Sediment Transport Analysis of Cable Installation for Block Island Wind Farm and Block Island Transmission System

Deepwater Wind was developing the Block Island Wind Farm, a 30 megawatt (MW) offshore wind farm located approximately 3 miles southeast of Block Island, Rhode Island in approximately 26 m of water. The BIWF consists of five, 6 MW wind turbine generators (WTGs), a submarine cable interconnecting the WTGs (Inter-Array Cable), and a 34.5-kV transmission cable from the northernmost WTG to an interconnection point on Block Island (Export Cable). 

As part of the Environmental Impacts Assessment, RPS performed a hydrodynamics and sediment transport modelling study to evaluate the sediment trajectory and fate from jet plow cable burial operations and near shore cofferdam re-fill activities (constructed for Horizontal Directional Drilling at the cable landfall). The analysis evaluated the Block Island Wind Farm activities, (the inter-array and export cable), and the Block Island Transmission System activities (between Block Island and the Rhode Island mainland), including several landfall alternatives. 

Timeline

  • 2011

    The project started in 2011.

  • 2012

    In May 2012, the modelling analysis was completed.

  • 2012

    In September 2012, meetings with CRMC and RIDEM were held.

  • 2014

    In September 2014, the dredging permit was awarded.

  • 2014

    The project was completed in September 2014.

1 /05
2011

The project started in 2011.

2012

In May 2012, the modelling analysis was completed.

2012

In September 2012, meetings with CRMC and RIDEM were held.

2014

In September 2014, the dredging permit was awarded.

2014

The project was completed in September 2014.

Key Details

Project Name

Sediment Transport Analysis of Cable Installation for Block Island Wind Farm and Block Island Transmission System

Client

Deepwater Wind through Tetra Tech

Sector

  • Renewables

Location

  • Block Island Sound and Narragansett Bay, USA

Services Provided

  • Hydrodynamic modelling 
  • Sediment transport modelling 
  • Environmental impact assessment 
  • Client support and presentation at regulatory hearings 

Challenge

Rhode Island’s Coastal Resources Management Council (CRMC) and Department of Environmental Management (RIDEM) require that certain thresholds of sediment concentration and sediment deposition depths not be exceeded, as a result of dredging activities in coastal waters. Accurate representation of local currents and circulation patterns, and quantification of the sediment grain size distribution along the cable burial route, are necessary to predict the material transport, water column concentrations and deposition footprint of the resuspended sediments. Reasonable estimates of the sediment loss rate from dredging activities (volume and rate of sediments that are resuspended), must be made to accurately determine the sediment mass in the water column that is free to be transported. 

Block Island Content (NAM).png

Solution

RPS supported the DWW permitting effort though accurate prediction of the cable burial operation impacts of sediment transport and fate, presenting the results to the RI CRMC and RIDEM. The hydrodynamic model was calibrated to acoustic doppler current profiler surface, mid, and bottom currents, at four stations, and the model predicted water surface elevations were calibrated to four NOAA tidal stations, all nearby the offshore wind farm development area. The sediment characterization along the route was determined through evaluation of 62 vibracore sampling data sets in the wind farm area and along the cable burial route. The sediment loss rates were determined through literature research, interviews with multiple dredging contractors and the US Army Corps of Engineers. 

Project Statistics

1st
offshore wind farm constructed in US waters
1st
time Block Island connected to the mainland grid
100%
of Block Island electricity now generated by the offshore wind farm
40%
reduction in electricity cost in the island

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