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Distribution Of Dredged Material At The Rockland Disposal Site May 1985

Disposal Area Monitoring System DAMOS

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DIF ho, 2

Contribution 50 March 1988

DOCUMENT LIBRARY Woods Hole Oceanagraphic Institution

US Army Corps of Engineers New England Division

MBL/WHOI

AIA

0 0301 0069485 7

DISTRIBUTION OF DREDGED MATERIAL AT THE ROCKLAND DISPOSAL SITE

MAY 1985

CONTRIBUTION #50

17 JANUARY 1988

Report No. SAIC - 85/7533&C50

Contract No. DACW-85-D-0008 Work Order No. 2

Submitted to:

Regulatory Branch New England Division U.S. Army Corps of Engineers 424 Trapelo Road Waltham, MA 02254-9149

Submitted by:

Science Applications International Corporation Admiral's Gate 221 Third Street Newport, RI 02840 (401) 847-4210

US Army Corps of Engineers

New England Division

TABLE OF CONTENTS

Page

INTRODUCTION Al. SITE CHARACTERIZATION aL Bo dk Bathymetry and Side Scan Surveys aL Dene Sediment Characterization 2 2703 Vertical Profiles of Temperature and

Salinity 3 DHA Current Regime 3 25 Discussion 4 SEDIMENT PLUME TRANSPORT STUDY 5 Sioa Introduction 5 Bye Methods and Analysis 7 Bio é Results ) 3.4 Discussion 10 CONCLUSIONS 12 FIGURES

TABLES

LIST OF FIGURES

Fagure)1—1! Rockland Disposal Area.

Raguse 2 —1 Bathymetric survey grid, Rockland Disposal Site.

Figure 2-2 Bathymetric contour plot of Rockland

Disposal Site, 20 May 1985.

Figure 2-3 Results of the side scan survey conducted at Rockland, ME on 20 May 1985.

Figure 2-4 Side scan record showing high reflectance at center of disposal area.

Figure 2-5 Side scan record showing circular area of reflectance indicating a single dump.

Figure 2-6 Side scan record showing result of disposal occurring underway in a turn.

Figure 2-7 Sediment sampling locations during the May 1985 survey.

Figure z-8 Sediment station sampling locations during the September 1984 survey.

Figure 2-9 Vertical temperature and salinity pro- files at Rockland Disposal Site, 21 May 1985.

Figure 2-10a Vertical temperature profiles at Rockland Disposal Site, 22 May 1985.

Figure 2-10b Salinity profiles at Rockland Disposal Site, 22 May 1985.

Figure 2-lla Vertical temperature profiles at Rockland Disposal Site, 23 May 1985.

Figure 2-11b Vertical salinity profiles at Rockland Disposal Site, 23 May 1985.

Figure 2-12 Vertical temperature and salinity pro- files at Rockland Disposal Site, 24 May TO 8 5ye

LIST OF FIGURES (cont. )

Figure 2-13 Three-hour low pass (3-HLP) time series of temperature, current speed, and direction at Rockland Disposal Site at a depth of 20m, 22 May col Wuner os Sr

Figure 2-14 Forty-hour low pass (40-HLP) temperature time series and current stick plots at Rockland Disposal Site at a depth of 10m, 21 May to 11 June 1985.

Figure 3-1 Actual ship's track during plume study.

Figure 3-2 Results of the plume study conducted at Rockland Disposal Site on 21 May 1985.

Figure 3-3 Photographs of the acoustic record for the plume study of 21 Mey 1985.

Figure 3-4 Results of plume study conducted at Rockland Disposal Site on 22 May i985.

Figure 3-5 Photographs of the acoustic record for the plume study of 22 May 1985.

Figure 3-6 Results of the plume study conducted at Rockland Disposal Site on 24 May 1985.

Figure 3-7 Photographs of the acoustic record for the plume study of 24 May 1985.

Table

LIST OF TABLES

Qualitative Sediment Characterization at Rockland Disposal Site, May 1985

Results of Sediment Chemical Analysis at Rockland Disposal Site, October 1984

Results of the Bivariate Analysis of 3-HLP Current Meter Data Collected at Rockland at a Depth of 10m, 21 May to Ines Se >

Estimates of Material in Water Column After Disposal

1.0 INTRODUCTION

The Rockland Disposal Site is located in the center of West Penobscot Bay (Fig. 1-1), 3.3 NM northeast of the Rockland

Breakwater. This site was first used during October 1973- February 1974 for disposal of approximately 69,000 m2 (90,200 yds?) of material from Rockland Harbor. The disposal site is a

0.5 nautical mile square centered at 44°07.01'N, 69°00.3'W. Water depths within the disposal area range from 65 to 80 meters. The disposal site is marked with a buoy deployed and maintained by the US Coast Guard.

An earlier baseline survey of the disposal site was

conducted during the period 24 September - 2 October 1984 to determine existing conditions of the bottom before the start of dredging projects from the Searsport area. This survey included

precision bathymetry, sediment characterization (chemical and physical), a side scan survey, and REMOTS® sediment profiling.

The present study was conducted during the period 19-24 May 1985 to assess the transport of dredged material during disposal operations and to perform bathymetric and side scan surveys of the area after approximately 360,000 ya3 (275,400 m?) of dredged material from the Searsport project had been

deposited. Vertical profiles of temperature and salinity were conducted, and current meters were deployed for approximately one month at the disposal site. The results of the data analysis

were used to estimate the percent of material expected to reach the bottom during disposal operations and to delineate the extent of dredged material throughout the disposal site.

2.0 SITE CHARACTERIZATION Pali Bathymetry and Side Scan Surveys

A bathymetric survey was performed over an area 1200 m by 1200 m surrounding the disposal site. The survey, comprised Of 50) Lanes; IANO) aul, aLohores,, spaced) 25 meters “apart, was

accomplished using a 24 kHz fathometer system operating in conjunction with the SAIC Navigation and Data Acquisition System which is based on an HP 9920 microcomputer system.

Figure 2-1 shows the bathymetric survey area in relation to the disposal site, the present location of the disposal buoy and the location of the Reference Site established for the sediment sampling program.

A contour chart of depths at the disposal site is shown in Figure 2-2a. The site is characterized by a depression which is well-defined in the northern portion of the site, but widens and shoals toward the south, completely losing its identity over

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the southern half of the site. Depths range from about 65 meters to 80 meters within the surveyed area. Comparison with the bathymetric survey conducted in September 1984 (Figure 2-2b) reveals no significant development of a disposal mound near the buoy location.

Figure 2-3 shows the results of the side scan survey at the Rockland site conducted in May 1985. The outlined area indicates the presence of intermediate acoustic reflectance that could be caused by soft natural bottom or old dredged material. The small dark areas show the location of higher acoustic reflectance characteristic of dredged material. The central area of the site is well covered by dredged material and is surrounded by individual disposal events. Long narrow areas probably indicate disposal that occurred underway or during a turn.

Figure 2-4 is a photograph of the side scan record at the center of the area showing this high acoustic reflectance. Figure 2-5 shows a circular area, probably the result of a single dump. Figure 2-6 shows the result of disposal occurring during a UG mM. The ‘side’ Scan “records! "did ~/not) sdeteces anys Vance accumulation in the form of a mound.

22 Sediment Characterization

Figure 2-7 shows the location of sediment samples taken at the Rockland Disposal Site to visually characterize the bottom and detect the extent of dredged material. Table 2-1 describes the sediment collected at each station with a 0.1 m2 Smith- MacIntyre grab sampler. The pattern of stations sampled was determined by visually identifying dredged material in the grab and proceeding until natural bottom was encountered. The spatial delineation of dredged material from the grab samples compared well with the areas of high reflectance measured with side scan sonar. Minor discrepancies resulted from the patchy nature of Single dump loads. The areas of intermediate reflectance could indicate the presence of old dredged material.

During the September 1984 survey, sediment samples were collected at stations on North-South, East-West, Northwest- Southeast, and Northeast-Southwest transects (Fig. 2-8), as well as at the Reference Site (2000m east). Table 2-2 contains the results of the chemical analyses of the sediment samples.

The results of the chemical and physical analyses indicate little variation throughout the site. The physical tests indicate the similarity between locations with a pattern common to most of the samples; olive in color, high percentage of fines (average about 90% and predominantly clays), low fine sand percentages (around 10%) and very little medium and coarse material (usually less than 1%). These values also occur at the

reference site. The only exception of note was at 200S where the amount of fine sand was 18%.

The chemical data also reflected this uniformity along the transects by showing rather small differences between individual location replicates. The concentrations for the trace metals were low throughout the area. COD and organics were typical for natural bottom silt/clay. An exception was at 200SW, where relatively high concentrations of oil and grease were found, most likely from the presence of dredged material. The results of the chemical analyses at a majority of the stations in the disposal area are similar with those from natural bottom. C:N ratios were mostly between 7 and 9 (typical of offshore natural sediment). Mg:Ca ratios averaged between 4-6, indicating relatively few shells. Dredged material may have influence at 400W; the Mg:Ca ratios here averaged about 1 (relatively high in shells) and the C:N ratio averaged about 11 (typical of inshore harbors and estuaries). The variation in Mg:Ca and Fe results at 400NE may be caused by local natural bottom conditions and not dredged material.

os} Vertical Profiles of Temperature and Salinity

On* seach day” “Orn the) perlodini2iy May, Y= 24 May alos 5. vertical profiles for temperature and salinity were measured at the disposal buoy to determine the structure of the water column. Figures 2-9 to 2-12 show the temperature and salinity data

collected each day. The temperature ranged from approximately Geer acy Che jbottom to!) Plc fat the surface wwalth” a thermocline present at about 10 meters by 23 May. Salinity ranged from

approximately 29.6 ppt at the surface to 32.2 ppt at the bottom. Throughout the tidal cycle, little effect was seen on the shape of the vertical profiles. Wind-driven water movement could account for changes in salinity and temperature at the surface. Certain vertical profiles are seen to contain fewer data points than others. This was due to intermittent data signals being received by the computer. Alternatively, data were recorded manually at specified depths.

2.4 Current Regime

During the ;period) 21 “May to.11 June’ 1985, current measurements were made at Rockland in order to determine the

overall current regime of the area. A string of two General Oceanics current meters at depths of 10 meters and 60 meters was deployed about 300m SE of the disposal buoy. Due to a tape

malfunction, no data were collected by the 60 meter instrument. Figure 2-13 presents the current speed and direction and temperature data for the observation period. These data have been 3-hour low pass filtered (3-HLP) to emphasize the tidal components.

The temperature trace reveals initial readings of approximately 6.4°C warming to an average value of 8.3°C with a maximum value of about 9.8°C. Higher temperatures are detected on the ebb tide as warmer waters from the shallower coastal areas flow by the disposal area.

The current direction varies in the N-S axis over the tidal cycle. The peak current velocities occur on the flood tide in the northerly direction with maximum values of approximately AOwmeMn/iSee (Lable wy 23). Table 2-3 presents the results of bivariate analysis, of the current data §(3-HLP) © revealing “the average current speed to be approximately 13 cm/sec. The current a@irection is northerly for approximately 503 (sum) (or @29%9) Vand 18.6%, see boxes in Table 2-3) of the period and southerly only 22%-(sum Of VoL? and 5.22) .0f) theiperiod. a horn s6> om (SUMmOtaire, 23.8 and 138%) of the period, the current velocitresswene anieche range of 4-16 cm/sec. Onily 913% / (sum iof 5-0), 93:56), W275), ele camamcl 0.4%) of the period saw velocities greater than 24 cm/sec (0.5 knots).

Figure 2-14 presents the temperature data and a current

stick plot for 40-hour low-pass filtered data (40-HLP). The temperature time series depicts the average values after the tidal component is removed. The net, non-tidal flow in the

GQisposal area is consistently to the north-northeast quadrant at a maximum of approximately 15 cm/sec.

DEaS Discussion

The results of the bathymetric survey did not reveal the development of a disposal mound near the location of the disposal buoy. Rough estimates from scow logs of the volume of dredged material deposited between the September 1984 survey and the present one are approximately 360,000 ya3 (275,400 m?). This large volume of material would be expected to create a mound if controlled point disposal was conducted. A combination of factors including, the -depth aati the dasposal: pone ai(7 0m m)sn cae wide scope (3 times the water depth) of buoys normally established by the US Coast Guard, and, apparently, a practice of depositing dredged material while the scow is underway has caused the material to be spread over a large area. Disposal of dredged material in a water depth of 70 m allows a significant amount of water to be entrained during the convective descent phase of disposal. A large percentage of the dredged material will then have a lower density due to the increased water content causing a

slower descent and increased spreading from the initial disposal location.

Figure 2-3 shows the wide distribution of dredged

material detected by the side scan sonar. The outline in the figure indicates the area of intermediate reflectance that usually signifies thin layers of dredged material. The pattern

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of distribution suggests that some scows began depositing the material before and/or after reaching the disposal buoy. However, a large percentage of the material has apparently accumulated southeast of the buoy.

A rough estimate of the amount and thickness of dredged material can be calculated using the approximate area of seafloor indicated by the side scan sonar survey to be covered with dredged material. Examination of Figure 2-3 shows an area roughly 700 by 700m square of intermediate acoustic reflectance, usually indicating dredged material. Assuming that approximately 275,400 m? of material (estimated from scow logs) was evenly spread over this area, a layer of dredged material of approximately 0.5 m thickness could be expected. When considering that both the loss of interstitial water during descent and compaction after impact with the bottom would reduce the actual volume of material expected to be seen on the bottom by as much as 40% (Tavalaro, 1983), the thickness of the dredged material may actually be approximately 0.3 m. It would be difficult to confirm the presence of this layer with bathymetry in a depth of 70 m due to the limitations of available fathometer systems and a combination of errors associated with the speed of sound, tide, and navigation. More detailed study of this area with precision bathymetric surveys at a smaller lane spacing (25 m) would detect any significant topographic features caused by dredged material disposal while REMOTS® sediment profiling would be needed to accurately measure the thickness of the dredged material layer and determine its areal limits.

3.0 SEDIMENT PLUME TRANSPORT STUDY 8}.5aL Introduction

In order to assess the potential impact of dredged material disposal on the surrounding environment, a plume study was conducted to track suspended material in the water column after a disposal event. Three studies were performed during the period of 21 May to 24 May 1985. Although attempts were made to track plumes on both the flood and ebb tides, coordination of ScOw arrival and dumping, weather, and tide prevented studies occurring during ebb tide. However, because the dominant current feature is the flood tide (with maximum peak velocities and long durations), emphasis on the flood tide was warranted due to its greater potential for transport.

Multi-frequency acoustic profiling has been under investigation since 1975 as a means of measuring concentrations

of suspended matter in the water column. Much of the work has been carried out by the NOAA Atlantic Oceanographic and Meteorological Laboratory in Miami. The work has included study

of diffusion properties and acoustic measurements, as well as development of a model to describe the relationship between

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concentrations of total suspended matter in the water column and acoustic profile observations. A diffusion model allowing prediction of diffusion velocities and spatial variation has been postulated and tested.

The following relevant observations are taken from a technical paper "SOME OBSERVATIONS ON DREDGED MATERIAL DUMPING IN THE NEW YOR! BIGHT", by Dr. John Proni and Dr. John Tsai at the NOAA-AOML facility.

ales Spatial» and .temporal, «variation: | dusing diffusion following a dump predicts a two- process diffusion with different diffusion

velocities and spatial variation. During the active phase, heavy materials settle through Gravitation, “~seaching, the, “boron: The

downward momentum may generate vertical mixing of the water column inside the plume, and resuspension from the bottom. During the passive diffusion phase, the sharp edges of the plume disappear and dissipate into the surrounding water. Rate of diffusion slows down as the particle size distribution changes from coarse heavy material to smaller, lighter particulate matter. This dual diffusion process was clearly observed during the dredged material dump in Massachusetts Bay in February 1983.

2s Acoustic backscatter measurements can be correlated with measurements of total suspended matter in the water column as:

eo or N|

where I is acoustic intensity, N is the number of particles per unit volume, anda is assumed to depend on particle shape, size, density, compressibility and frequency. Direct and linear relationships are found between observed acoustic intensity and measured total suspended matter.

In the Rockland Disposal Area study, the following tasks were attempted:

ive Track and observe the extent of plume dissipation and movement over a period of time following the dump.

Dis Measure ambient and in-plume concentrations of total suspended matter by analyzing water samples taken in selected locations where acoustic intensity could also be observed.

5 Using the measured values of total suspended matter as "ground truth", make estimates of total suspended matter concentration based on acoustic backscatter measurement alone.

SZ Methods and Analysis

The Datasonics Model DFS-2100 Acoustic Remote Sensing System was used at 200 kHz for performing the acoustic plume tracking. High power output, low receiver noise levels and calibrated control of signal level allows monitoring of extremely HOW concentrations - Of » material invecthe s waters =column “and acquisition of suspended sediment concentration levels when correlated with ground truth sampling.

in satcempting ste. relate = acoustic “backscatter measurements in a plume of suspended particulate matter to Guantitative “concentration “levels, ‘one “must “measure” the reflection, or backscattering characteristics of the material in the scattering volume of interest.

The echo or reverberation level received back at the towed vehicle transducer from particulate scatterers in the dredged material plume may be expressed as part of a standard sonar equation as follows: (See definition of terms below).

RL = SL - 40 Log R - 2eR + Sy, + 10 Log V (1) Equation 1 summarizes signal losses within the water

column. The acoustic receiver voltage output measured and recorded during a survey can be defined as follows:

out rms = RL + RS + GAIN Ors RL = out rms - RS - GAIN @)

Equating equations 1 and 2. out rms = SL-(40 Log R + 2aR) + (Sy, + 10 Log V) + RS + GAIN (3) Definition of Terms RL: Reverberation Level, or backscattered acoustic intensity

from a random, homogeneous distribution of scatterers throughout a defined volume of water.

SL: Source Level, a measure of on-axis acoustic intensity of the transmitted signal.

AO) CYC IR ae Aese Two way transmission loss from the transducer down to the scattering volume of interest and back to the transducer. The 40 Log R component is due to spherical spreading loss, while the 2eR loss is due to absorption (primarily a function of frequency).

Sy tneO Loge: A measure of the backscattering strength due to volume reverberation. Tt a5 thelyS, term) which! \wadslecom-elate with concentration of !scatterers, (jon totalaesuspended matters i is a measure of the backscattering volume which will increase with depth as the volume of water encompassed by the acoustic signal increases due to the transducer beam pattern.

RSE Receive Sensitivity or transfer function of the receiving transducer .in conversion (of) the secho, epressunesswaveR aeoggan electrical voltage.

GAIN: Overall receiver gain, including Time Varying Gain (TVG).

Examination of Equation 3, when using the Datasonics Model DFS-2100 Acoustic Remote Sensing System, reveals the following:

a The receiver gain incorporates TVG which compensates precisely for the 40 Log R + 2aR transmission loss term. These two terms then fall out of the equation. If overall gain is changed, however, the change must be factored into the calculations.

Die Receive Sensitivity is a constant, determined by laboratory measurement. This term can be ignored in making concentration calculations from acoustic observations because relative concentration measurements, with respect to measured total suspended matter samples, are being made.

Bo Source Level may be ignored so long as it remains constant. It must be accounted for whenever a change is made.

4. Vo rms, the measured receiver output, is then proportional gto (Se 04 510) eg Vy. Velsmva function of pulse width, equivalent beamwidth and range (R) Erom the jetrnansducersy sito scattering volume. V is a measure of the reverberating volume at any given depth.

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BOTAN ye EVEN a paint hekhen wSaZey uaduisitrn buitcsom\) | sathe concentration of total suspended matter will be proportional to the measured rms voltage output of the receiver, when corrected for depth.

Ses Results

The plume studies were conducted at disposal events when the towed scow was unloaded by opening the bottom doors. As soon as disposal was complete and the scow was underway, the research vessel, with navigational control and the Acoustic Remote Sensing System aboard, began executing patterns of parallel tracks to determine the boundaries of the plume. After the motion of the plume was determined, the ship's track was modified to continually encompass the boundaries. Figure 3-1 presents an actual ship's track followed during the first two hours of the plume study on 24 May 1985.

During each study, water samples were collected with Niskin water samplers as the acoustic sensing transducer passed through the plume. Sampling depths were determined from the acoustic results of a pass through the area immediately prior to sampling. The samples were analyzed for total suspended sediment @ng/Ah)== and “used = to callilbrate’ the’ “output voltages!” from “the acoustic record.

PA MAverL OS Sia ienP lL umMenSitudy.

The plume study began at 0920 EST as the scow began the disposal operation. One scow of 1575 cu yds (1205 m?) was dumped approximately 200 m east of the disposal buoy. The study was continued until 1051 EST. Low tide occurred at 0547 EST and high tide at 1157 EST. Flood tide was in progress during the survey, providing a N-NE flow. Figure 3-2 presents the results of the plume survey.

The area of heavy concentrations (averaging about 1000 mg/l, Fig. 3-3) occurred below a depth of 50m and did not extend far beyond the area directly beneath the disposal location. Within 15 minutes of the completion of the disposal operation, concentrations averaging approximately 40 mg/l (Fig. 3-3) occurred from a depth of 20 m to the bottom and extended approximately 300 m from the initial location in the N-NE quadrant. Within forty minutes of disposal, suspended sediment concentrations were averaging less than 10 mg/l (Fig. 3-3) at depths below 35 m and extended less than 500 m from the disposal point, still within the boundaries of the designated disposal site. Further surveying detected no significant levels of suspended sediment above background concentrations of 3-5 mg/l.

2:2) MayanliGisiobuaai=r a Plumem SituGiy:

The second plume study began at 0645 as one scow of 1900 cu yds (1450 m?) completed disposal operations approximately 150 meters south of the disposal buoy (Fig. 3-4). With low tide occurring at 0623 EST, flood tide was just beginning to develop. Again, the heavy concentrations of suspended material, averaging Greater than’ 1000 mg/l “(Rig. 93-—5)),) cccumned (directly Soeneatchysene disposal location throughout the water column. Within forty minutes of disposal, concentrations averaging greater than 25 mg/l (Fig. 3-5) occurred below 30 m to the bottom and extended approximately 350 meters to the north. One hundred minutes after disposal, concentrations averaging only 12 mg/l (Fig. 3-5) occurred at depths below 50 m and extended less than 500 meters north-northeast of the disposal point, still within the disposal site.

ZA MayanlO8 >in umMem Stuy

The third plume study began at 1025 EST as two scows (tandem: oad) containing “a > total wor 364 On vecumiydsEs (27/80 m?) completed disposal operations within 50m SE of the disposal buoy

(Fig. 3-6). .With low tide: occurring at.0744 ESTwand hugh trdemat 1355 EST, the flood tide was fully in progress, producing maximun tidal current velocities to the N-NE. Heavy concentrations

averaging about 1400 mg/l of suspended material (Fig. 3-7) were detected directly beneath the disposal location throughout the water column. Within one hour of disposal, concentrations of suspended material of 110 mg/l (Fig. 3-7) occurred from 20 m to the bottom and were centered at a point approximately 700 meters

north of the disposal location. After more than two hours, low concentrations of suspended material, averaging about 13 mg/l (Fig. 3-7), occurred below 50 m and were measured within a 400 m

radius around a point 1700 meters N-NE of the disposal location, or as much as 1000 meters beyond the northern boundary of the disposal site. Background levels of 3-5 mg/l were measured at the disposal buoy just before this survey was conducted and just outside of the disposal site when no disposal operations were being conducted.

3.4 Discussion

Of the three plume studies conducted at the Rockland disposal area during the period 21 to 24 May 1985, the first two, although occurring on the flood tide, did not detect any elevated concentrations of suspended material above background levels (3-5 mg/l) outside of the designated disposal area. The survey conducted on 24 May 1985 resulted in measurable concentrations (averaging 13 mg/l) of suspended material as far north as 1000 meters beyond the disposal site boundary.

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In an attempt to estimate the amount of material that could be transported out of the disposal area, preliminary calculations were made from the results of the plume studies. A representative range of values for bulk density of the dredged Maeerlale Of 1 S4 yO 16 g/cm? was used to calculate the dry mass

of material in the scow. The estimated scow volumes were obtained from NED. From each plume study, estimates of the depths where the suspended sediment concentration occurred were used in the calculations. Table 3-1 presents the estimates for

the percent of material present in the water column after the disposal event. These calculations included determining the mass of material (mass,) in the suspended sediment cloud as:

maSSo = A X D XGES

where A = the area of the sediment cloud (m2), D= the height of the water column (m) with suspended sediment, and S= eae suspended sediment concentration (mg/l or g/m-),

and determining the mass of material (mass,) in the scow as: mMaSSg = scow volume (m3) X bulk density (g/cm?) x 10° aneiy famalliy.:

% material = (mass, / maSsg) X 100.

The estimates for the percent of disposed material still in the water column vary widely for the three plume

studies. This is due to the estimated values for area and depth of the suspended sediment concentrations determined in Figures 3- Bn (BOG ehavel iSO Although the acoustic measuring system can

detect suspended sediment vertically from the surface to the bottom and along the ship's track (horizontally), another pass of the ship through the suspended material is needed to delineate the spatial area. The interval of time required for this allows the suspended material cloud to settle or spread. Qualitative judgments were made in order to graphically illustrate the best approximation of the suspended sediment clouds. Despite the variation in estimates of material in the water column, an important feature common to all three plume studies is that within two hours, 93% or more of the material was on the bottom and suspended sediment concentrations were similar to background levels. For the plume studies conducted on 24 May, tandem scow loads were deposited at the disposal site. Doubling the volume of disposed dredged material increased the initial sediment concentrations (1420 mg/l versus 950 or 1100 mg/l on 21 or 22 May, Table 3-1) due to more material being available for suspension and increased the distance from the disposal point that the suspended material was tracked (1700 m versus 500 m on

alta

De andy 22 eeMavin nalolehy aSi—5) irs The distribution of suspended sediment immediately after disposal would probably be less for a single scow load of the same volume because more material would make up the single descending plume and a smaller percentage of material would experience the entrainment of water at the water column/plume interface.

Results from the current meter data showed that the dominant flow was to the N-NE and that the maximum current velocities occurred on the flood tide. For the Rockland area, NOAA tidal current tablles estimate that slack water before flood tide occurs approximately 1 hour before low tide and that maximum flood tide current) velocities occur about 3 hours vatter Vow jeder This suggests that the sediment transport out of the disposal site during the 24 May plume study was caused by the peak tidal Velocities, occurring ulat: gchatiiatimer This period of maximum transport appears to be relatively short. The survey of 21 May occurred one hour later in relation to the stage of the tide, and no material was detected outside the disposal site. Current velocities greater than 24 cm/sec occurred only 13% of the time, or approximately 1.6 hours during each tidal cycle.

Examination of Figures 2-9 to 2-12 indicates that the thermocline usually occurred at or near the 10 m depth where the current meter was deployed. Therefore, the results of the current meter analysis would reflect the potential for movement of sediment that may be suspended at that depth. However, a closer look at the results of the acoustic records obtained during the plume tracking. (Figures 3=3', 3-5, and) 3-7) sadudamnoc reveal a significant accumulation of suspended sediment at the thermocline.

In order to estimate the long-term effect of disposal at this site based on conditions described above, a calculation was made to determine the percent of material that could be expected to leave the disposal site during a prolonged disposal project. If we assume that after disposal approximately 6% of the material will be in the water column and available for transport by current velocities greater than 24 cm/sec, then only 0.8% (or 6% times 13%) of the material disposed during the entire dredging project is available for transport outside the site boundaries. As mentioned earlier, this material would be so widely dispersed that detection of any accumulation would he almost impossible.

4.0 CONCLUSIONS

The results of the precision bathymetric survey conducted on 20 May 1985 did not detect any significant development of a mound of dredged material from disposal operations occurring since September 1984. The side scan survey revealed the presence of dredged material near the disposal buoy,

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as well as isolated patches surrounding the center of the area. Visual identification of dredged material from sediment grab samples correlated well with the results of the side scan survey. The distribution of intermediate acoustic reflectance (usually signifying thin layers of dredged material) suggests that disposal operations ranged out to more than 300 meters from the bpueyv (which had a Scope of up to 150 m) and did’ not create a disposal mound detectable with bathymetric survey procedures. The depth of the disposal location (70 m) also contributes to wider distribution of disposed material by the entrainment of water during descent and the subsequent reduction in the density of the material.

Results of chemical analysis of sediment samples collected in September 1984 did not reveal any significant elevations in chemical concentrations that could indicate the presence of contaminated dredged material. In general, no trends could be seen throughout the sampling area. Slightly higher concentrations of oil and grease at some stations indicate the potential for isolated patches of recently deposited dredged material existing.

The Rockland Disposal Area experiences its maximum (40 em/s)) ptidal current) velocities only | at maximum 9 flood) tide conditions in a N-NE direction. Because the flood tide is the dominant feature in the current regime, it yields the greatest potential for transport of suspended sediment introduced by dredged material disposal operations. Although the data from the bottom current meter was lost, bottom current velocities sufficient to resuspend and transport large amounts of dredged material out of the disposal site are not expected, based on data collected by the meter at the 10 m depth.

If disposal- occurred only on maximum flood tide (a worst case), an estimate of the material transported out of the disposal site would be approximately 6%. However, if disposal occurred evenly at all stages of the tide, this estimate reduces ieOmme lor. Once this small percentage of material has settled outside the disposal site, it would be so widely distributed as to be undetectable. The fact that no significant accumulations of sediment were detected in the N-NE direction from the disposal buoy by either bathymetry or side scan sonar supports this conclusion. Alternatively, if adverse levels of sediment accumulation were detected outside the disposal site, disposal operations could be scheduled to avoid peak flood tide.

In summary, although the potential for the movement of suspended sediment produced by disposal operations exists at the Rockland Disposal Site, results of the plume tracking experiments indicate that this potential is small. Analysis of current meter data suggests that any transport would occur to the N-NE, although no evidence of accumulation of dredged material in that

Als}

direction “was detected’ by bathymetry “orn ) siden iscanmysonaz: Finally, transport of suspended material out of the disposal site would only be expected to occur when disposal operations take place at maximum flood tide.

14

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Figure 2-3). Scale lines (vertical) are 7.5 meters apart.

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Figure 2-9. 21 May 1985.

Rockland Disposal Site,

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Actual ship's track during plume study.

ae

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950 mg/l at 09330 EST

Figure 3-2. Results of the plume study conducted at Rockland

Disposal Site on 21 May 985. (Axes tanrescentered at disposal location. Distances in meters:

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0 EST

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Figure 3-6. Results of the plume study conducted at Rockland Disposal Site on 24 May 1985. Axes are centered at disposal location. Distances in meters.

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Table 2-1

. . a . . * Qualitative Sediment Characterization

Station

200W 100W

50W CENTER 50E 100E

150E 200E 300E

400E 450E

500E

200N/150E 100N/150E 100S/150E 200S/150E 300S/150E

400S/150E 500S/150E

600S/150E

at Rockland Disposal Site May 1985

Sediment Description

Natural bottom

Dredged material; sandy silt with wood, clay, shale fragments

Soft sandy silt with clay clumps and gravel, possibly old dredged material

Sandy silt over silty sand with gravel, well-colonized

Dredged material, sandy silt with clay clumps and stones

Similar to 50E with hard packed clay clumps

Similar to 100E with cobble Large stone (25cm, incomplete grab)

Sandy silt with large clay clumps and gravel

Sandy silt with hard clay and stones

Soft natural bottom, well colonized, Nephtys, starfish, sea cucumber

Same as 450E

Natural bottom, starfish

Large stone, incomplete grab

Dredged material with clay clumps, stones Same as 100S/150E

Dredged material with coarse sand, gravel, clay clumps

Silty sand, stones, wood chips, colonized, possibly old dredged material

Dredged material with clay clumps, gravel, shell fragments

Natural bottom, starfish, worms

* . . Benthic organisms are mentioned when seen in grab sample.

Station

200S/300E 200S/150E 200S/50E 200S/50W 200S/150W

200S/250W 200S/350W

200S/450W 400S/150E 400S/150W

500S/250W

600S/350W

Table 2-1 (cont. )

Sediment Description

Natural bottom

Dredged material with clay clumps, stones Sandy silt, clay clumps, gravel

Large clay clump

Natural bottom with traces of dredged material

Possibly old dredged material colonized by tube worms

Clay clumps on natural bottom, tube worms, starfish

Natural bottom Old dredged material, wood chips, colonized

Dredged material with sand, gravel, clay clumps, wood chips

Dredged material with sand, gravel, clay clumps

Natural bottom, soft sediment, worms

Table 2-2

Results of Sediment Chemical Analysis at Rockland Disposal Site

Station

400N-A -B -C

Mean

Std.Dev.

200N-A -B iC

Mean

Std.Dev.

CTR -A 8} =¢

Mean

Std.Dev.

Mean Std. Dev.

400S-A -B —@

Mean

Std.Dev.

2000E-A -B AS

Mean

Std.Dev.

Fe 2n x104

2.66 134 2.60 160 2.88 137 Pe igfal 144 Or ai2 12 De 204 D313 1Oe) 2.52 149 Diao 158 0.08 34 2.38 139 2.59 159 3.00 137 2.66 145 0.26 10 2034 103 2.40 134 2.45 141 2.40 126 0.04 iy 2.75 123 2.318 132 252 146 2.55 134 O15 9 2.35 151 2 e5i 124 2.59 110 2.48 128 O10 ay)

October 1984

(ie

°

ONNNO ONNNN O1 U1}00 \/O OO oO WO}O OY)

ONINNN PON OU

RP WIR WwW Oo, OO O OW

OWWW A OO} Oo Ul

90 ON Oo NNIW OO WM

oO Pf} OW

OW|W~I 0 O1 alow Oo Wo

Polk ow

Station

400W-A -B -C

Mean

Std.Dev.

Mean

Mean Std.Dev.

400E-A —i5 =<

Mean

Std.Dev.

2000E-A -B AG

Mean

Std.Dev.

BrOO 3.40 0.30

Drape? Px) 0) 32.00 2.54 0.36

3)52'5 Bra27 0.65 Zo. MG BE

2.82 Sots 3.94 BiG Sik 0.47

4.10 By dual 4.06 Bi. 116, 0.46

3.66 3.40 2.50 2) 6 cue) O50

Table 2-2 Continued.

COD x10?

Fe x104

- indicates no data available.

O MW]o 10 e e b O}P op wb

ONIN NN ° oa |b Fr OO) OV

ONIN NN oO ojo YIN PAIN AW OOj;ON &

(2) fe) (a) (e) (J W WOld O W

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e

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Station

Mean Std.Dev.

CTR -A =B —

Mean

Std.Dev.

200SW-A -B -C

Mean

Std.Dev.

400SW-A -B —C

Mean

Std. Dev.

2000E-A -B —

Mean

Std.Dev.

2.64 Syeles, 0.54

Sis dal 7h 5 Se AES 37210)0 0.08

Srey2jD Socal OMG5: Bool eee

3738 2.28 2.82 2.83 0.45

302 = 174 4.07 3579/0 0.12

3.66 3.40 Zhc15)0) Boake) 0.50

Table 2-2 Continued.

CoD x10°

Fe x104

- indicates no data available.

154

(eye

ONIN ON OnION Fr

oN aA PON ®8 wo

ONN~ Oo WoW OO

ONINNN PON OU

OO & oO

NNW Oo U1

OW} NM & Ov Nir ON

oO fj Ww \O 0VjO U1 OV

POL ao» Oo wlw nw

Table 2-2 Continued.

% COD Fe

Vol. X10? x104

Station NED (ppm) (ppm) 4O0ONW-A S76 0.63 AG AUS) 3} 3.64 0.40 2.49

AG BL. 4 ORISw/i ZO)

Mean B15 15 0.47 2.39 Std. Dev. O22 O)5.ALZ Opaka 20O0NW-A PX (Oak 0.18 BEA -B PENS 0.64 Py Sh)

-C PRESS) 0.56 ene)

Mean PRINT) 0.46 PRAT Std.Dev. O35 O20 ORi2 CER =A Ba 0.40 PBS Sie} -B Bis2a) 0.38 BESS)

=¢C OS (SS) ORAS 31010 Mean Dice Sal: Onsi2 2.66 Std.Dev. 122 (OVS AL) ONZE 200SE-A By CAT 0.28 2.44 -B BEES Ons 2.48

-C Ba NS) 0.28 2.415

Mean Br 45 0233 2.46 Std. Dev. O23 OR 1O)7/ 0.02 400SE-A SrO2 0.38 Dedi2 -B Phe AE 0.38 22516

—C BLS Onan 253

Mean See (0)ib 0.39 2.54 Std. Dev. 0.64 OOM: 0.02 2000E-A 3.66 O35 7B S35) -B 3.40 Oe S7/ 7d Sy al

=C PAO) ORS7/ 2.59

Mean Sh ALG) 0.36 2.48 Std.Dev. 0.50 OO 0.10

- indicates no data available.

Gr

Pr OlF Oo POW

O wWla ~ 0 bO}, Bt

ONINNN NW 0V{O7) &

ONO ON O1 P| 0%

ONINNN RP ANA UW

oO AO}|O NO OR PN

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OM & oO MNIW OW

oO 8]R Bw OPI|O wow

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Pol ow owlyo yuo

Results of the Bivariate Analysis of 3-HLP Current Meter Data

FREQUENCY DISTRIBUTION

Table 2-3

Collected at Rockland at a Depth of 10m 21 May to 11 June 1985

Oe ometes, #8 5:4) 520/042. 3:6 119 1.2) 1-2) 1.0) 22 wd]

SOICOmemame 20420) Ie Deesby-2.) 0) 00) 0) 20> 920i «20 6.0

BUesvOMy lsh 4. Aasls2 24) 10) 720 50!) 1.0, ".0) 01" 20 9

POc(2OMmmte2n2s5 aise 8%. 2 0/0 20. Oe 60 <0 5.8 20EISOMMN NSO) 56 (2kS 9161.2 «0 20 1.0.0) 0. <0 7.9 150-180] 42.7 3 29 167 B10 8 2 OO 18.7 180-210 OMe 2eeloz il Sims D ste tereAS ec tee Qin a0) ay50 3.2 MOeTsOen Meaty 0) 72 1.2) 6052.0 60 0 9.0 60) 20 my =D OM eds 4 a2 20) 160) 1200050 | 20) 020. 20 1.0 Di0-S00Mme 50) .6) eeeOle01 4.0 0) 0.0 40) 60.0 6 SDOEISOMene 7 Ts) eA, 0.2 <s0ura0 1.0) 20) 1-020 1.0 FESO=S6O]p teed 2 7) 40000 2.5002.9 1.2 Hof L1,Si 1.2 542 [ee]

SPEED 0 16 20 32 «36 WO

; eens

cH/s dlsaest2_16)) 20-24 PERCENT dla mera tne 4.0 100.00 MEAN DIR 119 120 Isl 197 154 107 me Tey TOT Te STD DEV 9«:100 110-120 140 135 145 140 164 162 230

SUMMARY STATISTICS MEAN SPEED = 13.16 2A/S MAXIMUM 3 40.71 CH/S MINIMUM = 1.31 CH/S RANGE = 39.40 CH/S STANDARD DEVIATION = 8.37 CH/S SKEWNESS = 1,09

IN A COORDINATE SYSTEM WHOSE Y AXIS 13 POSITIONED

MEAN X COMPONENT = 2.19 CM/S MEAN Y COMPONENT = 4.68 CH/S

STANDARD DEVIATION = 3 STANDARD DEVIATION 2 14

00 DEGREES CLOCKWISE FROM TRUE NORTH

85 CH/S SKEWNESS = 13 13 CA/S SKEWNESS = 11

MEAN

SPEED

16.01

7,38

6.42

KIN SPEED

1.31

MAX SPEED

40.27

16.81

14.55

19.16

18.36

32.83

29.12

15.26

9.77

5.88

17.49

40.71

STD. DEV

9.24

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