12 



Division's (OLLD) National Water Level Observation Network. This network is 

 composed of 181 water level stations that are located throughout the Great Lakes and 

 coastal regions of United States interest. These stations are equipped with the Next 

 Generation Water Level Measurement System tide gauges and satellite transmitters that 

 have collected and transmitted tide data to the central NOAA facility every six minutes, 

 since 1 January 1994. 



Observed tide data are available 1 to 6 hours from the time of collection in a station 

 datum or referenced to Mean Lower Low Water (MLLW) and based on Coordinated 

 Universal Time (UTC). For the 1995 CLIS surveys, data from NOAA tide station 

 8467150 in Bridgeport Harbor, Bridgeport, CT, were used for tidal calculations. The 

 NOAA 6-minute tide data were downloaded in the MLLW datum and corrected to local 

 time, and tidal differences based on the entrance to New Haven Harbor, New Haven, CT, 

 were applied. 



In order to make valid comparisons between present and past bathymetric surveys of 

 the area, the July 1994 and March 1994 bathymetry models were corrected to observed 

 MLLW. The CLIS 1993 baseline survey of the project area was previously corrected to 

 MLLW using the predicted tides for those survey days; therefore, no re-calculation was 

 required. 



During the bathymetric survey, a Seabird Instruments, Inc. SBE 26-03 Sea Gauge 

 wave and tide recorder was used to collect tidal data on-site. The tide gauge, deployed in 

 the survey area, recorded pressure values every six minutes. After conversion, the 

 pressure readings provided a constant record of tidal variations in the survey area based on 

 a mean tidal level (MTL) datum. These observed tidal data were later used to compare 

 and verify the corrected NOAA data generated from the Bridgeport Harbor station (Figure 

 2-2). 



A Seabird Instruments, Inc. SEACAT SBE 19-01 Conductivity, Temperature, and 

 Depth (CTD) probe was used to obtain sound velocity measurements at the start, midpoint, 

 and end of each survey day. The data collected by the CTD probe were bin-averaged to 

 1 meter depth intervals to account for any pycnoclines, rapid changes in density that create 

 distinct layers within the water column. A mean sound velocity was then calculated using 

 the bin-averaged values. 



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 used to construct depth models of the surveyed area. A 



Monitoring Cruise at the Central Long Island Sound Disposal Site, September 1995 



