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Water Resources Research

Critical Shields values in coarse-bedded steep streams

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Abstract

[1] Critical Shields values ( inline image) 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 inline image (for the bed D50 size) as well as inline image and inline image (for the bed D16 and D84 sizes) with stream gradient Sx and also the stratification of inline image by relative flow depth and relative roughness are confirmed. Critical Shields values inline image are shown to exceed inline image by about threefold, while those for inline image are nearly half of inline image. However, it remains unclear to what extent physical processes or numerical artifacts contribute to determining critical Shields values. Critical bankfull Shields values ( inline image) back-computed from the average largest particles mobile at bankfull flow DBmax,bf approach inline image at steep gradients and inline image at low gradients and therefore increase very steeply with Sx. The relation inline image is stratified by bed stability (D50/DBmax,bf) and predictable if bed stability can be field categorized. Noncritical Shields values ( inline image) computed from bankfull flow depth and the D50 size differ from inline image and inline image. Only in bankfull mobile streams where D50/DBmax = 1 can τ*cbf, inline image, and inline image be used interchangeably. In highly mobile streams, substituting inline image by inline image 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 inline image 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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