Bogue Banks Restoration Project Phase II (2003)
View Graphic: Bogue Banks Restoration Project
EI passed a bond referendum in 2002 with special oceanfront and non-oceanfront tax districts to fund both Phase II and III of the Project. The tax rate for the oceanfront (Primary Benefit) and non-oceanfront (Secondary Benefit) tax districts are 0.48 and 0.03 cents per $100 valuation, respectively, that equates to an annual tax bill of $1,630 (Primary Benefit District) and $53 (Secondary Benefit District). Proceeds from the County's occupancy tax are also being used to defray project costs. The cost of Phase II and III, not including interest payments, is $17,000,000. The cost of Phase II was $11,711,630 and included expenditures for dune plantings, sand fence installation, mitigative measures, construction, and oversight. Coastal Science & Engineering (CSE) prepared the Environmental Impact Statement, Environmental Assessment, Biological Assessment, and construction documents for the project (CSE 2001 and 2002). The dredging contract was awarded to Weeks Marine, Inc.
(B) Project Background
Phase II can be conceptualized as 31,111 foot (5.9 mile) long project that essentially extends near the old IB Pier (just east of the IB/EI town boundary) at station 382+43 (or footmark 38,243), westward to Pinta Drive, located in the Columbus Square subdivision at station 693+54 (or footmark 69,354). Sand sources for the project were delineated in two (borrow sites B2 and A) of the three borrow sites (A, B1, and B2) depicted in the figure above. The project construction and post-project activities were conducted in accordance with original and subsequent modifications to the protocols and stipulations provided in N.C. State Permit Number 124-01 issued by the N.C. Division of Coastal Management, and Permit Number 200000362 issued by the U.S. Army Corps of Engineers.
View Graphic: Phase II Map
Specifications for the engineered beach were developed using a volumetric analysis (profile volume method) that was supplemented with historical erosion rate data. 111 transects at ~1,000-foot intervals were established along the entire length of Bogue Banks in 1999 to assess pre-project beach conditions (Fig. 4). At each transect, the profile geometry was determined by surveying the grade of the beach from the toe of the dune to the depth of closure estimated at -12 feet NGVD (National Geodetic Vertical Datum-1929). The depth of closure in this case was considered as the depth beyond which there is active motion of the seafloor for only 12 hours per year (CERC, 1984). Precise volumes were calculated by interpolating elevation boundaries for each cell and converting the distances and elevational differences to volumetric quantities. The profile volume method establishes site-specific criteria whereby volume deficits can be determined against a designed profile volume that accounts for the overall deficit and incorporates an additional beachfill volume to increase project longevity (Kana, 1993; Kana and Mohan, 1998).
View Graphic: Transect Map Insert
AB was determined as the healthiest portion of beach along Bogue Banks with the most sand volume along its profiles. AB was most recently nourished in 1994 as part of the Brandt Island beach disposal event that occurs every 8 to 10 years. Approximately 4.6 million cubic yards (mcy) were pumped onto the shores of Fort Macon and AB during this event. Development in AB sustained almost no damage as a result of the tropical storms/hurricanes that impacted Bogue Banks from 1996-1999. In contrast, approximately $10 million of damage was reported for oceanfront structures in PKS, IB, and EI (Kana et. al, 2002). Based on this information and additional profile analysis, CSE determined that 175 cubic yards (cy) per linear feet was a threshold minimum profile volume. Once this minimum profile volume was established, volume deficits were established for PKS, IB, and EI. The minimum deficit was augmented by an additional sand volume to increase project longevity based on erosion rates and final costs for cy of sand. The additional sand volume was estimated to account for ten years of historical erosion rates. Thus, the volume of sand placed on the beach for the project was engineered to provide adequate protection for ~10-year return period events. This analytical process is detailed in the Executive Summary - Shoreline Assessment and Preliminary Beach Restoration Plan and the Environmental Assessment (CSE, 1999 and 2001).
Based on the profile analysis and bid prices, 1,810,000 cy of sand were to be placed along the 5.9 mile stretch of shoreline in eastern EI. The profile volume method described above was utilized to delineate three distinct subcomponents of the beach where the designed volume of the non-tapered beachfill varied. The eastern, most critically eroded portion of Phase II that extends for almost 12,300 feet from the IB/EI town boundary westward was to receive approximately 83 cubic yards per linear foot (cy/ft). The middle zone encompassing almost 4,000 linear feet was scheduled to receive approximately 58 cy/ft, and a westernmost zone encompassing almost 12,900 linear feet was to receive approximately 35 cy/ft. Discontinuous dune construction was also proposed for the Project based upon pre-condition beach conditions for a cumulative stretch of approximately 8,400 feet. Precise volumes per construction transects (design templates) were included in the construction details prepared for the Project and were to be surveyed separately by Weeks Marine and CSE for payment verification. Dredged sand would be bulldozed into general construction specifications for subsequent grading into final contours and opened for recreational use. Completed sections were also scheduled to adjust to natural conditions before the upper reaches of the beach and newly constructed dunes were stabilized with sand fencing and/or dune plantings.
View Graphic: Cross-Section Graphic
Sand was to be dredged utilizing both hopper and cutterhead-suction dredging techniques. While the working permits did not stipulate dredging equipment specifications, EI pursued a maximum 6-foot cut within three subareas in borrow site B2 to facilitate the use of a cutterhead-suction dredge. Borrow site A was limited to a maximum cut of 3 feet which for the most part, constrains excavation alternatives to hopper dredging which is more productive at this depth. The 6-foot cut and the anticipated use of a cutterhead-suction dredge were sought in the interim between the construction of Phase I and II in hopes of; (1) diminishing the potential of turtle takings, and (2) increasing production rates that were experienced using hopper dredges for Phase I construction, which were perceived as "slow". Both delivery methods employ a buoyed pipeline with secondary "Y-valve" discharge pipes situated on the beach to transport material in one direction (east or west), then the other to complete ~1-2 mile sections. The cutterhead-suction dredge transports material directly to the beach via the buoyed pipeline. Conversely, hopper dredges utilize vacuum dragheads that collect sediments and temporarily stores borrow material in its hopper. Subsequent to completing a cut, the full hopper dredge will travel and affix to a bell joint end of the buoyed pipeline and propel the material to the beach.
View Graphic: Discharge
The Federal permit modifications obtained by EI also established sediment composition thresholds for the 3 or 4-foot and 6-foot cuts. A representative sediment sample was obtained during each day of operation along each newly constructed reach. Two samples were obtained per day if both cutterhead-suction and hopper dredging activities were being conducted along different reaches of the Project. Each daily sample was a composite of samples taken every 50 feet along a single shore normal transect from the toe of the dune to the berm crest. The target calcium carbonate percentage for material placed on the beach utilizing a cutterhead-suction or hopper dredge was 42 or 35 percent, respectively. There were no targets established for sediment texture (size, sorting, etc.).
(C) Construction History
Beach construction was initiated on January 13, 2003 when the cutterhead-suction dredge RS Weeks began excavating material from subarea B1 in borrow site B2. However, productivity remained below expectations (10,260 cy in-place/operation day through February 5, 2003) and it became apparent that additional equipment would need to be mobilized to complete the Project in the environmental window delineated in the working permits. Weeks Marine mobilized the hopper dredges RN Weeks and the BE Lindholm on February 5 and March 6, 2003, respectively. With the exception of a few trial cuts in borrow site B2 (4-foot maximum cut), authorized hopper dredge lanes were confined to borrow site A (3-foot maximum cut). Dredging productivity was at its highest for the Project from mid- to late-March, averaging approximately 50,000 cy in-place/operation day cumulatively for all three dredges.
The initial engineering criteria for dune construction was devised to insure the presence of a +15-18 feet NGVD dune along the entire Phase II Project area. Consistent with this approach, dune construction was not planned for areas where dunes located seaward of individual structures possessed a +15-18 feet NGVD maximum dune height. Locations below the +15 feet NGVD dune height threshold did receive a newly constructed dune at a +18 feet NGVD maximum height. Based on this criteria, dune construction was limited to the easternmost 3 miles of Phase II. This approach became problematic as the newly constructed dunes were placed seaward of adjacent existing dunes, providing for a discontinuous and irregular shore parallel dune system. Based on field reconnaissance and discussions with citizens, project engineers, and the dredging subcontractor; it was determined to distribute the newly constructed dune system along the entire Project reach extending from 10th Street to the old EI Pier (stations 440+43 to 532+18). The new construction methodology to achieve the continuous dune profile from 21st Street towards the old EI Pier (stations 501+00 to 532+18) included a 10-foot wide dune crest at +15 feet NGVD, with a 1 horizontal to 3 vertical slope. Re-grading and shaving the dunes previously constructed at +18 feet NGVD was completed in the oceanfront compartment located from the 10th to 21st Street reach (stations 440+43 to 501+00) to fill in the construction gaps in this area.
The Town of EI received a Notice of Violation from the N.C. Division of Coastal Management during construction of Phase II dated March 19, 2003 for placing incompatible sediment on beach in the area of 15th - 21st Streets (stations 468+00 to 501+00). A zone of calcium carbonate sand (shells and limestone fragments) was encountered by the cutterhead-suction dredge RS Weeks that was the basis of the violation. The calcium carbonate percentage was over the Project target of 42% and ranged as high as 87%. Sediment quality monitoring was being performed during the delivery of this material and the RS Weeks was instructed to relocate. Also, some of the limestone fragments encountered in this area were cobble- and boulder-sized. These rocks were manually removed from the beach per instructions from the U.S. Army Corps of Engineers in a correspondence that was also dated March 19th. A series of mitigative and restoration measures have been subsequently established for the area that predicated the Notice of Violation. These measures are described in a correspondence prepared by CSE on behalf of EI dated May 5, 2003 and include such provisions as additional compaction monitoring, the establishment of an additional beach biological sampling station, sediment sampling, and continued retrieval and disposal of rocks.
(D) Final Construction Summary
A total of 1,867,726 cy was delivered to the Phase II Project area by cutterhead-suction and hopper dredges from January 13 - March 27, 2002. The volume of sediment delivered by the cutterhead-suction dredge and hopper dredges were 877,831 and 989,895 cy, respectively (1,867,726 cy total). The geographic extents of the fill per placement method is depicted in the graphic below. Based on these data, hopper dredging provided approximately 20,632 linear foot of the fill or 66% of Phase II, while the cutterhead suction dredge RS Weeks provided approximately 10,479 linear foot of fill or 34% of the total Project. Other key construction facts and a summary of sediment quality data for Phase II is included in the following table.
View Graphic: Dredge Production
View Table: Fact Sheet
CERC, 1984. Shore Protection Manual. 4th Edition, U.S. Army Corps of Engineers, Coastal Engineering Research Center, Ft. Belvoir, Virginia, U.S. Governmental Printing Office, Washington, D.C., 2 vols.
Coastal Science & Engineering, 1999. Executive Summary - Shoreline Assessment and Preliminary Beach Restoration Plan, Bogue Banks, North Carolina. Columbia, S.C.
Coastal Science & Engineering, 2000. Technical Proposal, Coastal Engineering Services for a Beach Nourishment Project, Town of Indian Beach, North Carolina. Columbia, S.C.
Coastal Science & Engineering, 2001. Environmental Impact Statement - Bogue Banks Beach Restoration Plan. Columbia, S.C.
Coastal Science & Engineering, 2001. Environmental Assessment - Bogue Banks Beach Restoration Plan. Columbia, S.C.
Coastal Science & Engineering, 2001. Supplement to Bogue Banks Beach Restoration Plan Final Environmental Impact Statement. Columbia, S.C.
Coastal Science & Engineering, 2001. Biological Assessment - Bogue Banks Beach Nourishment Project, Carteret County, North Carolina. Columbia, S.C.
Kana, T.W., 1993. The Profile Volume Approach to Beach Renourishment. In Stauble, D.K. and Kraus (eds.), Beach Nourishment Engineering and Management Considerations, Association of Civil Engineers, New York, NY, p. 176-190.
Kana, T.W. and Mohan, R.K., 1998. Analysis of Nourished Profile Stability Following the Fifth Hunting Island (SC) Beach Nourishment Project. Coastal Engineering, v. 33, p. 117-136.
Kana, T.W., White, T.E., Forman, J.W., and McKee, P.A., 2002. Shoreline Erosion Along Bogue Banks, North Carolina. Proceedings from Solutions to Coastal Disasters Conference, Association of Civil Engineers, San Diego, CA., Feb. 25-27, 2002.