No way to stop sand migration
February 16, 2011
To the editor,
You would think that after all these years, we would resign ourselves to the reality of the opening and closing of Blind Pass which separates Sanibel and Captiva Islands. As many times as we dredge it open, the sand, carried south and east from Captiva by the littoral (coastal) current will settle in the Pass until the channel is completely closed. This is a natural phenomenon and there is no way to stop this sand migration.
Many years ago, a groin was constructed on the upstream, Captiva, side of the pass with the hope that would solve the problem. As can be clearly seen in aerial photos, it did slow the littoral flowso that the sand dropped out of suspension behind the groin until it completely filled in the area upstream of the groin, forming a beautiful beach on Captiva. It pushed the current further off shore around the groin. This flow returned to Sanibel’s shore a bit further south or east eroding the beach in that area. To replenish this loss, Captiva must provide Sanibel with 25,000 cubic yards of sand per year.
The source of the sand is the Captiva shore which is constantly eroded by the waves, both smalland those larger ones created by storms. Once the sand goes into suspension, it is carried toward Sanibel by the afore mentioned coastal current. The amount of sand transported in this manner is a function of the sand grain size and the velocity of the shore current. The courser grains require a higher current speed to stay in suspension while the finer sand grains stay in suspension even at relatively low current velocities. If we can get the current to slow down enough before it reaches the existing groin, most of the sand will drop out of suspension and settle to the sea bed before it reaches Blind Pass.
Q = How can we economically slow down the current?
A = By building a sand trap.
Q = What is a sand trap?
A = A sand trap is an area dredged deep and wide enough so that the water flowing over the dredged area will slow to the required velocity for the sand to drop out of suspension.
This solution to keep Blind Pass open will not eliminate the need to dredge. The trap itself must be dredged and the sand that drops into the trap must be periodically removed to keep the trap from filling with sand and becoming dysfunctional. The dredging, however, will be done at the location of the trap, just north of the rock groin, not in Blind Pass.
I have no data on the sand grain sizes, coastal current velocity nor profile of the sea bottom. The Captiva Erosion Protection District has, from time to time, consulted with some reputable coastal engineers and I would imagine they have this data. It should be possible for them to review the economic feasibility and practicality of such a project.
With all that is written above, it is most important to understand that the entire project can be disrupted by a hurricane or a sustained period of high winds and waves, natural events over which we have no control.