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 Restoration Design

Introduction

The grounding of the R/V Columbus Iselin impacted the reef's surface and crushed the structural framework below the vessel's hull, resulting in exposure of vulnerable sub-surface reef structure. This reef structure is composed of lightly, naturally cemented coral fragments. Eight areas of damage were identified, including four severely impacted spurs. The grounding impacted approximately 3,749 square feet of reef area with the loss of approximately 7,503 cubic feet of reef material (Coastal Planning & Engineering, Multibeam Survey Results, July 1997). Additionally, this damage made the reef structure susceptible to further reoccurring damage from storm events. Storm waves from Hurricane Georges scoured out reef material at the four severely damaged sites, almost doubling the material (volume) losses from the initial damage incident. Survey results indicate that the volume of lost reef material increased from 7,503 to 13,827 cubic feet following the hurricane. Until the reef spurs are repaired and stabilized, the damaged areas are at risk of further storm damage. This project is intended to restore and stabilize the structure of the damaged reef spurs.

Design Concept

The proposed repair plan involves the use of limestone boulders, placed in the four damage depressions in the reef and stabilized with a tremie pour of specialized marine concrete around the boulders. Each structure will be constructed by first placing a layer of boulders into the bottom of the damage hole. Concrete will be poured around this layer, filling interstitial spaces and attaching the layer to the sides and bottoms of the cavity to the greatest extent possible. The concrete fill will terminate just above the boulders' widest sections to stabilize the boulder layer and provide for secure placement of the next layer.

Side view of boulder layers placed in the damage depression in the reef. Click on the image to view a larger, more detailed version.

Next, corrosion resistant fiberglass rebar will be placed in the concrete for improved attachment to the next concrete/boulder layer. The concrete surface will also be rough to allow for improved adhesion to the upper layer. Layers will be constructed above one another in a similar manner until the reef spur is restored. When restoration is completed, the surface will consist mostly of limestone boulders, with concrete interstitial fill below the boulders' crests. Some of the repair units have sides that will be exposed to deeper areas. Along these sides, the structure will be formed to replicate, as closely as possible, the undisturbed reef slopes. The limestone boulders used in repair will weigh from three to five tons and generally feature diameters of about four feet. Once completed, each reef repair unit will be a solid structure.

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Stability Analysis

Once the reef repair structures are constructed, their stability in storms is of utmost importance. Wave climate and stability analyses were conducted to ensure restoration structures will be able to sustain a 50-year storm as well as other storms. The stability analysis was based on predictions for reef breakwaters and rubble mound structures fully exposed to storm waves, without the sheltering of adjacent reefs and at a shallower depth than the proposed reef repair structures. This provided a conservative approach.

The stability analysis assumes the most critical site conditions, a water depth of six feet National Geodetic Vertical Datum over the adjacent reef crest, and thus over the crest of the design fill structure. Assumptions also include a 50-year surge of 8.8 feet and depth-limited waves over the structure. The 50-year storm surge elevation estimate was based on 50-year return period stillwater elevations available for several onshore sites within seven miles of Looe Key reef (storm surge information was not available for the Looe Key site). The limestone rock density is assumed to be 140 pounds per cubic foot.

One stability analysis was based on predictions for reef breakwaters and free-standing rubble mounds fully exposed to storm waves. It assumes that if less than 100% of the rubble mound is destabilized, then the structure below is stable.

Three analyses were used to test the stability of the structures in storm waves. Two are based on work conducted at the Corps of Engineers Coastal Engineering Research Center and one on the Shore Protection Manual. The first of these analyses represents, as a relatively conservative approach, a prediction of the stability of a free-standing rubble mound, without the benefit of the concrete. With this approach, the post-storm shapes of rubble mound structures following ultimate wave damage are evaluated to determine structure stability. If at least the lower layer of the rubble mound structures are found to be stable, then the structure below the hypothetical rubble mound breakwater (the reef repair) should be stable. The analysis is conservative considering the boulders used in reef repair will be: placed in a reef depression; transformed into a single unit by the tremie pour concrete; and at a lower elevation than the breakwater boulders. Results indicated that the bottom layer of mound will be stable and, thus, the repair will be stable.

The second analysis was conducted to investigate the structural stability of the design, including the benefit of using concrete to create a single, solid structure at each site. The "fill" is defined as the combined boulder/concrete unit that will be created in each of the reef cavities. The interpretation of the results accounts for the fact that the effective sizing of the fill is increased beyond that of the individual boulders due to the adherence of the interstitial concrete. The adherence between the concrete and boulders is expected to be adequate to withstand the storm conditions, due largely to the irregular shapes and rough surfaces of limestone quarry boulders. The analysis indicated that to ensure conservatively adequate stability, an effective boulder size of 19 tons would be required. This would be represented by a boulder approximately 7 feet in diameter. In comparison, the smallest repair unit will be over 15 feet wide at the narrowest point, and several feet in average thickness. Thus, the stability of the structure fill material is predicted to be adequate.

The final analysis was similar to the second analysis. It is the most sensitive to storm wave height and was used to verify stability in severe storms. This analysis is based on the "Stability of Rubble Structures" relationship presented in the Shore Protection Manual. It involved applying the most severe predicted storm conditions at the Looe Key project area to determine the effective boulder sizing required to result in a stable boulder at Looe Key. Results indicated that boulders placed in such an environment would require sizing of 27 tons (about eight feet in diameter). The effective boulder sizing of the repair units will exceed this size (as discussed above), verifying adequate stability of the Looe Key reef repair. Additionally, based on volumes determined from a bathymetric survey, the weight of boulders and concrete used to fill the damaged spurs will greatly exceed 27 tons.

Each analysis approach was based on conservative applications of the potential wave climate. Although their particular results vary, all indicate the adequate stability of the proposed reef repair design.

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Construction Methods

Limestone boulders will be obtained from a quarry, most likely in the Key Largo area. The boulders will be washed at the quarry site and transported by truck to the contractor's barge. The barge will transport the boulders to the site for placement within the reef repair site.

At the project site, the barge will be positioned with five previously established mooring points. If the existing mooring points are insufficient for securing the barge, the contractor, in cooperation with sanctuary personnel, may establish additional mooring points.

Top view of perimeter boulders sealed with grout to create a "form" for the placement of tremie pour concrete within the cavity. Click on the image to view a larger, more detailed version.

Boulders will be lifted into place by a crane located on the barge. Initially, boulders and smaller chinking rocks will be placed along the perimeter of the damage site that coincides with the edge of the reef spur, and thus with deeper adjacent bathymetry. The spaces between and around the boulders on the perimeter will be grouted with concrete to form an impervious seal. The purpose of these sealed perimeter boulders is to create a "form" for the eventual placement of tremie pour concrete within the cavity. After the perimeter boulders have been sealed, interior three to five ton boulders will be placed within the damage site cavity. The boulders will be placed closely together, but sufficient space will be left

Fiberglass rebar will be inserted into the concrete to assist in attachment of concrete/boulder layers. Click on the image to view a larger, more detailed version.

between the boulders for concrete placement. After the first layer of boulders has been placed in the cavity, tremie pour concrete will be pumped between the boulders. The upper one-third of the boulders will not be covered with cement during the first pour so that this first layer of boulders will provide attachment to the second layer of boulders and concrete that follow. Additionally, rebar will be inserted into the concrete layers of the repair. This procedure will be repeated for the additional layers of boulders and will continue until the surface layer of boulders is placed. It is anticipated that no more than three layers of boulders will be required at any location. The top layer of boulders will be placed to emulate the relief of the natural reef.

The concrete formula to be used in the tremie pour is the same that was used at the Elpis-Maitland restoration in the Upper Keys in 1995. The concrete will be mixed aboard the contractor's barge and pumped through hoses to the voids between the boulders. The contractor is required to contain all materials on board the barge, to guard against accidental releases of water or concrete into the water column.

Following restoration of the physical reef structure, on-going monitoring for structural integrity and biological recovery will be conducted. Biological restoration of the Columbus Iselin site is planned for the future.

Top view of boulders, rebar and concrete placed in damage depression. Click on the image to view a larger, more detailed version.

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