WETLAND LOSS DIRECTLY ASSOCIATED WITH CANAL DREDGING 

 IN THE LOUISIANA COASTAL ZONE 



W. B. Johnson 

 J. G. Gosselink 



Center for Wetland Resources 



Louisiana State University 



Baton Rouge, LA 70803 



ABSTRACT 



This study addresses wetland losses directly resulting from canals, including initial 

 construction practices and subsequent canal bank erosion. The average actual width of 

 the newly dredged canals studied exceeded the width specified in the dredging permit by 

 13.4 m. The total width affected, including berm and spoil deposits, exceeded the 

 permitted canal width by an average of 81.7 m. 



As canals age, they widen through erosion. The history of three old canal systems 

 in coastal Louisiana was examined. All these canals continue to increase in width and 

 differences in their patterns of widening can be explained by boat traffic, length of time 

 since construction, and substrate differences. The widening rate in the Leeville oilfield 

 is directly related to the proximity of the canal to boat traffic. Canals in areas of greatest 

 boat activity widened at a rate of 2.58 m/yr, while those in areas of minimal boat 

 activity widened at a rate of 0.95 m/yr. 



INTRODUCTION 



Louisiana has 30% of the Nation's coastal wetlands (Turner and Gosselink 1975), but 

 they are being lost at an alarming rate. Numerous investigators (Gagliano and van Beek 

 1970; Adams et al. 1976, 1978, 1980; Craig et al. 1979, 1980; Gagliano et al. 1981; 

 Baumann and Adams in press) have examined Louisiana's land loss problems. These 

 investigations have generally relied on large-scale mapping procedures for data 

 extraction, concentrating on the entire coastal zone, shoreline sections, or hydrologic 

 basins. 



Manmade canals are a dominant feature of the Louisiana coast, and there is 

 considerable evidence (Craig et al. 1979, 1980; Scaife et al. in press) that this canal 

 network contributes significantly to wetland loss, both directly and indirectly. Direct 

 effects are the immediate conversion of wetland to canals and spoil banks during canal 

 construction (Darnell 1976), and the subsequent widening of canals as their banks erode 

 through time. Indirect effects (Morton 1977) are marsh deterioration from saltwater 

 intrusion and changes in waterflow patterns that result when deep straight canals are 

 dredged through wetlands. In this report we document the direct wetland loss associated 

 with dredging and historical erosion. Indirect effects are documented by Scaife et al. (in 

 press). 



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