indicates relative sea- level is rising 10 times faster in Terrebonne Parish 

 than the eustatic-sea-level rise average. 



The rapid rate of relative sea- level rise observed in Louisiana is 

 attributed to the contribution of compactional subsidence of the Mississippi 

 River delta plain. Louisiana directly overlies the entrenched Pleistocene 

 valley of the Mississippi River, which is infilled by a 300 m thick sequence 

 of Holocene deltaic sediments. A rapid acceleration in relative sea-level 

 rise can be documented by both USACOE and NOS data sets throughout Louisiana 

 and the Gulf of Mexico. (Authors). 



264 PENLAND, S., RAMSEY, K. E., McBRIDE, R. A., MESTAYER, J. T. , and 

 WESTPHAL, K. A. 1986. "Relative Sea-Level Rise, Delta Plain Development," 

 Louisiana Geological Survey, Coastal Geology Bulletin Number 1. 



The Louisiana Geological Survey at Louisiana State University conducted 

 a detailed investigation of sea-level rise, delta-plain development, and wet- 

 land sedimentation in the Terrebonne Parish region using tide gage records, 

 geomorphic information, stratigraphic information, radiometric dating tech- 

 niques, and geodetic leveling data for the Terrebonne Parish Consolidated Gov- 

 ernment. Recent research results indicate that global relative sea- level is 

 rising at a rate of 0.12 cm/year; in the Gulf of Mexico the relative rate of 

 sea-level rise is slightly higher at 0.23 cm/year. Relative sea-level rise is 

 defined as the combined effect of global or eustatic sea-level rise and local 

 subsidence. The Houma tide gage station on the Intracoastal Waterway measured 

 a relative sea-level rise rate of 1.28 cm/year. 



Using a eustatic correction factor of 0.23 cm/year for tide gage station 

 records from the Gulf of Mexico, the authors estimate rates of subsidence in 

 Houma at 1.03 cm/year. The results of the geodetic leveling analysis indicate 

 that the rate of subsidence ranges between 0.24 cm/year and 1.60 cm/year with 

 a mean rate of 0.92 cm/year. These rates correlate closely with the 

 eustatic-corrected tide gage results. From stratigraphic analysis, it was 

 determined a mean subsidence rate of 0.66 cm/year for the last 500 years and 

 0.19 cm/year for the period 500-4000 year B.P. Subsidence rates tend to 

 decrease east and v/est away from Houma and increase toward the coast as a 

 function of the underlying Holocene delta-plain thickness. A comparison of 

 relative sea- level rise and subsidence rates indicates that subsidence is the 

 major component driving relative sea- level rise under current conditions. 



The average wetland sedimentation rate is 0.86 cm/year in Terrebonne 

 Parish. Sedimentation rates increase east to west from 0.58 cm/year at 

 Cocodrie to greater than 2.0 cm/year in the Atchafalaya River area. It should 

 be noted that adjacent to the Atchafalaya River and east of Avoca Island 

 sedimentation rates decrease to 0.81 cm/year owing to the presence of the 

 levee system. In general, under the current sea-level rise conditions, marsh 

 accretion rates are not sufficient to maintain the coastal wetlands within the 

 Terrebonne Parish region except in isolated areas surrounding the Atchafalaya 

 River. (Modified Abstract). 



120 



