Dunes In Flows Research Articles

A note on megaripples in the surf zone: evidence for their relation to steady flow dunes

Megaripples in the combined flow environment of the nearshore are proposed to behave like dunes or large ripples in rivers, tidal estuaries, and deserts. Their profile basically is symmetric and thus significantly different from the traditional asymmetric triangular features observed in steady flows. Similarly their planform often exhibits little directionality, unlike crescentic or lunate steady flow dunes that point in the downstream direction. These characteristics are the result of complex combined flows in the nearshore, including oscillatory flows, wave skewness, and steady currents (undertow, rips and alongshore flows). Recent observations of megaripples in the nearshore suggest that they occur frequently. However, they are rarely considered in studies of flow resistance or sediment transport. In addition, megaripples are thought to be the source of hummocky cross-stratification in sedimentary sequences and are generally attributed to storm waves on inner continental shelves. However, observations show that they also exist inside the surf zone and under lower-energy conditions. A better understanding of their dynamics and thus their occurrence and characteristics would improve the understanding of nearshore wave and circulation dynamics, sediment transport, large-scale morphodynamics, and the resulting sedimentary sequences. It is hypothesized that megaripples in the nearshore are dynamically similar to steady flow features, which are observed in rivers, estuaries and deserts and have been studied in much more detail.

The stability of ripples and dunes

The stability of ripples and dunes in flows of finite depth are investigated for the linear, small-amplitude case and also for the more realistic finite-amplitude situation. For the former perturbation approach is taken, leading to the general result that, without the inclusion of some lag between the sediment transport rate and the boundary shear stress, all wavelengths are unstable. This results from an upstream shift in the stress relative to the bedform, leading to deposition at the crest. By assuming a realistic lag based on the mechanics of bed load transport, a fastest growing wave at ripple-scales results: similarly, if suspended sediment were included, longer lags result and a dune-scale instability could result. Some investigators have invoked a gravitational effect, arguing that sediment transport is enhanced downslope and retarded upslope. This creates an effective downstream shift in the transport rate and some argue that both ripple and dune instabilities result, however, a realistic evaluation of this effect shows that it is much too small to be effective. Naturally occurring bedforms are of finite-amplitude; typically they are asymmetrical, often causing flow separation. This highly non-linear process creates a vastly different flow environment from that assumed in the initial stability case. A wake-like flow structure develops downstream of the separation zone and an internal boundary layer develops beneath. The complex interaction of these two different flow regimes as well as the effect induced by the topography of the bedform itself, produces a shear stress field that dictates the stability of the feature itself.