Critical Shields values in coarse‐bedded steep streams

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