Net Submergence

 

Today, large increases of land loss in Louisiana have been attributed to flood control management strategies of the Mississippi River. However, subsidence is also influenced by sediment compaction, tectonic activity, and crustal sinking (Saucier, 1994). The Mississippi deltaic plain area has been affected by all five of these factors (Kolb and Van Lopik, 1958).


Picture of Levee on the Mississippi River courtesy of James
Addison, US Army Corp of Engineers
Presently the Mississippi River delta has two active deltas, the Belize delta and the Atchafalaya delta. Cahoon et al. observed that wetland loss is attributed to net submergence, a dominant process in the destruct ional phase of the deltaic cycle. Net submergence has three components including subsidence, sea level rise, and accretion of surface land through mineral and sediment deposition and organic matter production. It occurs in areas where subsidence and sea level rise exceed vertical growth (1995).

However, much of the Louisiana coast is undergoing net submergence because accretion is not able to counter the effects of subsidence and sea level rise through vertical growth. Submergence rates average 1 cm per year while vertical growth averages about 0.6 cm per year (Cahoon et al, 1995). These numbers do not take into account compaction that occurs in the upper few meters of the delta. This might imply that submergence rates are even greater as a whole because the potential for compaction is greater in the upper 2 meters of surface sediments (Turner 1991). Turner concluded that this is a product of organic decomposition, sediment compaction, and soil dewatering (1991).
Most observed submergence is not from eustatic sea levels rise (10-50%) but from subsidence (Penland at al., 1994). Subsidence rates are a reflection of the age and thickness of Holocene sediment deposits. The thickest Holocene sediment deposits have the greatest potential for compaction. Younger sediment deposits subside faster than older deposits because they have not been consolidated and compacted as extensively as the older deposits. Penland et al. has observed the thickest Holocene sediments (100 meters) are within the largest portion of the coast. Thus this area is subsiding at a faster rate than any other sections of the Mississippi Delta (Kuecher et al., 1993).
 
Map of Mississippi Delta with thickness of Holocene sediments in meters
Modified from Kuecher et al., 1995

 

 

Geomorphology
Net Submergence
Subsurface Fluid Extraction
Human Induced Subsidence
About the Author
References