Lidar in Scaled Physical Modeling: Applications, Advantages, and Development

Author(s): Bell, G.

Nearly every physical model done is scaled (1:1 scaled models are rare in that they require vast amounts of space and resources). The larger the scale, the larger the error is amplified from measurements at the model scale. Thus the need for a very high degree of accuracy in physical model data collection is of the highest importance.

When doing moveable bed modeling, it is advantageous to have data with high resolution and accuracy to evaluate bathymetric changes from hydraulic processes. Use of a lidar (light detection and ranging) system provides an accuracy of millimeters, minimizing measurement error that is magnified when scaled to prototype dimensions. Unlike the challenges of traditional bathymetric data collection methods, the high resolution of terrestrial scanning provides complete coverage of the domain.

Lidar scanning is a process that collects high resolution geometric, bathymetric, and topographic data. Lidar uses lasers to make measurements based on time of flight returns. The scanners used in this study each have a wavelength of ~ 1550 nm (or near infrared). These lasers cannot travel through the water medium (their energy is absorbed by the water). When the bathymetry is being scanned, the model must be drained of water (and if this is after testing, then the draining process can be long due to slow drainage in order to preserve post-test bathymetric results). These methods of collecting physical model bathymetric data have been proven to be accurate and efficient.

Lidar was also used in a coastal application in which a dune’s response to collision and over-wash erosional events was tracked. The focus of the study whether, and to what degree, vegetation alters the dune response to those erosional events. In addition to pre and post test lidar scanning, line-scanning was performed during the testing to track the dune configuration changes during the testing. Line-scan lidar continuously scans to provide instantaneous water levels and dune evolution during each wave burst. A MATLAB code was written to filter the returns of the lidar data, thus the position of the wave (water) could be pin pointed and continuous dune topography could be collected during the model testing.

I am undertaking a research study which involves using lidar scanning to measure water surface elevations in scaled physical models. Currently, water surface elevation measurements are typically made using some variation of stilling gage/pipes or a variety of piezometer boards. Measurements can be made via point gage, surveyed elevation, or some other point of control. Accuracy of these measurements is on the order of 0.5 mm. The biggest constraint of using this method is the lack of measurement locations that can be utilized. The pipes will cause flow disturbance, take up space, require routing, etc. The work that I have done up to this point has proven that by scanning materials floating on the water surface, it is possible to collect accurate (below 10 mm) measurements of the water surface elevation during live physical model testing.

The following points summarize what will be presented in this topic.

  1. Explain where lidar stands now in laboratory physical modeling.
  2. What improvements have been done, are being done, and will be done to this data collection tool.
  3. Where possibilities in the future lie.

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