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Erosion
Mobile Outer Bar Channel Dredging

Littoral Drift

To discuss this issue, two facts must be known:

  • Mobile Bay is a shallow body of water, measuring no more than 13 feet deep at its lowest points of natural depth.  The waters directly around Dauphin Island have traditionally always been shallow as well, compared to the deeper waters offshore.

  • There is a natural process by which sand and sediment moves east to west across the North Central Gulf Coast.  This is known as "littoral drift."

The Littoral Drift, Pre-Dredging
(click to enlarge)

(estimation)

Above is an estimated rendering of the littoral drift prior to the dredging which created the Mobile Harbor ship channel.

In the early 1900s, the U.S. Army Corps of Engineers began dredging a ship channel in the middle of Mobile Bay to allow commercial ocean traffic to navigate up to the Alabama State Docks area and points north.  Periodic dredging deepened the channel to 45 feet, and maintenance dredging has intensified in the last century.

This dredging is conducted not only in Mobile Bay, but also in the Mobile Outer Bar Channel, the entry point to the bay.  This is an area between Fort Morgan on the eastern shore of the bay and Fort Gaines on the western shore, extending out into the Gulf waters.  The sand dredged from this area is deposited in deep offshore waters.

Dauphin Island is "sediment-starved"

In 1978, the Corps did a study of the dredging.  The study concluded that the dredging had removed vital sand from the littoral drift system, making that sand unavailable for barrier island nourishment that would otherwise occur naturally.  Sand migrating westward from Florida was getting "caught" in the 40+ foot deep channel.  The Corps does periodic maintenance dredging to remove the sand filling up the channel.

The Littoral Drift, Post-Dredging
(click to enlarge)

(estimation)

Above is an estimated rendering of the littoral drift after the ship channel was built and then increasingly deepened and widened over the years. Without replenishment, the subsurface profile of the West End has become too steep. Sand does not collect on the shore like it did when the beach was wider and the surrounding waters shallower. Under these conditions, more sand slides past the island.

Additional studies have been conducted since 1978, with the most recent efforts also being conducted by the Corps in connection with the Mississippi Coastal Improvements Program Study (MsCIP) that was completed in 2009.

These and other studies have demonstrated that Dauphin Island is being gradually and chronically “sediment starved” by the consistent dredging of sands from the Mobile Outer Bar Channel. The dredging is the most significant contributing factor to the island’s inability to “heal itself” via natural replenishment over time. This could be mitigated if the sand dredged from the bay was deposited closer to the island, rather than in deeper offshore waters.

Dauphin Island’s Gulf shoreline has steadily experienced increasing coastal erosion, as breaking waves and longshore currents have continued to transport sands from the island shoreline to the west towards Mississippi without adequate replenishment from the east.  Not only has Dauphin Island been adversely affected by this process, but the downdrift Mississippi barrier islands have suffered as well from the removal of sand from Dauphin Island’s littoral environment.

What to do

The 1978 Corps study recommended that the disposal area for the dredged sand be moved from the deeper offshore waters to an area closer to the island, where the sand might migrate onto the south shore beaches.  This would restore some of the sand deficit caused by the dredging.

Current & Recommended Deposition Areas
for Channel-Dredged Sand
(click to enlarge)

Dr. Scott Douglass (Univ. of South Alabama), in recent comments concerning the GoMESA proposal submitted in Feb. 2010 by the Task Force, said the following:

"Appendix D discusses six possible sand sources for the proposed project. In this discussion, there is consideration of the use of sand dredged from the Mobile Ship Channel. Figure D-3 reproduces a 1978 Corps pullout map with a potential disposal area that is directly offshore of the west end of Dauphin Island.

This location is generally consistent with my recommendations to the Town made in September 2009. If that disposal area is moved closer to the island into shallower depths (say 15-20 feet instead of 30 feet), that is an excellent idea for truly “beneficial” disposal of dredged sands.

Sand placed there (15-20 feet deep immediately offshore of the beach) would likely migrate right onto the beaches of the island’s west end within a matter of months. [emphasis ours]

This estimate of timing is based on some other published University of South Alabama research (Douglass, S.L. 1996, Nearshore Placement of Sand, Proceedings of the 25th International Conference on Coastal Engineering, pp. 3708-3721). It may be prudent to begin discussions with the Corps concerning improved bypassing of sand now that the lawsuit settlement is complete."

We hope to have the State Legislature instruct the Corps to dump or pump (via pipelines) the sand dredged from the ship channel into the deposition sites recommended by the Corps' own report as well as by Dr. Douglass.


Related Links

More factors contributing to erosion

1978 Corps' report:
Note that these are large files (35MB each) and slower internet connections may time out upon attempted download.

Beach Erosion Part 1

Beach Erosion Part 2

 

 

 

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