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Title: Final Integrated Feasibility Report and Environmental Assessment : Jacksonville Harbor (Mile Point) Navigation Study, Duval County, Florida
Authors: United States. Army. Corps of Engineers. Jacksonville District
Keywords: Dredging
Inland navigation
Saint Johns River (Fla.)
Publisher: United States. Army. Corps of Engineers. Jacksonville District.
Abstract: The Mile Point Shoreline of Jacksonville Harbor (Jax Mile Point) is located between river miles four and five west of the Atlantic Ocean along the St. Johns River. This area on the north bank of the St. Johns River has been experiencing shoreline erosion. The confluence of the St. Johns River and the Intracoastal Waterway (IWW) is within the Mile Point study area. The IWW enters the main channel of the St. Johns River at an angle of approximately 45° from the north, out of Sisters Creek (Figure 1). From the south, the IWW enters out of Pablo Creek at an angle almost parallel to the main channel flow out of Pablo Creek, with flow usually running in the opposite direction of the flow of the river. On the south bank of the St. Johns River is the Mile Point training wall, also known as the Little Jetties. The U.S. Army Corps of Engineers (USACE) has been studying the continued erosion of the Mile Point shoreline and the effects of the navigation restrictions. The following erosion and navigation problems related to the existing Mile Point area necessitated this feasibility study. There have been catastrophic failures on the north Mile Point shoreline due to erosion. Figure 2 highlights a failure event at parcel no.8856 on the Mile Point shoreline. In this particular case, the head scarp appears to have eroded 75 to 100 feet back from the seawall. Other erosion events are documented in Figure 3 and date from 1986 to 1997. Due to difficult crosscurrents at the confluence of the IWW and the St. Johns River during the ebb tide (Figure 1), the St. Johns Bar Pilots have enacted navigation restrictions for inbound vessels with a transit draft greater than 33 feet to avoid transiting during the ebb tide. Due to the angle of entry of the flow at the confluence, there is a danger that large vessels could turn in response to the current and leave the channel resulting in groundings or collisions. The proposed actions of this report are in the national interest and can be constructed while protecting the environment from unacceptable impacts. Benefits of the proposed action would minimize the impacts of the flows out of the IWW during the ebb tide, slow or redirect the velocities away from the north bank, and slow the progression of erosion. Reducing or redirecting the difficult crosscurrents in the harbor would allow the pilots to reduce or eliminate navigation restrictions impeding the free movement of vessels. As is detailed in the Engineering Appendix (Appendix A) on page A-4, numerical modeling results indicate that the potentially dangerous crosscurrents exiting the IWW southern channel under ebb tide can be redirected to more closely parallel the alignment of the Federal navigation channel instead of being focused toward the erosion prone areas along the northern shoreline of Mile Point. Adverse impacts from the project, that would be mitigated, would include loss of salt marsh adjacent to the existing Mile Point training wall. Measures were taken to avoid, minimize, and compensate for adverse impacts. The restoration of Great Marsh Island is the least cost dredging alternative. It provides beneficial use of dredged material through the creation of habitat beyond the required mitigation. The creation of a flow improvement measure is proposed to prevent any adverse impacts on water quality within Chicopit Bay from restoring Great Marsh Island.
Description: Feasibility Report/Environmental Assessment
Rights: Approved for Public Release; Distribution is Unlimited
Appears in Collections:Environmental Documents

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