Sedimentary and geo-mechanical properties of Willapa Bay tidal flats

The mechanical and physical properties of sediments from a tidal channel and adjacent flats were measured in Willapa Bay, Washington State, USA. Yield strength, Young's modulus, and tensile fracture toughness measurements revealed seasonal changes in the behavior of these tidally influenced sediments. In summer, both flats and channel sediment behaviors were similar, while during winter, channel sediments were much weaker than sediments on the flats. Sediments typically displayed an increasingly elastic and fracture behavior with depth below the surface, with the exception of winter channel deposits, which deformed viscously. Porosity was inversely correlated to the measured geotechnical properties, resulting in increased strength with depth. In addition, each mechanical property was linearly proportional to one another. Average grain size did not differ within the field site, eliminating any effect that changing grain size might have on the mechanical behavior and strength. Overall, compressive and tensile strengths were low in shallow Willapa Bay sediments (<20 cm).

► Examination fine-scale mechanical behavior of tidal sediments.
► Comparison of mechanical behaviors between tidal flats and tidal channel sediments.
► Porosity is strongly correlated to the mechanical behavior in Willapa Bay.
► Tidal flats display predominantly elastic fracture behavior, channel sediments undergo viscous flow.
► Channels are depo-centers for weak sediment deposits that are easily removed and redistributed.