Channel Cross-sectional Shape Research Articles

Lithologic control of bedrock meander dimensions in the appalachian valley and ridge province: A comment on a comment

AbstractAbrahams' comment relates meander length to channel cross‐section shape and recurrence interval so that meander length can be both directly and inversely proportional to rock resistance. This reply notes that either meander length is directly proportional to rock resistance or it is not; one cannot have it both ways. Many Appalachian Valley and Ridge bedrock meanders are shown to be the same size as alluvial meanders, and appear to be somewhat underfit. A hypothesis is proposed where modest discharge increases may have accelerated bedrock meander cutting although present streams remain capable of slowly cutting the meanders.

Paleohydrology and Paleomorphology of a Carboniferous, Meandering, Fluvial Sandstone

ABSTRACT Analysis of an abandoned meander preserved in toto in the Harold sandstone of the Breathitt Formation allows 1) accurate determination of primary input variables in paleomorphology calculations, namely channel bankfull width, depth, and cross-sectional area, and 2) evaluation of established methodologies. Bankfull depth is equal to the thickness of the entire fining upward cycle and to the thickness of point bar accretion surfaces, within measurement error. Channel width is equal to 1.4 times the horizontal extent of point bar accretion surfaces, slightly less than the 1.5 value commonly used. Channel cross-sectional shape is closely approximated by a trapezoid. The continuity equation, Q = A V is used to calculate an average bankfull discharge, Q. Bankfull cross-sectional area, A, is accurately reconstructed from an abandoned meander channel and V is calculated from Middleton's (1976) ''criterion for suspension. Average flow velocity, V, estimated from this method is similar to flow velocity estimated from initiation of sediment motion curves and graphic relationships among grain size, bedforms and flow velocity. Close agreement exists among values of Q calculated from the continuity equation and several independent regression equations developed by Schumm (1972) for modern, fluvial environments. Estimates of stream length and drainage basin size are consistent with paleogeographic reconstructions of the southern Appalachians during middle ennsylvanian time. When compared to measured outcrop parameters empirical equations which employ M, the sediment load parameter, are consistently better estimators of paleomorphology and paleohydrology than empirical equations without M, provided that diagenesis does not obscure the original percent of silt-clay in the channel alluvium.

Meander spectra of the Angabunga River

Three sets of air photos spanning twenty-four years are used in an analysis of the dominant modes of meandering of the Angabunga River, in central Papua, using the method of the power spectrum. Spectra are constructed for successive reaches of the stream based on the auto-correlation of the direction of flow measured at equi-spaced points on the talweg of the channel. Such spectra display the meander intensity associated with each band of frequency, where frequency is the reciprocal of meander-wavelength, measured along the talweg. It is shown that the subjectively chosen characteristic meander wavelength used by previous workers is a poor indicator of the dominant frequencies of oscillation. All spectra show a number of stable peaks of meander intensity, of which the dominant one corresponds to a wavelength of the order of 90 times the root of the bankfull discharge, that is, about 50% longer than the expected “characteristic” wavelength. The most prominent spectral peaks are more stable in a downstream direction than are other parameters, such as channel width. The relative intensities of the various peaks, however, are subject to rapid change associated with change in channel cross-sectional shape. Parts of the channel with a lower width-depth ratio have relatively high intensity associated with high-frequency peaks, a fact which strongly influences the subjective judgement of meander wavelength.