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Home My WebLink About00 - Avon - AppxA - Geotech APPENDIX A GEOTECHNICAL DATA ANALYSIS Avon Village Beach Nourishment Dare County, NC Prepared on Behalf of: Dare County Board of Commissioners Bob Woodard, Chairman 954 Marshall C Collins Drive, Manteo, NC 27954 Prepared by: PO Box 8056, Columbia, SC 29202–8056 [2525–TASK 4–JULY 2021] Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina ii — THIS PAGE INTENTIONALLY LEFT BLANK — Coastal Science & Engineering Geotechnical Data Analysis [2403M–Task 2–Appendix A] Buxton, Dare County, North Carolina i SYNOPSIS This report contains geotechnical data on the native beach and proposed borrow area sediments for a proposed beach nourishment project along ~2.5 miles of shoreline in Avon, North Carolina. The maximum sand volume to be placed along the project area is 1.0 million cubic yards (cy). The proposed borrow source is a sandy ridge ~2–3 miles offshore of the Avon Pier, within state waters. The report presents detailed results of beach sampling collected in March 2021 and borrow area sampling via cores collected in April 2021. Beach samples obtained along the visible beach and inshore zone, as well as offshore borings, were all obtained in accordance with North Carolina Technical Standards for Beach Fill Projects (15A NCAC 07H .0312 – see Attachment 4) and National Park Service Sediment Management Framework (NPS 2021). Beach Sampling Five stations (transects) were established by Coastal Science & Engineering (CSE) in March 2021 along the Avon project area at 3,000-foot (ft) spacing (5 stations between Sta 1560+00 to 1670+00) for sampling at 14 cross -shore positions (NPS/USACE 2015). The mean grain size of beach sand at the Avon project area was 0.289 millimeters (mm) as of March 2021. Samples collected below –8 ft NAVD were finer than those collected from the upper portions of the profile , while samples collected within the Trough (below MLW) were coarser. Offshore Borrow Area Compatibility Analysis The proposed offshore borrow area encompasses ~250 acres and was sampled by twelve 3-inch borings spaced ~900 ft apart. Each boring was collected to a uniform depth of 10 ft beneath the seafloor. The borings were subsampled and ana lyzed for grain-size distribution and comparison with the existing beach sand, then pro-rated according to the length of each sample interval. This allows calculation of boring statistics to a specified “composite” depth, which is useful for the operational considerations of dredge vessels. After calculating the composite values to 6 ft, 8 ft, and 10 ft depths, seven borings were found to have beach-quality sand to a depth up to 10 ft. These borings constitute Borrow Area 1. The mean grain size of Borrow Area 1, composited to a 10-ft depth, is 0.308 mm with 16.4 percent shell material, 0.1 percent gravel (4 to 76 mm), and 2.6 percent granules (2 to 4 mm) by weight. Five borings were found to have beach-quality sand to a depth up to 6 ft. These borings constitute Borrow Area 2. The mean grain size of Borrow Area 2, composited to a 6-ft depth, is 0.331 mm with 16.8 percent shell material, 0.7 percent gravel (4 to 76 mm), and 5.1 percent granules (2 to 4 mm) by weight. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina ii The boring density in both borrow areas is approximately 1 core per 20 acres. Based on these descriptive statistics, the proposed borrow area meets the requirements of the North Carolina Technical Standards for Beach Fill Projects (15A NCAC 07H .0312 – see Attachment 4) and National Park Service Beach Nourishment Guidance (NPS 2021). The proposed 150-acre Borrow Area 1 would provide up to 2.4 million cubic yards of beach quality sand if excavation is permitted to a depth of 10 ft. The proposed 100-acre Borrow Area 2 would provide up to 1.0 million cubic yards of beach quality sand if excavation is permitted to a depth of 6 ft. The proposed ~250-acre borrow areas will provide sufficient volume to accomplish a ~1.0 million cubic yard project. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina iii TABLE OF CONTENTS SYNOPSIS ......................................................................................................................... i TABLE OF CONTENTS ....................................................................................................... iii 1.0 INTRODUCTION ........................................................................................................ 1 2.0 METHODS ................................................................................................................. 7 2.1 Beach Samples ............................................................................................................................ 7 2.2 Borrow Area Samples ............................................................................................................... 11 3.0 RESULTS – BEACH SAMPLES ..................................................................................... 17 3.1 Beach Sediment Statistics – March 2021 ................................................................................ 17 3.2 Selection of Native Mean Grain Size ........................................................................................ 24 4.0 BORROW AREA INVESTIGATIONS ............................................................................... 25 4.1 Mean Grain Size of the Proposed Borrow Area ....................................................................... 34 5.0 SEDIMENT COMPATIBILITY ....................................................................................... 41 REFERENCES CITED ........................................................................................................ 46 Attachment 1A) Grain Size Distributions (GSDs) – Individual Beach Samples Attachment 1B) GSDs – Beach Composites Attachment 2) Core Logs and Core Photos Attachment 3A) GSDs – Individual Core Samples Attachment 3B) GSDs – Composite Samples–Upper 6 ft of Section by Core Attachment 3C) GSDs – Composite Samples–Upper 8 ft of Section by Core Attachment 3D) GSDs – Composite Samples–Upper 10 ft of Section by Core Attachment 4) North Carolina Technical Standards for Beach Fill (15A NCAC 07H.0312) Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina iv — THIS PAGE INTENTIONALLY LEFT BLANK — Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 1 1.0 INTRODUCTION This report provides sediment data for the Avon, North Carolina, project area based on sampling and analysis in 2021. Samples were obtained along the beach and inshore zone in accordance with North Carolina Technical Standards for Beach Fill Projects (15A NCAC 07H .0312 – see Attachment 4) and National Park Service Beach Nourishment Guidance (NPS 2021). Five stations (transects) were established by CSE in 2021 along the Avon project area at 3,000-foot (ft) spacing (5 stations between Sta 1560+00 to 1670+00) for sampling at 14 cross-shore positions. An offshore borrow area encompassing ~250 acres was sampled by twelve 3-inch borings spaced ~900 ft apart (Fig 1.1). The borings were subsampled and analyzed for grain -size distribution and comparison with the existing beach sand. Nourishment success depends on finding a source of sand that is similar in character to the native beach. The degree to which a particular borrow sediment matches the native beach sediments strongly influences project longevity and environmental impacts. Three outcomes are possible (Fig 1.2) (cf, Dean 1991, 2002):  Borrow sediment is finer than native – The majority of fill will shift offshore and yield a more gently sloping profile. The dry beach will be the narrowest.  Borrow sediment is coarser than native – The majority of fill will tend to “perch” on the visible beach and yield a steeper profile through the surf zone. The dry beach will be the widest.  Borrow sediment matches the native sediment – The fill will tend to follow the natural contours of the profile and retain similar slopes and morphology. It is generally accepted that the environmental impacts of nourishment are most likely to be minimized if the borrow sediment “matches” the native (NRC 1995). However, the question of what constitutes “native” is still debatable. In some settings, such as many South Caro lina beaches, sediments exist over a very narrow size range between the foredune and inshore zone [eg – mean = 0.18–0.22 millimeters (mm) with well-sorted sand at Isle of Palms SC]. In these cases, it is relatively easy to distinguish between “coarser” and “finer” than native. Most northern North Carolina beaches, by contrast, exhibit more variable sediment size distribu tions. Fine sand may dominate in the dunes and offshore, while coarse sand dominates the inner surf zone (USACE 2010). Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 2 FIGURE 1.1. Map showing the Avon project area, stationing of beach profiles along the fill template, and proposed borrow area with location of borings. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 3 FIGURE 1.2. Effect of borrow material grain size (nourishment scale parameter, AF) on the width of the dry beach for a fixed volume of nourishment sand added per unit of beach length (from Dean 1991, Fig 25). In simple terms, coarser sand relative to the native sediment produces a wider visible beach than finer sand. [Note: 1 m ≈ 3.28 ft] Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 4 FIGURE 1.3. Grain size distributions along a profile at Duck, North Carolina, ~70 miles north of the Buxton project area, illustrating the variation in grain sizes as a function of position (from Birkemeier et al 1985). North Carolina beaches are typically composed of quartz sand in the medium size range [0.25 –0.5 millimeter (mm) mean diameter]. Northern Outer Banks beaches tend to be coarser than southern North Carolina beaches with a wider range of sediment grain sizes (USACE 2000, 2010). Waves and nearshore currents, as well as winds, sort the sediments of the littoral zone and introduce characteristic topography across the profile. Coarsest material tends to concen trate at the inshore “plunge” point of bre aking waves where energy dissipation is focused (Miller & Ziegler 1958, Greenwood & Davidson-Arnott 1972, Komar 1998). Finer sands are winnowed and shifted offshore, leaving coarser sediments near the low watermark (Fig 1.3). Sands washed up the profile across the berm at high tide dry out and become sorted by winds, leading to the accumulation of finer sand in the dunes. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 5 Each sand size typically finds its equilibr ium position across the profile, with accumulations developing a particular slope and geometry. The longshore bar is often composed of fine sand (<0.25 mm diameter), which equilibrates at a gentler slope than the swash zone. Any suite of sediment sizes introduced to a beach by natural or artificial means will similarly sort under waves and migrate across the profile. Figure 1.4 illustrates a typical profile across the littoral zone showing primary features such as the foredune, dry beach (berm), beach face, trough, and outer bar. The visible beach (ie – above low water) along most coasts tends to exhibit well -sorted (poorly graded) sands of some dominant size class. A composite mean grain size of samples from each position for Avon, as measured in March 2021, is shown at the bottom of Figure 1.4. In other locations along the Outer Banks near Avon, dune sands are typically ~0.3 mm in mean diameter, while swash zone samples are coarse (0.5–1.0 mm) or very coarse sand (1.0–2.0 mm). Seaward of the inner surf zone, sediments are consistently fine sand (0.12–0.25 mm). The peaks in grain size observed at the dry beach (‘Dune Toe’) and trough along Avon are reflective of breaking waves and high winds winnowing finer sediments landward and seaward. Under North Carolina rules and standards for beach fill projects, any sediments within the sand -size range (0.0625–2.0 mm) are considered acceptable for use in nourishment proj ects. However, borrow areas must meet three important criteria: 1) Borrow sediments must not contain more than 5 percent fine-grained sediment (<0.0625 mm) by distribution above ambient conditions. 2) Borrow sediments must not contain more than 10 percent granules (2 mm to 4.76 mm) and 5 percent gravel (4.76 mm to 76 mm) by distribution above ambient conditions. 3) Shell content (percent CaCO3 material) may not exceed 15 percent by distribution above ambient conditions (ref — 15A NCAC 07H .0312). The following sections provide detailed results of sampling and analyses performed to iden tify potential borrow sediments meeting state standards for beach fill. The potential borrow sediments are also evaluated in terms of their likely performance in widening the beach. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 6 FIGURE 1.4. [UPPER] Littoral profile showing eight representative sediment sampling positions based on morphology. The ‘beach face’ sample in the upper figure corresponds to the Mid Tide sample in the lower figure, and the ‘low tide terrace’ corresponds to the Mean Low Water (MLW) sample. [LOWER] Overall trends in mean grain size by position across the profile based on 5 transects along Avon. Note the predominance of finer sands in the underwater zone and coarsest sand in the trough, within the active surf zone. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 7 FIGURE 2.2. Uniform sediment samples were collected on the beach in the upper 6 inches (15 centimeters), mixed, and subsampled for laboratory testing. FIGURE 2.1. Sample positions for “beach” grab samples along Avon following North Carolina sediment sampling criteria rules. The Avon littoral profile typically exhibits a narrow berm (dry-sand beach) and deep trough separating the outer bar from the beach. Elevations and depths (y-axis) are relative to approximately Mean Sea Level. 2.0 METHODS 2.1 Beach Samples CSE collected beach samples along the Avon project area in March 2021, encompassing the entire littoral profile to a water depth of −24 ft NAVD. Samples were collected along five transects (70 samples total). Figure 2.1 shows the cross -shore sample locations, and Figure 2.2 shows the sampling tool used for surface grabs in the upper 15 centimeters (cm) (6 inches) of sub strate. Station locations for the 2021 samples are illustrated in Figure 2.3. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 8 FIGURE 2.3. Location of sediment sample transects (14 samples per transect) along the Avon project area. Samples were collected in March 2021. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 9 Samples were inspected for mud , then washed, dried, and weighed in splits for analysis of grain size, gravel, and shell content. The split for grain size (~100-gram sample) was mechanically sieved at 0.25-phi intervals (ie – ~21 sieves in the sand size range), and each subsample was split, weighed, and recorded on lab sheets. The split for shell analysis (~20 -gram sample) was immersed in diluted muriatic acid (ie – nearly pure hydrochloric acid–HCl). After ~24 hours or once there was no evidence of bubbling, the remainder was rinsed, dried, and reweighed to the nearest 0.01 gram. The difference represented the proportion of shell in the sample. Summary tables of results, including sediment size distribution statistics, shell percentages, and fines percentages, are given in Section 3.0 (Results). Fines are defined here as mate rial passing the US Standard Sieve #230 (ie – <0.0625 mm) and generally consists of minute fractions of silt. No beach samples were observed to contain measurable quantities of clays or organics. Gra nule and gravel percentage was determined from the split retained on the US Standard Sieve #5 (>2 mm). In some cases, additional coarse sieves were used in the analysis for a breakdown of the small gravel sizes. Sample splits were converted to percentages and graphed as frequency and cumulative fre quency distributions. Standard statistical measures were computed, including true-moment measures, graphic means, and standard deviations (ie – Inman 1952, Folk and Ward 1957). Results were reported in millimeters as well as standard phi units. Figure 2.4 shows a typical datasheet for one sample; the set of laboratory datasheets is given in Attachment 1. Statistical composites of groups of samples were determined mathematically by averaging results for each individual size class for a given group of samples, then calculating moment measures for the composite. Composites were developed for each morphological unit sampled (ie – all dune samples combined, all toe-of-dune samples combined, etc). Groups of morphological units , such as dune and toe-of-dune, were also composited mathematically. Results of composite size distributions are given after the individual sample results in A ttachment 1. In general, they are identified on the datasheets as a morphological group or all samples. Multiple groups include a numerical value in the name corresponding to the a pplicable number of samples represented by the result . Percent fines are given on the sample datasheets, and summary tables provide all key statistics, including mean, standard deviation, skewness, percent shell, and percent gravel. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 10 FIGURE 2.4. Representative datasheet for a sediment sample at Station 1590+00 along Avon beach obtained in March 2021. The location of this station is illustrated in Figure 2.3. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 11 Summary tables of results, including shell and gravel percentages, follow in Section 3.0. Consistent with North Carolina sediment standards , arithmetic (non-weighted) means of groups of samples were computed from tabulated results using simple statistics (ie – mean, standard deviation, and skewness). Mean is the commonly reported typical grain size; standard deviation is a measure of the degree of sorting, and skewness reflects the degree to which the sample contains higher proportions of coarse sediment or fine sediment. Most beaches tend to have well -sorted and slightly coarse (ie – negative), skewed sediments. Shell material often adds a coarse fraction, as do granules and pebbles, which are common on Dare County beaches. 2.2 Borrow Area Samples There are delineated borrow areas off Rodanthe for emergency nourishment in the area around the NC Highway 12 S-curve near Mirlo Beach (USACE 2014), but these are not close enough to provide an economical option for fill at the Avon project area. Prior to the current effort, the most detailed core data off Avon was collected by the North Carolina Geological Survey (NCGS) with funding by Minerals Management Service and others (eg – Boss & Hoffman 2000, Hoffman et al 2001). This reconnaissance grid of borings obtained in the 1990s between Oregon Inlet and C ape Hatteras is summarized in a report by NCGS (Hanna & Nickerson 2009). Core spacing for the Boss and Hoffman borings was relatively large (typically >4,000 ft), which means that sediment quality over broad areas is unknown. For the proposed nourishment, CSE delineated one search area located along the northern flank of a shore-oblique ridge ~2 to 3 miles off Avon Pier (CSE 2020; Fig 2.5). One of the NCGS borings near the ridge indicated beach -quality sediment deposits in the area at thicknesses of more than 10 ft below the sea floor. Initial sediment samples from the sand search area were collected using CSE’s proprietary boring system, which creates a partial vacuum that allows penetration of a 3-inch aluminum core barrel into the substrate. The cores are retrieved by removing the core device, then capping and sealing the ends before they are brought to the coring vessel. Fourteen borings were obtained via this method in October 2020, which indicated sand along the ridge was a good match for the beach at Avon (CSE 2020). In April 2021, CSE contracted American Vibracore Services (AVS) to collect 15 additional borings spaced ~900 ft apart across a ~300-acre area along the northern portions of the ridge. Twelve of these indicated the search area contained beach -quality sediment. CSE used these favorable borings to lay out two proposed borrow areas with a total area of ~250 acres. Seven of the borings contained beach-quality sand to a depth of 10 ft below the substrate, while five indicated beach - quality sand to a depth of up to 6 ft below the substrate. The remaining 3 borings did not contain beach-quality sand in the volumes needed for a beach nourishment project. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 12 With 12 borings collected to a 10 -ft depth across a ~250-acre borrow area, CSE’s sand search meets North Carolina Technical Standards for Beach Fill Projects (15A NCAC 07H .0312 – see Attachment 4) and National Park Service Sediment Management Framework (NPS 2021) with a boring density of one core per ~20 acres of seafloor . Cores were split, logged under the supervision of a NC-registered professional geologist, and subsampled for sediment analysis. The “saved” core half was photographed and archived in plastic sleeves. Subsamples representing the section lithology were taken from the other half of the core at full-section intervals as given on the core logs. Samples were dried, weighed, disaggregated (if mud was present) and/or washed of salts, dried, weighed, and subsampled (~100 grams) for grain-size analysis via dry sieves at 0.25-phi intervals in the sand size range and several intervals as appropriate up to the “pea” gravel range (−4.0 phi, or 16 mm). Any pebbles, cobbles, or shells greater than 16 -mm diameter were retained on the −4.0 phi sieve and included in the weight percentages. Visual inspections indicated that only trace amounts of mud occurred in most of the borings , so mud analysis was only performed on a few samples. A separate subsample (~20 grams) was taken for “shell” an alysis (CaC03 content) which was determined by acid-burning using dilute hydrochloric acid. Percent granule and gravel were determined by percent weight of particles 2 to 4 mm and 4 to 76 mm diameter, respectively . Figures 2.7 and 2.8 contain example photo-mosaic and core logs for one of the final 10-ft borings (CSE core AV-27). The location of this core is shown in the inset panels of Figure 2.7. Attachment 2 contains the set of photo-mosaic and core logs. Attachment 3 contains the set of grain-size distribu- tions (statistics, frequency, and cumulative frequency curves) for i ndividual samples. Sample results were composited (weighted by section length) for the upper 6 ft, 8 ft, and 10 ft of substrate (Fig 2.8). This provides a practical operational result for evaluating sediment quality under representative dredge cut depths. Sediment quality for beach nourishment can be evaluated using the analytical model of James (1975), which is a standard method adopted by the USACE (CERC 1984). The James method computes an “overfill factor” (RA), which uses two simple parameters (mean grain size and standard deviation) to compare a prospective borrow sediment with the native size distributions. The overfill factor, RA, compares these parameters (using phi units) with a prospective borrow material and yields a simple ratio between one and ten. A value of RA=1.0 means the prospective borrow material matches or exceeds the native beach in terms of its potential performance (not necessarily a duplicate size distribution). A value of R A=1.5 means that ~1.5 times more borrow material would have to be placed to provide performance equaling the native beach. Borrow material that is considerably finer than the native sediment may have R A’s >>1 and, consequently, require many Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 13 times more volume to yield the same performance as native sand. The overfill factor, RA, is consistent with Dean’s equilibrium profile predictions , as previously illustrated in Figure 1.2. The similarity between borrow sediments and native beach sediments was also evaluated by means of comparative size-frequency curves for composited samples, which offers a more critical comparison of the beach and borrow sediments. If the two frequency curves are similar, the nourished beach will generally maintain the same aesthetic qualities. In general, the broader the size distribution of the native beach, the less likely there will be a perfect match with prospective borrow areas (Kana & Mohan 1998). As Gravens et al (2008) report, sediment grain size is the most important borrow material characteristic. FIGURE 2.5. Location of 29 CSE borings off Avon obtained between October 2020 and April 2021, along with borings obtained in 2001 by the North Carolina Geological Survey (NCGS). Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 14 FIGURE 2.6. Example core photo log for one of the 10-ft borings (AV-27) obtained by AVS in April 2021. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 15 FIGURE 2.7. Core log for AV-27 showing the lithology, sample intervals, and mean grain sizes. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 16 FIGURE 2.8. Example grain-size distribution (GSD) for the upper 10 ft of one of the 10-ft borings (AV-27). Results were composited from individual samples (weighted) for the upper 6 ft, 8 ft, and 10 ft of substrate. See Attachment 3 for composite GSDs for each core calculated for the upper 6 ft, 8 ft, and 10 ft. These thicknesses are representative of typical dredging depths (excavation sections) for offshore sediments along the East Coast (USACE 2010). Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 17 3.0 RESULTS – BEACH SAMPLES 3.1 Beach Sediment Statistics – March 2021 The Avon project area grain-size distributions (GSDs) for March 2021 samples (70) are given in Attachment 1 and summarized in Tables 3.1 to 3.4. Table 3.2 lists the means by station and cross- shore position. Primary measures base d on the method of moments and graphical methods show that arithmetic mean grain size for all samples is 0.289 mm (medium sand) with 4.0 percent shell and 1.8 percent gravel (Table 3.3 “UPPER”). However, there is a range of mean grain sizes moving in a cross-shore direction. Averaging by sample position (Table 3.3 “LOWER”), mean grain size ranges from ~0.17 mm seaward of the bar to ~0.92 mm along the trough. Figure 3.1 plots the results of all samples by station (north to south). As Table 3.3 indicates, the trough contains a high shell percentage of 17.6 percent. The mean grain size (unweighted, arithmetic means, and standard deviations) by cross -shore sample position is shown in Figure 3.2. The relatively low standard deviation at each point indicates the results were fairly consistent from station to station. Compared with Nags Head (~50 miles north of Avon), the Avon results are comparable. Coarsest sediments along Nags Head were observed along the mean low water zone , and dune samples tested ~0.3 to 0.35 mm (typical; CSE 2019). The underwater samples (seaward of the bar to –24 ft NAVD–North American Vertical Datum) along Avon generally exhibit mean grain sizes <0.25 mm (fine sand), similar to underwater samples along Nags Head (see Fig 1.4). Table 3.3 and Figure 3.1 show the sample averages. Mean grain size increases slightly from north to south along Avon in the range of ~0.27–0.32 mm. The percent shell and gravel by station is shown in Figure 3.4. Shell content is relatively uniform from north to south, while gravel content (4 to 76 mm) is <1 percent and granule content (2 to 4 mm) increases from <1 per cent to ~3 percent from north to south. This trend likely reflects the ongoing chronic erosion south of Avon pier. Samples collected seaward of the trough (n=30) are fairly uniform in size from north to south, with mean grain sizes ranging from 0.14 mm to 0.33 mm (Table 3.1a and 3.2). The trough exhibited the largest grain sizes, as well as the largest variation in grain sizes, within a range from ~0.71 mm to ~1.17 mm. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 18 The trough at Avon contains material that is coarser than typical for North Carolina beach sand. These grain sizes occur in the trough because finer sands have been winnowed from the area under strong littoral currents , and storm waves are unable to transport larger sediments out of the relatively protected trough. The grain-size distributions (Attachment 1A) show nearly all gravel (sizes >2 mm, −1.0 phi) falls in the 2–8 mm (−1.0 phi to −3.0 phi) size range with few large shell clasts. No significant concentration of large clasts was observed along the beach at the time of CSE samplings in March 2021. A large clast survey was conducted in March 2021 per requirements under North Carolina sediment standards for beach nourishment. Photo processing and data analysis were performed in April – May 2021 under Grant Contract No. CW20490 between the North Carolina Department of Environmental Quality and the County of Dare. The main findings of the large clast survey are summarized herein, and the full conclusions are presented in a separate report (CSE 2021). Photographs of the samples were examined, and the sediments were classified into three groups by a licensed professional geologist. These groups are ‘shell,’ ‘sediment,’ and ‘anthropogenic.’ Basic diagnostic characteristics of shell material include a smooth shiny surface, s harp angular edges (in the case of shell fragments), scars from tube worms, radial ribs and/or growth rings (more useful for bivalves), and an apex or siphonal notch (more useful for univalves). Samples classified as ‘sediment’ were differentiated based on the common characteristics of marl and beach rock common along many southeastern beaches. These types of sediments commonly exhibit either more shell material or a more consistent grain size than anthropogenic materials like asphalt or paving cement. Along the Outer Banks, anthropogenic material is often found on beaches where chronic erosion has resulted in structural foundations or roads being undermined and eroded by surf. These clasts are distinguished from natural beach rock because they often exhibit different density/weight, surface roughness, and chemical makeup. There were 26 samples observed along the five transects at Avon project area in the March 2021 survey. Each transect averaged ~5 shell fragments greater than 3” in diameter, <1 sediment samples greater than 1” in diameter, and <1 anthropogenic sample greater than 1” in diameter. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 19 TABLE 3.1a. Avon project area beach sediment characteristics (statistical measures) in March 2021. See Attachment 1 for detailed frequency and cumulative frequency results of each sample. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 20 TABLE 3.1b. Avon project area beach sediment characteristics (descriptive) in March 2021. See Attachment 1 for detailed frequency and cumulative frequency results of each sample. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 21 TABLE 3.2. Mean grain size (mm) for March 2021 beach samples in the Avon project area. TABLE 3.3. [UPPER] Arithmetic mean grain size and related statistics based on 14 samples per transect (n=70). [LOWER] Arithmetic mean grain size and related statistics by sample position for 5 transects along the Avon project area in March 2021. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 22 FIGURE 3.1. Mean grain size by station and sample position in March 2021. FIGURE 3.2. Mean grain size by sample position based on 14 samples per station. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 23 FIGURE 3.3. Sediment mean grain sizes for 5 transects in the Avon area in March 2021. The trend shows relative uniformity in the medium sand range (0.27–0.32 mm). FIGURE 3.4. Percent shell, granule (2 to 4 mm), and gravel (4 to 76 mm) by station in March 2021 along the Avon project area. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 24 3.2 Selection of Native Mean Grain Size According to North Carolina sediment standards for beach nourishment, the native sand should be determined based on an arithmetic average of means for all cross -shore sand samples. This simple calculation yields a mean and standard deviation of 0.324 mm and 0.644 mm (respectively) for the March 2021 samples (see Table 3.3). However, grain-size distributions are non-linear, so an average of mean grain sizes does not equal the mean grain size of the same collection of samples mixed in a bin, then tested for grain-size distribution (and resulting mean). It can be shown that a more realistic mean for a group of samples (composite mean) can be computed using the results of each sample split. CSE combined results of all subsamples retained on each sieve to compute the total weight by each size class for a designated number of sam ples, the total weight of all subsamples, and the corresponding proportions of the total. Data were normalized to the weight of each sample. This yields means and standard deviations via the method of moments, analogous to combining all physical samples into one big sample for sieving by standard lab methods. All 70 samples (5 stations, 14 cross-shore samples each station) yielded a mean and standard deviation of 0.289 mm and 0.521 mm (respectively), marginally lower than the arithmetic mean. Under the Wentworth size classification, the composite means are in the medium sand -size range (0.25–0.5 mm). The subaerial samples have a composite mean in the medium sand range (0.25–0.5 mm). These ranges are comparable to those observed at Nags Head (USACE 2010, CSE 2019) and Bogue Banks (CSE 2001, 2004), where composite mean grain size was ~0.3–0.35 mm. Beach sediments in the southern part of North Carolina and the northern part of South Carolina (the Grand Strand) are typically in the range of 0.2–0.3 mm with less cross-shore variation (eg – USACE 1993). Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 25 FIGURE 4.1. Location of proposed borrow area and relevant borings off Avon. Lines labeled “Transect 1,” etc. are the locations of bathymetric profiles shown in Figure 4.2. Coordinates of individual borings are given in NC State Plane Projected Coordinate System, in Easting and Northing (feet). 4.0 BORROW AREA INVESTIGATIONS One search area was evaluated for the proposed Avon nourishment project during the feasibility study (CSE 2020). The search area was centered on a low ridge ~2 to 3 miles offshore from Avon Pier in ~35 to 50 ft water depth. Previous studies by Boss & Hoffman (2000) and CSE (2013) indicated this area may have beach-quality material. Water depths in this area range from ~30 ft to ~45 ft and are therefore considered well outside the normal limits of the beach zone. Deeper water (~50 ft) exists between the shoal and longshore bar over one mile landward of the sand search area. Based on these preliminary data points, in October 2020, CSE implemented a sampling scheme wherein 15 borings were collected at ~900 ft spacing. Twelve borings out of the total fifteen indicated a ~250-acre portion of the central search area contained beach-quality sediment at thicknesses of 10 ft below the sea floor in Borrow Area 1 and 6 ft below the sea floor in Borrow Area 2 . The following sections of the report provide the results and evaluation of the proposed borrow source. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 26 FIGURE 4.2. Bathymetric profiles measured along transects as shown in Figure 4.1. The borrow area is situated along the northern slope of a ~3,000-ft ridge with a maximum elevation of (~)–35 ft NAVD. The long axis of the ridge trends north- northeast, and drops slowly to a depth of ~50 ft along the northern and western boundaries of the proposed borrow areas. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 27 * AVS – American Vibracore Services, a division of Amdrill Inc. based in Brooksville FL using the vessel R/V Thunderforce The October 2020 samples, obtained via CSE’s proprietary coring methodology, were collected to an average depth of 7.0 ft. The April 2021 borings (ie – AV-16 to AV-30) were collected to a uniform depth of 10 ft via vibracore by subconsultant AVS*. The October 2020 samples are exploratory and were not collected with the intention of delineating a final cut depth and borrow area boundaries; the April 2021 sample s were collected with those goals in mind. The April 2021 samples indicated a ~100-acre portion of the central search area contained beach -quality sand at depths of at least 6 ft below the sea floor, and a ~150 -acre portion of the northern search area cont ained beach-quality sands to at least 10 ft below the sea floor. As such, CSE’s selection of a borrow area mean gra in size (Section 4.1) is based on composited statistics from the April 2021 samples calculated to those cut depths. Figure 2.7 presented one of the 10-ft cores (AV-27) situated in the eastern quarter of the proposed Borrow Area 1. For the most part, sediments in each core were light tan or grayish tan in color and consisted of various mixtures of medium to coarse sand with minor amounts of she ll material and fine-sized grains (<0.125 mm diameter). Munsell color numbers were interpreted for each sample. Granules and “pea” gravel occurred in some cores. Core logs and photos of each core are provided in Attachment 2. The core logs in Attachme nt 2 document visual breaks in sediment type. In general, the CSE cores tend to have only trace amounts of mud. A couple of cores contained muddy and/or shell-rich layers, particularly sites close to the western and northern boundaries of the proposed borrow areas in deeper water. Visually, the appearance of a majority of core sediments was similar to the beach. Cores contained similar mixtures of sand, granules, pea gravel, and shell fragments found on the beach along Avon with only trace fines except for an occasional thin mud lens, as noted in the logs (see Attachment 2). Shell percentages for individual samples range from 5.2 percent to 48.9 percent, gravel percentages were <10 percent, and granule content were <30 percent. The highest percentages of shell and coarse material occur in Borrow Area 2, generally at depths greater than the proposed 6 - ft cut depth. When measured to the proposed cut depths, approximately 40 percent of the samples (14 out of 35) tested >15 percent shell above the native beach average whereas 8 percent of the samples (3 out of 35) tested >10 percent granule above the native beach, and one sample contained gravel content >5 percent above the native beach average. Fine grain-sized material exceeds >5 percent above the native beach in only 2 of the samples. This indicates the borrow area contains material that meets the quality standards of the NC Technical Standards for Beach Fill. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 28 The low statistical agreement between shell and gravel percentages (R2≈0.3) suggests that while some shell material may be larger than 2 mm, it is predominantly small particles under 2 mm in diameter. Such material is referred to as shell hash and often is composed of Donax sp. These small, thin-walled shells break easily under the action of waves in the surf zone. Core photo logs (Attachment 2) further confirm nearly all samples contained negligible concentrations of coarse (>2 mm) shell material within the proposed cut depths. Grain-size distributions for each core sample are given in Attachment 3. Table 4.1a–b lists the key descriptive statistics for each core sample along with the Unified Soil Classification System (USCS) and Wentworth description and the interpreted Munsel color. The unweighted arithmetic mean grain size for all samples is 0.36 mm (medium sand size comparable to the overall average mean grain size for the beach as discussed in Section 3.0). The unweighted arithmetic mean shell and gravel percentages for all 12 cores (35 samples) within the proposed borrow areas are 19.9 percent and 5.4 percent (respectively). For purposes of evaluating sediment quality within the sand search area, CSE computed weighted composite statistics to 6 ft , 8 ft, and 10 ft depths below the seafloor. Under operational conditions, dredges excavate to a particular depth, mixing sediments before discharge along the beach. The results of random length core samples were weighted proportionally over the applicable section. This yields a “composite” mean grain size, shell content, and gravel content for each designated interval. Similarities between the results for each composite are a measure of the down -core consistency of the sediments. A goal of the design is to leave sedime nts at the base of the excavation , which are similar in size and character to the removed sediments. This improves the chance for rapid recovery of benthic organisms (Van Dolah et al 1998, NPS 2012). Table 4.2 a-c provides key statistics for each core to the composited lengths of 6 ft, 8 ft, and 10 ft. The composite grain-size distributions for each set of data are given in Attachment 3B. Table 4.3 provides weighted mean sizes, standard deviations, shell percentages, and gravel percent ages for individual samples and weighted composite samples. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 29 TABLE 4.1a. Proposed borrow area sediment characteristics (statistical measures) for 15 cores collected in April 2021 (12 cores and 35 samples within the final borrow area). See Attachment 3A for detailed frequency and cumulative frequency results of each sample. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 30 TABLE 4.1b. Proposed borrow area sediment characteristics (descriptive) for 15 cores collected in April 2021 (12 cores and 35 samples within the final borrow area). See Attachment 3A for detailed frequency and cumulative frequency results of each sample. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 31 TABLE 4.2a. Composited offshore core sediment statistics and descriptions to a 6 ft depth, based on weighted averages of individual samples. See Attachment 3B for size frequency curves. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 32 TABLE 4.2b. Composited offshore core sediment statistics and descriptions to an 8 ft depth, based on weighted averages of individual samples. See Attachment 3B for size frequency curves. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 33 TABLE 4.2c. Composited offshore core sediment statistics and descriptions to a 10 ft depth, based on weighted averages of individual samples. See Attachment 3B for size frequency curves. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 34 TABLE 4.3. Weighted mean grain size, standard deviation, shell percentage, and gravel percentage for the 10-ft borings (35 samples) off Avon (“All Samples”). Composites are weighted by the applicable length of individual samples for the upper 6 ft of core (Comp 6), the upper 8 ft of core (Comp 8), and the upper 10 ft of core (Comp 10). Composite statics are based on the 10-ft cores within the box shown in Figures 2.5 and 4.1. Core Samples Mean Size (mm) Std Deviation (mm) Shell (%) Gravel (%) Granules (%) Borrow Area 1 6 ft 0.338 0.501 17.2 0.2 2.8 Borrow Area 1 8 ft 0.317 0.497 16.5 0.1 2.7 Borrow Area 1 10 ft 0.308 0.494 16.4 0.1 2.6 Borrow Area 2 6 ft 0.331 0.385 16.8 0.1 5.7 Borrow Area 2 8 ft 0.325 0.364 15.6 0.9 5.9 Borrow Area 2 10 ft 0.311 0.358 14.9 0.7 6.1 4.1 Mean Grain Size of the Proposed Borrow Area Borrow area sediments must reasonably match the native beach sediments in order for a beach nourishment project to be successful. If borrow sediments are finer than the native material, the beach will adopt a shallower slope and eroded more quickly (Dean 2002). If borrow sediments are coarser, the opposite will occur. The final round of borings within the proposed borrow area were collected in April 2021 to a uniform depth of 10 ft below the seafloor. There were 35 samples extracted from 12 borings across the ~250-acre proposed borrow area, meaning each boring is assumed to represent ~20 acres of sediment characteristics to a 10-ft depth. Composite statistics calculated to 6-, 8-, and 10-ft depths indicated the entire search area may contain beach -quality sand to a depth of at least 10 ft below the sea floor. However, closer examination of individual samples as well as visual inspection of the lower portions of borings within the proposed Borrow Area 2 indicated higher quantities of shell and fine-grained material than desired at depths greater than 6 ft (see Table 4.1a). The composite statistics and descriptions of the borings, composited to a 10-ft depth within Borrow Area 1 and a 6-ft depth within Borrow Area 2, are provided in Table 4.4. Based on the calculated composite statistics, the proposed Borrow Area 1 mean grain size is 0.308 mm with a standard deviation of 0.494, granule (2 to 4 mm) content of 2.6 percent, and gravel (4 to 76 mm) content of 0.1 percent, and shell content of 16.4 percent to a depth of 10 ft beneath the seafloor. The proposed Borrow Area 2 mean grain size is 0.331 mm with a Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 35 TABLE 4.4. Composited offshore core sediment characteristics to 10 ft and 6 ft depths, based on weighted averages of individual samples. See Attachment 3D for size frequency curves. standard deviation of 0.385, granule (2 to 4 mm) content of 5.1 percent, gravel (4 to 76 mm) content of 0.7 percent, and shell content of 16.8 percent to a depth of 6 ft beneath the seafloor. These values, as well as the core spacing (~20 acres per core), meet the NC Technical Standards for Beach Fill Projects (15A NCAC 07H .0312 – see Attachment 4) and NPS Sediment Management Framework (NPS 2021). Figure 4.3 shows composite statistics for each core within the sand search area. To more easily visualize the results, Figures 4.4–4.6 provide color isopach maps of mean grain size, shell percentages, and gravel percentages for 6 ft and 10 ft composite results. Coarsest sediments are found over a broad area encompassing the northern ~50 percent of the search area. Mean grain size is lower along the southern and southeastern edges. Figures 4.5 (percent shell) and 4.6 (percent gravel) show distributions of shell and gravel are similar to those of the coarsest sand. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 36 FIGURE 4.3. Mean grain size, percent shell, and percent gravel for core composite samples to 10 ft in Borrow Area 1, and 6 ft in Borrow Area 2, based on borings obtained in April 2021. Composite results of the seven 10 ft vibracores and five 6 ft cores are listed in table on the upper left. The 10-ft vibracores are highlighted in with red dots, while the 6-ft vibracores are shown in blue. Three of the borings obtained in April 2021 did not contain beach-quality sand at operational depths for a dredging operation, and are shown in grey outside the proposed borrow areas. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 37 FIGURE 4.4. Isopach map of mean grain size to 6 ft (upper) and 10 ft (lower) based on 15 cores within the search area off Avon. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 38 FIGURE 4.5. Isopach map of mean shell content to 6 ft (upper) and 10 ft (lower) based on 15 cores within the search area off Avon. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 39 FIGURE 4.6. Isopach map of mean gravel and granule content to 6 ft (upper) and 10 ft (lower) based on 15 cores within the search area off Avon. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 40 — THIS PAGE INTENTIONALLY LEFT BLANK — Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 41 FIGURE 5.1. Grain-size distribution (frequency and cumulative frequency) for composited March 2021 beach samples showing the distribution of beach sand grain sizes. 5.0 SEDIMENT COMPATIBILITY CSE evaluated sediment compatibility by comparison of grain-size distribution (GSD) of all “native” beach and offshore borrow areas sampled. The composite GSDs of the beach samples are weighted using the results of individual sample splits to derive numeric al results for each size interval. Figure 5.1 contains the composited beach sample GSDs. Similar composite GSDs were computed for cores in the offshore sand search area. All cores in the proposed borrow area were evaluated. Weighted GSDs were computed for the upper 6 ft, upper 8 ft, and upper 10 ft composited length (Attachment 3). Based on the quality of these composite samples, compared with NC Technical Standards for Beach Fill Projects), those borings indicate beach-quality sand is available to a 10-ft depth within Borrow Area 1 and a 6-ft depth within Borrow Area 2. So, the Sediment Compatibility analysis will focus on the 6-ft and 10-ft composite statistics. Figure 5.2 contains the composited borrow area GSDs for Borrow Area 1 (10 ft cut) as well as Borrow Area 2 (6 ft cut). Composite borrow sediments are classified as poorly sorted and coarse skewed , typical of many littoral profiles (Komar 1998). Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 42 FIGURE 5.2. Grain-size distribution (frequency and cumulative frequency) for offshore borings to a composite depth of 10 ft in Borrow Area 1 [UPPER] and 6 ft in Borrow Area 2 [LOWER]. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 43 Figure 5.3 provides comparative frequency and cumulative frequency size distrib utions for native and borrow sediments. Beach samples are compared with the relevant composite borrow area statistics for all borings within the proposed borrow area (eg 6-ft n=5; 10-ft n=7). Note the fair match in D50 for each comparison (ie – where the two cumulative curves intersect the 50% line), evident in the matching shapes of the frequency curves. As previously discussed, sampling of the beach in March 2021 yielded a mean grain size of 0.289 mm, a result that is within 0.1 mm of the composited borrow area means in Figure 5.3. CSE also computed overfill factors, R A, for native beach GSDs and each 10-ft composited boring. The overfill factor, RA (CERC 1984), provides a measure of how a particular sediment will perform as beach nourishment. Low RA’s are generally preferred, with the ideal being equal to 1.0. To apply the method, a native sediment size must be assumed. As discussed in Section 3.3, the native beach grain size is 0.289 mm along the entire profile. Table 5.1 provides RA’s for each core composite to 10 ft and 6 ft, depending on where each core is located. Each table lists the applicable mean and standard deviation in phi units which are required in the James (1975) formulation (CERC 1984). RA values were calculated for compos ite core mean grain sizes compared to the native beach. Inspection of Table 5.1 shows that RA’s range from ~1.0 to ~1.5 for Borrow Area 1, and from ~1.0 to ~1.4 for Borrow Area 2. All of the core composites have low RA’s when compared with the native beach size distribution. While RA’s are not considered to be definitive in matching sediment texture for nourishment (Dean 2002), the results herein suggest the majority of cores will provide good nourishment performance. The overlap of the GSDs (Fig 5.3) further supports this finding. The proposed ~250-acre borrow area(s) will provide sufficient volume to accomplish the proposed ~1.0 million cubic yard project. The minimum excavation area to provide the design volume would be ~60 acres, assuming a dredge cut averaging ~10 ft deep, or ~100 acres with a dredge cut averaging ~6 ft deep. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 44 TABLE 5.1. Computed RAs (overfill factor) for each composite core sample (6- or 10-ft composite, for the respective borrow areas) compared with the composite native beach results. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 45 FIGURE 5.3. GSDs for Avon native beach samples (n=70) compared with Borrow Area 1 samples to 10 ft [UPPER] and Borrow Area 2 samples to 6 ft [LOWER]. In both cases, the borrow area sediments are expected to be coarser than the native beach initially. Over time, the grain size of the post-nourishment beach is expected to move closer to the historical grain size distribution around Buxton. Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 46 — THIS PAGE INTENTIONALLY LEFT BLANK — Coastal Science & Engineering Geotechnical Data Analysis [2525–Task 4–Appendix A] Avon, Dare County, North Carolina 47 REFERENCES CITED Birkemeier, W, H Miller, S. Wilhelm, A. DeWall, and C Gorbics. 1985. A user’s guide to the Coastal Engineering Research Center’s (CERC’s) Field Research Facility. Tech Rept CERC-85-1, USACE-WES, Vicksburg, MS. Boss, SK, and CW Hoffman. 2000. Sand resources of the North Carolina Outer Banks: assessment of Pea Island study area. 4th Interim Report for Outer Banks Task Force and the North Carolina Department of Transportation, 87 pp. CERC. 1984. Shore Protection Manual. 4th Edition, US Army Corps of Engineers, Coastal Engineering Research Center, Ft Belvoir, VA; US Government Printing Office, Washington, DC, 2 vols. CSE. 2001. 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