Layer: Skokomish Bathymetry DEM.tif (ID: 6)

Name: Skokomish Bathymetry DEM.tif

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Description: We performed bathymetric surveys using a shallow-water echo-sounding system (Takekawa et al. 2010, Brand et al. 2012) comprised of an acoustic profiler (Reson, Inc.; Slangerup, Denmark, Navisound 210), Leica Viva RTK GPS, and laptop computer mounted on a shallow-draft, portable flat-bottom boat (Figure 7). The RTK GPS enabled high resolution elevations of the water surface. The rover positions were received from the Leica Smartnet system (www.lecia-geosystems.com) or base station and referenced to the same bench mark used in the elevation surveys (Table 1). We mounted a variable frequency transducer on the front of the boat and connected it to the sounder; the sounder worked in areas of >10 cm of water. We recorded twenty depth readings and one GPS location each second along transects spaced 100 m apart perpendicular to the nearby salt marsh. We calibrated the system before use with a bar-check plate and adjusted the sound velocity for salinity and temperature differences. The bar-check plate was suspended below the transducer at a known depth that was verified against the transducer readings.We synthesized the bathymetry data to create a digital elevation model (DEM) of the nearshore regions at Port Susan, Skokomish, Nisqually, Grays Harbor, Willapa, and Bull Island using ArcGIS 10.2.1 Spatial Analyst (ESRI 2013, Redlands, CA) with exponential ordinary kriging methods (5 x 5 m cell size). We removed portions of bathymetry data that overlapped with elevation surveys conducted on the tidal marsh. In this report we present elevation data as local orthometric heights (NAVD88). At Padilla we mapped the nearshore area using the methodologies outlined in the Sections 5.2 and 5.3 as the dense eelgrass beds would have increased the error in acoustic measurements.We collected bathymetry data in the adjacent nearshore habitat at all sites (except for Siletz) for a total of 984.4 hectares surveyed (Figure 19, Table 5). We found large variation in the bathymetric conditions among site with areas ranging from large shallow mudflats to deep narrow channel systems. Elevation of the adjacent nearshore habitat ranged from -49.7 to +2.68 m. The average bathymetric range was 2.2 m for all study sites except Skokomish, which had a large measured range due to inclusion of offshore areas of Puget Sound. See appendices for detailed site specific results.

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