Timothy R. Nelson

Temporal and spatial evolution of wave-induced ripple geometry: Regular versus irregular ripples

Concurrent observations of inner shelf near bed hydrodynamics and acoustic imagery of the seabed are used to relate wave-induced ripple geometry (wavelength and orientation) to near bed directional wave velocities. The observations were collected on the continental shelf of the South Atlantic Bight at water depths of 9.5 and 30 m off the coasts of South Carolina (median size 177 µm) and Georgia (388 µm), respectively. 2-D spectral analysis techniques are performed on the imagery to automate detection of ripple wavelength, orientation, and irregularity. Our analysis shows that ripple irregularity is a time-dependent process dependent on magnitude, direction, and duration of wave forcing. During energetic events, ripple geometry changes rapidly and the ripples align with the main wave direction. During periods of low energy, close to the critical conditions for initiation of sediment motion, ripple evolution occurs at a much slower rate often leading to irregularities such as terminations and bifurcations along the ripple crest. Under constantly changing wave direction, the rippled bed becomes highly disorganized. Six types of ripples are defined based on newly developed irregularity parameters: linear, bifurcating-linear, linear-bifurcating, bifurcating and cross, irregular, and disorganized beds. Ripple irregularity depends on the time history of the bed; ripples remain irregular until their wavelength and orientation attain a nearly equilibrium geometry. The observations collected provide significant information on the response of the seabed to wave forcing and identify processes that should be reproduced by any time-dependent ripple prediction model. Ripple irregularity can only be predicted using such time-dependent models.

A spectral model for estimating temporal and spatial evolution of rippled seabeds

Existing time variable ripple prediction models have focused on ripple wavelength and height yet have not accounted for the effect of wave directionality on ripple evolution. These models use an equilibrium ripple sub-module that provides the target geometry of the seabed. Temporal variability of the hydrodynamic conditions cause ripple geometry to lag behind and not always agree with that predicted by equilibrium models. In this contribution, previous work is extended through the development of a new 2-D, time variable, spectral ripple model that allows for the estimation of ripple wavelength, height, and orientation. Since ripple irregularity is associated with directionality, the new model also has the ability to predict the irregularity of the rippled seabed. A comparison of the 2-D and existing time-dependent models is carried out using both synthetic and field ripple data that consist of time series representing a variety of wave-forcing conditions. Under strong wave forcing with no orientation changes, the 2-D model predicts ripple wavelength and orientations that agree with those of existing models. During periods of change in wave directions, the 2-D model predicts a significantly smaller ripple height during ripple reorientation. Under weaker energy flows, the 2-D model yields improved estimates especially for relict ripple geometry. In addition, the 2-D model’s spectrum yields a prediction of the seabed irregularity.