CANALS AND WETLAND EROSION RATES IN COASTAL LOUISIANA 



R. Eugene Turner 



R. Costanza 



W. Sea if e 



Center for Wetland Resources 



Louisiana State University 



Baton Rouge, LA 70803 



ABSTRACT 



Canals have increased in area from practically zero at the beginning of the century 

 to about 2M% of the Louisiana coastal surface area in 1978. The annual increase in canal 

 area is continuing to climb in 1981 as a result of new canal dredging and the widening of 

 existing canals. Land loss rates across the coastal zone since the 1890's, among 

 hydrologic units, and within areas of similar substrates and equal distances to the coast, 

 are all positively related to estimates of canal density. Further, estimates of land loss 

 at zero canal density (from regression equations) are similar to the 7,000 year coast-wide 

 rate of gain in land. Within 7 1/2' quadrangle maps, the new "holes" or ponds in the 

 marsh have appeared close to canals, not near natural channels. Coastwide, canal 

 surface is about 10% of the total land loss. Based on our analysis we conclude that 

 coastal erosion rates in Louisiana are largely an indirect result of canal dredging 

 activities or use. The mechanism for the effect probably involved an alteration in 

 wetland hydrology, but a complete understanding is presently lacking. Thus corrective 

 measures cannot be identified and implemented with confidence until more is known 

 about the mechanisms of canal and spoil bank effects on wetland hydrology. 



INTRODUCTION 



Canals are conspicuous features of the south Louisiana wetlands. At surface level, 

 in a boat, their great length, density, and diversity can go unnoticed. A few hundred feet 

 above the ground, however, they stand out as dominant geomorphic features. Most still 

 have some remnants of their original levees formed from the dredge spoil put aside 

 during construction. A few, notably gas pipeline canals, were filled in almost as soon as 

 the pipe was laid and are no longer evident; the plants there have regained their former 

 position in the reworked soil. Many canals are still in commercial and recreational use; 

 others are blocked at one or both ends. They lie straight in contrast to the twisting, 

 anastomotic natural channels which the canals often intersect. Water within canals rises 

 and falls with the tide, contains fish, and is not noticeably different from bayou water in 

 many respects. The linear structure of canals and the resulting effects on water and 

 sediment movement constitute the major difference between canals and natural drainage 

 systems. 



These canals were largely absent at the turn of the century. Almost all were 

 constructed to help in the recovery of mineral deposits located thousands of meters 



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