Letter From Engineer Analyzing Option of Temporarily Relocating the Channel

By Karen Erickison, PE.
Applied Technology & Management, Inc.
August 26, 1996

ATM analyzed the option of dredging a new primary tidal channel at a location below the existing position of mean high water approximately 600 feet north of its present location.

The cross-sectional area of the new channel would have a width and area that is comparable to the existing natural tidal channel.

Volumetric computations of sand volumes for excavation were based on the post-Bertha hydrographic surveys. A quantity of approximately 121,000 cubic yards of sand will require excavation excluding any infilling that would occur due to bank sloughing and wave action.

The sand fill quantity necessary to infill the existing primary tidal channel is estimated at 180,000 cubic yards. Although the geometric configuration of the fill template is only 120,000 cubic yards, the sand losses due to current induced sand redistribution along the submerged portion of the platform will require a large overfill. The overfill must account for fine sand losses that are easily carried by the strong discharge currents and the sand losses due to the infilling of an active tidal channel.

The net result is that the new channel will require a 50% overage to account for expected sand losses.

The cost for this work, including mobilization/demobilization at $250,000 and unit yard excavation and placement at $8.00 per yard, would be close to $2 million, including engineering and permitting.

This option would provide approximately 18 months of protection before the channel migrates south to the present position assuming that no major winter storms or hurricanes accelerate inlet migration.