Tidal flat thermodynamics

Abstract

Intertidal flats are important coastal margin environments linking the terrestrial and marine ecosystems. This dissertation studies the thermodynamic processes of tidal flats over tidal, fortnightly, and seasonal timescales through in situ field observations, remotely sensed measurements, and a numerical model of cross-flat tidal heat and mass transport. First, seasonal variation of sediment-water heat exchange is modeled at both sites using a numerical cross-shore model of tidal flat heat and mass fluxes. The model uses a convective heat transfer coefficient to evaluate sediment-water heat fluxes. The model accurately predicts water and sediment temperatures observed at both sites and estimates that exchange of heat between the sediment and water can be as great as 20% of the incoming forcing solar shortwave radiation. Seasonal variations in the net heat flux show that the tidal flats act as a net source of heat during the summer months and a net sink during the winter. This pattern is explained by the phasing of the exposure periods and daytime solar radiation whereby maximum flat exposure during the summer occurs during the daytime whereas flat exposure during the winter months occurs at night allowing significant cooling of the flats. The model is most sensitive to the choice of thermal conductivity indicating the importance of accurately determining tidal flat sediment thermal properties. Next, tidal time-scales are examined by focusing on heating at the leading edge of the flooding front. Field observations indicate that during the summer under clear sky conditions, the leading edge of the flood front is nearly 5 °C warmer than the sediment it inundates. Model results show that this process is related to the absorption of solar radiation in a thin film of water occurring at the front edge. This result only occurs under conditions where the light extinction coefficient is large enough to fully absorb all shortwave radiation in the water column and prevent its transmission to the sediment bed and is consistent with the qualitative observations of high turbidity at the leading edge of the front. Finally, tidal time-scale processes on the ebb tide are studied using novel remote sensing technique to determine surface fluxes in an incised intertidal channel. Ebb flows through incised channels off the flat continue throughout the low tide period and are difficult to measure as water depths are below 10 cm. A infrared imaging technique is used to determine surface velocities during these low depth periods. Flows off the flats exhibit two distinct dynamic regimes: ebb-tide flow and post-ebb discharge. Ebb-tidal flow occurs during the receding tide when downstream water elevations control the upstream flow velocities (M1 profiles). The post-ebb discharge continues throughout the low tide period and obeys uniform open-channel flow dynamics. Calculations of total volume fluxes and the use of the tidal heat flux model support the hypothesis that remnant water from the flat surface is the source of these discharges.