Hydroacoustic survey and bathymetric map creation for Brant Lake, New York

Transcription

1 Hydroacoustic survey and bathymetric map creation for Brant Lake, New York Holly A. Waterfield CLM 1 INTRODUCTION Brant Lake is located in Warren County, New York, within the bounds of the Adirondack Park. The Brant Lake Association contracted the Biological Field Station (RF Contract # ) to conduct a survey of the lake s bottom depths (bathymetry) and create a bathymetric map. This report details the methods used to collect hydroacoustic data and analyze those data to yield a bathymetric map with 2-meter contours; digital files of these map products as well as supporting files for use in Geographic Information Systems (GIS) software will be provided to the Brant Lake Association. Prior to this survey, bathymetric data available for Brant Lake were limited to those presented in the map available through the NYS DEC website, Figure 1, which has 20-foot (~6- meter) depth contours (NYS DEC 2015). A more detailed understanding of the lake s bathymetry was needed ahead of the development of a comprehensive lake management plan. The original map does not include islands, shoals, or other bathymetric and near-shore features; such features influence estimates of lake volume, a parameter that is essential to the full understanding of lake trophic state. Analyses of nutrient dynamics, evaluation of external and internal nutrient loading, oxygen dynamics, fish habitat, littoral zone (plant habitat) and many others are all based on an understanding of the lake s morphometry, specifically, the bottom characteristics. Figure 1. Bathymetric map of Brant Lake, New York, as accessed from the NYS Dept. of Environmental Conservation website (2015). 1 Research Support Specialist, SUNY Oneonta Biological Field Station, Cooperstown, NY.

2 METHODS Overview of Bathymetric Survey and Mapping Process Creation of the bathymetric mapped involved both an on-lake survey of bottom depths followed by data processing and mapping in GIS software. Details are provided below. Bottom depths were surveyed using hydroacoustic equipment (SONAR); depth measurements were obtained once per second, yielding over 56,000 data points. These points, along with those representing the shoreline, islands, and exposed rocks, were used to estimate depths across the entire lake (a continuous surface) and from this surface, contours were created. The survey was conducted over the course August 13, 19, and 26, Lake level and transducer depth were recorded at the start of each field day. Lake level was measured at the Rte 23A bridge, as the distance from top of the steel curtain to water surface. Transducer depth was measured from the transducer face up to the water surface. On 13 August and 26 August water temperature was measured every meter from the surface to 18 meters depth. The route surveyed is shown in Figure 2. Transects were irregular in shape and orientation; boat traffic on the lake was heavy at times making it difficult to maintain a regular course and pattern. Additionally, the route was altered as bottom features were discovered, as in the case of several off-shore shoals, and to survey around natural jetties and islands. Hydroacoustic data collection details are provided below in Table 1. Table 1. Data collection settings used during the Brant Lake hydroacoustic survey on 13, 19, and 26 August Transducer Frequency Transducer Beam Width (3 db) Ping Rate Pulse Width Survey Speed 123 khz 7.4 degrees 1 ping per second (pps) 0.4 ms 5 mph off-shore, 2-3 mph near-shore Using acoustics analysis software, Sonar5 Pro, raw data were corrected for sound speed (based on water temperature profile), transducer depth and heave (boat movement). Initial analysis for bottom detection was completed in Sonar5 Pro; the bottom line was inspected and edited before the data were exported for use in ArcMap. Excel files were added to ArcMap as tables, displayed, and then exported as shapefiles. All survey points were merged into a single shapefile and points were adjusted for the lake level on each survey date. The lake shoreline polygon was extracted from the National Hydrography Dataset waterbodies shapefile, accessed via The National Map (nationalmap.gov). High resolution orthoimagery (1-foot) downloaded from the New York State GIS Clearinghouse was used to edit the lake shoreline polygon and digitize islands, jetties, and exposed boulders. The shoreline polygon was densified (to increase the number of vertices) and then converted to a point file. The attribute table was modified to match that of the bottom depth points and all features were assigned a depth of 0.0 meters.

3 Figure 2. Survey route driven during August 2015 hydroacoustic survey of Brant Lake, NY. Data points were collected once per second while the collection gear were in operation.

4 Figure 3. Bathymetry of Brant Lake, as derived from August 2015 hydroacoustic survey data interpretation. Dashed line indicates estimated 1-meter contour; solid lines indicate 2-meter intervals from 2 to 18 meters.

5 A Triangulated Irregular Network (TIN) was created in order to generate a continuous surface from the point data; datasets used included shoreline, islands, exposed boulders, and all survey points. From this TIN, a gridded raster surface was created with a cell size of 5 meters, and contours were generated (as polygons) using the 3D Analyst Toolset. Contours were visually inspected and edited manually with depth points overlaid. Contours were smoothed minimally, with a 1-meter horizontal tolerance to preserve accuracy. The final map was generated with a dashed line indicating the estimated 1-meter contour and solid black lines representing 2-meter intervals from 2 to 18 meters (Figure 3). The maximum depth measurement was meters. Figure 4 illustrates the large bay on the northwestern shore of the lake; it was difficult to navigate due to irregularity of the bottom, particularly along the north and east of the bay, indicated by diagonal striping. The abundance of boulders along with deployment of our survey equipment beneath the boat made sufficient survey of this area nearly impossible. Depth contours in this area are not precise and do not necessarily reflect shallow waters over boulders; this map is NOT intended for use in navigation in such areas. Figure 4. Bathymetric map of Brant Lake enlarged to show the northwestern bay. Diagonal striping indicates a portion of the shoreline where contours may not accurately represent water depth. Boulders are prominent in this area; boaters should navigate with caution.