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Study Title: Numerical Modeling Evaluation of the Cumulative Physical 

Effects of Offshore Sand Dredging for Beach Nourishment

Report Title: Numerical Modeling Evaluation of the Cumulative Physical 

Effects of Offshore Sand Dredging for Beach Nourishment 

Contract Number: 1435-01-99-CT-30993 

Sponsoring OCS Region: Headquarters – International Activities and Marine 

Minerals Division 

Applicable Planning Area(s): Offshore Sand Borrow Sites from southern New 

Jersey to Cape Canaveral, Florida 

Fiscal Year(s) of Project Funding: FY1999 to FY2001 

Completion Date of Report: November 2001 

Costs: FY 1999: $37,074.90; FY 2000: $ 79,059.52; FY 2001: $ 83,806.76 

Cumulative Costs: $199,941.18 

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., Sean W. Kelley, M.S., John S. 

Ramsey, M.S. 

Key Words: wave modeling, sediment transport, offshore sand borrow sites, 

beach nourishment, cumulative impacts, dredging, Outer Continental Shelf 

Background: The potential for exploitation of sand resources as a source for 

beach and barrier island restoration has grown rapidly in the last several 

years as similar resources in State waters are being depleted or polluted. 

Extraction of sand resources in Federal waters may be preferred relative to 

State waters due to concerns over changes in physical oceanographic 

conditions resulting from large quantities of material dredged from resource 

sites impacted by waves and currents. This study examines the potential for 

negative impacts to coastal and nearshore environments, particularly from 

alterations to the local wave and sediment transport regime, due to long-term 

dredging and significant removal of sand from shoals offshore southern New 

Jersey, southeastern Virginia (Sandbridge Shoal), North Carolina (north of 

Oregon Inlet), and Cape Canaveral, Florida. 

Objectives: The most effective means of quantifying incremental and 

cumulative physical environmental effects of sand dredging from shoals on the 

continental shelf is through the use of wave transformation numerical 

modeling tools that recognize the random nature of incident waves as they 

propagate onshore. Although the interpretation of wave modeling results is 

relatively straightforward, evaluating the significance of predicted changes 

for accepting or rejecting a borrow site is more complicated. A substantial 

part of this study was aimed at assessing the significance of simulated 

changes between existing and post-dredging conditions versus natural 

variability in wave climate and potential sediment transport rates to 

determine the relative importance of predicted changes. It is expected that 

information generated will enable the MMS to assess potential impacts of 

long-term offshore dredging and to identify potential dredging alternatives 

aimed at minimizing or precluding adverse physical environmental impacts. 

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

State-Federal OCS boundary and within the Exclusive Economic Zone (EEZ), 

encompasses the project study areas offshore southern New Jersey, 

southeastern Virginia, northeastern North Carolina, and Cape Canaveral, 

Florida (Figure 1). The seaward limit of all borrow sites is generally within 

about 30 km of the shoreline and located between the 10- and 20-m depth 

contours. A four-phase approach was implemented to assess the potential for 

negative impacts from alterations to the local wave climate and sediment 

transport regime associated with offshore sand mining. First, a detailed 

evaluation of three readily-available, project-tested spectral wave models 

was conducted to determine the most effective simulation routine for 
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predicting potential wave impacts due to dredging at offshore borrow sites. 

Second, a standard method was developed to qualify the significance of 

changes associated with borrow site excavation to determine the influence of 

borrow site geometry on local wave refraction and sediment transport 

patterns. The method developed for this study is based on historical wave 

climate variability, as well as local wave climate changes directly 

attributable to borrow site excavation. The third phase of the project 

focused on wave spectra development, wave transformation modeling, and 

coastal sediment transport calculations. Wave transformation modeling and 

sediment transport potential calculations were performed for existing and 

post-dredging bathymetric conditions within the study area. In addition, a 

site-specific determination of acceptable limits for borrow site impacts 

relative to sediment transport potential was determined for each case. The 

fourth phase of the project addressed potential cumulative effects of sand 

dredging from offshore sand borrow sites. These analyses were designed to 

determine how potential wave impacts at individual sites may interact to 

produce additive physical environmental effects along the coast. 

Significant Conclusions: Potentially rapid and significant changes in 

bathymetry due to sand extraction from the OCS may have substantial impact on 

wave propagation patterns on the continental shelf and at the shoreline. As 

such, significant effort was spent understanding existing wave propagation 

patterns relative to those resulting from potential sand extraction 

scenarios. Overall, post-dredging wave model output for the four study sites 

illustrates reduced wave heights landward of borrow sites and increased wave 

heights at the longshore limits of the borrow site. This effect is more 

pronounced for cases with larger wave heights and longer periods. As waves 

propagate across a borrow site (deeper water than the surrounding area), wave 

refraction bends waves away from the center of the borrow site and toward the 

shallower edges. The net effect is to create a shadow zone of reduced wave 

energy immediately landward of the borrow site and a zone of increased wave 

energy updrift and downdrift of the borrow site. This shadowing effect is 

apparent in model results for offshore southern New Jersey. In the immediate 

vicinity of the borrow site, wave heights increased by a maximum of 0.20 m at 

the northern and southern edges of the borrow site and decreased by a maximum 

of 0.19 m behind the borrow site. 

 A predominant condition for simulating wave propagation across the 

southeastern Virginia continental shelf was a 1.8 m high, 10 sec peak period 

wave, approaching from the southeast. Maximum increased wave heights near 

Rudee Inlet are 0.5 m above the offshore condition. This zone of increased 

wave heights extends approximately 10 km on either side of Rudee Inlet. As a 

result of wave focusing by Sandbridge Shoal, there is a corresponding 

decrease in wave heights south of the shoals, shoreward to Sandbridge. An 

area of maximum decrease in wave height is indicated just seaward of 

Sandbridge, where wave heights are approximately 0.8 m less than at the 

offshore boundary. Modeled wave height changes relative to post-dredging 

conditions range between 0.2 and -0.35 m. The zone of influence along the 

shoreline for the two sites extends approximately 12 km, from just north of 

Rudee Inlet to Sandbridge. 

 For offshore North Carolina, the predominant wave condition has a 

significantly longer period than the New Jersey condition (Hs=1.5 m, Tpeak=12.8 

sec, θpeak=71 deg), and as a result, waves respond more rapidly to offshore 

bathymetry gradients. At the offshore shoals directly east of Oregon Inlet, 

wave heights increase approximately 0.7 m over offshore conditions. Wave 

height change from existing and post-dredging conditions at the two borrow 

sites illustrates an overlapping region of influence, as site 3 West is 

located directly landward of the site 3 East along the direction of wave 

propagation (Figure 1). The area of maximum increased wave heights for post-
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dredging conditions is located at the northern corner site 3 West, where wave 

heights increase 0.06 m over existing conditions. Similar to the southeastern 

Virginia examples, wave height increase is the result of the combined 

influence of the two sites. The area of maximum wave height decrease is 

located landward of site 3 West, where wave heights are reduced by 0.07 m 

from existing conditions. 

 Canaveral Shoals, the complex of ridges and troughs that extend southeast 

from Cape Canaveral, cause significant increases in wave heights as waves 

propagate over this area. The predominant offshore wave condition is a 1.6 m, 

14.3 sec wave propagating from the east-northeast. Vectors indicating wave 

direction illustrate that for some nearshore regions adjacent to the Cape, 

the direction of wave propagation changes more than 45 degrees, following the 

gradient in bathymetric contours. The largest waves in the model domain 

occur at the shoals north of Cape Canaveral, and are 1.3 m higher than 

offshore waves. At shoals in the vicinity of the borrow site, wave heights 

increase to a maximum of 2.8 m, 1.2 m above offshore conditions. Shoals tend 

to refract wave energy and focus it near the cusp of the Cape. Wave energy 

is focused away from the coast south of Cape Canaveral, and an area of 

reduced wave heights is created. Maximum changes in post-dredging wave 

heights were ±0.7 m, which are substantially greater than changes observed at 

previous sites. The area of greatest wave height increase occurs at the 

northwest corner of the site. The alongshore extent of influence extends 

approximately 13 km north of the Cape to about 5 km south of Port Canaveral. 

 To establish useful criteria for estimating impacts of sand mining on the 

nearshore littoral system, a comparison of sediment transport potential and 

long-term shoreline change was performed at each of the four sand borrow 

sites. For the southern New Jersey shoreline, erosion and accretion trends 

are predicted well at all locations, including in the vicinity of tidal 

inlets. Along the southeastern Virginia coast, model results predict similar 

trends as observed long-term shoreline change, where much of the coastline is 

stable or slightly erosional. Overall agreement between modeled trends and 

measured shoreline change also was achieved for the North Carolina coast. 

Discrepancies between predicted and measured results likely are a result of 

the significant historical migration of Oregon Inlet. Along the Cape 

Canaveral coast, STWAVE modeling was not capable of evaluating changes in 

wave climate resulting from wave diffraction processes across Canaveral 

Shoals. In addition, these shoals likely serve as a sediment source to the 

beach in this region. South of Port Canaveral, away from the influence of the 

Cape’s topographic and bathymetric features, the trends predicted by the 

sediment transport potential model match well with historical shoreline 

change. 

Study Results: Based on site-specific analyses for each of the four sites, 

the impacts associated with offshore dredging were deemed acceptable. For the 

southern New Jersey coastline, the significance criterion envelope was 

typically ±10 to 15% of the modeled annualized net transport potential, with 

values of about ±30% close to Great Egg Harbor Inlet. Overall, the 

significance criterion envelope (representing plus or minus one standard 

deviation about the mean) for New Jersey was relatively narrow compared with 

the other three modeled areas. This results from limited variability in 

inter-annual wave climate along the southern New Jersey coast. 

 For southeastern Virginia and North Carolina coastal regions, the 

dredging significance criterion envelope generally falls within ±20 to 40% of 

the computed net transport values. For the modeled sand excavation 

conditions, post-dredging model results fall within the significance 

criterion envelope for both cases, so again the modeled dredging plan would 

be acceptable. Model output for the region south of Port Canaveral, Florida 
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illustrates that in the area of greatest impact from the borrow site, the 

significance envelope is approximately ±20% of the mean computed net transport 

potential. Again, the modeled impacts in this region associated with 

potential offshore dredging are well within the significance envelope. 

However, due to limitations with the wave modeling effort, further analyses 

would be required to accurately assess impacts caused by dredging for the 

shoreline north of Port Canaveral. 

 To evaluate cumulative impacts associated with incremental dredging of a 

single site and/or the combined effects of dredging borrow sites in the same 

region, two types of borrow site configurations were investigated. The first 

involves the interaction of multiple sites in close proximity, such that they 

have overlapping areas of shoreline impact. The second involves multiple 

dredging events at a single site. At Sandbridge Shoal, the two borrow sites 

are oriented side-by-side and parallel to the shoreline. For offshore North 

Carolina, sites 3 East and 3 West are oriented front-to-back in a line 

perpendicular to the shoreline. To evaluate the influence of individual 

borrow sites relative to the combined influence of both borrow site 

excavations, wave model runs were performed for cases where each borrow site 

was excavated individually and both borrow sites were excavated in a single 

event. Annual sediment transport calculations were then performed for each 

wave modeling scenario. Superimposing the effects of individual borrow site 

excavations onto the sediment transport potential curve developed from the 

combined excavation model run was used as the basis for comparison. The 

results from these two cases (Virginia and North Carolina) suggest that 

borrow sites located in close proximity illustrate additive impacts. 

Therefore, the influence of multiple sites on sediment transport along a 

coastline is a simple additive effect, rather than a more complicated nonlinear effect or amplification. 

 The second type of cumulative impact analysis evaluated the effect of 

multiple dredging events at a single site. As a borrow site is excavated to 

greater depths, the impact that it has on sediment transport along the 

shoreline will increase. Site M8, offshore southern New Jersey, was used to 

evaluate a range of excavation depths for identifying the point where changes 

in sediment transport potential become significant relative to natural 

transport variability because the site is positioned close to shore, and it 

has a relatively large perimeter. Deep excavations at this location would 

have pronounced (and therefore easily observable) effects on modeled sediment 

transport patterns at the shoreline. From the five model runs, change in 

transport potential varied linearly with excavation depth and did not exceed 

the impact significance criterion until an excavation depth >5 m.

Study Products: Kelley, S.W., J.S. Ramsey, M.R. Byrnes, 2001. Numerical 

Modeling Evaluation of the Cumulative Physical Effects of Offshore Sand 

Dredging for Beach Nourishment. U.S. Department of the Interior, Minerals 

Management Service, International Activities and Marine Minerals Division 

(INTERMAR), Herndon, VA. OCS Report MMS 2001-098, 95 pp + 106 pp appendices.
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 a. Offshore southern New Jersey. b. Offshore southeastern Virginia. 

 c. Offshore North Carolina. d. Offshore Cape Canaveral, Florida. 

Figure 1. Location diagram illustrating potential offshore sand borrow sites 

in the U.S. East Coast study area.