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.

[1]  NON-LINEAR FRICTIONAL RESIDUAL EFFECTS ON TIDE PROPAGATION , 2003 .

[2]  Philippe Bonneton,et al.  Tidal bore dynamics in funnel‐shaped estuaries , 2015 .

[3]  R. Lindenbergh,et al.  Laser ranging for monitoring water waves in the new Deltares Delta Flume , 2013 .

[4]  H. Chanson Undular Tidal Bores: Basic Theory and Free-Surface Characteristics , 2010 .

[5]  D. Mouazé,et al.  Sedimentary signatures of tidal bores: a brief synthesis , 2015, Geo-Marine Letters.

[6]  M. Donelan,et al.  Measuring waves with pressure transducers , 1987 .

[7]  R. Almar,et al.  The influence of swash-based reflection on surf zone hydrodynamics: a wave-by-wave approach , 2017 .

[8]  Peter Teunissen,et al.  Incidence angle influence on the quality of terrestrial laser scanning points , 2009 .

[9]  Frédéric Frappart,et al.  High rate GNSS measurements for detecting non-hydrostatic surface wave. Application to tidal borein the Garonne River , 2016 .

[10]  Serge Tamari,et al.  Stage Monitoring in Turbid Reservoirs with an Inclined Terrestrial Near-Infrared Lidar , 2016, Remote. Sens..

[11]  P. Bonneton,et al.  Nearshore Dynamics of Tsunami-like Undular Bores using a Fully Nonlinear Boussinesq Model , 2011 .

[12]  Robert H. Weisberg,et al.  Coastal Ocean Observing Systems , 2015 .

[13]  Chris E. Blenkinsopp,et al.  Monitoring Individual Wave Characteristics in the Inner Surf with a 2-Dimensional Laser Scanner (LiDAR) , 2016, J. Sensors.

[14]  R. Slocum,et al.  Lidar and pressure measurements of inner-surfzone waves and setup , 2015 .

[15]  Per A. Madsen,et al.  On the solitary wave paradigm for tsunamis , 2008 .

[16]  William L. Peirson,et al.  Measurements of the time-varying free-surface profile across the swash zone obtained using an industrial LIDAR , 2010 .

[17]  Serge Tamari,et al.  Flash Flood Monitoring with an Inclined Lidar Installed at a River Bank: Proof of Concept , 2016, Remote. Sens..