Predictions of Bedforms in Steady, Tidal and Oscillatory Flows

Bedforms are ubiquitous in unconsolidated sediments. They range in size from small orbital ripples (l ~5-50 cm) to megaripples (l ~1-5 m) to large dunes (l ~10-100 m). Bedforms are important because they affect sediment transport, flow energy dissipation, and larger-scale hydro- and morpho-dynamics. Bedforms in different environments (eg, deserts, rivers and oceans) are thought to be dynamically similar, therefore modeling approaches from one environment can be used to predicted features in another (Gallagher 2011). Here, a self-organization model is used to simulate the formation and development of bedforms in the combined flows of the surf zone, tidal inlets and river mouths. Sediment flux is determined from combined wave and current flows using different formulations, but, interestingly, the transport formulation has little effect on model results. Random bed irregularities, either imposed or resulting from small variations in the flow representing turbulence, are seeds for bedform development. Feedback between the bed and the flow in the form of a shadow zone downstream of a bedform and increasing flow velocity with elevation over bedform crests alter the transport such that organized bedforms emerge. The model has been used to predict surf zone megaripples and bedforms in tidal inlets and river mouths.

Many bedform models (e.g., Hulscher et al. 1996, Nielsen 1981, Clifton 1976, Wiberg and Harris 1994), predict specific bedform characteristics for a given flow condition. However, many observations report multiple scales existing simultaneously. A recent study by Lefevbre et al (2013) found that the boundary layer thickness and resulting bedform-induced roughness was different for single and multiple bedform fields. The present model predicts multiple bedform scales. In this study, predictions of different time and length scales of bedform evolution and roughness, as a function of flow magnitude, direction and variability are being examined.