other factors.
As the transporting medium begins to slow, the sediment particles begin to fall. Eventually, the sediment particles stop moving all together. The settling of the sediment particles is called deposition. The cycle of detachment, entrainment, transportation and deposition can occur several times before the sediment particles eventually become part of a rock again.
Sediment and Pollution
Sediment particles themselves are one of the leading pollutants in the U.S. today. Erosion from construction sites, and eroded streambeds and banks can block waterways downstream. This causes localized flooding as drainage systems no longer function the way they were designed.
When sediments are suspended in water this produces another problem. Many substances that are harmful for people and the environment will bind to the sediment particles. This creates new problems.
Contaminated sediments present a greater risk for removal. Disturbing contaminated sediments offers the pollutants another chance at moving downstream. Contaminated sediments may also pose a risk to the workers responsible for removing the sediments. Sometimes, it is not know that sediments contain additional pollutants until the removal process has begun, and workers find themselves unprepared and unequipped for the risk.
Another problem posed by contaminated sediments is the slow release of the pollutants for long periods of time. Many pollutants remain in the environment for very long periods of time after they were originally released. Binding to sediments is one of the processes that assist this pollutant behavior.
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