Flow, Sediment Transport and Groundwater Modeling During the Remedial Design Process

Flow, Sediment Transport and Groundwater Modeling During the Remedial Design Process

OCEANS, LAKES & RIVERS. INNOVATION, EXCELLENCE & SERVICE.

the Remedial Design Processthe Remedial Design Process

National GridNew York Harbor - Gowanus Canal

Why Model?Why Model?

• The best practice approach to assessment and remedial design of contaminated sites includes numerical modeling of flow and sediment transport

• The Gowanus Canal is a complex site and many of the forces acting on it, like the Flushing Tunnel, are the forces acting on it, like the Flushing Tunnel, are not yet quantified

• Modeling is necessary to ensure a sustainable remedy

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Best PracticeBest Practice

• Guidance documents recommend numerical modeling for large or complex sediment sites:• EPA 2005• USACE 2008

• Models are an essential predictive design tool:• Models are an essential predictive design tool:• Where uncertainties exist • To evaluate differences between remedial

approaches• Provide a quantitative framework for design

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Gowanus Canal Site ConditionsGowanus Canal Site Conditions

• Conceptual Site Model shows that surface water-sediment processes are central to the fate and transport of contaminants in the Gowanus Canal

• These processes can be very complexcan be very complex

• Models assist us inunderstanding these processes during remedial design

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What Makes This Site Unique?What Makes This Site Unique?

• The impacts of the Flushing Tunnel (215 Mgal/d) on flows and sediments in the canal are unique to this site

• Restored Flushing Tunnel is not yet operational• RI sampling was during flushing cessation, so we have

not measured the impact of the Tunnelnot measured the impact of the Tunnel• Aside from the Tunnel, all net flow is from CSOs and

groundwater• A design cannot be finalized

without determining these impacts

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Flow in the CanalFlow in the Canal

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Erosion Potential in the CanalErosion Potential in the Canal

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Preliminary model results

WITHOUT FLUSHING TUNNELWITH FLUSHING TUNNEL

Design QuestionsDesign Questions

• How can we use flow and sediment transport analysis to explain the past and future sediment and contaminant distributions in the canal?

• How do CSOs/Flushing Tunnel/Groundwater flows influence these processes?influence these processes?

• What are the impacts of proposed remediation activities on flow and sediment movement in the canal?

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Why Use a Model During Design?Why Use a Model During Design?

• Many of the forces that will be acting on the Canal (like the Flushing Tunnel) have not been adequately quantified

• Predict long-term behavior beyond the period when physical measurements are possiblewhen physical measurements are possible

• The best tools to evaluate the behavior of proposed changes to the system are surface and groundwater models

• Only comprehensive way of evaluating ‘what if?’ scenarios during the design process

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Design ScenariosDesign Scenarios

• Remediation alternatives will need to consider:• Different tidal and flood conditions• Effects of flushing (both directions) • Effects of CSOs• Effects of CSOs• Effects on groundwater• Long-term sedimentation patterns

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Delft3D Gowanus ModelDelft3D Gowanus Model FLUSHING FLUSHING TUNNELTUNNEL

CSOs / OUTFALLSCSOs / OUTFALLS

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TIDES AND SURGETIDES AND SURGE

GROUNDWATERGROUNDWATER(FROM GEI MODEL)(FROM GEI MODEL)

Model InputsModel Inputs

• Physical observations • Bathymetry• Flow (tides)• Sediment transport• Bed sediment distribution & properties

• Groundwater model• CSO flows• Flushing Tunnel flows

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Importance of Groundwater ModelImportance of Groundwater Model• What are the potential hydraulic impacts of

remedial alternatives?• Flooding/groundwater mounding• Changes to groundwater flow direction

• How can a proposed remedy be designed to • How can a proposed remedy be designed to minimize hydraulic impacts?

• Provide groundwater input to hydrodynamic model• Evaluate contaminant fluxes from land-side sites• Determine upwelling velocity (critical for cap

design)

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Example of Future Model UseExample of Future Model Use• Any dredge and cap solution

will require optimization• Dredging too deep would have

implications for circulation and sedimentation in the canal –may reduce effectiveness of may reduce effectiveness of Flushing Tunnel

• The model is the ideal tool for design in this case• It allows us to balance competing

needs• It is a decision-making tool

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Particle TrackingParticle Tracking

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Modeling Expedites the Design ProcessModeling Expedites the Design Process• The models are ready to use

as tools in the design process once calibration is complete (October 2011)

• They provide the most • They provide the most effective way of pushing the design forward as they reduce uncertainty in the design process

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Picking a Sustainable SolutionPicking a Sustainable Solution

• How do we pick the remediation option with the best long-term performance?• The model is the best tool to evaluate the long-term

effectiveness of each proposed remediation approach:

• Benefits

• Longevity• Minimize negative consequences

• Risk / uncertainty evaluation

• The model also provides us predictions against which to measure the success of the project

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SummarySummary

• Modeling is an essential tool in understanding the processes in the Canal

• Best practice recommends this approach• Using models as design tools is the industry-

standard approach to remediationstandard approach to remediation• Modeling aids in optimizing the remedial design,

and in minimizing the potential negative impacts of remediation

• Modeling will expedite the design process

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