premature maintenance dredging. Placement of the spoil material is constrained by tine 

 length of the arm on the dredge. To some extent berm width is indirectly controlled by 

 this. Our regression analyses showed that berm and spoil bank together generally added 

 68 m to the width of the wetland corridor destroyed in canal construction. When the 

 extra unauthorized canal width was included, the total corridor width was 81.7 m wider 

 than the permitted canal. For a well-access canal permitted at about 21 m (65 feet) the 

 total impacted width was typically about 103 m or five times the permitted canal 

 width. Apparently, there has been almost no policing of canal construction, nor is there 

 a record showing whether permitted canals are ever dredged. Since habitat loss from 

 canals is much greater than permit records indicate, closer adherence to permit 

 dimensions should be enforced. In addition, we observe that sufficient numbers of spoil 

 bank openings to allow the flow of water across the marsh were seldom maintained, but 

 sheet flow over the marsh was severely impeded by all spoil banks visited. 



Boat traffic greatly influences canal widening rates as demonstrated in the analysis 

 of dead-end canals in the Leeville oil field. Dead-end canals off Bayou Lafourche and the 

 Southwestern Louisiana Canal, the two major nearby navigation routes, widened 1.46 

 m/yr faster than dead-end canals off oil field navigation canals and 1.63 m/yr faster than 

 dead-end canals some distance from boat traffic. 



The re-examination of the Rockefeller Refuge and Southwestern Louisiana canals, 

 and information gathered in other parts of the study, provide insights into the factors 

 that influence the widening of dredged canals in wetlands. The specific controlling 

 factors that have been identified are boat traffic, geologic environment, and width of the 

 spoil bank. The Humble canal system has more boat traffic than the Deep 

 Lake-Constance Bayou system and widened about 0.3 m/yr faster. The Southwestern 

 Louisiana Canal, with even more exposure to boat wakes, widened at a mean rate of 

 almost 3 m/yr. These trends support the findings from the Leeville oil field, but part of 

 the dramatic difference between the widening rates of the Rockefeller Refuge and the 

 Southwestern Louisiana canals may be the generally firmer substrates at the Rockefeller 

 Refuge (Gosselink et al. 1979). 



In the Southwestern Louisiana Canal, the initial period of slow widening followed by 

 more rapid widening may be explained by slow erosion through the consolidated spoil 

 banks, followed by an increased erosion rate once the canal edge reached open marsh 

 beyond the spoil. As shown in Table 6, a hypothetical canal permitted at 21.3 m in width 

 would have a berm and spoil bank 34.2 m wide on each side [=(100.6 - 32.3)/2]. At the 

 Initial slow widening rate of the Southwestern Louisiana Canal it would take 72 years for 

 the canal edge to erode through the spoil bank (compared with only 27 years at the 

 present rapid rate). The slower rate corresponds to the time between construction and 

 the dramatic increase in erosion rate of the Southwestern Louisiana Canal. Thus, we 

 hypothesize that once spoil banks are eroded away, one can then expect a dramatic 

 increase in canal widening rates. The Rockefeller Refuge canals are still eroding through 

 the spoils banks, as are most of Louisiana's oil field canals. Therefore, their widening 

 rates are relatively low and linear (Figures 3 and 4). We predict that when these canals 

 become 30 to 70 years old their associated land loss rates will begin to accelerate 

 rapidly. 



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