gowanus canal dredging alternativesoperations office (33cfr 117.787 gowanus canal). an exemption to...

Gowanus Canal Dredging Alternatives

July 2009

Table of Contents

1.0 Introduction ......................................................................................................... 1 2.0 Potential Working Depths and Dredged Material Volumes .................... 2 3.0 Existing and Without-Project Conditions .................................................... 3

Bathymetry ...................................................................................................................... 3 Sediment Quality ............................................................................................................ 3 Water Quality .................................................................................................................. 3 Bridges ............................................................................................................................ 5 Bulkheads ........................................................................................................................ 8

4.0 Dredging Constraints ........................................................................................ 8 5.0 Dredging Technology Evaluation .................................................................. 9

Mechanical Dredging Technology ............................................................................ 10 Enclosed Clamshell – Cable Crane Operated ........................................................... 10 Horizontal Profiling Grab (HPG) Bucket - Backhoe Operated ................................ 11

Hydraulic Dredging Technology .................................................................................. 12 Specialty Dredging Technology ................................................................................... 13

Amphibious Excavators ............................................................................................ 13 Submersible Pumps ................................................................................................... 14 Water Injection Dredging Technology ..................................................................... 15

6.0 Advantages and Disadvantages of Dredging Technologies ................ 15 7.0 Recommendations ........................................................................................... 18 8.0 References: To be revised ............................................................................. 19

Dredging Technology Review 1

1.0 Introduction

The New York District of the US Army Corps of Engineers is conducting a joint feasibility study with the New York City Department of Environmental Protection (NYCDEP) for restoration of the aquatic environment of the Gowanus Canal. The feasibility study evaluates dredging and capping alternatives that would result in a less degraded condition of Gowanus Canal sediments. The general purpose of the project is to isolate Gowanus Canal sediment contaminants through removal and capping thereby improving benthic habitat. The Gowanus Canal is an urban-industrial waterway located in the southwestern quarter of Brooklyn (Kings County), New York. The canal provides barge access for commercial and industrial interests along its bulkhead-confined banks. Once a world-renowned oyster ground and estuarine marsh, the area southwest of Hamilton Avenue and between Third Avenue and Gowanus Bay was described as marshland too shallow for navigation, as late as 1841. Construction of the Erie and Red Hook basins in the 1850’s greatly altered the shoreline of Gowanus Bay. The Gowanus Canal extending north from Hamilton Avenue in the general vicinity of the former Gowanus Creek was completed in 1870 (construction lasted from 1853-1870) for the dual purposes of providing drainage for 1,700 acres within the southern portion of the City of Brooklyn and to extend ship traffic into south Brooklyn. The River and Harbor Act of 1881 authorized a channel 18 feet deep at mean low water from Gowanus Bay to the Hamilton Avenue drawbridge. The River and Harbor Act of 1896 authorized a 26-foot deep channel from the junction with the Red Hook channel to the foot of Percival Street. The most current authorization, which includes a 30-foot deep main channel (Red Hook channel to the foot of Percival Street), a 30-foot deep branch channel (towards the Henry Street Basin), and an 18-foot deep channel (from Percival Street to the Hamilton Avenue Bridge), is based on a Chief of Engineers Report to the Secretary of the Army, dated 19 September 1950. The most recent maintenance dredging of the federal channels occurred in 1975. The Federal channel does not extend into the canal upstream of the Hamilton Avenue Bridge, which is the area considered for dredging in this analysis. The proposed dredging area, which is totally within the canal, runs for approximately 7,000 feet. The proposed dredging area covers a surface area of 21 acres, including the main-stem canal and adjacent turning basins. There currently are fewer water-dependent interests along the shoreline than were present historically. Historical industrial facilities along the canal were engaged in refining, manufactured gas distribution and/or bulk-storage of petroleum, coal storage, and chemical manufacturing. Untreated industrial and sanitary wastes were discharged directly into the Gowanus Canal for over a century. Twenty-six major industrial facilities (both operational and abandoned) were identified within the project area. Discharges of petroleum and other environmental contaminants have been documented at most of the facilities. As a result, the canal is now characterized by poor water quality, contaminated sediments, deteriorating bulkheads, a poor benthic community structure, extensive filling, hardened shorelines, and unpleasant odors.


2.0 Potential Working Depths and Dredged Material Volumes

Potential project alternatives identified by the District are based on three dredging and capping scenarios, each of which include Reach 1 (from the head of the canal to 3rd Street), Reach 2 (from 3rd Street to 9th Street), Reach 3 (from 9th Street to Hamilton Ave) and four basins (3rd Street, 5th Street, 6th Street, and 11th Street). The total area is 21.66 acres, of which 4.65 acres are in turning basins and 17.01 acres are in the main stem of the canal.

Alternative Depth Scenarios (all finish depths in feet relative to NAVD)

Reach

3rd St Basin Disposal

All navigation depths

Dredge to -30

Reach 1 -3 -3 -28 Reach 2 -16 -16 -28 3rd St. Basin +8 -3 -28 5th St. Basin -16 -16 -28 6th St. Basin -16 -16 -28 11th St. Basin -16 -16 -28 Reach 3 -16 -16 -28

One dredging depth scenario provides navigation depth (-16 feet) from Reach 2 through Reach 3, and uses the 3rd St. Basin as a dredged material disposal area for material coming from Reaches 1 and 2. The 3rd St. basin is assumed to be filled, including a cap, to the top of the bulkhead. Additional investigations will need to be conducted to determine if contaminated dredged material disposal is feasible in the 3rd St. Basin. Less dredging is required in Reach 1 (finished depth of -3 feet) because commercial navigation is assumed not to be a factor in this reach. The second dredging depth scenario provides navigation depth (-16 feet) from Reach 2 through Reach 3, and provides a finished depth of -3 feet in Reach 1 and in the 3rd St. Basin. In this scenario, the 3rd St. Basin is not used for dredged material placement or for commercial navigation. The third scenario presented by the District is based on dredging to a depth of -30 feet in all reaches and basins, which results in a finished depth of -28 feet throughout the canal. Potential dredged material volumes range from 230 thousand cubic yards to 845 thousand cubic yards.


Alternative Dredge Material Volumes (cubic yards)

Reach

3rd St Basin Disposal

All navigation depths

Dredge to -30

Reach 1 24,285 24,285 140,235 Reach 2 87,992 87,992 415,594 3rd St. Basin 0 5,953 66,914 5th St. Basin 34,263 34,263 68,846 6th St. Basin 27,019 27,019 45,783 11th St. Basin 6,891 6,891 11,483 Reach 3 49,000 49,000 95,863 Totals 229,450 235,403 844,719

3.0 Existing and Without-Project Conditions

Bathymetry The canal, located upstream of the Hamilton Avenue Bridge, is substantially shallower than the federal channel, which runs from the Red Hook Channel in Upper New York Harbor to the Hamilton Avenue Bridge. Bathymetry investigations conducted in 2003 indicate that the deepest areas upstream of the Hamilton Avenue Bridge are no deeper than ten feet below mean low water. There are areas at the head of the canal and in the turning basins where sediment grade is equivalent to mean low water.

Sediment Quality Sediment sampling conducted by the Corps (2002), Northern Ecological Associates (2003), and AMEC (2005) identified substantially degraded sediment quality throughout the canal. Sediment contaminants include horizontal and vertical mixtures of organic and inorganic chemicals combined with fine, clayey sediments. Constituents found to exceed sediment screening criteria include metals, semi-volatiles and volatile organic chemicals, pesticides, petroleum derivatives and PCBs. Canal sediments have not been tested for dioxins. The maximum extent of sample borings was 30 feet below the substrate surface. Contaminant concentrations were found throughout the length of the boring samples and may exist at deeper depths. Volatile compounds continually emerge to the surface. Some contaminants reside at substrate depths that may be practically unavailable to potential ecological receptor species however, other contaminants are at the surface and exposed at low tide.

Water Quality Despite positive improvements in water quality over the last several decades, there continue to be episodic discharges of untreated sewage from Combined Sewer Outfalls (CSOs). In Brooklyn, storm water and wastewater flow through a combined system, which is overwhelmed during periods of moderate and heavy precipitation. There are 11 CSOs in the Gowanus Canal which convey an estimated 293 million gallons of CSO


effluent annually into the Gowanus Canal. After planned CSO upgrades, the estimated volume of CSO effluent will be reduced to approximately 215 million gallons annually (NYCDEP, 2004). CSOs convey human pathogens, a variety of industrial wastes, and floatable materials into the waterways. Non-point source pollution from roads, construction sites, vacant lots and other disturbed areas provide additional sources of contaminants to the canal A single large-turbine pump moves water from Buttermilk Channel in the East River through a one-mile long tunnel to the northern head of the Gowanus Canal. Even with the flow contribution from the pumping system, the Gowanus Canal is classified by the NYSDEC as severely degraded (SD). This is the most restrictive New York State Saline Surface Water Standard use classification. Water bodies with use classification SD are not suitable for primary or secondary contact recreation (due to total coliform levels) or fish propagation (due to low levels of dissolved oxygen). SD designated waters are expected to only support fish survival. The Gowanus Canal does not meet the SD designation’s minimum dissolved oxygen standard of 3.0 mg/L. Flows from the pumping system and CSO discharges have eroded the substrate in areas adjacent to outfalls. Areas of low energy support deposition along bridge abutments, in dead-end turning basins, and on the inside bends of turns. The Coastal and Hydraulics Laboratory at the USACE Engineer Research and Development Center has conducted an existing conditions analysis of velocities and sheer stresses in the canal (Martin, Undated). The analysis concluded that the Gowanus Canal is a low velocity environment with the highest sheer stresses in the local vicinities of major CSO outfalls. A second pump is scheduled for operation by NYCDEP in 2009. The addition of a second pump will increase the volume of water brought into the Canal from the East River. Operation of the second pump coupled with dredging in the Gowanus Canal is projected to improve dissolved oxygen levels throughout the Canal. The Designated Use Standard could be upgraded to Class I (secondary contact recreation, fish survival and propagation). Minimum dissolved oxygen levels are projected to be greater than 4 mg/L and average bottom dissolved oxygen levels would be between 7 and 8 mg/L (NYCDEP 2004b). The second pump will substantially increase flow in the Canal. Currently, single pump operation produces an average flow of 140 MGD, with no flow occurring during low tide. After the facility upgrade, average flow is projected to be 215 MGD (40% increase), peak flow at high tide is projected to be 252 MGD (30% increase) and peak flow at low tide is projected to be 175 MGD with no shut down during low tide conditions, which is the current condition (NYCDEP 2004b). The pumps are designed for high volume low velocity flows and the addition of a second pump is not projected to significantly alter erosion and deposition patterns in the canal.


Bridges The Gowanus Canal has several bridges crossing over it (http://www.nyc.gov/html/dot/html/bridges/bridges.shtml). The general rule for bridge openings is that each bridge shall be opened on signal provided that a minimum of two hours advanced notice be given to the NYCDOT Radio Hotline or the NYCDOT Bridge Operations Office (33CFR 117.787 Gowanus Canal). An exemption to this rule exists for the Hamilton Avenue Bridge during construction renovations, which requires a minimum of four hours advanced notice (33CFR117.788 Gowanus Canal). The vertical clearance constraint for all Gowanus Canal bridges in the open position is 60 feet above mean high water at the Ninth Street Bridge. The Gowanus Canal bridges impart a channel width constraint of approximately 60 feet through the Hamilton Avenue and Ninth Street Bridges and approximately 40 feet through the remaining bridges. From an upstream to downstream direction the bridges are:

The Union Street Bridge (mile 2.1) The Carroll Street Bridge (mile 2.0) The 3rd Street Bridge (mile 1.8) The 9th Street Bridge (mile 1.4) The Hamilton Avenue Bridge (mile 0.2)

Union Street Bridge The Union Street Bridge is a double leaf Scherzer rolling lift bascule with a span of 56 feet. The Union Street Bridge was first opened in 1905. The bridge has two vehicular traffic lanes, each approximately 17 feet wide, and two sidewalks, each 6 feet wide. Both traffic lanes carry eastbound traffic. Channel width between the abutments is approximately 40 feet.


Carroll Street Bridge The Carroll Street Bridge is a retractile bridge, which was first opened to traffic in 1889. The maximum span is 45 feet. The Carroll Street Bridge, supports a 17 foot wide roadway and two 4.5 foot sidewalks. Channel width between the abutments is approximately 40 feet.

Third Street Bridge The Third Street Bridge is a double-leaf Scherzer rolling lift bascule initially constructed in 1905 and comprehensively rehabilitated in 1986. The bridge’s maximum span is 56 feet. It supports two vehicular traffic lanes, each approximately 16 feet wide, and two sidewalks, each six feet wide. Channel width between the abutments is approximately 40 feet.


Ninth Street Bridge The new (1999) lift bridge replaced a bascule span that was in an advanced state of deterioration. The new structure provides 60 feet of channel width and 60 feet of vertical clearance from mean high water. The bridge carries 3 lanes of traffic; 2 lanes westbound and 1 eastbound.

Hamilton Avenue Bridge The Hamilton Avenue Bridge is a bascule type bridge with two parallel leafs, one carrying the northbound roadway and the other carrying the southbound roadway. The Hamilton Avenue Bridge is located below the elevated portion of the Gowanus Expressway. The bridge connects Smith Street and Second Avenue over the Gowanus Canal and is the first canal crossing north of the Gowanus Bay. Currently, ongoing re-construction of the Hamilton Avenue Bridge is projected to be complete in 2009. Channel width between the abutments is approximately 60 feet.


Bulkheads A bulkhead inventory and assessment was conducted by a contractor (Adam Brown Marine Consulting) for the Gowanus Canal Community Development Corporation in 2000 to evaluate the stability of the thoroughly bulk-headed Gowanus Canal. The assessment showed that many portions of the bulkheads are in disrepair. A follow-up visual investigation by the New York District (May 2008) confirmed the general level of disrepair and inadequacy of the existing bulkheads. Replacement of the existing bulkheads would be an integral component of an environmental dredging project in the Gowanus Canal. Feasibility level engineering design of replacement bulkheads needs to be conducted prior to final selection of a recommended dredging plan.

4.0 Dredging Constraints Suitable dredging methods and equipment, which may be used in the Gowanus Canal, must be able to perform within existing condition constraints. These constraints are based on the physical dimensions of the canal, access and location, canal bathymetry, and sediment contamination. The physical dimensions of the canal that may constrain dredge plant access and operations are the narrow widths at the canal’s bridge abutments. The first two bridges, when entering the canal from the bay, have a maximum channel width of approximately 60 feet, although the canal itself has a width of approximately 100 feet in this area. The three upstream bridges have a channel width of approximately 40 feet between bridge abutments and the canal itself has a width of 80 to 100 feet in this area. The constraining vertical clearance for all canal bridges is 60 feet at mean high water at the Ninth Street Bridge, which would unlikely affect equipment access and operation. Landside access to the canal is limited by densely spaced urban land uses surrounding the canal, including industrial, commercial, and residential uses. Opportunities for landside staging areas along the canal are very limited and would need to be negotiated with existing private land owners. The only potential landside staging area that is publicly owned is the NYCDEP facility at the head of the canal. This facility is currently congested, but may provide some opportunity for limited staging and equipment storage. The main stem of the canal is currently no deeper than 10 feet below mean low water and canal sediments are exposed at low tide in the most upper reach of the canal and in the canal’s turning basins. One way to conduct shallow water dredging is to build a broad floating platform that sufficiently distributes vessel weight to greatly reduce operating draft. For example, the dredge equipment platform used in the shallow waters of New Bedford Harbor was built on-site using multiple pontoons to create an 80-foot by 80-foot platform capable of a minimum working draft of two feet (USACE 2000). The shallow water conditions at some locations in the canal may require specialized equipment or equipment configurations which are not needed in the deeper main stem reaches of the canal. Sediment contamination generates two major constraints on dredging methods and equipment operations. One constraint is the potential size and amount of debris buried in


canal sediments. Debris surveys, such as side-scan sonar or magnetometer, have not been conducted in the canal. However, the industrial history of the canal and the proximity to city streets (numerous streets dead-end at the canal) create a high likelihood that a substantial amount of debris, and possibly large debris, are contained within the sediments. A second and equally important constraint resulting from sediment contamination is the re-suspension of contaminants. The severely contaminated condition of canal sediments may require that all dredging be conducted within an enclosure, such as the sheet pile enclosure being used for the CERCLA non-time-critical removal action being conducted at the Passaic River CERCLA site. If an enclosure is not required, then the minimization of re-suspension will be a critical component of all dredging operations within the canal. Although not an actual constraint on dredging in the Gowanus Canal, dewatering of dredged sediments is an important consideration. The ultimate fate of sediments dredged from the Gowanus will determine whether dewatering of dredged sediments will be required, but it is highly likely that dewatering will be required. The severely contaminated Gowanus Canal sediments require special handling. Treatment and placement options for highly contaminated sediments are limited by regulations. Dewatering reduces the volume and weight of dredged material to be disposed and may be required for some placement options such as incineration or Subtitle C landfill disposal. A water treatment system may be required to clean decant water that has been removed from the dredged material. Effluent from the water treatment system would need to be tested prior to being discharged. Design specifications of the water treatment system would need to be based on the constituency of the decant water and the discharge standards of the receiving water body. 5.0 Dredging Technology Evaluation There are numerous alternatives for environmental dredging operations in the Gowanus Canal. Alternatives can be categorized by equipment type, such as mechanical, hydraulic, or pneumatic dredging equipment. Alternatives also exist within each equipment category. This assessment of dredging technologies borrows heavily from previous technology assessments conducted for other projects or other purposes. These previous technology assessments include:

Passaic River Phase I Engineering Evaluation/Cost Analysis, Tierra Solutions Inc. (2008);

Lower Passaic River Restoration Project Dredging Technology Review Report, NYJDOT (2004);

Equipment Choices for Dredging Contaminated Sediments, Michael Palermo (1991); and

Overdepth Dredging and Characterization Depth Recommendations ERDC/TN EEDP-04-37, USACE Dredging Operations technical Support Program (2007).

Criteria for assessing dredging technology alternatives for environmental dredging of the Gowanus Canal are based on the dredging project objectives. These objectives include:

Removal of contaminated material in a cost effective manner;


Minimize the re-suspension and migration of contaminated material during dredging operations;

Minimize the potential spillage of contaminated materials; Avoidance of damage to existing or retro-fit structures (bulkheads, outfalls,

building foundations, etc.); and Isolation of residual sediment contaminants (assumed to be conducted through

capping).

Mechanical Dredging Technology Mechanical dredging systems are commonly used for construction and maintenance of navigation channels and have also been used for removal of contaminated sediments. Mechanical dredging systems include numerous variations on clam shell bucket equipment (typically cable operated) and hydraulically operated grab buckets. A barge mounted crane with clam shell bucket was selected as the preferred dredging system for removal of contaminated sediments from the Lower Passaic River CERCLA site. The Lower Passaic River dredging will be conducted within a sheet pile enclosure, which resolves the resuspension problem associated with mechanical dredging systems. A hydraulically operated grab bucket system – a modified version of the dredge Bonacavor - was selected as the preferred method for removal of contaminated sediments at New Bedford Harbor. The Bonacavor is currently owned by Jerico Products Inc., a Petaluma California dredging and marine towing firm (http://www.jericoproducts.com/Home/tabid/68/Default.aspx). The Bonavacor is currently working in and around the San Francisco Bay region. Mechanical dredging systems can accommodate debris laden sediments, which may be an important consideration for sediment removal in the Gowanus Canal. Mechanical systems are also capable of working closely around structures, such as bridge abutments and bulkheads. Hydraulically controlled grab buckets provide better horizontal control than cable crane operated clam shell systems, and therefore may be more suited for working in confined areas within the canal. Although mechanical dredging systems provide a level of accuracy suitable for sediment removal in the Gowanus Canal, they also have the potential for a high level sediment resuspension and can be expected to create a disturbance depth of as much as 6 feet below target grade (ERDC 2007). Clam shell buckets and hydraulically controlled grab buckets can be out-fitted with sealing systems that reduce spillage and resuspension. The following discussion of specific mechanical dredging systems is adapted from Lower Passaic River Restoration Project Dredging Technology Review Report, NYJDOT (2004).

Enclosed Clamshell – Cable Crane Operated


This commonly used mechanical dredging system uses a crane-type configuration that drops the clamshell-shaped bucket into the water. The crane may be mobile on top of a barge platform, or it may be fixed to the barge on a swivel system. The bucket is equipped with a vent that prevents the formation of a pressure wave that would disturb exposed sediments.

The weight of the falling bucket causes the bucket to penetrate bottom sediments. The bucket is closed using cables or hydraulic rams. The bucket may be sealed with gaskets to minimize leakage of sediment and water, but debris can prevent closure of the bucket. After filling, the vents in the bucket are closed and the bucket is raised to surface. Dredged material is typically deposited onto a barge. The bucket may be dipped into a wash tank prior to being submerged for another sediment grab. Dredged material must be dewatered and the decant water treated to approved specifications. The Cable Arm system (http://www.cablearm.com/), a mechanical environmental dredging system developed by Cable Arm Inc., is equipped with water depth and sediment penetration depth sensors, and a bucket open/close switch, which can be used to minimize sediment resuspension. The bucket can also be equipped with video cameras, temperature sensors, and pH sensors. Whether the highest level of precision is appropriate or cost effective for environmental dredging in the Gowanus Canal would need to be determined. The effectiveness of high precision monitoring equipment may also be compromised by conditions at the Gowanus Canal, such as turbidity, and operator reluctance to continually use additional sensory equipment.

Horizontal Profiling Grab (HPG) Bucket - Backhoe Operated This mechanical dredging system consists of a hydraulically operated backhoe mounted on a barge. The backhoe may be mobile on the barge or it may be mounted on a swivel system. The backhoe is capable of producing downward and horizontal pressure on sediments and therefore does not rely on the weight of a falling bucket to penetrate bottom sediments. The bucket may be fitted with a trap door that can seal grabbed sediments within the bucket. The bucket is typically filled with a pulling motion. The dredge Bonacavor (owned at the

Bonacavor

Enclosed Clamshell


time by Bean Environmental, currently owned by Jericho Products Inc.) was successfully field tested in the shallow waters of New Bedford Harbor. In the New Bedford Harbor field test, dredged materials were converted into a slurry-mix that was pumped to the disposal site.

The Bonacavor was modified to meet the draft constraints of the New Bedford Harbor test site by being supported on an 80’ by 80’ pontoon platform, which distributed the dredge’s weight sufficiently so that the operating draft was 2’ or less. The depth of cut for the test ranged from 1 to 4 feet, however the Bonacavor has a reported working depth of 50 feet. The Bonavacor is approximately 45 feet wide, which may restrict its operations in the upper reaches of the Gowanus Canal.

Hydraulic Dredging Technology Hydraulic dredging systems use the suction generated by a centrifugal pump to remove bottom sediments. The centrifugal pump discharges a slurry-mix consisting of dredged material and water. The discharged slurry-mix may range from 5% to 50 % solids. High solids content may not be sustainable throughout dredging operations. The water content of dredged material generated by any dredging system in the Gowanus Canal would likely require treatment prior to discharge into receiving waters. Hydraulic dredging systems generate much higher water content in their dredged material output than mechanical dredging systems. The higher water content associated with hydraulic dredging systems would generate a greater amount of decant water requiring treatment than would be required by a mechanical dredging system. Hydraulic dredging systems are less able to remove debris than mechanical dredging systems. The two most common types of hydraulic dredging systems are cutterhead-pipeline dredges and hopper dredges. Cutterhead pipeline dredges operate with a rotating cutterhead followed by the centrifugal pump suction intake. The pump’s slurry output is directed through a pipeline to the disposal area. The cutterhead is swept from side to side across the area being dredged. Cutterhead pipeline dredging can be expected to create as much as seven feet of disturbance below target grade (ERDC 2007). However, cutterhead pipeline dredging systems typically have the lowest resuspension rates of standard dredging plants. The low resuspension rate is due to the close proximity of the cutterhead and the suction pump intake. One variation on the cutterhead pipeline dredge is the Matchbox dredge, which was evaluated for the New Bedford Harbor Superfund Pilot Study. The Matchbox dredge

Cutterhead Dredge


uses hydraulic pistons and a cutterhead to excavate sediments. Plates cover the suction intake and sides of the cutterhead to reduce sediment resuspension. However, evaluation of the Matchbox dredge during the New Bedford Harbor Superfund Pilot Study showed that the Matchbox dredge created substantially more sediment resuspension than the cutterhead dredge (Palermo 1991). Hopper dredges operate by pulling a drag-head and adjacent suction pump intake along bottom sediments. The pump’s slurry output is discharged onto a hopper which delivers the dredged material to a barge or on-board storage area. The dredged material must be further transported to the final disposal area. The drag head used by the hopper dredge generates the least disturbance depth (3 feet below target grade) but typically generates a substantial amount of re-suspended sediments due to the configuration of the drag head and suction pump intake. A third, less common, hydraulic dredging system is the horizontal auger dredge, which operates in a manner similar to the cutterhead dredge. The horizontal auger is guided along bottom sediments by a cable and is followed by the suction pump intake. The pump typically discharges the slurry-mix into a pipeline for transport to the disposal area. The Mudcat horizontal auger system (http://www.mudcat.com/) received the highest rating of all dredging systems in an analysis of dredging systems for the New Bedford Harbor environmental dredging project (Palermo, 1991). The horizontal auger system was not recommended by the analysis because the resuspension rate observed in field testing at New Bedford Harbor was greater than the cutterhead resuspension rate. Subsequently, the Mudcat horizontal auger system was used in the New Bedford Harbor dredging project and has been used in numerous other environmental dredging projects. The Mudcat series of dredges has a maximum nominal digging depth of 20 feet.

Specialty Dredging Technology

Amphibious Excavators One example of amphibious excavators is the Amphibex dredge

(http://www.normrock.ca/1/The_Amphibex/The_Amphibex), which can “walk” to the dredging location by means of legs, or propel itself over water at a speed of eight knots. The dredge has a transport width of 11’6” which would allow access through the narrow portions of the canal to the shallow areas

MC-2000 Mud Cat™

The Amphibex


at the head of the canal. The maximum working depth is 21’5”. The hydraulically actuated arms can be fitted with a rake, horizontal cutter, or small bucket. Sediment is transported away from the dredge by means of twin hydraulic pumps. A major complication concerning the Amphibex is that it is built on a Canadian hull and is therefore restricted from operating in US waters by the Jones Act. Another amphibious excavator, with a history of use in Europe and South Asia, is the CrawlCat which is manufactured by IHC Holland.

The Dredging Supply Company of Reserve, LA constructs and leases amphibious dredges (http://www.dscdredge.com/). The Amphibian uses a swing ladder dredge with an 8” by 8” dredge pump configuration, which is capable of low flow rates with high percentages of solids. The Amphibian can be transported on a single truck. This is a small dredge which may be useful in the shallowest upper reach of the canal.

Submersible Pumps Submersible pumps used for dredging applications operate by suction and are sometimes equipped with a cutting head. Dredge pumps operate with low re-suspension since the disturbed sediment is immediate to the suction intake. These dredges are typically mounted on ladders, as in the Amphibian discussed above, but can also be mounted on a floating platform or hung from a boom or crane. They are not capable of dredging debris.

EDDY Pump The intake nozzle of the EDDY Pump is submerged into the sediment and a spinning rotor forces sediments into the suction chamber. The EDDY Pump configuration generates a small amount of re-suspension during dredging operations. The pump is capable of moving material with a high percentage of solids content and highly viscous material. The 12” EDDY Pump dredge has a capacity of nearly 400 cubic yards per hour and is reported to generate more dredged material output per gallon of diesel fuel than mechanical dredging systems (http://www.tornadomotion.com/technology-comparison.htm). The dredge is capable of operating at depths between one foot and 45 feet. The dredge may be transported by truck on non-permit loads and may be assembled on-site.

Pneuma Pump The Pneuma Pump operates by changing air pressure in a system of cylinders. As a cylinder fills with sediment, the intake valve is closed and compressed air is forced

DSC Amphibian


through the cylinder, which propels the sediment through a discharge pipe. The operation is similar to a two-stroke piston. Due to the pump’s insertion into the sediment, re-suspension as a result of operation is minimal. Pneuma pumps are typically mounted on a ladder, but can also be suspended from a boom or crane. This type of dredge operates with very high solids content.

Oozer Pump A variation on the system employed by the Pneuma pump is the Oozer pump, which draws sediments into its dredge pipe by changing the air pressure within its two cylinders. This is also a low re-suspension, high solids content technology. Oozer pump technology has been developed and applied in Japan. References to the Oozer Pump tend to be in the 1980’s and 1990’s. This technology may not be currently available in the US.

Water Injection Dredging Technology Water injection dredging systems typically consist of injecting water into the sediments causing them to act in a fluid manner. The fluidized sediments can then be removed using suction dredging techniques or pushed to deeper portions of the harbor or waterway. This technology has been previously evaluated by ERDC (DRP 3-10, April 1993) and is generally not recommended for environmental dredging applications. 6.0 Advantages and Disadvantages of Dredging Technologies The advantages and disadvantages of alternative dredging systems which may be employed in the Gowanus Canal are based on each system’s operational capability within the canal. The following discussion highlights the advantages and disadvantages of each alternative dredging system.

Mechanical Dredging Systems The advantages of using a mechanical dredging system in the Gowanus Canal are based on the high potential for large debris contained in the sediments, the relatively low volume of water that would need to be decanted and processed, and the ability to work effectively within the physical constraints of the canal. Debris Handling Capability: Mechanical dredging systems, including both clamshell bucket and drag bucket based systems, are capable of hauling large debris, such as shopping carts, lengths of pipe, chunks of concrete, and other large urban and industrial debris, which are likely to be encountered during dredging of the Gowanus Canal. Low Water Volume: Most of the dredged material water content will need to be decanted and processed before being discharged into receiving waters. The ultimate placement or disposal of Gowanus Canal’s contaminated sediments will likely require that dredged material pass the “Paint filter test”, which tests for the presence of free liquids in the waste sample (EPA Method 9095B). Passing the paint filter test requires that no liquids from a sample of material pass through a paint filter after a five minute period.


It is important to note that the cost analysis conducted for dredging the Lower Passaic River CERCLA (Tierra, 2008) site estimated the cost of dewatering and treating the decanted water ($2.3 million) to be nearly equal to the cost of mechanically dredging the material ($2.4 million). The Lower Passaic River dredged material would be sufficiently de-watered to pass the paint filter test and the decant water treated sufficiently to be discharged back into the Passaic River. Although this alternative dredging method evaluation is based on effectiveness and not costs, there is an advantage to mechanical dredging systems which can be expected to operate at 80% or more solids content in the Gowanus Canal. Working Within Physical Constraints: Mechanical dredging systems have the capability of operating in the shallow and deeper waters of the canal and within the narrow areas between bridge abutments and in turning basins. Backhoe operated grab bucket systems, which have a high degree of horizontal control, are also especially able to work around bridge abutments, foundations, and bulkheads. Mechanical systems can also be sized appropriately for work in difference reaches of the canal. It may be required that a larger dredge and dredging platform work in the broader and deeper reaches of the canal (from Hamilton Avenue to 9th Street) and a much smaller dredge and dredging platform work in the turning basins and the reaches above 9th Street. The disadvantages of mechanical dredging systems are based on a relatively low production rate, double-handling of dredged material, and high resuspension rates. Low Production Rate: Mechanical dredging systems typically have a lower production rate (cubic yards per hour) than other systems of equivalent size (see for example Table 2 in Palermo, 1991). A lower production rate increases the amount of time equipment and labor must be on-site and increases the amount of time that navigation in the canal would be impacted by the dredging project. Double Handling: Mechanical dredging requires transport of dredged material from the dredge to the dewatering location. Typically, the mechanical dredge loads dredged material onto a barge, which transports the material to the dewatering or staging area. The dredged material must be re-handled when it is offload from the barge. Transport barges will need to be small enough to fit within the confines of the canal. Additionally, contaminated material handling protocols will need to be developed for material handling and spillage precautions. High Resuspension Rates: Mechanical dredging systems typically generate more re-suspended material than hydraulic dredging systems. Even the inclusion of bucket enclosures, which can substantially reduce resuspension, does not reduce mechanical dredging resuspension to levels generated by cutterhead dredges. The presence of large debris would also increase sediment resuspension during dredging operations. The plan for dredging at the Lower Passaic River CERCLA site resolves the high level of resuspension associated with clam shell bucket dredging by constructing a sheet pile enclosure around the entire area to be dredged. Dredging within an enclosure may be a solution to potential resuspension associated with mechanical dredging.


Hydraulic Dredging Systems The advantages of using hydraulic dredging systems in the Gowanus Canal are limited to cutterhead pipeline dredges and horizontal auger dredges. Hopper dredges are typically deployed in deeper waters and in areas where they can make long, uninterrupted runs. Hopper dredges are not suited to dredging in the Gowanus Canal. The advantages of using hydraulic dredges in the Gowanus Canal are related to relatively high production rates, the potential for direct placement of dredged material at the de-watering staging area, and low levels of sediment resuspension.

High Production Rates: Hydraulic dredging systems typically have higher production rates than mechanical dredging systems because the cutterhead is in constant contact with the sediments. Higher production rates result in less time on site for equipment and labor and less disruption to canal navigation. High production rates however, are contingent on there being no debris in the sediments. Debris can foul the cutterhead and/or pump, which would require stopping production, de-fouling the equipment (possibly making repairs) and removing the debris by other means (typically mechanical dredging equipment).

Pipeline Transport: Hydraulic dredging systems are capable of discharging dredged material (in slurry form) through a pipeline to the de-watering area. Pipelines can be floating or fixed and may be nearly any length if booster pumps are used. The use of pipeline transport greatly reduces material handling and spillage risks, if a continuous pipeline can be run from the dredging area to the de-watering area.

Low Resuspension Rates: The proximity of the cutterhead to the suction intake substantially controls the resuspension of sediments during dredging operations. The cutterhead dredge generated the least amount of re-suspended sediments during field testing for the New Bedford Harbor dredging project (Palermo, 1991). The use of a cutterhead dredge may not relinquish the need for additional sediment transport abatement measures that would be stipulated by the dredging permit.

The disadvantages of using hydraulic dredging systems in the Gowanus Canal are related to the inability to handle debris and the high water content of the dredged material slurry.

Inability to Handle Debris: Although debris surveys have yet to be conducted for the canal, a debris survey conducted for the Lower Passaic River CERCLA site may be indicative of Gowanus Canal debris. A magnetometer was used in a 100-foot wide search area in a sample survey. The survey resulted in 12 magnetic anomalies, of which three were classified as large and others were described as individual or groups of smaller objects, such as chain, wire, anchors, or pipes. In addition, two other objects were identified by a sub-bottom profiler. Other observations made in the same area have reported numerous objects lodged in the sediments, with tires and pilings being the most common (Tierra, 2008). Debris laden sediments will likely be the typical condition within the canal and the ability to handle debris effectively is a pre-requisite for any dredging system working in the canal.


High Water Content: De-watering dredged material and processing the decanted water is a substantial component of the Gowanus Canal dredging project. The relatively high water content of the dredged material slurry that would result from hydraulic dredging would require a larger de-watering and decant water processing effort than would be required by mechanical dredging.

Specialty Dredging Systems The specialty dredging systems identified in this analysis offer some advantages related to the size of equipment, high production rates, and low resuspension rates. Small Equipment Size: The amphibious dredging systems identified in this analysis are variations on either mechanical or hydraulic dredging systems, and therefore share the same advantages and disadvantages of those dredging systems. One distinct advantage of amphibious dredging equipment is that the dredging plant is compact, transportable over-the-road without the need for special load permits, and can operate in very shallow water depths. The small plant size and shallow water capability would allow an amphibious dredging system to be employed in the smallest turning basins and at the head of the canal. High Production Rates: Submersible pump dredging systems are similar to hydraulic dredging systems in that the dredge is in constant contact with the sediments. Submersible pump dredging systems are capable of high production rates, but with low water content because the pump suction intake is submerged in the sediments. The combination of high production rates and low water content is an advantage over hydraulic systems which have high production rates, but also have high water content. Low Resuspension Rates: Submersible pump dredging systems have the lowest sediment resuspension rates because the pump suction intake is submerged in the sediments. 7.0 Recommendations Selection of the appropriate dredging technologies to be used in the Gowanus Canal is based on the ability of each technology to operate effectively and achieve project objectives under the conditions that exist at the canal. The expected presence of substantial and large debris throughout Gowanus Canal sediments indicates that pump based dredging technologies, including hydraulic dredges and submersible pump dredges, will not be effective in the canal. Mechanical dredging systems, although they generate re-suspended sediments, are the dredging systems most suited to the Gowanus Canal. Mechanical dredging systems are capable of handling frequent and large debris, and equipment may be small enough to work in the shallow and narrow areas of the canal. A crane operated clam shell bucket dredge was selected for the Lower Passaic River dredging project. One of the main reasons that dredge system was selected was the ability to dredge debris laden sediments. In the Lower Passaic River project, re-suspended sediments will be contained and contaminant migration will be abated through the use of a sheet pile enclosure that will surround the entire area to be dredged.


Enclosure of portions of the Gowanus Canal where dredging is taking place may be a solution to contaminant migration through sediment resuspension. 8.0 References 33cfr117.787. Gowanus Canal. (Federal Register notice date 05 August 2000). Lally, John and Ikalaian, Allen. 2001. Field Testing Advanced Remedial Dredging and Sediment Transport Technologies at the New Bedford Harbor Superfund Site. Martin, Keith S. Presentation: Coastal and Hydraulics Laboratory USACE Engineer Research and Development Center - Gowanus Canal Existing Conditions. Undated. New Jersey Department of Transportation, Office of Maritime Resources. Lower Passaic River Restoration Project: Dredging Technology Review Report. June 2004. New York City Department of Environmental Protection. (2004a). Use and Standards Attainment Project: Gowanus Canal Waterbody/Watershed Assessment and Preliminary Facility Plan. Presentation to the Gowanus Canal Waterbody/Watershed Stakeholder Team: Meeting Number 5, April 20, 2004 New York City Department of Environmental Protection. (2004b). Gowanus Facilities Upgrade. Presentation to Community Board Number 6-Brooklyn: Public Safety and Environmental Committee. June 21, 2004 Palermo, Michael R. Equipment Choices for Dredging Contaminated Sediments. Remediation, Autumn 1991 pp 473 – 492. Palermo, Michael R. and Pankow, Virginia R. New Bedford Harbor Superfund Project Acushnet River Estuary Engineering Feasibility Study of Dredging and and Dredged Material Disposal Alternatives: Report Number 10 Evaluation of Dredging and Dredging Control Technologies Technical Report EL-88-15. November 1988. USACE, New England District. Final Pre-Design Field Test Dredge Technology Evaluation Report New Bedford Harbor August 2001. USACE, New York District (2008). Memorandum. Final Gowanus Trip Report: Bulkhead Investigation. 07June 2008. USACE, New York District (2005). Gowanus Bay and Canal feasibility Study: P-7 Briefing Document. March 2005. Tavolaro, John F. et al. Overdepth Dredging and Characterization Depth Recommendations: ERDC/TN EEDP-04-37. June 2007.


Tierra Solutions, Inc. Phase I Engineering Evaluation/Cost Analysis: CERCLA Non-Time Critical Removal Action – Lower Passaic River Study Area. November 2008.

gowanus canal dredging alternativesoperations office (33cfr 117.787 gowanus canal). an exemption to...

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