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Coastal Science & Engineering Preliminary Coastal Engineering Analysesi

AUGUST 2005 [TR2145-02] Nags Head, North Carolina

EXECUTIVE SUMMARY

This report is prepared in connection with planning for a possible locally funded beachnourishment project at Nags Head, North Carolina. In December 2004, Coastal Science& Engineering (CSE) submitted a “Draft Beach Restoration Plan” to the Town of NagsHead at the request of the town’s Board of Commissioners. The draft plan (CSE 2004a)was based on review of the US Army Corps of Engineers (USACE 2000) Dare Countyproject and other available data. CSE determined that the federal project greatly over-estimates the 50-year nourishment requirements and costs because:

• Long-term erosion rates for Nags Head have been much lower than projected inthe Dare County project (USACE 2000). Therefore, much less sand is neededover a 50-year period to keep pace with erosion.

• Quality sand resources for nourishment are located offshore in close proximity toNags Head. Experience with other projects suggests that these deposits could bedredged and placed on the beach in a reasonably cost-effective manner.

CSE’s alternative nourishment plan estimated an initial “ten-year” project that wouldrequire ~4 million cubic yards at a cost of (~)$21–26 million (ie, $23.5 million ±15 percent).

The alternate plan is intended to build on the existing federal plan such that the approvedenvironmental impact statement (EIS) would remain applicable (USACE 2000). Thismeans that a particular offshore borrow area (USACE 2000 offshore area S1) would bethe favored sand source. To remain consistent with the federal plan, the project wouldencompass ~10 miles (out of ~11.2 miles) of Nags Head shoreline. No nourishment wouldbe provided along the northernmost ~1.0 mile, consistent with the Dare County EIS.Under CSE’s plan, some sections of Nags Head would receive greater volumes of nourish-ment than other sections because of variations in the existing beach condition and differ-ences in erosion rates.

Because of uncertainties in CSE’s draft alternate plan and given large differences betweenthe federal and alternative plan formulation, the Town of Nags Head commissioned thepresent study. Its purpose is to verify historical sand losses, evaluate the potential qualityand compatibility of offshore sand for beach nourishment, and revise the alternate planaccordingly.

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The present report includes:

• Updated beach condition survey and calculation of volumetric erosion rates(Section 2.0).

• Extensive beach sampling and definition of the “native beach” sediment quality(Section 3.0).

• Results of offshore borings (60 sites) and sediment quality tests in potential borrowarea “S1" (Section 4.0).

• Results of sediment sampling and sediment quality tests in Oregon Inlet andUSACE disposal areas on Pea Island (Section 4.0).

• Engineering analyses of sediment compatibility for nourishment (Section 4.3), arevised project formulation, and an updated estimate of probable construction costs(Section 5.0).

Following are highlights of each section.

Nags Head Sand Losses and Erosion Rates (1994–2005)Using comparative surveys from August 1994 (courtesy USACE 2000) to April 2005(present study), CSE has determined that Nags Head has lost ~2.9 million cubic yardsover the past decade (Fig A). Approximately 70 percent of the loss occurred between theforedune and low-tide wading depth (ie, –5 ft NGVD depth contour). The balance occurredin the zone between wading depth and the 18-ft depth contour (ie, ~1,500 ft offshore).

Average annual erosion rates (measured in cubic yards per linear foot of shoreline peryear, cy/ft/yr) ranged from near-zero along the northernmost ~1 mile of Nags Head (Reach0) to about 12.0 cy/ft/yr along the southern ~2 miles of Nags Head. The central ~8 milesof Nags Head have eroded at an average of ~3.5 cy/ft/yr. Town-wide erosion averages~5 cy/ft/yr or about 275,000 cy/yr. These rates are about 29 percent of the projectedfuture losses under the federal Dare County project. In other words, if the past decadeis an indication, future erosion along Nags Head is likely to be about one-quarter theestimate assumed in the federal nourishment plan.

Figure B (upper) illustrates how much sand is present on the beach (1994 and 2005conditions) for each of the reaches delineated in Figure A. The corresponding volumetricerosion rates are given in Figure B (lower).

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FIGURE B [upper]. Average unit-width beach volume by reach to –18 ft NGVD (about 1,500–2,000 ft offshore) inAugust 1994 versus April 2005.

FIGURE B [lower]. Average annual unit-width beach volume change to low-tide wading depth and offshore inAugust 1994 versus April 2005.

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As Figure A shows, fully 60 percent of the shoreline (Reaches 0 and 1) accounts for onlyabout 27 percent of the sand losses. Reach 2, representing ~22 percent of the shoreline,accounts for ~27 percent of all sand losses. Reaches 3 and 4 (south Nags Head),representing ~18 percent of the shoreline, account for at least 45 percent of the sandlosses.

The present analysis indicates higher average erosion rates along south Nags Head thanestimated by CSE (2004a), but reaffirms moderate erosion rates (<2.5 cy/ft/yr) for 60percent of the shoreline. Accordingly, CSE recommends increasing the nourishmentvolume under the alternate beach restoration plan to approximate CSE’s (2004a) “upperscenario” as discussed in Section 5.0.

Summary of Native Beach Sediment QualityCSE analyzed the quality of sediments on the native beach using draft sampling protocolsof the NC Coastal Resource Commission (NC CRC 2005), as well as an alternate, morelimited sampling across the active beach zone (CSE–recommended criteria for the presentproject). Based on 110 samples encompassing the length of Nags Head between theforedune and 15-ft depth contour, the present study finds:

• There is a high degree of variation in mean grain size from station to station andfrom position to position across the profile.

• The mean grain size of dune samples (dune, toe dune) averages between 0.3 mmand 0.36 mm but individual samples exhibit a range of ~0.2 mm to >0.7 mm.

• The mean grain size of dry beach and swash zone samples (dry berm, MHW, LTT)average around 1.0 mm, but individual samples span a range from 0.27 mm to >3.5mm.

• Underwater samples (trough, bar, outer) show mean grain sizes that average 0.19mm to ~0.23 mm, with the range for individual samples between ~0.17 mm and>0.3 mm.

The alongshore and cross-shore trends in mean grain size are shown in Figure C. Theupper portion of Figure C shows the alongshore trend for three groups of samples:

• Dry beach to low-tide terrace (LTT) (ie, the breaker and swash zone)• Underwater (ie, trough, outer bar, and offshore)• All samples combined (ie, dune to offshore)

Coastal Science & Engineering Preliminary Coastal Engineering Analysesvi


FIGURE C. Overall trends in mean grain size by station and position across the profile. Red lines pool all samples.Trend line (dashed red line in upper graph) shows decrease in mean grain size from north to south.

Coastal Science & Engineering Preliminary Coastal Engineering Analysesvii


The results illustrate how coarse the swash zone samples are compared with the offshoresamples. Offshore samples tend to be relatively uniform in mean grain size (around 0.2mm). Swash zone samples, even when combined, still exhibit a wide range of grain sizes.Combining all samples (red line in Figure C, upper) smooths the trend and suggests an(arithmetic) average range of mean grain sizes of the order ~0.45–0.65 mm. Mean sedi-ment grain size tends to become finer toward the south, consistent with previous studies(cf, USACE 2000).

Figure C (lower) shows the cross-shore trend in mean grain size, giving the average of allsamples from a particular position along the profile. The cross-shore trend shows acharacteristic coarsening from the dune to the low-tide terrace (near wave plunge point),then a rapid fining of sediment seaward of the inner breaker zone.

For purposes of continued project planning, CSE elected to adopt two “native beach” sizedistributions for Nags Head, using the results in Section 2.0. Figure D shows the charac-teristic size distribution curves for the two composites. The top graph shows a compositenative size distribution based on toe of dune, dry beach, mean high water, low-tide ter-race, and trough samples, consistent with CSE’s prior practice. The lower graph showsa composite based on foredune to outer (offshore) samples, consistent with draft NC CRC(2005) sampling guidelines. Resulting mean grain sizes are 0.47 millimeters (mm) (CSEcriteria) and 0.36 mm (NC CRC criteria).

Borrow Area InvestigationsTwo potential borrow areas for nourishment are considered in the present study:

1) Offshore area “S1" as delineated by USACE (2000) for the federal DareCounty project (Fig E).

2) Oregon Inlet channels and shoals.

Area S1 was chosen for investigation because of previous studies and recommendationsby USACE (2000) as well as the fact that there is an existing EIS for the area whichshould facilitate permitting should the town elect to pursue a locally funded nourishmentproject.

The USACE (2000) first delineated the nearly ten-square-mile area S1 based on ~32borings. The Corps estimated that S1 contains as much as 100 million cubic yards ofbeach-quality sand within the upper 10 ft of the bottom. CSE was contracted to collect 60additional borings and further evaluate its sediment quality. In anticipation of an ~4 millioncubic yard project for Nags Head, only a small fraction of S1 would be required.

Coastal Science & Engineering Preliminary Coastal Engineering Analysesviii


FIGURE D. Nags Head composite grain-size distributions for the “native beach” as adopted herein. The lower graph,based on all samples, follows the draft NC CRC (2005) sampling protocols. The upper graph shows the result for a morelimited zone of sampling between the toe of dune and trough.

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FIGURE E. Location of offshore area S1 (delineated by USACE 2000) various subareas and the grid ofcores obtained in the present study. Subareas W–C, E–C, and S were the basis of CSE’s survey grid.Subgroups N, C, and S (red) were delineated based on sediment test results.

Coastal Science & Engineering Preliminary Coastal Engineering Analysesx


Oregon Inlet was considered as a potential borrow area for Nags Head because of ongo-ing federal dredging of the channel and the possibility of piggy-backing on the federalproject via Section 933, or some other funding means. Section 933 projects, under federalregulations, allow a local sponsor to obtain dredged material for the difference in cost be-tween what the federal government would pay for nearby disposal and what it costs toplace the spoil on the local beach.

Oregon Inlet is dredged on a regular basis, and material is usually disposed along PeaIsland about one-half mile downcoast of the channel. Because of these ongoing activitiesand the fact that the inlet is situated less than 5 miles from Nags Head, it may provide aneconomic source of sand. As part of the present study, CSE obtained sediment samplesand short borings from sites in the inlet and on the Pea Island disposal area for purposesof evaluating sediment quality.

Comparison of Potential Borrow and Native SedimentsCSE analyzed about 150 sediment samples from offshore area S1 and Oregon Inlet forcompatibility as nourishment material. Compatibility was evaluated by means of the over-fill factor RA (CERC 1984), which provides a measure of how a particular sediment will per-form as beach nourishment. RA’s of less than 1.5 are generally preferred, with ideal beingequal to 1.0.

To apply the method, a native sediment size must be assumed. In this case, two possiblenative size distributions were applied:

1) “Composite 69" representing the sediments found between the toe of duneand trough along Nags Head.

2) “Composite 110" representing all available beach samples (dune to off-shore) following draft recommendations and criteria of the NC CRC (2005).

In the first case, the mean grain size (Mz) is 0.474 mm. In the second case, Mz=0.362mm.

CSE subdivided ~125 samples from offshore area S1 into three subgroups (S, C, N) meet-ing the following approximate criteria:

• Coarse sand (mean grain size ~0.5 mm or greater)• Represents upper 2 ft or deeper substrate• Similar sediment quality in adjacent cores

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On the basis of subgroups S, C, and N, CSE determined that portions of area S1 will yieldhighly favorable sediments comparing the two native distributions (Fig F). Resulting RA’swere in the range 1.02–1.3 for subgroups S, C, and N. By comparison, RA’s for OregonInlet sediments average >7.0, meaning nearly seven times more Oregon Inlet sand wouldbe required to equal the performance of S1 sand.

Revised Plan & Estimate of Probable Construction Costs for a Locally Funded ProjectCSE recommends modifications to the draft alternative beach restoration plan in connec-tion with the findings in the present study. These modifications are based on refinementsin the measured erosion rates and sediment quality in the two potential borrow areas.Table A summarizes CSE’s recommended changes to the project formulation. Thechanges include:

• Minor modification of reach boundaries consistent with the boundaries usedto estimate updated erosion rates (cf, Fig A). [Note: These boundarieslikely will be modified further during final design.]

• Slight (~8 percent) reduction in the average “initial” volume to make up thesand deficit (revised @ 37.6 cy/ft versus draft plan @ 40.6 cy/ft).

• Increase in the “advance nourishment” volume based on the measured vol-ume losses since 1994 (revised @ 50.0 cy/ft versus draft plan @ 35.1 cy/ft).

• Revised average nourishment volume is 87.6 cy/ft versus draft plan at 75.7cy/ft.

• Revised total volume for a “ten-year” project is 4.6 million cubic yards [ie,the upper scenario in the original CSE (2004a) draft plan].

TABLE A. Revised project formulation for alternate locally sponsored nourishment at Nags Head.

Coastal Science & Engineering Preliminary Coastal Engineering Analysesxii


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Coastal Science & Engineering Preliminary Coastal Engineering Analysesxiii


The modified project formulation outlined in Table A is consistent with the goal of prolong-ing the life expectancy of the project by varying the rates of nourishment from reach toreach. It is important to recognize that if 4.6 million cubic yards of sediment similarto the native grain-size distribution are placed on the beach and annual sand lossescontinue to be in the range of 250,000–300,000 cy/yr, there will be a significantvolume remaining after ten years. If the quality of sediment placed on the beach is finerthan native, little or none of it will remain on the visible beach after ten years.

CSE updated the estimates of probable construction costs given in CSE (2004a) basedon the revised project formulation (Table B). The resulting nourishment volume and esti-mate of probable construction costs are:

Revised Plan: ~4.6 million cubic yards $26,750,000

No attempt is made in the present study to further refine mobilization costs or pumpingcosts in this estimate. Variations of 10–15 percent in these totals should be anticipated.

TABLE B. Nags Head – revised alternate locally funded beach restoration plan – “10-Year Project.”

The present report also provides an estimate of “equivalent performance” costs usingOregon Inlet as a borrow source. Because inlet sediments are much finer than S1, anoverfill factor should be applied. In other words, x-times more inlet sand would be re-quired to provide nourishment sediment comparable in performance to S1. CSE’s analysis

Coastal Science & Engineering Preliminary Coastal Engineering Analysesxiv


indicates that even an optimistic overfill factor of 3.0 and cost sharing under a Section 933project would yield costs much higher than a local plan using sand from area S1.

Figure G provides an updated summary of the revised alternate project formulation forNags Head using a portion of offshore area S1. Further refinement to the plan is antici-pated as additional engineering analyses are developed.

Recommendations and Project Requirements for ImplementationThe present study addresses erosion rates and sediment availability for nourishment ofNags Head. Based on the results herein, several refinements to the “ten-year” plan arerecommended, including:

• Increase nourishment volumes by 15 percent to better match the observed erosionrates between 1994 and 2005.

• Use portions of offshore area S1 having the coarsest sediments so as to bettermatch the native beach.

• Abandon consideration of Oregon Inlet dredge spoil because it will not providedesirable performance or adequate volumes to restore and maintain the beach.

Implementation of a locally funded project will require certain milestones as outlined byCSE (2004a). The time line for execution depends on establishing a funding plan andobtaining permits. Accordingly, CSE recommends the following critical path.

• Town review and adoption of a plan (either the recommended plan or somemodification of it to fit an approved budget)

• Town establishment of a funding plan and overall project budget• Use the elements of the present study and USACE (2000) to prepare a

permit application at the earliest time• Hold a funding referendum• Continue engineering and environmental coordination

– Conduct additional coastal engineering site investigations, culturalresource surveys, and environmental surveys

– Refine the design based on fill-erosion models and sediment budgets– Updated condition survey in spring 2006 (or prior to completion of

final design)– Finalize design

Coastal Science & Engineering Preliminary Coastal Engineering Analysesxv


• Obtain construction easements• Obtain permits• Prepare plans, specifications and construction documents• Receive bids from contractors• Revise final plans to match bids and funds available• Construct project• Postproject monitoring and maintenance

CSE believes the project will achieve significant cost savings if it can be constructed dur-ing fair-weather months (ie, April through September). Construction in winter will necessi-tate considerable weather delays and add to project costs. Additional environmentalprotection costs will be incurred for summer construction. However, these would beexpected to be far less than the costs associated with winter weather delays. The earliesttime for construction (assuming summer periods are allowed) would be approximatelyApril–September 2006. Such a schedule would require permits and funding to be in placewithin less than eight months from the date of this report. A more realistic schedule wouldbe for construction to occur in 2007. This would provide sufficient time for permitting andenvironmental coordination and give the Town of Nags Head more time to evaluate fund-ing options.

Coastal Science & Engineering Preliminary Coastal Engineering Analysesxvi


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