ACCESS NUMBER: M10PC00118 

STUDY TITLE: Best Practices for Physical Processes and Impact Assessment in Support of Dredging 

Operations on the U.S. Outer Continental Shelf

REPORT TITLE: Use of Morphological Models to Predict Physical Processes and Assess Impacts in 

Support of Dredging Operations on the U.S. Outer Continental Shelf

CONTRACT NUMBER: M10PC00118 

SPONSORING OCS REGION: Environmental Assessment Division 

APPLICABLE PLANNING AREA(S): U.S. Atlantic Coast Outer Continental Shelf 

FISCAL YEAR(S) OF PROJECT FUNDING: 2010-2015 

COMPLETION DATE OF REPORT: December 10, 2015 

COSTS: $436,357 

PROJECT MANAGER(S): John S. Ramsey, P.E. 

AFFILIATION: Applied Coastal Research and Engineering, Inc. 

ADDRESS: 766 Falmouth Road, Suite A-1, Mashpee, MA 02649 

PRINCIPAL INVESTIGATOR(S)*: John Ramsey; Gwen Simmons; Rebecca Quan; Fengyan Shi, Ph.D.; 

James T. Kirby, Ph.D.; Giovanni Vittori, Ph.D.; Julio Zyserman, Ph.D.

KEY WORDS: numerical modeling, morphological change, offshore borrow site, sediment transport, 

sand shoal mining, bathymetric change, dredging, hydrodynamic, simulations, wave conditions, 

calibration, near-field, far-field, ridged bed 

BACKGROUND: The goal of the overall project was to evaluate morphological response of sand and 

gravel shoal dredging along the Atlantic coast of the U.S. 

OBJECTIVES: The objective of this project was to test, tailor, and apply existing numerical 

morphological modeling tools and methods in order to provide robust and defensible predictions of 

morphological behavior in OCS sand extraction pits/areas, as well as the associated near-field and farfield impacts. 

DESCRIPTION: Several process-based morphodynamic modeling packages that describe sediment 

transport and morphological changes under the combined action of waves and currents were compared. 

The models included the public domain Nearshore Community Model (NearCoM) and Community 

Sediment Transport Modeling System (CSTMS) developed by various researchers in the United States, as 

well as commercial models from DHI Water and Environment, Inc. (MIKE 21) and the United States 

Army Corps of Engineers (USACE), the latter being available as the Coastal Modeling System (CMS). 

Based on an evaluation of the morphodynamic modeling tools, NearCoM was selected to evaluate 

demonstrative geometries typical of the U.S. Atlantic coast. These geometries included a range of borrow 

site aspect ratios, ridge geometries, and ambient tidal current conditions. Both near-field and far-field 

influences of the various geometries were assessed within the context of morphologic change. 

SIGNIFICANT CONCLUSIONS: Although previous studies have indicated that the application of 

either research or practical models in an un-tuned manner may not yield useful results or predictions, 

modeling tools that are ‘calibrated’ with site-specific field data greatly reduce uncertainty. A key to the 

overall morphological modeling effort was to ensure that a characteristic site representative of borrow 

areas and dredging along the U.S. Atlantic inner shelf provided the basis for model calibration and 

testing. The Long Bay, South Carolina Can South borrow site was found to be the only borrow area to 

meet all of the criteria needed and these sites were selected for further analysis utilizing the 

morphodynamic modeling tools. 

Three different morphological models were tested; model results compared with pre- and post-one year 

dredging surveys from the Cane South borrow area. The models selected were Nearshore Community 

Model (NearCoM), MIKE 21 Coupled Model FM (MIKE 21), and Coastal Modeling System (CMS). 

Based upon the Cane South application NearCoM was selected to model the demonstrative geometries, 

due to the ability of NearCom to alter the sediment transport equations, not need a model “speed up” or 

calibration factor, and its ability to accurately reproduce the morphological patterns seen in the measured 

data at the Cane South borrow area. 
ACCESS NUMBER: M10PC00118 

Nine (9) scenarios were evaluated to assess the effect of borrow pit aspect ratio and varying tidal current 

speeds on a flat bed in the near-field region for a one-year period. An additional seven (7) model 

scenarios were run to investigate potential near-field impacts associated with sand ridge fields utilizing 

non-linear process-based modeling techniques. 

Utilizing a combination of the results from the model sensitivity assessment and results from the 

evaluation of borrow area geometries, general guidance was developed for the input necessary to 

effectively incorporate morphological modeling tools into the decision-making process for sand 

extraction within the U.S. Atlantic coast region. It should be noted that the assessment used a limited 

number of scenario cases and fixed hydrodynamics and sediment characteristics were considered and can 

only give an approximate idea of the near-field and far-field effects of dredging. To develop a more 

complete understanding of the influence of the different parameters and processes on the hydrodynamics 

and sediment transport further investigations would be necessary. However, the scenarios developed 

provide an in-depth overview of how specific parameters related to pit aspect ratio, pit centroid position, 

and pit orientation influence the morphodynamics within the borrow pit and surrounding areas for both 

flat bed and ridged bathymetry. This evaluation of model parameters governing morphodynamic 

simulations at borrow areas can assist with developing best management practices for the assessment of 

potential physical impacts associated with borrow site dredging. 

STUDY RESULTS: Based on the in-depth data assessment and morphodynamic model evaluations, the 

following factors should be considered in morphodynamic modeling and are critical to effective model 

simulation of both regional and local physical processes: 

 Collection of site-specific data is critical to characterize both the wave climate and ambient tidal 

currents at a particular borrow area. For the generally wave-dominated sediment transport regime 

along much of the U.S. Atlantic coast, wave information, especially during storms, is important to 

characterize accurately for a particular borrow area. 

 Large-scale hydrodynamic models can be utilized to provide boundary conditions for projectscale morphodynamic models, but it is important to ensure that the model simulation time period 

corresponds to (a) a relevant post-dredging period when bathymetry surveys are available at the 

beginning and the end, and (b) hydrodynamic and wave field data are available at the site or at 

least within the general vicinity of the borrow site excavation. 

 Bathymetry data collected prior to dredging, immediately after dredging, and (likely) on an 

annual basis following the dredging event provides the necessary information to calibrate the 

model to in situ morphologic changes. 

 In the case of Long Bay, South Carolina, bathymetry data collected using a single beam 

fathometer for all surveys became problematic because (a) bathymetry track lines were not 

consistent and (b) full bathymetric coverage was not achieved for every survey. To evaluate 

bathymetric change accurately, it is critical to ensure that single-beam survey coverage is 

performed in a consistent and complete manner. Multi-beam bathymetric surveys with complete 

borrow area coverage would alleviate concerns related to line spacing. 

 Based on model simulations, it appears that bathymetry surveys that extend to an area 

approximately twice the size of the borrow site (i.e. the overall survey area would be two times 

the excavation area and mimic the shape of the borrow area) incorporates a majority of the 

potential bathymetric change. Outside of this area, it appears that bathymetric change would be 

limited to several centimeters in overall depth change. 

 It was possible to ‘tune’ each of the morphodynamic models assessed to simulate general trends 

of erosion and deposition; however, this requires that model calibration be performed for each 

specific site. A priori calibration of the models would be required prior to dredging to provide 

meaningful site-specific information that could be utilized to predict post-dredging morphologic 

change. 

 Simulated bed elevation changes in the borrow area are small (of the order of 10% of the pit 
ACCESS NUMBER: M10PC00118 

depth over the 1-year simulation period). At water depths similar to ‘typical’ U.S. Atlantic coast 

borrow areas, recovery of the borrow areas bathymetric conditions to the pre-dredging surface 

may take several years and perhaps more than a decade in sites dominated by wave-induced 

transport. However, this is highly dependent on frequency of storms and sediment transport rates. 

 The influence area of the borrow area both on wave height and on tidal current increases as the 

aspect ratio of the pit increases. Since the wave and current conditions are based on information 

derived from Cane South (Long Bay, South Carolina), where the borrow area aligns obliquely 

with the incoming wave field and ambient tidal currents, the larger aspect ratios cause a greater 

perturbation to both the background wave and current fields. 

 For flat bed scenarios, maximum accretion and erosion at the 5 m contour was more dependent on 

borrow pit aspect ratio than tidal current magnitude. This difference in influence appears to be 

related to wave refraction effects associated with the different aspect ratio borrow areas. 

 For ridged bed scenarios, the borrow area influence extended significant distances alongshore (up 

to 2,000 meters) ; therefore, process-based numerical models should consider computational 

domains with a large alongshore extent to ensure that model boundary conditions do not influence 

results. 

 For the ridged bed far-field scenarios, it appears that the comparatively greater dimension of the 

rectangular borrow area boundary exposed to the incoming wave field caused both more 

deposition within the borrow area, as well as more correlating erosion landward of this region. In 

general, borrow areas oriented with the longer dimension aligned parallel with the predominant 

incoming wave crests will infill more readily for wave-dominated sites. In addition, waveinduced sediment transport is highly dependent on water depth and shallower borrow areas 

located at the shoal crest (versus shoal flank or trough) will exhibit higher sediment transport 

rates (i.e. increased erosion and deposition potential). 

STUDY PRODUCT(S): Final Report as listed above. 
ACCESS NUMBER: M10PC00118 

Figure I-1. Sand and gravel shoal mining sites evaluated for the morphological modeling analysis 

including Sandbridge Shoals (Virginia), Long Bay (South Carolina), Canaveral Shoals 

(Florida), and Jupiter Island (Florida). 

 

*

 P.I.’s affiliation may be different than that listed for Project Manager(s).