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• W2065866218 abstract "[1] Curran and Wilcock [2005] report an experimental study on the formation of step-pool patterns under active transport of all sediment sizes. They focus on the geometrical properties (step height, H, and step spacing, L) of the bed forms and argue against pattern regularity invoked by previous researchers. [2] Two major findings emerge from this work: (1) that the formation of a step appears to be equally likely at any location on the bed and, more important, (2) that there exists a so called “exclusion zone” larger than the scour hole (pool), next after each step, in which the presence of a new step must be excluded. [3] Although they measured all relevant flow parameters during experiment, Curran and Wilcock [2005] declare unable to find a significant relationship between them and step spacing. The aim of this comment is to provide such a relationship using a quite general result by Giménez-Curto and Corniero [2003]. [4] Giménez-Curto and Corniero Lera [1996] have studied the fluid flow over irregular fixed surfaces by introducing spatially averaged Reynolds equations which consider the variation of the fluid domain of averaging, thus allowing the treatment of the flow between bed features. They showed that besides the well known mean viscous and turbulent Reynolds stresses there exists a form induced stress, representing the mean momentum flux due to (non turbulent) flow disturbances introduced by boundary irregularities. This stress requires the existence of vorticity in the disturbed motion to be different from zero and becomes the prevailing stress in cases with bed irregularities of very high amplitude, provided that flow separates from bed features. This is called the jet regime. [8] This corresponds with the so called exclusion length observed by Curran and Wilcock [2005]. By applying equation (5), with f from (1), to their measurements we obtain values of Lmin between 18.2 cm and 25.4 cm, in good agreement with observation. [10] Therefore, as the ratio 〈H〉/〈L〉 grows approaching the static absolute limit, the gap between 〈H〉/〈L〉 and 〈H〉/Lmin narrows which means that the pattern becomes more regular and the crests more symmetrical. As a consequence, 〈L〉/Lmin must decrease with increasing values of the ratio 〈H〉/〈L〉. Figure 2 demonstrates this statement. Besides Curran and Wilcock's [2005] data we include in Figure 2 the observations by Zimmermann and Church [2001] on some natural streams in British Columbia and also those of Abrahams et al. [1995] in order to increase the range of data. It must be pointed out that in the latter experiments the static absolute limit of the bed form height that imposes the angle of repose is about two times the above given value. This is because the lee side of the bed form consists of narrow wooden weirs. Therefore we use 〈H〉/〈L〉 as the abscissa, instead of 〈H〉/〈L〉, for Abrahams et al.'s [1995] data, thus allowing an homogeneous comparison. [12] If, as showed by Curran and Wilcock [2005], the step spacing distribution would depend on only two parameters (the minimum and the mean spacing) our results (5) and (8) prove that the entire geometric properties of any step-pool configuration are given from the mean step amplitude 〈H〉 and the fundamental parameter f/ɛ of the flow that has generated the step-pool pattern. [13] Very recently, Allen and Hoffman [2005] have applied the concept of maximum steepness as given by Giménez-Curto and Corniero [2003] to relate remarkable giant wave ripples found at stratigraphic levels associated with the aftermath of the Neoproterozoic glaciation with the wave climate that could generate them. Their study led to the conclusion that this climatic transit was characterized by extreme meteorological conditions. [14] This idea can also be applied to infer flood conditions in steep natural streams from just the geometrical properties of the bed. As an example we consider the Vogelbach, a small mountain stream in Central Switzerland with a mean width of 5.5 m and mean bed slope tan β = 0.187, whose step-pool morphology has been studied in detail by Milzow [2004]. The mean height and spacing of the five step categories that he was able to identify from the step height spectrum can be seen in Table 1. By applying equation (8), together with (1), we calculate the parameter ɛ corresponding to the flow that formed each step-pool category. Then it is obtained the length scale Λ0 from which the flow velocity and depth are immediately calculated, thus allowing the estimation of the flow rate Q (see Table 1). [15] From a comparison with observed floods (the maximum measured flow rate since 1984 is 6.3 m3/s [Milzow, 2004]) we conclude that category 1, the smallest steps, are the consequence of annual adjusting, like the observations of Zimmermann and Church [2001] in British Columbia. The second category corresponds to the largest flood of the last two years, whereas the third category appears to have been formed by the largest flood of the last ten years. Category 4 represents steps larger than the overall mean, which have been formed by floods with recurrence interval over hundred years. The largest steps of category 5 are due to extremely large floods with very large recurrence intervals, perhaps millenniums. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article." @default.
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• W2065866218 date "2006-03-01" @default.
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• W2065866218 title "Comment on “Characteristic dimensions of the step-pool bed configuration: An experimental study” by Joanna C. Curran and Peter R. Wilcock" @default.
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• W2065866218 doi "https://doi.org/10.1029/2005wr004296" @default.
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