Comparison of Three Techniques for Scour Depth Measurement: Photogrammetry, Echosounder Profiling and a Calibrated Pile

Abstract

Laboratory studies form an integral part of scour research, yet there is no standard technique for monitoring scour depth development during testing. This paper investigates the advantages and disadvantages of a variety of techniques, and compares results from three methods: photogrammetry, an echosounder, and a calibrated pile. A novel system involving an underwater camera is presented. This produces results in close agreement with the other techniques, and has the advantage of providing accurate measurements over the full scour hole while at the same time enabling qualitative information to be gained from the photographs.

High-resolution single-camera photogrammetry: incorporation of refraction at a fluid interface

“…As discussed by Porter et al (2014), the main drawbacks of using photogrammetry to measure a sediment bed under a water layer is the temporal resolution and the intrusiveness of the cameras on the flow or the refraction caused by the air-water interface. However, with the improvements made to the photogrammetric technique, these issues can be overcome.…”

“…Compared with planar laser sheet illumination (Younkin and Hill 2009), photogrammetry has the clear advantage of being able to measure the bed thickness over an area and no real disadvantage in terms of accuracy ). Photogrammetry has been compared with two more traditional techniques (calibrated pile and echosounder) to measure the scour hole around a pile (Porter et al 2014). In this case, photogrammetry proved to be superior in terms of accuracy to the echosounder for a much more affordable price.…”

“…Analytical, geometry-based corrections have also been implemented to account for refraction by Butler et al (2002) and Dietrich (2017). A solution to reduce the effect of refraction is to use underwater cameras Porter et al 2014), but this is not always possible. In a more recent study, the refraction at the water interface was taken into account during the calibration while using a pattern composed of parallel lines to measure underwater objects and the thickness of a sediment bed (Cebrián-Robles and Ortega-Casanova 2016).…”

Investigation into laboratory bathymetric measurement techniques

“…Altimeters are commonly used both in the field and in the laboratory and are required to keep the acoustic transducer in water to operate. The acoustic beam size (or spot size) is inversely proportional to the acoustic frequency (i.e., higher frequencies have a smaller spot size [Porter et al 2014]), but higher frequencies attenuate faster in water with high suspended sediment concentrations (McGovern et al 2012). Commercially available altimeters generally operate in the frequency range of 50 kHz 1 to 10 MHz and can have an advertised vertical resolution of less than 1 mm (Coleman et al 2003).…”

“…Stereophotography: Stereophotograpy is a type of photogrammetry that requires a minimum of two cameras to reconstruct a three-dimensional surface (Porter et al 2014). This technique has been used in a range of laboratory environments such as subaerial landslide surface reconstruction (McFall 2014;McFall et al 2018) but has more relevantly been used in laboratory studies to measure bathymetric changes around a pile (Faraci et al 2000;Baglio et al 2001;Porter et al 2014). Baglio et al 2001 used a laser and diffractive lens to create a grid of points on the sandy bottom, and the dynamic bathymetric measurements were taken using the changes in the spatial position of each point recorded on two cameras.…”