Local thermal non-equilibrium in sediments: Implications for temperature dynamics and the use of heat as a tracer

• We have derived a correlation for heat transfer coefficient at low Re.
• Local thermal equilibrium may not be a valid assumption in sediments’ heat transfer.
• Error in temperature derived velocity estimates may be obtained using LTE.

Understanding streambed thermal processes is of fundamental importance due to the effects of temperature dynamics on stream ecology and solute exchange processes. Local Thermal Equilibrium (LTE) between fluid and solid is usually assumed for modelling heat exchange in streambeds and for inferring pore water flow velocities from streambed temperature data. By examining well established experimental and theoretical relationships of the fluid–solid heat transfer coefficient in a numerical scheme for a range of Reynolds (Re) numbers (0.01 > Re > 0.001), we show here that, for a range of typical streambed conditions, LTE is not attained. Thus errors in velocity estimates obtained when inverting streambed temperature data assuming LTE can be considerable especially at relatively low flow rates. We show that for certain conditions were the LTE assumption is not valid, inferred pore water velocities of up to 1 m/d can be obtained with LTE assumption even if the actual velocities are much smaller or even zero. Ignoring the possibility of Local Thermal Non-Equilibrium (LTNE) will have consequences for the correct estimation of streambed pore water velocity and heat fluxes at low Re values. More laboratory studies are urgently needed to supplement the sparse existing data in this area and further test the findings of this study