Apra Harbor Current and Dredging Plume Study for Proposed CVN Berthing

In support of a Habitat Equivalency Analysis to support proposed berthing and maneuvering of CVN class vessels, Sea Engineering was responsible for completing a current measurement and dredging suspended sediment modeling study to assess possible dredging impacts. Specific project work tasks included the following:

• Current Measurement – SEI deployed a total of seven ADCPs to record currents throughout the project area and areas possibly impacted by the project. The deployment period was from November 1, 2007 to February 1, 2008 with the intent of capturing variable wind conditions during the transitional fall weather months of September to December, as well as typical trade wind conditions.

• Water quality and sediment sampling – Water quality profiling and sediment sampling were conducted as part of the field activities.

• Numerical Modeling – An EPA approved, state-of-the-art, three dimensional hydrodynamic model – EFDC – was applied to Apra Harbor. The model is 3-dimensional, which allows for variations in water properties and currents at different depths; it computes suspended sediment transport and deposition; and it allows input of variable suspended sediment load size distributions and concentrations. The model was calibrated with data collected in the current measurement program. Suspended sediment transport, settling and deposition was computed. Twenty modeling case studies were completed, bracketing a range of possible dredge locations; dredging plume suspended sediment grain size distribution, and initial size and concentration; winds – trade winds, calm; tides – spring, neap, tide phase; and dredging operational intervals.