15 



Observed tide data are downloaded through the Internet in a station datum or 

 referenced to Mean Lower Low Water (MLLW) and based on Coordinated Universal Time. 

 For the 1997 and 1998 surveys over the Seawolf Mound, data from NOAA tide station 

 8461490 in New London Harbor, New London, Connecticut were downloaded in the MLLW 

 and corrected for local time. Tide differences based on the entrance to West Harbor, Fishers 

 Island, New York were applied to the observed data. 



The bathymetric data were analyzed using SAIC's Hydrographic Data Analysis 

 System (HDAS), version 1.03. Raw bathymetric data were imported into HDAS, corrected 

 for sound velocity, and standardized to MLLW using the NOAA observed tides. The 

 bathymetric data were then processed to produce depth models of the survey area. A model 

 is a depth matrix used to generate graphical representations of the survey area (i.e., three- 

 dimensional plots and depth contours). A detailed discussion of the bathymetric analysis 

 technique is provided in the DAMOS Navigation and Bathymetry Standard Operating 

 Procedures (Murray and Selvitelli, 1996). 



The depth models constructed for each survey performed over the Seawolf Disposal 

 Mound were subjected to depth difference routines in HDAS to document the formation and 

 consolidation of the bottom feature over time. The end result of each depth difference 

 comparison was a graphical representation of the disposal mound or changes in mound 

 morphology. However, due to a variety of factors (tidal corrections, changes in sound 

 velocity through the water column, slope of the bottom, and vertical motion of the survey 

 vessel) comparisons of sequential bathymetric surveys can only reliably detect changes in 

 depth of 20 cm or greater. These factors often introduce artifacts that may appear to be 

 small areas of depth increase or decrease. As a result, the lateral extent of a disposal mound 

 or apron is often below the threshold of the bathymetric data products. Other monitoring 

 techniques are often employed to define the thinner margins of the disposal mound (i.e., 

 sediment-profile photography). 



2.2 Sediment Profile Photography 



REMOTS® sediment-profile photography is a benthic sampling technique used to 

 detect and map the distribution of thin (<20 cm) dredged material layers, map benthic 

 disturbance gradients, and monitor the process of benthic recolonization over the disposal 

 mound. This is a reconnaissance survey technique used for rapid collection, interpretation 

 and mapping of data on physical and biological seafloor characteristics. REMOTS® utilizes a 

 Benthos Model 3731 Sediment-Profile Camera, designed to obtain undisturbed, vertical 

 cross-section photographs (in situ profiles) of the upper 15 to 20 cm of the seafloor, for 

 analysis and interpretation. 



Monitoring Cruise at the New London Disposal Site, Seawolf Mound 1995 - 1998 



