CWPtionary - Understanding Sediment, Accretion and Subsidence

CWPPRA technicians, planners and engineers use a specialized vocabulary when discussing the restoration and protection of coastal wetlands. Three of the most commonly used terms are sediment, accretion and subsidence.

Sediment

Sediment is the primary ingredient used in building wetlands, whether naturally or artificially. Composed mostly of inorganic particles, such as sand, silt or clay, sediment is transported by a moving water source like a river or bayou. Sometimes these particles are also referred to as "suspended sediment" because they are trapped in fast-flowing water. As the water's speed decreases, the sediment settles to the bottom and accretes, or builds up. As sediment continues to accumulate, new land is formed.

Accretion

Accretion, also called sediment accretion, is the basic process in wetland formation. Under natural conditions, a river seasonally overflows its banks, carrying sediment-rich water into outlying areas. When the water later recedes, sediment is left behind on riverbanks and mudflats. As this process is repeated over time, the remaining sediments build the soil base for wetlands (see illustration below).

Modern levee systems, however, prevent this natural process, denying sediment to existing wetlands and making the creation of new wetlands impossible.

Subsidence

Subsidence is the process by which land surface becomes lower over time. As sediments compact, the level of the land will sink. This is a natural process, normally offset by accretion. However, if the rate of subsidence exceeds the rate of accretion - in other words, if the land sinks faster than it is built up - the affected land is said to suffer a sediment deficit. Eventually, land with a sediment deficit is flooded by rising sea water.

Because modern levee systems limit the rate of accretion, subsidence is occurring at an abnormal rate throughout much of coastal Louisiana