Critical Shields values in coarse-bedded steep streams

Critical Shields values (tau*(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 of tau*(c50) (for the bed D-50 size) as well as tau*(c16) and tau*(c84) (for the bed D-16 and D-84 sizes) with stream gradient S-x and also the stratification of tau*(c50) by relative flow depth and relative roughness are confirmed. Critical Shields values tau*(c16) are shown to exceed tau*(c50) by about threefold, while those for tau*(c84) are nearly half of tau*(c50). However, it remains unclear to what extent physical processes or numerical artifacts contribute to determining critical Shields values. Critical bankfull Shields values (tau*(cbf)) back-computed from the average largest particles mobile at bankfull flow D-Bmax,D-bf approach tau*(c16) at steep gradients and tau*(c84) at low gradients and therefore increase very steeply with S-x. The relation tau*(cbf) = f(S-x) is stratified by bed stability (D-50/D-Bmax,D-bf) and predictable if bed stability can be field categorized. Noncritical Shields values (tau*(bf50)) computed from bankfull flow depth and the D-50 size differ from tau*(c50) and tau*(cbf). Only in bankfull mobile streams where D-50/D-Bmax = 1 can tau*(cbf), tau*(c50), and tau*(bf50) be used interchangeably. In highly mobile streams, substituting tau*(cbf) by tau*(bf50) overpredicts the D-Bmax,D-bf size by up to fivefold and underpredicts D-Bmax,D-bf by the same amount in highly stable streams. A value of 0.03 is appropriate for tau*(cbf) only on low stability beds with S-x congruent to 0.01, but overpredicts D-Bmax,D-bf by 30-fold on highly stable beds with S-x congruent to 0.1. Differences in field and computational methods also affect critical Shields values.