Session: 06-16-03 Wave Mechanics, Modeling and Wave Effects III

Paper Number: 81143

81143 - Observations on Horizontal Velocities Under Waves in Shallow Water at a Wave Energy Site 

Wave orbital velocities transform into elliptical shapes during the shoaling process, becoming horizontal when approaching the surf zone.  These velocities can be described by linear wave theory; however it implies an important level of uncertainty. In cases where detailed wave shape information is available, non-linear approaches could better describe the orbital velocities but they also exhibit limited accuracy due to complexities of wave dynamics in very shallow waters.

Data has been collected from an exposed prospective marine energy site in the UK over periods of one and two years in 13 m water depth 500 m offshore by two wave-enabled ADCPs at 500 m distance between sensors. The two ADCPs were set up to measure both surface elevation and directional velocities at 1 Hz sample rate for 34 minutes every hour.  The data was then processed to return horizontal peak velocities under the waves on a wave-by-wave basis.  A representative number of waves was observed to allow for significant statistical analyses and to provide insights on the validity of the horizontal velocities as derived from linear wave theory.

For this research, data was grouped into bins containing individual combinations of wave heights and periods, and the associated horizontal peak velocities were extracted for each bin.  The percentage occurrence distributions are shown and compared with the calculated values. Results show that although the calculated approach correlates relatively well with a significant number of the measured values, there is also a considerable scatter around the single calculated values for each bin. Peak velocities were observed to importantly exceed the theoretically derived values.

This provides an important insight for the establishing of horizontal flow forces around potential coastal structures, moorings, cables, or also wave energy converters.  Improved understanding of these forces allows for more accurate design and fatigue calculations, with potential for a positive impact on survivability and longevity of infrastructure at exposed wave sites.  Furthermore, results also bear relevance on the assessment of sediment transport, as the fluctuation of velocities impacts on the initial sediment uplift and subsequent transport. To the best of the authors’ knowledge, this is the first time that such detailed data has been successfully gathered and analysed for the evaluation of horizontal peak velocities at an exposed high energy site. The velocity distributions presented in this work have the potential to become a useful contribution to future coastal engineering works.

Presenting Author: Arne Vogler MEECE ORE Catapult

Authors:

Arne Vogler MEECE ORE Catapult
Diego Miguez MEECE ORE Catapult
Ben Pepper University of Strathclyde