Self‐formed waterfall plunge pools in homogeneous rock

Abstract

Waterfalls are ubiquitous, and their upstream propagation can set the pace of landscape evolution, yet no experimental studies have examined waterfall plunge pool erosion in homogeneous rock. We performed laboratory experiments, using synthetic foam as a bedrock simulant, to produce self‐formed waterfall plunge pools via particle impact abrasion. Plunge pool vertical incision exceeded lateral erosion by approximately tenfold until pools deepened to the point that the supplied sediment could not be evacuated and deposition armored the pool bedrock floor. Lateral erosion of plunge pool sidewalls continued after sediment deposition, but primarily at the downstream pool wall, which might lead to undermining of the plunge pool lip, sediment evacuation, and continued vertical pool floor incision in natural streams. Undercutting of the upstream pool wall was absent, and our results suggest that vertical drilling of successive plunge pools is a more efficient waterfall retreat mechanism than the classic model of headwall undercutting and collapse in homogeneous rock.

A Mechanistic Model of Waterfall Plunge Pool Erosion into Bedrock

“…h sed is shown only for cases when h sed ≠ h BR (i.e., periods when sediment was deposited or predicted to be deposited in the pool) in Figures 3a and 3b; error bars in Figures 3a and 3c reflect ~20% uncertainty in laser scanner measurements (Scheingross et al, ). Figures 3c and 3d show experimentally measured plunge pool radii measured at the top of the pool ( r pool_lip ) and averaged radii based on pool volume ( r pool_avg ; Scheingross et al, ). Thin, solid vertical line in Figures 3b and 3d denotes timing of sediment supply ( Q s ) increase in Exp2, and gray shading denotes time when pool filled to levels outside of the model domain and no predictions are made.…”

“…Conceptually, our model builds on previously proposed ideas (Howard et al, ; Lamb et al, ; Scheingross et al, ; Scheingross & Lamb, ) and works as follows: approaching a free overfall, water accelerates due to the loss of hydrostatic pressure at the brink (Hager, ; Rouse, , ). The water detaches from the face of a bedrock step, forming a sediment‐laden waterfall that further accelerates during free fall (e.g., Stein et al, ).…”

“…The grains falling from the waterfall brink are assumed to be limited to the jet‐descending region where the waterfall jet impacts the pool floor ( r < δ ). For cases when plunge pools have radii smaller than the jet‐descending region, impacts from grains falling from the waterfall brink should act to rapidly widen plunge pools via sidewall impacts during their descent (Scheingross et al, ). Therefore, we assume that plunge pools must maintain a minimum radius, r min , that grows at the same rate as the expansion of the jet‐descending region in the ZOEF, that is, such that r min = δ ( z BR ) for z BR < z λ .…”

“…We additionally compare measurements of vertical erosion to predictions from the Lamb et al () plunge pool erosion model where we use the theory of Scheingross and Lamb () to solve for plunge pool sediment transport capacity. Full experimental details are described in Scheingross et al (); in brief, these experiments investigated the formation and evolution of waterfall plunge pools formed by sediment impacts into an initially planar surface using polyurethane foam as bedrock simulant (Lamb et al, ; Scheingross et al, ). Two experiments were performed varying grain size and sediment supply.…”

“…For the final 36.2 h of Exp2, sediment supply was increased by a factor of 5, forcing aggradation of the pool floor while lateral erosion continued. The range in plunge pool depth and radii achieved in the experiments results in approximately order of magnitude variation in the nondimensional variables that are thought to govern plunge pool sediment transport and are within the range observed in field surveys (Scheingross et al, ; Scheingross & Lamb, ), such that the dynamics observed in the laboratory should scale to field cases. To compare the theory to the experiments, we use the boundary conditions from the experiments (Table S2) and set an initial pool geometry of r pool = h BR = 1 cm.…”