Measuring Alongshore Variations in Swash Flows Using Stereophotogrammetry and Multi-Beam Lidar

The swash zone is the dynamic area of the foreshore that is alternatively submerged and exposed due to incident and infragravity wave action. During swash motion, water depths and current velocities rapidly change across the swash extent, which itself can move across the intertidal zone with varying tides and wave setup. In addition to the wide range of spatial scales and intermittent submergence, swash hydrodynamics are highly dependent on the inner surf-zone characteristics, making the swash zone one of the most challenging littoral regions to measure. Cross-shore arrays of pressure gauges, current meters, and more recently single line scan Lidars can provide high resolution, phase-resolving measurements in one dimension. Two-dimensional field measurements are limited due to the wide range of pertinent alongshore spatial scales O(10^0-2 m) which make large scale sensor deployment expensive and impractical. This low measurement resolution means forcing mechanisms, which are thought to be dependent on both alongshore and cross shore velocity and surface gradients, have rarely been measured in the field environment.

The presented project utilizes a novel configuration of remote sensing instrumentation to provide point clouds of the swash water surface highly resolved in time and the alongshore/cross-shore directions. Point clouds are generated by coupling measurements from multi-beam Lidars and RGB camera stereophotogrammetry. Multi-beam Lidar scanners can measure multiple cross shore surface profiles in the alongshore direction. Commercial off-the-shelf photogrammetry software and stereo-camera rigs have facilitated stereo imaging, providing measurements even in areas poorly resolved by Lidar (little surface texture, foam). It is anticipated the combined Lidar and camera instrumentation can cover wide enough range to capture alongshore infragravity/incident wave motion and high enough resolution to resolve surface gradients associated with gravitational swash flow which was previously impractical with in-situ sensors. This presentation will highlight preliminary measurements and methodology evaluation made at the U.S. Army Engineer Research and Development Center’s Field Research Facility in Duck, NC, USA. Funded by the U.S. Army RDT&E Military Engineering 6.1 Basic Research Program.