SEAFLOOR CHARACTERIZATION OF THE MERRIMACK RIVER EBB TIDAL DELTA

In the summers of 2009 -2011, a comprehensive ground truth survey was conducted for creating a controlled test site for seafloor characterization using remote sensing techniques. The ground truth survey included grab sample measurements and underwater video imagery at more than two hundred sites in the Merrimack River Embayment, Gulf of Maine, USA. There has been an ambitious effort by the state agencies in Massachusetts to do seafloor characterization as part of the MA Ocean Management Plan. This sediment data ground-truth’s the existing, recently-collected geophysical data at a finer scale than any previous sampling. The sediment from the grab samples was separated into fractions of quarter phi size resolution so grain size distributions could be analyzed. The underwater video imagery was used to describe the surface texture of the sediment. The sediment and video data from each ground truth station were then georeferenced and geospatially analyzed using ArcGIS.

Sediment samples primarily were composed of coarse iron-rich garnet sands and finer amphibole-rich grey sands. Grain size analysis included fraction, skewness, and kurtosis of the grain size distribution. Negative skewness around the delta is caused by an abundance of fine sediment in the Merrimack River derived sands, suggesting a fluvial, rather than tidal, origin of the particles.

Optic and acoustic remote-sensing surveys were used to gain complete spatial coverage of the seafloor, returning different values based on the depth, surface texture and reflectance. Airborne Lidar Bathymetry (ALB) collected by the United States Army Corps of Engineers in 2007 was used to map land and the seafloor up to a depth of 15 meters below sea level. Swath bathymetry and backscatter imagery data, collected by the United States Geologic Survey (USGS) from 2004-2005, was used for intermediate water depths ranging from 10 to 120 meters below sea level. These surveys were then used to segment sediment types based on the backscatter intensity using ArcGIS tools. The results of this work, along with the previous results from a physiographic study provided by the USGS, will support additional coastal research and aid in coastal zone management decisions.