Appendix B Bathymetry

Contents

Bathymetric Survey Report

Bathymetric Survey Report, Addendum 1, Comparison of Gowanus Canal Bottom Elevations, June 2003 versus January 2010

BATHYMETRIC SURVEY REPORT

GOWANUS CANAL

BROOKLYN, NEW YORK

3-D Visualization of a Portion of the Gowanus Canal Survey Area

Prepared for:

Henningson, Durham and Richardson Architecture and Engineering, P.C.

One Blue Hill Plaza, P.O. Box 1509

Pearl River, New York 10965

Prepared by:

CR Environmental, Inc.

639 Boxberry Hill Road

East Falmouth, MA 02536

April 2010

Bathymetric Survey Report

Gowanus Canal

Brooklyn, New York

CR Environmental, Inc.

April 2010

TABLE OF CONTENTS

Page

1.0 INTRODUCTION………………………………………………………………… 1

2.0 METHODS………………………………………………………………………… 1

2.1 Survey Vessel 1

2.2 Bathymetric Data Acquisition 2

2.3 Vertical Control 2

2.4 Bathymetric Data Processing 3

3.0 BATHYMETRIC RESULTS AND DELIVERABLES …………..…...………...… 4

4.0 CONCLUSIONS AND RECOMMENDATIONS ………………..…...…………… 5

FIGURES

Figure 1A Gowanus Canal Site Trackline Map (Southern Section)

Figure 1B Gowanus Canal Site Trackline Map (Northern Section)

Figure 2A Gowanus Canal Site Bathymetric Contour Map (Southern Section)

Figure 2B Gowanus Canal Site Bathymetric Contour Map (Northern Section)

LIMITATIONS

ATTACHMENT 1 GEOD Corporation Survey Information for the BM-1 Control

Point on the Carroll Street Bridge

PROJECT DATA CD

i

Bathymetric Survey Report

Gowanus Canal

Brooklyn, New York

CR Environmental, Inc.

April 2010

1.0 INTRODUCTION

CR Environmental, Inc. (CR) performed a bathymetric survey of part of Gowanus Bay

and all navigable portions of the Gowanus Canal, located in Brooklyn, NY, on January 5,

2010 for Henningson, Durham and Richardson Architecture and Engineering, P.C.

(HDR). The objective of the survey was to map seabed elevations within the Gowanus

Canal Study Area in support of the ongoing Remedial Investigation/Feasibility Study

(RI/FS). These data can be used to support volume calculations, estimates of changes in

depth over time, and to assist in planning sediment sampling investigations.

This draft report summarizes the methods used for bathymetric data acquisition and the

results obtained. Digital data products were transmitted electronically to HDR and CH2M

Hill, Inc. on January 25, 2010 and also on a CD with the draft report submitted February

2010. Data are projected to New York State Plane (Long Island), NAD-83, U. S. Survey

Feet. Seabed elevations are reported in U.S. Survey Feet relative to NAVD88 (estimated).

2.0 METHODS

The bathymetric survey was designed to meet or exceed the survey requirements

specified by HDR in an August 2009 Request for Proposal. The executed survey

methodology was in compliance with hydrographic survey standards promulgated by the

U.S. Army Corps of Engineers (USACE, EM-1110-2-1003).

Shore-parallel survey transects were spaced approximately 10 to 50 feet apart. Channel

cross-sections were spaced 100 feet apart. Navigation for the survey was accomplished

using Trimble AgGPS 132 and Trimble ProXRS 12-channel Differential Global

Positioning Systems (DGPS) capable of receiving U.S. Coast Guard (USCG) Beacon

corrections, and OmniStar subscription-based satellite differential corrections. Both

systems provided real time digital positions accurate to less than 1 meter horizontally.

2.1 Survey Vessel

Two vessels were utilized for the survey. The primary survey vessel was the M/V Donna

Miller, chartered from Miller’s Launch in Staten Island, NY. This 25-ft aluminum work

boat was used to conduct the survey in Gowanus Bay, and a portion of the work in the

lower Gowanus Canal. The 12-ft survey skiff was utilized in the shallower portions of the

Canal, and to conduct the survey work above the Carroll Street Bridge. Both vessels were

outfitted with over-the-side transducer mounts, DGPS antenna brackets, and data

acquisition computers. Neither vessel was equipped with ice breaking equipment, and

because of this, the surveys were limited by the extent of any ice encountered in the

Canal and turning basins.

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Bathymetric Survey Report

Gowanus Canal

Brooklyn, New York

CR Environmental, Inc.

April 2010

2.2 Bathymetric Data Acquisition

The echo sounders and DGPS were interfaced to the survey computers via RS-232 serial

and Ethernet ports. Most depth measurements on the Donna Miller were acquired by

simultaneous deployment of two survey-grade echo sounders in order to increase data

density, productivity, and usability. The two echo sounders on the Donna Miller were an

ODOM CV-100 precision echo sounder equipped with an 8-degree 200- kHz transducer,

and a SyQwest Bathy500 precision echo sounder equipped with a 3- degree 200-kHz

transducer. Depth measurements on the skiff were collected using a SyQwest HydroBox

precision echo sounder equipped with an 8-degree 200-kHz transducer.

The echo sounder transducers were mounted to the rails of the survey vessels

amidships using high-strength adjustable booms. The DGPS antenna on the skiff was

mounted directly over the transducer, eliminating the need to account for horizontal

offsets. The depth of each transducer below the water surface was measured both at the

start and end of the survey.

The accuracy of the CV-100, HydroBox, and Bathy500 was approximately 0.1% of the

water depth with a resolution of 0.1 foot. System accuracy was checked at the start and

end of the survey day by comparing each echo sounder’s water depth measurements to

known water depths obtained using the bar check method, in which a metal plate is

lowered beneath the echo sounder’s transducer to several known distances (e.g., 5, 10, 15

and 20 ft below the water surface). Based on these comparisons, the systems were

calibrated for shallow water conditions. Bar check calibrations for all three systems were

consistently accurate to within 0.1 foot throughout the survey.

Additional calibrations were conducted in-situ by collecting water column profiles of

sound velocity. Sound velocity in water can be determined based on measurements of

temperature and conductivity. Measurements of water column temperature and

conductivity were performed using a YSI, Inc. Model 6600 EDS-V2 water quality sonde,

and a YSI, Inc. Model 85 Handheld Water Quality Meter.

2.3 Vertical Control

Vertical control for this survey was accomplished by using a cost-effective combination

of techniques. GEOD Corporation, a NY Licensed Land Surveying firm, had established

three vertical control points in support of CR’s 2003 bathymetric surveying effort of the

Canal (see Attachment 1). One benchmark was located in outer Gowanus Bay at the

Columbia Street pier, one was located in the central reach of the Gowanus Canal at

Bayside Fuel Corporation (537 Smith Street), and one was near the head of the Canal at

the Carroll Street Bridge. GEOD was contacted and they conducted a site reconnaissance

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Bathymetric Survey Report

Gowanus Canal

Brooklyn, New York

CR Environmental, Inc.

April 2010

which concluded that only the benchmark in the Bay and the benchmark at Carroll Street

were still in existence and usable.

CR set two time-synchronized water level recorders, one near the upstream extent of the

survey area and one near the downstream extent of the survey area in order to evaluate

time and range offsets in water surface elevation during the survey. A new temporary

benchmark (TBM-1) was established near the downstream limit of the survey area,

adjacent to 629 Smith Street, and an In Situ, Inc. LevelTroll 500 water level recorder was

suspended beneath this benchmark. An In Situ, Inc. LevelTroll 100 water level recorder

was suspended beneath the upstream Carroll Street Bridge control point (BM-1). After

the survey was completed, data from the two synchronized tide recorders were compared.

CR determined that there was no quantifiable difference in tidal range or timing between

the two locations during the survey (both flood and ebb around high tide). Thus, for the

purposes of this survey, occupation of TBM-1 by a land survey firm was not required.

Note, however, that the tidal gradient in the Canal surrounding low tide or under different

wind conditions is unknown. The TBM-1 benchmark was clearly marked, and could be

surveyed at any time if needed.

Per the request of Project advisors from CH2M Hill, Inc., the elevations from the January

2010 survey have been reported as “estimated” NAVD88 elevations. The Carroll Street

benchmark elevation was reported by GEOD relative to NGVD29 (Attachment 1), and

three methods were used to convert the benchmark elevation to NAVD88. First, the U.S.

Army Corps of Engineers’ (ACOE) datum conversion routine, Corpscon, was run to

determine the offset between the two datums. Corpscon gave a result of -1.109 U.S.

Survey Feet between NGVD29 and NAVD88. CR then contacted GEOD Corporation,

and they confirmed that the separation between NGVD29 and NAVD88 was -1.11 feet.

This value has been used to convert the soundings to estimated NAVD88 seabed

elevations. The accuracy of the conversion was verified by comparing the NAVD88

adjusted tide data to NOAA’s Battery tide series (Station ID: 8518750), adjusted for

Gowanus Bay. This comparison suggested a -1.19 foot separation between datums.

2.4 Bathymetric Data Processing

Individual transect data were visually inspected in profile format, and components of

processing included:

Removal of outlying soundings associated with water column interference (e.g.,

fish, vegetation, or mid-water column debris);

Adjustments of soundings for variations in sound velocity;

Filtering of DGPS position data; and

Conversion of soundings to estimated NAVD88 elevations based on a benchmark

surveyed by GEOD Corporation in 2003, located near the Carroll Street Bridge.

See Section 2.3 for a discussion of the vertical control for this project.

3

Bathymetric Survey Report

Gowanus Canal

Brooklyn, New York

CR Environmental, Inc.

April 2010

Neither water column interference nor sound velocity gradients influenced data quality,

likely due to the cold dry weather which preceded the survey. However, DGPS position

uncertainty was substantially increased by overhead obstruction and signal multipath near

each of the bridge overpasses. Position data were filtered to retain only differentially

corrected fixes with low dilution of position values. The locations of soundings beneath

bridges were estimated based on interpolation between high-confidence DGPS fixes on

either side of each bridge.

After performing data adjustments, the processed bathymetric data were combined into

comma-delimited ASCII text files including fields for Northing, Easting, and Elevation.

The data were imported to Golden Software, Inc. Surfer V.9.3 Surface Modeling

Software and a digital elevation model (DEM) of seabed elevations was created using

triangulation interpolation methods. A contour map depicting bottom elevations using a

0.5-foot contour interval was created from the DEM and the map was exported in DXF

and SHP formats. A surface map of the bathymetric data was created using conventional

hydrographic shading, artificial illumination, and a 2x vertical exaggeration to better

illustrate bottom morphology. The surface map was exported as a georeferenced TIF

image file.

3.0 BATHYMETRIC RESULTS AND DELIVERABLES

Processed bathymetric data, map layers, figures, and a copy of this report are attached on

the project CD. Occupied survey transects (i.e., sounding tracklines) are depicted on

Figures 1A and 1B. The increased data density afforded through simultaneous use of two

echo sounders is readily apparent. Access to inner portions of all four turning basins was

precluded by ice. DGPS signal quality was compromised near all bridges. This

degradation was most severe beneath the 9th

Street Bridge.

Elevations reported during the survey ranged from approximately -0.13 feet to -38 feet

NAVD88, as illustrated by the contour and DEM layers presented on Figures 2A and 2B.

Prop scours from tug boat operations are visible on the seabed near the downstream limit

of the survey area.

Although single-beam bathymetry is not considered an effective tool for identifying

underwater obstructions, wreckage is clearly visible in data collected from the turning

basin between 5th

Street and 6th

Street. This feature extends above the water surface at all

tides and is clearly visible from shore and in aerial photographs. Submerged wreckage or

large debris is also suggested near the mouth of the upstream turning basin. Data suggests

the widespread presence of other debris throughout the survey area. Mapping of these

features will require side scan or multibeam sonar surveys.

Bathymetric data quality was assessed by comparing approximately co-located

soundings along perpendicular transects. These “cross-tie” comparisons were conducted

on the final merged data set, and included points collected using all three sounding

systems on both vessels over a tidal range of 2.6 feet. Examination of 337 of these co-

located soundings revealed a negligible arithmetic mean difference (depth bias) of -0.025

4

Bathymetric Survey Report

Gowanus Canal

Brooklyn, New York

CR Environmental, Inc.

April 2010

feet, and a standard deviation (SD) of the residuals of +0.30 feet. Considering cross-tie

comparisons across the full project depth range (38 feet), the root mean squared (RMS)

error was 0.62 feet at the 95% Confidence Interval (CI), less than the most stringent

Army Corps of Engineers (ACOE) requirement of 1.0 foot for navigation and dredging

projects in water depths of 15 to 40 feet. Data collected from the skiff in water depths less

than 15 feet had a RMS at the 95% CI of 0.27 feet, less than the 0.5-foot ACOE

specification for this water depth.

International Hydrographic Organization (IHO) Special Order specifications for a survey

in depths of 5 meters (16.4 feet) would call for a 95% CI of 0.8 feet (max SD =

0.42). IHO Special Order specifications for a survey in depths of 10 meters (32.8 feet)

calls for a 95% CI of 0.9 feet (max SD = 0.44).

We, therefore, conclude that the quality of data generated by this survey is compliant

with ACOE and IHO guidelines. Some of the variation in “cross-tie” comparisons is

associated with the steep slopes and engineered headwalls which dominate the Canal

margins. These slopes and headwalls cause vertical discontinuity between soundings run

in different directions. In addition, the slight variation between some soundings suggests

the presence of a surficial floc layer on the seabed which was intermittently penetrated by

the ODOM and SyQwest sounding systems.

4.1 CONCLUSIONS AND RECOMMENDATIONS

The January 2010 bathymetric survey of the Gowanus Canal generated an accurate

database and map of seabed elevations and morphometry. Ice prevented access to small

sections of the Canal’s turning basins, and active and derelict barges and other vessels

hindered navigation in some portions of the Canal’s main channel. Further bathymetric

survey work will be required to characterize these areas, and additional soundings should

be collected in the vicinity of the 9th

Street Bridge using a smaller vessel which does not

require Bridge openings for access. Finally, the use of side scan sonar to accurately locate

and characterize submerged debris which could pose a danger to sediment sampling or

potential future dredging efforts is recommended.

5

Bathymetric Survey Report

Gowanus Canal

Brooklyn, New York

CR Environmental, Inc.

April 2010

CR ENVIRONMENTAL, INC.

GEOPHYSICAL AND HYDROGRAPHIC SURVEY

UNCERTAINTIES AND LIMITATIONS

1. Acoustic methods of remote sensing in aquatic environments are influenced

by the chemical and physical characteristics of the waterbody and underlying

sediment. Water column characteristics which introduce uncertainty include:

biological interference (e.g. fish or debris in mid-water column);

conductivity/salinity and temperature as they affect sound velocity; and

weather and other surface conditions as they effect navigation. Sediment and

bottom characteristics which introduce uncertainty include supersaturated

flocculent sediments; sediments which contain entrained natural gases (e.g.,

sulfides, methane) which limit sonar penetration; steep slopes which may limit

sonar bottom penetration due to increased angle of incidence or scattering;

and rooted vegetation. CR Environmental, Inc. (CR) has made all technically

feasible attempts to minimize the above uncertainties as described in the

report.

2. The observations described in this report were made under the conditions

stated therein. The conclusions presented in the report were based solely upon

the services described therein, and not on scientific tasks or procedures

beyond the scope of services or the time and budgetary constraints imposed by

the Client.

3. The conclusions and recommendations contained in this report are based in

part upon the data obtained using acoustic remote sensing observations

obtained along survey transects spaced approximately 10 to 50 feet apart.

Information presented in this report between the survey transects (i.e., depth

contours) is based on mathematical interpolation, and thus may not be suitable

for navigation.

CR Environmental, Inc. respectfully reserves the right to amend this report should new

information become available.

ATTACHMENT 1

GEOD Corporation

Survey Information for the BM-1 Control Point

on the Carroll Street Bridge

08/06/2003 11:36 973838643< GEOD CORP PA<:£ 132/03

GEOD COR?ORATION

PHOTOGRAMt.lETRI C SCIENCES·SURVEYTECHNOLOGIES

August 5,2003

Mr, Chri Wrillhl CR EnvironmentalInc. 639 Boxberry Hill Road t:ast t-almoutn, MA •Jb:.!::ll$

RE: semNG ELEVATION REFERENCE MARKS ALONG THE GOWANUS CANAL

(GEOD PROJECT # 2089)

Dear Sir,

I As requested, GEOD Corporation established 3 elevation reference marl<s (ench marks) for the sub-surface survey to be pelforrned along the Gowanus Canalby CR Envirnmental.

These elevation reference marks were established In the NationalGeodetic iertlcel Datum of 1929 (NGVD 1929) and MLW Mean Low Wat.er Datum uncler the dlect upervision of a surveyor licensed in the State of New Yori<. •

Nome:

I i i

l'le83e do IIOt Msitate !¢ c.eo11tol11,., with your oomments or questions.

Very Truly Yours, I GEOD _ CORPORATION

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Stanley 8. Palinski Vlce President

OORPORAT& Ol'FICE, t&.24 Ka,,o.., Ad..,...wloundla,d. H.J.07435 (ffl) 69741:13 F"AX • (013)

OFFICES: SBR Rollrl SWOl, !!oslm MA Qa!29 {ti 1 /J // t'A!\Ul /J / fbo(..,14$4

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1t·24 K>Muse Road, Newl:i.NJ07435 (973) $91 2122 • l'ax:(973) 83&-6433

E-Mail: ooCgeo<lc<>rp.com

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CR Environmental, Inc. April 2010

BATHYMETRIC SURVEY REPORT

GOWANUS CANAL

BROOKLYN, NEW YORK

ADDENDUM 1

Comparison of Gowanus Canal Bottom Elevations

June 2003 versus January 2010

CR Environmental, Inc. compared Gowanus Canal bottom elevations recorded during

bathymetric survey efforts conducted in 2003 and 2010 to estimate areas of sedimentation

and erosion within the canal. The sections that follow provide the methods used to

compare the elevation data, and briefly describe the results of the comparison.

Methods

Prior to conducting the elevation data comparison, the June 25, 2003 data were re-

processed to adjust soundings to estimated NAVD88 elevations using correction factors

detailed in Section 2.3 of the April 2010 Bathymetric Survey for Gowanus Canal,

Brooklyn, New York report. These data were acquired along cross-sections spaced 25 to

200 feet apart (see Figure A1). The root mean squared (RMS) error of the reprocessed

data was calculated as approximately 0.3 feet using Army Corps of Engineers (ACOE)

methods (see Section 3.0), similar to the error value calculated for data collected in 2010.

The reprocessed 2003 data were then gridded using a 20-foot node interval. The extent

of the grid (surface model) was limited to the boundary of the 2010 survey effort.

The approximate thalweg (deepest continuous line) of the Canal as interpreted from the

2010 data was digitized. Points from the 2003 and 2010 grids which intersected the

thalweg were extracted and exported in ASCII text format. These points were used to

generate comparative profiles of thalweg elevation differences between the two surveys.

The points have been delivered digitally in MS Excel tabular format.

The grids from each survey were next converted to an ASCII format suitable for analysis

using IVS3D Fledermaus software. This software was used to compare the two surface

models. The elevation values from 2003 were subtracted from co-located values on the

2010 surface model, resulting in a new surface for which positive values indicated

shallower 2010 depths (sedimentation) and negative values indicated portions of the

canal which were deeper in 2010 (erosion, scour, dredging or other anthropogenic

activities). This elevation comparison grid was used to generate a contour map of

elevation differences (Figure A2). The map was imported to GIS in DXF and TIF

formats for comparison with bathymetry. These layers have been delivered

electronically.

Results

The distribution of bathymetric data points for the two surveys are shown on Figure A1

Detailed maps of the 2010 data distribution have been provided on Figures 1A and 1B in

CR Environmental’s April 2010 Bathymetric Survey Report for the Gowanus Canal,

639 Boxberry Hill Road, East Falmouth, MA 02536 1 508 563-7970 phone/fax www.crenvironmental.com

CR Environmental, Inc. April 2010

Brooklyn, New York). The substantially lower data density specified for the 2003 survey

effort is readily apparent, and is important to acknowledge during examination of the

surface model comparison results. The 2003 data density was highest upstream of the

Carroll Street Bridge where transect spacing was approximately 25 feet (Figure A1).

Figure A2 is a plan view comparison of the 2003 and 2010 surface models. Elevation

differences along the thalweg are depicted on Figures A3 and A4. The data comparisons

shown on Figures A2 and A4 highlight the area of highest uncertainty as +/- 0.6 feet, the

combined RMS error estimates for the two surveys. Despite efforts to minimize

modeling artifacts, the surface model comparison shown on Figure A2 shows regularly

spaced positive elevation difference “lobes” along each shoreline at intervals that mirror

the distance between the 2003 cross-sections. CR recommends that users of these data

simultaneously evaluate model data relative to sounding distribution in GIS to minimize

the likelihood of misinterpretation. Data comparisons along the thalweg are less subject

to grid interpolation errors.

Elevation differences appeared minor upstream of the floating boom adjacent to Sackett

Street (about 530 feet downstream of the head of the Canal). Some erosion was indicated

along the western shoreline (see Figure A2). Approximately 2 to 3 feet of sediment

accumulation was suggested between the boom and the Carroll Street Bridge (about

1,400 feet downstream of the head of the Canal). Data comparisons within this upper

portion of the Canal are considered robust from shore to shore due to the higher 2003

data density upstream of the Carroll Street Bridge.

Comparison of the two model surfaces along the thalweg suggests approximately 1 to 2 feet of sediment accumulation at the northern end of the Canal between Carroll Street and

3rd

Street (see Figures A3 and A4).

Elevation differences of approximately -1 to 1 foot dominated most of the reach between

3rd

Street and Interstate 278. Highest confidence is given to areas of elevation differences

greater than the summed RMS uncertainty of +0.6 feet (see Figure A4). Erosion in this

reach is suggested in an area subject to frequent tug and barge traffic adjacent to a gravel

shipping yard between 5th

Street and Huntington Street.

At the southern end of the Canal, the surface comparison suggests a wide (approximately

80,000 square foot) region of sediment accumulation along the thalweg adjacent to

Halleck Street (Figures A2 and A4). Also suggested in this southern reach were many

smaller areas of substantial elevation difference either erosional or depositional (Figure

A2).

Confidence for many of these areas decreases with increasing distance from the 2003

longitudinal centerline due to anomalies associated with interpolation between survey

transects spaced approximately 150 feet apart.

639 Boxberry Hill Road, East Falmouth, MA 02536

508 563-7970 phone/fax www.crenvironmental.com

984000 985000 986000 987000 988000

Extracted Thalweg Points (see Figures A3 and A4)

Union St.

Carroll St.

5th St.

3rd St.

Huntington St.

9th St.

I 278

Halleck St.

Feet 0 750 1,500

EROSION

SEDIMENTATION

984000

985000

986000

987000

988000

Bathymetric Surface Model Comparison - June 2003 and January 2010 Survey Data

Gowanus Canal Brooklyn, New York : NOTES: 1) Survey data collected by CR Environmental, Inc.

of East Falmouth, Massachusetts on June 25, 2003 and January 5, 2010. 2) Grid: NY State Plane (LI), NAD83, US Foot. 3) Elevation differences less than +/- 0.6 feet are transparent.

These values are within the combined RMS error range for the two surveys.

Figure A2

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