Engineering Considerations

Letter From An Engineer Regarding Project Design and Installation

By Karen Erickson, PE
Applied Technology and Management Inc.
August 27, 1996

Since NC prohibits all hard coastal protective structures except for sand fill and sand-filled containers and,whereby the sand fill is not a workable alternative due to technical, economical and permitting problems, the only viable measure to prevent further erosion of Shell Island is a revetment-type sand-filled container system.

The primary goals of the shoreline protection project are as follows:

1. Stabilization of the south channel bank shoreline at New Hanover County Public Beach Access on Shell Island for a period of 3 to 5 years.

2. Utilization of materials that are considered to be "soft" engineering solutions to provide erosion control and storm damage prevention.

3. Minimization of impacts to adjacent lands through the design of a container system that is user friendly for recreational beach users.

Erosion Trends

The southern migration of Mason's Inlet, from 1989 to 1996, has resulted in a shoreline erosion rate of 185 to 300 feet per year at Shell Island.

Hurricane Bertha (July 13, 1996) caused the dune and vegetation to erode an average of 45 feet along the south channel bank at Shell Island.

Mason's Inlet is strongly influenced by the expansive flood shoal located at the interior of the inlet and the shallow sand bar extending across the inlet's entrance to the Atlantic Ocean.

These sand laden shoals in concert with storm related sediment movement by longshore currents results in continuous infilling and bathymetric changes at the inlet.

The net result has been a high rate of migration of the inlet's tidal channels towards the south.

The hydrodynamic processes at Mason's Inlet are dominated by the net north-to-south littoral drift that controls the location of the natural tidal channel.

The tidal prism creates sufficient current to maintain an 8 to 10 foot controlling depth in the primary tidal channel. (Thus, Mason's Inlet is expected to remain open for the foreseeable future.)

Design Conditions

The inlet bank slope at Mason's Inlet is approximately 1V:8H. This is almost four times greater than the shape observed on open coast shores.

The engineering design provides for a crest elevation of +8 feet to protect the upland from wave run-up and over-topping and provides for -10 feet of toe penetration to prevent further undercutting and scour of the foundation.

The size and configuration of the geotextile tubes must be designed to remain stable during storm waves of six feet that can be combined with storm tides.

To avoid scour holes along the lower tube and the subsequent rolling and rupture of the tubes, a toe apron of filter-cloth material with a small diameter toe tube (18 inches) is required.

As the containers will be lying horizontally, in a stair step fashion, the sand-filled tubes will take on a flattened, elliptical shape.

The dimensions of the geotubes will be approximately 3 feet by 5 feet.

The tube must be placed on a mild slope (1H:3V) to prevent the individual containers from being displaced.

This slope will absorb and dissipate incoming energy, thereby reducing reflection of wave & tide induces scour currents. Reducing these energies will lessen sand losses adjacent to and at the toe of the containers.

Also, this slope size has been shown to facilitate sand deposits onto the face and at the toe of sloped erosion control structures.

This 1V: 3H slope, crest and toe elevations and toe scour results in a plan view footprint of approximately 65 feet.

Steeper slopes result in greater wave reflection and less energy dissipation causing a general steepening of the beach profile.

Geotextile containers placed in a step style fashion, are frequently used as a base for sitting, lying, or walking and considered beach user friendly.

Engineering Design Considerations

Geotextile fabrics and materials, for use in erosion control, are considered new and innovative materials based on prototype designs that have been developed, built and tested to determine strength and durability over the past twenty years.

The primary application has been to reduce the effects of erosion during relatively low energy events.

The geotextile container is designed to minimize impacts to adjacent properties and reduce toe scour. To accomplish this a gentle slope is required. This will dissipate energy and minimize reflected energy, thereby reducing scour, which results in the containers rolling down slope and rupturing.

The geotextile containers are also designed to minimize the potential for failure due to rupture and sand loss from tears by foreign objects, heavy abrasion and vandalism. However, rupture from foreign objects is expected to occur within 3 to 5 years.

The crest elevation of the structure is based on the expected wave run-up for incident wave conditions and periodic super-elevation of water due to astronomical and storm tides.

The position of the structure on the profile is based on an analysis of the site conditions. The objective is to follow the natural south channel bank alignment such that future erosion at the toe of the structure will be uniform. An uneven settlement of the lowest tubes would result in tearing of the geotextile fabric.

The overall length of the container system is approximately 400 feet.

Installation Design Considerations

The tubes are shaped in the typical pillow shape, a roughly elliptical cross-section that is thickest at its mid-section and tapered down to either end.

If multiple tubes are laid out abutting one another, a problem with joint and/or overlap separations usually develops. As a result, differential movements open gaps between adjacent segments caused by high water velocities from back rushing waves that tend to concentrate at these gaps. Therefore, a single length of tube for each tier is required.

If a smaller size of sand-filled tubes is employed (3 feet by 5 feet), a strap restraint system is necessary to assist in the overall structural integrity of the container system. The purpose of the straps is to tie the individual containers together to increase the overall stability of the individual units from displacement during high seas.

Due to the existing location of the shoreline at the north end of Shell Island, the construction of the project will require temporary measures to control bank stability for site preparation (trenching and excavation) and slope grading.

It is recommended that the tubes be filled with the high quality sand excavated from the site.

Upon completion of the structure, the upland face of the structure will be covered with a minimum of 18 inches of sand and sea oats planted along the length of the uppermost tubes.