Experimental evidence for fluvial bedrock incision by suspended and bedload sediment

Abstract

Fluvial bedrock incision sets the pace of landscape evolution and can be dominated by abrasion from impacting particles. Existing bedrock incision models diverge on the ability of sediment to erode within the suspension regime, leading to competing predictions of lowland river erosion rates, knickpoint formation and evolution, and the transient response of orogens to external forcing. We present controlled abrasion mill experiments designed to test fl uvial incision models in the bedload and suspension regimes by varying sediment size while holding fi xed hydraulics, sediment load, and substrate strength. Measurable erosion occurred within the suspension regime, and erosion rates agree with a mechanistic incision theory for erosion by mixed suspended and bedload sediment. Our experimental results indicate that suspension-regime erosion can dominate channel incision during large fl oods and in steep channels, with signifi cant implications for the pace of landscape evolution.

Experimental evidence that ooid size reflects a dynamic equilibrium between rapid precipitation and abrasion rates

“…4a-b) for kv = 9 x 10 5 and β = 2. This value of kv is similar to the range of values calculated for bedrock erosion (kv ~ 10 6 , Dietrich, 2004 andLamb et al, 2008; kv ~ 3 x 10 5 , Scheingross et al, 2014). As in the total-load bedrock erosion model (Lamb et al, 2008;Scheingross et al, 2014), the ooid precipitation-abrasion model accounts for viscous damping of particle-bed impacts using a threshold particle Stokes number, St =…”

“…S10) is Stc = 9, lower than that identified for total-load bedrock erosion (Stc = 75; Scheingross et al, 2014) , but consistent with the critical Stokes number, Stc ~ 5-10, identified as the threshold below which particle deformation during a collision is negligible (Davis et al, 1986;Joseph et al, 2001). The lower Stokes limit might also be due to the roughened bed in the ooid experiments, as compared to a smooth bedrock bed in the experiments of Scheingross et al (2014).…”

“…This is similar to the variation of experimental bedrock erosion rates (c.f. Table DR1 in Scheingross et al, 2014). Complexities relating to carbonate chemistry may also contribute to variations between trials.…”

“…The volume eroded per impact is defined as (Sklar and Dietrich, 2004) where Vp is particle volume, ρs is particle density (~2.8 g/cm 3 , see Table S3), wi is impact velocity normal to the bed, and εv is the kinetic energy per unit volume eroded, which is defined as = 2 2 (4) where σT is tensile strength (we chose 1 MPa following Torok, 2007), Y is Young's modulus of elasticity (we chose 20 GPa following Lebedev et al, 2014), and kv is a dimensionless coefficient that accounts for differences in material properties between the particles and the bed surface (e.g. Scheingross et al, 2014). As in Lamb et al, 2008, the number of impacts per unit time per unit particle area should scale as = 2…”

Exceptional river gorge formation from unexceptional floods

“…During the rapid phase of gorge formation, abrasion was considered to be minimal as the river sediment load would have been largely entrapped within the reservoir behind the dam. Although negligible, some erosion by abrasion may have occurred within the plunge pool associated with dislodgement and transport of the scoured blocks from the plunge pool 13,43,44 . None of the existing documentation or photographs observed within the archives suggests any evidence to support spring seepage as an active process at any stage of erosion.…”