Intermediate Scale Coastal Behaviour: Measurement, Modelling And Prediction

LONG-TERM GOAL: Our overall goal is to achieve a better understanding and better predictions of coastal behaviour at intermediate (event/season/year/decade) scales. We aim to bring together researchers from Europe and North America to gain the best possible benefit from developments in field observation, theory and numerical modelling.Award #: N00014-97-1-079

Direct estimation of the Reynolds stress vertical structure in the nearshore

Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 24 (2007): 102-116, doi:10.1175/JTECH1953.1.Measurements of the vertical Reynolds stress components in the wave-dominated nearshore are required to diagnose momentum and turbulence dynamics. Removing wave bias from Reynolds stress estimates is critical to a successful diagnosis. Here two existing Reynolds stress estimation methods (those of Trowbridge, and Shaw and Trowbridge) for wave-dominated environments and an extended method (FW) that is a combination of the two are tested with a vertical array of three current meters deployed in 3.2-m water depth off an ocean beach. During the 175-h-long experiment the instruments were seaward of the surfzone and the alongshore current was wind driven. Intercomparison of Reynolds stress methods reveals that the Trowbridge method is wave bias dominated. Tests of the integrated cospectra are used to reject bad Reynolds stress estimates, and the Shaw and Trowbridge estimates are rejected more often than FW estimates. With the FW method, wave bias remains apparent in the cross-shore component of the Reynolds stress. However, the alongshore component of Reynolds stress measured at the three current meters are related to each other with a vertically uniform first EOF containing 73% of the variance, indicating the presence of a constant stress layer. This is the first time the vertical structure of Reynolds stress has been measured in a wave-dominated environment. The Reynolds stress is, albeit weakly, related to the wind stress and a parameterized bottom stress. Using derived wave bias and bottom stress parameterizations, the effect of wave bias on Reynolds stress estimates is shown to be weaker for more typical surfzone conditions (with both stronger waves and currents than those observed here).Funded by NSF, ONR, and NOPP

Energy dissipation in the inner surf zone: new insights from LiDAR-based roller geometry measurements

The spatial and temporal variation of energy dissipation rates in breaking waves controls the mean circulation of the surf zone. As this circulation plays an important role in the morphodynamics of beaches, it is vital to develop better understanding of the energy dissipation processes in breaking and broken waves. In this paper, we present the first direct field measurements of roller geometry extracted from a LiDAR data set of broken waves to obtain new insights into wave energy dissipation in the inner surf zone. We use a roller model to show that most existing roller area formulations in the literature lead to considerable overestimation of the wave energy dissipation, which is found to be close to, but smaller than, the energy dissipation in a hydraulic jump of the same height. The role of the roller density is also investigated, and we propose that it should be incorporated into modified roller area formulations until better knowledge of the roller area and its link with the mean roller density is acquired. Finally, using previously published results from deepwater wave breaking studies, we propose a scaling law for energy dissipation in the inner surf zone, which achieves satisfactory results at both the time‐averaged and wave‐by‐wave scales

Practical sand transport formula for non-breaking waves and currents

Open Access funded by Engineering and Physical Sciences Research Council Under a Creative Commons license Acknowledgements This work is part of the SANTOSS project (‘SANd Transport in OScillatory flows in the Sheet-flow regime’) funded by the UK's EPSRC (GR/T28089/01) and STW in The Netherlands (TCB.6586). JW acknowledges Deltares strategic research funding under project number 1202359.09. Richard Soulsby is gratefully acknowledged for valuable discussions and feedback on the formula during the SANTOSS project.Peer reviewedPostprin

Intermediate Scale Coastal Behaviour: Measurement, Modelling and Prediction

N00014-97-1-079

Composite modelling of the interactions between beaches and structures

An overview of Composite Modelling (CM) is presented, as elaborated in the EU/HYDRALAB joint research project Composite Modelling of the Interactions Between Beaches and Structures. An ntroduction and are view of the main literature on CM in the hydraulic community are given. In Section 3, the case studies of CM of the seven partners participating in this project are discussed. The focus is on the methodologies used and their impact on the modeling approach, rather than the results of the experiment sperse. A further section presents reflections on key elements in CM, as they emerged in the various case studies. The related subject of Good Modelling Practice is summarized in Section5. Then guidelines are given on how to decide if CM may be beneficial, and how to set up a CM experiment. It is concluded that CM in the hydraulic community is still in its infancy but involves challenging research with significant potential

Mechanisms controlling crescentic bar amplitude

The formation of crescentic bars from self-organization of an initially straight shore-parallel bar for shore-normal incident waves is simulated with a two-dimensional horizontal morphodynamical model. The aim is to investigate the mechanisms behind the saturation process defined as the transition between the linear regime (maximum and constant growth of the crescentic pattern) and the saturated state (negligible growth). The global properties of the morphodynamical patterns over the whole computational domain are studied (“global analysis”). In particular, consideration of the balance of the potential energy of the emerging bar gives its growth rate from the difference between a production term (related to the positive feedback leading to the instability) and a damping term (from the gravity-driven downslope transport). The production is approximately proportional to the average over the domain of the cross-shore flow velocity times the bed level perturbation. The damping is essential for the onset of the saturation, but it remains constant while the production decreases. Thus, it is notable that the saturation occurs because of a weakening of the instability mechanism rather than an increase of the damping. A reason for the saturation of the crescentic bar growth is the change in bar shape from its initial stage rather than the growth in amplitude itself. This change is mainly characterized by the narrowing of the rip channels, the onshore migration of the crests, and the change in the mean beach profile due to alongshore variability. These properties agree with observations of mature rip channel systems in nature.Postprint (published version

Toward modeling turbulent suspension of sand in the nearshore

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C06018, doi:10.1029/2003JC002240.We present two depth- and phase-resolving models, based on single- and two-phase approaches for suspended sediment transport under water waves. Both models are the extension of a wave hydrodynamic model Cornell Breaking Wave and Structure (COBRAS). In the two-phase approach, dilute two-phase mass and momentum equations are calculated along with a fluid turbulence closure based on balance equations for the fluid turbulence kinetic energy k f and its dissipation rate ε f . In the single-phase approach the fluid flow is described by the Reynolds-Averaged Navier-Stokes equations, while the sediment concentration is calculated by an advection-diffusion equation for the conservation of sediment mass. The fluid turbulence is calculated by k f -ε f equations that incorporate the essential influence of sediment, which can also be consistently deduced from the two-phase theory. By adopting a commonly used sediment flux boundary condition near the bed the proposed models are tested against laboratory measurements of suspended sediment under nonbreaking skewed water waves and shoaling broken waves. Although the models predict wave-averaged sediment concentrations reasonably well, the corresponding time histories of instantaneous sediment concentration are less accurate. We demonstrate that this is due to the uncertainties in the near-bed sediment boundary conditions. In addition, we show that under breaking waves the near-bed sediment pickup cannot be solely parameterized by the bottom friction, suggesting that other effects may also influence the near-bed sediment boundary conditions.This research has been supported by NSF grants CTS-0000675 and OCE-0095834 to Cornell University. This paper is also a resulting product [R/CCP-9] funded under award NA16RG1645 from the National Sea Grant College Program of U.S. Department of Commerce’s National Oceanic and Atmospheric Administration to the Research Foundation of State University of New York on behalf of New York Sea Grant. The financial supports for Tian-Jian Hsu provided by Department of Civil and Environmental Engineering, University of Delaware, and the Coastal Ocean Institute of Woods Hole Oceanographic Institution are also acknowledged