Vertical gradational variability of fines deposited in a gravel framework

Abstract

Vertical gradational structures develop as sand infiltrates into static gravel beds. Understanding the vertical distribution of interstitial sand deposits will improve predictions of ecological suitability and hyporheic hydrodynamics. A series of flume experiments was performed to investigate fine infiltration processes. Four sand distributions were introduced into flows over gravel beds. After each experiment, bed cores were extracted and analysed in vertical layers to examine the gradational trends with depth. Vertical trends of fine content were highly sensitive to the relative grain-size distributions of the gravel bed and the introduced sand. For experiments with d 15gravel /d 85sand ratios 15AE4 and larger unimpeded static percolation was observed, where sand filled the voids relatively uniformly from the bottom of the gravel layer to the top. Experiments with ratios 10AE6 and smaller bridged. Sand clogged a thin layer of gravel pores near the bed surface, precluding subsequent infiltration. Interstitial sand deposits fined with depth of penetration for all experiments which was the result of three distinct but overlapping processes. (i) Granular sorting: As particles fell through the substrate, smaller material preferentially passed through the voids deeper into the gravel. (ii) Bed-load sorting: Size segregation occurs in the wake of the leading bed form as smaller particles saltate further and settle first. (iii) Hydraulic sorting: Smaller sand was transported preferentially as suspended load filling the deep voids of the furthest flume positions downstream. Finally, when the experiments that formed a bridge layer were replicated with higher bed shear stresses, less interstitial sand deposition was observed. Higher shear stresses transported coarse particles downstream more efficiently causing bridge layers to form earlier and allowing less time for suspended load to settle into the deeper substrate pores before the pathways were closed.

Transport of moderately sorted gravel at low bed shear stresses: The role of fine sediment infiltration

“…This behavior is consistent with the findings reported by Gibson et al (2009), who stated that bridging should occur for D 15 =d 85 Ä 10.6 (for the present H-G/S bed, D 15 =d 85 was equal to 4.5). The sand was too coarse to recirculate mixed with the water.…”

“…According to Gibson et al (2009), USP is the expected infiltration mechanism for D 15 =d 85 15.4, meaning that fine sediments should fill the beds from the bottom upwards. For these beds, the D 15 =d 85 ratio, representing the capacity of the coarsest fine grains .d 85 / to get through the finest coarse grains .D 15 /, was larger than 150.…”

“…To a lesser extent, one can find a number of field (Frostick et al, 1984;Lisle, 1989;Sear, 1993) and laboratory experiments examining the infiltration of cohesive sediments (Krishnappan and Engel, 2006). Depending on the fine-to-coarse particle diameter ratio (Gibson et al, 2009), two infiltration mechanisms can occur:…”

Direct observations of the effect of fine sediment deposition on the vertical movement of Gammarus pulex (Amphipoda: Gammaridae) during substratum drying

“…This reflects the high clogging potential of 0.5-1 mm compared to 0.125-0.5 mm particles and the ability of larger particles to bridge the interstitial spaces between grains, blocking pathways within the subsurface and reducing sediment porosity/permeability (Boulton et al, 1998;Bo et al, 2007;Vadher et al, 2015). The high clogging potential of the 0.5-1 mm particles was clearly exhibited in this study as this size fraction completely clogged the surface of mesocosms (bridging the majority of surface interstitial pathways) forming a physical barrier (Gibson et al, 2009) through which G. pulex could not penetrate. The deposition of fine sediment particles (\ 2 mm) within riverbeds has Table 2 Fisher's least significant difference (LSD) post hoc pairwise comparison of Gammarus pulex burrowing at the end of experiments in treatments 1-7 (see Table 1 Table 1.…”

Using multiple bed load measurements: Toward the identification of bed dilation and contraction in gravel‐bed rivers

“…The rate of supply, the grain sizes, the size and shape of the pores and the local morphology of the channel are among the many physical factors controlling the infiltration of fine particles (that can be of small gravel sizes) into alluvial frameworks [ Frostick et al , 1984]. In a flume experiment, Gibson et al [2009] observed rapidly filling zones of the bed just prior to or following the arrival of a bed load front. This process, combined with the natural heterogeneity of a gravel bed and the hyporheic exchanges associated mainly with the presence of bed forms, is an interesting hypothesis to explain the local increase or decrease of the proportion of fine particles within the bed.…”