MUDFLAT AND MARSH PROGRADATION ALONG LOUISIANA'S CHENIER PLAIN: 

 A NATURAL REVERSAL IN COASTAL EROSION 



John T. Wells 

 G. Paul Kemp 



Coastal Studies Institute 



Louisiana State University 



Baton Rouge, LA 70803 



ABSTRACT 



The chenier plain coast of southwestern Louisiana has been receiving sediment 

 intermittently from the Mississippi River for the last 5,000 years. A new influx of fine- 

 grained sediment, the first such sediment pulse in perhaps 500 to 1000 years, is leading to 

 localized coastal progradation along what has historically been one of the most rapidly 

 retreating shorelines in the United States. Carried as suspended sediment by the 

 "Atchafalaya mud stream," silts and clays from the Atchafalaya River are now 

 accumulating as mudflats along a segment of coast from Freshwater Bayou Canal to 

 Rollover Bayou. These transitory mudflats provide a buffer to incoming storm waves and 

 serve as a temporary storehouse for littoral sediments. 



Process-oriented field studies initiated in 1980, together with satellite imagery, 

 color infrared photography, and aerial overflights since 1974, are providing insight as to 

 present and future trends in sedimentation. Growth of the chenier plain appears initially 

 to be by a series of transitory mudflats, a few of which become welded to the shoreline. 

 Since 1969 the pattern of mudflat sedimentation has been increasing and shifting to the 

 west, consistent with the direction of coastal and wave-induced currents. Accelerated 

 growth of the chenier plain is expected when the subaerial Atchafalaya Delta outgrows 

 Atchafalaya Bay, thus allowing an even greater volume of sediments to enter the 

 dynamic shelf region seaward of the bay and to become entrained in the mud stream. 

 The time scale for widespread reversal in present coastal erosion is 50 to 100 years. 



INTRODUCTION 



Modern man has acquired a very unstable inheritance in the coastal plain of south 

 Louisiana, a landscape that expands and contracts in area at rates almost unequaled 

 anywhere else in the world. The potentials for land building via rapid sediment 

 deposition and for land loss through compaction and wind/wave erosion are both large. 

 The degree to which these land building/land loss potentials are individually realized at 

 any one time, as well as the degree to which they offset each other, determine the 

 coastline's position on a cyclic curve of alternating progradation and retreat (Kolb and 

 Van Lopik 1958). The works of Morgan and Larimore (1957), Gagliano and van Beek 

 (1970), and Adams et al. (1978) as well as many papers in this volume, establish that 

 the shoreline of Louisiana, taken as a whole, is currently retreating. These authors also 

 point out, however, that this retreat shows a high degree of spatial variability. For 



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