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- Title
A Regime‐State Framework for Morphodynamic Modeling of Seabed Roughness.
- Authors
DuVal, Carter B.; Trembanis, Arthur C.; Miller, Douglas C.
- Abstract
Seabed roughness in the dynamic near‐shore environment is continuously modified by the interplay of physical and biological processes including bedform morphodynamics and bioturbation. We introduce a novel regime‐state framework to determine the dominant modifier of seabed roughness based on observed or estimated physical forcing, which are classified by critical Shields parameter or wave mobility thresholds. The regime‐state framework is applied to a set of field observations containing time‐lapse sonar imagery of ripple growth, decay, and bioturbation. The rate of ripple decay varies in both physical and biological processes. The decay rate is sensitive to oscillatory forcing magnitude and direction relative to relict ripple orientation and time‐evolving ripple height. We found two distinct episodes of ripples decayed by 20%–30% within 4–5 days. Ripple decay models and observations did not agree in rate of decay up to and following the same time period. By varying decay rate using a time‐dependent diffusion coefficient determined by the dominant decay mechanism, the regime‐state framework resulted in better agreement to field observations. Benthic megafauna are also shown to create seabed roughness, excavating dense fields (>1 pit/m2) of pits (≤84 cm in diameter) that introduce acoustic roughness comparable to 8–12 cm high ripples. The spatiotemporal variability of megafaunal pit formation suggests diffusion‐based bioturbation models are not adequate and megafaunal pits require additional parameterization to mechanistically model. The regime‐state framework presented in this study serves as a framework for future investigations and modeling of ripple morphodynamics, bioturbation, and roughness. Plain Language Summary: Surface features on the seabed, such as sand ripples, plants, and animals, increase the textural roughness of the seabed. Although small in size, ripples and biology change near‐seabed currents and the movement of sediment by physically altering the smooth, flat seabed. The ability to accurately predict seabed roughness is a continuing topic of research since these processes are difficult to measure accurately over different spatial and temporal scales. We introduce a new method to better understand the connection between seabed ripples, biological activity (bioturbation), and near‐seabed currents, and how the interaction of these processes affects seabed roughness. We show that ripple height is decreased by both weak, near‐seabed currents and bioturbation. The direction and intensity of near‐seabed currents relative to the ripple size and orientation may also affect the rate of ripple destruction in ways not previously considered. In addition, we observed larger animals, likely a species of stingray, digging shallow pits in the seafloor when foraging for food. Although bioturbation is often considered to create less roughness than ripples and other seabed features, our findings show that animal excavation pits may alter seabed roughness on scales similar to ripple bedforms. Key Points: A regime‐state framework for seabed roughness is established to distinguish ripple formation from periods of ripple decay by physical or bioturbation processesThe rate of ripple decay varies in both physical and biological processes and is sensitive to oscillatory forcing magnitude and direction relative to relict ripple orientation and time‐evolving ripple heightBiota act as both modifiers of existing features and creators of seabed roughness approaching magnitudes similar to ripple bedforms
- Subjects
OCEAN bottom; BIOTURBATION; SEDIMENTOLOGY; RIPPLES (Fluid dynamics); WATER waves
- Publication
Journal of Geophysical Research. Oceans, 2021, Vol 126, Issue 5, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016769