Guillaume Detandt

High Frequency Field Measurements of an Undular Bore Using a 2D LiDAR Scanner

The secondary wave field associated with undular tidal bores (known as whelps) has been barely studied in field conditions: the wave field can be strongly non-hydrostatic, and the turbidity is generally high. In situ measurements based on pressure or acoustic signals can therefore be limited or inadequate. The intermittent nature of this process in the field and the complications encountered in the downscaling to laboratory conditions also render its study difficult. Here, we present a new methodology based on LiDAR technology to provide high spatial and temporal resolution measurements of the free surface of an undular tidal bore. A wave-by-wave analysis is performed on the whelps, and comparisons between LiDAR, acoustic and pressure-derived measurements are used to quantify the non-hydrostatic nature of this phenomenon. A correction based on linear wave theory applied on individual wave properties improves the results from the pressure transducer (Root mean square error, R M S E of 0 . 19 m against 0 . 38 m); however, more robust data is obtained from an upwards-looking acoustic sensor despite high turbidity during the passage of the whelps ( R M S E of 0 . 05 m). Finally, the LiDAR scanner provides the unique possibility to study the wave geometry: the distribution of measured wave height, period, celerity, steepness and wavelength are presented. It is found that the highest wave from the whelps can be steeper than the bore front, explaining why breaking events are sometimes observed in the secondary wave field of undular tidal bores.

High rate GNSS measurements for detecting non-hydrostatic surface wave. Application to tidal borein the Garonne River

Abstract This study presents the results of the use of high frequency Global Navigation Satellites Systems (GNSS) data for the detection and the characterization of tidal bores. The experiment took place the 31st of August 2015 in the Garonne River, at 126 km upstream the mouth of the Gironde estuary. The GNSS data acquired on a buoy at a sampling rate of 20 Hz were processed with a differential GNSS technique using the RTKLIB freeware. GNSS-based elevation of the free surface provides accurate estimates of the tidal bore first wave amplitude (1.34 m) and period (3.0 s). These values were in good agreement with values obtained using ADCP measurements.