ASSESSMENT OF GEOLOGICAL AND HUMAN FACTORS 

 RESPONSIBLE FOR LOUISIANA COASTAL BARRIER EROSION 



Shea Pen land 



Louisiana Geological Survey 



Louisiana State University 



Baton Rouge, LA 70803 



Ron Boyd 



Department of Geology 



Dalhousie University 



Halifax, Nova Scotia, Canada 



ABSTRACT 



Louisiana's coastal barrier systems are experiencing severe shoreline erosion and 

 land loss^ Between 1880 and 1980, total coastal barrier area decreased from 98.6 km"^ to 

 57.8 km , an overall loss of 41%. Coastal barrier land loss results from the natural 

 processes of deltaic transgression and marine erosion, combined with the impact of 

 human development. A three-stage model for the evolution of abandoned Mississippi 

 deltas describes deltaic transgression. Sand bodies deposited during delta building are 

 successfully transformed after abandonment into an erosional headland and flanking 

 barriers (Stage I), a transgressive barrier island arc (Stage 2), and a subaqueous 

 inner-shelf shoal (Stage 3). Barrier erosion trends closely correspond to the pattern of 

 sediment dispersal identified for each barrier evolutionary stage. Barrier islands in the 

 erosional headland and flanking barrier stage are essentially in a state of dynamic 

 equilibrium, due to the presence of a deltaic headland sand source. Transgressive barrier 

 island arcs do not contain such a sediment source, and hence suffer net erosion. The 

 principal mechanisms of transgression are subsidence combined with repeated erosion by 

 extratropical and tropical cyclones. Coastal barrier sediment loss, hence land loss, can 

 be attributed to the following mechanisms: (I) longshore loss into spits and tidal deltas, 

 (2) landward loss through overwash into a subsiding lagoon, (3) offshore loss due to an 

 inequality in offshore/onshore transport capacity, and (4) subsidence of the deltaic sand 

 sources. Human impacts that result in accelerated coastal barrier deterioration include 

 coastal structures, pipeline canals, and navigation channels. These manmade structures 

 disrupt sediment transport pathways and create additional sediment sinks. 



INTRODUCTION 



Louisiana is faced with the most serious coastal barrier erosion problem in the 

 United States (Figure I). Between 1880 and 1980, the total coastal barrier area of 

 Louisiana decreased 41%. Coastal barrier erosion and land loss results from the natural 

 processes of deltaic transgression and marine erosion, combined with the impact of 

 human development. The economic consequences of shoreline erosion and land loss are 



14 



