Flocculation on a muddy intertidal flat in Willapa Bay, Washington, Part I: A regional survey of the grain size of surficial sediments

Bottom sediments were collected on a muddy tidal flat in Willapa Bay (southwestern Washington State) and analyzed for grain size using a Coulter Multisizer. The disaggregated inorganic grain size (DIGS) distributions obtained from the sediment were then analyzed using conventional methods including median diameter, d50, the largest 25% of the grain size in the population, d75, skewness, and kurtosis. In addition, the inverse model of Curran et al. (2004) was used to provide process-based parameters for description of the size distributions. To examine small-scale spatial differences in grain size, the results of the analysis were plotted against seabed elevation, obtained from USGS LiDAR data. Results reveal a strong inverse correlation between the mass fraction deposited to the seabed as flocs, which is called the “floc fraction”, and elevation on the tidal flat but failed to show any correlation with conventional grain-size descriptors. The dependence of floc fraction on elevation arises mainly due to the differences in size distributions between secondary tidal channels and tidal flats. The extent of flocculation in sediments deposited in the channels of the Shoalwater tidal-flat system is seasonal. Greater precipitation in the winter months is associated with periods of increased suspended-sediment concentrations, which favours more extensive flocculation and the formation and maintenance of low-strength, high-water-content muds. During the summer dry season, lower suspended-sediment concentrations lead to reduced floc fractions in the channels, while the size distributions on the flats resemble those in winter.

► Bottom sediments were collected and analyzed for grain size on a muddy tidal flat.
► Results reveal a strong correlation between floc fraction and tidal flat elevation.
► Differences in size distributions exist between secondary channels and tidal flats.
► Channel sediments were more flocculated and finer grained.
► Porosity was also correlated with floc fraction.