# GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 343-6
Presentation Time: 9:00 AM-6:30 PM

# ESTIMATE OF GRAVEL TRANSPORT RATES AT BANKFULL FLOW IN MOUNTAIN STREAMS

BUNTE, Kristin, SWINGLE, Kurt W., ETTEMA, Robert, ABT, Steven R. and CENDERELLI, Daniel A., Dept. of Civil and Environmental Engineering, Colorado State University, Engineering Research Center, 1320 Campus Delivery, Fort Collins, CO 80523, kbunte@engr.colostate.edu

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 QB = a Q b (Eq. 1) were fitted to the sampled transport rates QB (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 QB,bf, and dividing by the bankfull width yields unit bankfull flow qbf and the unit bankfull transport rate qB,bf.

From a log-log scatterplot of worldwide data of qB,bf vs. A, a positive, slightly convex trend emerges when data are limited to central Rocky Mountain streams. The trend straightens when relating qB,bf to a modified unit stream power expression ω’ = ρ · qbf · S0.5 · %Dsub<8 that integrates the percentage of subsurface sediment < 8 mm (%Dsub<8). For most Rocky Mountain streams, measured data of qB,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 qB,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 qB,bf estimates.