Meandering: fluvial versus tidal. (Invited)
Abstract
Tidal meanders (Marani et al, Water Resour Res, 2002) display similarities as well as important differences from fluvial meanders (Seminara, J Fluid Mech, 2006). Like fluvial meanders they have characteristic wavelengths scaling with channel width: this is why the convergent character of tidal channels leads to meander wavelengths decaying landward. Unlike fluvial meanders, the typical curvature spectra of tidal meanders contain even harmonics: hence, meander skewing does non display any distinct correlation with the flow direction and the known Kinoshita curve, which approximates the shape of fluvial meanders, is not appropriate to tidal meanders. Additional constraints are brought up by the spatial gradients of the basic bed profile connected to the finite length of tidal channels at equilibrium. In fact, it has been theoretically established (Schuttelaars and De Swart, Eur J Mech, B/Fluids, 1996, Seminara et al, J Fluid Mech submitted, 2009) and confirmed by controlled laboratory experiments (Tambroni et al., J Geoph Res, 2005) that tidal channels closed at one end and connected at the other end with a tidal sea, evolve towards an equilibrium configuration characterized by a ‘slow’ landward decay of the average flow depth. An equilibrium length of the channel is then determined by the formation of a shoreline. Channel curvature affects the lateral equilibrium topography and gives rise to a pattern of point bars and scour pools resembling that of fluvial channels. With some notable differences, though. In fact, Solari et al (J Fluid Mech, 2001) showed that long sequences of weakly sinuous identical meandering channels subject to a symmetrical tidal forcing develop a symmetrical bar-pool pattern with small symmetrical oscillations during the tidal cycle. However, in the laboratory investigations of Garotta et al. (Proceedings RCEM5,2007) the bar-pool pattern was somehow unexpected. In a first experiment, it was in phase with curvature only in the inner half of the channel, whereas the seaward pattern displayed deposition at the outer bends and scour at the inner bends, a pattern which would clearly be planimetrically unstable if the channel walls were erodible. In a second experiment, in the final stage, close to equilibrium, point bars were out of phase with respect to curvature throughout the whole channel. A possible explanation of this striking observation is that asymmetry of an observed pattern must be associated with either flood- or ebb- dominance of the basic flow field: some indication, in this respect, comes from the observation that the bar-pool pattern changed in time with the hydrodynamics as the average bed profile evolved towards equilibrium. A second key to be explored is the very nature of the observed bar-pool pattern, recalling that the relationship of tidal alternate (free) bars to point (forced) bars differs from its fluvial counterpart: tidal free bars are non migrating features at equilibrium (Seminara and Tubino, J Fluid Mech, 2001), bar migration arising from the role of overtides (Garotta et al, Phys. of Fluids, 2006). Distinguishing free from forced bars is then harder than in the fluvial case and the issue of their possible coexistence needs be revisited. Finally, the plan form evolution of tidal meanders is typically slower than in the fluvial case: not surprisingly, as sediment transport is very weak close to channel equilibrium.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.H44C..06S
- Keywords:
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- 1824 HYDROLOGY / Geomorphology: general;
- 1825 HYDROLOGY / Geomorphology: fluvial;
- 1856 HYDROLOGY / River channels;
- 4235 OCEANOGRAPHY: GENERAL / Estuarine processes