Evolution of Small Scale Seafloor Topography and Sediment Transport under Energetic Waves: From ripples to sheet flow

University Of Delaware, Newark DE

Investigators

Abstract

Coastal communities throughout the world have experienced exceptionally severe shoreline retreat in the past decade due to accelerated sea-level rise and increased storm intensity. A comprehensive understanding of the exchange of sediments between inner-shelf and surf zone, which primarily occurs through migration of bedforms, can significantly improve the existing depth-of-closure concept and enhance our capability in predicting coastal response. Bedform migration is the major mode of sediment transport between the inner-shelf and the surf zone. The geometry of these bedforms also determines hydrodynamic dissipation in the inner shelf and its existence and evolution are key information for an accurate prediction of waves and currents. In the past two decades an extensive amount of work has been conducted in the surf zone to understand the dynamics of offshore and onshore sand bar migration over short time scales. In surf zone bed elevation changes are large (1 to 5 m), while the across-shore scale of the surf zone is relatively small (10's of meters to at most 1 to 2 km). On the inner shelf across shore scales are much larger as sediment can be mobilized up to 10's of kilometers offshore during energetic waves, yet large scale elevation changes due to storms are usually less than 10 cm. Quantifying the sediment transport between these two regimes is essential to predicting long term shore line retreat due to sea-level rise. If there is onshore transport across this transition, shorelines may retreat slower than predicted by current models. This study tightly integrates existing field observational data with numerical simulations to investigate three key hypotheses on the evolution of bedforms and transition to sheet flows. The numerical model, SedFoam, adopted in this study has already been disseminated as open-source model through the Community Surface Dynamics and Systems (CSDMS) model repository. This project will significantly enhance this open-source model with turbulence-resolving capability for ripple simulation and the new model (SedLESFoam) will also be disseminated as open-source via CSDMS. The project will also facilitate close international collaboration with scientists at the Grenoble Institute of Technology (France) on field/laboratory data analysis and co-development of SedLESFoam with alternative closure schemes. A Ph.D. student will receive broad training in computational fluid dynamics and field data analysis and an early career postdoc researcher will further his training on nearshore modeling. Finally, an undergraduate student will be recruited to create a dual-sphere model for natural sand grain and develop a hand-on landslide experiment for a participating Engineering Cool Stuffs Camp for middle school students. With anticipated increasing rates of sea level rise in the next century a comprehensive understanding of cross-shelf sediment transport processes of the inner shelf will become important to accurately predict coastal response. The primary goal of this study is to investigate the dominant mechanisms driving the migration and evolution of bedfroms and the transition to sheet flows. A newly developed two-phase model (SedlLESFoam) will be used to carry out simulations guided by comprehensive analysis of field observational data. Research outcomes will examine the following three hypotheses. First, transitions between bedform scaling regimes (e.g. orbital vs an-orbital) are determined by the relative amounts of near-bed load and suspended load transport within a wave cycle. The ratio of transport via suspended load to near-bed load is the key parameter for describing the evolution and migration of bedforms under skewed wave, streaming and combined wave and current forcing. Second, bedform migration in coarse grained environments with orbital scale ripples is typically onshore irrespective of wave velocity skewness and asymmetry, which can be directed onshore or off-shore, indicting either onshore directed wave-forced bottom boundary layer streaming is an important mechanism for forcing ripple migration, or a spectral decomposition of bottom stress is required, whereby low frequency motions have a lower friction factor than high frequency motions. Third, in energetic conditions approaching sheet flow with bedforms still present and in sheet flow conditions without bedforms, unique combination of forcing can drive momentary bed failure which further leads to large transport rate and rapid migration/evolution of bedforms. These transport processes cannot be solely parameterized by the conventional shear-stress-based approach. A tightly integrated research effort of analysis of previously collected field data and numerical simulation will be implemented to understand evolution and transport from bedforms to sheet flows. The field data sets encompass a total of nearly 5-month duration of rotary imaging sonar to measure bedfoms, and hydrodynamic forcing. More recent data sets include high vertical resolution flow and suspended sediment fields obtained by convergent beam Pulse Coherent Doppler Profiler. The profiler data contains some of the first in-situ field measurements of wave boundary layer streaming over orbital scale ripples, in addition to resolution of vortex ejection eddies from the ripples. Re-analysis of these field data will be carried out and produce segment of events relevant to the proposed research questions. A novel turbulence-resolving (or turbulence-averaged) Eulerian two-phase sediment transport model without a priori assumption of bedload and suspended load will be validated and its high resolution 3-dimensional and 2-dimensional flow fields will be used to interpret events observed in the field. Findings will then be used to inform the creation of new parameterizations that can be adopted by coastal evolution models.