Lake Mapping Report

Bathymetry mapping can provide useful information for water quality managers to address sedimentation issues on Iowa’s Lakes and Reservoirs. The Iowa Water Science Center of the United States Geological Survey (USGS) began a lake bathymetric mapping program in June 2001 on Lake Delhi in east central Iowa resulting in a published bathymetric map and report (Schnoebelen and others, 2003). In addition, recent advancements in hydro-acoustic technology and equipment have given rise to several new applications being developed. These advancements have given the Iowa Water Science Center an opportunity to use a simple and compact system for determining an estimate of sediment thickness and volume in lakes and reservoirs. The creation of bathymetry and sediment thickness contour maps involves field data collection and data processing in the office.

Current Work

Fourteen lakes across Iowa have been surveyed since the lake bathymetric mapping program began. Both bathymetric and sediment thickness data have been collected at thirteen of these lakes. Contours maps will be the eventual products of this effort. Seven other lakes across Iowa are slated to be surveyed in 2005.

Equipment:

Bathymetry data are collected using the Bathymetric Survey System (BSS+5) with a 200 kHZ intelligent depth sounder (IDS) and a differential GPS (Novatel DGPS beacon or omnistar receiver) (Specialty Devices, Inc., 2003, Bathymetric Survey System BSS+5 with omnistar manual: Specialty Devices, Inc., Plano, Texas, 38 p.). The differential GPS has a horizontal accuracy of less than 1 meter (3.281 ft). Bathymetry data are collected and stored in Hypack Max software (ver. 4.3) (Hydrographic Survey Software, User’s Manual, Hypack Max, Coastal Oceanographics, Inc., Middlefield, CT).
The sediment thickness data are also collected using the BSS+5 bathymetric survey system with a 200kHZ intelligent depth sounder (IDS) with the addition of a broad-spectrum sub-bottom profiler with 48 kHZ, 24 kHZ, 12 kHZ, and 4 kHZ transducers. The software SDIDepth (ver. 3.3, Specialty Devices, Inc., 2003, Bathymetric Survey System BSS+5 with omnistar manual: Specialty Devices, Inc., Plano, Texas, 38 p.) records the digital data of the lake sub-bottom.

Bathymetry:

Data are collected along viewable planned transect lines that were set-up in Hypack Max prior to data collection. Thousands of data points (or multiples thereof) are collected throughout the lake along the transect lines. Files of processed data from Hypack® Max software are converted into ESRI-ArcGIS point coverages representing discrete point locations of bathymetry. The point coverages are input into TIN model applications within the GIS software to produce three-dimensional surfaces representing bathymetry. The surfaces are then contoured and adjusted for any interpretive errors. Volumetric calculations are also performed within the tin model applications.

Sediment Thickness:

Seismic data are collected at the same time the bathymetry data are collected along the viewable planned transect lines that were set-up in Hypack Max prior to data collection. Thousands of data points (or multiples thereof) are collected throughout the lake along the transect lines. The seismic data are stored in the SDIDepth software. The data sediment thickness data are processed back in the office using the Depthpic 3.2 software (Specialty Devices, Inc., 2003, Bathymetric Survey System BSS+5 with omnistar manual: Specialty Devices, Inc., Plano, Texas, 38 p.). The bottom of the recent sediment layers are digitized for each transect of data collected. Sediment cores are collected to assist in the interpretation of the seismic data. Data processed in the Depthpic software are converted into ESRI-ArcGIS point coverages representing discrete point locations of sediment thickness. The point coverages are input into grid model applications within the GIS software to produce a surface representing sediment thickness. The surfaces are then contoured and adjusted for any interpretive errors. Calculations are performed within the gridding applications to estimate sediment volume.