Access Number 30843 

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Study Title: Collection of Environmental Data Within Sand Resource Areas 

Offshore North Carolina and the Environmental Implications of Sand Removal 

for Coastal and Beach Restoration 

Report Title: Collection of Environmental Data Within Sand Resource Areas 

Offshore North Carolina and the Environmental Implications of Sand Removal 

for Coastal and Beach Restoration, Volume I: Main Text, Volume II: Appendices 

Contract Number: 1435-01-97-CT-30843

Sponsoring OCS Region: Minerals Management Service, Leasing Division, Sand 

and Gravel Unit 

Applicable Planning Area(s): Offshore Dare County, North Carolina 

Fiscal Year(s) of Project Funding: FY1998 to FY2003 

Completion Date of Report: September 2003 

Costs: FY 1998: $ 6,626; FY 1999: $197,807; FY 2000: $167,275; 

 FY 2001: $ 50,472; FY 2002: $ 42,027; Cumulative Costs: $499,999 

Project Manager: Mark R. Byrnes, Ph.D. 

Affiliation (Project Manager): Applied Coastal Research and Engineering, Inc. 

Address: 766 Falmouth Road, Suite A-1, Mashpee, MA 02649 (Applied Coastal) 

Principal Investigators: Mark R. Byrnes, Ph.D.; Richard M. Hammer, Ph.D. 

Key Words: sand resources, offshore North Carolina, sediment transport, wave 

modeling, bathymetry change, dredging, benthic, pelagic, recolonization 

Background: The North Carolina Geological Survey (NCGS) characterized four 

sand resource areas in Federal waters offshore Dare County, North Carolina as 

potential borrow sites for beach nourishment. The main purpose of this study 

was to address environmental concerns associated with dredging sand from 

these Outer Continental Shelf (OCS) sites. 

Objectives: Provide MMS environmental information to assess the potential 

impacts of offshore dredging activities and to identify ways in which 

dredging operations can be conducted to minimize or preclude long-term 

adverse physical and biological impacts to the environment. Five primary 

study elements were developed for addressing environmental concerns 

associated with offshore sand dredging: 1) evaluate potential modifications 

to waves due to offshore sand dredging within the proposed sand resource 

areas; 2) evaluate the potential impact of offshore dredging and consequent 

beach replenishment on sediment transport patterns, sedimentary environments, 

and local shoreline processes; 3) characterize benthic ecological conditions, 

using existing data sets and data collected from field work, in and around 

the proposed sand borrow sites; 4) evaluate benthic infauna present in the 

proposed sand resource areas, and assess the potential effects of offshore 

sand dredging on these organisms; and 5) develop a schedule of best and worst 

times for offshore sand dredging in relation to transitory pelagic species. 

Description: The inshore portion of the continental shelf, seaward of the 

Federal-State OCS boundary and within the North Carolina Exclusive Economic 

Zone (EEZ), encompasses the project study area. The seaward limit of the 

study area is generally within about 15 km of the shoreline. Sand resource 

areas are located on the North Carolina OCS between the 10- and 20-m depth 

contours. The continental shelf surface contains a number of shore-oblique 

sand ridges formed during the Holocene transgression. Sand ridges 2- to 5-m 

high and 0.5- to 1.5-km apart represent the primary sand resource targets of 

this study. For sand resource areas on the northern North Carolina 

continental shelf, average shoal relief is about 2 to 3 m. 

 Four potential sand resource areas were identified offshore Dare County, 

North Carolina, in Federal waters by the NCGS and evaluated to determine the 

potential impacts of offshore sand mining for beach replenishment (Figure 1). 

Resource areas contain borrow sites with the greatest potential for future 

use. Existing physical processes and biological data were compiled and 

analyzed to assess the potential impacts of offshore dredging activities to 

minimize or preclude long-term adverse environmental impacts. Wave 

transformation and sediment transport numerical modeling were employed to 
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simulate the physical environmental effects of proposed sand dredging 

operations offshore and along the coastline landward of resource areas to 

ensure that OCS sand resources are developed in an environmentally sound 

manner. Biological field surveys were conducted to characterize benthic 

infauna and epifauna at each resource area.

Significant Conclusions: Potentially rapid and significant changes in 

bathymetry due to sand extraction from the OCS may impact wave propagation 

patterns on the continental shelf and at the shoreline. As such, substantial 

effort was spent understanding existing wave propagation patterns relative to 

those resulting from potential sand extraction scenarios. Because large 

natural spatial and temporal variability exists within the wave climate at a 

particular site, determination of physical impacts associated with sand 

mining must consider the influence of process variability. A method based on 

historical wave climate variability, as well as local wave climate changes 

directly attributable to borrow site excavation, was applied to determine 

appropriate criteria for assessing impact significance. 

 Under existing conditions wave transformation simulations, bottom 

topography offshore Bodie Island modifies incident waves as they propagate 

shoreward. For example, the shoal near Borrow Site 1 refracts waves, causing 

a slight focusing of wave energy behind the feature. Wave heights behind the 

shoal are about 0.4 m greater than wave heights at the northern and southern 

limits of the shoal. Post-dredging model simulations indicate that wave 

heights within borrow sites are reduced relative to existing conditions, and 

this effect was more pronounced in cases that had greater wave heights. As 

waves propagate across a borrow site, wave refraction guides wave energy away 

from the center of the excavation site and toward the shallow edges. The net 

effect is to create a shadow zone of reduced wave energy immediately landward 

of a borrow site and a zone of increased wave energy updrift and downdrift of 

the borrow site. Near Site 2, for example, wave heights increased by a 

maximum of 0.05 m at the northern and southern edges of the borrow site and 

decreased by a maximum of 0.06 m behind the borrow site. During storm 

conditions, where wave period was significantly larger than normal wave 

characteristics, waves were more affected by offshore bathymetric gradients. 

 Because spectral wave simulations were employed, and because different 

frequencies in a spectrum are refracted by varying degrees at each borrow 

site, regions of increased and reduced wave energy gradually diffuse as the 

wave field approaches shore. As such, borrow sites farther offshore affect a 

greater length of shoreline, however the actual magnitude of the impact is 

reduced because the affected wave field has a greater distance to diffuse. 

For example, Borrow Site 1 is larger than Site 2, but farther offshore, and 

therefore, wave height change from existing to post-dredging conditions at 

the shoreline was less than changes induced by Site 2. The area of maximum 

increased wave heights for post-dredging conditions was located at the 

northernmost corner of Site 1, where wave heights increased 0.09 m over 

existing conditions. Model output for storm wave conditions indicated that 

the length of affected shoreline where borrow site shadows propagate extended 

about 40 km to the north, starting from about 3 km north of Oregon Inlet. 

 Three independent sediment transport analyses were completed to evaluate 

impacts due to sand mining, including quantifying net sediment transport 

trends using historical bathymetry data sets; documenting sediment transport 

patterns at proposed offshore borrow sites using wave modeling results and 

existing current measurements; and predicting longshore sediment transport 

potential using wave modeling output. Sediment transport at borrow sites will 

experience rapid changes after sand dredging is completed. Local sediment 

that replaces dredged material will fluctuate based on location, time of 

dredging, and storm characteristics following dredging episodes. Average 

transport rates near borrow sites ranged from a minimum of about 38,000 m3

/yr 

(Sites 3 East and 4) to a high of about 123,000 m3

/yr (Site 2), while 
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infilling times varied between 37 to 98 years. Site 2 had the greatest 

infilling rate due to its shallow depth relative to other sites and its large 

perimeter. Difference in longshore transport potential between existing and 

post-dredging conditions resulted in a maximum change in computed net 

sediment transport potential of 34,000 m3

/yr. Impact significance was 

determined by comparing actual change in transport potential between existing 

and post-dredging conditions to a significance envelope of one-half the 

standard deviation (±0.5σ) along the shoreline. Based on this comparison, it 

was determined that no significant changes to longshore sediment transport 

would result from the modeled borrow site configurations, except when Sites 3 

West and 3 East were dredged simultaneously. 

 Results of field surveys for biological characterization agreed well with 

previous descriptions concerning benthic assemblages associated with shallow 

shelf habitats offshore North Carolina. Numerically dominant infaunal groups 

included numerous crustaceans, echinoderms, mollusks, and polychaetes, while 

epifaunal taxa consisted primarily of decapods, sea stars, and squid. 

Potential benthic effects from dredging will result from sediment removal, 

suspension/dispersion, and deposition. Effects are expected to be short-term 

and localized. Seasonality and recruitment patterns indicate that removal of 

sand between late fall and early spring would result in less stress on 

benthic populations. Early-stage succession will begin within days of sand 

removal, through larval recruitment dominated by opportunistic taxa. Later 

successional stages of benthic recolonization will be more gradual, involving 

taxa that generally are less opportunistic and longer lived. 

 Zooplankton, squids, fishes, sea turtles, and marine mammals were groups in 

the pelagic environment considered to be potentially affected by offshore 

dredging. No cumulative effects to any of these pelagic groups are expected 

from multiple sand mining operations offshore North Carolina. 

Study Results: Minimal physical environmental impacts due to potential sand 

dredging operations have been identified through wave and sediment transport 

simulations. Under representative wave conditions for each of the model grids 

offshore Dare County, North Carolina, it was determined that no significant 

changes in longshore sediment transport potential would result from modeled 

borrow site configurations, where Sites 3 West and 3 East were modeled 

separately. For sites dredged simultaneously, the impacts were estimated to 

exceed ±0.5σ, and therefore would require mitigation along the affected 

shoreline or a redesign of the borrow site configuration, likely a reduction 

in maximum design depth at one of the sites. 

 Data collected, analyses performed, and simulations conducted for this 

study indicate that proposed sand dredging at sites evaluated on the OCS 

offshore North Carolina should have minimal environmental impact on fluid and 

sediment dynamics and biological communities. Short-term impacts to benthic 

communities are expected due to the physical removal of borrow material, but 

the potential for significant cumulative benthic impacts is remote. 

Additionally, no cumulative effects to any of the pelagic groups are expected 

from potential sand mining operations. 

Study Products: Byrnes, M.R., R.M. Hammer, B.A. Vittor, S.W. Kelley, D.B. 

Snyder, J.M. Côté, J.S. Ramsey, T.D. Thibaut, N.W. Phillips, J.D. Wood, and 

J.D. Germano, 2003. Collection of Environmental Data Within Sand Resource 

Areas Offshore North Carolina and the Environmental Implications of Sand 

Removal for Coastal and Beach Restoration. U.S. Department of the Interior, 

Minerals Management Service, Leasing Division, Sand and Gravel Unit, Herndon, 

VA. OCS Report MMS 2000-056, Volume I: Main Text 256 pp. + Volume II: 

Appendices 69 pp. 

*

P.I.’s affiliation may be different than that listed for Project Manager. 
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75°40'

75°40'

75°35'

75°35'

75°30'

75°30'

75°25'

75°25'

75°20'

75°20'

35°50'

35°50'

35°55'

35°55'

36°

36°

36°5'

36°5'

1

2

3 west

4

Bodie Island

Roanoke Island

Pea Island

Oregon Inlet

Nags 

Head

Kitty

Hawk

Atlantic Ocean

Platt 

Shoals

3 east

Depth (m NGVD)

2 to 0

0 to -2

-2 to -4

-4 to -6

-6 to -8

-8 to -10

-10 to -12

-12 to -14

-14 to -16

-16 to -18

-18 to -20

-20 to -25

-25 to -30

-30 to -35

Below -35

Bathymetric Surface

1970/75/96 

N

2000 0 2000 Meters

FEDERAL-STATE 

BOUNDARY

1980 SHORELINE

SAND 

RESOURCE AREA

POTENTIAL 

BORROW SITE

UNIVERSAL TRANSVERSE 

MERCATOR, ZONE 18

NAD 1983

Figure 1. Location diagram illustrating potential sand borrow sites relative 

to shelf morphology offshore Dare County, NC.