Critical Shields values in coarse‐bedded steep streams

Critical Shields values ( τ c * ) 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 ofτ c 50 *(for the bed D 50 size) as well asτ c 16 *andτ c 84 *(for the bed D 16 and D 84 sizes) with stream gradient S x and also the stratification ofτ c 50 *by relative flow depth and relative roughness are confirmed. Critical Shields valuesτ c 16 *are shown to exceedτ c 50 *by about threefold, while those forτ c 84 *are nearly half ofτ c 50 * . However, it remains unclear to what extent physical processes or numerical artifacts contribute to determining critical Shields values. Critical bankfull Shields values ( τ c b f * ) back‐computed from the average largest particles mobile at bankfull flow D Bmax , bf approachτ c 1 6 *at steep gradients andτ c 84 *at low gradients and therefore increase very steeply with S x . The relationτ c b f * = f ( S x ) is stratified by bed stability ( D 50 / D Bmax , bf ) and predictable if bed stability can be field categorized. Noncritical Shields values ( τ b f 50 * ) computed from bankfull flow depth and the D 50 size differ fromτ c 50 *andτ c b f * . Only in bankfull mobile streams where D 50 / D Bmax  =  1 can τ * cbf ,τ c 50 * , andτ b f 50 *be used interchangeably. In highly mobile streams, substitutingτ c b f *byτ b f 50 *overpredicts the D Bmax , bf size by up to fivefold and underpredicts D Bmax , bf by the same amount in highly stable streams. A value of 0.03 is appropriate forτ c b f *only on low stability beds with S x ≅ 0.01, but overpredicts D Bmax , bf by 30‐fold on highly stable beds with S x ≅ 0.1. Differences in field and computational methods also affect critical Shields values.