Measurement of gravel-bed topography: an evaluation study applying statistical roughness analysis

Abstract

In this two-part study, experiments are conducted to evaluate available topography measurement techniques for gravel beds in a laboratory flume and to study their suitability for statistical roughness analysis. The available instruments for this study include (1) an acoustic bed profiler; (2) a hand-held laser scanner; and (3) two digital consumer cameras forming a stereophotogrammetric system, and are employed to obtain digital elevation models (DEMs) of water-worked gravel beds. In the first part of the study, the three measurement techniques are reviewed and their feasibilities for future grain-scale roughness work assessed, based on the obtained elevation datasets. Water-worked gravel-bed topographies are measured with all three available measurement techniques. The analysis of the DEMs concentrates on using probability distribution functions (PDFs) and second-order structure functions of bed elevations. Roughness coefficients are determined and used as a benchmark for comparison of the three measurement techniques. Although, visually, differences in the DEMs obtained with different measurement techniques are observed, the results of the chosen statistical analysis do not disclose the visual differences to the same extent. It is shown that the used stereophotogrammetric system, although theoretically allowing a fast and high-resolution recording process, lacks behind in accuracy. Thus, the second part of the study identifies and presents steps to improve the quality of the obtained stereophotogrammetric DEMs. A checklist is provided, highlighting the improvements made in the follow-up study, in order to obtain a high-quality stereophotogrammetric DEM. The overview will be useful for other researchers to make use of available low-cost and high-quality consumer camera equipment, to setup their own, non-proprietary stereophotogrammetric system.