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STUDY TITLE: Field Testing of a Physical/Biological Monitoring Methodology for Offshore Dredging and

Mining Operations

REPORT TITLE: Field Testing of a Physical/Biological Monitoring Methodology for

Offshore Dredging and Mining Operations

CONTRACT NUMBER: 1435-01-02-CA-85050

SPONSORING OCS REGION: Minerals Management Service, Leasing Division, Marine Minerals Branch

APPLICABLE PLANNING AREA: Generic, Applicable to All Planning Areas

FISCAL YEAR(S) OF PROJECT FUNDING: FY199 to FY200

COMPLETION DATE OF REPORT: July 2006

COSTS: FY2001: $84,666; FY2002: $227,119; FY2003: $248,326; FY2004: $119,018; FY2005: $119,018;

FY2006: $4,729; Cumulative Cost: $683,858

PROJECT MANAGER: Carl H. Hobbs, III

AFFILIATION (PROJECT MANAGER): Virginia Institute of Marine Science, College of William & Mary

ADDRESS: P.O. Box 1346, Gloucester Point, VA 23062

PRINCIPAL INVESTIGATORS: Robert J. Diaz, Jerome P.-Y. Maa, C.Scott Hardaway, Jr., Jesse E. McNinch,

and Carl H. Hobbs, III

KEY WORDS: sand resources, dredging, mining, monitoring, beach nourishment, beach profile, shoreline change,

radar wave monitoring, biological resources, fish

BACKGROUND: In 2001, under contract to the Minerals Management Service, Research Planning, Inc. of

Columbia, South Carolina, developed a protocol for monitoring offshore borrow areas along the east and Gulf of

Mexico coasts. The proposed protocol addressed six issues: 1) bathymetric and substrate surveys, 2) sediment

sampling and analysis, 3) wave monitoring and modeling, 4) shoreline monitoring and modeling, 5) benthic

communities and their trophic relationships to fish, and 6) marine mammal and wildlife interactions during dredging. 

The present study was initiated for the dual purposes of exercising and reviewing the draft protocols and of adding to

the knowledge and understanding of the Sandbridge Shoal study area.

Two sections of Sandbridge Shoal have been dredged four times from 1996 to 2003. Area A on the south

end of the shoal had 841,500 m3

 (1,100,000 yd3) dredged in 1998. Area B on the north end of the shoal had 619,500

m (810,000 yd3

) dredged in 1996, in 2002 another 1,530,000 (2,000,000 yd3) dredged, and in 2003 another 535,500

m3 (700,000 yd3). A total of 3,527,000 m3

 (4,610,00 yd3) has been dredged, mostly from Area B.

OBJECTIVES: To test and evaluate the draft protocols for physical/biological monitoring for offshore dredging

and mining operations particularly as regards beach nourishment projects, to enhance the knowledge and

understanding of the Sandbridge Shoal study area, to assess the recovery of the biological resources following

dredging, to organize and interpret data on regional shoreline change, and to develop tools and techniques for radar

observation and analysis of nearshore waves.

DESCRIPTION: In order to assess the protocol for monitoring the benthic communities and their trophic

relationships to fish and study the long term and before-after, control-impact (BACI) of sand mining on Sandbridge

Shoal, data were collected during four intervals (“cruises”): June 2002, about 6 months before dredging, August

2003, about 4 months post-dredging, June 2004, about 2 months after another dredging project, and June 2005, about

4 months after dredging. Two grab samples were collected at each site, one for community structure the other for

grain-size and organisms for stable isotope analysis. Fish were sampled with a 4.9m otter trawl having a 2.5 cm

stretched mesh body and a 0.5 cm mesh liner and a 10 minute haul. Fish and crustaceans were identified to species

and measured length. A video sled was used to assess the association of fishes with substrate and for classifying

bottom habitats. Additional information was collected with a digital sediment-profile camera. The samples were

analyzed following the draft protocol and standard professional procedures.

The radar wave observation study necessitated determining and acquiring radar and data acquisition

systems appropriate for the project, developing software for the physical operation of the system, data collection, and

analysis, and testing and deploying the system. The analytical system was tested with mathematically generated,

synthetic data. Field data were collected by mounting the system on the roof of a hotel near Rudee Inlet.

The geologic and geomorphic studies involved acquiring shoreline position data with an RTK-GPS system

mounted on an all-terrain-vehicle (ATV), obtaining historic shoreline, beach-profile, and similar data from various

sources, and operating chirp-seismic, sidescan, and interferometric-swath-bathymetric systems to develop

information on the sediment surface and antecedent geology. The historic data were rectified to a standard datum

and were entered into a geographic information system (GIS) to allow comparisons across dates. Data on shoreline

monitoring programs in other states were acquired, tabulated, and assessed for common factors etc. 

SIGNIFICANT CONCLUSIONS: A consistent set of criteria for a standard, minimum monitoring program

appears to be beneficial for both broad and local analysis of the impacts of offshore sand mining and beach
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nourishment. Such a set of criteria, or protocol, needs to be adaptive in order to facilitate inclusion of new methods

and the consideration of new questions. However the establishment of a standard protocol requires that another suite

of questions be addressed. Why monitor at all? The basic reasons for the monitoring will define the dataset that

needs to be acquired. Who pays? The tax payer is the ultimate payer, but the agency sponsoring the project likely

would seek a smaller, shorter (less costly) monitoring program than would a permitting agency. What happens to the

data? A robust monitoring system would benefit from uniform data formatting across projects and a continuing online availability of all data at a single URL, either in place or by link.

The effort on biological monitoring and evaluating the biological monitoring protocol indicated that there

was little discernable difference between areas that had been disturbed by sand mining and nearby areas that had not. 

No negative environmental impacts were observed for macrobenthos or demersal fishes. Although substantial

quantities of sand had been removed from the designated source areas, no negative environmental impacts have been

observed for macrobenthos or demersal fishes. Also, although much of the suggested protocol proved valuable,

there is a need to provide for a more mobile strategy that better facilitates responses to unanticipated changes in

planned dredging activities

The major conclusions of the wave monitoring portion of the present study are 1) marine X-band radar for

wave measurements is a promising technique, 2) the X-band radar set up used in this study has a temporal resolution

< 2 Hz and spatial resolution > 8 m, 3) for offshore use, a small (6 kw) X-band radar might be the best option, 4)

experience on calibrating the X-band radar indicates that it is impossible to get a perfect correlation between the

Signal to Noise Ratio (SNR) and the significant wave height thus more work is necessary to determine why, and 5)

other wave analysis techniques for use with radar image analysis should be verified with mathematically generated

wave fields.

The analysis of the draft protocols for monitoring the shoreline and nearshore areas identified three major

considerations: 1) documentation of survey datums (metadata) is crucially important; 2) an initial, preferably, predredging, survey of the study area using some form of side-scan sonar or swath bathymetry is important as it will

provide data with which to set parameters, such as profile spacing, for future surveys; and 3) the actual tools and

techniques of data acquisition are less important than the actual spatial distribution of points at which there is

accurate and reasonably precise x-y-z bathymetric data. The monitoring protocol must be adaptive as every site is

not the same as every other and as conditions at individual sites might change. The alongshore spacing of beach

profiles needs to be close enough to capture the onshore consequences of nearshore bars and shoals. 

STUDY RESULTS: It was our intention to collect Spring/Summer and Summer/Fall data over a two-year period. 

However, another dredging event occurred during our initial study. The protocols should emphasize an adaptive

management strategy able to cope with these types of unforeseen events. What actually occurred over the four-year

period of our study is a good example of adaptive management where our field design and effort were redirected to

extract as much environmental information on dredging impacts as possible. The first pre-dredging survey was in

June 2002, about 6-months prior to dredging. The first post-dredging survey was in August 2003, about 4-months

post-dredging and about five months prior to another dredging episode. A third survey was in June 2004 about 2-

months post-dredging, and the fourth survey was in June 2005 about 14-months post-dredging. Protocols suggest

sampling at 1, 3, 5, and 7 years post-dredging, however, as our study highlights, the protocols must be able to

accommodate change.

The sea floor on and around Sandbridge Shoal was dominated by physical processes. Substrates were

predominantly fine to medium sands with little to no silt-clay. The sediments of the shoal and the area to the east

averaged medium-sand. The sediments west of the shoal were finer sands, contained 6% silt-clay, and tended to

have more biogenic structures. The area was habitat to macrobenthic and fish populations. The relative uniformity

of the substrate lead to about equal habitat value throughout the study area. From year to year, there appeared to be

a regional trend in the total abundance of macrofauna in that populations would rise and fall across all strata. 

Abundances averaged 3,700 individuals/m2

 in 2002 and declined to 2,300 individuals/m2 in 2005. Community

composition on and around Sandbridge Shoal for 2002-2005 was similar to that reported in previous work in the area

and for the shallow continental shelf habitats along the Atlantic Coast.

The study focused on possible trophic shifts that may be related to sand dredging by collecting data on fish

feeding preferences, secondary production, and stable isotopes of nitrogen and carbon. There were differences

between fish species in their diets with mysids and other planktonic larvae being the most abundant food items. 

Other important prey were decapods, mainly sand shrimp, amphipods, polychaetes, lancelets, and molluscs. 

Differences in feeding patterns were examined for 2004 data for spotted hake, sea robins, smallmouth flounder, and

pinfish but there were no differences between what was eaten between habitats. The trophic value of the dredged

Area B was the same as the other three habitats examined. Stable isotope signatures of both carbon and nitrogen for

the fauna on and around Sandbrige Shoal were typical for marine food webs. The general pattern of isotopic
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enrichment combined with gut content analysis confirmed that the demersal fishes on and around Sandbridge Shoal

fed and relied primarily on the local epibenthic invertebrates. There were no significant differences between habitats

for either stable isotope ratios for any of the taxonomic groups. There were two trophic levels beyond the primary

producers present on or around Sandbridge Shoal. The top trophic level species appeared to be spotted hake and

weakfish.

Commercially available X-band (9.41 to 10.5 GHz), marine radar can provide a valuable tool for remote

observation of waves. Longer antennas yield greater spatial resolution in the radial direction and higher antenna

rotation rate yields greater temporal resolution. Antenna elevation and output power are the main factors that control

the range of useful observation. Although the wave-energy directional distribution and wave period at the peak

frequency can be obtained relatively directly, significant wave height has to be calculated with the signal-to-noise

ratio. Further work needs to be done on to improve that calculation.

The region of complex nearshore morphology at Sandbridge indicates potential shortcomings of the

proposed protocol. The profiles should be part of a three dimensional monitoring program to validate comparisons

of subsequent profiles in areas of complex and variable relief. While shore-normal profiles accurately describe the

beach profile at discrete locations, such profiles assume little variation in alongshore. A shore reach might be better

represented by variably spaced profiles with greater density in more complex areas. An initial, high resolution

evaluation of the morphology will allow refinement of the site-specific protocol, provide a better understanding of

framework geology, and improve assessment of shoreline change. Complex nearshore morphology will make

shoreline change modeling arduous, since most numerical models assume smooth, parallel bottom contours. As

complex bathymetry and irregular bedforms impact wave transformation and energy attenuation, a solid knowledge

of the nearshore environment is essential. At Sandbridge Beach, cross shore transport appears to be non-uniform

and tenuously related to subaerial shoreline movement.

The combined use of two-dimensional and three-dimensional data in monitoring has inherent problems, as

does the use of data collected in different vertical datums. Errors, even cm scale, in converting between vertical

datums can account for significant volumes of sediment. Another problem is the spatial and temporal disconnect

between data collected from the beach and from the nearshore. It is important to conduct beach and nearshore

surveys as close in time as possible, to use the same vertical and horizontal datums and tidal corrections, and to

ensure that the two surveys overlap. Thus it is important that the terrestrial profiles be carried as far into the water as

possible. Shore monitoring protocols should endeavor to follow the recommendations developed by the National

Research Council.

Numerous state, regional, local, and federal entities and academic institutions employ a variety of methods

including beach profiles and digital orthophotography to monitor shoreline change. Time series shoreline change

and monitoring data are used to evaluate short-term and long-term beach and nearshore accretion and erosion,

determine building setbacks and the like, develop and maintain land management and stewardship practices,

establish insurance rates, and evaluate the performance of beach nourishment projects. For a number of reasons, the

scale and duration of the monitoring programs varies. A suite of barriers, primarily variability in survey resolution

and frequency, data projection, formatting, and metadata serves to diminish exchange across programs. 

Promulgation of a suite of protocols for monitoring the biological, physical oceanographic, and geological

consequences of marine sand mining and beach nourishment has the potential to be of substantial benefit to the

research and resource management communities. The protocols must allow adaptation to local needs and changing

circumstances. Specification of a minimum data set, data format, and metadata needs is important. Compatible pre

and post dredging surveys also will allow assessment of the pre-dredging predictive models.

STUDY PRODUCTS: Hobbs, C.H., III, ed, 2006, Field Testing of a Physical/Biological Monitoring 

Methodology for Offshore Dredging and Mining Operations. U.S. Dept. of the Interior, Minerals Management

Service, Leasing Division, Marine Minerals Branch, Herndon, Va. OCS Study MMS 2005-056.

Browder, A.G. and J.E. McNinch, in press. Linking framework geology and nearshore morphology: Correlation of

paleo-channels with shore-oblique sandbars and gravel outcrops. Marine Geology.

Browder, A.G., 2005. Linking Framework Geology and Nearshore Morphology : Correlation of Paleo-channels

with Shore-oblique Sandbars and Gravel Outcrops. unpub. MS Thesis, Virginia Institute of Marine Science, College

of William & Mary, Gloucester Point, VA, 72p.
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Location of the study with detail shown on a portion of National Oceanic and Atmospheric Administration, National

Ocean Survey, Coast Survey chart 12207, Cape Henry to Currituck Beach Light. The locations of Sandbridge Shoal,

Sandbridge Beach, and Rudee Inlet (all areas of importance to the study) and annotations of latitude and longitude have

been added to the chart.