American Geophysical Union Fall Meeting, 1999:

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Modeling Rip Channel Development

A.B. Murray and C. Guillon

Channels can cause the discrete, isolated zones of offshore-directed flow called rip currents. In a channel, which can be a gap in an alongshore bar or a deeper area extending across an otherwise planar beach, the water carried onshore by the waves can most easily escape the surf zone. Conversely, observations suggest that rip currents can cause the development of channels, an aspect of the problem that has been less well studied. Strong, narrow rip currents can occur on approximately planar beaches, and under some wave conditions, narrow channels subsequently appear. We are investigating channel initiation and development, using a model in which rip currents can occur either in the presence or absence of channels. The hydrodynamic model can, for the present purpose, be considered as a black box that produces rip currents with basic characteristics that match those observed on natural beaches. By linking to this model a commonly used equations for sediment transport in the presence of waves, currents and bed slopes, developed originally by Bagnold, and modified by Bailard and others, we have examined whether the processes represented by this relationship can explain/predict the formation of rip channels.

In numerical experiments involving initially planar beaches, the pattern of sediment transport predicted by this relationship does not produce rip channels. Analysis shows that, according to these equations, where alongshore flow converges to produce divergence in an offshore flow on an offshore-sloping beach, the alongshore convergence of sediment flux can balance the cross-shore flux divergence. Analytically, inclusion of an incipient channel increases the alongshore convergence relative to the cross-shore divergence, further inhibiting channel development. Observations of natural rip currents suggest that non-local effects, not included in the local application of these equations, may be important. Strong rip currents produce plumes that extend outside the surf zone, into a relatively low energy environment. These plumes can remain visible partly because of the sediment suspended near the surface-sediment that was most likely suspended throughout the water column by turbulence generated by wave breaking and the strong current within the surf zone, not by the local conditions. Incorporating into the model a relationship between the current velocity and the degree to which the suspended load responds immediately to downstream changes in a strong current does produce rip channels.

In experiments starting with an alongshore bar, with dimensions approximating measurements made at the Army Corps of Engineers Field Research Facility at Duck, NC, gaps in the bar develop even in the absence of non-local effects. In this case, as a rip current moves from the trough to the crest of the bar, the decrease in depth over the onshore-sloping bed enhances cross-shore divergence of sediment flux (the reverse of the effect on a planar beach, where depth increases offshore). Across the crest of the bar, the change in bed slope tends to give divergence in cross-shore sediment flux. The channels and related features that result after days of simulated time match qualitative observations of rip channels on natural barred beaches.

Supported by the Andrew W. Mellon Foundation.