Sampling variability in estimates of flow characteristics in coarse‐bed channels: Effects of sample size

Abstract

Adequate description of hydraulic variables based on a sample of field measurements is challenging in coarse‐bed streams, a consequence of high spatial heterogeneity in flow properties that arises due to the complexity of channel boundary. By applying a resampling procedure based on bootstrapping to an extensive field data set, we have estimated sampling variability and its relationship with sample size in relation to two common methods of representing flow characteristics, spatially averaged velocity profiles and fitted probability distributions. The coefficient of variation in bed shear stress and roughness length estimated from spatially averaged velocity profiles and in shape and scale parameters of gamma distribution fitted to local values of bed shear stress, velocity, and depth was high, reaching 15–20% of the parameter value even at the sample size of 100 (sampling density 1 m−2). We illustrated implications of these findings with two examples. First, sensitivity analysis of a 2‐D hydrodynamic model to changes in roughness length parameter showed that the sampling variability range observed in our resampling procedure resulted in substantially different frequency distributions and spatial patterns of modeled hydraulic variables. Second, using a bedload formula, we showed that propagation of uncertainty in the parameters of a gamma distribution used to model bed shear stress led to the coefficient of variation in predicted transport rates exceeding 50%. Overall, our findings underscore the importance of reporting the precision of estimated hydraulic parameters. When such estimates serve as input into models, uncertainty propagation should be explicitly accounted for by running ensemble simulations.

Quantifying the restoration success of wood introductions to increase coho salmon winter habitat

“…At bankfull flow, the increased floodplain connectivity was associated with more heterogeneous flow fields characterized by wider distributions of velocity and shear stress with overall lower mean values. The shapes of pre-LW velocity distributions for all sites were similar to those observed in small mountain streams with large frequency at both intermediate and low velocity values (Cienciala and Hassan, 2016). The shape of the velocity distributions changed dramatically, with post-LW being characterized by a higher proportion of lowvelocity areas in all three sites.…”

“…Flume experiments as well as field simulations have also reported reductions in flow velocity with increasing large wood obstacles (He et al, 2009;Davidson and Eaton, 2013;Hafs et al, 2014). The distributions of shear stress also changed dramatically from closely resembling those observed in single-thread streams pre-LW (Lisle et al, 2000;Mueller and Pitlick, 2014;Segura and Pitlick, 2015;Cienciala and Hassan, 2016) to resembling complex braided channels (Paola, 1996;Nicholas, 2003;Mueller and Pitlick, 2014;Tamminga et al, 2015) post-LW. The shift towards a greater frequency of low shear stress is likely attributed to shear stress partition by the channel banks and LW form drag (Kean and Smith, 2006;Yager et al, 2007;Ferguson, 2012;Scheingross et al, 2013).…”

Hydraulic studies and their influence for regional urban planning: a pratical approach to Fynchal’s rivers

“…Despite the difficulty in determining the coefficient of roughness, several scientificdocuments such as books, articles and studies have been published over the years (Harun-ur-Rashid, 1990;ACPA, 1997;Martins, 2000;Lyra, 2003;IHB, 2005;Abayati, et al, 2006;Szydłowski & Magnuszewski, 2007;De Doncker, et al, 2009;Hossain, et al, 2009;Baioni, 2011;Acevedo-Espinoza, 2014;Aupoix, 2015;Li, et al, 2015;Neal, et al, 2015;Cienciala & Hassan, 2016;Dimitriadis, et al, 2016;Verschoren, et al, 2016;Wei, et al, 2016), contributing to a significant improvement in applied methods, methodologies and materials, focusing not only technical aspects but also their influence in urban planning processes.…”

Vegetation and Geomorphic Connectivity in Mountain Fluvial Systems

“…These effects can persist for decades after the initial disturbance, especially when large quantities of coarse material are supplied [247,248] (Figure 8D). Such changes, in turn, alter disturbance regimes in riverine ecosystems [234,[249][250][251][252], which are tightly linked to sediment dynamics [253][254][255]. Therefore, a better understanding of these cascading effects is critical also from the point of view of ecosystem conservation and restoration [256,257].…”