Comparison of high rate GNSS and GNSS-R measurements for detecting tidal bores in the Garonne River

<p>The Reflected Global Navigation Satellite System (GNSS-R) is a bi-static radar system in which the receiver collect GNSS signals reflected from the Earth surface and compares them with corresponding direct signals. Measurements can be performed on the waveforms to determine the elevation of the free surface, leading to applications such as ocean altimetry, inland water level variations, soil moisture, snow depth and atmospheric water changes. This study presents the potential of in-situ GNSS-R for tidal bore detection and characterization, and compares it to high rate GNSS observations and other reference datasets.</p><p>The data we used were acquired on 17<sup>th</sup> and 18<sup>th</sup> October 2016 in the Garonne River, at 126 km upstream the mouth of the Gironde estuary. We processed GNSS-based elevations from data acquired on a buoy at a 20 Hz sampling rate using differential GNSS (DGNSS) technique. Acoustic Doppler Current Profiler (ADCP) measurements as well as pressure data were used for validation purposes. These techniques show good results in estimating the amplitude of the first wave, the period of the tidal bore and the oceanic tides. All of these datasets were compared to the retrieval of GNSS-R signals above the river. We have processed the changes in water height throughout the acquisition using <em>Larson et al.</em> (2013) and <em>Roussel et al.</em> (2015) techniques. We finally separate the atmospheric component from the tidal bore and the oceanic tides ones.</p><p> </p><p>Larson, K. M., Löfgren, J. S., and Haas, R. (2013). Coastal sea level measurements using a single geodetic gps receiver. Advances in Space Research, 51(8):1301–1310.</p><p>Roussel, N., Ramillien, G., Frappart, F. et al. (2015). Sea level monitoring and sea state estimate using a single geodetic receiver. Remote Sensing of Environment, 171:261 – 277.</p>

5. Tidal bores

A tidal bore is perhaps the most spectacular tidal phenomenon that can be readily observed. When a large tide enters a shallow, funnel-shaped estuary with a gently sloping bottom, its waveform is distorted and this can lead to an impressive rolling ‘wall of water’, travelling upriver. ‘Tidal bores’ explains that estuary shape and a large tidal range are important for tidal bore formation. Tidal bores can be smooth, non-breaking ‘undular’ waves or a variety of breaking forms of increasing violence. Famous examples are seen along the Qiantang River in China, the Amazon River in Brazil, and the River Severn in the UK. The impact of tidal bores on estuarine processes and ecosystems is also discussed.