Assessing the Spatial Variability in Geotechnical Sediment Properties on a Sandy Hardbottom Intertidal Beach

An intertidal beach with hardbottom rock outcrops and composed of mixed sand-shell hash sediments was examined for surficial (~10 cm depth) cross-shore variations in geotechnical properties. Areas adjacent to intertidal hardbottoms contained significantly higher shell hash content with coarser particle sizes compared to areas lacking these features. In situ moisture content measurements revealed notably low values between 0% and 12%. Furthermore, the expected seaward increase of moisture content across the beach typically observed on sandy beaches was disrupted in areas of higher shell hash concentrations. Direct shear test results produced a friction angle of 33° for the pure sand; however, sediments containing high concentrations of coarse shell hash exhibited consistent friction angles between 35° and 36°. The findings suggest that intertidal hardbottoms likely influence shell hash distribution along beaches and that increased shell hash content affects sediment shear strength.