Effective discharge in Rocky Mountain headwater streams

The anatomy of effective discharge: the dynamics of coarse sediment transport revealed using continuous bedload monitoring in a gravel‐bed river during a very wet year

“…For flows in excess of barfull the linear rating breaks down rapidly so that the overall power law in Fig. has a low exponent compared, for instance, with those in mountain streams (1.6, here, for impacts, versus 3.3–18.3 for load in Bunte et al ., ). Reasons for the breakdown in bedload rating in flows above barfull may relate to issues of local or catchment sediment supplies.…”

“…Such discordance is not uncommon (Pickup and Warner, ; Orndorff and Glonek, ; Soar and Thorne, ) and deconstructing the effective discharge process provides justification for the relatively low value. In summary, the result is likely attributable to: (i) the numerous bar‐topping and high in‐bank events during the wet year that maintained a comparatively loose bed material that was easily entrained (Reid et al ., ); (ii) an abundant supply of finer gravels capable of mobilisation in low flows coupled with evidence of periodic exhaustion during extreme flows that both reduce the rating exponent (Bunte et al ., ); (iii) a pool–riffle channel character and a relatively low magnitude of flow variability that are inherently suited to lower effective discharges and; (iv) an omission of procedural filters (e.g. insensitive instrumentation, long time period discharge interval) that might intrinsically promote a higher effective discharge.…”

Do the coarsest bed fractions and stream power record contemporary trends in steep headwater channels?

“…The coarsest fraction is responsible for the origin of stepped-bed channel morphology and in this way acts as an additional important roughness element that dissipates the flow energy even during high water stages (Chiari and Rickenmann, 2011;Comiti and Mao, 2012). In contrast to alluvial rivers where bankfull discharge is understood as channel-forming and is the most-effective discharge, confined, steep boulder-bed channels that are usually under sediment supplylimited conditions shape their morphology as a consequence of higher magnitude floods and, to a certain extent, other agents (e.g., colluvial transport processes, bedrock outcrops or the occurrence of large amounts of woody debris) (Grant et al, 1990;Kavage-Adams and Spotila, 2005;Bunte et al, 2014). Lenzi et al (2006a) demonstrated two stages of effective discharges in an Alpine torrent (A = 5 km 2 , mean S = 0.136 m/m), where the first stage equal or slightly higher (Q 1.5 -Q 3 ) than bankfull flow is responsible for maintaining the channel form in terms of pool depth and step-pool steepness, and the second one (Q 30 -Q 50 ) transforms the channel in terms of step destruction/ creation, adjustments of channel width, and planform changes.…”

“…Bunte et al (2014) documented the importance of high-magnitude flood events in mountain streams when they referred to maximal discharges as effective discharges for coarse-bedded, steep channels under a snowmelt regime, except for some step-pool channels that contained a high proportion of fine sediments. Lenzi et al (2006a) suggested definition of two dominant discharge ranges for steep headwater streams based on long-term observance of suspended and bedload transport in a steep Alpine channel.…”

Applicability of bed load transport models for mixed‐size sediments in steep streams considering macro‐roughness

“…Channel slopes range from about 0.05% to 11%, bankfull flows range from 0.3 to 114 m 3 s −1 , and the median grain size of the surface D 50 surf ranges from 0.004 to 0.2 m. The selected streams are compiled from the bed load trap data provided by Bunte et al . [, 2008, , hereinafter Bunte] including 10 U.S. streams in Colorado, Wyoming, and Oregon, as well as the data from Oak Creek, Oregon, USA [ Milhous , ], and data from two Swiss streams, Erlenbach (denoted EB ) and Riedbach (denoted RB ). To extend the range of bed gradients and grain sizes, the data set was complemented by data from King et al .…”