11844 101 modeling position paper final to epa 04102011x

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Baird

o c e a n s

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N a v i g a t i n g N e w H o r i z o n s

Gowanus Canal Superfund Site

Numerical Surface Water Modeling

October 4, 2011

11844.101


N a v i g a t i n g N e w H o r i z o n s

Gowanus Canal Superfund Site EPA Index No.: CERCLA-02-2010-2009/ EPA ID No.: NYN000206222

Prepared for

National Grid Prepared by

W.F. Baird & Associates Coastal Engineers Ltd.

For further information please contact

Alex Brunton at (905) 845-5385

11844.101

This report was prepared by W.F. Baird & Associates Coastal Engineers Ltd. For National

Grid. The material in it reflects the judgment of Baird & Associates in light of the information

available to them at the time of preparation. Any use which a Third Party makes of this

report, or any reliance on decisions to be made based on it, are the responsibility of such Third

Parties. Baird & Associates accepts no responsibility for damages, if any, suffered by any

Third Party as a result of decisions made or actions based on this report.


G o w a n u s C a n a l S u p e r f u n d S i t e P a g e 1 1 1 8 4 4 . 1 0 1

1.0 INTRODUCTION

Urban estuary systems like the Gowanus Canal are complex water bodies, through which regular

and irregular flows of water and sediment are focused. However, many of the forces governing

flow and sediment transport in the Gowanus Canal, such as the restored Flushing Tunnel or any

potential remediation activities, are either not yet active or not yet quantified. Since contaminants

can be tightly bound to sediment particles, pathways of water and sediment in the Canal must be

evaluated and understood in order to understand the fate and transport of contaminants in the

study area. Indeed, the best practice approach to assessment and remedial design of contaminated

estuarine sites includes numerical modeling of flow and sediment transport. Remediation of

contamination in an effective, sustainable and economical manner cannot be undertaken without

such evaluation.

The Physical and Chemical Conceptual Site Model (CH2M Hill, 2011) that is a key component of

the Remedial Investigation of the Canal shows that surface water-sediment processes and

interactions are central to the fate and transport of contaminants in the study area (Figure 1).

Figure 1. Physical and Chemical Conceptual Site Model (CH2M Hill, 2011). Red box indicates processes

requiring quantification using numerical modeling



The Flushing Tunnel will have a significant impact on the surface water and sediment processes

described in the Conceptual Site Model, and this is one factor that makes the Gowanus Canal

Superfund Site unique as a remediation site. The sampling in support of the Remedial

Investigation was undertaken during Flushing Tunnel cessation, so the impact of the Tunnel has

not yet been fully determined, particularly since the restored Flushing Tunnel will have different

operational characteristics to the historic tunnel. Models are an essential predictive design tool

where uncertainties such as this exist, and also where we need to evaluate differences between

remedial approaches.

A numerical model is a tool that helps engineers, scientists and risk managers understand these

fluxes and interactions, and a model can assist in predicting long-term system behavior for periods

when physical measurements are not available (historically or in the future). As the conceptual

model identifies the processes in operation, a numerical model assists in quantifying these

processes. The numerical modeling will establish the existing hydrodynamic forces and sediment

transport processes in operation in the Canal under a range of different flows and tidal conditions.

Evaluation of the nature of these processes is essential to understanding the historic, current, and

future behavior of the Canal.

A numerical model is required to evaluate the physical and chemical processes in the Canal because

it is not possible to assess the effects of past events or proposed remediation activities on flow and

sediment movement in the Canal through empirical observation. Accordingly, the numerical

model is imperative to be able to predict how any proposed remediation activities will potentially

affect existing sediments and contaminants in the system. In addition, the model assists in

assessing the long-term performance of different remediation options to ensure that the final design

will be both effective and sustainable. These are central objectives for mitigating the problems in

the Canal, and they are critical measures of how successful the remediation will be.

The objective of the modeling study is to develop and apply a verified numerical model capable of

evaluating hydrodynamic flow patterns and sediment transport behavior for the Gowanus Canal

for a range of remediation options. Such a model will assist EPA in selecting the most appropriate,

effective and sustainable solution to the issues in Gowanus Canal.



2.0 HOW MODELING INFORMS THE REMEDIATION PROCESS

The U.S. Environmental Protection Agency (EPA, 2005) guide on contaminated sediment

remediation for hazardous waste sites recommends numerical modeling for large or complex sites,

especially where it is necessary to predict contaminant transport and fate over extended periods of

time to evaluate relative differences among possible remedial approaches. Modeling can help in

answering the following fundamental questions:

• What potential beneficial outcomes to flow, water quality and sediment movement are there

from the remediation?

• How can we identify and constrain the negative effects of the remediation activities?

• How do we optimize our approach, and how can we make it adaptive to changing

conditions?

• How do we effectively predict the environmental benefits versus project cost (how and

where do we best spend the remediation funds)?

From these fundamental questions, several technical issues may be identified and then addressed

through application of the model:

• How can we predict sedimentation, erosion and sediment transport over long periods of

time (such as years or decades) or during episodic, high-energy events (e.g., convective

storms, surges or low-frequency flood events)? The impacts of extreme events on flow and

sediment movement in the study area have not yet been evaluated;

• Identifying data gaps and gaps in our understanding of existing conditions (such as

highlighting unaccounted sediment sources or volumes);

• How do sediment characteristics and contaminant concentrations vary spatially at a site?

Empirical observations provide useful benchmarks that can be interpolated or modeled to

get a better understanding of the distribution of contaminants; and

• Can we compare modeled results to observed measurements to show convergence of

information? Both modeling results and empirical data usually will have a measure of

uncertainty, and modeling can help to examine the uncertainties (e.g., through sensitivity

analysis) and refine estimates, which may include indications of where to sample next (EPA,

2005).

Answering these questions is fundamental to the success of the project, and without the model to

assist in evaluating the potential merits of the remediation design it is not possible to outline the

measurable outcomes of the remediation process. Proceeding without modeling would preclude

being able to predict the effects of the remediation project on water quality, sediment accumulation

and sediment quality following remediation. The optimum solution in terms of balancing

contaminant removal with future water quality and sediment accumulation in the canal cannot be

designed without the numerical model.



Furthermore, at some sites, significant uncertainties may exist about site characterization data and

the processes that contribute to the relative effectiveness of available remedial alternatives (EPA,

2005). This is the case in the Gowanus Canal, where there are significant uncertainties about the

sources, transport and deposition of sediments and contaminants, and there are potentially

controversial outcomes of the remediation process. Numerical models can help fill gaps in

knowledge and allow investigation of the relationships among contaminant sources, exposure

pathways, and receptors, and processes at a site that cannot be fully understood through empirical

investigations. These models are often used to predict and quantify the likely response of the area

to various cleanup options (EPA, 2005).

Based on the activities described below, the numerical model will provide important information

throughout the remainder of the Feasibility Study and into Remedial Design. In addition to being a

valuable design tool, the model can assist in proving a framework for goal-based decision making

throughout this process. Understanding the potential for scour and erosion of sediments in the

canal under existing conditions and different remediation scenarios is necessary for evaluating

different design alternatives with respect to the long-term viability of each option.

A three-dimensional hydrodynamic and sediment transport model has been set up to examine

circulation patterns in the Canal and surrounding channels. The numerical model is being used to

simulate tidal- and flushing-driven circulation in the Canal over spring and neap tide cycles, and

for flood and surge scenarios (and Canal overflow events when relevant). Some examples of issues

in which the model may facilitate evaluation and design are:

• The influence of the flushing tunnel and Combined Sewer Overflows (CSOs) on canal flows

and sedimentation;

• Identifying and evaluating the effects of historic and ongoing sources of contamination in

the Canal;

• Determining the impacts of the flushing tunnel flow on the potential effectiveness of

different remediation activities – that is, whether the remediation selected will have an

impact on how the flushing tunnel will need to operate);

• Evaluation of the stability of capping material versus uncapped sediment;

• Defining areas of the canal where cap armoring may be required;

• Evaluation of potential remedies, including potential dredge and capping activities (see

below for further details);

• Impacts of the presence of construction-related structures such as temporary coffer dams on

flows;

• Transport of re-suspended sediment and associated contaminants from dredging and

capping operations and the need for control measures such as silt curtains;

• Evaluation of whether sediments need to be capped or armored, given their spatially-

varying erodibility and exposure to different flow characteristics.



3.0 PROJECT MODEL SELECTION

In order to have the appropriate quantitative information needed to answer the above questions,

the Delft3D numerical model was set up to evaluate the flow and sediment characteristics in the

canal. After consideration of the varying capabilities of seven available three-dimensional models,

the Delft3D model was selected as being best suited for use on this project. Delft3D is a three-

dimensional hydrodynamic and sediment transport model developed by Delft Hydraulics in the

Netherlands. The model recently became an open-source model, which is an important

consideration for Federal agencies. The model uses a curvilinear grid system, which is suitable for

the shoreline boundary conditions in this project. Sediment transport (cohesive and non-cohesive),

morphologic change and water quality processes can be included in the model. This model system

has been used and tested worldwide, and it is considered to be an industry-standard model for

applications such as the Gowanus Canal study. Finally, Delft3D is widely considered to be the best

available model for the prediction of sediment transport and morphologic change, particularly in

estuarine conditions.

NYCDEP (2007) has previously used the ECOM-RCA model to evaluate at flow and water quality

in the canal. While this model was used to consider flow, salinity, temperature and water quality in

the canal, it has not been applied to study sediment transport beyond a basic consideration of total

suspended solids. The model has not been applied to evaluate sedimentation, erosion (re-

suspension) and transport of sediments in detail. In addition, the ECOM-RCA model grid

resolution was much too coarse (and there was no discretization of grid cells across the width of the

Canal) to allow for consideration of detailed flow and sediment transport characteristics. The

Delft3D model offers some advantages over the ECOM model in that numerous particle size

fractions may be considered for sediment transport (versus two in ECOM), and the Delft3D model

also includes bed load transport, which may be a significant transport process in the canal.

4.0 MODEL IMPLEMENTATION

National Grid will apply the model to aid in understanding flow and sediment transport conditions

in the Canal, and also to evaluate the likely performance of proposed remedial activities in the

study area. Preliminary hydrodynamic model results suggest that bed sediments as large as sand

may be mobilized on a spring tide in the Canal (Figure 2). With operation of the flushing tunnel in

addition to a spring tide, particles as large as gravel may be moved along the bed in certain

locations (Figure 3), and there is a net flux of sediment towards the mouth of the Canal. When the

field data from the Acoustic Doppler Current Profilers (ADCPs) and laboratory tests of sediment

erodibility are complete, the hydrodynamic and sediment transport model will be calibrated to

represent the existing conditions in the area. After the model is calibrated and validated, it will be

used to undertake a detailed analysis of sediment movement in the Canal, and to assess flow and

sediment transport behavior for different options for remediation.



The preliminary model results highlight the importance of using a numerical model to understand

the behavior of sediments in the Canal under a variety of conditions. These include the relative

effects of the flushing tunnel, tidal cycles, wet weather events (CSO flows) and flood/surge

conditions, and how these processes interact under existing and design conditions. It is especially

important to be able to determine the potential causes and consequences of mobilizing sand, silts

and clays in the study area under different design scenarios. As the Gowanus Canal receives flows

from the surrounding watershed, the contaminants present in any inflow will often adhere to

sediments in suspension and on the bed of the Canal. As a result, sediment scour, transport and

deposition processes will affect contaminant pathways in the system, and these processes need to

be accounted for in the remediation design process. To address this, the model will also be able to

predict where new sediment will tend to accumulate under post-remedy conditions, the rates of

accumulation of new sediment, and also where any accumulated sediment is likely to move

through scour, transport and deposition under an extreme episodic event.

An example of the value of the model as a feasibility and design tool is illustrated by considering

the benefits of using the model to evaluate the impacts of potential capping alternatives for the

Canal, which might include some dredging to accommodate the cap. If dredging is being

considered as part of a potential remediation option, the U.S. Army Corps of Engineers’ Technical

Guidelines for Environmental Dredging of Contaminated Sediment (USACE, 2008) states that the

hydrodynamic behavior of the site should be characterized with respect to waves, currents, and

fluctuating water levels to determine whether dredging is feasible, or whether constraints should

be imposed on equipment selection. The USEPA Guidance for In-Situ Capping of Contaminated

Sediments (USEPA 1998) similarly states the hydrodynamic behavior of the site should be

characterized for purposes of cap design. The potential for episodic events (i.e. storms and surges)

will also affect the design of control measures such as silt curtains. In addition, USACE (2008)

recommends that the potential change in hydrodynamics following completion of dredging should

also be considered, especially if dredging is a component of a capping remedy. The application of a

numerical hydrodynamic and sediment transport model is the most defensible and effective

method of undertaking these analyses.

Any potential solution which includes dredging and capping will require evaluation because

dredging too deep would have implications for circulation and sedimentation in the Canal, and

could undermine ageing bulkheads in the area. If the beneficial effects of the flushing tunnel

circulation were to be compromised by over-dredging, then the benefits of the remediation

activities would also be compromised. Conversely, any evaluation should consider navigability in

the Canal, and the effects of propeller scour and flow scour in the vicinity of flushing tunnel. There

are several aspects of this which require a model for further evaluation. For example, the

downstream hydrodynamic impact of the flushing tunnel on the channel with different caps and

dredge depths can be evaluated for both scour and water quality. In addition, the model can be

used to evaluate the location and likely rate at which sediment will be deposited in the canal (due

to future supply from the outfalls and flushing tunnel) once capping is complete, and therefore it

can estimate the ongoing maintenance obligation of the project. Finally, the model will provide



important information about potential capping materials and cap design (e.g., options for capping

portions of the Canal).

5.0 CONCLUSION

A numerical surface water flow and sediment transport model is being developed for the Gowanus

Canal Remediation Effort. Preliminary model results suggest that factors such as the Flushing

Tunnel have significant effects on flow and sediment movement in the Canal. Because of this, Baird

believes that the model will be a critical tool to be used in designing and evaluating remedial

approaches for the Canal. Given the present state of knowledge and knowledge gaps regarding the

Canal system, numerical evaluation of remedial technologies is most appropriate and imperative to

ensure that a viable and successful remedy is designed and constructed for the Gowanus Canal.

6.0 REFERENCES

CH2M Hill. 2011. Gowanus Canal Remedial Investigation Report. Volume 1. Draft prepared for

U.S. Environmental Protection Agency.

EPA. 2005. Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. United

States Environmental Protection Agency Report # EPA-540-R-05-012.

NYCDEP, 2007. City-Wide Long Term CSO Control Planning Project Receiving Water Quality

Modeling Report. Volume 4: Gowanus Canal. The City of New York, Department of Environmental

Protection, Bureau of Engineering Design & Construction, September 2007.

USACE, 2008. Technical Guidelines for Environmental Dredging of Contaminated Sediments.

United States Army Corps of Engineers Report ERDC/EL TR-08-29.

B a i r d & A s s o c i a t e s


Figure 2. Preliminary model results showing bed shear stress on a spring tide (no flushing tunnel). Color shades show areas where bed shear

stresses are large enough to potentially mobilize different sizes of bed sediment. Note potential to mobilize silts and sands from the bed of

the Canal.

B a i r d & A s s o c i a t e s


Figure 3. Preliminary model results showing bed shear stress on a spring tide (with flushing tunnel). Color shades show areas where bed

shear stresses are large enough to potentially mobilize different individual sizes of bed sediment. Note potential for mobilization of gravel in

the Canal.

11844 101 modeling position paper final to epa 04102011x

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