Foti, Enrico

Fredsøe, Jørgen

Locatelli, Luca

Musumeci, Rosaria E.

Petersen, Thor Ugelvig

Sumer, B. Mutlu

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 20, 2017Backfilling of scour holes around pilesSumer, B. Mutlu; Petersen, Thor Ugelvig; Locatelli, Luca; Fredsøe, Jørgen; Musumeci, Rosaria E.; Foti,EnricoPublished in:Proceedings of the 33rd International Conference on Coastal EngineeringPublication date:2012Link back to DTU OrbitCitation (APA):Sumer, B. M., Petersen, T. U., Locatelli, L., Fredsøe, J., Musumeci, R. E., & Foti, E. (2012). Backfilling of scourholes around piles. In Proceedings of the 33rd International Conference on Coastal Engineering  AmericanSociety of Civil Engineers.Backfilling of scour holes around piles  B. Mutlu Sumer, Thor U. Petersen, Luca Locatelli and Jørgen Fredsøe，Technical University of Denmark, Section for Fluid Mech., Coastal and Maritime Engng., 2800 Kgs. Lyngby, Denmark, Corresponding author: bms@mek.dtu.dk  Rosaria E. Musumeci and Enrico Foti, Department of Civil and Environmental Engineering, University of Cantania, 95125 Catania, Italy.  INTRODUCTION Observations show that, when the correct conditions exist, a previously generated scour hole around a pile may be backfilled; e.g., a scour hole generated by a steady current is backfilled when the flow conditions change from steady current to waves, see Figure 1. Great many works have been devoted to scour around piles. A detailed account has been given in the books of Whitehouse (1998), Melville and Coleman (2000) and Sumer and Fredsøe (2002). While much has been written on scour, only few studies have been reported on backfilling. The backfilling comes into focus when the time variation of scour depth in a continuously changing flow climate (current, waves and combined current and waves) is considered. Scour around offshore wind turbine (OWT) foundations is a typical example; the bed around an OWT foundation continuously experiences scour and backfilling in an alternating fashion under the ever changing wave and current climate. To the authors’ knowledge, no study is yet available investigating, in a systematic manner, backfilling around piles. The purpose of the study is twofold. One is to gain an understanding of the backfilling process around a pile when the pile is exposed to a changing flow environment (the flow environment changing from, e.g. a steady current to a wave; or from a steady current to combined waves and current; or from a wave to a smaller wave; etc.) The second purpose is to collect data for various characteristics of the backfilling process, including the time scale, for the purpose of establishing design diagrams.   Figure 1 – Scour hole generated by steady current (1). Scour hole backfilled by waves (2).   EXPERIMENTAL SETUP Experiments were, for the most part, conducted with piles with diameters of D =15 mm, 25 mm, 40 mm and 75 mm. However, some supplementary experiments were also conducted with a large pile with D = 310 mm. Irregular waves were used in the experiments to avoid bed undulations associated with very long testing times, up to 85 hours. The grain size of the sediment (fine sand) used in the experiments was 𝑑50 = 0.17 mm with a geometric standard deviation of 𝜎𝑔 = 𝑑84 𝑑50⁄  = 1.3. The model pile was embedded vertically in the sediment bed. It was rigidly fixed, extending down to the base bottom of the flume. The 40 mm and 75 mm diameter piles were transparent, enabling the scour and backfilling processes to be monitored by a mini video camera placed inside the pile with the help of a 45° mirror. The overall time development of the scour hole was monitored by a second camera placed outside the pile. With this setup, the scour and backfilling process was videotaped at the offshore side of the pile with a viewing area of approximately 100° in plan view. In the case of waves and combined waves and current, the orbital velocity of water particles at the bed was measured, using a Laser Doppler Anemometer (LDA). The development of backfilling also was monitored in a few experiments, using a 3D optical technique developed at the University of Catania.  RESULTS The investigation has shed light onto the mechanism behind the backfilling process. The results show that the scour depth corresponding to the equilibrium state of backfilling is the same as that corresponding to the equilibrium state of scour around the pile for the same wave (or combined wave and current) climate. The time scale of backfilling has been determined as a function of three parameters, namely (1) the Keulegan-Carpenter number, 𝐾𝐶𝑖 , of the initial wave (which generates the initial scour hole), 𝐾𝐶𝑖 being 𝐾𝐶𝑖 = ∞ if the initial flow climate is a current; (2) the Keulegan-Carpenter number of the subsequent wave which backfills the scour hole, 𝐾𝐶𝑓 ; and (3) the Shields parameter associated with the latter wave, for live-bed conditions. In the case of combined wave and current, the current-to-wave-velocity ratio is also involved. The results show that the time scale of the backfilling process is completely different from that of scour. The time scale of backfilling is much larger than that of scour when the backfilling Keulegan-Carpenter number is 𝐾𝐶𝑓 = O(10) or less (typical wind farm application), while the trend is opposite when 𝐾𝐶𝑓  >> O(10). A major paper summarizing the results is to appear in Sumer et al. (2012).  REFERENCES Melville, B. W. and Coleman, S. E. (2000): Bridge Scour, Water Resources Publications, LLC, CO, USA. Sumer, B. M, and Fredsøe, J. (2002): The Mechanics of Scour in the Marine Environment, World Scientific, Advanced Series on Ocean Engineering – Volume 17. Sumer et al. (2012): Backfilling of a Scour Hole around a Pile in Waves and Current, to appear in Journal of Waterway, Port, Coastal and Ocean Engineering, ASCE. Whitehouse, R. (1998): Scour at Marine Structures, Thomas Telford. 

Backfilling of scour holes around piles

http://orbit.dtu.dk/ws/files/51555572/65.pdf

Backfilling of scour holes around piles

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology