Wave Run-up measurement with 2D laser scanner

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Application of LiDAR (Light Detection And Ranging) technology for measurements in coastal research and hydraulics is now becoming a standard approach. LiDAR measurement has been approved to be able to obtain the time-varying water surface profile in swash zones and areas in which the water surface remission of light is high enough for the scanner sensors to detect the surface. With the correct application, a single 2D liDAR system could be an alternative for an array of several point measurement gauges [1]. This method is also reported in literature like in [1], [2], [3], and [4]. This technique was also used in Hydralab+ large-scale experimental project WaLoWa (Wave Loads on Walls). A major part of the written material in this page is taken from the virtual learning video provided by Hydralab+ [5], which may be also seen below.

Figure 1: A SICK 2D LiDAR sensor


Theory[edit]

the LiDAR systems are also known as time-of-flight systems because they utilize the measurement of the time required for light to travel from a source to a reflecting target surface and back to a light detector in order to calculate the distance [6]. Figure 2 shows the basic measurement principle of a LiDAR system. if the time delay or time-of-flight from the source to the measuring point and back to the detector is , then the distance between the measured point and the reflector may be calculated as

,


and the elevation of the measured point could be calculated as


.


Figure 2: The 2D LiDAR scanning geometry

in order to produce a complete 2D profile of a surface the laser beam is deflected by an internal mirror rotating in a 2D plane to take a 180° fan-scan (See Figure 3).

Figure 3: The 2D LiDAR scanning mechanism Source:Wikipedia

Mounting and measurement[edit]

The mounting technical details could be found in the instrument instructional documents (for example see [7]). Two mounting parameters are important to get good results [5]:

  • scanning plane angle (Figure 4), which should be more than 15°. The inclination of the scanning plane reduces direct reflection of the laser beam from the water surface, which causes measurement errors [1].
  • appropriate distance to the target area. there is a limit in scanning range which depends on the scanning target remission. the scanning range should be considered based on the water turbidity, length of the profile and target angular resolution.
Figure 4: the scanning plane inclination angle

Configuration[edit]

before the measurement, the scanner settings should be configured with the manufacturer's utility. the SICK scanners have a user interface tool named SOPAS for such task. The software automatically detects the scanner and then the user logs in as the authorized user with the factory password 'client'.

Data record configuration[edit]

The configuration starts with data record configuration. the main parameters of record configurations are sampling rate and maximum recording time.

Figure 5: the scanning plane inclination angle

Scan range[edit]

The scan area to be saved in the output data could be adjusted, which is important in order to optimize the size of data. This could be set in the output data configuration window with assigning the start angle and stop angle of the output view.

Figure 6 the scanning plane inclination angle

Tutorial video[edit]

See also[edit]

References[edit]

  1. 1.0 1.1 1.2 Blenkinsopp, C. E., Turner, I. L., Allis, M. J., Peirson, W. L., and Garden, L. E. (2012). “Application of LiDAR technology for measurement of time-varying free-surface profiles in a laboratory wave flume.” Coastal Engineering, 68, 1–5.
  2. Blenkinsopp, C. E., Mole, M. A., Turner, I. L., and Peirson, W. L. (2010). “Measurements of the time-varying free-surface profile across the swash zone obtained using an industrial LIDAR.” Coastal Engineering, 57(11), 1059–1065.
  3. Hofland, B., Diamantidou, E., van Steeg, P., and Meys, P. (2015). “Wave runup and wave overtopping measurements using a laser scanner.” Coastal Engineering, 106, 20–29.
  4. Vousdoukas, M. I., Kirupakaramoorthy, T., Oumeraci, H., de la Torre, M., Wübbold, F., Wagner, B., and Schimmels, S. (2014). “The role of combined laser scanning and video techniques in monitoring wave-by-wave swash zone processes.” Coastal Engineering, 83, 150–165.
  5. 5.0 5.1 Hydralab Project. (2017). How to use a water surface scanner. [1]
  6. Vosselman, G., and Maas, H.-G. (2010). Airborne and Terrestrial Laser Scanning. Whittles Publishing.
  7. https://www.sick.com/media/docs/4/14/514/Operating_instructions_LMS5xx_LASER_MEASUREMENT_SENSORS_en_IM0037514.PDF