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Title: Final Environmental Assessment : Maintenance Dredging, St. Augustine Inlet and Adjacent Intracoastal Waterway, St. Johns County, Florida
Authors: United States. Army. Corps of Engineers. Jacksonville District.
Keywords: Dredging
Dredging spoil
Inland navigation
Saint Augustine Inlet (Fla.)
Publisher: United States. Army. Corps of Engineers. Jacksonville District.
Abstract: The U.S. Army Corps of Engineers (Corps), Jacksonville District, is proposing to conduct periodic maintenance dredging of St. Augustine Inlet and the adjacent Intracoastal Waterway (IWW) in St. Johns County, FL. This would include IWW Cuts SJ-28 to SJ-30, a portion of the Inlet flood shoal, and a portion of the inlet entrance channel along Porpoise Point (see Figure 1, Project Map). Beach compatible dredged material would be placed along the shoreline within Anastasia State Park (ASP) and St. Augustine Beach between Florida Department of Environmental Protection (DEP) monuments R-132 to R-152. Non-beach compatible material would be placed in a near-shore placement area between DEP monuments R-141 to R-146. The IWW channel would be maintained to its authorized dimensions of 125-feet wide by 12-feet deep plus 2-feet of allowable over-depth at mean lower low water (MLLW). The inlet entrance channel is authorized to be maintained at a “best fit” alignment within the confines of a 600-foot-wide area, between the north and south jetties. The entrance channel bottom width is to be maintained at 200 feet wide by -16 feet deep MLLW (plus 2 ft of allowable over depth for a total project depth of -18 ft MLLW), along with 50 feet wide settling basins along the north and south sides of the channel. The accumulation of sediment, commonly referred to as shoaling, has restricted the width of the project channels and reduced their depths. In addition, the sediment which has accumulated along Porpoise Point is trapped there by the man-made inlet and is restricting it’s width. The relatively high rate of shoaling within the IWW and St. Augustine Inlet necessitates frequent maintenance dredging. Last dredged in 2005, the most recent examination survey documented a total in situ shoaling volume of approximately 700,000 cubic yards (cy) within the authorized channels. Minimum depths recorded from the project channels are less than 4 ft causing navigation problems for commercial and recreational vessels. Vessels are currently being forced outside the authorized channels in search of deeper water, waiting for high tides, or prop dredging through the channels. Removal of the shoal material would maintain the navigable capacity of the project channels. In addition, the sediments accreting on the Porpoise Point shoal (Figure 2) are effectively being removed from the near-shore sediment transport system. So, bypassing this material to the critically eroded down-drift beaches would restore (or mimic through regional sediment management - RSM) the natural transport process. A DEP Bureau of Beaches and Coastal Systems (BBCS) report (June 2009) on Critically Eroded Beaches in Florida, identified 9.8 miles of critically eroded shoreline in St. Johns County. The proposed beach placement area between R-132 to R-152 (Figure 1) composes 3.8 miles of the 9.8 mile DEP designated critically eroded area. In addition, the St. Augustine Inlet Management Plan (Taylor, 1997) recommends to: 1) “Continue to bypass sediment to the down-drift beaches.” and 2) “Restore the down-drift beaches, designated by the Department as experiencing critical erosion, to mitigate the effects of the inlet."
Description: Environmental Assessment
Rights: Approved for Public Release; Distribution is Unlimited
Appears in Collections:Environmental Documents

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