Thermal observations of drainage from a mud flat

Incised channels on tidal flats create a complex flow network conveying water on and off the flat during the tidal cycle. In situ and remotely sensed field observations of water drainage and temperature in a secondary channel on a muddy tidal flat in Willapa Bay, Washington (USA) are presented and a novel technique, employing infrared imagery, is used to estimate surface velocities when the water depth in the channel becomes too shallow for ADCP measurements, i.e., less than 10 cm. Two distinct dynamic regimes are apparent in the resulting observations: ebb-tidal flow and the post-ebb discharge period. Ebb tide velocities result from the surface slope associated with the receding tidal elevation whereas the post-ebb discharge continues throughout the low tide period and obeys uniform open-channel flow dynamics. Volume transport calculations and a model of post-ebb runoff temperatures support the hypothesis that remnant water on the flats is the source of the post-ebb discharge.

► In situ (acoustic) and remotely sensed (IR) velocity measurements from a tidal flat quantify runoff at low tide.
► IR techniques measure post–ebb drainage velocity well when the water depth is too shallow for acoustic instruments.
► Dynamic analysis separates the runoff into periods of uniform (bed slope) and tidally influenced (surface slope) open channel flow regimes.
► A thermal flux model of runoff temperatures supports the hypothesis of remnant surface water as the source of channel drainage.