Critical Shields values in coarse-bedded steep streams

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Abstract

[1] Critical Shields values ( math formula) suitable for specific applications are back-calculated from representative bed load samples in mountain streams and a flow competence/critical flow approach. The general increase of math formula (for the bed D50 size) as well as math formula and math formula (for the bed D16 and D84 sizes) with stream gradient Sx and also the stratification of math formula by relative flow depth and relative roughness are confirmed. Critical Shields values math formula are shown to exceed math formula by about threefold, while those for math formula are nearly half of math formula. However, it remains unclear to what extent physical processes or numerical artifacts contribute to determining critical Shields values. Critical bankfull Shields values ( math formula) back-computed from the average largest particles mobile at bankfull flow DBmax,bf approach math formula at steep gradients and math formula at low gradients and therefore increase very steeply with Sx. The relation math formula is stratified by bed stability (D50/DBmax,bf) and predictable if bed stability can be field categorized. Noncritical Shields values ( math formula) computed from bankfull flow depth and the D50 size differ from math formula and math formula. Only in bankfull mobile streams where D50/DBmax = 1 can τ*cbf, math formula, and math formula be used interchangeably. In highly mobile streams, substituting math formula by math formula overpredicts the DBmax,bf size by up to fivefold and underpredicts DBmax,bf by the same amount in highly stable streams. A value of 0.03 is appropriate for math formula only on low stability beds with Sx ≅ 0.01, but overpredicts DBmax,bf by 30-fold on highly stable beds with Sx ≅ 0.1. Differences in field and computational methods also affect critical Shields values.

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