ESTIMATE OF GRAVEL TRANSPORT RATES AT BANKFULL FLOW IN MOUNTAIN STREAMS

From stream restoration to watershed management, many studies require knowledge of the bankfull gravel transport rate. Ryan (2007) showed that for a steep watershed in central Colorado bankfull bedload transport increased with watershed area A. Expanding on her findings, our study compiled a worldwide set of 55 gravel transport relations measured in mountain streams using samplers best suited for coarse beds: bedload traps, vortex, baskets, and pit-type samplers. Power functions in the form of Q_B = a Q ^b (Eq. 1) were fitted to the sampled transport rates Q_B (g/s) and the discharge Q (m³/s) at the time of sampling in each data set. a-coefficients and b-exponents are empirically determined. Solving Eq. 1 for bankfull flow yields the bankfull gravel transport rate Q_B,bf, and dividing by the bankfull width yields unit bankfull flow q_bf and the unit bankfull transport rate q_B,bf.

From a log-log scatterplot of worldwide data of q_B,bf vs. A, a positive, slightly convex trend emerges when data are limited to central Rocky Mountain streams. The trend straightens when relating q_B,bf to a modified unit stream power expression ω’ = ρ · q_bf · S^0.5 · %D_sub<8 that integrates the percentage of subsurface sediment < 8 mm (%D_sub<8). For most Rocky Mountain streams, measured data of q_B,bf vs. ω’ plot within an envelope two orders of magnitude wide. A few outlier data in Colorado are explained by disturbances that caused overly large (after a log jam burst) and overly small (upstream gravel entrapment) transport rates. The plot shows geographical variations: small step-pool streams in the forested Pacific Northwest transport little gravel per unit width at bankfull flow. The upward extension of the data envelope encloses Alpine step-pool and plane-bed streams where large basin portions are above tree line. However, most mountain torrents (wide gravel-cobble beds with an incised step-pool low-flow channel) have larger bankfull transport rates than Rocky Mountain streams.

The plotted relation of q_B,bf vs. ω’ facilitates an estimate of bankfull unit transport rates, especially if watershed sediment supply (e.g., hillslope-channel connection), active bank erosion, and downstream gravel conveyance potential (e.g., obstruction by beaver dams) are assessed from aerial photography. Stream type classification does not improve q_B,bf estimates.