Bathymetric Surveys-

Objectives and Accomplishments:

Bathymetric surveys are crucial in the environmental assessment of any marine/terrestrial system. The sediment-water interface can be determined from any vessel equipped with a fathometer, plumb line or measured staff, but environmental parameters such as tide and pitch and roll of the vessel should be obtained simultaneously with the survey to accurately assess actual depth and for temporal comparison of bathymetric datasets.

In 1996 and 1997, bathymetry was acquired in Lake Pontchartrain onboard the G. K. Gilbert during underway sediment sampling operations, discussed later. The transects were three-quarter miles apart and covered the entire lake. The data was acquired continuously and tied to DGPS every ten seconds. The data is uncorrected for environmental parameters. At the same time a separate water depth reading was acquired when the vessel slowed at each sediment sampling site.

The following year extensive bathymetric surveys were conducted along the southern shore of Lake Pontchartrain, from South Point to Bonne-Carre Splillway. The surveys focused on locating and defining the dredge pits along the south shore, as well as a high resolution survey across the southern portion of the lake. The surveys were conducted from a twenty-one foot boat (U.N.O.) as described below. Grid surveys covered the dredge pits with transects less than one-half kilometer apart. The regional survey consisted of shore perpendicular transects approximately one-half kilometer apart with two or three shore parallel tie-lines.

The bathymetric surveys were conducted to establish a baseline bathymetry along sensitive areas of the southern shoreline and to monitor temporal change. Tentative grid sets of the two BCS surveys, uncorrected for environmental parameters, have been developed and compared for bathymetric change during discharge of Mississippi flood waters via the Bonnet-Carre spillway in 1996. The datasets will also be compared to the two underway sediment sampling system datasets, and to the National Oceanic and Atmospheric Administration (NOAA) Gulf Coast Low Water datum. The 1998 dataset is currently being processed by researchers at the University of New Orleans and will provide a baseline grid for temporal change models.

Method:

Bathymetric surveys in 1995 were conducted onboard the G. K. Gilbert. The signal from two 200 Khz transducers located in the sea chest in the boat’s hull was received by an Innerspace 448 fathometer which recorded about 20 readings a second. Navigation was recorded at about one fix per second from a Trimble GPS receiver using Geolink mapping software. The boat maintained a speed of three to four knots throughout the survey. Fathometer depth below waterline was corrected, but no corrections were made regarding tide, pitch or roll. The bathymetry surveys conducted in 1996 and 1997 during the underway sediment sampling surveys were similar to the 1995 bathymetric survey, except a PLGR (Rockwell) DGPS was used acquire navigation using Geolink software.

The 1997 bathymetric survey off the Bonnet-Carre spillway was a repeat survey of the 1995 survey lines. As with the 1998 survey, the data was acquired from a twenty-one foot fiberglass V-hull boat. Speed was maintained at about 4 knots. One E Sound 103 fathometer received a continuous signal from a 200 Khz transducer with a resolution of ±50cm. Navigation was acquired every two seconds using a PLGR (Rockwell) DGPS receiver. Data was collected using Fugawi software package.