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Title: Final Supplemental 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 and along South Ponte Vedra (SPV) and Vilano Beach (VB) between R-84 to R-110 and R-110 to R-117. Non-beach compatible material would be placed in a near-shore placement area between DEP monuments R-141 to R-146 south of the inlet or R-84 to R-110 (SPV) and R-110 to R-117 (VB) north of the inlet. 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. The relatively high rate of shoaling within the IWW and St. Augustine Inlet necessitates frequent maintenance dredging. Last dredged in 2013, the most recent examination survey documented a total in situ shoaling volume of approximately 200,000 cubic yards (cy) within the authorized channels. Minimum depths recorded from the project channels are -2.1 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 in the project channels are effectively being removed from the near-shore sediment transport system. So, placing this material on the adjacent critically eroded beaches would restore (or mimic through regional sediment management - RSM) the natural transport process. A Florida Department of Environmental Protection, Division of Water Resource Management report (June 2014) on Critically Eroded Beaches in Florida (DEP 2014), identified 11.5 miles of critically eroded shoreline in St. Johns County. The proposed SPV and VB placement areas (Figure 1) compose 3.4 miles of the 11.5 mile DEP designated critically eroded areas. In addition, the St. Augustine Inlet Management Plan (IMP; DEP 2014) was revised based on an updated sediment budget (USACE 2012) which found that a maximum of 278,000 cubic yards (CY) per year could be dredged from the inlet system which would naturally replenish itself without adverse erosion on the adjacent beaches. The revised IMP recommends to: 1) Continue to transfer sediment to the adjacent beaches with a placement ratio of approximately one-third of material placement to the north and two-thirds of material placement to the south; and 2) Inlet sand transfer material shall be placed in designated critically eroded areas to the north or south of the inlet between R84 and R152.
Description: Environmental Assessment
Rights: Approved for Public Release; Distribution is Unlimited
Appears in Collections:Environmental Documents

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