Regional Variation in Gravel Riverbed Mobility, Controlled by Hydrologic Regime and Sediment Supply

Abstract

The frequency and intensity of riverbed mobility are of paramount importance to the inhabitants of river ecosystems as well as to the evolution of bed surface structure. Because sediment supply varies by orders of magnitude across North America, the intensity of bedload transport varies by over an order of magnitude. Climate also varies widely across the continent, yielding a range of flood timing, duration, and intermittency. Together, the differences in sediment supply and hydroclimate result in diverse regimes of bed surface stability. To quantitatively characterize this regional variation, we calculate multidecadal time series of estimated bed surface mobility for 29 rivers using sediment transport equations. We use these data to compare predicted bed mobility between rivers and regions. There are statistically significant regional differences in the (a) exceedance probability of bed‐mobilizing flows (W* > 0.002), (b) maximum bed mobility, and (c) number of discrete bed‐mobilizing events in a year.

The impact of inter‐flood duration on non‐cohesive sediment bed stability

“…The greatest rate of change in critical shear occurs for the shortest antecedent durations which is in line with previous stress history research (Paphitis and Collins, 2005;Monteith and Pender, 2005;Haynes and Pender, 2007;Ockelford and Haynes, 2013;Masteller and Finnegan, 2017). However, interestingly the rate of change is also grade specific where the unimodal bed response to increasing inter-flood flow duration is 2.5 times faster than the bimodal bed and three times as fast as the uniform bed.…”

“…Whilst Masteller and Finnegan (2017) fitted a linear model to their cumulative bedload flux data as a function of increased conditioning flow, they do state that an exponential decline function also fitted their data, albeit with lower model skill. Grass, 1970;Paintal, 1971;Graf and Pazis, 1977;Lavelle and Mofjeld, 1987;McEwan et al, 2004;Paphitis and Collins, 2005;Bottacin-Busolin et al, 2008) or to the fact that a population of high protruding grains is always available for transport (Masteller and Finnegan, 2017). Such behaviour is similar to that of many degradation experiments (Tait et al, 1992;Proffitt and Sutherland, 1983;Pender et al, 2001).…”

“…Reid and Frostick, 1984;Reid et al, 1985;Willetts et al, 1987;Oldmeadow and Church, 2006;Pfeiffer and Finnegan, 2018) and laboratory (Paphitis and Collins, 2005;Monteith and Pender, 2005;Haynes and Pender, 2007;Masteller and Finnegan, 2017) data which have both indirectly and directly suggested that antecedency may be an important control on entrainment thresholds and bedload flux. Analysis shows that all three grain size distributions responded to changes in antecedent duration.…”

“…The changes to entrainment threshold have been linked with bed re-organization during the subthreshold flow period (Hassan and Church, 2000;Haynes and Pender, 2007;Ockelford and Haynes, 2013;Masteller and Finnegan, 2017). Given that the applied antecedent flow in this article was set at τ* c50 , active, large-scale processes of re-organization as a result of grain entrainment are unlikely and thus significant bed surface composition change should not occur (Sutherland, 1991;Hassan and Church, 2000;Whiting and King, 2003).…”

“…Paphitis and Collins (2005) studied the entrainment threshold for uniform sand beds subjected to antecedent flow durations of up to 120 minutes. Using a unimodal gravel distribution and conditioning flow periods between 1 and 200 minutes Masteller and Finnegan (2017) noted an 86% reduction in bedload flux. Similarly, Monteith and Pender (2005) and Haynes and Pender (2007) exposed a bimodal sand-gravel mixture to increasing antecedent conditioning flow durations; up to a 48% increase in critical bed shear stress was noted as antecedent duration was increased from 0 to 5760 minutes.…”