The TRR 181 seminar is held by Prof. Dr. Falk Feddersen (Scripps Institution of Oceanography, UC San Diego) on October 19, 11 am at Bundesstr. 53 20146 Hamburg, room 22/23.

The effect of wind on overturning wave shape - Field scale observations and modeling

Abstract

The shape of depth-limited breaking-wave overturns is important for turbulence injection, bubble entrainment and sediment suspension. Wave overturns can be large or small relative to wave height and have varying aspect ratios. Traditionally overturn shape is thought to be related to the incident waves and the bathymetry through parameters such as the Iribarren number. Cross-shore wind direction (offshore/onshore) and magnitude affect laboratory breakpoint location Xbp. In the surfing community, offshore or onshore wind are well understood to affect how the wave overturns, with offshore wind being conducive to large overturns good for "tube riding". However, wind effects on overturning wave shape were previously unstudied scientifically. Here, I report on field-scale experiments at the Surf Ranch wave basin with fixed bathymetry and ≈ 2.25 m shoaling solitons with small height variations propagating at C = 6.7 m s−1. Observed non-dimensional cross-wave wind Uw was onshore and offshore, varying realistically (−1.2 < Uw/C < 0.7). Georectified images, a wave staff, and lidar are used to estimate Xbp, wave height to water depth ratio (H/h), overturn area A and aspect ratio for 22 waves. The non-dimensionalized Xbp was inversely related to Uw/C. The non-dimensional overturn area and aspect ratio also were inversely related to Uw/C, with smaller and narrower overturns for increasing onshore wind and larger more circular overturns for offshore wind. In the second half of the talk, I will transition to our efforts to model wind effects on overturning wave shape using the 2-phase CFD model basilisk. The model is set up in a nondimensional domain analogous to that of the surf ranch with a soliton initial condition. Varying onshore and offshore wind is spun up from a prior simulation and used as in initial conditon. We examine wave overturn properties as a function of wind speed and direction, similar to the field studies. The model allows us to gain insight into the air-sea interaction mechanism that leads to these overturn shape changes. Lastly, I argue that through wave-overturning impacts on turbulence and sediment suspension, coastal wind variations could be relevant for near-shore morphology.