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Erosion
Mobile Outer Bar Channel Dredging
Littoral Drift
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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The
Dauphin Island Restoration Task Force
