High resolution river surface roughness characterization for neart-nadir Ka band interferometry CAL/VAL

Jean Negrel (LEGOS/CNRS, France)


Jean-Francois Cretaux (LEGOS/CNES, France); Stéphane Calmant (LEGOS/IRD, France)

Event: 2014 Ocean Surface Topography Science Team Meeting

Session: Science Results from Satellite Altimetry: Inland waters (multi-mission and long-term monitoring)

Presentation type: Type Poster

Water surface roughness strongly impacts microwave backscattering process over rivers. Therefore, developing and applying radar interferometry techniques over continental water to determine river longitudinal slope (cross-track interferometry) or surface velocity (along-track interferometry) requires a detailed characterization and understanding of water surface roughness, its relation with river flow conditions and wind conditions, and its impact on directional backscattering coefficient.

In situ measurement of river surface roughness is a complex task as surface topography is both rapidly changing and sensitive to obstacles or contact measurements. Furthermore laboratory measurements, although highly informative and valuable, fall short to represent the diversity of river flow and wind conditions.

In the framework of the SWOT mission preparatory phase and AirSWOT experiments, a first method relying on a set of immersed pressure sensors has been developed for field measurement of river surface roughness. After being tested and validated in laboratory conditions and implemented in natural conditions on the Rhône river, this method presented strong limits in characterizing small surface structures (<10cm) and required a swallow depth measurement.

This limits appears critical for the SWOT/AirSWOT Ka band instrument. Smaller structures need to be characterized in order to better understand the backscattered signal. To overcome this limitation, we are currently developing and testing a new method based on high resolution stereo-photogrammetry to characterize the smallest surface structure. The main challenge in such a method is the correlation between the two pictures to reach the higher precision surface modelling.

The second part of the present work is to study the effects of water flow and wind speed on the surface structures. The previous field measurements (may 2011) demonstrated an important on correlation length and standard deviation of the heights. Although the correlation appeared clearly, the flow speed must also have its own impact and must be studied.
Jean Negrel