Bed evolution measurement with flowing water in morphodynamics experiments Francesco Visconti, 1 * Luana Stefanon, 2 Carlo Camporeale, 1 Francesca Susin, 2 Luca Ridolfi 1 and Stefano Lanzoni 2 1 Dipartimento di Idraulica, Trasporti e Infrastrutture Civili, Politecnico di Torino, Turin, Italy 2 Dipartimento di Ingegneria Idraulica, Marittima, Ambientale e Geotecnica, Universitá di Padova, Padua, Italy Received 14 February 2011; Revised 15 December 2011; Accepted 19 December 2011 *Correspondence to: Francesco Visconti, Dipartimento di Idraulica, Trasporti e Infrastrutture Civili, Politecnico di Torino, corso Duca degli Abruzzi 24, Turin, Italy. E-mail: francesco. visconti@polito.it ABSTRACT: A new approach for the profiling of movable sediment beds in laboratory experiments is presented. It couples a triangulation laser sensor and an ultrasonic level transmitter, and allows a non-intrusive, fast and accurate measurement of bed topography without stopping the experimental runs. The distortion of the laser beam due to the refraction at the water surface is corrected by contemporaneously measuring the elevation of the water surface through the ultrasonic level transmitter and taking advan- tage of geometrical relations involving the water depth, distance of the sensors from the water surface, and the angles that the emitted laser beam forms with the vertical before and after refraction. Several tests, under either still- or flowing-water conditions, as well as increasing/decreasing water surface elevation, were carried out to evaluate the accuracy of the measurements. These tests indicate that good-quality measurements are obtained for flow depths in the range 0 < D < 60 mm, typical of morphodynamic laboratory experi- ments. Finally, two relevant applications to movable bed experiments carried out under either lagoonal or fluvial conditions are presented that show the effectiveness of the proposed profiling technique. Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS: morphodynamics; laboratory experiments; channel evolution; laboratory measures; bedforms Introduction The interactions between an erodible boundary and a flow field usually lead to complex and fascinating patterns such as those observed in fluvial, coastal and lagoon environments or created by the blowing of the wind in deserts (Allen, 1984; Garcia, 2008). Meandering planforms and sediment waves (ripples, dunes and bars) typically occur both in fluvial and tidal environ- ments, affecting significantly the structure of the flow field as well as sediment fluxes, and hence bearing important theoretical (Seminara, 2010), engineering (Thorne et al., 1997) and environ- mental (e.g. Allen, 1984) consequences. Morphodynamics aims at investigating the physical pro- cesses generating the various landforms and shaping a given landscape. The experimental approach is fundamental in these investigations. Indeed, the morphological evolution of a sediment bed through laboratory tests allows one to measure the interactions between the water stream and the movable boundary under controlled conditions, thus eliminating the variations in external forcings usually characterizing real environments. Another advantage of movable bed experi- ments is the shortening of the evolutionary timescales: in a few hours it is possible to observe morphological changes that in nature develop over several years. Moreover, laboratory models can provide useful information on strongly nonlinear dynamics that can hardly be simulated by mathematical models, because of the presence of rather different space scales and timescales and the difficulties in describing thoroughly turbulent flow fields. Finally, experimental results are not as place sensitive as field studies, which usually strongly depend on the specific studied environment. The quality of data gathered from movable bed experiments is strictly linked to the reliability and accuracy of the adopted measuring systems. Several well-tested instruments and meth- odologies are currently available for measuring the flow field (e.g. particle image velocimetry, acoustic Doppler velocimetry) as well as for bed profiling. In this latter case, however, it is necessary to distinguish between instruments that can measure bed topography with or without the presence of overlying water. In fact, classical bed measurements are generally performed after the removal of surface water. A number of instruments can be used in this latter case, ranging from the simple point gage to the sophisticated laser scanner or digital photogrammetric techniques (e.g. Lane et al., 2001). The bed profiling can be done at the end of the run (Friedkin, 1945) or following a stop-and-go approach, i.e. stopping the run at regular times, removing the water, and performing the measures (Bertoldi and Tubino, 2005). Clearly, this approach cannot be used when short-timescale bed changes have to be captured: experiments should in fact be stopped at time inter- vals that have the same order of magnitude as the bed deforma- tion timescale. Therefore, when highly dynamic processes are investigated, the experiments should be halted so frequently that the reliability of the runs would be compromised. Con- versely, servo-controlled profile indicators (e.g. Delft PV-09) and, more recently, acoustic profilers and acoustic scanners allow bed profiling also in the presence of water overlying the bed (e.g. Lanzoni, 2000; Thorne and Hanes, 2002). These EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms (2012) Copyright © 2012 John Wiley & Sons, Ltd. Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3200