I. INTRODUCTION

The water levels in Lake Okeechobee are at record low levels this summer (2001) making hydrographic surveying ideal in that less area must be covered by boat to fully map the lake floor. This condition also allows the upper elevations of the lake, which are now exposed, to be mapped by traditional survey techniques or airborne topographic LIDAR technology. Conversely, due to the very flat slope of the lake floor, much of the lakebed is now less than 0.6m (2') deep making surveying by any method very difficult at best.

The U. S. Geological Survey (USGS) has developed a hydrographic survey system specifically designed to map in very shallow water. The system can acquire data in water depths of ~25cm (10"), but in practice boat/motor draft limitations prevent surveying in water depths less than 45cm (18"). Precise differential GPS receivers are used to measure boat position and dynamic elevation, a survey quality 200 kHz depth sounder acquires water depth measurements, and a motion sensor measures heave, pitch, and roll of the boat. A measurement is collected each second or about every 3m (10') along a survey line. The vertical accuracy of the system is +/- 8cm (3") and +/- 4cm (1.6") inches horizontally. To achieve this accuracy, the boat cannot operate more than 20 Km from any one GPS base station. It is anticipated that two GPS base stations will be used for the Lake Okeechobee survey, located probably upon the lake levee. Several new survey benchmarks will be established around the lake perimeter for GPS base station occupation.

Previous reports indicate that a portion of Lake Okeechobee's lakebed consists of fluid mud. This situation is not uncommon in low energy water bodies but does require some criteria on where to define the bottom. It is suggested that some testing be done with the USGS depth sounder compared to penetrometer or other depth measurement devices in order to derive this criteria. The advantage of using a digital depth sounder is that the measured depths will be consistently derived based upon a specific water/mud density.

II. SCOPE OF SERVICES

III. RESPONSIBILITIES OF THE SFWMD

1. Provide funds to the U.S. Geological Survey Center for Coastal and Regional Marine Studies (USGS-CCRMS) for the performance of the work as outlined herein.
2. Arrange for permission for ingress and egress at each dock site for the USGS boat.
3. Arrange for permission for USGS personnel to place survey monuments (benchmarks) where needed near the lake perimeter, and on islands or platforms in the lake.
4. Arrange for permission for USGS personnel to deploy and retrieve, on a daily basis, GPS base station equipment located on survey monuments.
5. Although not anticipated, provide personnel and equipment as needed to support the work, e.g., field coordination and logistics, local knowledge.
6. Assist in the preparation of final report.

IV. RESPONSIBILITIES OF THE USGS-CCRMS

1. Provide equipment and personnel as necessary to conduct high resolution bathymetric surveying for a minimum of 1550 kilometers. It is anticipated that the survey will be completed within 25 working days, assuming no down days for weather or equipment failure. The anticipated start date will be September 5, 2001.
2. Provide post processing of the benchmark and hydrographic data.
3. Store and maintain all data and provide access to this data for reproduction as needed by the SFWMD.
4. Provide a final data package to include the following:
   1. Hard copy plot of locations of surveyed tracklines.
   2. Digital ASCII XYZ data in UTM Zone 17, GRS80/NAD83 horizontal coordinates, and NAVD88 vertical coordinates in tabular format.
   3. Copies of raw and processed digital data files on CD-ROM

V. DELIVERABLES

1. A minimum of 1550 kilometers of bathymetric survey data as indicated on the attached "Exhibit A".
2. A final data package will be prepared by the USGS that will include the items listed in Part IV, Section 4. An optional USGS Open File report may also be included as a final deliverable.

VI. TIME FRAME