Thermal diffusivity and strength of tidal flat sediments during a tidal simulation

Coastal margins and tidal flat sediment systems are some of the most complex, heterogeneous and energetically dynamic regions on earth. Tidal flats are repositories of terrigenous and biogenous sediments that are shaped by tides, waves and storms and utilized by birds and benthic organisms. They often lie adjacent to rivers that enable inland passage for ships and access to spawning grounds for fish. As such, they are subject to numerous anthropogenic effects, such as fishing, clamming, beach combing, and automobile traffic. Depending on their morphology and tidal range and periodicity, tidal flats are inundated or exposed for variable amounts of time and over widely different areas. To better understand the properties and distribution of the sediments within this setting, an ongoing study is being conducted to determine the relationship between thermal and geotechnical properties of tidal flat sediments. Our specific objectives are: 1) to determine how to assess thermal properties of laboratory-simulated tidal flat sediments and 2) to assess the relationship between sediment composition and undrained shear strength. The ultimate goal of these efforts is to remotely predict tidal flat trafficability (humans or vehicles) from the temperature signature. To understand how mineralogy influences thermal properties of sediments, several sediment types were tested. To simulate the heterogeneity of the tidal flat, a range of sand-clay mixtures was evaluated. The sand-clay percentages in these mixtures ranged from 100:0 to 0:100 with fractional percentages decremented or incremented by 10 or 20% until all possible sediment mixtures were achieved. Though more complicated scenarios can be simulated, the initial experiments only considered fully saturated sediments. Each sediment tested was exposed to a heat lamp and the resulting temperature gradient was measured every two centimeters with a vertical thermistor array. Measurements were recorded at 5-second intervals during a warming cycle (~20°C change in temperature) and were used to calculate the thermal diffusivity of the sediment using the one-dimensional heat equation. To mimic lateral continuity of a real-world tidal flat and to satisfy the one-dimensional requirement of our numerical method, the sediment was insulated on its sides and base while remaining open at the top to a focused heat source. Homogeneous sediments of varying mineralogy showed distinct differences in thermal conductivity. The ratio of saturated and dry conductivities for sand was 3 times greater than the same ratio for kaolin, suggesting that measurements of sediment temperature at high and low tides might provide insight to tidal flat sediment type. Furthermore, data collected with sediment mixtures showed a non-linear decrease in thermal conductivity with increasing clay fraction and an inverse relationship between thermal conductivity and shear strength. Finally, sediment bearing capacity and shear strength increased with packing density with the highest shear strength occurring at depth within the sediments. The greatest difference in sediment strength occurred in the pure clay sample with the surface strength being ~53% lower than the strength at 10 cm below the surface. Additionally, as clay content increased, sediment strength increased as well. On going work will continue to evaluate the relationship between thermal and geotechnical properties of tidal flat sediments. Field work will be conducted in the upcoming months to assess whether thermal properties of sediments may be correlated with sediment strength and trafficability.