Collaborative Research: Shoreward Sediment Transport: Combining Highly Resolved Field Observations and Modeling to Examine Fundamental Processes Controlling Shoreline Adjustment

Oregon State University, Corvallis OR

Investigators

Abstract

Shoreward sediment transport is responsible for building and maintaining shorelines on wave-dominated coasts. Due to their oscillatory motion waves move sediments back and forth and the net transport, averaging over many waves and conditions, is responsible for accretion or erosion of beaches. Shoreward sediment transport is critical to understanding how beaches rebuild in response to rising sea levels, storms, and the construction of engineered coastal structures. Improved understanding of shoreward transport, the focus of this study, will enable better model parameterizations to predict this response and forecast the potential recovery of the coastline following a storm. The fundamental processes underlying wave-driven shoreward transport are among the least understood and most elusive processes in nearshore oceanography. Numerous competing theoretical mechanisms are likely to contribute, but there is a critical lack of validation data to determine their relative importance. The proposed work is an interdisciplinary collaboration that brings together oceanography, fluid dynamics, and engineering to address the scientific fundamentals of a problem with significant societal implications. Field measurements of turbulence and sediment will be carried out on a natural beach. The experimental data will be combined with advance numerical modelling techniques that will allow identification of the most important wave-driven shoreward mechanisms on a natural beach. Apart for the scientific and engineering values of the study, the project will provide contribute to the training PhD students and it will expose undergraduate students into field research. Public outreach will be conducted through the development of short documentary-style video. Modeling shoreward transport requires representation of multiple transport processes that act across a broad range of scales, from small-scale turbulence, sediment mobilization, and fluid-sediment feedbacks, to wavelength-scale nonlinear wave processes. In the past, direct investigation of such processes has only been possible in laboratory studies, which have uncertain applicability to real nearshore environments due to scaling effects and simplified dynamics. Full-scale experiments, on the other hand, represent realistic dynamics but often sacrifice resolution, e.g. by only measuring average transport over many wave periods, leaving uncertainty as to what processes are contributing to transport. The key components of this study are the utilization of a purpose-built, broadband pulse-coherent acoustic Doppler profiler (MFDop), and detailed process models, which together resolve the full suite of transport mechanisms that have been hypothesized to occur on natural beaches. The MFDop system is a profiler, capable of measuring sediment concentration and velocity fluctuations at high frequency (50 Hz) and with a vertical resolution of 4 mm capable of resolving the wave boundary layer that extends a few cm above the seabed. The MFDop system and supporting instrumentation will be deployed on a natural beach in Oregon to measure the vertical structure of boundary layer velocities and sediment suspension as a function of the free-stream velocity and pressure gradient. The data collected will guide modeling experiments simulating flow and sediment variability at wavelength and cm-scales (Reynolds-averaged 1-D vertical simulations) and at sub-cm to sub-mm scales capable to resolve boundary layer eddies and sediment particles (Large eddy simulation (LES) modelling). A nested approach will be developed aiming at adding wavelength-effects into LES modelling while avoiding computational limitation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.