4. Key attribute alterations 



Key attribute alterations induced by this phase of oil and gas 

 operations involve changes in land elevations. Borrow ditches and 

 marsh buggy tracks allow an increase in frequency and area of inun- 

 dation by salt water. First-order effects include removal of vege- 

 tation and creation of a standing-water habitat. If the increased 

 salt-water flows are not confined by topographic features or other 

 means, large areas of marsh may be affected. Long-term changes in 

 vegetation and consumer groups would occur. Long-term turbidity 

 increases would also result. 



Levee construction inherently results in higher land elevations. 

 Burial of flora is less significant than possible alterations of 

 waterflow regimes. Poorly planned placement of the roadway may 

 result in blockage of existing tidal inundation and/or runoff patterns, 

 These blockages will lead to long-standing changes in types and/or 

 amounts of vegetative cover, followed by corresponding changes in 

 consumer composition. 



Site access by canal and wellsite dredging. 



1. Activity sequence 



Surveyors enter the marsh by marsh buggy or foot, stake the wellsite 

 location and access route, and leave the area. The pathway may or 

 may not be the most direct one possible. Usually there is no need 

 to clear vegetation, but vegetation may be trampled and crushed in 

 adjacent zones. 



Site location, equipment availability, spoil placement requirements, 

 and economic factors may dictate which type and size of dredge is 

 used in each phase of the excavation. If entry is from bayside to 

 the marsh, a barge-mounted bucket dredge or a hydraulic dredge is 

 used to cut the channel to the marsh edge. Spoil is placed on both 

 sides of the channel and is usually submergent. Maintenance dredging 

 may eventually lead to subaerial spoil levees or "islands." 



Once the dredge reaches the marsh proper, the spoil levees will be 

 subaerial. If the marsh soils are firm enough to support the weight, 

 a buggy-mounted or track-mounted bucket dredge may be utilized. 

 Canal depths must be about 3 m (8-10 ft); typical widths approach 

 23 m (70 ft); a 10-m (30-ft) berm typically exists between the canal 

 edge and the inside foot of the continuous levee (both sides of the 

 canal ). 



183 



