fill suitability characterization of a complex … · 2018. 2. 23. · fill suitability...

FILL SUITABILITY CHARACTERIZATION OF A COMPLEX COASTAL AREA: DARE COUNTY, NORTH CAROLINA

Donald K. Stauble1, William Birkemeier2, Michael F. Forte2 and William A. Dennis3

ABSTRACT

As part of the design and pre-project monitoring for a beach nourishment project, the native beach sediment needs to be characterized and compared with sediments from a borrow area to see if they are suitable for use. In preparation for a beach nourishment project along two sections of the Dare County, North Carolina coast, a total of 670 sediment samples were collected. Ten samples were collected along each of 67 profile lines along a stretch of coast from Southern Shores, NC to the northern side of Oregon Inlet in May 2006 as part of the pre-fill monitoring. Surface grab samples were collected by hand at the toe of the dune, berm crest, mean high water line, mean sea level line, and at the mean low water line. In the nearshore, samples were collected with a Ponar dredge from -6, -12, -18, -24 and -30 ft depths. This large number of native beach sediment samples has allowed for a detailed study of both cross shore and alongshore variability. The area features complex geology with variability in the morphology of the nearshore area. Several hot spots have been identified where erosion exceeds the average rate and their location can be tied to the morphology differences. The complex sediment data is related to this varied morphology and is characterized to provide data for design and monitoring of a stable beach nourishment project. The fill design can be adjusted in the future during the operations and maintenance of the project based on the results of this pre-fill monitoring. INTRODUCTION To assess fill suitability and document the composition of the natural beach, pre-fill beach sediment needs to be characterized and compared with sediments from a borrow area. The ideal situation would be for sediment in the borrow area to have the same grain size distribution and composition of the existing beach where the fill is to be placed. When the native beach and/or the borrow area have high variability in the distribution of grain sizes or composition of sediment, a more involved analysis procedure is needed to correctly characterize the beach and determine the amount of material needed to provide for a stable fill. This paper will provide techniques used to characterize the grain size of a native beach in an area that has a complex sediment distribution and to understand the reasons behind the associated complicated shoreline morphology. The goal is to provide the best analysis for monitoring a beach nourishment project that will provide the desired level of shore protection and damage reduction. A compatible fill will extend the interval between renourishments and reduce the overall cost of the project. __________________________ 1 U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199, 2 U.S. Army Engineer Research and Development Center, Field Research Facility, 1261 Duck Road, Kitty Hawk, NC 27949 3 U.S. Army Engineer District, Wilmington, 69 Darlington Ave, Wilmington, NC 20402

DARE COUNTY PROJECT Dare County, North Carolina is located in the northeast part of the state along the Atlantic Ocean. Portions of the beachfront are experiencing erosion with loss of beachfront homes and commercial structures from storm activity. The study area covers 44 km (27 miles) from Southern Shores in the north to Oregon Inlet in the south (Figure 1). The monitoring plan has divided the study area into a North of Project area, where no fill will be placed. The Project Area includes two areas (North Fill and South Fill) that will receive fill, separated by a gap of approximately 3 km (1.9 miles) between the fills where the shoreline has been stable and no fill will be placed. The monitoring continues in the South of Project area through the Cape Hatteras National Seashore to Oregon Inlet. No fill will be placed in this area, but it is downdrift of the net longshore transport and will be monitored to document any gain in sand on the National Seashore beach and Oregon Inlet ebb shoal from the fill placement. DATA SAMPLING SCHEME The physical monitoring program includes the surveying of 144 beach profiles spaced approx. 305 m (1,000 ft) apart. Profiles originate from a known benchmark landward of the primary dune and continue offshore to the -10 m (-32 ft) depth contour. Real Time Kinematic Global Positioning Satellite system (RTK GPS) is used to obtain position and elevation data with horizontal and vertical accuracies between 2-6 cm (0.05–0.20 ft). The dune and dry beach are surveyed with a backpack mounted RTK GPS system. The bathymetric data is collected with an Army Lighter Amphibious Resupply Cargo (LARC)

Figure 1. Location map and sampling scheme for the Dare County Project

Hyde

Pender

Onslow

Brunswick

Dare

Carteret

Currituck

New Hanover

ProjectLimitsNORTH

CAROLINA

Oregon Inlet

Cape Hatteras

Cape Fear

Cape Lookout

North of Project

Project Area

North Fill Area

South Fill Area

SouthernShores

KittyHawk

NagsHead

KillDevilHills

Cape HatterasNational Seashore

AtlanticOcean

AlbemarleSound

Croatan Sound

SouthNagsHead

Line -140

Line 20

Line 265

Line 29

Line 289

Line 489

South of Project

Line -10

Line -159

Line 670

Line 1009

vessel equipped with RTK GPS, echosounder, and motion reference unit measuring heave pitch and roll. The speed of sound through the water column was determined from measurements of conductivity, temperature, and salinity (CTD). The LARC has the advantage of being able to survey continuously from the base of the dune through the surf zone to the seaward limit of the profile. The backpack obtained portion of the profile was combined with the overlapping LARC beach portion of the profile to obtain a continuous profile from the landward side of the dune out to the seaward limit of the profile. The profile lines used in this study were collected during April 2006. Sediment samples were collected along 67 of the profile lines (locations shown on Figure 1) during May 2006. Ten samples were collected along each profile with a total of 670 samples collected for grain size analysis. Surface grab samples were collected by hand at specific elevations based on the NAVD88 tidal datum. Sample locations included the Dune Toe (+3.96 m, +13 ft), Berm Crest (+2.13 m, +7 ft), mean high water line (MHW) (+0.76 m, +2.5 ft), mean sea level line (MSL) (+0.15 m, +0.5 ft), and at the mean low water line (MLW) (-0.3 m, -1.0 ft). In the nearshore, samples were collected with a Ponar dredge off the LARC at -1.83, -3.66, -5.49, -7.32 and -9.14 m (-6, -12, -18, -24 and -30 ft) depths (Figure 2). GPS was used to obtain positions of the sediment samples and they were entered into a Geographic Information System (GIS). With this large number of native beach sediment samples, a detailed study of cross shore and alongshore variability in the native beach grain size distributions could be completed.

Figure 2. Sediment sample locations across profile GRAIN SIZE ANALYSIS All 670 sediment samples were washed and sieved at ½ phi intervals from -2.5 phi (5.66 mm) to 4.0 phi (0.0625 mm) at the Coastal and Hydraulics Sedimentation Research Laboratory. About 10% of the samples were also run through the Coulter LS100 laser particle counter to analyze for silt size material. Sediment statistics including mean grain size and sorting were calculated by method of moments.

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Elev

atio

n (m

)

Distance Offshore (m)

Dare County, NC

-30

-24-18

-12-6

MLWMSLMHW

BERM

DUNE TOE

NAVD88

Line -140 North of ProjectSediment Samples

-30 ft

-24 ft

-18 ft

-12 ft

-6 ft

LARC Ponar Samples (5)

Beach Grab Samples (5)

Dune ToeBerm

MHWMSL

MLW

The grain size distributions were highly variable in both the cross shore and long-shore directions. Figure 3 shows representative frequency curves across four profiles from the a) North of Project, b) North Fill, c) South Fill and d) South of Project areas. The nearshore samples all had similar curves showing a fine grained, well sorted sand. The beach samples showed the variability with coarser, more poorly sorted sands on the northern portion of the study and finer, more-well sorted sands to the south. The distribution of the mean grain size across the profile, using all 670 samples shows that the most variability and coarsest material are in the MHW, MSL, MLW and -6 ft samples (Figure 4). The means became slightly finer toward the dune toe but still had a wide spread. The nearshore samples from -3.66 to -9.14 m (-12 to -30 ft) were all fine sands with much less mean grain size variability. This spatial distribution of mean grain sizes was found on other ocean beaches. This difference in the grain size distribution as one proceeds from the dune base, across the beach and continues offshore was described by Bascom (1959). The coarsest grains are usually found in an area just seaward of the backwash/surf interaction zone, at the shore break plunge point, an area of high turbulence (Bascom, 1959; Zarillo, et al, 1985; Stauble, 1992; Stauble and Bass, 1999). The berm crest area also contains significant coarse material due to runup sediment transport dynamics. Finer, better-sorted material is found in the dune area owing predominantly to wind transport processes. Seaward of the nearshore bar, sediments become more uniform, with a narrow range of fine grain sizes (well sorted), between the -3.66 to -9.14 m (-12 to -30 ft) samples. This extends to depth of closure (the seaward limit of significant depth change) on most of the profiles (Birkemeier et al, 2007).

Figure 3. Variability in individual grain size distributions along study area

Pre Installation Sediment DataDare County, NC

0

10

20

30

40

50

60

70

-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50 5.50 6.50 7.50 8.50 9.50 10.50 11.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

VERY COARSE SAND

COARSE SAND

MEDIUMSAND

FINESAND

VERY FINE SAND

16.00 4.00 0.25 0.00035mm

phi

8.00 2.00 0.50 0.031

GRAVEL SILT

LINE 670 - Nags Head (Main South Fill)May 2006 (Start of Phase I)

1.00

CLAY

0.0039

Pre & Post Sediment DataDare County, NC

0

10

20

30

40

50

60

70

-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50 5.50 6.50 7.50 8.50 9.50 10.5 11.5

Grain Size

Freq

uenc

y W

eigh

t Per

cent

VERY COARSESAND

COARSE SAND

MEDIUMSAND

FINE SAND

VERY FINE SAND

16.00 4.00 0.25 0.063mm

phi

8.00 2.00 0.50 0.125

GRAVEL SILT

LINE 159 - Kitty Hawk - North fillMay 2006

CLAY


0

10

20

30

40

50

60

70

-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50 5.50 6.50 7.50 8.50 9.50 10.50 11.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

VERY COARSE SAND

COARSE SAND

MEDIUMSAND

FINESAND

VERY FINE SAND

16.00 4.00 0.25 0.00035mm

phi

8.00 2.00 0.50 0.031

GRAVEL SILT

LINE -140 - North ControlMay 2006

1.00

CLAY

0.0039


0

10

20

30

40

50

60

70

-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50 5.50 6.50 7.50 8.50 9.50 10.50 11.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

VERY COARSE SAND

COARSE SAND

MEDIUMSAND

FINESAND

VERY FINE SAND

16.00 4.00 0.25 0.00035mm

phi

8.00 2.00 0.50 0.031

GRAVEL SILT

LINE 29 - Cape Hatteras NS (South Control)May 2006 (north side Oregon Inlet)

1.00

CLAY

0.0039

a) Line -140 North of Project (Southern Shores)

b) Line 159 North Fill (Kitty Hawk) d) Line 29 South of Project (Oregon Inlet)

c) Line 670 South Fill (Nags Head)

Coarse Fine

Dune Toe

Berm

MHW

MSL

MLW

x

-6ft

-12ft

-18ft

-24ft

-30ft

x

Figure 4. Mean vs. sorting plot of cross-shore trends CHOOSING A COMPOSITE SAMPLE To reduce some of the variability in grain size distributions, composite curves were constructed by mathematically combining the weight percents of each sieve size for several samples across the profile. The typical composite sample is the Profile Composite where all 10 samples (Figure 5a) were combined and averaged to produce a new composite frequency distribution curve (blue line in Figure 5d). Due to the abundance of fine well sorted nearshore samples, the composite curve is finer and more poorly sorted than the individual curves. This tended to make all the curves similar for each profile line and did not help in understanding the grain size distribution along the project. A Foreshore Composite is often used to represent the intertidal area of the beach profile where most fill sand will be placed during a project. This curve is a composite of the MHW, MSL and MLW samples (Figure 5b). This composite curve (green line in figure 5d) represents the combined grain size distributions of the intertidal area of the beach and eliminates the finer nearshore samples. Since there was high variability in the Berm Crest and -6 ft samples along the Dare County beaches, a Beach Composite was constructed that included the Berm Crest, MHW, MSL, MLW and -6 ft samples (Figure 5c). This curve (red line in Figure 5d) fell between the coarser foreshore composite and the finer profile composite. It was thought this curve best represented the sediment composition along each profile line and was used in the analysis for this paper.

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

-35.0-30.0-25.0-20.0-15.0-10.0-5.00.05.010.015.020.0

Elevation about NAVD (ft)

Mea

n

V Fine Gravel

V Coarse Sand

Coarse Sand

Medium Sand

Fine Sand

V Fine Sand

DuneToe

Berm MHWMSL

MLW-6 ft

-12 ft -18 ft -24 ft -30 ftFine Gravel

L109

L199L109

phi0.063

mm

0.125

0.25

0.5

1.0

2.0

4.0

8.0

L159

L869

Fine

Coarse

4.66.1 3.1 1.5 -4.6 -6.1

ft

m-7.6 -9.1 -10.70.0

Medium SandMedium Sand

All 670 Samples

0

5

10

15

20

25

30

35

40

45

50

-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

-140Dune Toe -140Berm -140MHW -140MSL -140MLW-140-6ft -140-12ft -140-18ft -140-24ft -140-30ft

VERY COARSE SAND

COARSE SAND MEDIUMSAND

FINE SAND VERY FINE SAND

16.00 4.00 0.25 0.063mm

phi

8.00 2.00 0.50 0.125

GRAVEL SILT

LINE -140 - North ControlMay 2006a) Line -140

all curves

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Elev

atio

n (m

)


Dare County, NC

DC -140 20060413 COMBINED

-14

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-8

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-4

-2

0

2

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6

8

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Elev

atio

n (m

)


Dare County, NC


MSL

-24-18

-30

-12-6

DUNE TOEBERM MHWMLW

0

5

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30

35

40

45

50

1 3 5 7 9 11 13 15 17 19

Grain Size

Freq

uenc

y W

eigh

t Per

cent

-140MHW -140MSL -140MLW

VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

LINE -140 - North ControlMay 2006b) Line -140

intertidal curves

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Elev

atio

n (m

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Dare County, NC


MLWMHW

MSL

Line -140

-14

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8

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Elev

atio

n (m

)


Dare County, NC


-6MLWMHW

BERMMSL

d) Composite SampleCurves

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50

-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

Composite -140 Composite -140 Composite -140

VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT


Composite of MHW, MSL & MLW

Composite of Berm, MHW, MSL, MLW & -6 ft

Composite of Dune Toe, Berm, MHW, MSL, MLW, -6 ft, -12 ft, -18 ft, -24 ft & -30ft

0

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1 3 5 7 9 11 13 15 17 19

Grain Size

Freq

uenc

y W

eigh

t Per

cent

-140Berm -140MHW -140MSL -140MLW -140-6ft

VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT


Foreshore Composite: MHW, MSW, MLWBeach Composite: Berm, MHW, MSL, MLW, -6ft

Profile Composite: Dune Toe, Berm, MHW, MSL, MLW, -6, -12, -18, -24,-30

c) Line -140beach curves

Figure 5. Composite sample types examined for study

SEDIMENT DISTRIBUTIONS AND PROFILE CHARACTERISTICS Due to the high variability in the individual sediment samples along each profile line, the Beach Composite was constructed for each profile. To identify spatial patterns on this project, the analysis of the composite grain size curves was divided into the four zones of the project.

North of Project Area The North of Project area included profile lines -140 to -10. The 10 composite samples in this zone are shown in Figure 6. Even with the use of composites there is still a high degree of variability and no discernable pattern in the grain size distributions in this zone. There are three peaks in the grain size distributions showing a predominance of very coarse sand on profile lines -110, -70 and -20 (lines w/red tones). A second group of samples had a peak of coarse to medium sand on lines -130, -100, -80 and -50 (lines w/blue tones). A third group had a peak in the fine sand range on lines -140, -40, and -10 (lines w/green tones). All of these samples were poorly sorted indicating a wide range of grain sizes found on the beach between the Berm Crest and the -6ft sample. The profile lines are shown in an insert on Figure 6 and show that the profiles are all uniform in shape in this area, with a dune, steep foreshore slope (indicative of the coarser sediments) and a single nearshore bar. The nearshore slope is uniform seaward of the bar.

North Fill Area Fill material will be placed south of the vicinity of profile line 109 and extend to the vicinity of profile line 329. Eight composite samples from profile lines 20 to 189 are shown in Figure 7 that covers the area in Kitty Hawk. As in the North of Project area, the sediments are highly variable, with very coarse to coarse composites at lines 109, 138 and 159. Coarse to medium sands are located at lines 20, 50 and 189, while lines 80 and 169 are medium to fine sands. In Kill Devil Hills the sediment composites become a little finer, but there is still a high variability in grain sizes distributions (Figure 8). Profile line 260 is composed of coarse sand and profile line 289 is bi-modal with predominately medium sand but has a coarse component. Profile lines 199, 219, 249, 279 and 309 are predominately medium sands and line 229 has a fine grain peak. This zone exhibits complex 3D nearshore bathymetry with ridges and swales or “moguls” running obliquely to the beach as shown in the two figures in both plan views and in profile. Generally, the coarser beach composites are along the profile lines that transect the swales and the finer beach composites are along the lines that transect the ridges, but there are some exceptions. As will be shown later, this area is associated with the subsurface trace of the ancestral Albemarle River channel (Snyder, 1997).

South Fill Area The gap between the North and South Fill area is represented by the beach composites on profiles 340 to 460 (Figure 9). These composites are either predominately coarse grained as in lines 369 and 400 or predominately medium sands as in lines 340, 429, and 460. The profiles along this section show a smoother nearshore with a single nearshore bar/trough configuration but still contain complex sediment grain size distributions.

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Elev

atio

n (m

)


North of Project North of Project ––Southern ShoresSouthern Shores

-140

-20

-40-50

-130

-70-80

-100-110

-10

0

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-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

Composite -140 Composite-130 Composite-110 Composite-100 Composite-80Composite-70 Composite-50 Composite-40 Composite-20 Composite-10

VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINES - North ControlMay 2006

Composite of: Berm, MHW, MSL, MLW and -6 ft samples

-140-130

-110-100

-80-70

-50-40

-20-10

Profile Line:

Grain SizeVery Fine SandFine SandMedium SandCoarse SandVery Coarse SandVery Fine GravelFine Gravel

Grain SizeVery Fine SandFine SandMedium SandCoarse SandVery Coarse SandVery Fine GravelFine Gravel

Figure 6. North of Project beach composite variation and related beach profiles

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Elev

atio

n (m

)


North Fill Area North Fill Area -- Kitty HawkKitty Hawk

0

5

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25

30

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-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

Composite 20 Composite 50 Composite 80 Composite 109Composite 138 Composite 159 Composite 169 Composite 189

VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINES - Kitty Hawk - Line 109 Start of N. FillMay 2006

Composite of: Berm, MHW, MSL, MLW & -6 ft

20

50

80

109

138

Grain SizeVery Fine SandFine Sand

Medium SandCoarse Sand

Very Coarse Sand

Very Fine GravelFine Gravel



Very Coarse Sand


High : -4.262

Low : -13.51

Depth (m)

Source FRF

1592050

80109

138 169189

169

159

189

Profile Line:

Figure 7. North Fill (Kitty Hawk) beach composite variation and "mogul" bathymetry

Figure 9. Area between Fills beach composite variation and related beach profiles

North Fill Area North Fill Area ––Kill Devil HillsKill Devil Hills



Very Coarse Sand




Very Coarse Sand


0

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-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

Composite 199 Composite 219 Composite 229 Composite 249 Composite 260 Composite 279 Composite 289 Composite 309

VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINES - Kill Devil HillsMay 2006


199

279260

249229

219

High : -4.262

Low : -13.51

Depth (m)

Source FRF

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Elev

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289309

199219

229249

260279

289309

Profile Line:

Figure 8. North Fill (Kill Devil Hills) beach composite variation and "moguls"

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-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

Composite 340 Composite 369 Composite 400 Composite 429 Composite 460

VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINES - Kill Devil Hills/Nags Head (Between Fills)May 2006




Very Coarse Sand




Very Coarse Sand


Between Fill Areas Between Fill Areas ––Nags HeadNags Head

340

429

400

369

-12

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-4

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2

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12

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Elev

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460

340

369

400

429

460Profile Line:

The South Fill will start in the vicinity of profile line 489 and extend to near profile line 1020 in the Town of Nags Head. The native beach composite grain size distributions in this zone are more skewed to medium to fine sand sizes. In the north end of the South Fill, only lines 489, 549 and 609 of the nine profiles had peaks in the coarse size range (Figure 10). The rest of the samples are predominately medium to fine sands. The beach profile shapes in this northern part of the South Fill have a more complex nearshore bar/trough configuration, with the trough width varying along each profile. The central section of the South Fill area also has a similar beach composite set, with only one very coarse sample at line 829 (Figure 11). There are a few coarse to medium samples at lines 670, 709, and 729. The other five composites are skewed to the finer grain size distributions. The profiles in this central South Fill area showed a smooth nearshore and single nearshore bar/trough shape. Line 829 is the northernmost line to show the shore-attached shoal at the seaward end of the profile. The remaining composites along the profiles in the southern section of the South Fill are shown in Figure 12. The composites are all composed of predominately fine grained material and show better sorting of grain sizes. This set of composites, containing no coarse material, is distinctly different from the composites in the north part of the study area. The profiles have the characteristic bar/trough configuration in the nearshore, but the slope of the nearshore is variable and the seaward end of the profiles show the edge of Pratt Shoals.

Figure 10. South Fill (north) beach composite variation and related beach profiles

South Fill Area South Fill Area ––Nags Head (north)Nags Head (north)

0

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-4.50 -3.50 -2.50 -1.50 -0.50 0.50 1.50 2.50 3.50 4.50

Grain Size

Freq

uenc

y W

eigh

t Per

cent

Composite 489 Composite 509 Composite 529 Composite 549 Composite 569Composite 589 Composite 609 Composite 630 Composite 649

VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINES - Nags Head (South Fill - North)May 2006




Very Coarse Sand




Very Coarse Sand


489

589

569

549

529

509

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-8

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Elev

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609

630

649489509

529549

569589

609630

649Profile Line:

Figure 11. South Fill (central) beach composite variation and related beach profiles

South Fill Area South Fill Area ––Nags Head (south)Nags Head (south)



Very Coarse Sand




Very Coarse Sand


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-4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5

Grain Size

Freq

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y W

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t Per

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VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINES - Nags Head (South Fill - South)May 2006


869

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890

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Elev

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989

1009

850869

890909

929951

970989

1009Profile Line:

Figure 12. South Fill (south) beach composite variation and related beach profiles

South Fill Area South Fill Area ––Nags Head (central)Nags Head (central)



Very Coarse Sand




Very Coarse Sand


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t Per

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VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINES - Nags Head (South Fill - Central)May 2006


670

789

771

749

729

709

689

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809

829

670689

709729

749771

789809

829

Profile Line:

South of Project Area Sediment samples were collected along 9 profiles in the South of Project area within the Cape Hatteras National Seashore property. The beach composites show more variability than the south end of the southern fill, but are still predominately fine grained sand. Only lines 234, 204, and 174 have a small percentage of coarse sand (Figure 13). The profiles show the influence of the northern edge of the ebb shoal at Oregon Inlet with a high degree of variability of nearshore bar positions and slopes.

HOT SPOTS Hot spots along a beach are areas of the shoreline that erode higher than the background rate along a stretch of coast. Two hot spots have been identified along the Dare County coast within the project area. Figure 14 shows the location of the two hot spots on nearshore bathymetry surveyed by the United States Geological Survey (USGS). The irregular offshore bathymetry in the North Fill area is due to the ancestral Albemarle River channel (Browder and McNinch, 2006; McNinch, 2004). A USGS shoreline change study comparing the 1970 to 1997 shoreline also confirms a higher rate of erosion in this region (Morton and Miller, 2005). Rates of erosion between 2 and 4 m/yr (6 and 13 ft/yr) were measured in the mogul area. The underlying geology appears to control the highly variable bathymetry, which in turn affects the sediment distributions in this area. The second hot spot is associated with the shore-attached shoal at the southern end of the project. This hot spot may be the result of wave refraction over the shoal feature.

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South of Project South of Project ––Cape Hatteras NSCape Hatteras NS

Oregon InletGrain Size

Very Fine SandFine Sand


Very Coarse Sand




Very Coarse Sand


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VERY COARSE SAND



16.00 4.00 0.25 0.063mmphi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINES - Cape Hatteras NS (South Control)May 2006


265

5985

114

144

174

204

234

265234

204174

144114

8559

2929

Profile Line:

Figure 13. South of Project area beach composite variation and related beach profiles

Similar patterns were found at Ocean City, MD, where two separate shore-attached shoals were associated with two hot spots (Stauble and Bass, 1999). Shoreline erosion rates in this hot spot area were measured to range between 0.5 to 4 m/yr (1.6 to 13 ft/yr). The uniform sediment distributions do not reflect this southern hot spot, but there is a general fining trend toward the south from Southern Shores to Oregon Inlet. The finest sediment composites are found in the southern South Fill and South of Project areas. FILL SUITABILITY Typical fill suitability techniques use the mean and sorting of borrow and native sediments obtained from grain size analysis. The overfill ratio or fill factor methods use the single value of the mean and sorting values of both borrow and native grain size distributions. The Dare County sediment data provide an ideal opportunity to evaluate this approach since there is a wide variation in the grain sizes present ranging from gravels to fine silt. Use of the entire grain size distribution should improve both the accuracy in identifying compatible sediments, and the performance of nourishment projects in areas with complex and variable beach sediments. A large number of cores have been collected and analyzed to identify suitable borrow areas. A complete analysis of the borrow area is beyond the scope of this paper. Figure 15 illustrates how this analysis could be done. A single composite core grain size distribution is shown which is achieved by mathematically combining several samples taken at selected depths in the core. This core composite is compared with two

S b li h

_ _ p-100 to -8 -8 to -6 -6 to -4 -4 to -2 -2 to -0.5 -0.5 to 0 0 to 0.5 0.5 to 2 2 to 4 4 to 6 6 to 8 8 to 100

Shoreline Change Rate (ft/yr)1970-1997

Erosion

SouthernShores

NorthHotSpot

SouthHotSpot

KittyHawk

KillDevilHills

NagsHead

CapeHatterasNationalSeashore

Accretion

No ChangeNo Change

North Project Limit

South Project Limit

Source:USGSShoreline Change RatesBathymetry

-28 - -27

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Depth (m)

Oregon Inlet

AncestralAncestralAlbemarle ChannelAlbemarle Channel

ShoreShore--attached Shoalattached Shoal

Figure 14. Location of hot spots related to bathymetry and shoreline erosion rates

representative native beach composite samples to show how compatibility could be evaluated. A typical analysis would consist of assembling a borrow composite of several cores from a nearshore area that would be compared to multiple beach composites. A more extensive analysis of the borrow sediments can be found in U.S. Army (2000). Offshore borrow area core NDCS_01_v_16 (dashed light blue line) from the southern shoal area is compared with a representative beach composite sample from the coarser North Fill area (Line 609 green line), and a finer representative beach composite from the South Fill area (Line 909, dark blue line). Looking at the entire distribution of each composite, shows that this single borrow core composite has less coarse material than the single North Fill beach composite, but has more medium sand and less fines. The representative finer South Fill beach composite has more fine grained material than this borrow area core.

CONCLUSIONS This study was able to characterize the pre-fill highly variable “native” conditions which appear to be related to the underlying geology of the area. This data established a pre-fill baseline to evaluate project performance when the project is constructed. The behavior of the fill can be compared to the native beach to identify renourishment requirements to maintain the design level of protection. It is anticipated that the fill will have a variable rate of loss as a function of the location of the hot spots. With the characterization of the native fill by profile, a measure of the fill stability can be done after fill placement. With the high variability in the native beach, the post-placement beach sediment distributions are suspected to also be complex. The ability to quantify


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Composite 909 16_comp Composite 609

VERY COARSE SAND



16.00 4.00 0.25 0.063phi

8.00 2.00 0.50 0.125

GRAVEL SILT

Sediment LINE 909 - Nags Head (South Fill - South)May 2006


Core NDCS_01_V_16 Composite (South Borrow Area)

Composite of 8 layers within core

Sediment LINE 609 - Nags Head (South Fill - North)May 2006

mm

Composite Sediment LINE 609 Composite Sediment LINE 609 –– Nags Head (South Fill Nags Head (South Fill –– north end)north end)May 2006 (May 2006 (BermBerm, MHW, MSL, MLW & , MHW, MSL, MLW & --6ft)6ft)

Composite Sediment CORE NDCS_01_V_16 Composite Sediment CORE NDCS_01_V_16 (South Borrow Area S1)(South Borrow Area S1)Composite of 8 samples within coreComposite of 8 samples within core

Less FinesLess FinesExcess CoarseExcess Coarse

Less CoarseLess Coarse

Overfill Factor ~ 1.2Overfill Factor ~ 1.2

Composite Sediment LINE 909 Composite Sediment LINE 909 –– Nags Head (South Fill Nags Head (South Fill –– south end)south end)May 2006 (May 2006 (BermBerm, MHW, MSL, MLW & , MHW, MSL, MLW & --6ft)6ft)

x

Figure 15. Example comparison of borrow core with beach composites

storm protection and benefits realized along this highly variable beach could not be possible without first understanding the details of the native beach sediment variability. With this understanding of the details of sediment distribution and profile response, guidance on the management of complex fills, including erosional hot spots can be made with more certainty. ACKNOWLEDGEMENTS Mr. Ben Lacky, Wilmington District, provided helpful information and discussions on the beach and borrow sediment samples and suitability analysis. Permission to publish was granted by the Chief of Engineers. REFERENCES Bascom, W.N. 1959. “The Relationship between Sand Size and Beach Face Slope,”

American Geophysical Union Transactions 32(6), 866-874. Birkemeier, W A., Forte, M.F., Miller, H.C. 2007. “New Mid-Atlantic Observations of

the Depth of Closure,” Proceedings 2006 International Conference on Coastal Engineering, ASCE.

Browder, A., and McNinch, J.E. 2006. “Linking Framework Geology of the Nearshore:

Correlation of Paleo-Channels with Shore-Oblique Sandbars and Gravel Outcrops,” Marine Geology 231, 141-162.

McNinch, J.E. 2004. “Geologic Control in the Nearshore: Shore-Oblique Sandbars and

Shoreline Erosional Hotspots, Mid-Atlantic Bight, USA,” Marine Geology 211, Issues 1-2, 121-141.

Morton, R.A., and Miller, T. 2005. “National Assessment of Shoreline Change: Part 2,

Historic Shoreline Change and Associated Coastal Land Loss Along the U.S. Southeast Atlantic Coast,” USGS Open file 2005-1401, U.S. Geologic Survey, St Petersburg, FL.

Snyder, S.W. 1997. “Evaluation of Potential Resources for Beach Nourishment off

North Dare County Coastline,” In: Final Feasibility Report and Environmental Impact Statement on Hurricane Protection and Beach Erosion Control, Dare County Beaches, North Carolina (Bodie Island Portion), Appendix I: Geotechnical Engineering, U.S. Army Engineer District, Wilmington, September 2000.

Stauble, D.K. 1992. “Long-Term Profile and Sediment Morphodynamics: Field

Research Facility Case History,” Technical Report CERC-92-7, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS, 252p.

Stauble, D.K., and Bass, G.P. 1999. “Sediment Dynamics and Profile Interactions of a

Beach Nourishment Project,” Coastal Sediments ’99, ASCE, 2,566-2,581.

Zarillo, Gary A., Liu, James, and Tsien, Hsiao-Shu. 1985. “A New Method for Effective Beach-Fill Design,” Coastal Zone ’85, ASCE, 985-1,001.

U.S. Army. 2000. “Sand Compatibility Analysis,” In: Final Feasibility Report and

Environmental Impact Statement on Hurricane Protection and Beach Erosion Control, Dare County Beaches, North Carolina (Bodie Island Portion), Appendix E, U.S. Army Engineer District, Wilmington, September 2000.

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